q and a for power quality shneider

8/4/2019 Q and a for Power Quality Shneider

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Wipeout Harmonics to Improve Power Quality

While the technical information contained herein is believed to be accurate,

no warranty, either express or implied, as to the accuracy or fitness forany particular purpose of the information is made.

Questions and Answers


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Introduction

The Wipeout Harmonics to Improve Power Quality seminar

held on May 3 was a tremendous success. Prior to theevent, numerous participants submitted questionsregarding harmonics, harmonic mitigators, and AccuSineactive harmonic filters many more than we could

address in such a short period of time.Please find these questions, with their corresponding

answers, in the following pages.


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How are TVSS affected by harmonics and are there new developments with

TVSS that relate to harmonic cancellation using a modified TVSS?

TVSS have no interaction with harmonic current. By design, they limit interaction tothe fundamental frequency.

TVSS are not capable of performing harmonic mitigation. They are designed toperform transient protection for large amplitude, short duration spikes of voltage that

are carried on power lines at the fundamental frequency.

TVSS that are capacitive based may have difficulties with the ripple currentsproduced by the 10 or 20 kHz switching frequencies of active harmonic filters. Theinteraction is dependent upon the location of the current transformers (CT) used foractive filters. If the CT is downstream from the TVSS, no interaction is expected.


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Do you have a sample calculation of the energy savings provided by eliminating

or mitigating harmonics?

Harmonic mitigation does not generate energy savings. Harmonics in no way affectthe real power drawn by the loads. Some claim that the I2R losses saved byreducing the apparent power current is energy savings. This has never beenscientifically documented. It reduces the current flow (VA) but the reduction is inreactive or harmonic power not in real power.


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What is the cost difference between active and passive systems? What added

future costs are there with active systems?

It is very difficult to compare the cost of active filters to passive filters. Passive filtersdo not approach the harmonic reduction performance level of active filters. Only18-pulse variable frequency drives (VFD) approach active filters in performance butthese devices are still dependent upon the source impedance and thus are not aseffective. Active filter performance is not dependent upon source impedance, but

rather on the harmonic producing loads attached. When active filters are applied asbus solutions where multiple nonlinear loads are present, the active filter is lesscostly and more effective than any other device, and requires less physical space.

Added future costs are similar to those of other power electronic devices like VFDand UPS. Annual inspections are required to ensure terminal tightness and the

removal of dust and dirt.


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We have three 750 HP 480 volt chillers on a 2500 KVA utility transformer. We are

putting drives on these three units but the mechanical engineers failed toorder up frontline filters. York claims worst case 29% THD current. In youropinion (say the transformer is 5.7%) will I have excessive distortion when allthree chillers are online possible to the point of interfering with the drivesperformance?

Assuming the VFD all have 3% input line reactors, the 29% Total Demand Distortion

(TDD) is about right. If there are no input line reactors or DC bus chokes, the TDDwould be a lot higher. Checking the information for IEEE 519-1992 selectioncriteria, the short circuit ratio is 19:5. In order to comply with IEEE 519-1992 Table10-3 level criteria, no more than 5% TDD is acceptable.

Compliance with IEEE 519-1992 is meant to limit the voltage distortion THD (V) to

5% due to the level of TDD defined in Table 10-3. This level (5% THD (V)) is notexpected to effect electronic equipment on the electrical system.

However, it is unlikely in my opinion that THD (V) will effect the operations of theVFD even if higher levels exist.


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Which is the better metric: THD or K-Factor?

K-factor is a method of sizing a transformer so that the transformer can withstandthe Total Harmonic Current Distortion levels (THD (I)). K-factor is based upon theTHD (I) present. K-factor does nothing to mitigate harmonics. It permits thetransformer to operate in the presence of harmonics and still not overheat.


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What is the correct method for monitoring and interpreting harmonics data so

that the equipment selected meets the customers needs?

The method defined in IEEE 519-1992 is total demand distortion (TDD). TDDcompares the amplitude of the harmonics at any operating point to the demand (full)load fundamental current of the loads. TDD is a hand calculation based on themeasured levels that define the harmonic current amplitudes and the maximumfundamental current of the loads. Metering devices do not measure TDD.

NOTE: THD (I) is neither used nor defined by IEEE 519-1992


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If we measure harmonic distortion with PowerLogic power management

equipment and find 6% total harmonic voltage distortion (THD (V)), how couldwe tell if part of the distortion comes from the utility company? How could weget them to mitigate theirs?

If PowerLogic or any other harmonic meter measures 6% THD (V), it does notdefine where that THD (V) originates. The loads may cause some harmonics, whileothers may be from the power source. The only way to measure what is coming

from a utility is to shut off all nonlinear loads in the plant and measure the THD (V).Utility standards according to IEEE 519-1992 are limited to 5% THD (V) for allelectrical systems less than 69 kV. If the THD (V) level from the utility is less than5%, the utility probably will not see it as a problem.


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What are the characteristics of harmonics generated by variable frequency

drives (VFD), especially regenerative VFD?

Harmonic currents caused by VFD depend upon the impedance of the electricalsystem and the impedance in the VFD and/or the impedance in front of the VFD.Typically, PWM VFD with diode rectifiers do not have (standard) DC chokeimpedance and input line reactors. These VFD will cause TDD levels between90120% on a typical utility supply. Adding a line reactor rated at 3% or a DC choke

of 5% impedance will lower the TDD levels to about 3237% TDD.

Regenerative VFD with IGBT rectifiers will draw current with TDD levels of about5%. However, active converters directly generate voltage distortion on the AC lines.An input line passive filter is required to limit the THD (V) to 5%. If no line filter ispresent, THD (V) levels can approach 20%.


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What is the best way to handle power factor correction for FVNR in a low voltage

MCC when a significant number of loads are running on VFD (there alwaysseems to be an issue with the harmonics effecting the power factor correctioncapacitors)?

Power factor correction capacitors (PFCC) can interact with all harmonic currentspresent. If the harmonic current distortion is high enough, nuisance fuse clearing,resonance, or capacitor destruction caused by harmonic heating are possible.

Typically, if the content of the loads is greater than 15% nonlinear, then detunedPFCC systems are required to protect the capacitors. Furthermore, detunedsystems are not typically installed as part of the MCC but rather as separate stand-alone units. If the MCC has greater than 50% nonlinear loads, then active filtersincorporated into the MCC should be considered for both power factor correctionand harmonic filtering purposes.


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What effect do harmonics have on Total Power Factor? What effect doharmonics have on Displacement Power Factor?

Total Power Factor is defined by displacement power factor (Cos ) and theharmonics factor (Cos ).

Harmonics have no effect on displacement power factor (DPF). DPF is the measureof the phase shift between the fundamental current and the fundamental voltage.

Correcting harmonics improves total power factor.


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With the development of the AccuSine, is there still a need for shielded powercables to variable frequency drives (VFD) and/or VFD drive motors?

Harmonic filters do not limit the harmonic current drawn by nonlinear loads. Linefilters have no effect on the output of the VFD to the motor. There is no reason tomodify present wiring practices.


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Can harmonic currents from large variable frequency drives (VFD) actuallycause 1520% voltage swells at your service entrance equipment? OurPowerLogic system is constantly seeing 20% voltage swells in a building withseveral 125 hp, 6-pulse drives and a high amount of distortion on both currentand voltage.

Cyclical loads may cause swells in harmonic current that result in high levels ofvoltage distortion. This may be internal or external to the plant in question. THD (V)

may be the result of local loads or caused by the power source. If it can be provento be an external cause, then the utility should be approached to investigate. If theTHD (V) is from the utilitys supply, then it is their responsibility to implement asolution.


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How do active filters compare to TVSS? Should they be used in conjunction withone another?

TVSS are designed to suppress momentary surges and clip the amplitude of thevoltage for the loads. Harmonics are noise at frequencies other than thefundamental frequency. The two are not related.

Take care when using harmonic filters with TVSS that are capacitive in nature.

Active harmonic filters may interact with the TVSS and overload the capacitor in theTVSS with resonance induced by the active harmonic filter. The current transformerthat senses current for the active filter needs to be on the load side of the TVSS.


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What are the implications of harmonic issues in hospitals with integratedcommunications/ data/control systems?

Nonlinear loads draw harmonic current from the source and the current path issubject to harmonic-induced heating. Therefore, thermal trip devices (i.e. circuitbreakers and fuses) could activate to remove the loads on that path from the lines.Also, because of the impedance of the electrical system, THD (V) could develop andreflect to the entire electrical network. THD (V) could reach levels that cause

adverse logic systems reactions, including data loss, power interruption, and evencircuit board failure.


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My customer has problems with PLC S908 remote I/O dropouts, and Square Dhas recommended a power study. What does this consist of and what are theusual steps for follow up?

A power study may take many forms. The objective is to review the electrical powersystem at critical branches of the system to see what levels of harmonic (currentand voltage) and displacement power factor are present. The study may detectother types of problems as well, such as poor grounding.

The technician performing the power study will provide a full report ofmeasurements, analysis, and recommendations for addressing any problemsdiscovered. Once of the recommendations are implemented, a follow-up visit canensure the problems have been fixed.


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Can you explain the benefit, if any, of installing active harmonic filters in aretrofit application? What additional engineering is required beyond theswapping of transformers?

Generally, retrofits are performed when harmonics related issues exist ormodifications add more nonlinear devices.

The engineering required could involve an electrical system study (harmonic

analysis) and computer modeling of the electrical system. When nonlinear loads areadded, it is wise to review the electrical system contingencies to insure thatproblems are minimized.


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Can active filters be installed with other manufacturers variable frequencydrives (VFD)?

Generally, all PWM VFD are of the same design. They employ diode rectifiers tocharge a DC bus so that managed voltage and frequency are delivered to themotors. Some manufacturers install DC chokes that reduce the harmonic currentdraw. Others install input line reactors that limit harmonic current draw. Most do notprovide either. These VFD draw harmonic current that may be 90120% TDD. If

harmonic reduction devices are installed per the manufacturer, compatibility of thefilters must be reviewed to ensure the filters do not interact adversely.


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Can you provide an exact cost and space comparisons between 6-pulse,12-pulse, 18-pulse, isolation transformer, regenerative VFD, parallel tunedfilter, etc.?

Exact cost and space comparisons between alternatives can be performed perproject. General statements were addressed during the seminar. Please contact usat [email protected] to discuss your specific situation.


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I am using pumps for the irrigation of my golf course. All of my primary neutralsare overheating or burning up. How far will harmonics travel?

Neutral heating can be the result of single-phase nonlinear loads or other problems,i.e. poor grounding issues. If single-phase nonlinear loads are present, theharmonic heating generally effects the neutral cables, distribution panels, and stepdown transformers.

If there are no phase shifting transformers (such as delta-wye), the harmonicstraverse the entire electrical system from the source to the nonlinear loads.Typically, single-phase loads are connected line-to-neutral and are downstreamfrom isolation transformers (60 Hz systems). The neutral harmonics will not be seenon the primary but will be seen on the secondary of this transformer. If the single-phase loads are directly connected line-to-neutral from the 480 V or 400 V system,

then neutral harmonics are present at these voltages.


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Do active harmonic filters fulfill the requirements of the welding industry,specifically, the dynamic nature of momentary loads and the large transientsthat are generated?

Schneider Electric has two products that will provide support for the very high in-rush current requirements of welders:

AccuSine, or

AccuSine and a detuned passive PF capacitor bank (Hybrid VAR Compensation system)

Generally, in-rush current support is necessary so that the line voltage does notcollapse and cause poor quality welds or flicker. Contact us at [email protected] and one of our PQC specialists can assist you in selecting the rightproduct for your unique application.


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Do Servo and stepper motor driver units in control systems give off or sufferfrom harmonics in the same way as large AC drive systems?

All products that rectify AC power to DC power draw harmonic current through theelectrical system. Single-phase rectifiers can generate up to 170% TDD. If thedevice is connected to the neutral, then the 3rd order harmonic is carried on theneutral and can be larger than the fundamental current. This can cause harmonicheating and destruction of the neutral if the neutral is not double the size of the line

cables. As a general rule, the smaller the load rating the higher the %TDD levels.This becomes a problem when the voltage distortion increases or the cablesoverheat.


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Where can I find a complete information (book), about the harmonics problems,and solutions?

There are some very good textbooks that define the subject and demonstrateapplication through actual case studies. Try one of the following:

Power System Harmonics, Jos Arrillaga and Neville R. Watson

Power System Harmonics: Fundamentals, Analysis and Filter Design, George J. Wakileh


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What is the proper way to size the feeders to the AccuSine units and whereshould current transformers (CT) be mounted?

AccuSine reduces the total rms current draw through upstream feeders. AccuSinehas very little effect on downstream feeders. The nonlinear loads must drawnonlinear current and AccuSine provides it. As far as branch circuit protection forAccuSine and the cables connecting AccuSine to an electrical system, they need tobe 125% of the AccuSine rating.

The seminar discussed placement of CT with diagrams for simple installations.More complex installations, such as Main-tie-Main or Dual Power sources, shouldbe discussed with one of our Power Quality Specialists at [email protected].


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What are some of the key things to look for with a PowerLogic monitor?

The primary worry from a harmonic perspective is the THD (V). If this value is morethan 5% and other equipment is malfunctioning, then further investigation isnecessary to review the cause and potential solutions for limiting the harmoniccurrent.


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How can I filter reflected power from variable frequency drives (VFD) ? How can Iprotect instrumentation without deviating from the nominal indicated line

voltage? How can I address "voltage gain" problems and the associated errorin the peak voltage?

Harmonic current is very similar to reactive current. The power source must provide both unlessthere are other devices in the electrical system to provide them. PF caps provide reactive currentfor the loads that need it, such as AC motors. Harmonic filters provide specific harmonics so thatthe source does have to provide.

Voltage distortion usually is the cause of problems for instrumentation. Protection requires limitingthe harmonic current that causes the voltage distortion. Whether this is beyond the normal or notcould involve much discussion. Its reasonable to expect to gain a benefit from the devices thatdraw the nonlinear current. There may be no other (easier) way to do what the nonlinear devicedoes. With nonlinear loads, it is normal to be wary and review the harmonic situation.

Voltage gain may mean a voltage increase caused by too much capacitance within the electricalsystem. In other words, there is more capacitance installed than needed. This causes the linevoltage to rise. The results are leading displacement power factor (Cos ) and resonance.

Nonlinear loads draw current in pulses. The amplitude is higher than a sinusoidal current and thusthe peak currents are larger than sinusoidal currents. This may reflect into voltage distortion thatcould increase the peak amplitude of the voltage.


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What is the most economical method of minimizing harmonics in hospitalfacilities/college classrooms/airport terminals?

Its impossible to say there is a most economical solution for a general installation.

Harmonics depend upon the source capacity, the electrical system impedance, thetype of nonlinear loads installed, the type and quantity of linear loads installed, andthe sensitivity of other loads installed. General statements can be made based

upon experience, but specific evaluations are required per project. Contact one ofour Power Quality Specialists at [email protected] to help you selectthe right solution for your application.


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How accurate are the models that estimate or determine the need for harmonicfilters or similar equipment? Can a model be "calibrated" to account for actual

or measured field conditions?

Harmonic models vary greatly in accuracy. Some vendors of nonlinear loadmeasurement or monitoring equipment may shape the output to justify a singlesolution that is advantageous to the company. However, the general modelsdeveloped and sold as software for harmonic analyses that are developed by

nonaligned companies (have no product to sell) are as good as the data entered.Accuracy of information is the limiting factor for these programs.

The programs can be tweaked through good information. Harmonic site studiesprovide accurate data so that a program can correctly model any electrical system.


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How "pure" does a power source have to be without affecting other equipment?What type of equipment is more sensitive to harmonics and what are the

acceptable limits for harmonic distortion for sensitive equipment?

IEEE 519-1992 addresses the utility supply through voltage distortion discussions inChapter 11. For electrical systems 69 kV and lower, utilities are required to provideTHD (V) no higher than 5%.

There are no standards for equipment tolerance to harmonics in North America.Tolerance levels will vary even within a single product provided by one vendor.Generally, electronic products are more susceptible to harmonics than other types ofproducts.


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How much longer do you think it will be before government regulation goes intoeffect demanding that major users clean up their power quality?

In the United States, there is no plan or law to require harmonic clean up and localutilities manage the situation. Around the world, some governments haveimplemented regulations as law that limit the levels of harmonic current drawn by allusers.


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What are the pros and cons of transformers (k-rated, zero sequence) forharmonic reduction?

As a general statement, transformers do nothing except offer impedance forharmonics. K-factor transformers do very little for harmonics. Total HarmonicCurrent Distortion (THD (I)) is the same on the primary as on the secondary.The K-factor merely defines the amount of harmonic current the transformer canhandle and not burn up.

Zigzag (or zero sequence) transformers are designed to shunt triplen harmonicsfrom the neutral to the AC lines. This reduces the harmonic current on the neutralbut increases the THD (I) on the AC lines.

Some transformers are used as phase shifting transformers for multi-pulse nonlinearloads. The effect is to reduce the harmonic current on the primary side of thetransformer.


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Can active filters be used with power factor correction capacitors?

Active filters can be used in conjunction with PF capacitor systems. AccuSine,specifically, can be used to remove the harmonic current such that the PF caps donot see harmonic current and thus will not overheat or resonate.

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