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WARNING:Please read the License Agreementon the back cover before removing

the Wrapping Material

Power Quality Implications ofOvercompensated System

1001683

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EPRI Project ManagerA. Sundaram

EPRI 3412 Hillview Avenue, Palo Alto, California 94304 PO Box 10412, Palo Alto, California 94303 USA800.313.3774 650.855.2121 [email protected] www.epri.com

Power Quality Implications ofOvercompensated Systems

1001683

Technical Update, February 2003

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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES

THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS ANACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCHINSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THEORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM:

(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I)WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, ORSIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESSFOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON ORINTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUALPROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'SCIRCUMSTANCE; OR

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ORGANIZATION THAT PREPARED THIS DOCUMENT

EPRI PEAC Corporation

This is an EPRI Technical Update report. A Technical Update report is intended as an informal report ofcontinuing research, a meeting, or a topical study. It is not a final EPRI technical report.

ORDERING INFORMATION

Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 WillowWay, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169,(925) 609-1310 (fax).

Electric Power Research Institute and EPRI are registered service marks of the Electric PowerResearch Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric PowerResearch Institute, Inc.

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CITATIONS

This report was prepared by

EPRI PEAC Corporation942 Corridor Park Blvd.Knoxville, TN 37932

Principal InvestigatorW. Grady

B. JohnsonC. PerryA. Mansoor

This report describes research sponsored by EPRI.

The report is a corporate document that should be cited in the literature in the following manner:

Power Quality Implications of Overcompensated Systems, EPRI, Palo Alto, CA: 2003. 1001683.

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REPORT SUMMARY

BackgroundElectric utility companies are under pressure to correct feeder power factors, to minimize losses,and improve voltage regulation. These objectives cannot be met for end-use customers usingonly substation capacitors. Rather, switched capacitors and line-regulating transformers must bedistributed along the feeder, correcting power factors and voltages at locations as close to theloads as possible.

But power-factor correction, loss minimization, and voltage regulation are not the only

considerations in reactive power compensation. Switched capacitors cause ringing transients thatcan trip adjustable-speed drives. Once energized, a capacitor bank may interact with theinductive components of the power system to form a tuned circuit that resonates at anunfortunate frequency. This may cause voltage distortions when nonlinear loads inject harmoniccurrents at the tuned frequency.

A second consideration is that of the propagation of capacitor switching transients when moreand larger capacitor banks are present on feeders. There are many cases of capacitor-switchingproblems documented in the literature. Furthermore, there has been speculation that systemdamping will be reduced with overcompensation, allowing ringing transients to propagate greaterdistances and affect a larger number of sensitive loads such as adjustable-speed drives (ASDs).

Objective To identify the potential benefits of using high levels of capacitive shunt compensation in a

distribution system

To investigate detrimental effects that are related to high levels of shunt compensation,particularly those relating to power quality

To demonstrate the capabilities of selected analytical tool that can be used to study theseproblems

To describe the capabilities and limitations of other engineering tools that can be used todesign and analyze distribution compensation

ApproachThe report begins by indicating some of the benefits of capacitive shunt compensation and whyconnecting capacitors to the distribution system can be particularly advantageous. It thendiscusses the potential problems related to increased levels of compensation. A variety ofanalytical methods and software packages are available for analyzing distribution systems withcapacitors. The report gives a brief overview of some of these methods. The report also considersseveral analytical techniques that may be used to select the best locations and sizes fordistribution-system capacitor banks.

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ResultsFor several reasons, the level of capacitive shunt compensation in distribution systems may beincreased significantly in the coming decades. The inductive components of the power systemimpedance may interact with a capacitor bank during normal operation to form a tuned circuit. Ifthe resonant frequency of the tuned circuit coincides with the frequency of the harmonic current

injected into the system, the normally sinusoidal voltage waveforms may be distorted. In mostsituations, third-harmonic currents produced by single-phase loads and transformer magnetizingpose no threat because feeders are not resonant near the third harmonic. However, high levels ofcapacitive compensation may move the resonant point to the third harmonic, giving cause forconcern.

Feeders with distributed capacitor banks have many resonant frequencies, and their frequencyresponses depend on voltage class and load level. In the simulations performed using actual testsystems, it appears that averagevoltage distortion levels on feeders pass through several peaksand valleys as the power factor is corrected from 0.90 to 1.00 and then on to 0.95 leading.

The frequency at which the impedance reaches a minimum may be as important as the resonantpeak. Many devices that produce harmonic currents also have filters that are designed to shunt

the harmonic currents to ground. If the system impedance at characteristic harmonics is low, theharmonic currents may enter the system instead of being shunted to ground by the filters.

Simulations indicate that as the total amount of shunt capacitance increases, the overshoot due toswitching in each additional identical bank is relatively unchanged, but the ringing frequency islowered, and the transient takes longer to decay. Also, more switching operations will probablybe required with increased levels of compensation. Because the voltage transients will be morefrequent and last longer, it is likely that increased levels of capacitive compensation willaggravate the power quality problems related to capacitor switching.

EPRI PerspectiveBy providing utilities with a clear assessment of using more shunt-connected capacitor banks in

the distribution system, EPRI enables utilities to evaluate the feasibility and effectiveness of

increasing the level of load compensation. Increased use of capacitive compensation that isstrategically located may benefit utilities and their customers by increasing the power-transfer

and improving power quality.

KeywordsCapacitorsDistributionPower qualityOvercompensationPlacement

Harmonics

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EPRI Licensed Material

CONTENTS

1INTRODUCTION....................................................................................................................1-1

The Benefits of Adding Shunt Compensation in the Distribution System.......................................1-1

Adding Capacitors to the Distribution System Instead of the Transmission System ........................1-2

Limitations on Shunt Compensation ..........................................................................................1-3

Report Organization.................................................................................................................1-4

2 IMPACT OF OVERCOMPENSATION ON CAPACITOR SWITCHING TRANSIENTS .......2-1

Introduction.............................................................................................................................2-1

Simulation Software and Methodology ......................................................................................2-1

Description of the Test System..................................................................................................2-1

Simulation Results ...................................................................................................................2-4

3IMPACT OF OVERCOMPENSATION ON HARMONIC RESONANCE AND

VOLTAGE DISTORTION .........................................................................................................3-1

Introduction.............................................................................................................................3-1Simulation Software and Methodology ......................................................................................3-2

Description of the Test Cases ....................................................................................................3-6

Simulation Results ...................................................................................................................3-8

Results of Type A Simulations ..................................................................................................3-9

Results of Type B Simulations ................................................................................................3-10

4INCORPORATING PQ SCREENING CRITERIA IN DISTRIBUTION CAPACITOR

PLACEMENT ALGORITHMS ..................................................................................................4-1

Optimization of Capacitor Placement Using Distribution Feeder Analysis Software .......................4-1Overview of Available Applications ..........................................................................................4-5

Example Using Advantica Stoners SynerGEE ...........................................................................4-5

5CONCLUSIONS AND RECOMMENDATIONS......................................................................5-1

The Move to Higher Levels of Shunt Compensation on Distribution Feeders.................................5-1

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Power Quality and Other Problems That May Be Caused by High Levels of Shunt

Compensation .........................................................................................................................5-2

Tools and Techniques for Analyzing Systems With High Levels of Shunt Compensation ...............5-5

6REFERENCES ........................................................................................................................6-1

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LIST OF FIGURES

Figure 2-1 Test System for Studying the Impact of Overcompensation on Capacitor Switching

Transients ............................................................................................................................2-3

Figure 2-2 300kVAr Capacitor Switched at Customer 4 (With 300kVAr On-Line at Each of the

Other Three Customer Busses)...............................................................................................2-6

Figure 2-3 Zoom-In of Figure 2-2 ...............................................................................................2-7

Figure 2-4 300kVAr Capacitor Switched at Customer 1 (With 300kVAr On-Line at Each of the

Other Three Customer Busses)...............................................................................................2-7

Figure 2-5 1200kVAr Capacitor Switched at Customer 4 (With 1200kVAr On-Line at Each of

the Other Three Customer Busses)..........................................................................................2-8

Figure 2-6 1200kVAr Capacitor Switched at Customer 1(With 1200kVAr On-Line at Each of






Figure 3-1 Resonance Brought About by Shunt Capacitors ...............................................................3-1

Figure 3-2 Interface Screen for PCFLO ..........................................................................................3-4

Figure 3-3 Figure Modified Interface Screen for PCFLO..................................................................3-5

Figure 3-4 Diversified Current Injection Current Waveform for Six-Pulse Current-Source ASD ..........3-7

Figure 3-5 Diversified Current Injection Current Waveform for Single-Phase Electronic Load

Through GY-GY Transformers ..............................................................................................3-7

Figure 3-6 Diversified Current Injection Current Waveform for Single-Phase Electronic

Load Through D-GY Transformers ........................................................................................3-8

Figure 3-7 Type A Simulations Showing Variation of THDV With Corrected Load Power Factorfor the SKI, CHL, and OIL Systems .......................................................................................3-9

Figure 3-8 Type A Simulations Showing Variation of Fundamental Voltage Magnitude With

Corrected Load Power Factor for the SKI, CHL, and OIL Systems ..........................................3-10

Figure 3-9 Type B Simulations Showing Variation of THDV With Corrected Load Power Factor

for the SKI System..............................................................................................................3-11Figure 3-10 Type B Simulations Showing Variation of THDV With Corrected Load Power Factor

for the WAT System ...........................................................................................................3-11


for the FND System ............................................................................................................3-12


for the HAY System............................................................................................................3-12

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LIST OF TABLES

Table 2-1 Corrected Power Factors Versus kVAr for the Test System................................................2-5

Table 2-2 Comparison of Feeder KV-Milliseconds Above Normal for the Capacitor Switching

Cases...................................................................................................................................2-6

Table 3-1 Seven Test Cases for Harmonics .....................................................................................3-6

Table 4-1 Capabilities of Various Capacitor-Placement Methods ......................................................4-4

Table 4-2 Distribution Feeder Analysis Software Reviewed for This Report ......................................4-5

Table 4-3 Results of Capacitor-Placement Analysis Showing Sizes of Recommended Capacitor

Banks ..................................................................................................................................4-7Table 4-4 Results of Capacitor Placement Analysis Showing Recommended Locations ......................4-8

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1INTRODUCTION

The Benefits of Adding Shunt Compensation in the Distribution System

Before deregulation of the power industry, utilities could build new transmission lines with thenear certainty that they would be compensated for their efforts. The utility rates were regulated toguarantee a reasonable rate of return on any investment that was deemed to be prudent. Thosedays are gone. The rules governing electric utilities are still evolving. At this point it is still notclear how, or if, a builder of new transmission will be compensated for its investment.

Also, it is becoming more and more difficult to build new transmission and distribution lines.This is true because the parties that may be adversely impacted by new lines are increasinglywell organized and financed. This is not just the NIMBY (not in my back yard) folks who havehistorically opposed new transmission lines. Transmission bottlenecks often benefit someindependent power producers because they face less competition from generators on the otherside of the bottleneck. They thus have a strong financial incentive to see that new transmissionthat might relieve the bottleneck does not get built. As a result, very little new transmission isnow being built.

Because very little new transmission is likely to be built in the near term, there is an increasingneed to maximize the amount of power that can be transferred over the existing circuits. Power-

transfer capability can be limited by low steady-state voltage, thermal overloads, rotor-angleinstability, or voltage instability. Shunt capacitors can be used in moderation to address each ofthese problems and thus to increase the transfer capability of the bulk power system as well asthe distribution system. In most cases, the most advantageous location for the shunt capacitors isclose to the load (in the distribution system).

Power transfer may be limited because voltage would be too low with additional power transfer.Shunt capacitors can be used to increase the voltage in the bulk power system as well as in thedistribution system. The series impedance of transmission lines is mostly inductive, theimpedance of distribution lines also includes a significant inductive component, and the leakageimpedance of transformers is mostly inductive. The voltage drop in an inductive circuit is due

mostly to the reactive current flow. Shunt capacitors in strategic locations can decrease thereactive power flow and thus raise the voltage. The best place for the capacitors is usually nearthe reactive load because the capacitor will then reduce the reactive power flow all the way to thegenerator from the load.

Line and transformer losses raise the temperature of these devices and waste costly energy.Thermal overloading of transmission or distribution circuits can damage equipment, so the

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Introduction

power transfer capability must be limited. Electrical, I2R losses are proportional to the square of

the current flow through the equipment. The sinusoidal current has two components:

1. A component that is in phase with the sinusoidal voltage; this is the component that doesuseful work.

2. A component that is 90 degrees out of phase with the sinusoidal voltage; this componentdoes no useful work but still contributes to electrical losses.

A shunt capacitor can be used to supply the second component of current locally, so it does nothave to flow through the distribution and transmission systems and add to the losses. Up to apoint, adding shunt capacitors will decrease the reactive current flow in the network; more in-phase current can then flow without overloading the circuits. Because useable power is equal tothe product of the in-phase components of current and voltage, increasing the in-phasecomponent of current will increase the amount of useable power that can be delivered. But thereis a bonus; because the voltage drop is less with no reactive current flow, the voltage will also behigher at the load. Both the voltage and the usable current can be higher, so the increase in

power-delivery capacity is multiplied.

The rotor-angle stability of the bulk power system is marginally better if shunt capacitors areadded to compensate the reactive power load in a power-importing area. Adding capacitors closeto the generation in power-exporting areas may, however, actually decrease the system stability.

Adding Capacitors to the Distribution System Instead of the TransmissionSystem

As indicated in the previous section, shunt capacitors can be used to increase the transfer

capability, improve the voltage profile, and reduce losses. If they are added in the distributionsystem, they can often provide these benefits both for the distribution system and for thetransmission network. Capacitors connected at the transmission level will not significantlyreduce the voltage drop and losses in the distribution system. In addition to compensating thereactive power requirements of the load, it may be desirable to also apply capacitors tocompensate for some of the reactive power losses in the power system.

It is usually less expensive per kvar to install capacitors at the distribution level than at thetransmission level because the voltage rating does not need to be as high. It is also often lessexpensive to connect shunt capacitors to a 13.8-kV tertiary winding of an autotransformer in thebulk power system. However, when communications have to be provided to monitor and switcheach bank, a large number of small units connected to the distribution system may sometimes be

more expensive than a large bank connected to the transmission system.

There are several problems associated with shunt capacitors that can occur during outages orafter certain switching operations:

Very high voltage may result if due to a switching operation, a large capacitor bank remainsconnected at the open end of a line.

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Introduction

For certain switching operations, capacitors may also form a ferroresonant circuit with themagnetizing reactance of a transformer. This can result in very high damaging voltagespikes.

A small generator that remains connected to a relatively large capacitor bank will self-exciteif the pair disconnect from the network and load.

Such problems are almost never severe if a significant amount of load remains connected nearthe capacitors. They are thus much less likely to occur if the shunt capacitors are located in thedistribution system, close to the load.

For all of the reasons listed above, there is a drive to add more shunt capacitors at the distributionlevel. Most utilities do not presently install enough shunt capacitors in their distribution systemsto compensate for the entire reactive power load. Compensation to unity power factor is,however, used in some cases. To help support the transmission system, it may be desirable toeven overcompensate the reactive power load (that is, to connect more capacitors to adistribution circuit than are required to cancel the reactive load). The extra compensation can be

used to make up for the reactive power losses in the network. Some utilities are designed so theoperators can switch distribution capacitors on during transmission system emergencies eventhough these capacitors would normally be off without the emergency.

Limitations on Shunt Compensation

It is, of course, possible to have too much of a good thing. The voltage magnitude may get toohigh if too many shunt capacitors are connected at a particular location or region. The magnitudeof the reactive current flow, and losses, will increase again if the reactive current supplied by theshunt capacitors exceeds the amount of reactive current required by the load. Capacitor banks areincremental in size, whereas the load varies more or less continuously. It is therefore usually not

possible to precisely balance the reactive load and the shunt capacitance at every location. Somereactive power flow and voltage variation across the system therefore needs to be tolerated. Theneed to have a reasonable bank size and the need to have reasonable voltages and lossestherefore need to be balanced.

When the load is heavy, more shunt capacitors than can be tolerated during light load may beneeded. Shunt capacitors may therefore be switched to remove the surplus during light loadperiods. The switching operation itself can produce additional problems if the system is notcarefully designed. Ringing voltage transients may result if the switching is not done on the zerocrossings of the fundamental voltage waveform. These may cause problems such as the trippingof adjustable-speed drives. If too much capacitance is switched at one time, the jump in the

steady-state voltage magnitude before and after the switching operation may also be excessive. Ifback-to-back capacitor banks are switched, the current inrush from one bank to another may beexcessive. The current inrush for nearby system faults may also be excessive.

The inductive components of the power system impedance may also interact with a capacitorbank during normal operation to form a tuned circuit with a resonance or anti-resonance at anunfortunate frequency. Magnetic saturation, rectification, and other nonlinear processes generateharmonic currents at characteristic frequencies, which are a multiple of the fundamental

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Introduction

frequency, 60 Hz. If the resonant frequency of the tuned circuit coincides with the frequency ofthe harmonic current injected into the system, the normally sinusoidal voltage waveforms may bedistorted. The harmonic current may also be rerouted or magnified so that the current flowing atsome network locations exceeds that injected by the nonlinear device.

Common power quality problems caused by harmonic interactions include harmonic currentflows, stray neutral-earth voltage, and capacitor-switching transients. The switching transientsmay cause equipment such as adjustable-speed drives to malfunction or trip. The neutral-to-earthvoltage can result in electric shocks. Zero-sequence harmonic current flows are particularlyobjectionable because they flow in the neutral circuit and may be easily coupled magnetically toother electrical circuits. Harmonic current flows:

Can cause telephone interference.

Can cause electronic equipment to malfunction.

Can increase system losses and thermal overloads.

Can cause earth-fault relays to malfunction.

Other common power quality problems related to shunt capacitor banks include overvoltage andflicker.

All of these potential problems need to be considered when designing shunt compensation. Arecent utility survey [1] indicated that the most common problem with distribution capacitors isthat a fuse blows when the bank is not faulted. The report indicates that the out-rush current, dueto nearby faults, may be a major cause. Another common problem is that the controller does notswitch the capacitors on or off when it should. Other problems that were noted with moderatefrequency include steady-state overvoltage, harmonic interference, and switching impacts oncustomers.

Report Organization

Some of the ramifications of connecting more capacitive shunt compensation to distributionfeeders are considered in this report. The effects of overcompensating the distribution feeder loadare of particular interest. A variety of analytical methods and software packages are available foranalyzing distribution systems with capacitors. The report gives a brief overview of some ofthese methods and provides examples that show how selected methods can be applied. Thevoltage transient that can result when a capacitor bank is connected to the system is analyzed inSection 2. The results of EMTP simulations modeling several capacitor-switching events areshown and discussed in that section. Section 3 considers harmonic resonance with distribution-system capacitors. Simulation results showing the resonant frequency shift with various levels ofcapacitive compensation are shown and discussed in that section. Section 4 considers some ofthe techniques that are available to select the best locations and sizes for distribution-systemcapacitor banks. These techniques typically consider the capacitor costs and how the capacitoraffects the system losses and voltage profile. Section 4 also discusses how harmonic effectsmight be factored into the analysis. Section 5 summarizes the results and conclusions.

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2IMPACT OF OVERCOMPENSATION ON CAPACITOR

SWITCHING TRANSIENTS

Introduction

When an uncharged capacitor is suddenly switched on, it momentarily pulls the feeder voltage tozero at its location. Almost immediately, a ringing voltage transient is induced whose first peakis approximately twice the peak voltage on the feeder. The ringing transient is the result ofenergy oscillation between the capacitor and the net inductance of the system. Since maximum

stored capacitor energy is proportional to capacitor size, large capacitors tend to produce strongtransients that propagate further.

There are many documented cases where ASDs have been tripped off line due to suddenincreases in their DC bus voltages caused by capacitor switching transient overvoltages. Sinceovercompensation will require more capacitors, and probably larger capacitors, there is a concernthat overcompensation will reduce the quality of power provided to ASDs and other sensitiveloads.

Simulation Software and Methodology

EMTP-ATP [2], ATPDRAW [3], and TOP [4] are chosen to perform the analysis. These threepowerful programs are widely used and are available royalty-free.

Unlike the harmonics cases, where actual systems were analyzed, the strategy for studying thecapacitor switching problem is to construct a fictitious feeder using actual data, vary switchedcapacitor sizes and locations, and make general observations based upon the simulation results.

Description of the Test System

A one-line diagram of the test system is shown in Figure 2-1. The impedance of the transmission

system is assumed to be small enough to be ignored for our study purposes. The feeder has a36-MVA substation transformer, with Z = 4% on the transformer base and an X/R ratio of 10.Converted to a system base of 100 MVA, 13.8 kV, the transformer impedance becomes

pujZtrans 11.001.0 +=+

Eq. 2-1

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Impact of Overcompensation on Capacitor Switching Transients

Using the system base ohms of = 9044.1100

8.13 2, the transformer impedance can then be

represented in ATP as

mHLRtranstrans

56.0,02.0 == Eq. 2-2

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Impact of Overcompensatio

7967Vrms

Sub&Tran Feeder

Cust 1

Feeder

Cust 2

Feeder

Cust 3

Feeder

Cust 4

300kVAr 300kVAr 300kVAr 300kVAr

Voltage and current probes to

confirm base case conditions Tim

swit

Run for 0.1s, using integration step size of 1s

7967Vrms

Sub&Tran Feeder

Cust 1

Feeder

Cust 2

Feeder

Cust 3

Feeder

Cust 4

300kVAr 300kVAr 300kVAr 300kVAr

Voltage and current probes to

confirm base case conditions Tim

swit

Run for 0.1s, using integration step size of 1s

Figure 2-1Test System for Studying the Impact of Overcompensation on Capacitor Switching Transients

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The symmetrical fault current at the load point is known to be 9400 A, which (assuming an X/Rratio of 3) yields an impedance at the load point of

pujZloadpt

42.014.0 +=+ Eq. 2-3

Thus, the impedance of the feeder segment between the transformer and the load point is thedifference,

pujjjZfeeder

31.013.0)11.001.0()42.014.0( +=++=+ Eq. 2-4

Converting to basic units for ATP use yields

mHLR feederfeeder 57.1,25.0 == Eq. 2-5

The customer load is P = 2.5 MW @ pf = 0.90, thus having reactive power load Q = 1.2 MVAr.To model the load P in ATP as a shunt resistor (per phase), the load P is converted to

= 76loadR Eq. 2-6

The reactive power component of the load model is ignored because studies have shown it tohave little effect in ATP as far as capacitor switching transients are concerned.

As a starting point, 300 kVAr of power factor correction capacitance is chosen for each of thefour load busses. Converting to basic units, this becomes 4.18 F per phase in a grounded-wyeconfiguration.

Finally, to approximate an actual feeder, three identical feeder segments and customers are thenrepresented to the right of the first customer, spaced at equal distances apart.

Simulation Results

The simulations are performed with 300 kVAr, 1200 kVAr,and 1800 kVAr at each load bus. Thecorresponding corrected power factors are shown in Table 2-1.

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Table 2-1Corrected Power Factors Versus kVAr for the Test System

Shunt kVAr(at Each Load Bus)

Net pf

0 0.90 (lagging)

300 0.94 (lagging)

1200 1.00

1800 -0.97 (leading)

The simulations begin by switching on the 300-kVAr capacitor at the Customer 4 bus (i.e., Cust4), while the 300-kVAr capacitors at each of the other load busses are already on-line. Theresults are shown in Figure 2-2, with a zoom-in shown in Figure 2-3.

Next, the 300-kVAr switching event is repeated, but this time at the Customer 1 bus instead ofCustomer 4 bus. The results are shown in Figure 2-4. Comparing Figure 2-4 with 2-3 shows thatswitching the Customer 1 capacitor yields a higher overshoot voltage and a higher ringingfrequency (because for frequency, the system inductance is lower at Customer 1).

The switching experiment is repeated in Figures 2-5, 2-6, and 2-7, 2-8 for 1200 kVAr and 1800kVAr, respectively. It is obvious in these figures that as shunt capacitance increases, theovershoot is relatively unchanged, the ringing frequency is lowered, and the transient takeslonger to decay.

In addition to comparing peak voltage overshoot and frequency, a meaningful way to comparethe significance of these switching transients is to numerically determine the time integral ofvoltage above normal sinusoidal voltage and then add the integrals for all four load busses plusthe substation 13.8-kV bus. This sum gives a relative basis for comparing the additional energythat might be expected to flow into a capacitive-filtered, rectified load during the disturbance.Performing the calculation yields Table 2-2.

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Table 2-2Comparison of Feeder KV-Milliseconds Above Normal for the Capacitor Switching Cases

Switching EventkV-Milliseconds Above

Normal

300 kVAr at Customer 4 3.0






As seen in Table 2-2, there is a strong correlation between the amount of kVAr switched and the

subsequent kV-milliseconds.

-15000

-10000

-5000

0

5000

10000

15000

0 20 40 60 80 100

Q_COMP>XX0003(Type 4)

V

o

lta

ge

(V

)

Time (ms)

Q_COMP>XX0003(Type 4) Q_COMP>XX0005(Type 4) Q_COMP>XX0009(Type 4)

Q_COMP>XX0013(Type 4) Q_COMP>XX0017(Type 4)

Figure 2-2

300kVAr Capacitor Switched at Customer 4 (With 300kVAr On-Line at Each of the OtherThree Customer Busses)

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0

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Customer 4

Figure 2-3Zoom-In of Figure 2-2

0

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49 50 51 52 53 54

NONAME>XX0003(Type 4)

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NONAME>XX0003(Type 4) NONAME>XX0005(Type 4) NONAME>XX0009(Type 4)

NONAME>XX0013(Type 4) NONAME>XX0017(Type 4)

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Customer 1

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Figure 2-4300kVAr Capacitor Switched at Customer 1 (With 300kVAr On-Line at Each of the OtherThree Customer Busses)

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o

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Figure 2-51200kVAr Capacitor Switched at Customer 4 (With 1200kVAr On-Line at Each of theOther Three Customer Busses)

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Figure 2-61200kVAr Capacitor Switched at Customer 1(With 1200kVAr On-Line at Each of theOther Three Customer Busses)

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3IMPACT OF OVERCOMPENSATION ON HARMONIC

RESONANCE AND VOLTAGE DISTORTION

Introduction

Resonance is produced by series and parallel combinations of inductors and capacitors. Fordistribution feeders with several shunt capacitor banks, many series and parallel resonancesoccur. However, for illustration purposes, it is helpful to consider the simplest practical situation,as shown in Figure 3-1.

Watch out for 3rd

harmonic resonance as

more caps are added.

Every transformer is a

source of 3rd harmonic,

especially as the

fundamental voltagerises. Single phase

electronic loads may

also become an issue.

Is 0.95 PF Correction coming?

Figure 3-1Resonance Brought About by Shunt Capacitors

In Figure 3-1,a dedicated feeder serves one large customer, and the reactive power load of thecustomer is corrected by shunt capacitor Q. The graph shows the frequency scan at thecustomer bus, which is a plot of impedance magnitude seen at that bus as frequency is varied.When Q is zero (i.e., no pf correction), then the impedance magnitude increases more-or-lesslinearly with frequency because feeder inductance dominates. As shunt capacitance (i.e., Q) isadded, a resonant peak develops and moves toward the left because of

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Impact of Overcompensation On Harmonic Resonance and Voltage Distortion

With the first modification, the user has the ability to replace the actual nonlinear loads in anexisting test case with globally distributed nonlinear loads, distributed in proportion to bus load.This feature permits more generalized conclusions than do studies made with present-daynonlinear load placement.

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Figure 3-2Interface Screen for PCFLO

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Impact of Overcompensation On Harmon

PCFLO modifications for Q compen

Figure 3-3Figure Modified Interface Screen for PCFLO

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For this study, the following three types of injection currents are employed:

1. Six-pulse current-source ASDs to represent the industrial sector

2. Single-phase electronic loads, connected through grounded-wye to grounded-wye (GY-GY)

transformers, to represent residential PCs, televisions, etc

3. Same as item 2 but connected through delta-to -grounded-wye (D-GY) transformers torepresent the commercial sector.

The corresponding PCFLO current injection waveforms are shown in Figures 3-4, 3-5, and 3-6,respectively. The single-phase, GY-GY electronic load current has a strong 3

rdharmonic

component, but the other two currents have no 3rdharmonic because of their inherently delta

connections.

Description of the Test Cases

Seven test cases are analyzed in this report. Each case was previously studied to resolve orpredict the likelihood of a specific harmonics-related or power quality problem. A summary ofthe cases is given in Table 1-1. All have multiple feeders.

Table 3-1Seven Test Cases for Harmonics

Case DescriptionNo. of

BussesSystem Power

(MW)

SKI 12.5-kV and 25-kV predominantly undergroundsystem with ski areas interconnected by overheadsub-transmission. Total 15MW of ASDs.

454 64

CHL 25-kV, lightly loaded underground system with 5000-HP ASD chiller motor. *

33 10

OIL 12.5kV overhead system with 2000-HP ASD oilpipeline pump. *

18 10

WAT 35-kV overhead system with water pumping station 49 9

FND 12.5-kV overhead system with large foundry arcfurnace load

162 8

HAY 12.5-kV overhead rural system with large hay cubingmachine

84 1

TV 12.5-kV and 25-kV overhead system with TVtransmitter

111 18

* Because zero-sequence data were not available, 3rdharmonic studies were not performed on the CHL and OIL

cases

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The first three cases (SKI, CHL, and OIL) were extensively studied and well documented in thepast, and field measurements confirmed the simulation results.

Figure 3-4Diversified Current Injection Current Waveform for Six-Pulse Current-Source ASD

Figure 3-5Diversified Current Injection Current Waveform for Single-Phase Electronic Load ThroughGY-GY Transformers

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Existing power factor correction capacitors are removed.

Power factor correction is globally made at each bus.

Transformer magnetizing current harmonics are ignored in all Type A and Type B simulations.

Results of Type A Simulations

Type A simulations were performed on the three highest-confidence cases where fieldmeasurements confirmed, or partially confirmed, the original base case condition. The firstgraph, Figure 3-7, shows the variation of average total harmonic voltage distortion (THDV)versus power factor correction level. Only the lightly loaded CHL system shows high sensitivityto compensation level.

In addition to THDV, the highest offending harmonic in the system is shown. The general trendis for the highest offending harmonic to move lower as shunt capacitors are added.

The second graph, Figure 3-8, shows the sensitivity of average fundamental voltage magnitudeversus power factor correction level. The heavily loaded SKI system, which has a highconcentration of ASDs, shows double the sensitivity of fundamental voltage when compared tothe other two cases.

0

5

10

15

20

25

SKI

CHL

OIL

pf = 0.90 pf = 1.00 pf = 0.95

7 13 5 5

5

7 5

Strong resonant peak at light load condition

Harmonicwith largest

voltage

identified

0

5

10

15

20

25

SKI

CHL

OIL

pf = 0.90 pf = 1.00 pf = 0.95pf = 0.90 pf = 1.00 pf = 0.95

7 13 5 5

5

7 5

Strong resonant peak at light load condition

Harmonicwith largest

voltage

identified

55

Figure 3-7Type A Simulations Showing Variation of THDV With Corrected Load Power Factor for theSKI, CHL, and OIL Systems

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90

95

100

105

110

SKI

CHL

OIL

Overcompensation can lead to undesirably high voltages

pf = 0.90 pf = 1.00 pf = 0.95

90

95

100

105

110

SKI

CHL

OIL

Overcompensation can lead to undesirably high voltages

pf = 0.90 pf = 1.00 pf = 0.95pf = 0.90 pf = 1.00 pf = 0.95

Figure 3-8Type A Simulations Showing Variation of Fundamental Voltage Magnitude With CorrectedLoad Power Factor for the SKI, CHL, and OIL Systems

Results of Type B Simulations

As explained previously, Type B simulations are made under the assumption that 20% of theconnected load at each bus is nonlinear. The two situations are either 1) 20% ASDs or 2) 10%single-phase GY-GY plus 10% single-phase D-GY. The ASD cases have no 3

rdharmonic or its

odd multiples (i.e., 9, 15), but the single-phase cases include these so-called triplen harmonics.

The simulation results are shown in Figures 3-9 through 3-13. Specific comments follow:

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0

1

2

3

4

5

6

7

8

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ASD

1P

35

pf = 0.90 pf = 1.00 pf = 0.95

0

1

2

3

4

5

6

7

8

9

10

ASD

1P

35

pf = 0.90 pf = 1.00 pf = 0.95pf = 0.90 pf = 1.00 pf = 0.95

Problem

switches to

3rd harmonic

Problem

switches to

3rd harmonic

Figure 3-9Type B Simulations Showing Variation of THDV With Corrected Load Power Factor for theSKI System

Figure 3-9 (SKI).100% power factor compensation will cause intolerable resonance for the 3rd

harmonic. The voltage distortion will rise as feeder linear load decreases.

0

1

2

3

4

5

6

7

8

9

10

11

12

ASD

1P

15

9

3

13 11 7 5

Resonant frequency shifts to lower harmonics as more capacitors are added

pf = 0.90 pf = 1.00 pf = 0.95

0

1

2

3

4

5

6

7

8

9

10

11

12

ASD

1P

15

9

3

13 11 7 5

Resonant frequency shifts to lower harmonics as more capacitors are added

pf = 0.90 pf = 1.00 pf = 0.95pf = 0.90 pf = 1.00 pf = 0.95

Figure 3-10Type B Simulations Showing Variation of THDV With Corrected Load Power Factor for theWAT System

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Figure 3-12 (HAY).Same conclusion as Figure 3-11.

0

5

10

15

20

25

30

35

40

ASD

1P

5

3

pf = 0.90 pf = 1.00 pf = 0.95

0

5

10

15

20

25

30

35

40

ASD

1P

5

3

pf = 0.90 pf = 1.00 pf = 0.95pf = 0.90 pf = 1.00 pf = 0.95

Figure 3-13Type B Simulations Showing Variation of THDV With Corrected Load Power Factor for theTV System

Figure 3-13 (TV).This system demonstrates severe sensitivity to the 3rdharmonic. The reason

for this unusual sensitivity has not been identified. However, the motivation for the originalstudy was a 3

rdharmonic problem that manifested itself at a television broadcast station whose

constant-voltage transformer resonated when the system was upgraded to 25 kV and the station12.5-kV D-GY transformer was replaced with a 25-kV GY-GY transformer. The transformer

resonance caused the broadcast picture to fluctuate in an annoying manner. The problem wasresolved by quickly replacing the D-GY transformer with a GY-GY transformer, thus isolatingthe transformer from the 3

rdharmonic equivalent circuit of the feeder.

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90

95

100

105

110

WAT

FND

HAY

TV

pf = 0.90 pf = 1.00 pf = 0.95

90

95

100

105

110

WAT

FND

HAY

TV

90

95

100

105

110

WAT

FND

HAY

TV

pf = 0.90 pf = 1.00 pf = 0.95pf = 0.90 pf = 1.00 pf = 0.95

Figure 3-14Type B Simulations Showing Variation of Fundamental Voltage Magnitudes With CorrectedLoad Power Factor for the WAT, FND, HAY, TV Systems

Figure 3-14.The fundamental voltage magnitude of the TV system is unusually sensitive topower factor correction level.

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4INCORPORATING PQ SCREENING CRITERIA IN

DISTRIBUTION CAPACITOR PLACEMENTALGORITHMS

Optimization of Capacitor Placement Using Distribution Feeder AnalysisSoftware

There are many analytical techniques and computer programs that are designed specifically to

optimize the placement of capacitor banks on distribution feeders. A general-purpose program,such as an optimal power flow (OPF) or a security constrained power flow, in the hands of anexperienced user, can also be used for this purpose. These applications may consider the voltageprofile on the feeder, the change in voltage due to switching, reduction of feeder losses, thedesired power factor range, available capacitor bank sizes, the available locations, andeconomics to determine the optimal capacitor bank sizes and locations. Each method of analysishas its advantages and disadvantages.

Numerous approaches and algorithms have been proposed for selecting distribution capacitorlocations. Forty-one references describing various approaches are listed in reference [6].Reference 5 separates the approaches into four different categories:

1. Analytical methods

2. Numerical programming methods

3. Heuristic methods

4. Artificial intelligence techniques

The so-called analytical methods are simplified, back-of-the-envelope approaches that do notrequire the heavy number-crunching capability of a computer. They may be particularly usefulfor sanity-checking the results of more sophisticated techniques. The 2/3 rule for minimizing

feeder losses is an example [1]. It states that for uniformly distributed feeder load, the optimallocation for a single capacitor bank is two-thirds of the way down the feeder, and the optimalkVA rating is two-thirds of the total feeders reactive power loading. Similar rules are availablefor connecting more than one capacitor and for other regular load distributions along the feeder(such as continuously increasing the load along the feeder).

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Incorporating PQ Screening Criteria in Distribution Capacitor Placement Algorithms

0

2

0

2reactive line

flow

Mvar

Uniform load2/3s rule for placing one capacitor

Substation Feeder end

0

2

0

2reactive line

flow

Mvar

Without capacitors

2-Mvar bank

Figure 4-1Capacitor Location and Rating for Minimum Losses on a Uniformly Loaded Feeder

Note that if a feeder with a uniformly distributed load is fully, 100%, compensated using onlyone capacitor bank, the losses and voltage profile will be significantly worse than they are with2/3 compensation. However, a higher level of compensation can be effectively applied if severalsmall capacitor banks are applied. If the uniformly distributed load could be compensated by auniformly distributed capacitance, the optimal level of compensation would be 100%. This is notpossible, of course, because real capacitor banks come in incremental sizes, but then again, sodoes real load. There is a tradeoff between the increased costs per kvar of smaller capacitorbanks and the reduced losses and better voltage profile that the smaller banks can provide.

Smaller banks will often be needed to effectively apply higher levels of shunt compensation.

Some of the simplifying assumptions that are used by the analytical methods to make theproblem manageable can be eliminated if a computer is used to do repetitive work, such assolving large systems of equations or iterative procedures. Numerical programming methodssuch as OPF and the security constrained power flow can be formulated to address the capacitor-placement problem. These techniques essentially find a capacitor-placement solution thatminimizes cost while obeying voltage, power flow, and/or other specified constraints. Theminimized cost may include the costs of losses (both energy and capacity costs may be factoredin), the costs of the capacitors themselves, as well as penalties for unacceptable voltages orundesirable capacitor-placement patterns such as locating capacitors in too many places.

Reference [6] defines heuristic methods as being rules of thumb that are developed throughintuition, experience, and judgment. The artificial intelligence techniques used for capacitorplacement include genetic algorithms, expert systems, artificial neural networks, fuzzy settheory, and simulated annealing.

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Common assumptions implicit in various methodologies that may or may not be appropriatedepending upon the application include:

The feeder has no branches, or the feeder is radial.

The load is uniformly distributed along the feeder.

Capacitor banks are not switched.

Capacitors can be added anywhere on the feeder.

Capacitor banks can be any size.

The amount of load is constant.

No credit is given for the capacity savings made possible by loss reduction.

The annual losses can be estimated based upon the losses at one or two levels of loading.

The tradeoff is often between ease of application and detail of representation. Simplifying

assumptions need to be made for every type of analysis. The key question may be howreasonable are the assumptions for the questions being considered.

Multiple applications of a simple approach can sometimes be used instead of one application ofan all-encompassing approach. For instance, with some approaches the feeder may berepresented with a heavy system load for one calculation and represented again at light load foranother calculation. A manually driven iterative procedure may be necessary using suchindividual calculations to arrive at a solution that will work for all levels of loading. On the otherhand, an all-encompassing approach or a computer-driven iterative procedure maysimultaneously consider the impact of capacitor additions at more than one level of loading.

The appropriate analytical tool for a particular problem will often depend upon: The type of information that is readily available

The presumed accuracy of the available information

The required accuracy for the analysis

The purpose of the analysis (for example, planning or operations)

The tool(s) that the engineer doing the analysis has access to and is most familiar with

Table 4-1 taken from [6] indicates the capabilities of various methodologies. The techniques aredescribed in detail in the references given in that paper.

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be switched at various locations without producing unacceptable jumps in voltage. A switchingsurge program or EMTP type of analysis may also be used to help define the maximum bank sizethat can be used at various locations. This information may then be used to limit the options forcapacitor placement until feeder locations and bank sizes are found that are in all waysacceptable. A future generation of programs may automatically consider these other factors whenchoosing capacitor locations and sizes.

Overview of Available Applications

There are many software applications available that provide the tools required by a distributionengineer for proper planning and design of distribution facilities. While the capabilities of theseapplications vary from manufacturer to manufacturer, all of those reviewed for this discussioninclude the following: load flow, short-circuit analysis, motor-start analysis, and capacitorplacement.

Table 4-2Distribution Feeder Analysis Software Reviewed for This Report

Vendor Application

ABB FEEDERALL

Advantica Stoner SynerGEE

Cooper Power Systems V-CAP

CYME International CYMDIST

Milsoft Utility Solutions WindMil

The capacitor-placement applications in these packages can optimize the use of capacitor banksbased on feeder loss reduction, voltage-profile improvement, power-factor correction, andeconomics. They usually operate based on a set of user-defined limits for voltage, leading andlagging power factor, and total number of capacitor banks allowed on the feeder.

These applications contain some form of harmonic-analysis capability, either in the base productor in an add-on module. The capability of this module is usually limited to a frequency scan ofthe circuit impedance at a particular point on the feeder. The harmonic analysis is not included inthe capacitor-placement optimization routine. Therefore, it is possible for the software torecommend capacitor locations that would cause an undesirable harmonic resonance.

Example Using Advantica Stoners SynerGEE

A distribution feeder (Figure 4-2) was modeled in SynerGEE to illustrate how a capacitor-optimization application works and to show the possible harmonic ramifications of addingcapacitors. The feeder originally had no capacitor banks installed.

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C

A

B

Figure 4-2Example Distribution Feeder With Capacitor Locations Shown (A=900 kvar, B=1200 kvar,C=1200 kvar)

The first step was to perform a harmonic analysis on a section of the feeder. The first sectionoutside of the distribution station (upper right-hand corner of Figure 4-2) was selected for thisanalysis. The positive-sequence, driving-point impedance measured at this point before adding

any capacitors is shown in Figure 4-3. The frequency response of the driving-point impedance islinear with no resonant points.

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Figure 4-3Results of Harmonic Analysis for Feeder With No Capacitors

The next step was to run the capacitor-placement application on the same feeder. The applicationpreferences were set to allow no more than a 0.90 leading power factor, place no more than eightcapacitor banks, and to optimize feeder losses. The results are shown in Table 4-3 and Table 4-4.

Table 4-3Results of Capacitor-Placement Analysis Showing Sizes of Recommended Capacitor

Banks

Capacitor Size (kvar) Number Placed

1200 2

900 1

600 None

450 None

300 None

50 None

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Table 4-4Results of Capacitor Placement Analysis Showing Recommended Locations

Section Total Added kvar

Name Nominal Actual

Feeder New Liberty

00159 900 956

00296 1200 1275

00233 1200 1270

The analysis recommends the installation of three capacitor banks on this feeder: 900 kvar onsection 159 (location A), 1200 kvar on section 233 (location B), and 1200 kvar on section 296(location C) in Figure 4-2.

The next step was to add the capacitors to the model, one at a time, and to perform a harmonicanalysis each time. First, the 900-kvar capacitor was added at location A in Figure 4-2. Thedriving-point impedance measured at the first feeder section outside the distribution substation isshown in Figure 4-4. There are now two resonant points: one at 7.5

thharmonic and one at the

11.75thharmonic. Any loads that create harmonic currents at or near these frequencies could

cause objectionable harmonic voltages in this section.

The frequency at which the impedance reaches a minimum may in some cases be equallyimportant. Many devices that produce harmonic currents also have filters. These filters have alow impedance to ground for critical harmonic currents. They are designed to ground theseharmonic currents locally so that they do not propagate through the system. However, if thesystem impedance is very low at characteristic harmonics, the harmonic currents will enter thesystem instead of being grounded. These harmonic currents may create problem such asmagnetically coupling to telephone circuits to produce interference.

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Figure 4-4Harmonic Analysis for Feeder With 900-kvar Capacitor at Location A

Next, a 1200-kvar capacitor was added to the model at location B in Figure 4-2. The harmonicanalysis was repeated for the first section of the feeder. These results are shown in Figure 4-5. Asyou can see, the most prominent resonant point has moved to a lower frequency near the 8

th

harmonic.

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Figure 4-5Harmonic Analysis for Feeder With 900-kvar Capacitor at Location A and a 1200-kvarCapacitor at Location B

Finally, the 1200 kvar was added to the model at location C in Figure 4-2. The harmonic analysisfor the first feeder section outside the substation was repeated. The results are in Figure 4-6. Thedominant resonant point has moved to a lower frequency, the 6.5

thharmonic.

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Figure 4-6Harmonic Analysis for Feeder With 900-kvar Capacitor at Location A, a 1200-kvarCapacitor at Location B, and a 1200-kvar at Location C

The following plots (Figure 4-7 to Figure 4-9) show the driving-point impedance at eachcapacitor location, with all capacitors in service.

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Figure 4-7Harmonic Analysis for Location A With All Recommended Capacitors in Service

Figure 4-8Harmonic Analysis for Location B With All Recommended Capacitors in Service

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Figure 4-9Harmonic Analysis for Location C With All Recommended Capacitors in Service

Note that some resonant peaks are higher at each capacitor location than at the head of thefeeder, but different peaks are magnified at each capacitor location.

For grounded capacitor banks, a similar analysis could be done to find the resonant points for the

zero-sequence driving-point impedance. The resonant points for the zero sequence will not ingeneral be at the same frequencies as they are for the positive sequence because the networkimpedances are different for the positive and zero sequence. The triplen harmonic currents (3

rd,

6th, 9

th, and so on) injected by three-phase devices are predominantly zero-sequence. Some of the

undesirable effects of harmonics are also related mostly to the zero-sequence flows (such ascoupling to telephone circuits or stray neutral voltages). Zero-sequence resonance and anti-resonance may therefore be particularly important.

Possible solutions to a harmonic resonance problem include:

Moving the capacitor bank to a different location on the distribution feeder This works bestif the feeder is connected to a strong, low-impedance transmission system. If the bulk powersystem impedance is larger than the distribution feeder impedance, then moving the capacitorlocation on the feeder will have little effect.

Using a different size capacitor bank.

Using an ungrounded capacitor bank if the problem is caused by zero-sequence harmonics.

Putting a small reactor in series with the capacitor bank so that it acts as a filter or moves theresonant point.

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It is sometimes cheaper to pay the telephone company to alleviate a telephone interferenceproblem from their side.

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5CONCLUSIONS AND RECOMMENDATIONS

The Move to Higher Levels of Shunt Compensation on Distribution Feeders

It is often nearly impossible to install a new utility transmission or distribution circuit. There istherefore an increasing need to maximize the amount of power that can be transferred over theexisting circuits. Power-transfer capability can be limited by low steady-state voltage, thermaloverloads, rotor-angle instability, or voltage instability. Shunt capacitors can often be used toalleviate these problems; the power-transfer capability of the network can thus often be increased

by adding shunt capacitors.

Shunt capacitors supply reactive current, which is 90 out of phase with the sinusoidal voltage.By supplying reactive current locally, shunt capacitors in strategic locations can reduce theamount of reactive current flowing through the distribution and transmission network. In aninductive network, the voltage drop is mostly due to the reactive current flow, so decreasing thereactive current flow will normally raise low system voltages. By the same token, if improperlyapplied, shunt capacitors can create overvoltage problems.

Useable electric power is proportional to the component of current that is in-phase with thesinusoidal voltage. By decreasing the reactive current flow, which is not in phase, shunt

capacitors reduce the I2R losses and thermal loading on the circuit. More in-phase current

(useable power) can then flow without thermally overloading the circuits.

The best location for shunt capacitors is usually near the load; a capacitor in the distributionsystem will often reduce the reactive power flows in both the distribution system and thetransmission system. The active power-transfer capability of both systems may therefore beincreased. In addition, the following problems, which can be potentially damaging anddangerous when capacitors are connected to the transmission system, can usually be easilyavoided if capacitors are located in the distribution system, close to the load:

Very high voltage when a large capacitor bank remains connected at the open end of a longline without load

Ferroresonance with the magnetizing reactance of a transformer for certain switching

configurations

Generator self-excitation when the system separates leaving a small generator connected to arelatively large capacitor bank with no load

These problems are almost never severe if the capacitors are not separated from the load. Theyare thus much less likely to occur when the shunt capacitors are located in the distributionsystem, close to the load.

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Conclusions and Recommendations

It is usually less expensive per kvar to install capacitors at the distribution level than at thetransmission level because the voltage rating does not need to be as high. There are exceptions,however. It may be economic to connect shunt capacitors to a 13.8-kV tertiary winding of anautotransformer in the bulk power system. Also, when communication has to be provided tomonitor and switch each bank, a large number of small units connected to the distribution systemmay be more expensive than a large bank connected to the transmission system.

For the reasons given above, the capacitive shunt compensation in distribution systems may beincreased significantly in the coming decades. Extensive distribution-capacitor-placementprograms are now in place at several utilities [7, 8]. To help support the transmission system, itmay be desirable to even overcompensate the reactive power load (that is, to connect morecapacitors to a distribution circuit than are required to cancel the reactive load). The extracompensation can be used to make up for the reactive power losses in the network. Some utilitiesare designed so that the operators can switch distribution capacitors on-line to help support thesystem during transmission system emergencies, even though these capacitors would normallybe off without the emergency.

Power Quality and Other Problems That May Be Caused by High Levels ofShunt Compensation

When a large capacitor bank is connected to the system, the jump in the steady-state voltagemagnitude before and after the switching operation may be excessive. This may make it difficultto coordinate with other voltage-control devices. It may also cause a visible change in the outputfrom incandescent light bulbs, which can become objectionable. On the other hand, moreswitching operations may be required if capacitor bank sizes are too small; this can also be aproblem.

If back-to-back capacitor banks are switched, the current inrush from one bank to another may beexcessive. The current out-rush for nearby system faults may also be excessive. This may make

it difficult to coordinate fuses and other protective devices.

Ringing voltage transients may result if shunt capacitors are switched on-line when thesinusoidal voltage waveform is not at a zero crossing. The ringing transient is the result ofenergy oscillation between the capacitor and the net inductance of the system. When anuncharged capacitor is suddenly switched on-line, it momentarily pulls the adjacent feedervoltage to zero. Almost immediately, a ringing voltage transient is induced whose first peak cantheoretically (in a lossless system) approach twice the normal peak-voltage on the feeder. Thesimulation results shown in section 2 indicate that for a realistic system, the peaks will typicallybe 1.6 times the rated peak-voltage. Simulations indicate that as the total amount of shuntcapacitance increases, the overshoot due to switching in each additional identical bank is

relatively unchanged, but the ringing frequency is lowered, and the transient takes longer todecay. There are many documented cases where adjustable-speed drives have disconnectedbecause of transient overvoltages caused by capacitor switching. Because the voltage transientslast longer, it is likely that increased levels of capacitive compensation will aggravate the powerquality problems related to capacitor switching. There will also be more switching operationswith increased levels of compensation.

The inductive components of the power system impedance may interact with a capacitor bankduring normal operation to form a tuned circuit with a resonance or anti-resonance at an

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unfortunate frequency. Magnetic saturation, rectification, and other nonlinear processes generateharmonic currents at characteristic frequencies, which are an integer multiple of the fundamentalfrequency, 60 Hz. If the resonant frequency of the tuned circuit coincides with the frequency ofthe harmonic current injected into the system, the normally sinusoidal voltage waveforms may bedistorted.

The resonant points, where the system impedance peaks, shift to lower frequencies as moreshunt-connected capacitors are added. With the load power factor corrected to 0.95, the lowestresonance is typically between the 5

thand 7

thharmonic. In most situations, 3

rdharmonic currents

produced by single-phase loads and transformer magnetizing pose no threat because feeders arenot resonant near the 3

rdharmonic. However, as can be seen in Figure 3-1, power-factor

compensation values of 1.0 or higher may move the resonant point to the 3rdharmonic, giving

cause for concern.

The frequency at which the impedance reaches a minimum may in some cases be equallyimportant. Many devices that produce harmonic currents also have filters. These filters aredesigned to have a low impedance to ground for critical harmonic currents. They are supposed toshunt the harmonic currents to ground locally so that the currents do not enter the system.

However, if the system impedance at characteristic harmonics is less than the filters, theharmonic currents will enter the system instead of being grounded.

Harmonic current flows:

Can cause telephone interference

Can cause electronic equipment to malfunction

Can increase system losses and thermal overload

Can cause earth-fault relays to malfunction

Zero-sequence harmonic current flows are particularly objectionable because they flow in theneutral circuit and may be easily coupled magnetically to other electrical circuits. The neutral-to-earth voltage can cause electric shocks. Zero-sequence resonance and anti-resonance maytherefore be particularly important.

The resonant points for the zero sequence will not, in general, be at the same frequencies as theresonant points for the positive sequence because the network impedances are different for thetwo sequences.

The triplen harmonic currents (3rd, 6

th, 9

th, and so on) injected by three-phase devices are

predominantly zero-sequence. If the fundamental-frequency phase voltages are unbalanced,

however, some of the triplen harmonic current can be positive- or negative-sequence. Mostcommercial loads and industrial three-phase sources are connected to the network usinggrounded wye-delta transformers. These transformers ground the zero-sequence currents so thatthey do not enter the network. As the simulations in section 2 show, third-harmonic voltagedistortion is generally not a problem with this type of load.

If the circuit is resonant at the third harmonic, single-phase harmonic sources can be a problem,however. The triplen harmonic currents from a single-phase-to-ground nonlinear load include a

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zero-sequence component as well as positive- and negative-sequence components. There will beno zero-sequence component, however, if the loads are connected phase-to-phase. The grounded-wye-to-grounded-wye transformer connection normally used for residential distribution systemsdoes not divert the zero-sequence currents to ground, so the harmonic current for all threesequences enter the distribution network. The zero-sequence triplen harmonic currents fromloads on different phases may tend to add, while the positive- and negative-sequence currents forthese harmonics will tend to cancel. The triplen harmonic currents flowing in the network maytherefore be mostly zero-sequence unless the loads are connected phase-to-phase. Third-harmonic voltage distortion may therefore be severe if the networks zero-sequence impedance isresonant at the third harmonic. Harmonic analysis of distribution networks with single-phase-to-ground loads should therefore consider both the positive-sequence and zero-sequence networkimpedances.

Possible solutions to a harmonic resonance problem include:

Moving the capacitor bank to a different location on the distribution feeder. This works bestif the feeder is connected to a strong, low-impedance transmission system. If the bulk power

system impedance is larger than the distribution feeder impedance, then moving the capacitorlocation on the feeder will have little effect.

Using a different size capacitor bank.

Using an ungrounded capacitor bank if the problem is caused by zero-sequence harmonics.

Putting a small reactor in series with the capacitor bank so that it acts as a filter or moves theresonant point.

It is sometimes cheaper to pay the telephone company to alleviate a telephone interferenceproblem from its side.

Special equipment and operating procedures may be needed with high levels of capacitive

compensation. Switching strategies for capacitor banks have traditionally been based upon thetime of day, the voltage at the capacitor, or the local reactive power flow. This does not alwaysresult in the lowest feeder losses or the best feeder voltage profile, particularly when thecapacitor has to coordinate with other voltage-control devices. Extensive distribution-capacitor-placement programs are now in place at several utilities [7, 8]. These programs sometimesinclude remote control and monitoring for each capacitor bank. As the load varies, individualphase capacitors can then be switched at the best time to maintain the optimum voltage profileand reactive power flow on the feeders. Some utilities also monitor the harmonic content of thevoltage and current. Armed with this information, the operators can change the capacitor orsystem configuration to move the resonant frequency when problems are detected. Provision isalso sometimes made to energize capacitor banks on the voltage-crossovers to avoid switching

transients.Many smaller capacitor banks at different locations will often have to be used instead of onelarge bank at a central location. There is a tradeoff: Smaller capacitor banks cost more per kvar,but the system losses may be less with smaller capacitor banks and they can also provide a bettervoltage profile. If added capacitive compensation is concentrated at only a few feeder locations,the losses and thermal loading of the feeder may actually increase instead of decrease. Often,with more total shunt compensation, smaller capacitor banks will be needed at more locations toavoid local voltage problems and excessive reactive power flows.

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Tools and Techniques for Analyzing Systems With High Levels of ShuntCompensation

There is an increased need to study high levels of capacitive shunt compensation to avoidpotential problems and to maximize the potential benefits. There are many software applications

available that provide the tools required for planning and designing these facilities. This reportdescribes some of the analytical tools and demonstrates how selected tools can be applied. Mostof the tools highlighted in this report are inexpensive and readily available.

There are many analytical techniques and computer programs that are designed specifically tooptimize the placement of capacitor banks on distribution feeders. In the hands of an experienceduser, a general-purpose program, such as an Optimal Power Flow (OPF) or a SecurityConstrained Power Flow, can also be used for this purpose. These applications may consider thevoltage profile on the feeder, the change in voltage due to switching, reduction of feeder losses,the desired power-factor range, available capacitor bank sizes, the feeder locations that canphysically accommodate added capacitor banks, and economics to determine the optimalcapacitor bank sizes and locations. Each method of analysis has its advantages anddisadvantages. The trade-off is often between ease of application and detail of representation.Some of the tools are not very user-friendly, and a great deal of skill or insider knowledge maybe required to properly apply and interpret the results from some of the more complex methods.Simplifying assumptions need to be made for every type of analysis. The key question may behow reasonable are the assumptions for the questions being considered.

Numerical programming methods such as OPF and the security-constrained power flow can beformulated to address the capacitor-placement problem. These techniques essentially find acapacitor-placement solution that minimizes cost while obeying voltage, power-flow, and/orother specified constraints. The cost that is minimized may include the costs of losses (bothenergy and capacity costs may be factored in), the costs of the capacitors themselves, and

penalties for unacceptable voltage or undesirable capacitor-placement patterns. A security-constrained power flow can consider several system loading levels at once.

Automated capacitor-placement programs typically do not include harmonic resonance as one ofthe placement criteria. It is therefore possible for the software to recommend capacitor locationsthat would cause an undesirable harmonic resonance. Harmonic analysis may, of course, be doneseparately to identify the locations on the feeder where capacitors of a specific size should not belocated. But this approach is cumbersome and may involve several iterations. A futuregeneration of programs may automatically consider harmon

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