transformer, where the secondary windings are connected in an open delta. Another
possibility is to measure the three phase-to-earth voltages, and calculate the
corresponding residual voltage internally in the IED. ROV2PTOV has two steps
with separate time delays. If the residual voltage remains above the set value for a
time period corresponding to the chosen time delay, the corresponding TRIP signal
is issued.
The time delay characteristic is individually chosen for the two steps and can be
either definite time delay or inverse time delay.
The voltage-related settings are made in percent of the base voltage, which is set in
kV, phase-phase. The set UBase value is divided by sqrt(3) before the set value is
calculated.
7.3.3.1 Measurement principle
M15331-6 v6
The residual voltage is measured continuously, and compared with the set values,
U1> and U2>.
To avoid oscillations of the output START signal, a settable hysteresis has been
included.
7.3.3.2 Time delay
M15331-10 v11
The time delay for the two steps can be either definite time delay (DT) or inverse
time delay (IDMT). For the inverse time delay four different modes are available:
• inverse curve A
• inverse curve B
• inverse curve C
• customer programmable inverse curve
The type A curve is described as:
IECEQUATION2422 V1 EN-US (Equation 33)
where:
Un> Set value for step 1 and step 2
U Measured voltage
The type B curve is described as:
1MRK 505 384-UEN A Section 7
Voltage protection
Breaker protection REQ650 2.2 IEC 191
Technical manual
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