There are several unbalance detection schemes and methods. ANSI/IEEE C37.99 shows unbalance detection schemes. This includes schemes for larger banks and higher voltage banks as well as the methods we have standardized on for distribution voltages. We have essentially settled on a handful of methods. These are as follows:

A PT is connected between the neutral of the capacitor wye and the system ground. This method cannot be used on three phase, three wire systems or three phase, four wire systems with unigrounded neutral. The system has to be three phase, four wire, and multigrounded. This needs to be ascertained before quoting or, explained in the quote.

Also, this unbalance detection should not be used on resistance grounded systems.

An unbalance detection scheme should be offered on any capacitor bank connected floating wye. At least as an option. A table should be provided to show the unbalance percentage of units as capacitors fail in a phase to emphasize the problem.

This method will be affected by system unbalances and the relay will require harmonic restraints on filter banks.

See details on article GES6233

A CT is connected between the neutral point of the capacitor bank to the system ground. The grounded wye connected capacitor bank can only be used on the three phase, four wire multigrounded system and this unbalance detection scheme is limited to grounded banks.

Unbalance detection schemes for grounded wye bank are not to be considered as normal options. Also, this detection scheme should not be used on filter applications.

This detection scheme will be affected by system unbalances.

See details on article GES7233

Normally a CT is used, connected between the neutrals of two wye capacitor sections. The wye sections are generally equal, but do not have to be so. There are some customers that prefer a PT to read the unbalance.

Either way the neutral bus will not be affected by system problems such as system unbalances and harmonics. The relay does not have to have harmonic restraints for this connection.

This method can be used on any three-phase system.

See details on article GES19233

CT’s are mounted in the line connected to the capacitors. We can use two or three CT’s for the sensing. See GES delta unbalance paper. The CT’s will sense a drop in current when a capacitor fails in a leg.

This method can be used on any three-phase system.

This detection scheme will be affected by system unbalances.

See details in article GES4233