Published by Dranetz Technologies, Inc., Application Note
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Introduction
IEEE Std. 519-2014, IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems is the most current recommended practice from the IEEE related to harmonics measurement and compliance. Although the recommended practices from the IEEE are usually only followed in North America, IEEE 519-2014 is used in Latin America, Asia, India and other parts of the world. As a result, there is a lot of interest in IEEE 519 2014 and how it is applied.
IEEE 519-2014 includes the harmonic measurement techniques of the widely used IEC 61000-4-7. IEEE 519-2014 adds two additional parameters, along with new requirements for statistics reporting for harmonic compliance. Therefore, instruments that only measure to IEC 61000-4-7 are missing required capabilities.
The Dranetz HDPQ Plus and SP are among the few instruments that fully comply with the measurement and reporting requirements of IEEE 519-2014. This application note focuses on the requirements of IEEE 519-2014 and how they apply to Dranetz instruments.
IEEE Harmonic Measurement Standards
The original IEEE 519-1992 recommended practice included descriptive information about harmonics and harmonic compliance limits. However, instrument measurement methods were not defined. The result was different manufacturers used different harmonic measurement techniques that produced different and inconsistent results.
IEEE 519-2014 addresses this by referencing the harmonic measurement techniques of IEC 61000-4-7. IEC 61000-4-7, and the broader IEC 61000-4-30 power quality measurement standard are well established, and have been adopted around the world. IEEE 519- 2014 states “…any instrument used should comply with the specifications of IEC 61000-4-7 and IEC 61000-4-30.” As a global supplier of power quality monitoring products, Dranetz products have complied with both of these standards since their inception in the early 2000’s.
IEC 61000-4-7 defines a harmonic measurement window width of 200ms, which is 12 cycles at 60Hz and 10 cycles at 50Hz. Each (DFT, Discrete Fourier Transform) harmonic analysis uses this 200ms window of data for its computations and this window is the smallest resolution for harmonic measurements in the Dranetz HDPQ family. Harmonic analysis is based on the evaluation of continuous 200ms windows without gaps.
Continuous 200ms windows without gaps
New Parameters for IEEE 519-2014
For statistical analysis, IEEE 519-2014 adds two parameters to the measurement methods of IEC 61000-4-7: Very Short Time Harmonics and Short Time Harmonics. It is important to note that these two new parameters are unique to IEEE 519-2014 and are not part of IEC 61000-4-7. This means that an instrument measuring to IEC 61000-4-7 does not necessarily comply with IEEE 519-2014.
Very short time harmonics are assessed over a 3- second interval and include 15 consecutive 200ms (12/10 cycle) windows. Without going into the specific details, harmonic components are aggregated over this interval and are then used for statistical evaluation.
Short time harmonics are assessed over a 10-minute interval and are an aggregation of 200 consecutive very short time values. Short time harmonics are aggregated over the required (10 minute) interval and then used for statistical evaluation.
IEEE 519-2014 Harmonic Statistical Evaluation
IEEE 519-2014 states that very short time harmonics should be accumulated over a one day period and the 99th percentile values should be calculated for each individual harmonic to the 50th each day. Short time harmonics should be accumulated over a one week period, and the 95th and 99th percentile values should be calculated for each individual harmonic to the 50th each week.
IEEE 519-2014 also includes recommended harmonic limits measured at the PCC (Point of Common Coupling). The intent is to manage the harmonics at the interface point between utility system owners and end users, so the specified limits apply only at the PCC and not for individual loads.
For voltage harmonics, the recommended limits for individual harmonics and VTHD are specified at various bus voltages measured at the PCC. Acceptable limits are based on the bus voltage, with higher bus voltages having lower acceptable limits (they are farther away from the harmonic generating loads).
Similarly, recommended limits for individual current harmonics are specified at several different bus voltage levels at the PCC, but TDD (Total Demand Distortion) is referenced and not ITHD. This means that TDD must also be computed by the instrument which is not part of IEC 61000-4-7. Current harmonics measured at higher bus voltage levels have lower acceptable limits (they are farther away from the harmonic generating loads).
The actual compliance limits are outside of the scope of this application note, so see section 5 of IEEE 519- 2014 for more details.
Dranetz HDPQ Plus & SP IEEE 519-2014 Reports
Although Dranetz products have measured harmonics to IEC 61000-4-7 for many years, IEEE 519-2014 adds the two new harmonic measurement parameters and new statistical evaluation requirements. The only Dranetz portable product that can fulfill these requirements are the Dranetz HDPQ Plus & SP family.
The Dranetz HDPQ Plus & SP have built in harmonic statistics reports that include very short time and short time harmonics. The report produces pass/fail daily and weekly voltage and current harmonic compliance in full accordance with IEEE 519-2014.
To view the report, press the Harmonics StatisticsReport button in the View Data page.
You will then see a list of daily and weekly harmonic statistic reports over the month 31 days, with each indicating pass or fail. There will be one very short time 99th percentile report for each day, and two short time reports for each week (one for the 99th percentile and the other for the 95th percentile).
Harmonics Statistics
Simply select a report in the list and press OPEN to view the details of the compliance for each harmonic to the 50th.
Harmonic Statistics Details
Dran-View 7 software also includes IEEE 519-2014 harmonic compliance reporting that uses the statistical data compiled by the HDPQ Plus & SP instruments (only).
Summary
IEEE 519-2014 is a significant update to the original version from 1992. In addition to specifying new harmonic measurement parameters and compliance limits, IEEE 519-2014 includes the harmonic measurement techniques of the globally accepted IEC 61000-4-7 harmonic standard.
The Dranetz HDPQ Plus & SP family are among the few products available that fully conform to both the measurement and compliance reporting requirements of IEEE 519-2014.
Published by Prof Jan Desmet, Hogeschool West-Vlaanderen, Email: jan.desmet@howest.be & Prof Angelo Baggini, Università di Bergamo, Email: angelo.baggini@unibg.it, June 2003
Source: Leonardo Power Quality Initiative (LPQI) Power Quality Application Guide Harmonics Neutral Sizing in Harmonic Rich Installations 3.5.1
Introduction
This section discusses the sizing of neutral conductors in the presence of power quality problems such as ‘triple-N’ – that is, currents with a harmonic order that is a multiple of three current harmonics. This issue is particularly important in low voltage systems where harmonic pollution by single phase loads is an increasingly serious problem. Triple-N harmonic currents add arithmetically in the neutral conductor rather then summing to zero as do balanced fundamental and other harmonic currents. The result is neutral currents that are often significantly higher, typically up to 170%, than the phase currents.
The sizing of conductors is governed by IEC Standard 60364, Part 5-52: Selection and Erection of Electrical Equipment – Wiring Systems. This Standard includes rules and recommendations for sizing conductors according to the current required by the load, the type of cable insulation and the installation method and conditions. Some normative rules are provided for sizing the neutral in the presence of harmonics, together with informative guidance in Annex D. National standards follow IEC 60364 closely but there is a significant time lag, so most national standards still do not deal with the neutral sizing issue in a comprehensive way. Since few installers and designers have easy access to the IEC standards, relying only on their national codes, they must depend on their own knowledge and experience when sizing neutral conductors. This application note is intended to clarify the issues involved and present the IEC guidance to a wider audience.
Theoretical background
In a star-connected three-phase system, the current in the neutral conductor is the vector sum of the three line currents. With a balanced sinusoidal three-phase system of currents, this sum is zero at any point in time and the neutral current is therefore zero (Figure 1).
Figure 1 – With a balanced three-phase load the neutral current is zero
In a three-phase power system feeding linear single-phase loads the current in the neutral conductor is rarely zero because the load on each phase is different. Typically the difference is small and is in any case far lower than the line currents (Figure 2).
Figure 2 – With an unbalanced three-phase load the neutral current is not zero, but it is smaller than the phase current
Where non-linear loads are being supplied, even when the load is well balanced across the phases, there is likely to be substantial current in the neutral conductor. With non sinusoidal currents, the sum of the three line currents, even with the same rms value, may be different from zero. For example, currents with equal rms values and square shape will result in a significant neutral current (Figure 3).
Figure 3 – With a non-linear three-phase load the neutral current is not zero and can also be larger than the phase current because of homopolar harmonics
In fact, the third harmonic components (and all other harmonics where the order is a multiple of three – the sixth, ninth, etc.) of the line currents are all in phase with each other (i.e. they are homopolar components), so they sum arithmetically rather than cancelling by vector addition (see Figure 4).
Figure 4 – Third harmonic currents in the neutral conductor
The neutral current amplitude may exceed the phase current in amplitude at the supply frequency due to the third harmonic.
The requirements of the Standard
IEC 60364-5-52:2001, ‘Electrical Installations in Buildings – Part 5-52: Selection and Erection of Electrical Equipment – Wiring Systems’, is concerned with the safe installation of circuits from the point of view of installation techniques and conductor sizing. The installation method frequently affects the thermal conditions in which the cable operates and so affects the cable carrying capacity of the conductor or circuit. Where cables of several circuits are installed in the same conduit, trunk or void, the current carrying capacity of each cable is reduced because of the mutual heating effect. In other words, the current carrying capacity of a cable is determined by the amount of heat generated by the current flowing and the amount of heat that can be lost from the cable by convection. Together, these determine the working temperature of the cable which, of course, must not exceed that appropriate to the insulation material, 70 °C for thermoplastic insulation (such as PVC) or 90 °C for thermosetting insulation (such as XLPE). The ratings and adjustment factors given in the Standard are based on practical tests and theoretical calculations based on typical conditions and need to be modified in the light of known installation conditions. Since the presence of triple-N harmonics in the neutral conductor results in higher heat generation, cable size selection must make allowance for this.
Reference to sizing the neutral conductor in case of non-sinusoidal currents can be found in IEC 60364-5-524. Clause 524.2 indicates that the neutral conductor shall have at least the same section as the phase conductors:
in two-conductor single-phase circuits and for all conductor cross-sections
in multi-phase circuits and in three conductor single-phase1 circuits when the cross-section of the phase conductors is equal to or less than 16 mm2 for copper or 25 mm2 for aluminium.
1 i.e. a centre tapped single phase supply where the centre point is neutral.
Clause 524.3 states that, for other multi-phase circuits, the neutral conductor may have a reduced crosssection if all the following conditions are met:
the maximum expected current, including harmonics, if any, in the neutral conductor during normal service is not greater than the current carrying capacity of the reduced cross sectional area of the neutral
the neutral conductor is protected against overcurrent
the size of the neutral is at least 16mm2 in copper or 25mm2 in aluminium.
These clauses are normative – in other words they provide regulations that must be followed in order to comply with the Standard. However, complying with these clauses requires knowledge of the type and number of loads that will be in use after the installation is put into service – unfortunately, this information is rarely available. The Standard also includes an informative annex – information provided to help the designer in the form of guidance and recommendation rather than regulation – that provides a methodology for sizing cables correctly.
This section presents this guidance with the addition of worked examples and some observations regarding de-rating in shared ducts and the effects of voltage drops.
Guidance from the Standard
The functioning of an electrical component or conductor can be significantly influenced by disturbances to the system, the supply, or the load. Of all of the electromagnetic disturbances that affect energy cables, the presence of current harmonics is one of the most important. The effects of this phenomenon can lead to overload of both phase and neutral conductors. Here, attention is focused on the sizing of the neutral conductor.
It should be noted that the current rating tables given in the Standard make many assumptions and it is the responsibility of the designer to recognise when these assumptions are not valid and make appropriate corrections. The most important assumption is that, in a four or five core cable, only three cores carry current; in other words, the load is assumed to be balanced and linear. In the situation where the load is unbalanced but linear the unbalance current flows in the neutral, but is offset by the fact that at least one phase conductor is carrying less load. Assuming that no phase conductor is overloaded, the total Joule loss in the cable is not excessive. When the load is non-linear there is a neutral current contributing to thermal loss as well as the full effect of the three line currents.
Under the conditions of current distortion described in paragraph 1.2, heat dissipation in the conductor due to the Joule effect is larger compared to the ideal linear load conditions, and the line capacity is therefore reduced. In addition, neutral conductors, often previously undersized with respect to the phase conductors in existing buildings (paragraph 1.3), can be overloaded even without the neutral current exceeding the rated phase current.
It is impossible to determine the neutral current in absolute terms unless the real or theoretical waveform of the load currents is known. However, as an approximation, the neutral current can be 1.61 times the phase current in the case of loads such as computers, but can reach the value of 1.73 times the phase current in the worst conditions with controlled rectifiers at high control angles, i.e. low DC voltages (α ≥ 60°).
The simplest way to solve the problem is to apply appropriate corrective coefficients to the cable current carrying capacity. Annex D of IEC Standard 60364-5-52 also gives a methodology for determining the appropriate derating factor. For simplicity, the approach assumes that:
the system is three-phase and balanced
the only significant harmonic not being cancelled in the neutral is the third one (i.e. the other triple-N harmonics have relatively low magnitudes and other harmonics are approximately balanced and sum to zero) and,
the cable is 4 or 5 core with a neutral core of the same material the same cross-section as the phase conductors.
In the strictest sense, calculation of the current harmonic effects should also take account of skin effect, which will reduce capacity as a function of the conductor size but, as a first approximation, it can be neglected.
To calculate the capacity of a cable with four or five conductors where the current in the neutral conductor is due to harmonics, multiply the standard current carrying capacity of the cable by the correction factor.
For phase currents containing 15% or less triple-N harmonics, the standard does not suggest any increase in neutral cross-section. Under these circumstances, the neutral current might be expected to be up to 45% of the phase current, and an increase of about 6% in heat generation compared to the normal cable rating. This excess is normally tolerable except in situations where the cable is installed in areas with poor ventilation or where there are other sources of heat nearby. An additional safety margin may be desirable in, for example, confined spaces.
For phase currents containing 15% to 33% triple-N components, the neutral current may be expected to be similar to the phase current and the cable must be de-rated by a factor of 0.86. In other words, for a current of 20 A, a cable capable of carrying 24 A would be selected.
Where the triple-N component of the phase currents exceeds 33% the cable rating should be determined based on the neutral current. For phase currents containing from 33% up to 45% triple-N harmonics, the cable size is determined by the neutral current, but de-rated by a factor of 0.86. At 45% triple-N current the cable is rated for the neutral current, i.e.135% of the phase current, derated by 0.86.
For even higher triple-N components, for example the typical worst case of 57%, the cable size is determined solely by the neutral current. There is no need for a correction factor because the phase conductors are now oversized.
Since the data for the reduction factors has been calculated on the basis of the third harmonic current value only, higher order triple-N harmonics at a higher level than 10% would further reduce the acceptable current. The situation described can be particularly critical when the neutral is used in common by several circuits (where this is permitted by local regulation).
Tables 2 to 5 show how the current rating changes with and without 3rd harmonic currents. The current ratings are calculated according to the IEC 60364-5-523 Standard. The ratings listed are for a 4 core 0.6/1kV cable with thermosetting (90 °C) insulation.
When using single core cables the choice of the neutral and phase conductor cross sections becomes independent. On the other hand the mutual thermal interaction is more difficult to model analytically because of the varying relative positions.
The most direct way to proceed is independent sizing of the neutral conductor, always bearing in mind that the thermal performance and the reactance of the circuit depends on the relative positions of the conductors. Additional factors that should be taken into account include:
When the cable is grouped with other cables, the greater current flowing in it (i.e. the harmonic current in the neutral) produces more heat so there is an effect on other cables. This must be taken into account by using enhanced grouping factors.
Table 2 – Current rating (A) with 3rd harmonic up to 15% (0.6/1kV 4 cores, 90 °C)
Table 3 – Current rating (A) with 3rd harmonic up to 33% (0.6/1kV 4 cores, 90 °C)
Table 4 – Current rating (A) with 3rd harmonic equal to 45% (0.6/1kV 4 cores, 90 °C)
Table 5 – Current rating (A) with 3rd harmonic equal to 60% (0.6/1kV 4 cores, 90 °C)
The voltage drop in the neutral caused by all triple-N harmonics becomes harmonic voltage distortions on all phases of the supply. This may require a further increase in neutral cross-section for long cable runs.
Particular attention has to be given to armoured or metal-screened cables. The contribution of harmonics to eddy currents in the screen or armour may be considerable. Therefore, whenever a load current distortion is expected, the neutral conductor should never have a cross-section smaller than the corresponding phase conductors. The same holds, of course, for all accessories of the neutral circuit.
When the design dimensions of the neutral circuit increases beyond that of the corresponding phase components, as can happen even in standard electrical systems, it is difficult if not impossible to find suitable commercial components available that are capable of correctly integrating into the system. The only suitable alternative is to limit the load or to size for the largest cross-section. Protection should, of course, be sized correctly for the smaller cross-section of the phase conductor.
For final circuits, separate neutrals for each line and separate circuits for each distorting load should be planned. This also ensures the best possible electromagnetic independence among both disturbing and susceptible elements. The use of the best possible balance of the loads avoids further contributions to the neutral current due to unbalance. The above considerations are just as important and applicable for large cross-section cables as they are to modest cross-section cables. They can also be applied, at least as a good approximation level, to busbars.
Numerical example
Consider the following example: a three-phase circuit with a 39 A load rating to be installed using a 4-core PVC (70 °C) insulated cable laid directly onto the wall. In the absence of harmonics, it is common practice to use a copper conductor cable with a 6 mm2 cross-section with a capacity of 41 A.
With 20% of the third harmonic, applying a 0.86 reduction factor, the equivalent load current is:
39.0 / 0.86 = 45 A
for which a cable with a 10mm2 cross-section would be necessary.
With a third harmonic equal to 40%, the cable section should be chosen according to the neutral current equal to:
39 × 0.4 ×3 = 46.8 A
and applying a 0.86 reduction factor a rated current:
46.8 / 0.86 = 54.4 A
so a cable with a 10 mm2 section is also suitable for this load. With 50% of third harmonic, the cable section to be chosen still depends on the neutral current:
39 × 0.5 × 3 = 58.5 A
requiring a 16 mm2 cable. (In this case the reduction factor is equal to 1.)
Conclusions
The discussion in this paper has pointed out how common design solutions, valid without power quality problems, become meaningless when the theoretical hypotheses upon which they are based are not fulfilled. In this instance, the assumption that voltages and currents have ideal waveforms is not valid.
In the case of neutral conductor sizing, common ‘old’ practice would advise the choice of a cross-sectional area smaller or equal to that of the corresponding phase conductors and the use of a scheme with the neutral shared among more lines. On the other hand, a correct consideration of the electromagnetic effects occurring with non-linear loads requires the selection of a neutral conductor with a cross-section larger than, or equal to, that of the corresponding phase conductors and based on the real current that is flowing in it. The use of a separate neutral conductor for each line (previously mandatory in some countries) is also required. The numerical example shows that the problem can arise on both important sections of a plant and on the final circuits of any electrical system.
References
[1] P Chizzolini, P L Noferi: Ottimizzazione degli interventi sulla rete di distribuzione mirati al miglioramento della continuita’ del servizio elettrico. LXXXVII Riunione AEI, Firenze 1986. [2] N Korponay, R Minkner: Analysis of the new IEC drafts for 185 (44-1) and 186 (44-2) instruments transformers in relation to the requirements of modern protection systems – Journée d’ études: Les transformateurs de mesure E2-20 SEE novembre 1989. [3] T M Gruzs: “A survey of neutral currents in three-phase computer power systems”, IEEE Transaction on industry applications, vol. 26, n° 4 July/August 1990. [4] IEC 364-5-52 – Electrical Installations in Buildings – Part 5-52: Selection and Erection of Electrical Equipment – Wiring Systems.
Published by Carelabs (Carelabz), Website: carelabz.com
Image: Carelabz – Zenith Controls, Inc. power switching systems
A transfer switch’s main job is to redistribute power from a grid to a backup source of power. The control panel system of a transfer switch is what makes the unit automatic in nature. Power failures can be detected immediately with the help of automatic transfer switch (ATS) and the shift to generator power from utility power is smooth.
The control panel’s job is to detect a power failure and initiate procedures to start the new or used generator’s engine. Once the used or new generator reaches the right frequency or voltage, the control system will signal to change to generator from the normal source of power.
The Automatic Transfer Switch (ATS) is a critical piece of equipment that alternates the origin or the source of power, typically between your utility power and backup power, ensuring your system’s ability to stay online. It is inherently important in making sure that this part of your emergency system is working properly. Also, because this part of your system is usually relatively complicated in nature, they are rarely examined or tested after the ATS has left its manufacturing facility.
Why Testing of Automatic Transfer Switch Important?
It is important to determine if your ATS is within manufacturers terms in the course of a planned examination, to eliminate the risk of determining this when an unpredicted outage occurs, and you are incapable to switch your system to its secondary power.
Occasional testing of your ATS guarantees that your emergency system is dependable. Carelabs is manufacturing facilities, power plants, and industrial businesses first choice when it comes to the electrical testing of ATS’s. Our technicians and engineers are skilled on emergency systems, specifically transfer switches. With our staff’s abundant experience and training in the field contributes as a definite recipe for success.
What is Done During Automatic Transfer Switch Testing?
Few of the steps in ATS testing incudes:
Contact to pole resistance test,
Settings and operations verifications
Control device Examinations
Manufacturer’s standards and specifications checks
Calibration services
Tap connections resistance measurements
Verifying engine start sequence
Time delay and retransfer functions checks
Mechanical parts inspections
Anchorage and groundings review for impairment or damage
Corresponding parts are properly lubricated and clean of debris or contaminants and many steps more.
Automatic Transfer Switch Tests Includes mainly three steps. The visual inspection, the electrical tests and the operational or functional tests:
Visual and Mechanical Inspection
Verify mechanical and physical.
Verify alignment, anchorage, required clearances and grounding.
Verify the unit is clean.
Verify appropriate lubrication on moving current-carrying parts and sliding surfaces.
Verify that manual transfer warnings are attached and visible.
Perform manual transfer operation.
Check positive mechanical interlocking amid alternate and normal sources.
Electrical Tests
With respect to ground execute insulation resistance tests on control wiring entirely.
Perform a contact/pole-resistance test.
Verify settings and operation of control devices.
Calibrate and set all relays and timers.
Check phasing, phase rotation and synchronized function as needed.
Perform automatic transfer tests:
Simulate loss of normal power.
Return to normal power.
Simulate loss of emergency power.
Simulate all forms of single-phase conditions.
Verify correct operation and timing of the following functions
Normal source voltage-sensing relays.
Engine start sequence.
Time delay upon transfer.
Alternate source voltage-sensing relays.
Automatic transfer operation.
Interlocks and limit switch function.
Time delay and retransfer upon normal power restoration.
Engine cool down and shutdown feature.
NFPA 110 has the following rules stated in terms of Automatic Transfer Switch testing
Operational Inspection and Testing.
EPSSs, including all components, should be exercised under load at least monthly and inspected weekly.
If the generator set is used for peak load shaving or for standby power, such use should be recorded and should be allowed to be replaced for scheduled functioning and testing of the generator set.
Transfer switches shall be operated monthly.
The periodic test of a transfer switch should constitute of electrically functioning the transfer switch till the alternate position from the standard position and returning.
How is Automatic Transfer Switch Testing Performed?
General Inspection
The inspection work should be conducted externally and internally on the transfer switch.
External Inspection
The transfer switch should be kept in good condition by performing a weekly overall examination of the unit. This inspection must consist of inspecting for signs of excessive heat, vibration damage, any level of deterioration, any leakage or contamination.
Any accumulations of dirt or dust must be removed. Dirt, dust and any other contaminants should always be removed from the outside and inside with a vacuum cleaner, dry cloth or brush. One should not use compressed air to blow away contaminants and dirt. This can result in debris being lodged in components resulting in damage to the switch.
If the inspection exposes damaged or loose components contact a trained professional to perform the repair work.
Any worn, broken or missing external sections must be substituted with manufacturer’s recommended components.
Contact the local authorized distributor or dealer for the specific part number to order.
Internal Inspection
All power sources must be turned OFF afore any internal inspection.
Verify to see if any external glitches found have disturbed internal components, while opening the switch door.
A trained service technician must be called to perform any service work. If any of the following conditions are detected:
Dirt, dust, moisture and other contaminants accumulating on the surfaces of the unit and components
Any signs of corrosion
Loose, missing or broken components
Deterioration of wiring or insulation due to cuts, abrasion or wear
Indications of overheating due such as melted plastic, discolored metal or burning odor
Any other evidence of damage, wear or malfunction of the transfer switch and its components
Only a trained technician must carry out internal service work and inspection on a standby system that doesn’t permit power interruption in the course of required inspection.
Inspections beyond visual inspections
When inspections are internal or more than just a visual inspection by the operator, they should be performed by an authorized distributor or dealer under a scheduled preventative maintenance agreement.
Have an approved dealer or distributor replace or repair all damaged internal parts with the manufacturer’s suggested components.
Disabling the Generator Set
Accidental starting can lead to severe injury and even death
Safety measure must be acquired chosen to prevent the generator set starting in the course of maintenance by a remote start/stop switch, an ATS, or another remote start engine command.
Afore operating on the generator set or any of its attached parts, like the transfer switch, detach the generator set as follows:
Move the generator master switch to the OFF-position
Disconnect the power to the battery charger
Remove battery cables starting with the negative (-) lead first
Transfer Switch Automatic Control System Testing
The transfer switch automatic control system should be tested monthly. The test should verify the following:
The necessary sequence of functioning happens when the load shifts to the emergency source which results in primary source failure
Verify indicator LEDs on the transfer switch operates properly
Watch and eavesdrop for any unnecessary vibration or noise in the course of operation
Finish the test once the switch transfers the load to the standby source and check the foreseen sequence of operations happening as the transfer switch shifts the load to the primary power source and signals the generator set to turn OFF later or after a cool down period.
Check if the time delay in the OFF position works while load is transferred to the standby source and transferred back to the favored source, in the case of systems with programmed time transitions.
Functional Test
The transfer switch functional tests comprise of electrical and manual tests. A manual operator handle is provided with the transfer switch for maintenance purposes only. Before it is operated electrically, manual operation of the switch must be checked.
A usual method of an automatic transfer switch functional test for a standby generator is explained below:
To begin the test, close the normal source circuit breaker. The switch controller will light up the available LED when right voltage is sensed. If the source 1 stages the automatic transfer switch mechanism, the LED at source 1 will turn on. Verify the phase to phase voltages at the utility line terminals.
Start the engine generator after closing the alternate source breaker. The S2 (Alternate) Available LED will illuminate when correct voltage and frequency levels are sensed. Turn OFF the engine generator after both sources have been validated and place the generator’s start control in the automatic position.
Replicate a utility failure by opening the Source 1 i.e. the normal side breaker. The delay to engine start timer begins its timing cycle. After the timer has completed its timing cycle, the engine start contacts close to start the generator.
When generator frequency and voltage touch the fixed reinstate points the Source 2 available LED lights up. Simultaneously, the delay to generator timer begins its timing cycle. When the time delay is completed the ATS will transfer to Generator, the S1 position LED goes off, and the S2 position LED illuminates. Systems shall transfer in no less than 10 seconds where failure of the equipment to perform could result in loss of human life or serious injuries.
Reclose the Source 1 breaker to re transfer to the normal source. The delay to utility timer begins its timing cycle. When the timer has completed its timing cycle, the ATS will transfer. The S2 position LED goes off, and the S1 position LED illuminates.
The delay engine stop timer will begin its timing cycle. The generator runs unloaded for the duration of this timing cycle. The generator will Turn OFF, once the timer finishes its timing cycle, The S2 Available LED goes off. A minimum time delay of 5 minutes should be provided for unloaded running of the EPS prior to shutdown to allow for engine cool down (NFPA 110). The minimum 5-minute delay is not required on small air-cooled prime movers 15 kW or less.
Precautions
Inspection and Cleaning Before doing any work on the transfer switch, de-energize all sources of power.
The switch must be checked for any moisture, dirt or dust and must be vacuumed or wiped with a soft brush or dry cloth.
DO NOT use a blower since debris may become lodged in the electrical and mechanical components and cause damage.
Any surface deposits must be removed with a clean cloth.
Published by Mirus International Inc., [2010-01-08] MIRUS-FAQ001-B2, FAQ’s Harmonic Mitigating Transformers, 31 Sun Pac Blvd., Brampton, Ontario, Canada. L6S 5P6.
Figure 7-1 shows how the sinusoidal currents on the phases of a 3-phase, 4-wire system with linear loads sum to return on the neutral conductor. The 120° phase shift between the sinusoidal load currents causes their vector sum to be quite small. In fact it will be zero if the linear loads are perfectly balanced.
Examining the dashed vertical lines in Figure 7-1 clearly demonstrates that the instantaneous sum of the currents in the three phases taken at any moment will also be zero if the linear loads are perfectly balanced. If they are not, then there will be a small residual neutral current as shown.
Figure 7-1: How non-linear load currents add in the neutral
With linear loads, the neutral conductor can be the same size as the phase conductors because the neutral current will not be larger than the highest phase current. Unfortunately, this is definitely not true for non-linear phase-to-neutral loads.
120VAC non-linear loads like the SMPS used in computers and in monitors draw current in two distinct pulses per cycle. Because each pulse is narrow (less than 60 degrees), the currents in the second and third phases are zero when the current pulse is occurring in the first phase. Hence no cancellation can occur in the neutral conductor and each pulse of current on a phase becomes a pulse of current on the neutral.
Even if the phase currents of the SMPS loads are perfectly balanced in RMS amperes, the RMS value of the neutral current can be as much as √3 times the RMS value of the phase current because there are 3 times as many pulses of current in the neutral than in any one phase. If the phase current pulses do overlap because they exceed 60 degrees in width, then there will be some cancellation so that the neutral current will be less than √3 times the phase current. Overlapped or not, because there are 3 times as many pulses in the neutral than in a phase, the predominant component of the neutral current will be the 3rd harmonic (180Hz for a 60Hz system). This is evident in the waveforms of Figure 7-1 since the linear current completes only 2 cycles in the same time period that the non-linear neutral current completes 6 cycles or 3 times the fundamental.
Often, in new construction this situation is addressed by simply doubling the neutral conductor ampacity. In existing facilities however, it is most often very difficult and too costly to implement this solution, therefore an alternate method is usually necessary.
Harmonics and Harmonic Mitigating Transformers (HMT’s) Questions and Answers
This document has been written to provide answers to the more frequently asked questions we have received regarding harmonics and the Harmonic Mitigating Transformer technology used to address them. This information will be of interest to both those experienced in harmonic mitigation techniques and those new to the problem of harmonics. For additional information visit our Website at www.mirusinternational.com.
Published by Jürgen Blum, Product Manager, Power Quality, A. Eberle GmbH & Co. KG
Image: A. Eberle (EV Charger – Electric Car Charger)
In the first part of the measurement campaign of 2016, various electric vehicles were evaluated in terms of their charging behavior and their loading effects on the power grid. The evaluation covered from DC up to the 50th harmonic or supraharmonics up to 20 kHz. As some electric vehicles use a chopper frequency much higher than 20 kHz, another measurement campaign was started at the University of Applied Sciences Bingen in September 2017 to measure the emissions ranging up to 150 kHz. In addition, the mutual interference between the different vehicles and between the electric vehicle and a solar inverter was investigated at the University of Applied Sciences Bingen.
Basis of the evaluation of the loading effects
The IEC 61851-21-1 standard (Electric vehicle on-board charger EMC requirements for conductive connection to AC/DC supply) will apply to electric vehicles in the future. This standard has already received the status of FDIS and will be published shortly.
At this time, the limits for current harmonics as given by IEC 61000-3-2 (Class A) up to 16 A and IEC 61000-3-12 (asymmetrical devices, Rsce = 33) for 16 A up to 75 A apply to electric vehicles. These same compatibility levels are also used in IEC 61851-21-1. There are limits up to the 40th current harmonic (2 kHz) and compatibility levels from 150 kHz to 30 MHz. There are no limits now on the emitted interference from electric vehicles in the frequency range from 2.5 kHz to 150 kHz. This range is also not regulated for the public power grid using compatibility levels. However, there are efforts underway now in the standards committee aimed at closing this gap as quickly as possible using compatibility levels.
There are many examples today of the mutual interference between different electronic devices. For example, the frequency converter of a CNC machine emits an interference level > 2.5 kHz into the power grid and a kitchen appliance malfunctions, or a solar inverter can automatically switch touch-dimmer lamps on and off. Who is responsible for this problem?
Is the kitchen appliance improperly equipped for interference resistance or is the CNC machine causing too large an interference level in the power grid at its connection? There can only be a fair regulation with the forthcoming limits for this frequency range. The interference can be remedied at both ends. Install a power filter on the device experiencing interference or reduce the emitted interference at its origin. Different customers always ask the question, „Who has to pay for it?“
The current draft of IEC 61000-2-2 (Compatibility levels for the public power grid) defined limits for the following ranges:
Frequency range (kHz) at 50 Hz
Compatibility level (%)
2 kHz to 3 kHz
1,4 %
3 kHz to 9 kHz
1,4 to 0,65 % Logarithmic drop-off with logarithmically increasing frequency
Frequency range (kHz)
Compatibility level (dBμV)
9 kHz to 30 kHz
129 to 122 dBμV Linear drop-off with the logarithm of the frequency from 9 to 30 kHz
The range of 30 kHz to 150 kHz is being prepared; compatibility levels are being added in real time.
In the standard, intentional emissions, for example, PLC signals for communication, are distinguished from nonintentional emissions.
Power utilities make use of a power line signal in the frequency range up to 148 kHz for signal transmission over the power grid. So that this signal can be detected unambiguously by the receiver, there must be a gap between the nonintentional emission from the power electronics, such as that caused by electric vehicles, and the PLC signal. Consequently, two limit curves are given in the standard.
Voltage (mV)
dBμV
1000 mV
120 dB
100 mV
100 dB
10 mV
80 dB
Sample table – conversion from volts to dB/μV
Measuring technology
Today, there are not many power quality measurement devices for permanent, uninterrupted monitoring of frequencies from DC to 150 kHz. This comes from the fact that there are no specifications as to how to evaluate in the future standard-compliant levels > 2.5 kHz to 150 kHz.
The measuring procedure for the frequency range from 2 kHz to 9 kHz is described in the standard for harmonics, IEC 61000- 4-7 in the informative Annex B. In this case, a grouping procedure for frequency bands of 200 Hz is used.
For the range > 9 kHz to 150 kHz, there is a suggestion in the Annex to IEC 61000-4-30, Ed. 3. Here, a grouping procedure for bands of 2 kHz is suggested. The final measuring procedure will only be specified in a few years in the future Edition 4 of IEC 61000-4-30. The frequency bands of 200 Hz or 2 kHz are under discussion. While a 200 Hz frequency band provides greater resolution in the spectrum, 10 times the quantity of data is measured than with a 2 kHz frequency band. Because of this, each procedure has advantages and disadvantages.
The PQ Box 300 from the A. Eberle company was used for the measurement campaign at the University of Applied Sciences Bingen. The power quality network analyzer measures frequencies from DC to 170 kHz with high accuracy. The grouping procedure of the measurement device can be configured for either 200 Hz or 2 kHz frequency bands. In this way, the different measurement results coming from the different grouping procedures can be verified.
For charging the various electric vehicles, a charging station with a type 2 charging plug and various 32 A CEE outlets and single-phase outlets was available.
The charging station was connected to the power grid of the University and uses a 25 mm² cable. The short-circuit performance at the charging station is about 2.5 MVA. A single phase 5 kW inverter is connected at a distance of about 10 m. The distance, that is the length of the cable connections, between the electric vehicles connected to the charging station, was usually 10 m, 2 times a standard charging cable of 5 m.
Measurement results
The following electric vehicles were evaluated:
Renault Zoe, Nissan Leaf, BMW i3, Audi e-tron, VW Golf GTE, Ford Focus e-lectric, Mitsubishi Outlander and the Tesla Model 90D.
Measurements were performed in the following configurations:
The electric vehicle alone to the power grid
The electric vehicle in parallel with other electric vehicles
The electric vehicle in parallel with the solar inverter of the University
The chopper frequencies of the vehicles measured were between 8 kHz and a maximum of 50 kHz for the different manufacturers.
In addition, there were large differences in the levels of the switching frequencies for the various vehicles. These were between 2 volts for vehicles with poor filtering and high interference levels as well as values that were undetectable by measurement in vehicles with an extremely low interference level.
Fig. 1: Electric vehicle with a 50 kHz chopper frequency; shown with 2 kHz frequency bands
Fig. 1 depicts the voltage using 2 kHz frequency bands up to 170 kHz A chopper frequency of 50 kHz can clearly be seen on an electric vehicle. The harmonics of 100 kHz (2 x 50 kHz) and 150 kHz (3 x 50 kHz) can also be seen.
The measuring procedure must be clearly defined for future evaluations of the compatibility levels of electric vehicles or other power electronics connected to the power grid. For example, if you compare the measurement results of 5 Hz, 200 Hz or 2 kHz frequency bands with one another, it is obvious that the results for level can differ substantially from one another. A broader frequency band of 2 kHz usually yields a larger measurement result than a narrow 200 Hz band for the same interference signal in the power grid (see Fig. 1 and Fig. 2 for comparison). With a broad frequency band, several spectral lines are quadratically added to one measurement value.
Fig. 2: Electric vehicle with a 50 kHz chopper frequency; shown using 200 Hz frequency bands
Fig. 2 shows the identical 50 kHz loading effect as the electric vehicle shown in Fig. 1. In each case, only the grouping procedure was switched in the measurement device from 2 kHz bands to 200 Hz bands.
Mutual interference between electric vehicles
During the measurement campaign, different electric vehicles were also charged in parallel to be able to evaluate the mutual interference between the vehicles.
Connection configuration 1: Electric vehicle No. 1 on the power grid alone
The vehicle is charged using one phase (C). In the figure, you can see the voltages A, B, C and the charging current C for the electric vehicle. No pronounced loading effects can be seen on the current. The switching frequency of this vehicle was 50 kHz.
Fig. 3: Electric vehicle No. 1 connected to the outlet alone
Connection configuration 2: Electric vehicle No. 1 is connected to the outlet and electric vehicle No. 2 in connected in parallel to the charging station using the type 2 plug.
The following Fig. 4 shows a clear change in the current consumption of electric vehicle No. 1 due to the loading effects from electric vehicle No. 2. The RMS value is virtually unchanged but there are large peaks in the charging current at the switching frequency of vehicle No. 2.
Vehicle No. 1 is connected to the power grid acting as an interference sink for the supraharmonics of the adjacent vehicle. In this example, a 10 kHz switching frequency can barely be seen in the voltage but is very pronounced in the current.
There are reports from practical use that sporadic interruptions in charging may occur if various different electric vehicles are charged in parallel.
Fig. 4: Electric vehicles No. 1 and No. 2 connected in parallel to the charging station and the outlet
Additional striking features
Almost all vehicles using single-phase charging used phase A. Only one vehicle used phase C for single-phase charging. This can later become a real problem by overloading phase A in the power grid with a greater number of vehicles in a supply power grid.
In addition, a large power grid imbalance occurs at the stub-end feeders. Even some vehicles, which were charged in the beginning using 3 phases, switched automatically toward the end to single-phase charging. This charging then usually used phase A.
In the engineering guides for connecting customer systems to the low-voltage grid of power utilities (VDE-AR 4100), the maximum connection power for single-phase loads is limited to <= 4.6 kVA. Electrical loads or generation systems 4.6 kVA must be connected using three phases. As a result, some of the electric vehicles from the measurement campaign may not be operated at a maximum connection power of up to 7.2 kVA.
Example: Electric vehicle with a single-phase charging current of 30 A on phase A
Fig. 5: Electric vehicle, single-phase charging at 6.9 kVA
Audible disturbance effects and summation with electric vehicles having the same switching frequency
One electric vehicle with a pronounced switching frequency of 10 kHz was clearly audible for the human ear in this frequency range during the charging process. Two vehicles of the same type were charged in parallel. The summation of the switching frequency in the power grid as well as the audible disturbance were evaluated.
As the switching frequency of both vehicles is not precisely 10 kHz but rather varies around 10 kHz, the level of the power grid loading effect fluctuates between a minimum of 1 V and a maximum of 2 V. One vehicle alone with a constant loading effect was about 1.4 V. As a result, both switching frequencies could add over time or also partially offset depending on the instantaneous phase difference of the frequencies.
If you stood exactly in the middle between the two vehicles during charging, you could hear the volume of the perceptible whistle change. In one case from practice, some vehicles from the same manufacturer caused a disturbance in an office building adjacent to several charging stations. This disturbance was so loud that the windows on the lower floor of the building could no longer be opened during office hours (reason: noise pollution).
Fig. 6: Chopper frequency of 2 electric vehicles of the same type and the photovoltaic inverter
Fig. 6 shows the 10 kHz chopper frequency of two electric vehicles of the same type and the 16 kHz of the solar power plant at the University of Applied Sciences Bingen. This amplitude of the switching frequency for both electric vehicles fluctuated in the measurement values between 1 V and 2 V at a frequency of about 0.5 Hz.
Mutual interference between electric vehicles and the solar inverter
The following figure shows the initial power grid load with an inverter and a solar power plant. You can see the chopper frequency of 16 kHz and its harmonics at 32 kHz, 48 kHz, and so on.
Fig. 7: Chopper frequency of the photovoltaic inverter
Fig. 8: Chopper frequency of the photovoltaic inverter and one electric vehicle
Now, an electric vehicle is connected to the charging station. You can now clearly see the 10 kHz switching frequency of the vehicle with 1.6 V in the power grid. The 16 kHz level of the solar inverter was, however, reduced by 50% from 1.4 V to 0.7 V.
Summary
Electric vehicles and modern power electronics may generate loading effects on the power grid far above 2.5 kHz. In the future, there will be compatibility levels for the public power grid for these frequencies. The measuring procedure for this is not been defined at this time. The PQ Box 300 from A. Eberle measures the range up to 170 kHz as a continuous measuring task. The measurement device can be set to frequency bands of 200 Hz or 2 kHz. This feature makes it ready for future changes to the standards. In addition, it is possible to select a different measuring procedure than for the parallel online measurement for recording.
Example: Continuous measurement of the 2 kHz bands and, in parallel, 200 Hz bands for online monitoring of a load. The network analyzer uses a continuous measuring procedure as the basis for calculating the FFT analysis.
VFD motor controls have become very popular for both retrofits and in new applications. They offer increased energy efficiency, potential for less motor wear, lower startup currents and more. Potential disadvantageous include producing harmonics and damaged motors if they are not controlled properly.
VFD’s use a Pulse Width Modulated (PWM) voltage for more precise control of the motor to meet the dynamic needs of the application. The varying voltage frequency of the VFD presents problems for most PQ analyzers, but not the Dranetz HDPQ family.
HDPQ® Power Quality Analyzers – Dranetz HDPQ ® Xplorer Plus
Eight channels, 4 voltage & 4 current (Colored Cables & Connector Panel)
Most PQ analyzers can only measure a stable 50/60Hz voltage source. The Dranetz HDPQ family can also synchronize to the current which is very beneficial for use in VFD and other applications.
PQ analyzer VFD testing applications include troubleshooting problems with controls, evaluating efficiency, harmonics, maintenance, and more. The Dranetz HDPQ family is ideally suited for VFD and many other related applications.
Configuring an HDPQ for VFD applications is simple – see application note (below).
Dranetz HDPQ Family VFD Output Measurements
Application Note
INTRODUCTION
Most power monitoring instruments synchronize their measurements to a stable 50/60Hz voltage source. The reason is to get accurate readings between the voltage and current phases. This is fine for the majority of applications, including when measuring the input of a Variable Frequency Drive (VFD), but poses a problem when attempting to measure the output of a VFD – by design, the VFD output voltage frequency varies, making it difficult for the power monitor to synchronize.
Unlike most power monitors, the Dranetz HDPQ product family can also synchronize to the current which is very beneficial for use in VFD and other applications. The instruments default to synchronizing to the voltage, but can be easily changed to synchronize to the current. Doing so involves a simple setup change, and this Application Note describes the required setting.
WHY MEASURE THE POWER AND POWER OF A VFD?
Many VFD monitoring applications are at the VFD’s input to measure the power consumption, harmonics, and power line fluctuations. Being at the VFD input, a stable 50/60Hz voltage source is available for power monitor synchronization. However, when evaluating VFD efficiency, susceptibility to PQ issues, and troubleshooting issues with the controlled outputs, it is necessary to measure at the VFD’s output, which does not have a stable 50/60Hz voltage output. Significant errors will result when using most power monitors.
It turns out that the VFD output current has a stable 50/60Hz frequency. See the snapshots below – the top is the voltage and the bottom is the current.
VFD output snapshot – the top is the voltage and the bottom is the current
DRANETZ HDPQ SETUP
The required current synchronization setting is located in the Nominal & Frequency tab of the Dranetz HDPQ Wizard setup. The Sync Channel setting defaults to Channel A Volts, but as shown below, when measuring the output of a VFD, select Channel A Amps as the sync channel instead. This is the only setting change that is required, and all other settings are as usual.
Sync Channel setting – measuring the output of a VFD, select Channel A Amps as the sync channel instead.
Website: Dranetz.com , Call 1-800-372-6832 (US and Canada) or +1-732-287-3680 (International)
Published by Carelabs (Carelabz), Website: carelabz.com
Image: Carelabz – Contact Resistance
What is Contact Resistance
Contact resistance is the resistance to current flow, due to surface conditions and other causes, when contacts are touching one another (in the closed condition of the device). This can occur between contacts of:
Breakers
Contactors
Relays
Switches
Connectors
Other switching devices
Contact resistance testing also known as Ductor testing, measures the resistance of electrical connections – terminations, joints, connectors, busbar sections or cable connections and so on. These can be connections between any two conductors, for example, cable connections or busbar sections. The instrument which is used to perform the ductor test is called an Ohmmeter, and since its function is to perform the ductor test, the ohmmeter is also known as a ductor tester.
The ductor tester can be found in many variations such as Micro, Mega and Milli- Ohmmeters, static resistance tester or DLRO which stands for Digital Low Resistance Ohm Meter. Is used to measure resistance in different applications of electrical testing. This tester consists of a DC ammeter and a few other components. The test measures the resistance at the micro- or milli-ohm level and is used primarily to verify that electrical connections are made properly, and can detect the following problems:
Loose connections
Adequate tension on bolted joints
Eroded contact surfaces
Contaminated or corroded contacts
The term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance, which is an inherent property, independent of the measurement method. This effect is often described by the term Electrical Contact Resistance or ECR and may vary with time, most often decreasing, in a process known as resistance creep. The idea of potential drop on the injection electrode was introduced by William Shockley to explain the difference between the experimental results and the model of gradual channel approximation. In addition to the term ECR, “Interface resistance”, “transitional resistance”, or just simply “correction term” are also used. The term “parasitic resistance” has been used as a more general term, where it is usually still assumed that the contact resistance has a major contribution.
Why You Need Contact Resistance Test?
The contacts in the circuit breaker needs to checked periodically to ensure that the breaker is healthy and functional. Poorly maintained or damaged contacts can cause arcing, loosing phase, and even fire.
This test is especially important for contacts that carry large amounts of current (e.g. switchgear busbars) because higher contact resistance can lead to lower current carrying capacity and higher losses. Ductor testing is usually performed using a micro/milli-ohmmeter or low ohmmeter.
Measurement of the contact resistance helps in identification of fretting corrosion of contacts, and allows contact corrosion to be diagnosed and prevented. Increase in contact resistance can cause a high-voltage drop in the system, and that needs to be controlled.
What is Done During Contact Resistance Testing?
The two common checks conducted on the contacts of a circuit breaker are the visual inspection check and the contact resistance check.
The Visual inspection check involves examining the contacts of the circuit breaker for any pitting marks due to arcing and worn or deformed contacts.
The second check is the contact resistance measurement. This involves injecting a fixed current, usually around 100A, 200A and 300 A through the contacts and measuring the voltage drop across it. This test is done with a special contact resistance measuring instrument. Then, using Ohm’s law, the resistance value is calculated. The resistance value needs to be compared with the value given by the manufacturer. The value should also be compared with previous records.
Both these tests need to be done together. As there are cases of contacts having good contact resistance yet being in a damaged conditions. Thus, for a contact to be certified healthy, it needs to have a good contact resistance and should clear the visual inspection test.
Ductor Tester
There are two types of ductor testers in general:
Series Type Ohmmeter has 4 resistors, internal battery voltage – E, and output terminals, A and B. When connected the A and B terminals with the R1 and R2 resistors, the battery forms a simple series circuit.
Shunt Type Ohmmeter, used for measuring small values of current resistance. When the A and B terminals are closed, the needle reads zero because the current flows only through the resistor RX. When these two terminals are opened, there is no current flowing through the RX resistor, thus the reading on the ductor tester is marked as infinite.
How We Conduct Contact Resistance Test?
Test Criteria
The criteria for evaluating the contact resistance of electrical connections largely depends on the type of connection (e.g. bolted, soldered, clamped, welded, etc.), the metallic contact surface area, the contact pressure, etc.. These will differ by equipment and manufacturer and there is no code or standard that mandates minimum contact resistances. As such, manufacturer recommendations need to be consulted. For example, manufacturers sometimes quote a maximum contact resistance of 10 micro-ohms for large bolted busbar joints.
Contact resistance measurement and its application domain are fairly extensive.
Electrical Connections
The electric connections of circuits have various ways and means, such as connected by welding, by pressing, by plug in and blot tightly and so on. If you want to know the quality of a connector and its conduction characteristic, you just need to measure its contact resistance. The contact resistance was often applied in quality testing of switches, relays and PCB pads.
At the aspect of the machinery assembly, the contact resistance of metals contact surface can be used in estimating the reliability and tightness of the machinery assembly. The contact resistance is associated with the conduction characteristic of contact surface. The larger area and the less impurity of the pair metals surface is, the better conductivity and the lower resistance are, and vice versa.
By the ways of measuring contact resistance we can qualitatively analyse the reliability and tightness of the machinery assembly. This technique has been already applied in quality test of the shield assembly for EMC. Measurement methods for different application are not the same. For example, in the case of measuring contact resistance of high-power switches and relays, high current should be used, a pair of contact, things just like the condition what is actually happening in working status. In the case of a dry circuit connector, the test current should be low to prevent the joint to be melted by heat, (the current less than 100mA).
Machinery Assembly
In the case of testing the machinery assembly quality, the different test circuits should be selected according to the different structures. There are two kinds of structure, the loop structure is close, and the non-loop structure is open. Their measurement methods are different completely.
How to Measure the Contact Resistance which Includes in a Loop Circuit, but no Changing the Circuit?
A new method will solve it. This method is very useful for measuring contact resistance in complicated machinery assembles. The contact resistance is defined as the ratio the voltage across the contact to the current flown through a closed pair of contacts. It accorded with Ohm’s law. There is an interface between the metal 1 and metal 2. The current, I, which coming from the current source flows through this interface, can be read from a current meter. And then the voltage drop across the interface can be read from a voltage meter as, U. Then the contact resistance value, Rx, can be calculated by.
Rx = U / I
Because the contact resistance changes with environment and the current pass through, the condition for measurement should be close as the condition in use. The four-terminal measurement technique and eliminating thermally EMFs technique must be used in accurate measurement. This indirect measurement method can be applied in measuring contact resistance or loop resistance. It needs three test points, three steps and three formulas. This method had been approved correct, and can also be used in calibrating the loop- resistor standard.
Typical Method for Contact Resistance Test
The four-wire (Kelvin) DC voltage drop is the typical method used by micro-ohmmeters for the contact resistance test, which ensures more accurate measurements by eliminating the own contact resistance and resistance of test leads.
The contact resistance test is performed using two current connections for the injection, and two potential leads for the voltage drop measurement; the voltage cables must be connected as closer as possible to the connection to be tested, and always inside the circuit formed by the connected current leads.
From the measurement of the voltage drop, the microprocessor controlled micro-ohmmeters calculate the contact resistance, while eliminates the possible errors due to thermal EMF effects in the connections (thermal EMFs are small thermocouple voltages which are generated when two different metals are joined together) they will be added to the total voltage drop measured, and will introduce errors into the contact resistance test if they are not subtracted from the measurement through different methods (reversal of polarity and averaging, directly measuring of thermal EMFs magnitude, etc.)
If low resistance readings are obtained when testing the breaker contact resistance using a low current, then it is recommended to re-test the contacts at a higher current. Why would we benefit using a higher current? A higher current will have the ability to overcome connection issues and oxidation on terminals, where a lower current may produce false (higher) readings under these conditions.
It is very important in the contact resistance test to maintain consistent measurement conditions, to be able to compare with previous and future results for trending analysis. Therefore, when taking periodic measurements, the contact resistance test must be performed in the same position, with the same test leads (always with the calibrated cables supplied by the manufacturer), and in the same conditions, to be able to know when a joint, connection, weld or device will become unsafe.
Conclusion
Measurements of thermal conductivity are also subject to contact resistance, with particular significance in heat transport through granular media. Similarly, a drop in hydrostatic pressure (analogous to electrical voltage) occurs when fluid flow transitions from one channel to another.
Contact resistance tests provide information about how healthy the contacts are and their ability to handle their rated current.
The maximum contact resistance should be verified against manufacturers’ specifications. Rated current should not be exceeded and testing at 10% of the rated current is recommended.
The minimum DC test current should be used according to manufactures specification; however, the IEC and ANSI recommended levels are: 50 A IEC 60694 100 A ANSI.
Published by Michael Daish, Summit Technology, Inc, Walnut Creek, CA
Introduction
Our modern technological world depends on systems and processes that require 24 x 7 x 365 operation. When their electrical infrastructures need maintenance it must inevitably be performed on live circuits. This article discusses a procedure to keep workers safe when performing “hot work”. NFPA 70E Article 130 FPN.2 discusses the justification for live work on “continuous processes” for “performing diagnostics”. Examples include start-up tests and trouble-shooting, verifying voltage quality, measuring circuit current loading capacity, and energy consumption studies. Here’s how to best to deal with the practical implications of complying with CSA and NFPA safety regulations to ensure success:
Document the Testing
An Electrical Hot Work Approval Form (EHWAF) must be completed first. It is a work plan with a step by step job description for installing and removing test equipment. The EHWAF must declare safety boundaries; the Restricted Approach Boundary, and the Prohibited Approach Boundary. No worker shall cross the Prohibited Approach Boundary – meaning no live work can be done beyond this boundary. The region between the Restricted Approach Boundary and the Prohibited Approach Boundary is where PPE (Personal Protective Equipment) is mandatory. The form requires approval, usually by the facility owner (or manager) who approves and grants permission to perform live work, and authorizes designated workers. The form keeps safety awareness uppermost in workers minds, and warns others of the proposed work.
Equip For Safety
CSA Z462 and NFPA 70E requirements are in force for the protection of electrical workers. Arc flash hazard labels indicating arc flash hazard severity and the required protective equipment should be present at the testing location. If no label exists a qualified engineer must assess what workers need to wear until a complete Arc Flash study is arranged. The following table is a guide:
Table: Arc Flash study
[1] Layering generally gives more protection than the sum total of the ATPV values of the individual garments being layered. However, this needs tested for each specific garment being layered and thus is never explicitly included in layered calculations, Nomex fabric and layering FR clothing grants you some extra protection. [2] Hard hat w/arc rated face shield + hearing protection + safety glasses (UV rated) + insulating gloves w/leather protectors + leather shoes. [3] Sock Balaclava [4] Arc rated (25 cal) arc flash hood and 25 cal flash suit. [5] Arc rated (40 cal) arc flash hood and 40 cal flash suit.
Best practices endorse the “buddy system” i.e. having another worker to assist, confirm the steps being taken, and be close by in case of a mishap. In addition to the PPE above hard hats are required with amber-tinted face shields that will protect against blasts and radiation that can damage corneas. Hearing protection should not be overlooked either as blasts generate high pressures that can rupture eardrums. Safety boots need to be chosen carefully for electrical insulation, and using rubber floor mats is a prudent precaution. PPE also applies to the selection of tools so the meter or test instrument must be chosen with the appropriate CAT safety rating.
Hot Testing
The following procedure from the EHWAF describes the steps to be followed when installing and removing a power monitor.
Steps to be followed installing power monitor
Steps to be followed when removing power monitor
Before connecting the test meter, an initial inspection should note the condition of the panel, conductors, debris, and obstacles that would interfere with safety. A thermographic scan with an IR camera will indicate hot spots due to loose or deteriorated connections that may cause flash-overs if disturbed. The presence of ozone odor may warrant an ultrasound test to locate corona discharges. Before any testing an assessment needs to be made to rectify potentially unsafe conditions.
Distance is Safety
Fact: Workers dislike wearing claustrophobic PPE. Vision is restricted by face shields, while thick gloves limit tactile feel and manipulation of tools. Plus, it gets hot inside those suits – workers want relief! Thankfully, wireless technology has come to their aid. A new generation of monitoring instruments now incorporate wireless technology (Wi-Fi or Bluetooth) to allow testing remotely, up to 25 feet, from a notebook PC, Netbook, PDA etc. Hoods, visors, and gloves can be removed allowing measurements and testing to be performed from a safe distance.
Safely perform testing via wireless from a laptop or an inexpensive Netbook PC.
Verify the Data
Verify the data integrity before leaving the site and make sure useful data has been captured by the monitor. If it hasn’t, it may be due to incorrect set-up, incorrect connections, or the monitor got disturbed and lost its connections during the test. To avoid wasted studies doublecheck the connections before monitoring by viewing a phasor diagram; errors become apparent immediately. The more intelligent meters automatically check connection errors and inform the user of missing signals or incorrect phase rotation errors.
To verify the data it may not be necessary to always remove the panel covers. Sometimes enough RF leakage occurs at gaps in panels, so data transfer is possible with panel covers closed. If the panel is tightly sealed no wireless signal escapes, covers must be removed requiring PPE. Before leaving the site view the data on the notebook PC to make sure the downloaded data is good. A report can be generated on the PC to display log graphs and events. If the data looks good then the deinstallation of the meter can proceed and the tech can rest assured sufficient useful data has been captured for subsequent analysis.
Conclusion: A Safer, Better, User-Experience
Electrical testing on live circuits must comply with CSA and NFPA safety requirements. By establishing a process, via a set of properly planned procedures as suggested above, testing can be performed on live circuits in complete safety. Working on live circuits in PPE is daunting and uncomfortable. Using test instruments with wireless communications for remote control provides workers with welcome relief from wearing safety gear for long periods. With wireless-enabled test tools, workers are safer, more comfortable, and thus more productive. Thanks to PowerCET Corporation for assistance with this article (www.powercet.com).
Published by Mirus International Inc., [2010-01-08] MIRUS-FAQ001-B2, FAQ’s Harmonic Mitigating Transformers, 31 Sun Pac Blvd., Brampton, Ontario, Canada. L6S 5P6.
Most power systems can accommodate a certain level of harmonic currents but will experience problems when they become a significant component of the overall load. As these higher frequency harmonic currents flow through the power system, they can create problems such as:
Overheating of electrical distribution equipment, such as cables, transformers, standby generators, etc.
Overheating of rotating equipment, such as electric motors
High voltages and circulating currents caused by harmonic resonance
Equipment malfunctions due to excessive voltage distortion
Increased internal losses in connected equipment resulting in component failure and shortened lifespan
False operation of protection equipment
Metering errors
Lower system power factor preventing effective utilization
Voltage regulator problems on diesel generators
Inability of automatic transfer switches to operate in closed transition
Harmonics overheat equipment by several means. For example, in electric machines and transformers, harmonic currents cause additional power losses by (i) increasing the eddy currents that flow in their laminated cores, (ii) through increased leakage currents across insulation and (iii) by producing skin effect in conductors.
The incidence of hot transformers and neutral conductors has been especially common. Even under less than full load conditions, a transformer can run surprisingly hot. One of the reasons is its winding configuration. The overwhelming majority of distribution transformers are DELTA primary, GROUNDED WYE secondary. The delta winding has some undesirable characteristics when significant amounts of 3rd harmonic (and other zero sequence currents) are present on the load side. These harmonics return along the neutral conductor and are trapped in the primary DELTA winding where they circulate causing significant extra heating. They do not flow through to the primary system, but they also are NOT cancelled (Figure 6-1).
Figure 6-1: Zero sequence currents trapped in the transformer’s delta winding
Since additional heating will reduce the life-span of a transformer, it must either be derated (not operated at its full nameplate rating), built to tolerate this additional heating (K-rated transformer) or designed to prevent the primary side circulating currents from being induced (harmonic mitigating transformer). A guide for derating has been proposed by CBEMA (Computer and Business Equipment Manufacturers Association) with the intent to provide users the ability to protect existing transformers which service non-linear loads. The relationship is as follows:
Since many of today’s multimeters can measure both peak and TRUE-RMS current, the derating factor can be quickly calculated. When a transformer feeds personal computers and other electronic equipment, typical values range from 0.5 to 0.7 meaning that the transformer should be loaded no more than 50 – 70% of its nameplate full-load rating to prevent damage due to premature aging.
The fact that harmonic currents create voltage distortion as they flow through the power system’s impedance makes their impact even more serious. It is voltage distortion, not current distortion, that will affect the connected equipment on the power system.
Harmonics and Harmonic Mitigating Transformers (HMT’s) Questions and Answers
This document has been written to provide answers to the more frequently asked questions we have received regarding harmonics and the Harmonic Mitigating Transformer technology used to address them. This information will be of interest to both those experienced in harmonic mitigation techniques and those new to the problem of harmonics. For additional information visit our Website at www.mirusinternational.com.
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