Power Supply Quality - ABB REG670 Applications Manual

1MRK 502 071-UEN -
10.3.3.4
10.3.3.5
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
3 3 3 11
Uo = ⋅ U =
−
Max Ph Ph
IECEQUATION2395 V1 EN
2. One VT input is to be used in the IED. The VT ratio is to be set according to the
open delta winding ratio. For this application correct primary and secondary
rating values are 19.05 kV and 110 V respectively.
3. For the base value, a generator-rated phase-to-phase voltage is to be set. Thus for
this application UBase=11 kV.This base voltage value is not set directly under the
function but it is instead selected by the Global Base Value parameter.
4. ROV2PTOV internally divides the set voltage base value with √3. Thus, the
internally used base voltage value is 6.35 kV. This is three times smaller than
maximum 3Uo voltage. Therefore, if the wanted start is 5 percent from the neutral
point the ROV2PTOV start value is set to U1>=3·5%=15%(that is, three times
the desired coverage).
5. The definite time delay is set to 0.5 seconds.

Power supply quality

The setting must be above the highest occurring "normal" residual voltage and below
the highest acceptable residual voltage, due to regulation, good practice or other
agreements.
High impedance earthed systems
In high impedance earthed systems, earth faults cause a neutral voltage in the feeding
transformer neutral. Two step residual overvoltage protection ROV2PTOV is used to
trip the transformer, as a backup protection for the feeder earth-fault protection, and
as a backup for the transformer primary earth-fault protection. The setting must be
above the highest occurring "normal" residual voltage, and below the lowest
occurring residual voltage during the faults under consideration. A metallic single-
phase earth fault causes a transformer neutral to reach a voltage equal to the nominal
phase-to-earth voltage.
The voltage transformers measuring the phase-to-earth voltages measure zero voltage
in the faulty phase. The two healthy phases will measure full phase-to-phase voltage,
as the faulty phase will be connected to earth. The residual overvoltage will be three
times the phase-to-earth voltage. See figure 206.
19 05 .
⋅ kV = kV
Section 10
Voltage protection
(Equation 246)
415