4.5.1.6 Application
Overvoltage in a network occurs either due to the transient surges on the network or due
to prolonged power frequency overvoltages. Surge arresters are used to protect the
network against the transient overvoltages, but the relay's protection function is used to
protect against power frequency overvoltages.
The power frequency overvoltage may occur in the network due to contingencies such as:
• The defective operation of the automatic voltage regulator when the generator is in
isolated operation.
• Operation under manual control with the voltage regulator out of service. A sudden
variation of load, in particular the reactive power component, gives rise to a
substantial change in voltage because of the inherent large voltage regulation of a
typical alternator.
• Sudden loss of load due to the tripping of outgoing feeders, leaving the generator
isolated or feeding a very small load. This causes a sudden rise in the terminal voltage
due to the trapped field flux and overspeed.
If a load sensitive to overvoltage remains connected, it leads to equipment damage.
It is essential to provide power frequency overvoltage protection, in the form of time
delayed element, either IDMT or DT to prevent equipment damage.
4.5.1.7 Signals
Table 512: 59 Input signals
Name
Type Default Description
V_A_AB SIGNAL 0 Phase to ground voltage A or phase to phase voltage AB
V_B_BC SIGNAL 0 Phase to ground voltage B or phase to phase voltage BC
V_C_CA SIGNAL 0 Phase to ground voltage C or phase to phase voltage
CA
BLOCK BOOLEAN 0=False Block signal for activating the blocking mode
Table 513: 59 Output signals
Name
Type Description
TRIP BOOLEAN Trip
PICKUP BOOLEAN Pickup
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 615
Technical Manual
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