Shunt capacitor bank fundamentals and the application of differential voltage protection of fuseless single star earthed shunt capacitor banks
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Date
2010-02-16T13:24:56Z
Authors
Baker-Duly, Phillip William
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Abstract
The research investigates reactive power compensation and protection of shunt
capacitor banks. The characteristics of capacitors including, formulae, design,
manufacturing, and testing is presented. Capacitor units using extended foil solder
type elements have losses as low as 0.1 watt/kVAr. Failure of capacitors generally
occurs due to overvoltage stress. The type and aging properties of the dielectric
determines the lifespan of the capacitor. Polypropylene film is commonly used as the
dielectric. Basic capacitor bank design calculations are presented. A detailed
discussion on the configurations and protection philosophies is described for single
star earthed, single star H-bridge, double star, and C-type filter H-bridge capacitor
banks. A novel approach to unbalance voltage detection and the protection of
fuseless single star earthed shunt capacitor banks is investigated, engineered and
tested. This methodology explores the potential evolution towards distributed
protection. This involves two programmed multifunction protection relays
communicating via the IEC 61850 Ethernet protocol. One relay receives voltage
measurements from the high voltage busbar. The other relay receives voltage
measurements from the low voltage capacitor tap point. The two relays share their
measurements via the Ethernet link. The difference in measurements is used to
initiate alarm and trip operations. The relay protection function satisfies criterion for
reset-ability, selectivity, stability, accuracy, and loss of potential blocking. Spurious
operation occurs when the Total Harmonic Distortion level is above 8%. The major
short coming is the cyclic processing of the logic function. The algorithm processing
duration is 396ms as opposed to an anticipated time of 60ms. This application has a
competitive overall cost advantage. This is based on the number of components
required, manufacturing and testing times, and onsite installation and commissioning
works. It is recommended to further investigate the cyclic processing of the logic
functions, as well as, to test the protection function on a power system simulator.
Future prospects involve using the programmability and flexibility of the onboard
relay PLC to count capacitor element failure, on a discrete basis, instead of detection
and protection based on analogue threshold settings. This will mitigate ambiguous
measurements and spurious operation.