Capacitor units in filter banks may require additional duties due to harmonic contents. In these applications, higher voltage capacitor ratings and fuse current ratings may be required. Generally, the additional overloading requirements on component parameters should compensate for higher peak voltages and the increased losses imposed on the reactors and resistor assemblies.
The overcurrent and overload relay in each phase of the filter bank serves two desired protective functions. Fast tripping is initiated for high-level short-circuits near the circuit breaker terminal.
In case of lowmagnitude overcurrents, properly coordinated, time-delayed alarm and tripping signals are initiated due to the abnormal condition on the filter bank. Special emphasis is given to the response of the overcurrent protection (50/51 relay) to harmonic current distortion for some types of solid state electronic relays based on peak current measurements.
In effect, undesired filter bank tripping may occur from these relays as the electronic level detection is influenced by the harmonic phase angle relationships for low-order frequencies (second to fifth) along with the maximum current level detection of the relay.
Digital relays may be influenced by harmonic distortions if the predominant orders of harmonic oscillations are not predicted. The digital band-pass filter response provided in the relay may have a low attenuation factor at a given harmonic frequency and cause undesired tripping of the filter banks.
The expected reliability of the relay protection depends on the protection technology used, the design and manufacturing approaches, and the environment in which the relays are applied. To improve the availability of filter banks, appropriate data and experience related to harmonic distortion shall be considered in the selection of the protection scheme.
Figure below illustrates one possible primary protection scheme for a filter bank. A redundant protection scheme (not illustrated in the figure) may also be provided. The reactor and C2 are tuned to the fundamental frequency to minimize the losses in R at the fundamental frequency in this example.
Sample protection scheme for a filter bank
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