A capacitor bank is not protected from system voltage by the unbalance relay. IEEE C37.99-2000 contemplates an overvoltage function (59), which in extra-high-voltage banks is supplemented by the bus 59B, and a time-delayed undervoltage function (27B) for loss of voltage. Neither locks out the bank, because the trip is not caused by an internal fault. The standard also requires care in how the voltage transformers feed the relays.
Short answer
Unbalance protection watches the inside of the bank. It does not see that the bus has more voltage than the capacitors can withstand, or that the bus has lost voltage. For that, IEEE C37.99-2000 (clauses 7.2.2, 7.2.4 and 7.2.5) contemplates system voltage protection:
- Inverse-time 59 (overvoltage), which removes the bank when system voltage exceeds what the capacitors or the neighboring equipment can withstand.
- 59B in extra-high-voltage banks, which watches the bus voltage and can trip fast on an extreme overvoltage.
- 27B (undervoltage), with a delay, for loss of bus voltage.
In all three, the bank is disconnected without being locked out, because the trip is not caused by a fault inside the bank.
Why the bank needs its own voltage protection
A capacitor is designed to operate at its rated voltage or below. The standard states that units withstand continuously up to 110 % of rated rms voltage, harmonics included.
System overvoltage has a double effect. It damages the capacitor, and the bank in service sustains it or raises it nearby. That is why the standard says that, if system voltage exceeds what the capacitor or the equipment can withstand, the bank be removed with the shortest possible delay. Removing the bank lowers system voltage in that area, and the other equipment suffers less overvoltage.
Overvoltage: 59 and 59B
The inverse-time relay
In some cases inverse-time overvoltage relays are needed to protect the units from severe power-frequency overvoltages. The standard says the relay must have suitable protection algorithms and be set according to the capabilities in the IEEE 1036 guide or from the capacitor manufacturer.
If the units receive a voltage that combines fundamental and harmonics, ask the manufacturer for the peak voltage they withstand as a function of time and temperature. Modern film capacitors are sensitive to peak voltage. That is why the standard recommends relays that measure the peak value of voltage in this application. The rms value alone does not reflect the peak.
The 59B in extra-high-voltage banks
In very large extra-high-voltage banks, the standard advises installing three-phase overvoltage relays (59B) that watch the bus voltage. In the standard’s example scheme (Figure 6), the 59B can trip the bank fast on an extreme overvoltage. In some cases the trip carries a timer so it does not act during transient overvoltages.
Because this trip is not caused by a fault inside the bank, the bank is not locked out.
The continuous voltage limit of the units is in the capacitor limits: 110 % voltage and 135 % kvar; the voltage on the healthy units when others are lost is explained in bank with external fuses. To calculate the voltage on the capacitor, you can use our capacitor bank tool.
Loss of voltage: 27B
The standard states that, in some cases, it may be necessary to trip the bank when the supply bus loses voltage. Two situations should be considered:
- Re-energizing a bank that holds trapped charge.
- Energizing a bank with no load in parallel through a transformer that was de-energized.
The standard asks you to consider the breaker reclosing schemes and the capacitor discharge means, to avoid nuisance trips or equipment damage when bus voltage is lost with high-speed reclosing.
The 27B detects the loss of system voltage and trips the bank after a delay. The delay prevents the bank from tripping for system faults external to it. The relay must be set so that it does not operate at voltages that require keeping the bank in service. Like the overvoltage trip, this one does not lock out the bank.
Summary of the three functions
| Function | What it watches | When it applies | Delay | Bank lockout |
|---|---|---|---|---|
| Inverse-time 59 | System voltage, better with peak measurement | When the system can exceed what capacitors or equipment withstand | Inverse: shorter time at higher voltage | No |
| 59B | Three-phase bus voltage | Extra-high-voltage banks | Fast; in some cases with a timer against transients | No |
| 27B | Loss of bus voltage | High-speed reclosing, trapped charge or energization through a de-energized transformer | Delayed for external faults | No |
- What happens to the bus voltage?
- It rises above what the bank or equipment withstand: Is it an extreme overvoltage?
- Yes: 59B trips fast, with a timer against transients
- No: The inverse-time 59 trips along its curve
- It is lost: Does it persist after the delay?
- Yes: 27B trips the bank
- No: The bank stays in service
- It is normal: The bank stays in service and unbalance protection watches its inside
- It rises above what the bank or equipment withstand: Is it an extreme overvoltage?
With all three voltage trips the bank is not locked out. Its later closing follows the logic of automatic control of capacitor banks, including the closing block while the capacitor has not discharged.
The place of these functions among the other protections is in bank protection matrix: threat and device; overcurrent and breaker failure in a bank is their complement.
How the voltage relays are supplied
Clause 7.2.5 of the standard deals with a point that is often overlooked. Many protection schemes for large banks are sensitive to loss of voltage. For that reason:
- Primary fuses should not be installed on the voltage transformers of this protection.
- The relay voltage circuit should run directly from the transformer to the panel, with no other loads connected. That way cable voltage drops, especially with varying loads, do not affect the sensitive unbalance relays.
- If another standard requires primary fuses, their operation must not cause loss of protection. A transformer with an open primary can present a high impedance between the secondary terminals and block the relay.
- Secondary fuses are usually acceptable: they are sized to protect the cable, normally 30 A, and the unbalance protection trips before they blow.
A failure of the fuse or breaker of a voltage transformer produces a loss-of-potential alarm, which should not be confused with a real loss of bus voltage.
Common mistakes
- Measuring only rms in a bank with harmonics. Film capacitors are sensitive to peak voltage.
- Expecting unbalance to protect against system voltage. The unbalance relay watches the inside of the bank and does not see the bus voltage.
- Locking out the bank for a system cause. The bank has no fault of its own and is taken out of service unnecessarily.
- Taking the voltage at the panel with other loads. Cable voltage drops upset the relay balance.
What we do at Tiza Group
In our remote relay configuration service we review the voltage protection of your bank and make it consistent with the unbalance protection and the substation reclosing. We work remotely, worldwide. Contact us and we will quote the review.
Related guides
- IEEE C37.99: shunt capacitor bank protection guide
- Bank protection: threat, device and remarks
- Overcurrent and breaker failure in a capacitor bank
- Capacitor limits: 110 % voltage and 135 % kvar
- Overvoltage protection 59 (PTOV): what it is and how it works
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks, clauses 7.2.2, 7.2.4 and 7.2.5, Figure 6.
- IEEE Std 1036-1992, IEEE Guide for Application of Shunt Power Capacitors.
- IEEE Std 18-1992, IEEE Standard for Shunt Power Capacitors.
Frequently asked questions
Why does an overvoltage or loss-of-voltage trip not lock out the bank?
Because the cause is in the system, not inside the bank. IEEE C37.99-2000 states that these trips are not due to a bank fault, so the bank is not locked out as it is for an internal fault.
What is undervoltage used for in a capacitor bank?
To take the bank out of service when bus voltage is lost. The standard asks you to consider two situations: re-energizing a bank that holds trapped charge, and energizing an uncharged bank in parallel through a transformer that was de-energized.
Can primary fuses be used on the voltage transformers of the bank protection?
The standard says they should not be installed. If another standard requires them, their operation must not leave the bank unprotected, because a voltage transformer with an open primary can present a high impedance at the secondary and prevent the relays from operating.