Relay Element Settings 2-7
SEL-300G Instruction Manual Date Code 20020206
between the grounding transformer secondary and the relay, calculate PTRN by
multiplying the grounding transformer ratio by the auxiliary transformer ratio to one.
In resistance grounded applications, calculate PTRN directly using the neutral voltage
transformer ratio to one.
The synchronism checking PT may be connected phase-to-phase or phase-to-ground on
the system side of the generator circuit breaker. Select a PT ratio so the voltage
presented to the relay is nominally equal to the phase-to-phase or phase-to-ground
voltage presented to the relay by the generator PTs. The relay does not require that the
synchronism PT connection match the generator PT connection. For instance, you may
elect to use open-delta connected generator PTs and a phase-to-ground connected
synchronism PT.
Nominal Machine Voltage (80.0–140.0 V l–l) VNOM = 115.0
Calculate VNOM by dividing the generator rated line-to-line voltage by the phase
transformer ratio, PTR. In a typical 13.8 kV generator application, when PTR = 120,
VNOM = 115. The relay volts/hertz, voltage-restrained overcurrent, and directional
power elements use the VNOM setting.
Nominal Machine Current (2.5–10.0 A; 5 A Models) INOM = 5.0
(0.5 – 2.0 A; 1 A Models)
Calculate INOM using the following equation:
A
CTR
kV•73.1
1000•MVA
INOM
ú
û
ù
ê
ë
é
=
where
MVA = generator rated output, MVA
kV = generator rated phase-to-phase voltage, kV
CT = phase current transformer ratio to one
The relay directional power, negative-sequence overcurrent, and differential elements
use the INOM setting.
To Next Page
