HIGH RESISTANCE GENERATOR GROUNDING PROTECTION METHOD

Method I: High-resistance grounded (distribution-transformer grounded)
Grounding method I utilizes a distribution transformer with a primary-voltage rating equal to, or greater than, the line to- neutral voltage rating of the generator, with a secondary rating of 120 V or 240 V. The distribution transformer should have sufficient overvoltage capability so that it does not saturate on phase-to ground faults with the machine operated at 105% rated voltage.

Secondary resistors are usually selected so that for a single-phase-to-ground fault at the terminals of the generator, the power dissipated in the resistor is equal to, or greater than, the zero-sequence reactive volt amperes in the zero-sequence capacitive reactance of the generator windings, its leads, and the windings of the transformers that are connected to the generator terminals.

This arrangement is considered to be high-resistance grounding, and it limits the maximum single-phase-to ground fault current to a value in the range of approximately 3 to 25 primary amperes. This is not of sufficient magnitude to operate standard generator differential relays.

In general, the W-Z current transformer will have a ratio of unity and the R-S current-transformer ratio is usually selected so that its secondary current will be approximately equal to the primary current in the generator neutral.

A generator system grounded through a distribution transformer with a secondary resistor has certain characteristics that may have the following desirable features:

a) Mechanical stresses and fault damage are limited during phase-to-ground faults by restricting fault current.

b) Transient overvoltages are limited to safe levels.

c) The grounding device is more economical than direct insertion of a neutral resistor.

A disadvantage of this grounding scheme is that surge protective equipment must be selected on the basis of higher temporary overvoltages during ground faults.

Method II: High-resistance grounded (neutral-resistor grounded)
Grounding method II is functionally equivalent to that of method I. In method II, the resistor is sized directly to limit the single-phase-to-ground fault current to the same magnitude as in method I without the use of a distribution transformer.

However, the voltage-transformer voltage ratings are selected on the same basis as those for the distribution transformer in method I. The W-Z current-transformer ratio is generally selected to be unity.