The reverse power protector provides continuous surveillance for ac
generators operating in parallel or for boosting mains supplies. On site
adjustment of the trip point and time delay ensures accurate protection
against ‘motoring’ in the event of engine failure and prevents tripping
from surges during synchronising.
Reverse power protectors provide continuous surveillance of ac generators against
motoring. Reverse power relays are used to detect the failure of the prime mover
(engine) when active energy (Watts) flows into the generator causing rotation – the set
will operate like an electric motor which can cause significant mechanical damage.
This relay offers an adjustable reverse power set point between 2% and 20% of
nominal power and time delay adjustment range of 0 to 20 seconds.
As soon as the reverse power level increases above the set point limit, the time delay is
activated, after which a trip will occur. The time delay prevents the relay from tripping for a
predetermined period to prevent nuisance tripping. The products also feature an internal
differential (hysteresis) setting of 1% to reduce nuisance tripping if the measured signal is
noisy or unstable. These units are powered from the measuring supply.
The protector relay estimates the power level in the system by measuring current and
power factor, but does not actually measure the system voltage. When the reverse
power level exceeds the set point, the time delay is started. When the time has
elapsed, the relay will energise and the red LED will illuminate to indicate the trip
condition. The relay will automatically re-set once the power level falls below the set
point minus the differential. The LED will extinguish and the relay de-energises.
The time delay is not active when resetting. The reverse power level will trip as
expected at the calibrated point for unity power factor. However, the system power
factor does affect the trip point calibration. The relay becomes more sensitive at lagging
power factors, as almost all systems exhibit inductance. At leading power factors, this
relay is less sensitive.
The ‘% set’ potentiometer trimmer on the front label is calibrated as a percentage of
the input current rating e.g. of 5A and not of the forward kW. Adjust the ‘% set’
trimmer to the required tripping value, 7.5% to 10% is normal. Setting accuracy can be
checked by reversing the current lead connections and, with forward power, measuring
the trip point value on a suitable ammeter (reconnect leads on completion). Adjust the
‘Delay’ to the required time delay, 10 seconds is normally adequate.
250 series protector relays offer various customised options to suit individual
- Adjustment ranges – different adjustment ranges are possible for the set point
and time delay controls.
- Relay operation – standard models are fail safe, but the relays can be customised
to de-energise on trip.