Application - ABB RELION 620 Series Technical Manual

1MAC504801-IB E
4.2.5.5
620 series ANSI
Technical Manual

Application

27PS can be applied for protecting a power station used for embedded generation when
network faults like short circuits or phase-to-ground faults in a transmission or a
distribution line cause a potentially dangerous situations for the power station. A network
fault can be dangerous for the power station for various reasons. The operation of the
protection can cause an islanding condition, also called a loss-of-mains condition, in
which a part of the network, that is, an island fed by the power station, is isolated from the
rest of the network. There is then a risk of an autoreclosure taking place when the voltages
of different parts of the network do not synchronize, which is a straining incident for the
power station. Another risk is that the generator can lose synchronism during the network
fault. A sufficiently fast trip of the utility circuit breaker of the power station can avoid
these risks.
The lower the three-phase symmetrical voltage of the network is, the higher is the
probability that the generator loses the synchronism. The positive-sequence voltage is
also available during asymmetrical faults. It is a more appropriate criterion for detecting
the risk of loss of synchronism than, for example, the lowest phase-to-phase voltage.
Analyzing the loss of synchronism of a generator is rather complicated and requires a
model of the generator with its prime mover and controllers. The generator can be able to
trip synchronously even if the voltage drops by a few tens of percent for some hundreds
of milliseconds. The setting of 27PS is thus determined by the need to protect the power
station from the risks of the islanding conditions since that requires a higher setting value.
The loss of synchronism of a generator means that the generator is unable to operate as a
generator with the network frequency but enters into an unstable condition in which it
operates by turns as a generator and a motor. Such a condition stresses the generator
thermally and mechanically. This kind of loss of synchronism should not be mixed with
the one between an island and the utility network. In the islanding situation, the condition
of the generator itself is normal but the phase angle and the frequency of the phase-to-
phase voltage can be different from the corresponding voltage in the rest of the network.
The island can have a frequency of its own relatively fast when fed by a small power
station with a low inertia.
27PS complements other loss-of-grid protection principles based on the frequency and
voltage operation.
Motor stalling and failure to start can lead to a continuous undervoltage. The positive-
sequence undervoltage is used as a backup protection against the motor stall condition.
Section 4
Protection functions
293