Intermittent Current Imbalance 2

[GMacD]

Just got home from a ship where we had an intermittent
electrical phenomenon which caused me some concern.
The ship has 3 Diesel Alternators (910Kw/1500A each)
Occasionally during periods when all 3 generators were
on the board and load sharing, No1 Generator would
experience an increase in current 200-300A higher than
the other 2 generators which remained steady,the increase
did not coincide with any load change visible from the
governors as they all remain steady. After a short period
the current will drop back to the same level as the other
generators who do not pick up any of the mysterious extra
current which just vanishes.
The offline voltage of the alternators is trimmed to
456VAC which provides the switchboard with 450VAC online.
The phase angle is 0.8 and the majority of the load is
inductive from AC induction motors.
My concern was that the AVR was on the way out,
I've priced a new one and been quoted £1400,
I'd like some advice just incase i'm going down the
wrong road with this problem.

Any help from an electrical brain would be greatly
appriated by one of your marine engineering brethren

 

[waross]

My first thought is an intermittent over-voltage on the generator field.
The cause may be a failing voltage regulator but I would first check your quadrature circuit. That is the circuit behind the switch that is labeled "Single/parallel" or something similar.
The most likely possibility is that the switch itself is failing.
There is a resistor inside the voltage regulator that is often a wire wound adjustable resistor. One of the wires from the "Single/Parallel" switch connects to one end of this resistor.
If this resistor is accessible check it also for damage.
Your problem sounds like generator #1 is being over excited and taking more than its share of the reactive current. The voltage may be rising slightly and the current on the other machines may be dropping slightly.
An intermittent in the quadrature circuit could be expected to cause these symptoms. You may also have a CT failing.
respectfully

 

[catserveng]

Before you change the AVR, check the droop circuits and the voltage adjust potentiometers. If you can while under load, monitor the field output voltage at F1 and F2 and see if in fact the excitation is increasing.

How is the system installed for VAR sharing, does it use droop, cross current compensation, or some other method?

Also check your sensing leads to the AVR, an intermittant open in the sensing or reference can cause the AVR output to increase.

Hope that helps.