In an ungrounded wye capacitor bank, losing a unit shifts the neutral point and a voltage appears between neutral and ground. IEEE C37.99-2000 (cl. 8.2.6.1) calls for a voltage device with the lowest possible ratio but rated for full system voltage, because the neutral can rise to 2.5 per unit during switching. The open-delta VT variant triples the signal and does not respond to system unbalance.
Short answer
In an ungrounded wye capacitor bank, when a unit goes out of service the neutral point shifts and a voltage appears between neutral and ground. The neutral voltage unbalance relay measures that voltage. IEEE C37.99-2000 (clause 8.2.6.1) recommends that the measuring device have the lowest possible ratio to gain sensitivity, but to withstand full system voltage, because the neutral can rise up to 2.5 per unit during switching. Its main limitation is that it also measures the system voltage unbalance, which must be compensated.
How it works
In an ungrounded wye bank with the three phases equal, currents and voltages are balanced and the neutral sits at ground potential. If a unit is lost in one phase, its capacitance drops, the balance is broken and the neutral shifts relative to ground. That displacement voltage grows with the number of units lost in the same phase.
The protection measures that displacement with a device connected between neutral and ground and feeds it to a time-delayed overvoltage relay. Clause 8.2.6.1 accepts three types of device:
- Voltage transformer (VT).
- Capacitive potential device.
- Resistive potential device.
The general principle of the scheme, and the associated ANSI 60 function, are in the guide on capacitor bank unbalance protection. This guide focuses on how the neutral voltage is measured and on what the standard asks of the sensor.
The connection this scheme belongs to is compared with the others in bank connection: grounded wye, ungrounded wye or delta.
What the standard asks of the voltage device
The lowest possible ratio. The lower the transformation ratio, the larger the signal per lost unit and the better the sensitivity. But the device must still withstand the transient and continuous overvoltages of the neutral.
Full system voltage. A VT in this application must be rated for the system voltage, because in switching the neutral can rise up to 2.5 per unit. A lower-voltage VT saturates deeply under those conditions. Clause 10.4 complements this: the device connected from neutral to ground must withstand, without abnormal operation, switching overvoltages of 0.5 to 2.5 times the system phase-to-neutral voltage.
A side effect at high voltages. At higher system voltages, VTs with a higher ratio are needed. The standard warns that this can make the neutral voltage method insensitive and that special relay techniques may be required (clause 10.4). That is why, above certain levels, the practice moves to differential schemes, such as voltage differential in a grounded wye or the extra-high-voltage schemes.
Resistive divider: with care. An undersized resistive divider with a voltage limiter on the secondary can allow the relay to operate with an open phase toward the bank. The divider must withstand that overvoltage.
The two connections of figure 12
| Connection | What it measures | Advantages | Limits |
|---|---|---|---|
| (a) Three line-to-neutral VTs, secondaries in open delta, overvoltage relay | The sum of the three phase voltages, equal to three times the neutral displacement | Not sensitive to system voltage unbalance. With the same VT ratio, it triples the sensitivity | The VTs must be rated for line-to-line voltage; three VTs instead of one |
| (b) One neutral-to-ground VT | The neutral displacement | A single device | Responds to system unbalance, which shifts the neutral even when the bank is healthy |
The standard clarifies that, with (b), the sensitivity is affected by system unbalance. In (a), the zero-sequence component of the system is compensated by construction, although the manufacturing tolerance of the units is still present. The topic is expanded in the guide on inherent unbalance.
What else affects the signal
- Harmonics. The relay must filter out other frequencies and keep the fundamental. Neutral harmonic voltages can confuse the relay if there is no power-frequency bandpass filter or other suitable filtering (clause 8.2.3 c).
- System compensation. For a single ungrounded bank, the neutral voltage caused by system unbalance is the zero-sequence component. An equivalent signal can be obtained with three line-to-ground VTs and secondaries in open delta, and subtracted from the neutral signal (clause 8.2.3 d).
- Settling time. Some capacitive VTs have their own transient response, and the relay may need a delay on initial energization (clause 8.2.4 d).
- Single-pole failure or open phase. A switching device that does not close all its poles, or a blown main fuse, can sustain a continuous voltage higher than the relay can withstand. The lockout logic must de-energize the voltage relay (clause 8.2.4 g).
To separate what comes from the system from what comes from the bank, see inherent unbalance and measurement error. In extra-high-voltage banks the voltage differential unbalance in a grounded wye scheme is preferred, which is the basis of protection of extra-high-voltage capacitor banks. Alarm and trip times are in alarm and trip times of unbalance protection.
Ferroresonance
In extra-high-voltage banks, the standard mentions series resistors with the VTs to avoid ferroresonance problems (clause 8.2.7). For the neutral VT of an ungrounded bank, the selection of the VT and its burden is part of the design. The guide on ferroresonance in voltage transformers explains the phenomenon.
What not to do
- Do not use a VT rated below the system voltage at the neutral.
- Do not set the sensitivity without knowing the inherent unbalance of the bank.
- Do not forget the harmonic filter in the relay.
- Do not use an undersized resistive divider without verifying that it withstands an open phase.
When to contact us
If your ungrounded wye bank alarms with no fault, or if you do not know whether the neutral VT is the right one, at Tiza Group we review the scheme, the sensor and the relay. We do remote relay configuration and quote the work according to its scope. Write to us.
Related guides
- IEEE C37.99: capacitor bank protection guide
- Inherent unbalance of a bank: why to compensate it
- Voltage differential unbalance in a grounded wye
- Bank connection: grounded wye, ungrounded wye or delta
- Neutral overvoltage 59N (3V0): what it is and how it works
References
- IEEE C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks, clauses 8.2.3, 8.2.4, 8.2.6.1 and 10.4, figures 11 and 12.
Frequently asked questions
Why must the neutral VT be rated for full system voltage if the neutral is at low voltage?
Because during switching the neutral can reach up to 2.5 times the phase-to-neutral voltage. IEEE C37.99-2000 warns that a lower-voltage VT goes into deep saturation. Clause 10.4 asks that the device withstand switching overvoltages between 0.5 and 2.5 times that voltage without malfunction.
What advantage does the open-delta connection have over a single neutral-to-ground VT?
With three line-to-ground VTs and their secondaries in open delta, the signal to the overvoltage relay is three times the neutral displacement, and the connection is not sensitive to system voltage unbalance. In exchange, the VTs must be rated for line-to-line voltage.
Is a resistive divider a good option for this measurement?
With care. The standard warns that an undersized resistive divider with a voltage limiter on the secondary can let the relay operate if one phase of the bank opens. The divider must withstand that overvoltage.