Arcteq Generator Commander, Instruction Manual

Page: 350 / 561

Signal Description Time base
U
4
Measured voltage U
4
/V 5ms
The selection of the used AI channel is made with a setting parameter. In all possible input channel
variations the pre-fault condition is presented with a 20 ms averaged history value from -20 ms from
ALARM or TRIP event.
Pick-up
When a fault appears in the power system and some areas are disconnected, normally the remaining
generators connected to the network must supply the area disconnected from the utility side supply.
This results in an instantaneous demand of power that the generators must tackle. The
excitation and the mechanical systems cannot answer such a huge demand of power quickly even if
there were enough reserve power. The worst of the situation is received by the rotors of the generator
units: they suffer a torsion torque that can even break the rotor and cause subsequent damage not only
for the generator but for the entire power plant too.
Figure. 5.5.8 - 182. Generator islanding.
As can be seen in the example above, only phase-to-phase voltages L1-L2 and L3-L1 have been
reduced, while voltage L2-L3 remains the same. This means that the problem occured in phase L1
of the network. The voltage level is not reduced to zero, nor is the voltage in any phase is totally lost.
The phases without the fault condition remain normal with the same value. On the other hand,
the frequency can sag as can be seen in the figure above.
The Δα setting parameter controls the pick-up of the vector jump function. This defines the minimum
allowed rapid measured voltage angle change before action from the function. The function constantly
calculates the ratio between the Δα
set
and the measured magnitude (Δα
m
) for each of the selected
voltages. The function's stage trip signal lasts for 20 ms and automatically resets after that time has
passed. The setting value is common for all measured amplitudes.
Figure. 5.5.8 - 183. Vector jump from the relay's point of view.
Genera Generattor Commander or Commander
Instruction manual
Version: 2.07
348 © Arcteq Relays Ltd
IM00001