Grounded wye capacitor banks of 345 kV or more must detect the loss of a single unit, or of a few elements, with a voltage differential between the bank tap and the bus, supported by low-voltage capacitors. IEEE C37.99-2000 (cl. 8.2.7 and figure 6) adds a redundant neutral voltage backup, bus 59B and 27B relays that trip without lockout, 50/51 and 51N overcurrent and breaker failure.
Short answer
In a grounded wye capacitor bank of 345 kV or more, unbalance protection must detect the loss of a single unit if the fuses are external, or of a few elements if they are internal or there are no fuses. IEEE C37.99-2000 (clause 8.2.7) solves this with a voltage differential between the bank tap and the bus, with low-voltage protection capacitors, and a redundant backup based on neutral voltage. Figure 6 of the standard completes it with bus 59B and 27B relays, overcurrent and breaker failure.
Why a scheme of its own is needed
The higher the bank voltage, the more units there are in series and in parallel, and the smaller the signal produced by the loss of one unit. A simple neutral scheme, which works in small banks, becomes insensitive, and with higher-ratio voltage transformers to withstand the system voltage the signal drops even further (clause 10.4). For these banks, the standard sets the criterion: detect the lost unit or the few elements before the healthy units are overexposed.
Main scheme: voltage differential
The main scheme of figure 6 compares the voltage at the bank tap with the bus voltage, through the secondaries of voltage transformers connected at those two points. How the tap is made depends on the bank type:
| Bank type | Tap |
|---|---|
| With external fuses | In many cases, one VT per phase connected across one or more series groups, depending on the sensitivity required; low-voltage capacitors are also used |
| With internal fuses | Low-voltage protection capacitors |
| Fuseless | Low-voltage protection capacitors |
The standard mentions that series resistors with the VTs can be used to avoid ferroresonance problems. The differential principle, its compensation and its taps are in the guide on voltage differential in a grounded wye.
The measuring principle and its compensation are in voltage differential unbalance in a grounded wye, and the inherent unbalance it compensates is in inherent unbalance and measurement error.
Redundant backup with neutral voltage
For these banks, the standard states that a redundant backup protection of neutral voltage unbalance is normally added. The unbalance relay is connected to a voltage transformer that measures the voltage across a low-voltage capacitor at the neutral of the bank. It is set to detect the loss of a specified number of units or elements, depending on the bank type, and trips with a delay if the remaining units see an overvoltage above what is allowed.
Details noted by the standard:
- There must be a short delay so as not to trip a bank that is already in an alarm state because of a system overvoltage transient.
- The overvoltage relay must have a fundamental-frequency bandpass filter. A third-harmonic block may not prevent all false operations.
- Some relays compensate for the effect of system unbalance: they compare the neutral voltage with the open-delta voltage of the bus VT in grounded wye banks, or with that of another low-voltage capacitor in double wye banks.
The setting criterion is developed in clauses 8.2 to 8.6 and depends on each setting philosophy; it is not reproduced here.
The backup scheme is explained in neutral voltage unbalance in an ungrounded wye; alarm and trip times, in alarm and trip times of unbalance protection.
Bus and system protections
Figure 6 includes functions that do not depend on a fault inside the bank:
Bus overvoltage (59B). For very large EHV banks, the standard advises installing three-phase overvoltage relays that monitor the bus voltage and trip fast on an extreme overvoltage. To avoid trips on transients, a timer is used in some cases. Since the trip is not due to a fault inside the bank, the bank is not locked out (clause 7.2.2). Removing the bank also lowers the voltage in the area. If the capacitors are exposed to combined fundamental and harmonic voltages, the standard recommends relays that measure the peak value, because modern film capacitors are sensitive to that value. The general function is explained in overvoltage 59.
Bus voltage loss (27B). In some cases the bank has to be tripped if the bus voltage is lost: on re-energization with trapped charge, or when energizing the bank with no load in parallel through a transformer. A 27B relay trips with a delay, so as not to act on system faults outside the bank, and it does not lock out the bank either (clause 7.2.4). The function is described in undervoltage 27.
Overcurrent (50/51 and 51N). For a major fault, such as a phase-to-phase or ground fault, external protection is required. In some large banks there are two redundant sets of three-phase 50/51 relays. The neutral 51N carries a low burden to limit the voltages on the CT secondary caused by the high-frequency currents the bank delivers to an external fault (clause 7.2.3).
Breaker failure. If the bank switching device is a circuit breaker or a circuit switcher, breaker failure protection can be added as local backup: if the device fails, the adjacent breakers are tripped after a time. In some cases a remote transfer trip may be required. The standard asks to verify the capacitive switching capability of those adjacent breakers (clause 7.2.6). The function is explained in breaker failure 50BF.
A detail about the voltage transformers
Many schemes for large banks depend on voltage. For that reason the standard states that primary fuses must not be installed on the VTs of these protections, and that the potential circuit must run directly from the VT to the relay panel, with no other burdens, so that the cable voltage drop does not affect the unbalance protection. If another standard requires primary fuses, their operation must not cause the protection to be lost: a VT with an open primary can present a high impedance on the secondary and prevent the relay from operating (clause 7.2.5).
What not to do
- Do not assume that the scheme of a small bank is enough in an EHV bank.
- Do not omit the redundant unbalance backup.
- Do not lock out the bank on 59B or 27B: they are not internal faults.
- Do not put primary fuses on the protection VTs without ensuring that their operation does not leave the scheme blind.
- Do not forget the capacitive switching capability of the adjacent breakers in the breaker failure scheme.
When to contact us
If you have a high-voltage or extra-high-voltage bank, or are planning one, at Tiza Group we review the complete protection scheme and leave the relays in order. 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
- Voltage differential unbalance in a grounded wye
- Neutral voltage unbalance in an ungrounded wye
- Ferroresonance in voltage transformers: causes
- Overvoltage protection 59 (PTOV): what it is and how it works
References
- IEEE C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks, clauses 7.2.2, 7.2.3, 7.2.4, 7.2.5, 7.2.6, 8.2.7 and 10.4, figure 6.
Frequently asked questions
Why does an extra-high-voltage bank not use just a simple neutral protection?
Because it needs to detect the loss of one unit (with external fuses) or of a few elements (with internal fuses or fuseless). At such high system voltages, the signal of a simple neutral scheme is small. IEEE C37.99-2000 (cl. 8.2.7) proposes a voltage differential between the bank tap and the bus, with neutral voltage backup.
Does the bus 59B relay trip with bank lockout?
No. The standard states that the bus overvoltage trip is not due to a fault inside the bank, so the bank is not locked out. It may include a timer to avoid trips on transient overvoltages. The same applies to the loss-of-voltage trip (27B).
What are the series resistors with the tap VTs for?
To avoid ferroresonance problems in the voltage transformers that measure the tap. The standard mentions them as an option in these schemes (cl. 8.2.7).