Power Capacitors

CAP-Banks

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Outline

DefinitionPower capacitor applicationsTypical applications of:

Series capacitorsShunt capacitors

Capacitor Placement / SizingStandards

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Definition

Power System Capacitor is an assembly of dielectric Materials and Metal-electrodes in a container (casing), with terminals brought out, that is intended to introduce capacitance into an electric power circuit. [1]

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Fig.1 Power Capacitor in Service[2]

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Nature of the power capacitors

The power capacitor can be considered to be a VAR-GEN (reactive power Source), since it actually supplies needed-magnetizing current requirements for inductive loads.[3]

The fundamental function of power capacitor is to provide needed reactive power compensation.

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Motivations

The first use of capacitors on power circuits was in 1920.[4]

The initial motivation for power capacitor application was for power factor correction.

Until present, Fixed- shunt capacitor banks for Power Factor Correction has been a well established approach.

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Motivations

Power factor correctionFeeder-Loss ReductionRelease of System capacity Voltage- Stabilization/RegulationEfficient Power UtilizationPower Quality EnhancementPower Harmonic Filtering

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Series Capacitors

Series capacitors are used on transmission lines to compensate for inductive reactance/ Enhance Power Transfer.

It is best to think of series capacitor as voltage regulator which regulate Series voltage (Boost/Buck) based on polarity and Bank Size.[5]

Series capacitor can be used as a blocking filter for offending harmonics

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Series Capacitors

For steady state Power Flow and transient state stability, the real power flow (P) over a transmission line is given by formula (1), where Es is the sending end voltage, ER is the receiving end voltage, X is the line reactance and β is the angle between Es and ER .

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Series Capacitors

The power transfer can be increased considerably by the use of series capacitor banks.

Series capacitors are suited practically to circuits where light flicker is encountered due to Dynamic load fluctuations, such as frequent inrush motor starting, electric welders and arc-furnaces.

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Shunt Capacitors[5]

The shunt capacitors are used to supply a Capacitive type-Leading VAR reactive power to the AC Power system at the point of Connection, in order to:

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Shunt Capacitors[5]

Reduce the lagging component of the circuit current

Increase the voltage of the load busImprove bus-voltage regulation and/or

power factorReduce Transmission lossesReduce Electricity Billing cost based on

KVA Demand

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Switched-Type Capacitor

A lot of dynamic/adaptive controller are used to switch or modulate Capacitor-Banks connected to the AC-systems.

Schemes are employed to change the compensation Level/topologies of the Capacitor Banks based on Load-voltage and current variations / excursions or sudden system faults.

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Capacitor Placement

The Capacitor Location or Placement for low voltage systems determines capacitor type, size, location and control schemes.

Optimal capacitor placement is generally a hard combinatorial .optimization problem that can be formulated as a nonlinear/Search Minimization problem.[6]

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Capacitor Placement

Almost all the methods to solve capacitor placement problems are based on the historical data of the load models and associated cost of the energy and the cost of capacitor banks.

Cost $/Kvar for Power savings and Losses (Power losses/Energy losses)

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Capacitor Placement

However, Historical Data and Load models are uncertain and may change in reality.

To account for such load model and load pattern/cycles uncertainties Soft-Computing AI Based algorithms using fuzzy sets/Neural networks/Genetic Algorithm can be utilized.[7]

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Capacitor Placement

In general, capacitor placement problems can be solved in two steps:[8]Use of load flow model and find the V,P,Q

at all the buses and also the feeder lossesMinimize the cost function-Jo-min - subject

to constraints, like practical limits of voltage and capacitor size!

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Limitations and Cautions

Resonance-Series or Shunt TypeCapacitor inrush current transients

caused by Capacitor SwitchingCapacitor Sinking harmonic currents Transient Recovery Voltage

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Resonance

Resonance is a condition whereby the capacitive reactance of the AC-System, offsets its inductive reactance leaving only the resistive elements in the AC-Network as the only impedance.

The resonant frequency can be calculated by as

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Resonance

If the resonant frequency happens to coincide with one generated by an existing harmonic source, then the dangerous voltages and currents will increase disproportionately, causing damage to capacitors and other electrical equipments. [3]

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Capacitor Switching Transients

Capacitor switching transient typically occurs when energizing a large capacitor on the high voltage side of the power system (the utility side) result in magnification/intensification of the transients at lower voltage capacitors.

The magnified transient at lower voltage end can reach up to 400%.

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Standards

There are two standards provide a basis for uniformity in design and testing of shunt power capacitors:• IEEE Std. 18-2002 IEEE Standard for

Shunt Power Capacitors• IEEE Std. 824-1985 IEEE Standard for

Series Capacitors in Power Systems

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Conclusion

Power capacitor are very useful for power factor correction, loss reduction, voltage profile improvement and distribution system-capacity release/increase.

Best location and sizing of the capacitor bank should be carefully selected.

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Reference

• [1] IEEE standards, IEEE Standard for Shunt Power Capacitor, IEEE Std. 18-2002

• [2] http://www.abb.com• [3] M.H.Shwehdi and M.R.Sultan, “Power Factor

Correction Capacitors; Essentials and Cautions”, 2000 IEEE Power Engineering Society Summer Meeting, Vol.3, pp.1317-1322

• [4] S.W.Cichanowaki and G.R. Newcomb, Power Capacitors, Electrical Electronics Insulation Conference, 1991. Boston '91 EEIC/ICWA Exposition, Prodeedings of the 20th

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Reference

• [5] Dr. G. Thomas Bellarmine, Optimum Series Compensated High Voltage Transmission Lines, Southeastcon '97 'Engineering New Century', Proceedings, IEEE 12-14 April 1997 P307-309

• [6] Ramon A. Callego, Alcir Jose Monticelli and Ruben Romero, “Optimal Capacitor Placement in Radial Distribution Networks”, IEEE Transactions on power systems, Vol.16, No.4, November 2001

• [7] H.N.Ng.M.M.A.Salama, “Fuzzy Optimal Capacitor Sizing and Placement”, Electrical and Computer Engineering, 1995. Canadian Conference on Vol.2, 5-8 Sept. 1995 P680-683

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Reference

• [8] A.M.Sharaf and S.T. Ibrahim, “Optimal capacitor placement in distribution networks”,1996 Electric Power System Research 37, 181-187