Published by Electrotek Concepts, Inc., PQSoft Case Study: General Reference – Performing Power Quality Audits, Document ID: PQS0306, Date: February 4, 2003.

---

Abstract: Power quality investigations often require monitoring to identify the exact problem and then to verify the solutions that are implemented. Before embarking on extensive monitoring programs, it is important to develop an understanding of the customer facility, equipment being affected, wiring and grounding practices, and operating considerations. Often, power quality problems can be solved without extensive monitoring by asking the right questions when talking to the customer and performing an initial power quality audit.

This document provides a guide for performing a power quality audit.

PERFORMING POWER QUALITY AUDITS

Power quality investigations often require monitoring to identify the exact problem and then to verify the solutions that are implemented. Before embarking on extensive monitoring programs, it is important to develop an understanding of the customer facility, equipment being affected, wiring and grounding practices, and operating considerations. Often, power quality problems can be solved without extensive monitoring by asking the right questions when talking to the customer and performing an initial site survey. Audit procedures generally include the following steps:

1. Obtaining customer input
2. Defining objectives (scope) of the work
3. Collecting data and conducting an equipment inventory
4. Wiring and grounding inspections
5. Determining monitoring locations
6. Selecting appropriate monitoring equipment
7. Preparing an audit report

Utilities are generally blamed for power quality problems. They are not necessarily responsible. There are four sources for most customer-encountered problems:

1. Natural phenomena (e.g., inclement weather)
2. Normal utility operations (e.g., automatic protection system operations)
3. Neighboring customers (e.g., welding equipment adjacent to an office)
4. Customer’s own equipment and facilities.

While most problems have nothing to do with the utility, customers often blame the utility for causing or contributing to the problem. In fact, eighty percent of all power quality related problems in commercial and industrial facilities are caused on the customer side of the meter. In residential facilities, eighty percent of the problems are due to weather and weather-related actions.

The first step is to understand how customers perceive power quality problems. Customers rarely see or understand these problems. They see symptoms of them and the resulting difficulties in their businesses and homes.

Some of the more common symptoms include:

− Equipment damage
− Blinking digital displays
− Data or information loss / software glitches
− Loss of instructional programming and controller timing
− An abnormal number of service calls on sensitive equipment
− Disk drive problems / computers re-booting
− Static shock

Performing a Site Survey

Data Collection Process

The initial site survey should be designed to obtain as much information as possible about the customer facility and the problems being experienced. Specific information that should be obtained at this stage includes:

1. Customer information:
   a. company and contact name
   b. address and phone/fax/e-mail

2. Nature of the problems:
   a. data or information loss
   b. nuisance trips of motor drives or other power-electronic devices
   c. electronic component failures
   d. control system malfunctions
   e. equipment damage

3. Impact on operations:
   a. stop or slow production
   b. lost production or sales
   c. reduced product quality
   d. scrap / restart

4. Characteristics of the sensitive equipment experiencing problems:
   a. equipment design information
   b. equipment ride-through characteristics
   c. equipment application guides or installation/user manuals

5. Frequency and timing:
   a. time of day, day of week/month, dates of occurrence
   b. repetitive (e.g., same time every day)

6. Coincident problems or known operations that occur at the same time:
   a. motor starting or slowing down
   b. capacitor switching
   c. lights blink on and off momentarily
   d. weather conditions

7. Possible sources of power quality variations within the facility:
   a. motor starting
   b. capacitor switching
   c. power electronic equipment operation (e.g., ASDs, PCs, electronic ballast fluorescent lights)
   d. arcing equipment (e.g., magnetic ballast fluorescent lights, arc furnaces, etc.)
   e. copy machines, HVAC

8. Power conditioning equipment being used:
   a. surge suppressors (e.g., TVSS, arresters, etc.)
   b. motor-generator sets
   c. ferroresonant transformers (also know as CVTs)
   d. UPS systems
   e. isolation transformers / chokes

9. Electrical system data:
   a. oneline or facility wiring diagrams
   b. transformer information (e.g., size and impedance)
   c. load information
   d. capacitor information (e.g., size, connection, and placement)
   e. feeder/cable data

Data Recording Process

Once this basic data is obtained through discussions with the customer, a site survey should be performed to verify the oneline diagrams, electrical system data, wiring and grounding integrity, load levels, and basic power quality characteristics. Data forms that can be used for this initial verification of the power distribution system are provided in Figure 1 through Figure 4.

Performing Wiring and Grounding Inspections

Wiring and grounding problems are responsible for many power quality variations within customer facilities. Some electric utility engineers have estimated that 80% of all the power quality problems reported by customers are found to be due to their own wiring and grounding problems. While end-users may have a different opinion, it is commonplace for many power quality problems to be resolved by simply tightening a loose connection, removing an unnecessary ground connection, bonding ground conductors, or replacing a corroded conductor. Therefore, the first step in most power quality investigations is to evaluate the wiring and grounding practices of the facility.

Wiring and grounding problems are identified by physical inspections of wiring, connections etc.; infrared scans to identify heating that may be caused by overloaded conductors or bad connections; and measurements to characterize circuit loading and identify grounding problems. Steps for a wiring and grounding inspections include:

1. Check rms voltage levels.

2. Check for extra neutral-ground bonds. There should be only one neutral-to-ground bond per separately derived system. This is a common problem that causes load currents to flow in the building ground system, creating the potential for serious interference problems. This can be checked by measuring the current in the green wire grounds at the service entrance or at the source of the separately derived system. These currents should be very close to zero. If any current is flowing in the ground, the source of the current should be found and corrected.

3. Check for overloaded neutral conductors. In three-phase, four wire systems supplying single-phase electronic loads, the neutral currents can be as high as 173% of the rms phase current. This can cause overloading of the neutral conductor because the code does not require the neutral conductor to be rated for currents higher than the phase conductor. The neutral currents should be measured with a true rms meter and checked against the ampacity of the neutral conductors. This problem can be corrected by filtering the harmonics of the electronic loads, using a zig-zag transformer, reducing the load, or increasing the neutral conductor capacity.

4. Checking grounding electrode system. The grounding electrode system consists of all the grounded elements of the building that are bonded together to form a grounding system. This can include ground rods, metal water pipe, building steel, concrete-encased electrodes, a ground ring, etc. All of these things should be bonded together to form the best equipotential reference for equipment in the building as possible. It is not advisable to have separate, isolated ground rods for individual equipment in the facility. If a separate ground rod is driven for equipment, it should be bonded with the overall building grounding electrode system. Guidelines for the grounding electrode conductor are provided in Table 1.

5. Check isolated ground receptacle wiring. Isolated ground receptacles are a good way to provide a separate, clean ground for sensitive equipment. These receptacles require a separate ground wire in addition to the safety ground. The isolated ground is insulated from the case of the receptacle and should go back to the ground of the separately derived system, where it is tied in to the building grounding electrode system.

Table 1 – Grounding Electrode Conductor for AC Systems

6. Check overall circuit layouts. Are sensitive equipment loads on separate circuits from disturbing loads? Loads that are switched or that have power electronic components can create transient disturbances that can impact the operation of some sensitive equipment. Loads like switched motors, copiers, laser printers, elevators, etc. should be on separate circuits from sensitive equipment. The separate circuits provide isolation for high frequency transients and a clean ground reference for the sensitive loads.

7. Check for use of separately derived systems. Separately derived systems permit the bonding of the ground and neutral. In circuits with significant neutral currents (e.g., single-phase electronic loads), a significant neutral-to-ground voltage will build up if there is a significant length between the loads and the supplying transformer. Using an isolation transformer close to the loads minimizes the neutral-to-ground voltage and provides isolation for transient overvoltages.

8. Check for ground loops. Ground loops are probably the most common cause of interference in network systems and the most common problems with multi-port devices in general. Multi-port devices have more than one type of interface. For instance, a television has a power input and a cable input; a computer has a power input and a phone input for the modem and a network input for a LAN. All of these ports require a ground reference. This multiple ground reference scenario creates the potential for serious ground loop problems. Ground loop problems are best avoided by making sure all equipment that is tied together through other ports (e.g., on a LAN) has the same ground reference. This means that all the equipment is part of the same separately derived system.

9. Apply protection to data/communication lines if there is ground loop potential. Sometimes, the ground loop problem described above cannot be avoided. In these cases, protection for data circuits, communication circuits, etc. should be applied. Optical coupling provides the most isolation and prevents the ground loops completely. Where ground loop problems exist, data lines should be protected with baluns, ferrite cores, or data line surge protectors.

A data form that can be used for recording the power distribution and grounding information is provided in Figure 5.

Problems with Conductors and Connectors

The first things to look for when inspecting the service entrance, panel boards, and equipment wiring during a site survey are problems with conductors or connections. A bad connection (faulty, loose, or resistive connection) will result in heating, possible arcing, and burning of insulation. Table 2 summarizes some of the wiring problems that can be uncovered during a site survey.

Table 2 – Problems with Conductors and Connectors

Missing Safety Ground

If the safety ground is missing, a fault in the equipment from the phase conductor to the enclosure results in line potential on the exposed surfaces of the equipment. No breakers will trip and a hazardous situation results.

Multiple Neutral to Ground Connections

Unless there is a separately derived system, the only neutral to ground bond should be at the service entrance. The neutral and ground should be kept separate at all panel boards and junction boxes. Double neutral-to-ground bonds result in parallel paths for the load return current where one of the paths becomes the ground circuit. This can cause misoperation of protective devices. In addition, during a fault condition, the fault current will split between the ground and the neutral that could prevent proper operation of protective devices (a serious safety concern). This is a direct violation of the NEC.

Ungrounded Equipment

Isolated grounds are sometimes used due to the perceived notion of obtaining a clean ground. Procedures which involve an illegal insulating bushing in the power source conduit and replacing the prescribed equipment grounding conductor with one to an Isolated Dedicated Computer Ground are dangerous, violate code, and are unlikely to solve noise problems.

Additional Ground Rods

Ground rods for a facility should be part of a grounding system, connected where all the building grounding electrodes are bonded together. Multiple ground rods can be bused together at the service entrance to reduce the overall ground resistance. Isolated grounds can be used for sensitive equipment, as described previously. However, these should not include isolated ground rods to establish a new ground reference for the equipment. The most important problem with additional ground rods is that they create additional paths for lightning stroke currents to flow. With the ground rod at the service entrance, any lightning stroke current reaching the facility goes to ground at the service entrance and the ground potential of the whole facility rises together. With additional ground rods, a portion of the lightning stroke current will flow on the building wiring to reach the additional ground rods. This creates a possible transient voltage problem for equipment and a possible overload problem for the conductors.

Ground Loops

Ground loops are one of the most important grounding problems in many commercial and industrial environments that include data processing and communication equipment. If two devices are grounded via different paths and a communication cable between the devices provides another ground connection between them, a ground loop results. Slightly different potentials in the two power system grounds can cause circulating currents in this ground loop. Because the communication signal levels can be quite low (e.g., five volts), very low magnitudes of circulating current can cause serious noise problems. The best solution to this problem is to use optical couplers in the communication lines, thereby eliminating the ground loop.

Insufficient Neutral Conductor

Switched-mode power supplies and fluorescent lighting with electronic ballasts are becoming increasingly prevalent in commercial facilities. The high harmonic currents produced by these loads can have a very important impact on the required neutral conductor rating for the supply circuits. The most important harmonic component in these load currents is the third. Third harmonic currents in a balanced system appear in the zero sequence circuit. This means that third harmonic currents from three single phase loads will add in the neutral, rather than cancel as is the case for the 60 Hz current. In typical commercial buildings with a diversity of switch-mode power supply loads, the neutral current is typically in the range 140%-170% of the fundamental frequency phase current magnitude. CBEMA has recognized this concern and has prepared a brief to alert the industry to problems caused by harmonics from computer power supplies.

Preparing an Audit Report

The result of a power quality audit is often a formal written report to a customer. The report may be as simple as a one or two page summary letter, or as detailed as a multi-section report. A standardized format for writing audit reports is recommended. In addition, since power quality is very technical and often confusing for a non-technical person, the report should be presented in easily understood language and organization. A suggested outline for an audit report includes the following sections:

− Executive Summary
• Description of the Problem
• Objectives of the Investigation
• Important Conclusions and Recommendations

− System Description
• Overview of the Utility Supply System
• Overview of the Customer System

− Engineering Analysis Summary
• Power Quality Concerns and Related Symptoms Evaluated in the Report
• Summary of Computer Simulation Results (if applicable)
• Mitigation Alternatives
• Economic Analysis (cost/benefit)

− Monitoring Results Summary
• Monitoring Period
• System Voltage Performance
• Summary Harmonic Distortion Levels
• Summary of Voltage Sags and Interruptions
• Summary of Transient Overvoltages
• Discussion of Major Events

− Appendices
• Glossary of Terms

The Executive Summary section should acknowledge that the utility or company is interested in helping the customer use electricity without problem, and is pleased to assist the customer in that pursuit. This section should provide a brief history of the events leading up to the audit work, and a description of all work that was done. Include any information that helps the reader understand the purpose and use of the audit report.

The Important Conclusions and Recommendations subsection may be organized by each conclusion drawn from the engineering and monitoring effort. In this structure, the rationale for each conclusion is stated and a specific recommendation is made. Each conclusion should state the cause of the problem, and its relative impact and importance to the customer. Recommendations should be described completely and leave no ambiguity about what actions should be undertaken.

The System Description section should include an overview of both the utility and customer systems, including all power system data collected during the investigation. Oneline or facility wiring diagrams should be included where appropriate.

The Engineering Analysis Summary section should present exactly what was found in each step of the diagnostic process. For example, what types of power quality problems or disturbances were found, and where. Include specific information to help the reader understand what was done. Photos and references to specific points on wiring diagrams or facility layouts are helpful. This section is also a good place to discuss the estimates made by facility personnel of the actual or estimated cost associated with each identified problem. This information is valuable in establishing a cost/benefit analysis for the customer and demonstrating the value of problem mitigation. This is particularly true if the solution to a specific problem involves the outlay of capital funds.

The Monitoring Results Summary section should include summary results for the relevant steady-state and disturbance quantities.

The Appendices section(s) should include supplemental information such as a glossary of terms and detailed monitoring results.

REFERENCES

IEEE Standard 1100. IEEE Recommended Practice for Powering and Grounding Sensitive Equipment (The Emerald Book).
IEEE Standard 1159. IEEE Recommended Practice on Monitoring Electric Power Quality.
ANSI/NFPA 70-1993, National Electrical Code.
Power Quality Considerations for Adjustable Speed Drives, EPRI Publication CU.3036.4.91, Electric Power Research Institute, 1991.

---

RELATED STANDARDS
IEEE Standard 1159
IEEE Standard 1346
IEEE Standard 1250
IEEE Standard 1036
IEEE Standard 519

GLOSSARY AND ACRONYMS
ASD: Adjustable-Speed Drive
CVT: Constant Voltage Transformer
GPR: Ground Potential Rise
IEEE: Institute of Electrical and Electronics Engineers
MOV: Metal Oxide Varistor
PWM: Pulse Width Modulation
TVSS: Transient Voltage Surge Suppressors
UPS: Uninterruptible Power Supply

Published by Electrotek Concepts, Inc., PQSoft Case Study: General Reference – Power Quality Assessment Procedure, Document ID: PQS0608, Date: July 1, 2006.

---

Abstract: Power quality is a frequently used term that means different things to different people. Common power quality problems include all of the issues that arise from the incompatibility between a utility’s power and the customer’s energy-using equipment that result in impaired operation. These include transients, sags and swells, harmonics, and short- and long-term voltage variations and outages. Also included under this broad area are issues of power reliability.

This case describes a general procedure for performing on-site case studies of power quality concerns.

POWER QUALITY ASSESSMENT PROCEDURE

This case describes a general procedure for performing on-site case studies of power quality concerns. The power quality assessment procedure is based on the variety of different power quality concerns that can exist and focuses on a combination of monitoring and analysis to characterize these concerns. Once the power quality concerns are characterized, the analysis procedures developed can be used to evaluate possible solutions to the power quality problems. These solutions must then be evaluated from both a technical and an economic perspective. There are a number of important areas that must be addressed in the power quality assessment procedure. These include:

• Data collection requirements
• Important power quality concerns as a function of the type of customer
• Equipment sensitivity
• Important parameters of the power quality concern
• Roles of measurements and simulations in evaluating the concern
• Implementation of possible solutions to solve the problem
• Technical considerations
• Economics
• Politics

The procedure presented here, along with the information included in the previous sections, provides the framework for performing case studies in a variety of customer categories. It is worthwhile to group customers in categories that involve the application of similar equipment and have similar electrical system designs. With this in mind, the following list of customer categories is provided as an example of possible customer segments:

• Office Buildings
• Hospitals
• Semiconductor Manufacturers
• Data Processing/Banking
• Telecommunications
• Computer Manufacturers
• Point of Sale Retail Operations
• Food Processing
• Pulp and Paper Mills
• Refinery and Chemicals
• Automotive Plants
• Printing/Publishing
• Steel Mills
• Rubber/Plastics Plants

Besides classifying the case studies by the type of customer involved, it is also possible to categorize the studies by the type of sensitive equipment involved or the specific type of power quality variation of concern. The results of multiple case studies can be combined to develop descriptions of general power quality concerns for various classes of customer equipment. Some of the most important equipment categories to consider include:

• Adjustable speed drives – harmonic distortion concerns
• Adjustable speed drives – sensitivity to transient voltages
• Electronic controls, ASDs, robotics, and PLCs – sensitivity to voltage sags
• Switch-mode power supplies – harmonic current generation and neutral current concerns
• Fluorescent lighting (especially with electronic ballasts) – harmonic generation
• Power factor correction capacitors – switching transients and magnification
• Power factor correction capacitors – harmonic distortion concerns (resonance)
• Motor contactors – sensitivity to voltage sags
• Power conditioning equipment selection – matching to requirements of protected equipment
• Data processing equipment – UPS system specification
• Electronic equipment – sensitivity to high frequency transients
• Transformers – harmonic heating
• Motors – voltage imbalances and harmonic heating

The power quality assessment procedure provides a general framework that contains all the possible elements that may be needed in a power quality case study. Each case study will have unique requirements, depending on the type of customer, equipment sensitivity, and other factors. These unique requirements will influence the level of effort needed for each step or may even permit bypassing a particular step of the overall procedure. The following sections summarize the general steps involved in the procedure.

Identify Power Quality Concerns

The specific power quality concerns that need to be evaluated will be different from customer to customer. A review of the types of equipment used by the customer, process requirements, and economic impacts of problems will lead to a list of concerns that need to be studied. The concerns can include possible problems with both the utility distribution system and the customer facilities. Possible power quality problem categories to be evaluated include the following:

• Voltage transients caused by circuit switching and load switching within the customer facility.
• Harmonic distortion from adjustable speed drives or other nonlinear loads.
• Transformer heating caused by harmonic current levels.
• Transient magnification at low voltage capacitor banks.
• Sensitivity of adjustable speed drives and control systems to utility capacitor switching transients.
• Transients and notching associated with power electronics equipment operation.
• Neutral conductor overloading due to harmonic producing loads in commercial installations.
• Voltage flicker from arc furnace loads and arc welding loads.
• Voltage sags due to faults on parallel circuits on the same distribution system or faults on the transmission system.
• Momentary interruptions at industrial and commercial installations due to recloser operations on feeder circuit breakers.
• Coupled voltages at customer facilities due to lightning transients on the primary distribution systems.

Identification of the particular concerns involved for an installation provides a focus for the study. Development of a model for analysis of the problem is dependent on the frequency range of the power quality variations that need to be studied. The model can be for computer simulations, hand calculations, or application of simple rules. Analysis of voltage sags often requires modeling all the way up to the utility transmission system. Analysis of high frequency transients might only require a model for a local part of the customer facility.

Monitoring requirements are also based on the particular concern involved. If harmonic distortion is a concern, monitoring of steady state conditions with a harmonic analyzer is required. Analysis of disturbances requires a disturbance monitor. The duration of monitoring depends on how often the problems occur. Some problems with voltage sags or momentary interruptions might only occur a few times per year due to faults on the transmission system while problems caused by capacitor switching might occur every day. Other voltage variations of interest probably fall somewhere between these extremes.

Collect Data/Develop Initial Models

A representation of the customer system and important parts of the utility system should be developed for preliminary analysis. This model can be used for preliminary simulations or analysis to predict power quality problems and evaluate possible solutions to problems. In cooperation with the customer, the data for the model is collected and compiled into a database for convenient reference during the analysis. Important information includes:

1. Load characteristics
   − Motors
   − Power electronics
   − Process controls
   − Computers
   − Adjustable speed drives
   − Lighting
2. Transformer sizes/ratings
3. Conductor lengths, characteristics
4. Customer capacitor sizes, locations, switching procedures
5. Customer equipment and circuit switching
6. Power conditioning equipment
   − Surge suppressors (arresters, varistors, etc.)
   − Isolation transformers
   − Constant voltage transformers
   − Voltage regulators
   − Power conditioners
   − UPS systems
   − Harmonic filters
7. Distribution system characteristics
   − Primary Voltage
   − Underground/Overhead
   − Protection practices, switching procedures
   − Capacitor applications (locations, sizes, switching)

Perform Simulations

Simulations provide a convenient means to characterize power quality problems, predict disturbance characteristics, and evaluate possible solutions to problems. They should be performed in conjunction with monitoring efforts and measurements for verification of models and identification of important power quality concerns. The models required for the simulations depend on the system characteristics and the power quality variations being evaluated. The simulations fall into three major categories:

1.Transients. Transient simulations can be performed with the Electromagnetic Transients Program (EMTP). This is a valuable tool for analysis of circuit switching operations, capacitor switching, lightning transients, and transients associated with power electronic equipment operation.

2.Harmonics. Harmonic PLCs is usually performed using steady state analysis techniques at the individual harmonic frequencies. The SuperHarm program can be used for these simulations. Harmonic producing loads can be modeled as harmonic current sources and the simulations used to predict harmonic voltages and currents throughout the customer and utility systems.

Overloading of neutral conductors, transformer heating considerations, resonances caused by capacitor applications, and harmonic currents injected onto the utility system can be evaluated in the simulations.

3.60 Hz Voltage Variations. Variations in the fundamental frequency voltage can be evaluated with conventional analysis tools. Load flow programs give system voltages as a function of load levels on the system. Fault programs can calculate system voltage profiles during fault conditions for analysis of voltage sag concerns.

The models being developed during the case studies in the second phase of this project will serve as templates for future case studies by utility personnel. The models will be verified and refined using the results of monitoring at customer sites.

Develop Monitoring Program

The utility and customer systems being evaluated should be monitored to characterize the power quality variations and to verify the analytical models developed for simulations. The measurement program should be designed based on initial simulation results and on the particular sensitive loads existing at the customer facilities. Monitoring will typically be performed at the customer service entrance and close to particular sensitive loads in order to characterize disturbances coming from the utility system and disturbances that are localized at the sensitive loads. A measurement program plan should be developed which specifies:

• quantities to monitor
• monitoring durations
• threshold levels which will trigger recording of disturbances
• waveform sampling and data storage requirements
• analysis procedures and data presentation formats

Available monitoring instruments should be evaluated for the measurements required. The problem of obtaining adequate representation of both harmonic and transient conditions must be addressed in particular if both of these concerns exist at a facility.

A customer site survey should be part of the measurement program design. The site survey should characterize the wiring and distribution system integrity and provide basic information about circuit and equipment loading. The data collection forms provided in Chapter 8 can be used to assist with this data collection effort. The site survey should also include discussions with facility personnel regarding characteristics of equipment problems and known customer system conditions at the time power quality variations have occurred.

The actual monitoring effort requires close cooperation between the customer and utility personnel. Monitoring sites and instrumentation should be selected based on the particular concerns being characterized. The duration of monitoring will depend on the parameters that can affect the power quality concerns. It is likely that the customer will need to be responsible for making sure the monitor is operating properly on a day-to-day basis. The monitoring results should be compiled and analyzed for verification of analytical models and to provide a concise description of the possible concerns.

The customer should participate in the monitoring effort by keeping a log of equipment problems during the monitoring period. It is very important to correlate actual equipment problems with power quality variations and with operations on the customer system or the utility system.

Evaluate Measurements Results

The measurement results are analyzed along with the results of simulations to correlate customer problems with the utility system power quality levels. The initial measurements and the site survey are used to identify the phenomena involved and the important parameters. The subsequent measurement results are used to verify the model and characterize the actual power quality variations. Using this information, the model can then be used for more detailed simulations of possible solutions to the power quality problem. The simulations provide the means to evaluate a range of possible solutions from a technical point of view.

Once the range of technical solutions is identified, economic analyses need to be performed to evaluate the possible alternatives for solving customer power quality problems. These alternatives will generally include the following options:
− Power conditioning and/or filtering at the sensitive loads.

• Central power conditioning and/or filtering at the customer service entrance.
• Changing operating procedures or system design on the utility distribution system.
• Modification to the design of loads to make them less sensitive to power quality variation

The requirements for each of these options will be developed from the simulation effort and the analysis of measurement results. Power conditioning in this case includes surge suppression, voltage regulation, and possibly backup for momentary interruptions. Harmonic filtering to solve harmonic problems can be applied either at individual loads or at the main service for a facility. Customer system design modifications, such as changing power factor correction procedures and equipment, can have an important impact on power quality variations. If particular loads are much more sensitive that other loads in the facility, either power conditioning at the particular load or design changes to the load equipment should be considered.

Momentary interruptions and voltage sags deserve careful consideration. Distribution system modifications could include implementation of switching procedures to minimize transients associated with capacitor switching events or addition of current limiting devices to minimize the voltage sags that occur during faults on parallel feeders. The impact of protection practices on power quality levels experienced by customers should be evaluated carefully using both the analytical and measurement results. Conclusions will be developed from this effort regarding optimum locations for power quality improvement and the impact of distribution system design practices on customer power quality levels.

SUMMARY

The procedure described in the previous sections is summarized by the block diagram in Figure 1.

REFERENCES

IEEE Standard 100. Terms and Definitions
IEEE Standard 1100. IEEE Recommended Practice for Powering and Grounding Sensitive Equipment (The Emerald Book).
IEEE Standard 1159. IEEE Recommended Practice on Monitoring Electric Power Quality.

---

RELATED STANDARDS
IEEE Std. 1159
IEEE Std. 1346
IEEE Std. 1250
IEEE Std. 1036
IEEE Std. 519

GLOSSARY AND ACRONYMS
ASD: Adjustable-Speed Drive
CVT: Constant Voltage Transformer
GPR: Ground Potential Rise
IEEE: Institute of Electrical and Electronics Engineers
MOV: Metal Oxide Varistor
PWM: Pulse Width Modulation
TVSS: Transient Voltage Surge Suppressors
UPS: Uninterruptible Power Supply
VCR: Video Cassette Recorder

Published by Horia Andrei¹, Costin Cepisca² and Sorin Grigorescu^2
1Valahia University of Targoviste, ²Politehnica University of Bucharest, Romania

---

1.Introduction

The chapter covers general issues related to power quality in Electric Arc Furnaces. The use of electric arc furnaces (EAF) for steelmaking has grown dramatically in the last decade. Of the steel made today 36% is produced by the electric arc furnace route and this share will increase to 50 by 2030.

The electric arc furnaces are used for melting and refining metals, mainly iron in the steel production. AC and DC arc furnaces represent one of the most intensive disturbing loads in the sub-transmission or transmission electric power systems; they are characterized by rapid changes in absorbed powers that occur especially in the initial stage of melting, during which the critical condition of a broken arc may become a short circuit or an open circuit. In the particular case of the DC arc furnaces, the presence of the AC/DC static converters and the random motion of the electric arc, whose nonlinear and time-varying nature is well known, are responsible for dangerous perturbations such as waveform distortions and voltage fluctuations.

Nowadays, arc furnaces are designed for very large power input ratings and due to the nature of both, the electrical arc and the melt down process, these devices can cause large power quality problems on the electrical net, mainly harmonics, inter-harmonics, flicker and voltage imbalances.

The Voltage-Current characteristic of the arc is non-linear, what can cause harmonic currents. These currents, when circulating by the electric net can produce harmonic voltages, which can affect to other users.

In evaluation and limitation, there are some definitions and standards to quantify the disturbance levels, such as (IEC, 1999), (IEEE 1995), and (***IEEE, 1996). and. The total harmonic distortion (THD), short-term voltage flicker severity (Pst), and long-term voltage flicker severity (Plt) are used. However, sometimes it is desired to record voltage and current waveforms in the specified duration to track the disturbance levels.

2.Electrical arc furnaces

2.1 Construction and typical steelmaking cycle
An electric arc furnace (EAF) transfers electrical energy to thermal energy in the form of an electric arc to melt the raw materials held by the furnace. The arc is established between an electrode and the melting bath and is characterized by a low voltage and a high current. Arc furnaces differ from induction furnaces in that the charge material is directly exposed to an electric arc, and the current in the furnace terminals passes through the charged material. Sir Humphrey Davy conducted an experimental demonstration in 1810 and welding was investigated by Pepys in 1815. Pinchon attempted to create an electrothermic furnace in 1853 and, in 1878 – 79, William Siemens took out patents for an electric arc furnaces. The first electric arc furnaces were developed by Paul Héroult, with a commercial plant established in the United States in 1907. While EAFs were widely used in World War II for production of alloy steels, it was only later that electric steelmaking began to expand. Of the steel made today 36% is produced by the electric arc furnace route and this share will increase to 50 by 2030.

A schematic cross-section through an EAF is presented in figure 1: three electrodes (black), molten bath (red), tapping spout at left, refractory brick movable roof, brick shell, and a refractory-lined bowl-shaped hearth.

The furnace is primarily split into three sections:

• the shell, which consists of the sidewalls and lower steel ‘bowl’;
• the hearth, which consists of the refractory that lines the lower bowl;
• the roof, which may be refractory-lined or water-cooled, and supports the refractory delta in its centre, through which one or more graphite electrodes enter.

Separate from the furnace structure is the electrode support and electrical system, and the tilting platform on which the furnace rests. Possible configurations: the electrode supports and the roof tilt with the furnace, or are fixed to the raised platform.

A typical alternating current furnace has three electrodes (Hernandez et al., 2007). The arc forms between the charged material and the electrode, the charge is heated both by current passing through the charge and by the radiant energy evolved by the arc. The electrodes are automatically raised and lowered by a positioning system and a regulating system maintains approximately constant current and power input during the melting of the charge, even though scrap may move under the electrodes as it melts. Since the electrodes move up and down automatically, heavy water-cooled cables connect the bus tubes/arms with the transformer located adjacent to the furnace.

The energy diagram shown in Figure 2 indicates that 70% of the total energy is electrical, the remainder being chemical energy arising from the oxidation elements such as carbon, iron, and silicon and the burning of natural gas with oxy-fuel burners. About 53 % of the total energy leaves the furnace in the liquid steel, while the remainder is lost to slag, waste gas, or cooling.

A mid-sized modern steelmaking furnace would have a transformer rated about 60 MVA, with a secondary voltage between 400 and 900 volts and a secondary current in excess of 44,000 amperes. To produce a ton of steel in an EAF requires approximately 440 kWh per metric tone; the theoretical minimum amount of energy required to melt a tone of scrap steel is 300 kWh (melting point 1520°C).

Electric Arc Furnaces (EAF) are being greatly improved at a fast pace. Only 20–30 years ago today’s EAF performance would be impossible to imagine (Hurst, 1994). Owing to the impressive number of innovations the tap-to-tap time has been shortened to 30–40 min. for the best 100–130 ton furnaces operating with scrap. Accordingly, their hourly and annual productivity increased. Electrical energy consumption got reduced approximately in half, from 580–650 to 320–350 kWh/ton. Electrical energy share in overall energy consumption per heat dropped to 50%. Electrode consumption was reduced 4–5 times – Figure 3. Typical steelmaking cycles are:

• arc ignition period (start of power supply) – figure 4a
• boring period –figure 4b
• molten metal formation period – figure 4c
• main melting period – figure 4d
• meltdown period –figure 4e
• meltdown heating period – figure 4f

Electrodes are initially lowered to a point above the material, the current is initiated, and the electrodes bore through the scrap to form a pool of liquid metal. The scrap itself protects the furnace lining from the high intensity arc. Subsequently, the arc is lengthened by increasing the voltage to maximum power. In the final stage, when there is a nearly complete metal pool, the arc is shortened to reduce radiation heat losses and to avoid refractory damage and hot spots.

After melt dawn, oxygen usually is injected to oxidize the carbon in the steel or the charged carbon. This process is an important source of energy; the carbon monoxide that evolves helps minimize the absorption of nitrogen and flushes hydrogen out of the metal. It also foams the slag, which helps minimize heat loss.

The random movement of the melting material has as consequence that no two cycles of the arc voltage and current waveforms are identical. The impact of these large, highly varying loads has a direct impact on the power quality of the interconnected power system. The abrupt initiation and interruption of current flow provides a source of harmonic currents and causes considerable disturbance to high-impedance circuits. Voltage and current waves deviate considerably from symmetrical sinusoidal patterns. Disturbances are worst during early meltdown, and they occur at varying frequencies.

Generation of harmonics may result in further flicker problems, and equipment on the power system may also be damaged. If static capacitors are to be used to improve the power factor, an analysis to ensure that resonance does not exist at any of the harmonic frequencies should be made. Harmonics contribute to wave distortion and to the increase in effective inductive reactance. This increase is often in the 10 to 15% range and has been reported as high as 25%. Current into the furnace is therefore less than what would be expected from calculations based on sinusoidal wave shapes, and losses in frequency sensitive equipment such as transformers are higher than the sinusoidal wave shape would produce. Generally, the initial period of melting causes the most electrical disturbances. As the scrap temperature begins to rise, a liquid pool forms, and disturbances begin to diminish. This is generally about 10 minutes or so after power-on and can vary depending on power levels and practices.

After about 20 minutes, most electric furnaces will have begun converting scrap to liquid metal. Hence, wide swings in disturbances will diminish considerably. When sufficient molten metal exists the arc is shortened by an adjustment to the electrode regulators. The current will rise since overall resistance is reduced, and the power factor and arc power will decline.

2.2 Perturbations
The majority of electric and electronic circuits (arc welders and furnaces, variable speed controllers, PC’s, medical equipment, etc) use switch mode techniques which act as a non linear load or disturbance generator which degrades the quality of the electricity supply.

In these electro energetic steady state circuits, the importance of the inconvenience caused by the non sinusoidal system of running is directly correlated to the amplitude of the harmonics. Also, it is of utmost importance to determine the variation of the apparent power at non defined node, in accordance with the presence of the current and voltage harmonics. Understanding the current harmonics and voltage harmonics is of utmost scientific importance both to the beneficiaries, who thus can prevent the undesirable effects of non sinusoidal steady state in a given network, and to the possible consumers as for as the corresponding measurement and pricing are concerned. Hence the elaboration of certain rules and prescription as regards the influence of the harmonics upon the fundamental component (first harmonic).

Such combinations of traditional and non-traditional loads, coupled with fluctuating loads, causes problems often classified as “random” or “sporadic” (problems with sensitive devices), annoying (light flickering) or as “strange” or “without apparent reason” (problems with cabling, capacitor banks, tripping, signaling etc.). The electric arc furnace produces strong disturbing effects featured by non-symmetries of currents and voltages, harmonics, flickers, voltage drops and over-voltages, characteristic parameters of power quality.

Many ways exist to reduce the effects of the arc disturbances. These are determined by the utility system to which the furnace or furnaces are to be connected, and they are influenced mainly by the size and stability of the power grid. Some sizable shops require no particular flicker control equipment. It is quite possible that, if a furnace shop is fed from a 220 kV or higher system with a short-circuit capacity of 6500 MVA or more, the utility will experience very little load disturbance, and the steelmaker can have considerable flexibility in configuring his internal plant power system.

Most utilities require power factor correction. Shops with large electric furnaces would more than likely use static capacitors; synchronous condensers of sufficient capacity would be prohibitively expensive for a multi-furnace shop. Before such systems are installed, transient analysis is required to determine:

• Capacitor bank configuration
• Need for harmonic tuning of sections
• Switching procedure

If additional regulation is needed, VAR control equipment would probably be required. However, if plans have already been made for power factor capacitors, including tuning reactors, then the thyristors and main reactor are the only further additions required. The perturbations caused by electric arc furnaces are of random nature and encompass a frequency range from DC to a few hundreds of Hz. Depending on whether AC of DC is used to supply the electric arc furnace there are unbalances, harmonics, inter-harmonics or voltage flicker.

2.3 Arc furnace models
For the design of EAF is necessary to utilize a suitable model. In this regard, numerous models have been presented to describe the electric arc (Lazaroiu & Zaninelli, 2010); (Math et al., 2006); (Hooshmand & Esfahani, 2009); (Sankaran, 2008). In general the models can be classified into:
a. Time domain analysis methods:

• Nonlinear Resistance Model: The approximation on the V-I characteristic of the arc,
  performed by piecewise linearization, neglect of the voltage rising time or nonlinear
  approximation. This method uses the numerical analysis method to solve the
  differential equation which is used to describe the furnace system with the assumed V-I
  characteristic.
  However it is a primitive model and does not consider the time-varying characteristic
  of arc furnaces;
• Current source models: An EAF is typically modelled as a current source represented by
  the Fourier series where the coefficients may change randomly during every period.
  This model is perfectly suited to size filter components and to evaluate voltage
  distortions resulting from the harmonic current injected into the system.
• Voltage Source Models: The voltage source model for an EAF is a Thévenin equivalent
  circuit where equivalent impedance of the furnace load impedance including the electrodes. The voltage source can be modelled in different ways. One possibility is to form it by major harmonic components that are known empirically. This method loses the stochastic characteristics of arc furnaces like the nonlinear resistance model does.
• Nonlinear Time Varying Voltage Source Model: The arc voltage is defined as a nonlinear
  function of the arc length. The time variation of the arc length is modeled with
  deterministic or stochastic laws.
• Nonlinear Time Varying Resistance Models: Arc furnace operation can be described by
  three basic states: open circuit, short circuit and normal operation. During normal
  operation the arc resistance can be modelled following an approximate Gaussian
  distribution. The random fluctuation in arc resistance accounts for the short-term
  perceptibility flicker index P_st.

b. Frequency domain analysis methods represent the arc voltage and current by their harmonic components (Key & Lai, 1997). The Harmonic Voltage Source Model first applies the Fourier transform to the arc voltage to obtain its harmonic components. Then the current harmonic components are calculated through the arc voltage harmonic components. Calculations provide an equivalent circuit for the fundamental frequency component consisting of an equivalent arc resistance and a reactance. The equivalent circuit for the calculation of the different order harmonics consists of a harmonic voltage source and the system impedance for that harmonic frequency. The model is simple, but suitable for steady-state iterative harmonic analysis.

c. Power balance method.
This model provides a harmonic domain solution method of nonlinear differential equation. The arc furnace load model is developed from the energy balance equation, which is actually a nonlinear differential equation of arc radius and arc current. This model uses some experimental parameters to reflect the arc furnace operation, but it neglects the influence of its supply system.

3.Basic principles for the power quality analysis

3.1 Power quality and harmonic distortion
One of the most important problems in nowadays consumers power supply is to ensure the power quality. Together with the power suppliers, the consumers are interested to use, to produce and to transport the electrical power as clean as possible. Any perturbation produced in the power system by any of its elements (components) may seriously affect the power quality consumed by the other elements especially those closely situated to the perturbing component (Filipski, et al., 1994).

The Power Quality has concerned the experts from power engineering area as far back as first years of using the energy, in a large amount of applications, the alternating current; during the last decade, we can observe several ascertainments to the involvement for this domain, owing to development based on power electronics.

Institute of Electrical and Electronic Engineers (IEEE) Standard IEEE 1100 define power quality as “a concept of powering and grounding sensitive electronic equipment in a manner suitable for the equipment”. But this is not the only interpretation. Another simple and more concise definition might state: “Power quality is a set of electrical boundaries that allows equipment to function in its intended manner without significant loss of performance or life expectancy”, definition that embraces two things that we demand from electrical equipment: performance and life expectancy. Another definition of power quality, based on the principle of EMC, is as follows: power quality refers to a wide variety of electromagnetic phenomena that characterize voltage and current at a given time and at a given location on the power system. IEC 61000-4-30 defines power quality as ”the characteristics of the electricity at a given point on an electrical system, evaluated against a set of reference technical parameters” (Toulouevski & Zinurov, 2010); (***IEEE, 1995). Power quality can be interpreted by the existence of two components:

• Voltage quality. It expresses the voltage deviation from the ideal one and can be
  interpreted as the product quality delivered by the utilities.
• Current quality. It expresses the current deviation from the ideal one and can be
  interpreted as the product quality received by the customers.

The main Power quality disturbances are:

• harmonics;
• under-voltages or over-voltages;
• flicker;
• transients;
• transients and voltage sags;
• voltage sags;
• interruptions.

Among the greatest electrical perturbations in a power system is the electrical arc furnace. Its perturbations are visible upon the reactive power flow, the load unbalance and the harmonics injected in the supply network. Also the random variation of the EAF electrical load, leads to the “flicker” phenomena characterized by variation in the field of 0.3-0.5% of the rated voltage and frequencies variations of 6 up to 10 Hz. Physically, the flicker phenomena is visible for the electrical bulbs that are rapidly changing the light intensity. Also, the side effects of the flicker are visible for the modern computation technique that could be damaged by the voltage variations.

At this moment we cannot talk about a united standardization of electrical energy quality on an international level and sometimes on national one. Currently, several engineering organizations and standard bearers in several parts of the world (IEEE, IEC, ANSI,…) are spending a large amount of resources to generate power quality standards. Some of them classify the events as steady-state and non-steady-state phenomena, in some regulations the most important factor is the duration of the event, other guidelines use the wave shape (duration and magnitude) of each event to classify problems and other standards (e.g., IEC) use the frequency range of the event for the classification. These documents come in three levels of applicability and validity: guidelines, recommendations and standards. In almost all the countries, the directives system of electrical energy quality is composed by several quantitative characteristics of slow or rapid variations of effective voltage value, the shape or symmetry as well as characteristics of slow or rapid frequency variations (IEEE-WG, 1996); (PE, 2004) (SREN, 1998); (CMP, 1987).

As it can be seen in Figure 5 there are presented the main causes of an improper electrical energy quality.

For the measurements of disturbances, IEC 61000-4-7 describes testing and measurement techniques for harmonics and inter-harmonics measurements and instrumentation, for power supply systems and equipment connected thereto.

3.2 The prominent power quality aspects
The prominent power quality aspects considered are the following:
a. Voltages and currents are non sinusoidal quantities, and can be expressed by relations:

where U_k , I_k are the RMS of each k-harmonic of voltage, respectively current, ω is the angular frequency, γ_k is the phase angle or each k-harmonic of voltage, k-harmonic of voltage, ϕ_k is difference of each phase angle of k-harmonic of voltage and current, t is the time.

where D = √S² − P² −Q² is the Budeanu distortion (deforming) power.

b. The presence of voltage and current harmonics is evaluated through a relative quantity, the total harmonic distortion (THD). Voltage harmonics are asserted with THDU, the ratio of the RMS value of the harmonic voltage to the RMS value of the fundamental, calculated by relation:

Everything presented for voltage harmonics is also valid for current harmonics and THD_I, the ratio of the RMS value of the harmonic current to the RMS value of the fundamental, calculated by relation:

Total harmonic distortion is the ratio between deforming residue and effective value of fundamental waveform:

Harmonic level is the ratio between effective value of the considered harmonic and the effective value of the fundamental:

c. Voltage imbalance. Applying the theory of symmetrical components, an unbalanced three-phase sinusoidal voltage system [Va, Vb, Vc] can be decomposed into a positive-sequence three-phase balanced system V₊, a negative-sequence system V-, and a zero sequence system V₀

d. Disturbance transiting among voltage levels: Rapid voltage changes, Transient overvoltages and voltage fluctuation and flicker.

3.3 Power quality measurements
A simple way for a technician to determine power quality in their system without sophisticated equipment is to compare voltage readings between two accurate voltmeters measuring the same system voltage: one meter being an “averaging” type of unit (such as an electromechanical movement meter) and the other being a “true-RMS (rms)” type of unit (such as a high-quality digital meter). Remember that “averaging” type meters are calibrated so that their scales indicate volts RMS, based on the assumption that the AC voltage being measured is sinusoidal. If the voltage is anything but sine wave-shaped, the averaging meter will not register the proper value, whereas the true-RMS meter always will, regardless of wave-shape.

The rule of thumb here is this: the greater the disparity between the two meters, the worse the power quality is, and the greater its harmonic content. A power system with good quality power should generate equal voltage readings between the two meters, to within the rated error tolerance of the two instruments.

Measurement and testing of supply voltage quality, according to EN 50160, requires specialized apparatus and measuring methods. This arrangement enables continuous monitoring, short time and long time, over 7 days, of the following parameters:

• frequency;
• total harmonic distortion factor THD_U and THD_I;
• voltage unbalance factor, which is a multiple of positive and negative sequence voltage
  components;
• fast and slow voltage variations, which are defined as short term (P_st) and long term
  (P_lt) flicker;
• severity factors.

This type of equipment, named digital power analyzer also enables measurement of voltage dips and outages, its frequency and duration.

The RMS values of voltages and currents can be determined correctly by digital methods in any harmonic content of waveforms. Also, with the results of RMS voltage and current can calculate the apparent power. The active power may be calculated and accurately measured in any circumstances of harmonic pollution. Unfortunately this is not the case for reactive power. For reactive power can be used different definitions and methods (Arrillage et. al., 2001); (Czarnecki, 1987); (Emmanuel, 1995); (Emmanuel, 1999); (Katic, 1994):

• reactive power measurement (Budeanu definition);
• Hilbert transform method;
• power triangle method;
• quarter period time delay method;
• low-pass filter method.

Table 1 presents the test conditions, voltage and current, used to test the measurement performances of the reactive power measurement solutions. Table 2 presents the errors obtained for different tests using notations: H- for Hilbert transform, LPF- for low pass filter, PT-power triangle, CTD- compensated time delay.

The traditional measurement methods, like Power triangle and the Time delay, comply with international standards but show limitations in the presence of harmonics or line frequency variation.

One can observe that Hilbert method give the best results, followed by the low pass filter method and then power triangle method. So, different analyzers implemented with different formulas can give discrepancies measuring the same loads.

Table 1.

Table 2.

4.Numerical simulations for energy calculation in power measurements

The model presented in (Vervenne et. al., 2007) is based on exponential-hyperbolic form which causes many problems in the power system quality. Also the model can describe different operations of the EAF and it does not need specific initial conditions.

The electric diagram of a electrical circuit supplying an EAF is illustrated in Figure 6. In this figure, bus 1 is the point of common coupling (PCC) which is the supplying bus of the EAF transformer. The arc furnace is also connected to the PCC through the transformer TS, (HV/MV). In this figure, X_C and RC are the reactance and resistance of the connecting cable line to the furnace electrodes, respectively. Also, X_Lsc is the short circuit reactance at bus PCC.

The electric arc is modeled by the following equations:

where V and i are arc voltage and current of the EAF, respectively. Also V_at is the voltage
threshold magnitude to which voltage approaches as current increases. Furthermore, I0 is
the current time constant in kA. It should be noted that the voltage V_at depends on the arc
length.

The constants C and D are corresponding to the arc power and arc current, respectively. These constants can take different values which depend on the sign of the derivative of the arc current.

As it can be seen in electric arc modeled equation, for the positive current and regarding the hysterias property of the arc, there are two cases. In the increasing current case, the hyperbolic equation and in the decreasing current case exponential equation is used. Hence, this model is called exponential-hyperbolic model. The proposed method has the capability of describing the EAF behavior in time domain using differential equation. In addition, it is able to analyze the behaviors in the frequency domain without solving the sophisticated differential equations.

Moreover, the proposed model can describe different operating conditions of the EAF such as initial melting (scrap stage), mild melting (platting stage) and refinement of the EAF. With the parameters of the system:

X_Lsc = 9.4245Ω, X_c = 2.356 mΩ, R_c = 0.4 mΩ, f_sys = 50 Hz

and:

V_at = 200 V, C_a = 190 kW, C_b = 39 kW, D_a = D_b = 5 kA, I_o = 10 kA

the voltage-current characteristic of the arc is obtained and shown in Figure 7. The voltage and the current of the arc are illustrated in Figure 8.

The characterization of flicker produced by an arc furnace is an extremely difficult operation (Alonso & Donsion, 2004); (Beites et. al., 2001); (Webster, 2004). The flicker is variable from one cycle to another and during melting stage very high peaks are produced. It depends on following parameters: quality and quantity of used scrap, reference operating points, quantity of injected oxygen, unpredictable consequences due to crumbling of the scrap during melting.

Consequently it is recommended to evaluate the level of flicker produced during at least one week of operation, representing several tens of operation cycles. LabVIEW and MATLAB software are used for simulation on EAF (Andrei et. al., 2006); (Andrei et. al., 2006); (Andrei et. al., 2006); (Beites et. al., 2001); (Bracale et. al., 2005); (Buzac & Cepisca, 2008).

5.Results of measurements in a real electric installation of arc furnace

5.1 Measurement method and equipment
The three-phase power analyzer is used for the analysis of power quality with compatible software analysis. The following quantities are necessary to be measured: voltage, current, flicker (IEC 68, IEC 61000-4-15-PST and PLT), THD, waveform snapshots and harmonics up to the minimum order of 64, frequency, transient events (Chi-Jui Wu & Tsu-Hsun Fu, 2003); (Pretorius et al., 1998).

The strategy of measurements was to carry out recordings on EAF with all electrical quantities: RMS voltage, RMS current, flicker, frequency, THD voltage, THD current, current and voltage waveforms, powers kW, kVAR, kVA, power factor, voltage and current vectors for the short and long time (Cepisca et al., 2004); (Cepisca et al., 2006).

One example of measurement equipment is a multifunctional Power Quality Analyzer METREL, shown in Figure 9, one advanced instrument for measuring quality of electrical power in compliance with the EN60150. It incorporates a number of different measurement instruments for calculating various electrical parameters which is based on current and voltage measurements.

5.2 Results of the measurements in a real electric installation of EAF
The electrical power networks of arc furnaces are presented in Figure 10 (Cepisca et al., 2008).

5.2.1 The real measurements of voltage and current harmonics, and of the powers
Figure 11 presents the current (a), the voltage (b) and Figure 12 presents the powers for a technological cycle of arc furnace. This cycle presents two phases: melting phase (6-8 minutes) and phase of stable arc burning (12-15 minutes). The electrical quantities are strong variation in the melting phase, with an important voltage fall. In the phase of stable arc burning the variation of electrical quantities are more reduced (Cepisca et. al., (2007); (Grigorescu et al., 2006); (Grigorescu et al., 2009); (***PE, 2004).

5.2.2 The real measurements of wave forms of voltage and current, and of the THD_U and THD_I for melting phase of the technological cycle of arc furnace
As regard to the wave forms of the voltages, shown in Figure 13, a, and, respectively the wave forms of the currents shown in Figure 13, b, on the 30 kV voltage supply line in the melting phase is found a strong distortion of currents.

The Figure 14 presents: (a) the total harmonic distortion calculated for voltages (THD_U, 2,8…3%), and (b) the total harmonic distortion calculated for the currents (THD_I, 10….11%).

5.2.3 The real measurements of wave forms of voltage and current, and of the THD_U and THD_I in the phase of arc burning of the technological cycle of arc furnace
In the phase of the electric arc stable burning (Figure 15, a, and b), that appears towards the final of the heat’s making, is found that the distortion that appear in the currents and voltages wave forms are more reduced. In this phase, the amplitude of the three phase currents and voltages are closer as value, fact which shows that the load impedance is more balanced.

The TDH for voltages and for currents in the arc stable phase are presented in Figure 16, a, and b. We observe that in the arc stable phase the THD_U is reduced (1…2%) and THD_U are an acceptable value (4…5%). One can reach to the conclusion that the deformation of the current and voltage waves is smaller in the stable burning phase also by the fact that the distorting power is smaller in this phase, in conditions where the apparent, active and reactive power is higher.

As regard the voltage on the 30 kV line, in the melting phase one can observe the presence of the important harmonics while in the oxidation phase is found practically only the presence of the fundamental. In the current’s case, the important values of harmonics demonstrate that in this phase the current is strongly deformed.

The variation form of powers measured values presented on the heat time presents in the first period, corresponding to the melting phase, a smaller apparent power. The electrodes are more lifted-up, in order to ensure protection against breaking and this determining a smaller value current. In the stable phase the apparent power is approximately constant and higher than in the melting phase. The variation of the voltage, as well as of the arc current, is reflected partially in the variation of active and reactive powers during the heat.

5.2.4 The variation of the THD_U and THD_I, and the variation of the power factor
The THD_U and THD_I (Figure 17) are higher in the melting phase than in the stable burning phase, bat the reactive power is higher in the stable phase than in the melting phase.

The power factor value (Figure 18) is higher in the stable arc phase and lower during the melting phase. For this reason results that on the 30 kV line the currents wave is more distorted than the voltages wave.

In different moments of technological process, following the measurements, were obtained values for THD_I within 1-21% for current and 1-6% for voltage. Comparing these values with the standard results that the furnace is not matched in the national and international standards.

References

Alonso, M. & Donsion, M. (2004). An Improved Time Domain Arc Furnace Model for Harmonic Analysis, IEEE Transaction on Power Delivery, 19(1), 2004, 367-373
Arrillage, J.; Watson, N. & Chen, S. (2001). Power System Quality Assessment, John Wiley & Sons, New York
Andrei, H.; Spinei, F.; Cepisca, C. & Caciula, I. (2006). 3-D mathematical model of the power factor in electro-energetical systems, Proceedings of VI World Energy System Conference, pp. 257-261, Torino, Italy, July 10-12, 2006
Andrei, H.; Cepisca, C.; Chicco, G.; Dascalescu, L.; Dogaru, V. & Spinei, F. (2006). LabVIEW measurements in steady state nonsinusoidal regime, WSEAS Transactions on Circuits and Systems, 11 (5), 2006, 1682-1687
Andrei, H.; Cepisca, C. & Spinei, F. (2006). The modelling of the power factor in steady state non sinusoidal regime with Mathcad techniques, Proceedings of IEEE-TTTC International Conference on Automation, Quality and Testing, Robotics AQTR THETA 15- Tome I, pp. 58-62, Cluj Napoca, Romania, May 25-28, 2006
Beites, L. F.; Mayordomo, J. G.; Hernandes, A. & Asensi, R. (2001). Harmonics, Inter harmonic, unbalances of arc furnaces: a new frequency domain approach, IEEE Transactions on Power Delivery, 16(4), 2001, 661-668
Bracale, A.; Carpinelli, G.; Leonowicz, Z.; Lobos, T. & Rezmer, J. (2005). Waveform distortions due to AC/DC converters feeding, Proceedings of International Conference Electrical Power Quality and Utilization, pp. 130-136, Cracow, Poland, June, 1-2, 2005
Buzac, E. & Cepisca, C. (2008). The importance of accurate measurement of electrical energy and the performance of modern electricity meters, OIML Bulletin, vol. XLIX, no.1, 2008, Paris, 5
Cepisca, C; Andrei, H.; Ganatsios, S. & Grigorescu, S. (2008). Power quality and experimental determinations of electrical arc furnaces, Proceedings the 14th IEEE Mediterranean Electrotechnical Conference – MELECON, vol. 1 and 2, pp. 546-551, Ajaccio France, May 5-7, 2008
Cepisca, C.; Ganatsios, S.; Andrei, H.; Cepisca, C. I.; Dogaru, V. & Lefter, E. (2004). The measurements of electrical nonsinusoidal signals, The Scientific Bulletin of University of Pitesti, Romania, Metrology, Measurements system and quality, 1, 2004, 22 -26
Cepisca, C.; Grigorescu, S. D.; Seritan, G.; Banica, C. & Argatu, F. (2006). Experimental results of harmonic pollution in the electric networks by the electric arc furnaces, Proceedings of the 5th International Symposium Advanced Topics in Electrical Engineering-ATEE, pp. 34 40, Bucharest, Romania, November 14-16, 2006
Cepisca, C.; Covrig, M.; Grigorescu, S. D.; Predescu, C.; Banica, C. & Argatu, F. (2007). Harmonic pollution in the electric networks by the electric arc furnaces. Experimental results, Proceedings of 7th WSEAS/IASME International Conference on Electric, Power Systems, High Voltage, Electric Machine – POWER’07, pp.233-238, Venice, Italy, April 20-22, 2007
Chi-Jui Wu & Tsu-Hsun Fu (2003). Data compression applied to electric power quality tracking of arc furnace load, Journal of Marine Science and Technology, vol.11, no.1, 2003, 39-47.
Czarnecki L. S. (1987). What is wrong with Budeanu’s concept of reactive and distortion power and why it should be abandoned. IEEE Transaction on Instrumentation and Measurement, IM-36 (3), 1987, 345-352 Emmanuel, A.E. (1995). On the assessment of harmonic pollution, IEEE Transaction on Power Delivery , vol. 10 (3), 1995, 1693-1698
Emmanuel, A. E. (1999). Apparent power definition for three-phase systems. IEEE Transactions on Power Delivery, 14 (3), 1999, 762-772
Filipski, P.S.; Baghzouz, Y. & Cox, M.D. (1994). Discussion of power definitions contained in the IEEE dictionary, IEEE Transaction on Power Delivery, vol. 9 (3), 1994, 1237-1244
Fuchs, E. F. & Masoum, M.A.S. (2008). Power Quality in Power Systems and Electrical Machines, Elsevier Academic Press, Amsterdam
Grigorescu, S. D.; Cepisca, C.; Potirniche, I.; Ghita, O. & Covrig, M. (2009). Numerical simulations for energy calculation in power measurements, Proceedings of the European Computing Conference (ECC09) and Proceedings of the 3rd International Conference on Computational Intelligence (CI09), pp.152-158, Tbilisi, Georgia, June, 20-22, 2009
Grigorescu, S.D.; Cepisca, C. & Ghita O. (2009). Strategies for energy quality monitoring in decision-making networks nodes, Proceedings of 5th International Conference Metrology&Measurement Systems – METSIM, pp. 174-178, Bucharest, Romania, 14-15 November, 2009
Hernandez, A.; Mayordomo, J.G.; Asensi, R. & Beites, L.F. (2005). A Method Based on Interharmonics for Flicker Propagation Applied to Arc Furnaces, IEEE Transactions on Power Delivery, 20(3), 2005, 2334-2342
Hurst, R. (1994). Power Quality and Grounding Handbook, The Electricity Forum, Toronto, Canada
Katic, V. (1994). Network harmonic pollution – A review and discussion of international and national standards and recommendations, Proceedings of Power Electronic Congress- CIEP, pp. 145-151, Paris, France, October 24-26, 1994
Key, T.S. & Lai, J.S. (1997). IEEE and international harmonic standard impact on power electronic equipment design, Proceedings of International Conference Industrial Electronics, Control and Instrumentation -IECON, pp. 430-436, London, England, May, 25-27, 1997
Lazaroiu, C. & Zaninelli, D. (2010). DC arc furnace modeling for power quality analysis, Scientific Bulletin of Politehnica University of Bucharest, Serie C, vol.72, Issue 1, 2010, 56-62
Math, H. J.; Bollen, Irene. & Yu-Hua Gu. (2006). Signal Processing of Power Quality Disturbances, Wiley-Interscience, New York
Pretorius, J.H.C.; Van Wyk, J. D. & Swart, P.H. (1998). An evaluation of some Alternative Methods of Power Resolution in a Large Industrial Plant, Proceedings of the Eights International Conference on Harmonics and Quality of Power-ICHQP-VIII, pp. 331-336, Athens, Greece, vol. I, October, 7-8, 1998
Hooshmand, R.A. & Esfahani, M.T. (2009). Optimal Design of TCR/FC in Electric Arc Furnaces for Power Quality Improvement in Power Systems, Leonardo Electronic Journal of Practices and Technologies, Issue 15, 2009, 31-50
Sankaran, C. (2008). Power Quality, CRC Press, London
Toulouevski, Y.N. & Zinurov, I.Y. (2010). Innovation in Electric Arc Furnaces, Springer- Verlag, Berlin, Heidelberg
Vervenne I.; Van Reuse K. & Belmans R. (2007). Electric Arc Furnace Modeling from a Power Quality Point of View, pp. 1-6, Proceedings of IEEE Conference on Electrical Power Quality and Utilisation, Lisbon, Portugal, September, 21-23, 2007
Webster, J. G. (2004). Electrical measurement, Signal processing and Displays, CRC Press, New York
***IEC. (1999). IEC 61000-4-30, Testing and measurement techniques-power quality measurement method
***IEEE. (1995). IEEE 1159:1995, IEEE recommended practice for monitoring electric power quality
***IEEE-WG. (1996). IEEE Working Group on Nonsinusoidal Situations. (1996). Practical Definitions for Powers in Systems with Nonsinusoidal Waveforms and Unbalanced Loads, IEEE Transactions On Power Delivery, vol. II, no. 1, 1996, 79-101
***IEEE. (1996). IEEE 100-1996 The IEEE Standard Dictionary of Electrical and Electronics Terms, Sixth Edition
***PE. (2004). PE 143/2004, Romanian norm for limitation of harmonic pollution and unbalance in electrical networks
***SREN. (1998). SREN 50160, Characteristics of supplied voltage in public distribution networks, October, 1998
***EN. (2004). Standard EN50160-Power Quality Application Guide, Voltage Disturbance, July 2004.
***CMP. (1987). Understanding electric arc furnace operations for steel production, Center for Metals Production-CMP, vol.3, no.2, 1987