Ground relaying of generators in unit connection

A-C GENERATORS in modern power plants usually are operated in unit connection directly on high-voltage bus bars, that is, alternator and transformer form one unit so that the circuit breaker at generator voltage is eliminated. A preferred method of ground protection for such units has been indirect high-resistance neutral grounding by connecting the primary winding of a distribution transformer between generator neutral and ground, and by loading the secondary of this transformer with a resistance of such magnitude that the resulting primary ground current for a solid ground fault at machine terminals becomes 10 to 20 amperes. This grounding method was first suggested
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in this country by L. F. Hunt as a means of minimizing fault damage within generators. The sensitivity of this scheme of protection depends on the lowest permissible pickup current of a relay activated by the current through the resistance. This pickup current indicates the parts of the machine windings which are protected. The straight line in Figure 1 shows that the assumed relay setting would lead to relay operation for faults anywhere between machine terminal (100) and location a, whereas ground faults between the neutral (0) and a would lie in the unprotected or dead zone.