ABB Relion 670 Series Applications Manual page 239

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1MRK 511 401-UEN Rev. K

This protection function is applied to capacitor banks that have CT's to measure the unbalance

current between equal impedance strings of the capacitor bank. The connection types of SCB

are considered with reference to the

the shunt capacitor banks .

For an ungrounded double WYE connection, neutrals of the two WYE sections are tied

together and a normal CT is placed to measure the unbalance current. For a grounded double

WYE connection, neutrals of the two WYE sections are grounded independently, so that a

differential CT is placed to measure the difference of unbalance currents flowing through the

two neutrals.

IEC19000196 V1 EN-US

Figure 109: Grounded/ungrounded double WYE connected SCB with CT arrangement

In the above configurations, the unbalance current is proportional to the positive sequence

current. Hence, the positive sequence component of current is considered as the reference to

measure the unbalance current.

For a grounded/ungrounded single WYE connection, each phase is divided into two main

strings. A differential CT is placed to measure the difference between the unbalance currents

through two strings.

Bay control REC670

Application manual

IEEE standard C37.99-2000 guide for the protection of

Differential CT

IEC19000196-1-en-us.vsdx

Unbalance protection

Relion rec670