ABB Relion 670 series, Technical Reference Manual

Page: 477 / 1232

7.11.1 Introduction
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 reverse power protection is to protect the turbine and not to protect the
generator itself.
Figure 238 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
Operating point
without
turbine torque
Margin Margin
Operate
Line
Operate
Line
Operating point
without
turbine torque
IEC06000315-2-en.vsd
IEC06000315 V2 EN
Figure 238: Protection with underpower IED and overpower IED
1MRK505222-UUS C
Section 7
Current protection
471
Technical reference manual