Capacitor bank unbalance protection 60: how it works

What unbalance protection protects

A capacitor bank is made of many units in series and in parallel, and each unit of many elements. When an element fails, the capacitance of its group changes and the voltage is shared differently: the remaining elements in that group see more voltage. If they keep failing, the overvoltage grows until it causes a cascading failure and, in the worst case, the rupture of a unit with an internal arc.

Unbalance protection protects the bank units against that progressive overvoltage. It alarms on the first failures, so replacement can be scheduled, and trips the bank breaker before the healthy units exceed their limit. It is the main protection of the bank; overcurrent and overvoltage cover other risks.

• ANSI/IEEE C37.2: 60, voltage or current balance relay.
• IEC 61850: there is no specific logical node for bank unbalance; it is modeled on overcurrent or overvoltage classes (PTOC, PTOV) and the bank itself is represented by ZCAP.
• IEC 60617: no dedicated symbol.

Where it is applied

At low voltage, banks are protected by fuses and by the controller devices, and element-level unbalance protection is rarely applied. The connection and its variants are explained in capacitor bank.

Operating principle

All schemes look for the same thing: a quantity that is almost zero with the bank healthy and that grows with each failure.

• Current between neutrals (double wye): the bank is split into two equal wyes with their neutrals tied through a CT. When healthy, no current flows; when an element fails in one wye, current appears through the CT.
• Neutral voltage (ungrounded wye): a VT between neutral and ground measures the neutral shift caused by an unbalanced leg.
• Neutral current (grounded wye): a CT in the neutral-to-ground connection.
• Voltage differential: in each phase, the voltage at a tap point of the bank is compared with the scaled bus voltage. It is used in large grounded banks and identifies the faulted phase.
• H-bridge: a CT in the center branch of each phase measures the unbalance between its two halves.

Inherent unbalance compensation. A small signal already exists with the bank healthy. The relay measures it and subtracts it as a phasor; some schemes also compensate for system voltage unbalance.

Internally fused, externally fused or fuseless. With external fuses, each failure removes a complete unit and the signal per failure is large. With internal fuses only one element is removed and the signal is small, so the scheme must be more sensitive and the inherent unbalance compensation more careful. In fuseless banks, the failed element is shorted and its voltage is shared by the other series groups of the string; the signal per failure is clear, but the bank tolerates fewer failures. The fuse type defines how many steps fit before the trip.

The characteristic

The characteristic is best understood as a staircase: on the horizontal axis, the number of failed elements or units in the most affected group; on the vertical axis, the unbalance signal and the overvoltage on the healthy elements. Each failure climbs one step.

• Alarm: at the step that corresponds to the first failure, or to the number considered acceptable.
• Trip: at the step where the overvoltage on the healthy elements would reach the limit the unit can withstand, with margin.
• Time delay: short, to avoid operating on switching transients, and coordinated with the fuses.

Setting criteria

The step calculation depends on the bank configuration, the number of units and elements and the fuse type. The general criteria of IEEE C37.99 are:

• Calculate the unbalance signal and the overvoltage on the healthy elements for each number of failures.
• Set the trip to act before the unit overvoltage limit is exceeded, and the alarm on the first failures.
• Check that the first failure gives a signal clearly larger than the inherent unbalance.
• Review coordination with the fuses.

You can estimate the behavior with our free capacitor bank tool. The calculation for a real bank is part of a protection coordination study.

Interaction with other functions

• Bank overcurrent 50/51: covers faults in the connections and between phases, not the elements.
• Overvoltage 59: protects the bank against high system voltage. See overvoltage 59.
• Undervoltage 27: opens the bank on loss of voltage, so it is not re-energized while still charged.
• Automatic control: the controller must not reconnect the bank before the discharge time. See automatic capacitor bank control.
• Breaker failure 50BF: initiated by the unbalance trip.

How each manufacturer implements it

• Siemens SIPROTEC (DIGSI): capacitor bank protection functions with current and voltage unbalance.
• SEL (AcSELerator QuickSet): capacitor bank relays with unbalance elements, compensation and faulted-phase identification.
• Hitachi Energy and ABB Relion (PCM600): in the 615 series, current unbalance is CUBPTOC; in the 670 series, dedicated capacitor bank functions.
• GE Vernova Multilin (EnerVista): bank unbalance elements with compensation.
• Schneider MiCOM and Easergy: current unbalance or neutral voltage stages.

IEC 61850 logical node

The bank is represented by ZCAP and its breaker by XCBR. Unbalance is modeled on PTOC (current) or PTOV (voltage), with manufacturer-specific names such as CUBPTOC in the ABB 615. Alarm and trip go through PTRC and reach XCBR by GOOSE or by a hardwired output.

How it is tested

In general the test verifies:

• The capacitance of each unit and each branch, and the inherent unbalance, before energizing.
• The ratio, polarity and insulation of the neutral CT or VT.
• With secondary injection, the alarm and trip thresholds, the times and the compensation.
• After energizing, the actual signal with the bank healthy and its compensation.

Typical mistakes

• Not compensating the inherent unbalance, or compensating it with a failed bank.
• Steps calculated for another fuse type or another configuration.
• Neutral CT ratio too high, giving no useful signal on the first failure.
• Replacing units without measuring capacitance, which changes the inherent unbalance.
• Repeating the compensation after a trip without finding the failed unit.

Did your bank trip on unbalance, or do you need its steps reviewed? At Tiza Group we review events and settings remotely, worldwide. See our remote relay configuration service or contact us.

Related guides

• Automatic capacitor bank control
• Overvoltage protection 59

References

• IEEE C37.99, Guide for the Protection of Shunt Capacitor Banks: unbalance schemes and step calculation.
• IEEE 18, Standard for Shunt Power Capacitors: unit limits.
• IEC 60871-1, Shunt capacitors for a.c. power systems having a rated voltage above 1 000 V – Part 1: General.
• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations: device 60.
• IEC 61850-7-4, Compatible logical node classes and data object classes: ZCAP, PTOC, PTOV and PTRC nodes.

Frequently asked questions