Handbook of Electrical Engineering

322 HANDBOOK OF ELECTRICAL ENGINEERING

power transfer into the generator being from other generators or utility sources in the network. This
can occur for example just after synchronising an incoming generator or unloading a generator prior
to taking it out of service.

Reverse power protection also protects a gas turbine from failure of its governor control system
to regulate its speed e.g. component failure, sluggish response to speed changes. When power is fed
back to the prime mover it will tend to cause the shaft speed to rise, and the governor control
system will attempt to reduce the fuel supply by closing the fuel valve to its lower limit. In these
circumstances the prime mover is effectively without speed control.

Gas-turbine generators up to above 35 MW are usually driven by the prime mover with a
speed-reducing gearbox, because the generators are usually 4-pole low-speed machines. The design
of the gearbox and the couplings may not permit prolonged reverse power operation.

The relay is usually set for 1 to 5% of rated power and with a tripping time of up to 5 seconds.

12.2.6 Negative phase sequence relay

A negative phase sequence relay (46) protects a generator against overheating of its rotor pole faces
and damper bars. This form of overheating is due to the presence of unbalanced stator currents,
which create a negative phase sequence (NPS) flux in the air gap. This flux rotates in the opposite
direction to the rotor but at the same absolute speed. Hence the rotor poles and damper bars have
double-frequency currents induced into them, which rapidly cause localised heating and eventually
distortion of the rotor and slot damage. The NPS current has a heating characteristic similar to the
familiar positive phase sequence overcurrents and fault currents, i.e.:-

Kn=I^22 t

WhereKnis typically 50 to 60 for air-cooled generators.

Salient pole generators used with gas-turbine drivers can tolerate NPS currents up to 40% of
full-load current, when defined by:-

I^22 t≤ 0. 4 I^21 t, for large values of timet.

The relay characteristic therefore has a negative slope of 2 on a log-log scale, and the value
ofKnis determined by biasing the sloping line up or down on the log-log scale. A setting forKn
in the relay is chosen between 5% and 30% depending upon the actualKnvalue of the generator. A
typical relay has an operating time of 10 seconds when 100% NPS current flows in its circuit, and
the time multiplier setting (TMS) is set at 100% or 1.0 pu. The relay should be insensitive to zero
sequence and third harmonic currents (otherwise externally connected star/delta interposing current
transformers can be used to achieve this requirement).

12.2.7 Stator earth fault relays

12.2.7.1 Standby earth fault relay

High voltage generators used in the oil industry are usually earthed by connecting an impedance,
which is invariably a resistor, between its star point and the ‘ground’. Occasionally generators are