PAGE 4
tripped off by removal of voltage to the respective under voltage relay.
d.
The 120 volt input to each generator undervoltage relay has a normally closed contact of the shore power
auxiliary relay (BR-SP) in series, When the shore power breaker is closed the BR-SP relay contact opens
which prevents generator voltage from operating the respective under voltage relay. The generator breaker
cannot be closed when its shunt trips circuit is energized. This interlocking between breakers is provided to
prevent paralleling any generator with shore power.
4.
Reverse Power Relay
(RPR)
a.
Each generator has a reverse power relay which senses current in phase A and voltage across phase A &
B.
b.
Reverse power can occur when two or more generators are running in parallel. If one generator does not
accept load, usually because its unloaded running speed is less than the other generators (called speed
droop), the other generators will motorize the drooping generator which is called reverse power. Reverse
power not only creates extra loads on the other generators but can hurt the engine being powered.
c.
The reverse power relay is screw driver adjustable from 4% to 20% of normal forward power with a
separate screwdriver adjustment from 0.5 to 20 seconds. Adjustment should be made to over ride normal
reverse power surges when breakers are first closed or when loads change. Motor start up currents can
cause reverse power surges during the time required for a motor to go from start to full speed.
d.
A normally closed contact of the reverse power relay will shunt trip the breaker by removing voltage from
the under voltage relay.
e.
A normally open contact of the reverse power relay will signal the auto start to shut the engine down on
reverse power.
5.
Shunt Trip Relay
(STR)
a.
The shunt trip relay operates from the Power Monitor relay. When the Power Monitor operates from loss of
phase or reverse phase, the shunt trip relay will operate to shunt trip the shore power breaker.
6.
Breaker Relay
(BR-SP)
a.
The breaker relay operates from an auxiliary 'a' contact in the shore power breaker. When the shore power
breaker is closed the breaker relay operates to shunt trip all three generator breakers.
b.
The electrical interlock between the shore power breaker and the generator breakers prevent the shore
power and generators from operating in parallel at anytime.
