ABB Relion 670 series, Technical Manual

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8.11.2 Functionality
SEMOD155787-4 v6
The task of a generator in a power plant is to convert mechanical energy available as a torque on a
rotating shaft to electric energy.
Sometimes, the mechanical power from a prime mover may decrease so much that it does not
cover bearing losses and ventilation losses. Then, the synchronous generator becomes a
synchronous motor and starts to take electric power from the rest of the power system. This
operating state, where individual synchronous machines operate as motors, implies no risk for the
machine itself. If the generator under consideration is very large and if it consumes lots of electric
power, it may be desirable to disconnect it to ease the task for the rest of the power system.
Often, the motoring condition may imply that the turbine is in a very dangerous state. The task of
the low forward power protection is to protect the turbine and not to protect the generator itself.
Figure 288 illustrates the low forward power and reverse power protection with underpower and
overpower functions respectively. The underpower IED gives a higher margin and should provide
better dependability. On the other hand, the risk for unwanted operation immediately after
synchronization may be higher. One should set the underpower IED to trip if the active power from
the generator is less than about 2%. One should set the overpower IED to trip if the power flow
from the network to the generator is higher than 1% depending on the type of turbine.
When IED with a metering class input CTs is used pickup can be set to more sensitive value
(e.g.0,5% or even to 0,2%).
Underpower IED Overpower IED
Q Q
P P
Tripping point
without
turbine torque
Margin Margin
Trip
Line
Trip
Line
Tripping point
without
turbine torque
ANSI06000315-1-en.vsd
ANSI06000315 V1 EN-US
Figure 288: Protection with underpower IED and overpower IED
1MRK 502 066-UUS B Section 8
Current protection
543
Technical manual