Application - ABB RELION REX640 Technical Manual

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1MRS759142 C

Technical Manual

The Blocking mode setting has three blocking methods. In the "Freeze timers" mode,

the operation timer is frozen to the prevailing value. In the "Block all" mode, the

whole function is blocked and the timers are reset. In the "Block OPERATE output"

mode, HCUBPTOC operates normally but the OPERATE output is not activated.

Application

Shunt capacitor banks (SCBs) are widely used in transmission and distribution

networks to produce reactive power support. Located in relevant places such as in the

vicinity of load centers, SCBs have beneficial effects on power system performance:

increased power factor, reduced losses, improved system capacity and better voltage

level at load points.

The capacitor bank terminology 'Bank', 'Unit' and 'Element' is shown in

GUID-B4ABB7B9-CB4C-4404-93ED-2DF3BA69DB65 V1 EN

Bank, unit and element of a capacitor bank

HCUBPTOC is designed for the protection against internal faults in H-bridge-

connected capacitor banks. This unbalance protection detects an asymmetry in the

capacitor bank, caused by blown fuses or short-circuits across bushings, or between

capacitor units and the racks in which they are mounted.

Normally, the capacitor units are designed to withstand 110% of the nominal voltage

continuously. When an element inside a capacitor bank fails, the remaining healthy

elements experience an increase in voltage across them. If the voltage exceeds 110%

of the nominal voltage, it can lead to a failure of the healthy elements of the bank and

in turn fail the entire capacitor bank. Since the capacitor unbalance current is directly

proportional to element failures, unbalance protection is an effective way of detecting

capacitor element failures. The current unbalance protection function is usually used

along with the three-phase capacitor bank overload protection function to increase the

sensitivity of protection for capacitor banks.

Protection functions

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