GENERATOR DISTANCE TYPE BACK UP BASICS

One zone of distance relaying with a mho characteristic is commonly used for system phase-fault backup. These relays are usually connected to receive currents from current transformers in the neutral ends of the generator phase windings and potential from the terminals of the generator.

If the generator is connected to the system using some means other than a delta-wye step-up transformer (i.e., direct connection, wye-wye transformer, etc.), then standard ct and vt connections made to a standard Mho distance relay will provide accurate measurement of impedances for system faults (neglecting infeed).

However, if there is a delta grounded-wye step-up transformer between the generator and the system, special care must be taken in selecting the distance relay and in applying the proper currents and potentials so that these relays see correct impedances for system faults. With some relay designs, the phase angle of the voltages applied to the relay have to be shifted so that they are in phase with the system voltages in order for the relay to see system faults correctly.

If required, this phase shift is accomplished by using auxiliary voltage transformers connected in delta-wye as shown in figure 1.

Application of system back-up relays— Unit generator-transformer arrangement

NOTE—This is a phase shifting transformer only. The turns ratio is chosen so that the line-to-line voltages on either side of the auxiliary vts are 1:1.

When a generator is connected directly to a system, the connections to the relay are shown in figure 2.

Application of system back-up relays—Generator connected directly to the system

In both cases, for the connections shown, the relay will not only provide backup for system faults but it will also provide some backup protection for phase faults in the generator and generator zone before and after the generator is synchronized to the system.

GENERATORS SEQUENCE IMPEDANCE REPRESENTATION

Positive-, negative-, and zero-sequence impedances are usually provided as identified values on the generator manufacturer’s data sheet for the machine. If negative and zero are not readily available, a couple of guidelines may be used to approximate values.

The negative-sequence reactance of a synchronous generator is defined in Park’s equations as the mean of the direct and quadrature axis subtransient reactances. For smooth rotor machines (i.e., 3600 r/min machines on 60 Hz systems), the direct and quadrature reactances are nearly equal; and in the absence of better data, the negative-sequence reactance may be assumed to be equal to the subtransient reactance.

Zero-sequence reactance of a synchronous generator is also defined in Park’s equations, but the definition is more complex. For most machines, its value is on the order of one half of the subtransient reactance. When looking into the terminals of a generator (in the figurative sense), the actual zero sequence impedance is a combination of the zero-sequence impedance of the generator plus the zero-sequence representation of the generator’s neutral grounding device. Neutral grounding devices are treated separately.

Positive-sequence reactance of a synchronous generator is also usually available from the generator manufacturer’s data sheet, but normally several values exist from which to choose.

As noted in Table 2-1, the value to be used in a calculation of short-circuit currents depends upon the intended use of the result of that calculation.

The definitions given for positive-, negative-, and zero-sequence voltage and current phasor sets also suggest another important consideration in the representation of synchronous machines in symmetrical component terminology. Positive-sequence voltages correspond to actual system voltages and currents, whereas negative- and zero-sequence voltages are physically fictitious.

Generators are a source of voltage on the power system, and the only sequence to include a voltage source is the positive sequence. Induction generators are finding their way more commonly into both industrial and utility applications. Induction generators should be treated as induction motors for fault calculations.

Table 2-1—Short-circuit impedances for protective device application and evaluation