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
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