mechanical or thermal damage to the insulating material or the anti-corona paint on a
stator coil. T
urn-to-turn faults, which normally are difficult to detect, quickly develop
into an ground fault and are tripped by the stator ground-fault protection. Common
practice in most countries is to ground the generator neutral through a resistor, which
limits the maximum ground-fault current to 5-10 A primary. Tuned reactors, which
limits the ground-fault current to less than 1 A, are also used. In both cases, the
transient voltages in the stator system during intermittent ground-faults are kept within
acceptable limits, and ground-faults, which are tripped within a second from fault
inception, only cause negligible damage to the laminations of the stator core.
A residual overvoltage function used for such protection can be connected to different
voltage transformers.
1. voltage (or distribution) transformer connected between the generator neutral point
and ground.
2. three-phase-to-ground-connected voltage transformers on the generator HV
terminal side (in this case the residual voltage is internally calculated by the IED).
3. broken delta winding of three-phase-to-ground voltage transformers connected on
the generator HV terminal side.
These three connection options are shown in
Figure 205. Depending on pickup setting
and fault resistance, such function can typically protect 80-95 percent of the stator
winding. Thus, the function is normally set to operate for faults located at 5 percent or
more from the stator neutral point with a time delay setting of 0.5 seconds. Thus such
function protects approximately 95 percent of the stator winding. The function also
covers the generator bus, the low-voltage winding of the unit transformer and the high-
voltage winding of the auxiliary transformer of the unit. The function can be set so low
because the generator
-grounding resistor normally limits the neutral voltage
transmitted from the high-voltage side of the unit transformer in case of an ground fault
on the high-voltage side to a maximum of 2-3 percent.
Units with a generator breaker between the transformer and the generator should also
have a three-phase voltage transformer connected to the bus between the low-voltage
winding of the unit transformer and the generator circuit breaker (function 3 in
Figure
205). The open delta secondary VT winding is connected to a residual overvoltage
function, normally set to 20-30 percent, which provides ground-fault protection for the
transformer low-voltage winding and the section of the bus connected to it when the
generator breaker is open.
The two-stage residual overvoltage function ROV2PTOV
(59N) can be used for all
three applications. The residual overvoltage function measures and operates only on
the fundamental frequency voltage component. It has an excellent rejection of the third
harmonic voltage component commonly present in such generator installations.
1MRK 502 071-UUS A Section 10
Voltage protection
Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062 447
Application manual
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