Harmonic Filter Protection per IEEE C37.99

Short answer

A power harmonic filter is a capacitor bank with a reactor and, sometimes, a resistor. Its capacitors withstand harmonics in addition to the fundamental voltage. IEEE C37.99-2000 (clause 9) asks that filter protection cover five things: overcurrent and overload, ground fault, resistor overload, overvoltage and capacitor unbalance. The difficulty is doing so without the harmonics themselves tripping the filter.

The filter design, that is, the tuning and the values of its components, is outside this guide. It is in harmonic filters: IEEE 1531 criteria.

Why a filter demands more than a compensation bank

The standard states that the capacitor units in a filter may be more heavily stressed because of the harmonic content. They may require a higher rated voltage and a higher rated fuse current. The additional overload ratings of the components must make up for the higher peak voltages and the greater losses in reactors and resistors.

The standard’s example (Figure 39) shows a filter with a high-voltage capacitor C1, a low-voltage capacitor C2, a reactor and a resistor R. In that example, the reactor and C2 are tuned to the fundamental frequency to minimize losses in R at that frequency. The scheme includes a breaker (52), overvoltage (59), resistor overload (51R), differential (87), overcurrent (50/51) and two unbalance relays (60/61).

The protections, one by one

Overcurrent and overload

The relay of each phase performs two functions. It trips fast on high-level short circuits near the breaker terminals. For low overcurrents it gives timed, coordinated alarm and trip for an abnormal filter condition.

The standard puts emphasis on how the 50/51 responds to distortion:

• Some solid-state electronic relays measure the peak current. The phase relationship between low-order harmonics (second to fifth) changes what they see and can trip the filter with no fault.
• Digital relays can be affected if the dominant harmonics are not anticipated. Their bandpass filter may attenuate little at a given frequency and cause unwanted trips.
• Expected reliability depends on the relay technology, its design and the environment. To improve filter availability, the standard asks that harmonic distortion data and experience be taken into account when choosing the scheme.

Ground overcurrent

The ground relay carries a delay and is coordinated with the other system protections, so that it does not trip on system ground faults or on transients. The phase and ground relays back up the differential protection.

Resistor overload

The damping resistor of each phase is protected against overloads from fundamental-frequency current and harmonics. That relay must respond to the true rms current through the resistor, and its setting must match the overload capability of the resistor.

Overvoltage

Each filter has an overvoltage relay with one of two tasks:

• Protect the filter against continuous power-frequency overvoltages, with a delay and coordinated with the system automatic voltage control.
• Protect the capacitors against severe overvoltages that subject them to excessive stress. The standard recommends relays that measure the peak, set according to the capability declared by the manufacturer. Overload and overvoltage can also be obtained by integrating the current, to calculate the real voltage the units withstand.

C1 and C2 unbalance

Unbalance is covered in detail in clause 8 of the standard. In the filter, C1 and C2 are protected with separate unbalance schemes. There are two examples:

• Neutral voltage shift (Figure 40). A general function detects the change in neutral voltage caused by capacitor failures on the high-voltage or low-voltage side. It can include an optional compensation for system voltage unbalance. The relay must detect failures in C1 or in C2 and trip the bank.
• Current in the resistor branch (Figure 41). An unconventional scheme measures the current in the resistor branch as an indicator of capacitor failure. The effect of the deviation from 60 Hz and of the variation of capacitance with temperature must be carefully evaluated. With compensation, filter availability improves greatly.

In digital relays, you need to know the response time of the numerical filter and its attenuation, to coordinate them with the unbalance delay.

The bank voltage protection (59, 59B and 27B) and the overcurrent and breaker failure in a bank also apply to a filter. For an SVC with thyristor-switched capacitors, see capacitors in SVC and TSC.

Multi-frequency filters

In applications such as high-voltage direct-current converter stations, two or more harmonics are shunted. The high-voltage capacitor serves both frequencies and the low-voltage section is tuned for more than one. Standard protection is applied to the high- and low-voltage sections. The standard gives five notes on the schemes (Figure 42):

1. Neutral unbalance. It can be used, but the relay needs a narrow bandpass filter at 60 Hz. The filter is large, so it must have a compensation circuit for system and natural unbalance. It detects a shorted turn in the reactors and damaged capacitors.
2. H-bridge on the high-voltage capacitor. It serves for alarm and trip and identifies the phase with the problem. It only detects capacitor bank faults, not those of the reactor or of the low-voltage section. It may require double-bushing capacitor units.
3. H-bridge on the low-voltage capacitor. As above, it does not detect a shorted turn of the reactor.
4. Voltage differential. It is not recommended in a multi-frequency filter, because the low-voltage section makes it hard to balance the two voltage sources.
5. Voltage transformers across the low-voltage capacitor. The three voltages are summed and only the fundamental is measured; the relay is balanced on energization. It is sensitive: it detects capacitor failures, shorted turns of the reactor and large impedance changes in the high-voltage section. It requires only single-bushing units.

Common mistakes

• Setting the 50/51 without knowing the harmonics of the filter. The relay can trip with no fault.
• Measuring only the fundamental in the resistor. The relay must respond to the true rms current.
• Protecting only C1. Unbalance protection must detect failures in C1 and in C2.
• Copying a compensation bank scheme onto a multi-frequency filter. There are schemes the standard does not recommend in that case.

Resonance between a bank and the network can be checked with our capacitor bank tool.

To check system distortion, use our harmonic distortion (THD) tool.

What we do at Tiza Group

In our engineering design and studies service we study the harmonics of your network, the filter design and the protection scheme that fits it. We work remotely, worldwide. Contact us and we will quote the study.

Related guides

• IEEE C37.99: shunt capacitor bank protection guide
• Harmonic filters: IEEE 1531 criteria
• Active or passive harmonic filter: which to choose
• Capacitors in SVC and TSC: protection and filters
• Resonance in capacitor banks: why they fail

References

• IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks, clauses 9.1 and 9.2, Figures 39 to 42.
• IEEE Std 1531, IEEE Guide for Application and Specification of Harmonic Filters.

Frequently asked questions