What is Short Circuit Analysis, and Why is it Done?

Published by Carelabs (Carelabz), Website: carelabz.com

Image: Carelabz

A Short circuit analysis is used to determine the magnitude of short circuit current, the system is capable of producing, and compares that magnitude with the interrupting rating of the overcurrent protective devices (OCPD). Since the interrupting ratings are based by the standards, the methods used in conducting a short circuit analysis must conform to the procedures which the standard making organizations specify for this purpose. The American National Standards Institute (ANSI) publishes both the standards for equipment and the application guides, which describes the calculation methods. 

Short-Circuit Currents are currents that introduce large amounts of destructive energy in the forms of heat and magnetic force into a power system. A short circuit is sometimes called a fault. It is a specific kind of current that introduces a large amount of energy into a power system. It can be in the form of heat or in the form of magnetic force. Basically, it is a low-resistance path of energy that skips part of a circuit and causes the bypassed part of the circuit to stop working. The reliability and safety of electric power distribution systems depend on accurate and thorough knowledge of short-circuit fault currents that can be present, and on the ability of protective devices to satisfactorily interrupt these currents. Knowledge of the computational methods of power system analysis is essential to engineers responsible for planning, design, operation, and troubleshooting of distribution systems. 

Short circuit currents impose the most serious general hazard to power distribution system components and are the prime concerns in developing and applying protection systems. Fortunately, short circuit currents are relatively easy to calculate. The application of three or four fundamental concepts of circuit analysis will derive the basic nature of short circuit currents. These concepts will be stated and utilized in a step-by step development. 

The three phase bolted short circuit currents are the basic reference quantities in a system study. In all cases, knowledge of the three phase bolted fault value is wanted and needs to be singled out for independent treatment. This will set the pattern to be used in other cases. 

A device that interrupts short circuit current, is a device connected into an electric circuit to provide protection against excessive damage when a short circuit occurs. It provides this protection by automatically interrupting the large value of current flow, so the device should be rated to interrupt and stop the flow of fault current without damage to the overcurrent protection device. The OCPD will also provide automatic interruption of overload currents. 

Short-circuit calculations are required for the application and coordination of protective relays and the rating of equipment. All fault types can be simulated. Carelab’s short-circuit study provides a detailed report identifying breaker ratings, breaker fault duties, discussions, and recommendations for any deficiencies found 

Risks Associated With Short Circuit Currents 

The building/facility may not be properly protected against short-circuit currents. These currents can damage or deteriorate equipment. Improperly protected short-circuit currents can injure or kill maintenance personnel. Recently, new initiatives have been taken to require facilities to properly identify these dangerous points within the power distribution of the facility. 

Why Is A Short Circuit Dangerous? 

A short circuit current can be very large. If unusually high currents exceed the capability of protective devices (fuses, circuit breakers, etc.) it can result in large, rapid releases of energy in the form of heat, intense magnetic fields, and even potentially as explosions known as an arc blast. The heat can damage or destroy wiring insulation and electrical components. An arc blast produces a shock wave that may carry vaporized or molten metal, and can be fatal to unprotected people who are close by. 

Fault current calculations are necessary to properly select the type, interrupting rating, and tripping characteristics of power and lighting system circuit breakers and fuses. Results of the fault current calculations are also used to determine the required short-circuit ratings of power distribution system components including bus transfer switches, variable speed drives, switchboards, load centres, and panel boards. In calculating the maximum fault current, it is necessary to determine the total contribution from all generators that may be paralleled and the motor contribution from induction and synchronous motors. 

Short Circuit Analysis is performed to determine the currents that flow in a power system under fault conditions. If the short circuit capacity of the system exceeds the capacity of the protective device, a dangerous situation exists. Since growth of a power system often results in increased available short-circuit current, the momentary and interrupting rating of new and existing equipment on the system must be checked to ensure the equipment can withstand the short-circuit energy (see Device Evaluation). Fault contributions for utility sources, motors and generators are taken into consideration.  

A Short Circuit Analysis will help to ensure that personnel and equipment are protected by establishing proper interrupting ratings of protective devices (circuit breaker and fuses). If an electrical fault exceeds the interrupting rating of the protective device, the consequences can be devastating.  It can be a serious threat to human life and is capable of causing injury, extensive equipment damage, and costly downtime. 

On large systems, short circuit analysis is required to determine both the switchgear ratings and the relay settings.  No substation equipment can be installed with knowledge of the complete short circuit values for the entire power distribution system.  The short circuit calculations must be maintained and periodically updated to protect the equipment and CC the lives. It is not safe to assume that new equipment is properly rated. 

The results of a Short Circuit Analysis are also used to selectively coordinate electrical protective devices. 

What is Short Circuit Analysis? 

Short circuit analysis essentially consists of determining the steady state solution of a linear network with balanced three phase excitation. Such an analysis provides currents and voltages in a power system during the faulted condition. This information is needed to determine the required interrupting capacity of the circuit breakers and to design proper relaying system. To get enough information, different types of faults are simulated at different locations and the study is repeated. Normally in the short circuit analysis, all the shunt parameters like loads, lime charging admittances are neglected* Then the linear network that has to be solved comprises of  

  • Transmission network  
  • Generator system and  
  • Fault. By properly combining the representations of these components we can solve the short circuit problem

Carelabs allows you to perform a per unit calculation on any system you are working with. We automatically converts the entire system (panel boards, transformers, generators, motorized items and cables) into a unique impedance unit from which you can obtain the rating of the short circuit current at any given point. The process is simple, efficient and will save you both money and time. 

Carelabs provides short-circuit calculations for single and multiple faults, together with a number of reporting options. As short-circuit calculations are needed for a variety of purposes, the short-circuit calculation in Carelabs supports different representations and calculation methods based on a range of international standards, as well as the superposition method (also known as the Complete Method).

What Are Bolted, Arcing and Ground Faults?

A bolted fault typically results from a manufacturing or assembly error that results in two conductors of different voltages being “bolted” together, or a source of power being directly connected (bolted) to ground. Since the connectors are solidly bolted there in no arc created and the high current quickly trips a protective device limiting the damage. 

An arc fault is one in which the short circuit creates an arc. An arc is a flow of electricity between two conductors that are not in contact. The resulting intense heat can result in a fire, significant damage to the equipment, and possibly an arc flash or arc blast resulting in serious injuries. 

A ground fault is when electricity finds an unintended, low resistance, path to ground. When that path goes through a human body the resulting heat can cause serious burns, and the electrical shock can disrupt the functioning of the human heart (fibrillation). 

What Are Symmetrical and Asymmetrical Currents?

A polyphase system may experience either a symmetrical or an asymmetrical fault. A symmetrical fault current is one that affects all phases equally. If just some of the phases are affected, or the phases are affected unequally, then the fault current is asymmetrical. 

Symmetrical faults are relatively simple to analyse, however they account for very few actual faults. Only about 5% of faults are symmetrical. Asymmetrical faults are more difficult to analyse, but they are the more common type of fault. 

What Are Protective Devices for Short Circuit Analysis?

Protective devices are designed to detect a fault condition and shut off the electric current before there is significant damage. There are a number of different types of protective devices, the two most common are: 

Fuses and Circuit Breakers

Fuses and circuit breakers are used to protect an electrical circuit from an over-current situation, usually resulting from a short circuit, by cutting off the power supply. Fuses can only be used once. Circuit breakers may be reset and used multiple times. 

Ground Fault Interrupter (GFI)

This is a device that detects when the current flow in the energized conductor does not equal the return current in the neutral conductor. The GFI protects people by quickly cutting off the current flow preventing injuries resulting from shock.  Ground Fault Interrupters are typically used in homes for bathroom, kitchen, and outdoor electrical sockets. The GFI will typically be built into the electrical socket. 

A GFI does not provide over-current protection, and the circuit that includes a GFI will also include a fuse or circuit breaker. 

In addition to fuses, circuit breakers, and GFIs, there are electrical protection devices that: 

  • detect changes in current or voltage levels 
  • monitor the ratio of voltage to current 
  • provide over-voltage protection 
  • provide under-voltage protection 
  • detect reverse-current flow 
  • detect phase reversal 
When are Short Circuit Analysis Needed? 

The first short-circuit analysis should be performed when a power system is originally designed, though this should not be the only time. These studies need to occur with any facility expansion or with the addition of any new electrical equipment such as circuit breakers or new transformers and cables. Without any new additions or changes, short circuit studies still need to occur on a regular basis of at least every 5-6 years. 

How Is Short Circuit Current Calculated? 

Short-circuit calculations are required to correctly apply equipment in accordance with NEC, and ANSI standards. Depending on the size and utility connection, the amount of detail required to perform these calculations can vary greatly. Carelabs short-circuit analysis will include calculations performed in accordance with the latest ANSI standards. 

Switches, fuses, and breakers that need to interrupt or close into a fault are of special concern. Cables and buswork also have short-circuit withstand limitations, and a thorough study will examine non-interrupting equipment, as well as switches and breakers. Standards such as ANSI C37.010 and C37.13 outline the recognized calculation methods for these equipment-rating analyses. 

These short circuit studies are performed using power system software as per IEEE standards. For larger systems, these short circuit calculations to be performed for both switch gear ratings and relay settings. Knowledge of the computational methods of power system analysis is essential to engineers responsible for planning, design, operation, and troubleshooting of distribution systems. A short-circuit study is an analysis of an electrical system that determines the magnitude of the currents that flow during an electrical fault. Comparing these calculated values against the equipment ratings is the first step to ensuring that the power system is safely protected. Once the expected short-circuit currents are known, a protection coordination study is performed to determine the optimum characteristics, ratings and settings of the power system protective devices. 

NEC 110 requires that a short circuit analysis be done for all electrical equipment and panels. The two most common standards for short circuit current calculations are the ANSI/IEEE C37.010-1979 standard and the International Electro-technical Commission (IEC) 60909 standard. 

The ANSI C37.010 standard was intended to be used for power circuit breaker selection, but it does provide the information needed for NEC 110 required labelling. The IEC 60909-3:2009 standard is more generic. It is intended to provide general guidelines for short-circuit analysis of any asymmetrical short circuit in a three-phase 50 Hz or 60Hz A.C. electrical system. 

Either the ANSI or the IEC short circuit calculation method can be used. They have been compared and found to produce similar results. The ANSI method is commonly used in short circuit current calculation software.  

Our short circuit analysis service: 

  • Is done with support of IEC 60909 (including 2016 edition), IEEE 141/ANSI C37, VDE 0102/0103, G74 and IEC 61363 norms and methods 
  • Is calculation of short-circuit currents in DC grids according to IEC 61660 and ANSI/IEEE 946 
  • We do complete superposition method, including dynamic voltage support of generators connected via power electronics 
  • Multiple fault analysis of any kind of fault incl. single-phase interruption, inter-circuit faults, fault sweep along lines, etc. 
Diakoptic Model For The Short Circuit Analysis (Do We Use This?)

In the short circuit analysis, it is customary to neglect the loads and other shunt parameters to the ground. Under this condition, impedance representation for the transmission network with ground as reference does not exist. However, connection to the ground is established at the generator buses, representing the generator as a constant voltage source behind appropriate reactant. Hence let us consider the combined transmission-generator network and while tearing the network, let us ensure that each sub-network has at least one generator. In practice this should pose no difficulty since large power system 84 networks normally consist of different areas having generations in each area.  

Neplan

Short circuit analysis is performed so that existing and new equipment ratings were sufficient to with stand the available short circuit current. This short circuit analysis can be done either through hand calculations or through known software like NEPLAN. 

Using NEPLAN we can perform short circuit studies on electrical systems in a quick time and effective manner in four steps. 

  • Data Collection and SLD Preparation 
  • Short circuit calculations 
  • Relay Coordination Studies 
  • Load flow Analysis  
Why chose Carelabs for Short Circuit Analysis? 

At Carelabs we differ from competitors in our size and structure and this allows us to be more responsive to change. It also allows us to provide personalized and superior services to you. We follow NFPA-70E and IEEE 1584 guidelines in order to guarantee that we always meet the highest industry standards.  

Benefits of Short Circuit Analysis 

Conducting a short circuit analysis has the following benefits: 

  • Helps avoid unplanned outages and downtime
  • Is critical for avoiding interruptions of essential services
  • Reduces the risk of equipment damage and fires
  • Increases safety and protects people from injuries
  • Determines the level and type of protective devices that are needed
  • Provides the information needed for NEC and NFPA required labels 
  • Keeps you in compliance with NEC requirements 
  • Reduces the risk a facility could face and help avoid catastrophic losses
  • Increases the safety and reliability of the power system and related equipment

Source: https://carelabz.com/what-short-circuit-analysis-done-why/

Rules, Indices and Approach for Assessment and Improvement of Power Quality

Published by Antonio Ardito, Stefano Malgarotti, Angelo Schiappacasse,1

1 This paper was developed within the activity Ricerca di Sistema DM 26/01/2000. Authors are with CESI spa, via Rubattino 54, 20134 Milano (MI), Italy, (e-mail: malgarotti@cesi.it, tel. +39-02 2125-5517)

ABSTRACT

Power Quality is of concern in both open market and in vertically integrated environment, since the quality of the energy supply is on one side a prerequisite to establish a market, and, on the other side, needed to support the investments for industries requiring an acceptable power supply quality, due to their sensible production processes.

Power Quality issue deals with both “continuity of supply” and “voltage quality”. Normally the standard levels of the electric power supply are defined by the competent National Authority, that has to define the indices of continuity of supply and the relevant standard levels. The assessment of the actual power quality levels has to be performed both for network areas, to which a set of Client/Users are connected, and for specific Client/Users, in their point of common coupling with the public network. The final scope is not only to assess the actual power supply quality, but to stimulate its improvement, through technical provisions related to investments. The paper, making use of CESI experience, deals with power quality rules and indices applied or under development, with the possible methods to assess the actual power supply quality, even by measurements, and with the technical provisions to be taken in the sub-transmission and distribution network in order to improve the power quality.

These provisions consist of network reinforcements, improvement of network schemes, enhancement of relay protection system, change in the neutral earthing practice, adoption of power electronic devices (Custom Power), enhancement of insulation co-ordination with reference mainly to lightning performance and to the adoption of suitable surge arresters.

1.INTRODUCTION TO EMC AND POWER QUALITY

The matter of Electromagnetic Compatibility (EMC) has been developed over a long period of time and it is rather complicated because it covers conducted and radiated phenomena over the whole frequency spectrum in use (from 0 to 400GHz), excluding problems related to telecommunications like the allocation of frequencies, transmission techniques and other specialised subjects. In Europe a strong impulse to the development of the EMC subject has been given by the European Union (EU) Directives 85/374 and 89/336 [1], [2]; in any case it is not yet complete from the point of view of the practical application which still has unfulfilled requirements.

Power Quality is conventionally intended as the quality of the electric power supply to customers/users in terms of technical characteristics that have to comply with levels fixed or recommended by standards, rules and contractual obligations: EMC standards cover Power Quality.

With the purpose to provide a self-contained description concerning the EMC subject, some basic definitions are given here under:

Electromagnetic disturbance: any electromagnetic phenomenon, which may degrade the performance of a device, equipment or system or adversely affect living or inert matter. (An electromagnetic disturbance may be an electromagnetic noise, an unwanted signal or a change in the propagation medium itself);

Disturbance level: the value of a given electromagnetic disturbance, measured in a specified way. (Disturbance levels are generally designated as 95 % probability values on a basis of a time statistics);

Electromagnetic Compatibility (EMC): the ability of an equipment or system to function satisfactorily in its electromagnetic environment and without introducing intolerable electromagnetic disturbances to anything in that environment;

Electromagnetic Compatibility Level: the specified disturbance level at which an electromagnetic compatibility should exist. (The compatibility levels are reference values for purpose of coordinating emission and mainly immunity of equipment; they are generally designated as 95 % probability values on a basis of time and system locations statistics);

Planning level: The specified disturbance level used mainly for planning purposes in evaluating the impact on the system of all disturbing consumers or equipment.

(Planning levels are:

  • internal reference values of the utilities, generally
    equal to or lower than compatibility levels,
  • not standardized, but only indicative values, often used for emission co-ordination in determining the emission allowed to the consumers, generally designated as 95 % probability values on a basis of a time statistics);

Total disturbance level: the level of a given electromagnetic disturbance caused by the superposition of the emissions of all pieces of equipment in a given system (Total disturbance levels are generally designated as 95 % probability values on a basis of a time statistics);

Conducted disturbance: Electromagnetic phenomen which propagates along the electricity supply conductors and/or signal-control connections. In transmission and distribution systems conducted disturbances can propagate along power lines and in some cases across
transformers so that they may affect equipment distant from their source. The main types of conducted disturbances are harmonics, interharmonics, voltage fluctuations, voltage dips and short supply interruptions, voltage unbalance, mains frequency variation;

Emission level: level of a disturbance injected into the surrounding space or into the supply conductors (Emission levels are generally designated as 99% or 95 % probability values on a basis of a time statistics);

Immunity level: the maximum level of a given electromagnetic disturbance, incident in a specified way on a particular device, equipment or system, at which no degradation of its specified performance occurs;

Susceptibility: the inability of a device, equipment, or system to function without degradation of its specified performance in the presence of an electromagnetic disturbance.

Utilities are concerned particularly with the electromagnetic disturbances which are described as “conducted disturbances”. These are so called because they are carried by the power conductors and are in this way distinct from other disturbances, like those consisting
of electromagnetic fields affecting the space around their sources.

Table 1 : Conducted Disturbances

Conducted low Frequency PhenomenaHarmonics, Interharmonics
Voltage fluctuations/flicker
Voltage unbalance
Voltage dips and interruptions
Signalling systems
Power frequency variations
Induced low frequency voltages
Direct Current in AC networks
Conducted high frequency phenomenaUnidirectional transients
Oscillatory transients
Magnetic fields
2.INTERACTION BETWEEN PARTIES

The distributors are the operators of a network which is provided for the sole purpose of delivering an electricity supply to the consumers, but which unintentionally on their part becomes the medium on which electromagnetic disturbances are conducted from their sources to susceptible equipment. In fact, particularly for low frequency conducted disturbances that are prone to summing up together, the disturbances at the supply terminals are the result of a population of sources spread along the network; practically the distribution network becomes a collector of disturbances mainly generated by consumers and conveys them to the sensitive equipment. Often distributors are concerned with large installations (Users) which may be the source of significant levels of disturbance emission.

It is mandatory, and usually is a condition of supply, that these emissions be maintained below levels which, in combination with emissions from all other sources, would interfere unduly with the performance of other users’ equipment. It is generally necessary, therefore, to set down specific emission limits for each large User on the basis of connection regulations; a strict co-operation between electric utility and consumer is often necessary. Distributors are also concerned with an increasingly large proportion of conducted disturbances that have their source in the section of the environment described as residential, commercial and light industry. As the equipment in this environment is comprised of large numbers of individually small items, the only feasible way to promote electromagnetic compatibility is to ensure that the design of the equipment is such that disturbance emissions are adequately controlled, having regard to the additive effect of emissions from multiple sources and to the adequate equipment immunity to disturbances. This implies that appropriate standards for both emission and immunity are adopted.

On the basis of what above reported, compliance with EMC requirement in distribution networks implies an interaction between several parties as:

  • Standardization bodies (environment definition, immunity and emission of single equipment)
  • Supply utilities (rules for connection of the installation with disturbing loads)
  • Users in collaboration with designers of plants and equipment manufacturers (choice of equipment and mitigation methods if required).

Figure 1 taken from [3], shows the relationships and the mutual effects between the standardisation process, the supply system and the user installation.

Figure 1: Relation between standardization, management of voltage characteristics, user equipment and installation options [3]

3.INTERNATIONAL STANDARDS RELEVANT TO POWER QUALITY

3.1 CENELEC standards relevant to the voltage characteristics of electricity supply

In the European Union (EU), with the goal to create a wide economic space without barriers to internal trade, a number of Directives have been issued by the Commission of the EU, to remove the differences in the legislation of the Member States, which could affect the free exchange of goods and services.

In particular the EU, recognising the energy market as an important component of the internal market, issued two Directives: Directive 85/374 EEC of July 25, 1985 on Product Liability [1], states in Article 2 that electricity is to be considered a product and, as a consequence, its characteristics have to be defined; Directive 89/336 EEC of May 2, 1989 on Electromagnetic Compatibility [2], declares that the Member States are responsible for ensuring that electricity distribution networks are protected from electromagnetic disturbances, which can also affect the connected equipment. An amendment introduced by Directive 92/31 has extended the transitory period to the end of 1995. On mandate of European Commission, CENELEC2 has been charged to develop a standard relevant to the voltage characteristics of electricity supplied by low and medium voltage public distribution systems. The mandate specified the different aspects to be covered, which were exclusively related to the following characteristics of the supply voltage: frequency, magnitude, waveform and symmetry of the three-phase-voltages.

The standard prepared by CENELEC is the EN 50160: “Voltage characteristics of electricity supplied by public distribution systems”, Second Edition, March 2000, [4]. The Scope of EN 50160 specifically excludes compatibility levels or emission limits. Its sole function is to give values for the main voltage characteristics of electricity supplied by LV and MV public networks.

The standard is applicable only under normal operating conditions of the supply system, excluding any condition outside the supplier’s control.

On the basis of IEC3 Standards, compatibility levels can be exceeded with a 5% probability in time and also in locations of the supply network, while voltage characteristics can be not complied with for 5% of the time in a specified observation period, but in all locations of the network. This explains why some of the EN 50160 voltage characteristics are less stringent than the compatibility levels.

Moreover the voltage characteristics serve as a reference concerning the electricity supply, as an indication on its expected performance and as a guiding criterion for selecting the immunity of user equipment.

2 European Committee for Electrotechnical Standardization
3 International Electrotechnical Commission

3.2 Standards relevant to compatibility level

Rules and recommendations presently in force and dealing with EMC compatibility levels for public networks are:

  1. IEC 61000-2-2 (Standard);
  2. IEC 61000-3-6 (Technical Report) and IEC 61000-3-7 (Technical Report).
  3. UNIPEDE “Report on EMC co-ordination in electricity supply systems”.

IEC 61000-2-2 Standard sets the compatibility levels for LV public supply networks with reference to harmonics, interharmonics, flicker, unbalance. Voltage dips and short interruption are dealt with by a statistical approach without setting limit values. The compatibility levels for MV and HV are under development.

IEC 61000-3-6 and IEC 61000-3-7 Technical Reports outline principles dealing with requirements for connecting large distorting loads to MV and HV public power systems. Both documents report the compatibility levels in LV and MV systems and the planning levels in MV, HV and EHV systems. The first document is relevant to harmonic voltages while the second one is relevant to flicker and rapid voltage changes.

UNIPEDE4 “Report on EMC co-ordination in electricity supply systems” deals with the co-ordination of the emission of low frequency conducted disturbances in public electricity supply systems with reference to harmonics, interharmonics, flicker and unbalance.

4.THE ROLE OF SUPPLY UTILITIES AND USERS

EMC requirement for conducted disturbances in distribution networks mainly consists in:

• emissions from each separate source of disturbance are such that the combined emissions from all sources do not exceed the conventionally accepted level of disturbance to be expected in the environment;
• equipment immunity is such that the appropriate level of performance at the conventionally expected level of disturbance is assured.

What above implies, from one side, to control the total disturbance in the distribution networks by connection, regulation and emission standards and, from another side, to adopt immunity level of the equipment assuring a margin with respect to the total disturbance level expected at the supply terminals.

4 International Union of Producers and Distributors of Electrical Energy

Figure 2, taken from [3], shows the relationships of the different above mentioned levels in the perspective of EMC co-ordination in a deterministic representation.

Figure 2: Deterministic representation of the co-ordination of conducted disturbances.

(1): Defined by Standards or Electricity Supplier (time statistic, 95% value)
(2): For a network location with a medium-high disturbance
(3): Defined by Electricity Supplier (time statistics, 95% value)
(4): Defined by Standards (time and location statistics, 95% value)
(5): Defined by Standards (time statistics, 95% value)
(6): Defined by Standards or agreed between User and Manufacturer

On the basis of what reported above, in EMC coordination relevant to the conducted disturbances, the roles of utilities and users can be very synthetically summarised as follows:

• Utilities have to control the total disturbance levels in the different system stages (different voltage levels), assuring the respect of the compatibility levels on the LV; this by defining reasonable and flexible disturbance emission limits for the users. In other words utilities have to assure that the supplied voltage at the user terminals presents characteristics being inside predefined ranges.
• Users have to respect emission limits and to adopt suitable immunity levels for equipment and machinery (including mitigation methods if required).

5. LOW FREQUENCY CONDUCTED DISTURBANCES

In this paper only the following types of low frequency conducted disturbances are considered: flicker; voltage dips; long and short interruptions; harmonics [4]. Flicker: a voltage fluctuation that causes changes of luminance of lamps giving unsteadiness of visual sensation. The flicker intensity is evaluated by UIE – IEC flicker measuring method through the following indices: Short Term Severity (Pst) measured over a period of 10 min; Long Term Severity (Plt) calculated from a sequence of 12 Pst values according to the following expression:

Voltage dips: a sudden reduction of the supply voltage to a value between 90% and 1% of the declared voltage followed by a voltage recovery in a short period of time;
Supply interruption: when the voltage at the supply terminal is lower than 1% of the declared voltage:

  • Long interruption – duration > 3min;
  • Short interruption – duration <3min and >1s;

Harmonic voltage: a sinusoidal voltage with a frequency equal to an integer multiple of the fundamental frequency.

Regarding the voltage levels of the systems, if not specifically stated, the following definitions apply:

  • Low Voltage (LV): VN ≤1kV
  • Medium Voltage (MV): 1kV<VN ≤35kV
  • High Voltage (HV): 35kV<VN≤230kV
  • Extra High Voltage (EHV): 230kV<VN

A short list of planning and compatibility levels from international standards is reported here under.

5.1 EHV and HV planning levels

Harmonics

Total harmonic distortion:

where Vh is the voltage amplitude of h harmonic order and Vl is the rms value of the fundamental frequency voltage. During each period of one week, the 95% of the 10 minutes rms values of each individual harmonic voltage (Vh) has to be equal or lower than the values in the previous table.

Flicker

5.2 MV planning and compatibility levels

Harmonics

Total harmonic distortion:

where the meaning of quantities and symbols is the same as in point 5.1.

Flicker

Supply voltage dips and interruptions

5.3 LV compatibility levels

Harmonics

Total harmonic distortion:

where the meaning of quantities and symbols is the same as in point 5.1.

Flicker

Supply voltage dips and interruptions

6.RULES ON CONTINUITY OF SUPPLY IN ITALY

In Italy the standard levels of quality of the electric power supply are defined by the competent Authority. Nowadays the Authority has already defined the indices of continuity of supply and some relevant standard levels, while, as far as the voltage quality is concerned, the Authority charged the Italian Independent System Operator (GRTN) to issue technical rules, now under elaboration, covering this specific aspect. Continuity of supply is defined as the absence of interruption of electric energy supply to the Users.

The aims of the Authority are:

  • to define an exhaustive and homogeneous method to detect the interruptions;
  • to define a method to record and store the above data, that have to be traceable and truthful;
  • to stimulate the improvement of the continuity of supply, making it nearer to the one of other Countries that nowadays show higher performance;
  • to reduce the differences in the continuity of supply among various Italian regions and areas;
  • to introduce an automatic mechanism for refunding the Users, when the standard levels are not fulfilled.

The distribution utility has the duty to set up and fill a data-base of the interruptions to its Users. The definition and classification of the interruptions are the following:

  • Interruption is the condition when the voltage, in the point of common coupling, becomes <1% of the nominal voltage, according to international standards.
  • The interruption is defined as prearranged when it is scheduled and noticed to the User:
    • 1 day in advance, at least;
    • through suitable means;
    • with indication of the starting time and duration.
  • The accidental interruptions (i.e. without notice) are classified as:
    • long, when the duration is > 3 min;
    • short, when the duration is < 3 min and >1 s;
    • transitory, when the duration is < 1s.
  • The Distribution utility has to detect and record all the interruptions: the prearranged ones and the accidental ones (long, short and transitory). Each record has to report the origin, the cause (only for accidental long interruptions), the number of HV and MV Users affected, the number of LV Users affected (limited to short and long interruptions only), the timing of the progressive restoration of the supply. The two indicators relevant to the continuity of supply are:
    • the number N of interruptions per User, for all kind of interruptions;
    • the cumulative duration D of the interruptions per User (limited to prearranged and accidental long interruptions only).
  • The Distribution utility has to calculate the values of the above indicators:
    • for each HV User (for all kind of interruption);
    • for each MV User (limited to prearranged and accidental long interruptions only);
    • the average value for both MV User (for all kind of interruption) and LV User (limited to prearranged and accidental long interruptions and short interruptions only).

Depending on the population of the municipality, different standard levels (thresholds) of the D indicator for LV User are already fixed, limited to accidental long interruptions:

  • highly populated municipality (>50.000 inhabitants): D = 30 min;
  • medium populated municipality (>5.000 and <50.000 inhabitants): D = 45 min;
  • lowly populated municipality (<5.000 inhabitants): D = 60 min.

For all other indices, the standard levels (thresholds) are not yet fixed.

7.POWER QUALITY IMPROVEMENT

As far as rural or semi-rural distribution systems are concerned, both continuity of supply and voltage quality are significantly affected by lightning. In fact such systems have typically overhead lines that are exposed to direct and indirect lightning which causes common mode overvoltages (i.e. between live conductors and ground). Such overvoltages produce insulation failures, thus causing transitory and short interruptions, or even long lasting ones when they cause damages to equipment; on the other side, they causes impulse overvoltages towards the users.

Electric utilities are making a big effort in order to improve the power quality with respect to this kind of phenomena, mainly with regard to the continuity of supply, that is the most sensitive issue. Different remedies have been investigated and then adopted.

Some utilities are substituting spark gaps by metal-oxide surge arresters on medium voltage systems in order to better protect transformers and other equipment, thus reducing the number of long (permanent) interruptions. However, even the arresters are not very effective to protect line insulation: they are ineffective against the lighting strokes that directly hit the line, while, as regards overvoltages induced by indirect lightning strokes that hit the ground near the line, arresters are able to protect a very short length close to themselves, due to the short time to crest of the induced overvoltages.

In particular, with regard to the Italian MV overhead line distribution network, the percentage of interruptions correlated to lightning is estimated between 25% and 30%, with higher values for the permanent ones. The comprehensive gain in terms of reduction of interruptions by replacing spark gaps by surge arresters is roughly estimated between 4% (for lines equipped with rigid pin insulators) and 7 % (for lines equipped with cap and pin insulators).

As far as the overvoltages transferred from the medium voltage to the low voltage network and thus to the LV users are concerned, a typical provision consists in the grounding of the LV system neutral not at the transformer itself, i.e. near the grounding of the MV surge arresters, but better at some distance, i.e. at the first LV line pole. As regards the state of the neutral of the medium voltage networks, now in Italy, as in other countries, electric utilities are turning to the resonant earthed neutral system due to many advantages, among which an expected improvement of the power quality to the users.

This provision, in fact, enables a reduction of the number of interruptions thanks to a higher extinction probability of single-phase to ground arcing fault. Beyond this main advantage, a decrease of permanent faults is expected due to a lower stress on MV equipment and mainly cables; this is a direct consequence of a lower fault current and a lower transient recovery voltage at fault extinction.

Shunt breakers, that are connected between each phase and ground at MV bus-bar, are used in MV isolated neutral systems from a long time and they prove to be very effective in clearing the single-phase transient fault before the switching of the main line breaker, but their main drawback lies in a higher stress on the insulation of the sound phases, thus sometimes causing a double phase-to-ground fault (“cross-country”) evolution.

Another very promising provision that is widely used in advanced countries and that is going to be applied in Italy too, consists in the use of line reclosers installed along the MV lines, that enable a fast recognition and clearing of the faulted line section between two subsequent reclosers, thus avoiding the switching of the feeder breaker in the station. This provision in practice allows a reduction of the line length switched off to clear the fault (and thus of customers involved).

The mitigation of the main disturbances affecting the distribution network and the connected loads, can be obtained by applying Power Electronic Devices. More precisely, the present available devices can be divided into the following main categories:

  • traditional devices – UPS (Uninterruptible Power Supply);
  • innovative devices, named “CUSTOM POWER”.

As regards this last category, various kinds of devices are applied in order to fulfil different functions:

  • Improvement of the supply power quality to susceptible loads;
  • Mitigation of the impact of disturbing loads on the distribution network;
  • Voltage control and reactive power and power factor (cos ϕ) compensation in weak distribution networks ;
  • Active power peak shaving.

These devices are applied mainly to MV distribution networks by now, and their rated power ranges from hundreds kVA up to few tens MVA. For LV applications the rated power reaches few hundreds kVA. A list of typical “Custom Power” devices with their main features and functions is reported here below

8.REFERENCES

[1] EUROPEAN UNION, “Council Directive 85/374 on the approximation of the laws of the Member States relating to the liability for defective products”, Official Journal (07.08.1985)
[2] EUROPEAN UNION, “Council Directive 89/336 on the approximation of the laws of the Member States relating to electromagnetic compatibility”, Official Journal, (23.05.1989)
[3] UNIPEDE – NORMCOM, Application Guide to the European standard EN 50160 on “Voltage characteristics of electricity supplied by public distribution systems”, C. Mirra, December 1994
[4] EN 50160, Second Edition, 2000-03 Voltage characteristics of electricity supplied by public distribution systems
[5] Report on EMC coordination in electricity supply systems UNIPEDE 1994
[6] IEC/TR3 61000-3-6, First Edition, 1996-10 – Assesment of emission limits for distorting loads in MV and HV power systems.
[7] IEC/TR3 61000-3-7, First Edition, 1996-10 – Assesment of emission limits for fluctuating loads in MV and HV power systems.
[8] UIE, Disturbances Working Group: Connection of fluctuating loads C. Mirra, 1988
[9] IEC Publ. 61000-2-2, 1990 and future second edition (at present
IEC 77A/324/CDV, 2000-09-29) : “Electromagnetic Compatibility (EMC). Part 2: Environment. Section 2: Compatibility levels for low-frequency conducted disturbances and signalling in public low voltage power supply systems”
[10] IEC Publ. 61000-3-3: “Electromagnetic Compatibility (EMC).- Part 3: Limits – Section 3: Limitation of voltage fluctuations and flicker in low-voltage supply systems for equipment with rated current ≤16 A per phase”, (December 1994)
[11] IEC 61000-4-15, First Edition, 1997–11. “Electromagnetic Compatibility (EMC).- Part 4: testing and measurement techniques- Section 15: Flickermeter – Functional and design specifications
[12] “Definition of the physical characteristics of electrical energy supplied by low and medium voltage public systems” – UIE Disturbances WG Connection of fluctuating loads; UNIPEDE DISNORM 10.
[13] J. Douglas: “Custom Power: Optimizing Distribution Services”, Epri Journal May/June 1996
[14] N.G. Hingorani: “Overview of Custom Power Applications”, IEEE Summer Meeting, San Diego (USA), 1998
[15] Cigrè SC 14 – WG 14.19: “Static Synchronous Compensator (STATCOM)”, Brochure Cigrè n° 144, August 1999
[16] I. Iyoda, M. Takeda, G.F. Reed: “Improved Power Quality Solutions Using Advanced Solid-State Switching and Static Compensation Technologies ”, IEEE Paper 0-7803-4893-1/99,
1999
[17] A. Sundaram et alii: “Custom Power: The Utility Solution ”, Cired 1995, Report 5.09
[18] L. Borgard: “Grid Voltage Support”, Transmission & Distribution World, October 1999
[19] R. Kenner: “Distribution System Static Var Compensator: Field
Experience”, IEEE Winter Meeting, Technical Panel Session of IEEE WG P1409, San Diego (USA), 1998
[20] K. Chan et alii: “Innovative Solutions for Power Quality Enhancement”, CIRED ’99, Nice (France)
[21] N.H. Woodley, L. Morgan, A. Sundaram: “Experience with an Inverter Based Dynamic Voltage Restorer”, PE–796–PWRD–0– 06-1997
[22] N.H. Woodley: “DVR Field Experience 0.6 to 6 MVA Systems”, Siemens presentation to the Technical Panel Session of IEEE WG P1409, New Orleans (USA), April 1999

What You Need To Know About Arc Flash Hazard Study And Analysis

Published by Carelabs (Carelabz)

Image: Carelabz

Arc flash study came into picture ten years ago when IEEE 1584, known as “IEEE Guide for Performing Arc Flash Hazard Calculations,” was first introduced. This landmark document defines equations and methods that have become key components of the arc flash study.

In order to identify the specific arc flash hazard at a given piece of equipment within a facility, an arc flash study must be performed. There are several software packages available like ETAP, similar to short circuit study and coordination study software packages, to facilitate this analysis by a qualified professional electrical engineer.

An arc flash hazard study is a complicated engineering survey and analysis that typically is performed by an engineering consultant. Preparing for the study in advance can make the process much easier and more accurate. Although an arc flash study can be time consuming, a correctly executed study can help improve plant safety by identifying where the most dangerous hazards are located and whether or not safety can be improved.

What is an Electric Arc?

An electric arc is a luminous bridge formed in a gap between two electrodes. An Arc Flash occurs during a fault, or short circuit condition, which passes through this arc gap. The Arc Flash can be initiated through accidental contact, equipment which is underrated for the available short circuit current, contamination or tracking over insulated surfaces, deterioration or corrosion of equipment and, or parts, as well as other causes. An Arc Flash event can expel large amounts of deadly energy. The arc causes an ionization of the air, and arc flash temperatures can reach as high as 35,000 degrees Fahrenheit. This is hotter than the surface of the sun.

This kind of temperature can set fire to clothing and severely burn human skin in fractions of a second at a significant distance from the event. The heat can also result in ignition of any nearby combustible materials. Arc Flash temperatures can also liquefy or vaporize metal parts in the vicinity of the event such as copper, aluminium conductors or steel equipment parts. This material rapidly expands in volume as it changes state from a solid to vapour, resulting in explosive pressure and sound waves. The pressure wave can knock workers off balance or off ladders and even throw them across the room against walls or other equipment

Why Arc Flash Study and Analysis?

Those involved with the design, implantation and evaluation of electrical distribution systems mush have an understanding of arc flash hazards and how to evaluate the hazards as well as minimize or mitigate the arc flash hazard to the electrical worker. In order to perform the arc flash study a short circuit and relay coordination study must first be performed or the results of these studies must be available to the engineer performing the arc flash study. At Carelabs, we use the tested and proven software ETAP to perform short circuit study, relay coordination study, and arc flash study.

The result of the arc flash analysis will categorize the hazard at specific equipment based on the incident energy, as well as identify the Arc Flash Protection Boundary (this is the closest approach allowed before PPE must be worn). Inside the Arc Flash Protection Boundary a worker must be wearing the proper clothing, or Personal Protective Equipment (PPE). The main objective of the PPE is to limit the burns to the body resulting from an arc flash event, to a survivable level. (i.e .2nd degree or less).

Carelabs provides complete arc flash engineering studies by performing a full, on-site assessment, arc flash modelling, equipment evaluation relative to the fault current and clearing coordination of your system with detailed one-line electrical model drawings.

We offer excellent services or will support your site team, local electrical contractor or engineering firm with any services you need. Our expertise is widely recognized in the industry and our engineers are members of IEEE, NFPA and IEC which set the standards for arc flash studies. We can provide arc flash studies using the latest ETAP software.

Tailored results by Carelabs allow you to reduce the cost of the services you need without paying for those you don’t need. We specialize in achieving fulfilment without excessive data and reports. These assessments are based on NFPA 70E, IEEE 1584, NESC. Studies can be customized to include any and all of the following:

  • Fault current and coordination analysis
  • Personal Protective Equipment (PPE) Level recommendations
  • Arc Flash Equipment Labelling
  • Arc Flash Incident Energy Mitigation Strategies
  • Written Electrical Safety Programs
There are two factors that have made arc flash study on focus:
  1. A greater understanding of arc flash hazards, and the risks they pose to personnel. Research into arc flash and arc blasts has begun to quantify the powerful forces they unleash.
  2. Increased vigilance on the part of OSHA. OSHA is using the requirements of NFPA 70E, the industry’s consensus standard for electrical safety, to judge whether the employer “acted reasonably” in protecting its workers from arc flash hazards. Noncompliance can result in substantial fines to employers.
When is Arc Flash Hazard Study & Analysis Needed
  • Arc Flash Analysis has not been performed in the past three years.
  • Short-circuit, protective coordination studies have not been performed in the past five years.
  • Changes have occurred to the electrical distribution system or electrical utility system.
  • A safety audit is required or the facilities insurance policy is up for renewal.
  • Modifications or expansions of the electrical distribution system are being considered.
What are the Processes Involved in Arc Flash Analysis?

Working within NFPA and IEEE guidelines, our experienced power systems engineers will perform accurate systematic Arc Flash Hazard Study & Analysis as follows:

  1. Short Circuit Calculations – ETS will either use your data or gather the data to make calculations for you to identify bolted and arcing fault levels at key points in a power distribution system.
  2. Protective Device Coordination – ETS can perform an electrical systems coordination study or use the customer’s study, to determine the duration of the arcing faults.
  3. Arc Flash Hazard Calculations – The incident energy level, the flash hazard boundary, and the PPE required are then calculated for each location.
  4. Documentation – The information above is compiled for you into a comprehensive report, which contains information necessary to comply with regulatory requirements.
  5. Unsafe Work Locations – The report will identify work locations having incident energy level in excess of available PPE ratings.
  6. Arc Flash Hazard Mitigation – Recommendations will be made to minimize Arc Flash Hazards by changes in system protection or operational procedures.
  7. Arc Flash Hazard Labels – In addition to the report, our customers will receive a field label containing the flash hazard boundary distance, incident energy level, and PPE category and shock category and shock hazard for each location.
Why is PPE Important?

Even where engineering controls and safe systems of work have been applied, some hazards might remain. These include injuries to:

  • The lungs, from breathing in contaminated air
  • The head and feet, from falling materials
  • The eyes, from flying particles or splashes of corrosive liquids
  • The skin, from contact with corrosive materials
  • The body, from extremes of heat or cold

The purpose of personal protective equipment (PPE) is to protect individuals, exposed to health and safety hazards, from the risk of injury by creating a barrier against workplace hazards.

When selecting and using PPE:

  • Choose products which are CE marked in accordance with the Personal Protective Equipment Regulations 2002 – suppliers can advise you.
  • Choose equipment that suits the user – consider the size, fit and weight of the PPE. If the users help choose it, they will be more likely to use it.
  • If more than one item of PPE is worn at the same time, make sure they can be used together, example wearing safety glasses may disturb the seal of a respirator, causing air leaks.
  • Instruct and train people how to use it, example train people to remove gloves without contaminating their skin. Tell them why it is needed, when to use it and what its limitations are.

In order to correctly apply the NFPA 70E Hazard/Risk Category task tables, knowledge of the available short circuit current and the opening time of the overcurrent protective device is required.

The IEEE 1584 method is a systematic approach which calculates the exact arc flash energies from the electrical power system parameters. Carelabs services recommends a complete data collection from the power system in order to generate short circuit and coordination studies in addition to arc flash energy calculations.

How do we Conduct Arc Flash Hazard Study and Analysis?
The arc flash hazard study involves following steps

Step 1: Data Collection

Initial steps are data collection and determining modes of operation. To perform an arc flash hazard analysis, data is collected about the facility’s power distribution system. The data includes the arrangement of components on a one-line drawing (These are simplified drawings that show items such as circuit breakers, conductors and power sources) with nameplate specifications of every device. Also required are details of the lengths and cross section area of all cables. The utility should be contacted for information including the minimum and maximum fault currents that can be expected at the entrance. The modes of operation are used to study worst case scenarios that may lead to arc flash, such as operations with circuit breakers open or closed or with certain motors or generators running or not running.

Accurate electrical system drawings are necessary to identify power sources, voltage levels, electrical equipment and protective devices. If you already have one-line diagrams, we will update the data and work from them, if possible

Qualified staff must gather data from all applicable electrical equipment. Required information includes:

  • Data from the utility, including available fault current, operating voltage, and specifics regarding the utility’s protective equipment at the point of service.
  • Specifics for each protective device in the electrical system, including manufacturer, model, available time/current settings, and short-circuit interrupting rating.
  • Transformer impedance, tap settings and ratings.
  • Conductor specifics, including lengths, sizes, and types of all overhead lines, and bus.

Step 2: Engineering Analysis of the Data

Once the data has been collected, a short circuit analysis followed by a coordination study should be performed. The resultant data can then be fed into the equations described by either NFPA 70E-2000 or IEEE Standard 1584-2002. These equations will produce the necessary flash protection boundary distances and incident energy to determine the minimum PPE requirement.

A short circuit study is required to determine the magnitude of current flowing throughout the power system at critical points at various time intervals after a “fault” occurs. These calculations will be used to determine the bolted fault current, which is essential for the calculation of incident energy and interrupting ratings of your equipment. A bolted fault current occurs as a result of two conductors becoming joined together. The bolted fault and the arc fault currents are calculated with the use of the data from previous steps. Comparison of equipment ratings with calculated short circuit and operating conditions will identify underrated equipment.

Step 3: Protective Device Coordination

Once the data is prepared and a flash hazard analysis has been performed, most likely it will be discovered that category 4 PPE will be required in most places. This is most unfortunate as this type of PPE is very unwieldy and could be costly in terms of time taken to perform work and the potential for mistakes. Prior to the new arc flash regulations, coordination studies were targeted at reliability with all settings adjusted towards the high side. Compliance with the new arc flash regulations means that not only does the coordination study need to be more accurate but it also needs to take into account the fact that the arc fault current is less than the bolted fault current. Protective device coordination should be performed to ensure selection and arrangement of protective devices limits the effects of an overcurrent situation to the smallest area. Results will be used to make recommendations for mitigation of arc flash hazards. Although this is an optional study, arc flash mitigation cannot be performed without completing this step. We perform this study in accordance with IEEE Std.

Step 4: Arc Flash Calculations

In this step we determine incident energies and flash protection boundaries, respectively, for all equipment. The incident energy is the energy needed for an arc flash to cause second-degree burns. The flash protection boundary is specified as the distance where the incident energy or second-degree burns are caused. These calculations are based on available short circuit current, protective device clearing time and distance from the arc. Calculations of incident energy levels and flash protection boundaries will be completed for all relevant equipment busses.

Step 5: Reporting

Upon completion of the calculations, we will prepare your Arc Flash Report. This will be supplied to you for a short review period, during which your team can review mitigation recommendations. At this point, we can hold a Management Summary meeting to interpret the report results. Upon approval, we will provide a final report and full sized one-line diagrams, stamped by our Registered PE. The drawings and report will also be supplied to you in a digital format.

Step 6: Label Installation

We will generate and install arc flash warning labels. These labels identify incident energy and working distance, nominal system voltage, and the arc flash boundary. In addition to standard requirements, our labels also include Limited, Restricted and Prohibited approach boundaries, date, upstream protective device and recommended personal protection equipment. We can also provide bolted fault current if desired.

Who Needs Arc Flash Analysis?

Most commercial, institutional, and industrial electrical systems have arc flash hazards. OSHA requires that those systems be individually analysed and, if hazards exist, labelled to identify the arc flash boundary. This will increase personnel safety, reduce shock hazards and reduce arc flash injuries.

Source: https://carelabz.com/what-you-need-to-know-about-arc-flash-hazard-study-and-analysis/

Carelabs is authorized provider of Electrical Installation’s Study, Analysis, Inspection, and Certification services in UAE, and offer arc flash study and analysis services.

Contact Carelabs at 971 4 514 9730 or +971 55 792 1054 (Call or WhatsApp) or email at info@carelabz.com

How PQView Was Born

With the annual PQView User’s Group Meeting only 3 weeks away we thought that we would share some of PQView’s rich history (below). The PQView User’s Group Meeting is on October 19 & 20, 2021. Space is still available. Click here for more details and to register.

In 1989, Electrotek was awarded a $5M project with Basic Measuring Instruments (BMI). This project was the EPRI Distribution Power Quality (DPQ) Project, which set out to establish a baseline of power quality at utilities across the United States in collaboration with 27 of those utilities. To achieve this, BMI PQNode 8010s were installed at 223 monitoring sites and collected data for two years.

Before long, the sheer volume of data was more than could be stored and processed on average computers at the time; even Microsoft Access had a limit of 1GB for its databases, and the DPQ project encompassed over 30GB of data by the end of the project. PQView was developed in order to process and visualize the data while it was stored across 200 separate databases as though it was integrated into a single source.

It’s hard to imagine today that a mere 30GB of data could be so troublesome, but necessity being the mother of invention, we’re glad, in the end, that the need presented itself. Both BMI—Dranetz-BMI, as of 1998—and Electrotek have the unusual demands of the EPRI DPQ project to thank for their maturation into the proud establishments they are today.

Now in its 4th major revision, PQView has come a long way; read more here about what it can do for you today!

Learn More

Strategies for Investigating Flicker

Published by Wayne LaFleur, Senior Engineer, Power Monitors, Inc.

Email: wlafleur@powermonitors.com, Website: powermonitors.com, (800) 296-4120, February 2014. White Paper.

ABSTRACT

Investigating f licker issues with PMI recorders can be straightforward if the right data is collected, and analyzed with ProVision. By enabling and examining Pst and IFL flicker data, in addition to RMS voltage and current, f licker can be quantified, and possible direction of the flicker source identified. Often the recordings will show that a potential problem exists but then comes the more difficult process of identifying the possible causes. PMI recorders can also be used to aid in the investigation of these causes. It is usually just a matter of knowing what data should be captured and how to interpret the results via Provision. Investigation into the possible source of flicker falls into this category of issues, and is described here.

THE PROBLEM

Flicker is an interesting issue. The name is unique in the sense that it perfectly describes the actual problem. A simple definition of flicker is that it is the visible fluctuation in light intensity due to voltage variations. Often the first indication that there is a problem is from direct observation although by default, PMI recorders will produce a flicker report which may indicate a problem as well. The default flicker report is designed to follow the industry standard GE Flicker Curve and the report will only trigger on events that fall outside of the curve. Flicker is the type of problem that can crop up suddenly based on changes in installed equipment or deterioration of components or connections. The problem may also be transitory and only manifest itself at certain times of the day. Since flicker itself is quantified based on measurements over time, a flicker investigation may require a recording covering many hours and sometimes days.

THE RECORDING

Once flicker has been reported (or is suspected) then the next logical step is to collect the data needed to identify the severity of the flicker and hopefully the location of the cause. Both the severity and direction from the point of monitoring can be determined by recording the correct data. The severity of f licker is captured with a Pst (Perceptibility Short Term) interval graph while the direction from the point of monitoring is captured using voltage minimum, current maximum and IFL (Instantaneous Flicker Level) interval graphs. See the white paper Flicker Standards Used by PMI Recorders for a more in depth definition of Pst and IFL themselves. Provision can be set up to capture each of these graphs in a single recording session via the Interval Graph tab when initializing a recorder. In some cases there may be saved recordings that do not contain Pst or IFL charts. These may still be useful for comparison based on the older GE flicker report contained in all recordings and/or the presence of voltage and current stripcharts. The Revolution, Guardian, and Eagle series (including the Eagle 120) all measure IFL and Pst flicker, and these stripcharts should be enabled in any recording where flicker is suspected. All PMI recorders also measure the older GE flicker curve, including older legacy devices, and this can be used as a fallback if needed.

THE ANALYSIS

As previously stated, a recording of sufficient time duration to encounter and capture any flicker issue is needed. This time will vary on a case by case basis. However, once the data has been captured and downloaded into Provision it can now be evaluated. A quick method to determine if a flicker problem is unlikely is to look at the Vmin, Vavg and Vmax stripchart. If the stripchart shows these three values tracking close together then flicker is probably not an issue. The Vmin and Vmax values are one cycle RMS readings, and if they are close to the average at each interval, then the voltage is not varying much, and thus flicker is unlikely. However, examining the voltage values alone only gives a clue about f licker but it cannot be ruled out on this basis alone. If the voltage variation continues at the most sensitive flicker frequency (8.8Hz), it only takes a 0.2% change to produce significant flicker. Having the additional stripcharts showing Pst or IFL will reveal any periods of voltage disturbance and accompanying flicker. An example of this can be seen in Figure 1 which shows voltage values along with both Pst and IFL to highlight this relationship. Typically a Pst greater than 1.0 is considered a possible problem but this can occur without any other indicators (such as customer complaints or equipment problems). The example shown indicates a severe flicker problem based on the indicators above.

Figure 1. Periods of voltage disturbance and accompanying flicker

Once a flicker issue has been confirmed then the next step is to determine where it is being introduced. The location of the cause of a flicker issue can be determined relative to the point of the recording. A problem upstream is toward the service hook up while a problem downstream is toward the load on the circuit. To discern the direction (upstream or downstream) a comparison of maximum current versus IFL is used. Being an instantaneous value, the IFL is useful when comparing against voltage or current fluctuations. The Pst value is computed over a time interval (typically 10 minutes) and therefore may not register a problem with momentary flicker. If spikes in the IFL graph happen at the same time as spikes in the maximum current then the issue is likely caused by something downstream from the point of the recording. Figure 2 shows an example of IFL compared with Imax and it clearly shows that in this case the current spikes correlate to the spikes in the IFL reading. This is a positive indication that the cause of the flicker is downstream from the point of the recording. Sometimes the IFL values are not clearly spiking with current as in the example so a comparison of Vmin with Imax can also be used to confirm the location of a flicker problem. If the low points of Vmin correlate to the high points of Imax (as seen in Figure 3) during periods of elevated Pst then the source of flicker is downstream from the point of the recording. If the low points of Vmin do not have accompanying spikes of Imax then the source of flicker is upstream from the point of the recording. Once the direction is determined it may be necessary to repeat the above steps to further pinpoint the cause of the issue, so well chosen recording points are crucial to an effective flicker investigation.

Figure 2. Example of IFL compared with Imax

Figure 3. Low points of Vmin correlate to the high points of Imax

CONCLUSION

Flicker can be a minor annoyance or quite literally a major headache (for some people). PMI recorders can be used to not only identify the severity of perceived flicker but also help to pinpoint the cause. When possible, enable Pst and IFL stripcharts, along with RMS voltage and current to capture flicker issues. Use the simple techniques outlined above to aid in your next flicker investigation.

What You Need To Know About Arc Flash

Published by Carelabs (Carelabz)

Image: Carelabz

Arc Flash is an energy release that takes place when electricity flows through the air and two live conductors causing short circuit. In a residential setting, arc flashes usually produce little more than a brief flash of light before extinguishing themselves harmlessly. In commercial or industrial setting, voltages are high, so electrical faults usually release high energy. As a result, arc flashes in data centers routinely produce powerful explosions marked by searing heat, toxic fumes, blinding light, deafening noise and massive pressure waves.

Human and Financial Repercussions of such Blasts can be Devastating
  • Injuries to employees: Without adequate protection, workers exposed to arc flash events can suffer third-degree burns, collapsed lungs, vision loss, eardrum ruptures, puncture wounds, and even death. Literally, electrical contact results in 3,600 disabling injuries annually and one workplace death every day in the U.S., according to statistics from the National Institute for Occupational Safety and Health.
  • Steep medical and insurance bills: Covering the cost of an injured employee’s medical leave after an arc flash incident can expensive. Further, businesses experienced an arc flash may difficult to get insurance and probably pay higher rates for any coverage they do acquire until they can prove proper safety measures have taken.
  • Exposure to fines and lawsuits: Any time employees would die or get injured during the job, costly lawsuits are almost sure to follow. Unless, it is important to pay fines from the Occupational Safety and Health Administration. Literally, Occupational Safety and Health Administration has known to impose fines aggregating millions of dollars after arc flash events, and has even great companies that have yet to suffer arc flashes for failure to comply arc flash safety standards.
  • Damage to equipment: Arc Flash events are likely to damage any servers, racks, networking gear and power distribution units in their immediate vicinity. Smoke condensation can cause further harm to sensitive electrical equipment at greater distances. Also, should an arc flash trigger your data center’s fire suppression system? You can complete easily with an entire room of valuable IT resources covered in water or thick flame proof foam.
  • Delays and downtime: Companies sometimes must wait for Occupational Safety and Health Administration to complete an investigation before repairing arc flash-related damage. That means organizations with inadequately redundant data centers can get through hours, days or even weeks of downtime after an arc flash event.
  • Impact on morale: Attracting and retaining qualified technicians can difficult if an arc flash incident causes current and prospective employees to view your data center as an unsafe place to work.
1.Perform a Hazard Analysis

Every Arc Flash reduction program should start by analysing risk aimed measuring how much energy an arc flash could release at various points along with power chain. Accuracy is essential with such measurements, so data centre managers who lack direct and extensive experience with arc flash incident energy assessment should always seek help from a qualified power systems engineer.

Beginning with integration of an arc Flash risk analysis, companies should take the following steps:

  • Equip data center staff with proper Personal Protective Equipment: Technicians should never come within range of a potential arc flash incident without wearing proper Personal Protective Equipment, such as flame-resistant clothing, eye protection and gloves. Personal Protective Equipment is available in different degrees of strength with different degrees of protection. Electrical engineers and fire safety professionals have developed two standards to help organizations analyse how much protection their employees need:
  • Institute of Electrical and Electronics Engineers 1584: Created by the Institute of Electrical and Electronics Engineers, one of the world’s most respected technical professional associations, Institute of Electrical and Electronics Engineers 1584 offers guidance on quantifying potential arc flash incident energy levels.
  • NFPA 70E: Produced by the National Fire Protection Association, a non-profit organization dedicated to fire, electrical, building and life safety, NFPA 70E defines thresholds for proper Personal Protective Equipment based on  severity of potential arc flash hazards. Drawing these two standards, data collected during an arc flash risk analysis, organizations can exactly analyse their risk factor, which would analyse what kind of Personal Protective Equipment their employees should wear while working in arc flash danger zones. Data centre managers should make sure that group from any vendors or third-party service who executes maintenance strategies on their server infrastructure wear exact Personal Protective Equipment at all times.
  • Post warning labels. To make sure data centre employees are always aware of potential arc flash hazards, companies should place warning labels on any piece of electrical equipment that poses an arc flash risk. They should record arc flash hazard zones on the floor so workers not wearing Personal Protective Equipment can clearly analyse how far from electrical equipment they must stand to avoid serious injury. Note that the NFPA 70E standard explicitly needs employers to post signage notifying employees of potential arc flash dangers. Organizations that ignoring this directive would dramatically increase their chances of paying serious fines and losing expensive litigation after arc flash incidents.
  • Execute an employee training program. To make sure data centre group understand arc flash dangers and know how to avoid them, be sure every existing and newly hired employee receives thorough arc flash safety training.
2.Reduce Available Fault Current

Even though it is not applicable to environments protection by fuses and electricity limiting breakers, data centers use Non-Current Limiting Breakers can decrease the risk of incident energy released during arc flashes by decreasing the amount of available fault electricity. The following four strategies can help data centers with NCLBs significantly reduce available fault current.

Operate with an open tie during maintenance: Managing two electrical feeds helps data centers to increase repetition of their power supply, and availability of their IT systems. The downside of this power architecture, however, is that it doubles the amount of current available when faults occur. In most of the situations, opening the tie between dual power feeds during maintenance strategies decreases arc flash dangers by cutting available fault current in half.

Of course, opening ties during maintenance also temporarily renders your power scheme less redundant, exposing IT equipment to heightened risk of failure. Given destructive human and financial toll arc flashes can take, most organizations trade-off worth making.

Switch to smaller kVA and/or higher impedance transformers: In the past, server power supplies commonly generated distortion that could overheat electrical transformers. To satisfy, data centers often used bigger and hence stronger transformers than their infrastructure needed. These days, most server power supplies “power factor corrected,” so they no longer pose a risk to transformers. As a result, data centers can safely use smaller transformers suiting their needs. Following such method would save money and improve arc flash safety. As a smaller transformer releases less energy during faults, lowering the scale and impact of arc flash events.

Employ High-Resistance Grounding: During ground faults, High-Resistance Grounding systems provide a path for ground current via a resistance that limits current significance. That greatly decreases the size of line-to-ground faults and associated arc flashes. High-Resistance Grounding can use on systems that service only three-phase loads. The U.S. National Electrical Code prohibits using High-Resistance Grounding on distribution systems serving loads that connected line-to-neutral, as are most servers. This limits practicality of High-Resistance Grounding system to the section of data center that powers cooling plants and other large three-phase loads.

Use current limiting reactors: Current-limiting reactors act as a bottleneck on electrical flows, restricting electricity failures. For example, low-voltage motor control centers can supply with three single phase reactors that restrict available short circuit electricity, resulting in less energy releases when errors occur.

3.Shorten Clearing Time

Just a smaller arc flashes release less energy, to do shorter ones. There are three techniques for shortening arc flash events by decreasing fault clearance times:

Utilize Zone Selective Interlocking: Zone Selective Interlocking is a security scheme that makes use of an “inhibit” signal transmitted from downstream breakers that see error to next breaker upstream. The upstream breaker analyse both the electricity failure and the inhibit signal and therefore delays tripping, allowing the downstream breaker to clear the fault. Should a fault occur between the downstream and upstream breaker, however, the downstream feeder doesn’t see the fault or send an inhibit signal to the upstream breaker. That causes the upstream breaker to bypass at any time, significantly decreasing arc flash incident energy.

Implement a bus differential scheme: These integrated zones of protection within an electrical system. If any error take place between main and feeder breakers, then the protective devices trip immediately, Resisting arc flash durations while confining arc flash damage to specific portions of your infrastructure. Bus differential systems are typically faster and more sensitive than Zone Selective Interlocking, but need further current transformers and relaying equipment. This generally makes them harder to apply and more expensive.

Deploy an Arcflash Reduction Maintenance System: An ArcFlash Reduction Maintenance System shortens faults by passing all time delays in the trip circuit at any time electricity exceeds maximal. That permits faults to clear even faster than a circuit breaker’s “instantaneous” function makes possible. Technicians must manually allow ArcFlash Reduction Maintenance System circuits before doing maintenance work and then disable them when work is complete, using familiar lockout strategies.

4.Adopt Remote Operation

Executing probably dangerous strategies casually can safeguard data center group from injuries. There are two ways to resist the number of maintenance activity, technicians must execute while in range of ArcFlash blasts:

Install remote monitoring, control and diagnostics software: Today’s sophisticated power management systems equip administrators to execute many administrative tasks remotely. They give companies to casually de-energize electrical equipment before data center staff approaches.

Use remote shaking devices: Normally, technicians should stand close to equipment, electrical connections shake and un-shaking breakers. Remote shaking devices allow operators to execute these extremely dangerous tasks from a safe distance.

5.Predict and Prevent Faults

One of the most effective ways to prevent arc flashes is to assume and ignore the conditions that cause them. The following three solutions permit data centers to spot possible ArcFlash risk before they have  chance to harm and keep group safely away from live connections.

Analyse insulation security: Deteriorating insulations is the leading cause of electrical failures. Analysing and repairing insulation before it fails could avoid arc flash explosions. Anticipative maintenance systems give warning of insulation failure in medium voltage switchgear, substations, generators, transformers and motors.

Monitor pressure junctions: Most of the electrical equipment has pressure junctions. Like, shipping splits, load lugs and compression fittings. Over time, vibration and thermal cycling can loosen these connections. When current flows through loosened connection, it cause overheating and ultimately produces an ArcFlash. Using non-conductors thermal sensors called pyrometers. Data centers can regulate pressure junctions regularly and receive advance notification of loose connections before they become very loose that they create an ArcFlash explosion.

Use infrared (IR) windows: Using contactless IR thermography technology, IR windows enable technicians to execute IR scans without removing switchgear side panels, lessening the likelihood of arc flash events caused by accidental contact with live bus.

6.Redirect Blast Energy

Equipment that directs arc flash energy away from data center group is called “arc resistant.” Arc-resistant switchgear, for example, utilizes sealed joints, top-mounted pressure relief vents, and reinforced hinges to have the energy and heat released by arc flashes and channel them by ducts to vacant area inside or outside the data center.

When all else fails, arc-resistant switchgear offers vulnerable data center employees a critical last line of defense from the explosive power of arc flash incidents. Anyhow, protective conditions said to effective only when equipment doors closed, it is important to train technicians to fasten doors safely while operating.

Source: https://carelabz.com/need-know-arc-flash/

Carelabs is authorized provider of Electrical Installation’s Study, Analysis, Inspection, and Certification services in UAE, and offer arc flash study and analysis services.

Contact Carelabs at 971 4 514 9730 or +971 55 792 1054 (Call or WhatsApp) or email at info@carelabz.com

Energy Platform EP1 Trend Data Time Interval

Published by Terry Chandler, Director of Engineering, Power Quality (Thailand) Co., Ltd.

Emails: terryc@powerquality.co.th,TChandler@Dranetz.com

The EP1 is Energy Platform has a slightly different time interval for the trended data from the other Dranetz portable products. The PX/PG/PV have a minimum intervals of 1 second, 10 seconds and 1 minute. Note: There are some limitations on the 1 second setting. The EP1 minimum interval is 1 minute. This application note explains the 1 second interval referred to in the manual.

Set up comparison.

Note: These RMS triggers are for voltage during monitoring. Nothing to do with Journal trended values triggers.

Minimum setting is 1 min.

EP1 User Manual States:

Power Values
Power Values Interval – Power parameter values are based upon all cycles during a 1 second
interval. Data is aggregated or summarized into min, max and avg values over the averaging period and stored at the end of the interval. Data measured include watts, power factors, as well as the 1 second RMS.

The EP1 measures every cycle of voltage and current by sampling it 256 times during the cycle. It uses these samples to calculate the minimum RMS value and the maximum RMS value of every cycle. It remembers the largest RMS value of one cycle and the smallest value every 1 second. This 1 second data in NOT brought out to the database, so neither the user interface nor DV6 can access the 1 second values. The 1 second values are aggregated for the time interval the user selects. (1 min, etc) So, the minimum time interval available to graph on the instrument or DV6. See graph below

The EP1 has additional capability based on the 1 second interval data to set thresholds and detect the exact time the value crossed the threshold.

Note: The user can set a threshold for any parameter shown for example harmonic power

When the value of this parameter exceeds the threshold, the EP1 saves the exact time of the event and the values at that time. The event list below shows an example of a Voltage sag and momentary interruption at the exact time. And the time is shown in between the Trend data points. Also, notice the trend data points for harmonics and voltage values are at different intervals.

The screen capture below shows the event graph and the event capture with exact time stamp.

This verifies the EP1 is recording the data in 1 second intervals and storing the event times in sub-second intervals.

Summary:

The EP1 measures and stores a lot of data that can be selected by the user for various reports and graphs. Not all this data is available on the user interface or to DV6. Critical data of abnormal events is available to both user interface and DV6.

Dranetz Energy Platform EP1 BrochureDownload

website: powerquality.co.th, Application note

Dranetz HDPQ Generator & Mission Critical Applications

Load stepping, power source switching and other tests required during the commissioning and troubleshooting of mission critical facilities presents unique challenges to the average PQ analyzer, but Dranetz doesn’t make average PQ analyzers. Most PQ analyzers can’t reliably capture the step load changes, don’t have a large enough recording buffer, can’t capture the effects on frequency, or have other deficiencies.

When combined with our Dran-View 7 Enterprise software, the Dranetz HDPQ Guide & Xplorer PQ analyzers are ideally suited for mission critical applications.

Benefits include:

  • High resolution measurements – capture the details of each transition
  • Waveshape change triggers – reliably capture each step load change
  • Large 10K cycle data buffer – capture before, during and after each step
  • Cycle-by-cycle frequency – see the frequency response of each step
  • Dran-View 7 Enterprise – analysis, data post processing, and reporting 

Click the button below to view our new application note and to learn more about the Dranetz HDPQ and mission critical testing applications.

View Application Notes
  • Load current steps
  • Steps frequency response
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