27
A situation of this kind can be critical for different reasons. The power plant
can be left to feed an isolated network due to a trip caused by a fault. In this
case, there is a risk that the isolated network, which is in asynchronous state
compared to the rest of the network, is reconnected to the network as a result
of an autoreclosure, for instance. In addition, the power plant can also fall into
an asynchronous state in a fault situation. These critical situations can be
prevented by disconnecting the power plant from the network fast enough by
tripping the connecting circuit breaker.
The benefit of this function is that the voltage value measured during or after
a network fault is a good measure of how critical the fault is for a smaller
power plant. When the positive phase-sequence voltage value falls below the
critical limit, the power plant has to be disconnected from the network.
REU 610 measuring the positive phase-sequence voltage complements other
methods used to disconnect smaller power plants. The application of these
methods is based on frequency and voltage measurement. The undervoltage
protection based on calculated positive phase-sequence voltage requires that
the relay is in three-phase use.
When the calculated positive phase-sequence voltage value U
1
falls below the
set pickup value of the element 27D, the element generates a pickup signal
after a ~ 50 ms
’
pickup time. When the set operate time at definite-time
characteristic or the calculated operate time at IDMT characteristic elapses,
the element generates a trip signal. The element is reset in 70 ms after the
calculated positive phase-sequence voltage value has exceeded the set pickup
value of the element.
The positive phase-sequence voltage is calculated as follows:
U
1
=
+ ×
+ ×
U
a U
a
U
ab
bc
ca
2
3
(2)
a
= 1
∠
120°
a
2
= 1
∠
-120°
On a symmetrical network, this formula scales the positive phase-sequence
value to the same amplitude as the measured voltage signal.
Element 27P-2 can be set to be blocked when the measured voltage values
fall below 0.15 × U
n
(VT). In addition, the tripping of element 27P-1 can be
set to be blocked by element 27P-2 pickup. The selection is made in SGF4.
Element 27D has a fixed drop-off/pick-up ratio of 1.04.
5.1.4.4.
Residual overvoltage protection
The residual overvoltage protection detects residual voltages caused by
ground faults.
1MRS755972
Voltage Relay
Technical Reference Manual - ANSI Version
REU 610
Summary of Contents for REU 610
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