Under IEEE C37.99-2000 (cl. 4.1), a power capacitor must operate continuously at up to 110% of its rated rms voltage, with a peak voltage of up to 1.2·√2 times that voltage, 135% of rated current and up to 135% of reactive power from the sum of overvoltage, harmonics and manufacturing tolerance. These limits are what set when a bank must be taken out of service.
Short answer
Clause 4.1 of IEEE C37.99-2000 sets what a power capacitor must withstand in continuous operation: up to 110% of rated rms voltage, a peak voltage of up to 1.2 × √2 times that voltage, 135% of rated current and up to 135% of rated reactive power, caused by the sum of overvoltage, harmonics and manufacturing tolerance. These limits are not academic data: they are the criterion for deciding how many units a bank can lose before it must leave service.
The limits of clause 4.1
The standard speaks of the unit (a case with terminals) and assumes it operates at or below rated voltage and frequency. It allows the unit to stay in service under contingency conditions of the system or the bank, as long as these values are not exceeded:
| Item | Continuous limit under 4.1 |
|---|---|
| Rms voltage at the terminals, with harmonics and without transients | Up to 110% of rated |
| Peak voltage | Up to 1.2 × √2 times the rated rms voltage (≈ 1.70) |
| Current | 135% of rated |
| Reactive power from the combined effect | Up to 135% of rated |
| Reactive power tolerance at 25 °C | Between 100% and 115% of rated |
| Temperature (units in several rows and tiers) | Designed for a 24-hour average of 40 °C on the hottest expected day, or −40 °C on the coldest expected day |
The standard adds two clarifications. The voltage capability of each series element inside a unit is its share of the total capability of the unit. And units manufactured under IEEE 18-1992 after the year 2000 have a maximum of 110% (not 115%) in the reactive power tolerance.
How the 135% is built up
The 135% reactive power limit is not obtained by multiplying the others. The standard describes it as the combined effect of three causes:
- Voltage above the nameplate at fundamental frequency, not exceeding 110% of rated rms voltage.
- Harmonic voltages superimposed on the fundamental.
- Manufacturing tolerance of reactive power, up to 115% of rated.
The reactive power of a capacitor with fixed capacitance is proportional to the square of the voltage. So 110% voltage means 1.10² = 1.21 times its power, that is 121%. The rest of the margin up to 135% is left for harmonics and tolerance.
Example. A unit rated 7,200 V can operate continuously up to 1.10 × 7,200 = 7,920 V rms. Its peak voltage must not exceed 1.2 × √2 × 7,200 ≈ 12,219 V, even if the waveform is distorted.
Why peak voltage matters
Modern film capacitors are sensitive to the peak value of the voltage, not only to the rms value. With harmonics, the peak can exceed the limit even when rms voltage is below 110%. That is why the guide recommends, for monitoring combined fundamental and harmonic overvoltages, relays that measure peak voltage (cl. 7.2.2), and that the manufacturer provide the allowable peak levels as a function of time and temperature.
Discharge and manufacturing ranges
Units above 600 V must have an internal discharge device that reduces the residual voltage to 50 V or less in 5 minutes. This is a unit design condition. Because the residual charge conditions a re-closing, the bank control takes it into account. See automatic control of capacitor banks.
The guide recalls the ranges found on the market: rated voltages from 240 V to 24,940 V and sizes from 2.5 kvar to about 1,000 kvar. There are units for special applications that exceed those ranges.
What these limits mean for the bank
The standard turns the 110% limit into a design and protection criterion:
- Bank design. In a bank with external fuses, isolating one unit must not put more than 110% of rated voltage on the remaining units of the group (cl. 4.3.1). That sets how many parallel units each group must have.
- Trip. Unbalance protection must take the bank out when the healthy units exceed 110% of their rated voltage, or the value the manufacturer recommends (Table 1).
- Bank without fuses. In an unfused bank, the protection must trip before the healthy units exceed 110% of their rated voltage or the level recommended by the manufacturer (cl. 4.4.4).
What the guide does not set are short-duration overvoltage values. It refers them to IEEE 1036-1992, applied with manufacturer data.
The 110% criterion for taking out a bank with external fuses is developed in externally fused bank and, with internal fuses, in internally fused bank. In voltage protection it is applied as bank voltage protection (59, 59B and 27B).
What not to do
- Treat 110% as a normal operating value. It is the maximum for contingencies. The standard states that capacitors are intended to operate at or below their rated voltage and frequency.
- Mix the 115% with the 110% tolerance. They depend on the edition of IEEE 18 under which the unit was manufactured. Check the nameplate and the data sheet.
- Check only rms voltage. Peak voltage and harmonics also limit.
- Choose the unit voltage without looking at the system. The maximum operating voltage of the system defines the real margin. To size the bank, see how to size a capacitor bank.
What we do
The real maximum system voltage, the harmonics present and the unit tolerance determine how much margin is left to lose units before tripping. At Tiza Group we resolve it within the bank’s engineering design and studies: we choose the unit voltage and the arrangement using the limits of this clause, and we quote the study.
Related guides
- IEEE C37.99: shunt capacitor bank protection guide
- Externally fused capacitor bank: design and limits
- Bank voltage protection: 59, 59B and 27B
- Capacitor bank unbalance protection (ANSI 60)
- How to size a capacitor bank
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: cl. 4.1 (unit capabilities), 4.3.1 and 4.4.4 (110% on the remaining units), 7.2.2 (peak overvoltage) and Table 1.
- IEEE Std 18-1992, IEEE Standard for Shunt Power Capacitors: rated classifications.
- IEEE Std 1036-1992, IEEE Guide for Application of Shunt Power Capacitors: short-duration overvoltage.
Frequently asked questions
Why is the power limit (135%) higher than the voltage limit (110%)?
Because the reactive power of a capacitor grows with the square of the voltage and also with harmonics and manufacturing tolerance. The 135% is the combined effect of the three causes, with voltage limited to 110%. It is not a limit that can be used for voltage alone.
Is a unit at 108% of rated voltage within the limit?
In continuous operation and without transients, yes: the limit is 110% of rated rms voltage, harmonics included. Even so, peak voltage has its own limit, and harmonics can use it up before the rms voltage reaches 110%.
What happens if the 110% limit is exceeded for a short time?
The 110% limit is for continuous operation. For short-duration overvoltages the guide refers to IEEE 1036-1992 or to the capacitor manufacturer. That standard defines how long each level is allowed.