Directional Underpower Protection Guppdup (37); Identification; Application - Hitachi Relion 670 Series Applications Manual

Section 8
Current protection
8.6

Directional underpower protection GUPPDUP (37)

8.6.1

Identification

Function description
Directional underpower protection
8.6.2

Application

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.
Steam turbines easily become overheated if the steam flow becomes too low or if the steam ceases to
flow through the turbine. Therefore, turbo-generators should have reverse power protection. There are
several contingencies that may cause reverse power: break of a main steam pipe, damage to one or
more blades in the steam turbine or inadvertent closing of the main stop valves. In the last case, it is
highly desirable to have a reliable reverse power protection. It may prevent damage to an otherwise
undamaged plant.
During the routine shutdown of many thermal power units, the reverse power protection gives the tripping
impulse to the generator breaker (the unit breaker). By doing so, one prevents the disconnection of the
unit before the mechanical power has become zero. Earlier disconnection would cause an acceleration of
the turbine generator at all routine shutdowns. This should have caused overspeed and high centrifugal
stresses.
When the steam ceases to flow through a turbine, the cooling of the turbine blades will disappear. Now, it
is not possible to remove all heat generated by the windage losses. Instead, the heat will increase the
temperature in the steam turbine and especially of the blades. When a steam turbine rotates without
steam supply, the electric power consumption will be about 2% of rated power. Even if the turbine rotates
in vacuum, it will soon become overheated and damaged. The turbine overheats within minutes if the
turbine loses the vacuum.
The critical time to overheating a steam turbine varies from about 0.5 to 30 minutes depending on the
type of turbine. A high-pressure turbine with small and thin blades will become overheated more easily
than a low-pressure turbine with long and heavy blades. The conditions vary from turbine to turbine and it
is necessary to ask the turbine manufacturer in each case.
142
IEC 61850
identification
GUPPDUP
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IEC 60617 ANSI/IEEE C37.2
identification device number
37
P <
2
SYMBOL-LL V2 EN-US
Phasor measurement unit RES670
1MRK511407-UUS Rev. N
SEMOD156693-1 v4
SEMOD158941-2 v4
SEMOD151283-4 v6
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