Stator Earth-Fault Protection Based On Residual Voltage Measurement - ABB REG670 Applications Manual

Section 10
Voltage protection
10.3.3.3
412
winding. The setting must be above the highest occurring "normal" residual voltage
and below the highest acceptable residual voltage for the capacitor.

Stator earth-fault protection based on residual voltage measurement

Accidental contact between the stator winding in the slots and the stator core is the
most common electrical fault in generators. The fault is normally initiated by
mechanical or thermal damage to the insulating material or the anti-corona paint on a
stator coil. Turn-to-turn faults, which normally are difficult to detect, quickly develop
into an earth fault and are tripped by the stator earth-fault protection. Common
practice in most countries is to earth the generator neutral through a resistor, which
limits the maximum earth-fault current to 5-10 A primary. Tuned reactors, which
limits the earth-fault current to less than 1 A, are also used. In both cases, the transient
voltages in the stator system during intermittent earth-faults are kept within
acceptable limits, and earth-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 earth.
2. three-phase-to-earth-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-earth voltage transformers connected on
the generator HV terminal side.
These three connection options are shown in
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-earthing resistor normally limits the neutral voltage
transmitted from the high-voltage side of the unit transformer in case of an earth 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
205). The open delta secondary VT winding is connected to a residual overvoltage
function, normally set to 20-30 percent, which provides earth-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 can be used for all three
applications. The residual overvoltage function measures and operates only on the
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
Figure 205. Depending on start setting
Application manual
Figure