Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

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4.56
SEL-700G Relay Instruction Manual Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
Figure 4.30 51V Element Voltage Restraint Characteristic
Loss-of-Field Element
Loss-of-field current causes the synchronous generator to act as an induction
generator. The rotor speed increases, active power output decreases, and the
generator pulls vars from the system. High currents are induced in the rotor,
and stator current as high as 2.0 per unit is possible. These high currents cause
dangerous overheating in a very short time.
The SEL-700G uses a pair of offset mho circles to detect loss-of-field.
Because loss-of-field affects all three phases, the condition is a balanced one.
The SEL-700G uses measured positive-sequence impedance to form the mho
circles.
Typically, Zone 1 and Zone 2 are offset from the impedance plane origin by a
value equal to half of the machine transient reactance, as shown in
Figure 4.32. Zone 1 is intended to operate with little time delay in the event of
a loss-of-field under full load conditions. Zone 2 reaches farther and operates
with a longer time delay. Zone 2 is intended to trip for loss-of-field conditions
that occur under light load conditions.
For compatibility with some existing electromechanical loss-of-field relays,
the SEL-700G Zone 2 element can be set with a positive offset, as shown in
Figure 4.33. When Zone 2 is used in this manner, the relay provides a
directional element with a settable angle characteristic. The Zone 2 element is
used together with an undervoltage element to provide faster tripping when
the system voltage is depressed during the loss-of-field condition.
The loss-of-field elements are supervised by the 40ZTC torque-control setting.
51V Pick
up
(per unit o
f 51VP set
ting)
Selected Phase-to-Phase Voltage
(per unit of VNOM_X)
1.00
0.50
0.25
0.125
0.0
0.125 0.25 0.50 1.00