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Application...

• Reduce ground fault current

When generators of different designs are operated in parallel, the line to ground fault current can be significantly higher than the three phase fault current in one of the generators.  This may result in excessive forces in that generator during a single line-to-ground fault.  The use of a neutral reactor will address this issue.

• Reduce excessive generator third harmonic currents

Although generators are designed to produce power at a fundamental frequency, such as 25, 50, or 60 Hz, they can also produce small amounts of other frequencies.  The highest of these other frequencies is often the third harmonic.  As a result, in certain system configurations high third harmonic currents can occur.  This high third harmonic current can result in overheating of the generators and the unexpected operation of ground fault relays.

The third harmonic is generally a zero sequence quantity.  Consequently, this issue of high third harmonic currents occurs when there is more than one ground source on a system that has at least one local generator.  A common configuration for this issue is illustrated in Figure 1.  In this example, two three-phase generators are shown supplying power to a three-phase load.  Each generator is connected in a wye-grounded configuration.

An equivalent circuit of the two generators at the third harmonic is illustrated in Figure 2.  Each generator can produce some small amount of third harmonic voltage, generally in the range of 2% to 5%.  The amount of third harmonic voltage is determined by the design of the generator, specifically the pitch of the generator windings.  Based on the difference in the two voltages and the zero sequence impedances of the generators, a third harmonic current will flow between the generators.  These currents can be on the order of 50% to 100% of the generator rated currents.  Generally, limiting the third harmonic phase currents to < 10% of the generator ratings is acceptable.  A relatively easy fix for the high third harmonic current is to insert a reactor in the neutral of one or both generators.  (See Figure 3.)

Excessive third harmonic currents can occur for a number of different system configurations.  They occur when there are more than two generators or when there is a generator(s) operating in parallel with a wye-grounded transformer from a utility source.

Qual-Tech Engineers will design and supply properly engineered reactors reducing the ground fault current and third harmonic currents.