Generator governor troubleshooting - AVR defects

[BK32]

Hi,

I have been recently made aware of an issue regarding erratic operation of a diesel alternator governor (or so it was reported) on a shipborne diesel alternator, part of a plant of three (3) 480kW DAs. The defect notice indicated that one of the DAs was load sharing eratically, continuing "to take on load or shed load when in parallel," and when not under load "frequency continues to reduce". Initially the report suggests a faulty governor.

However, in a follow up report, it suggests that the root cause could be a faulty AVR. I undertand enough to know that there can be quite a number of causes relating to frequency excursions, surges etc. However, noting I only have a general knowledge of generator control, I cannot fully understand how the operation of the AVR (or maloperation) would have an effect on the operation of the governor and visa versa.

I am aware that generally speaking the Governor will respond to kW load changes and the AVR KVAR load changes; consequently could someone please explain the relationship between the two and provide some reference that I may be able to do some further reading into? Further to this could someone explain how a defect in one may affect the operation of the other (i.e. how an AVR defect may affect the ability of the governor to maintain engine speed and consequently frequency)?

 

[waross]

First, a drop in frequency is a governor problem.
Second, we must be sure that we understand what we mean by "load".
The load is measured by a kW meter.
The KVA is related to the current as shown by the the ammeter and the KVA will never be less than the kW.
When two sources with differing voltages are connected in parallel, be they transformers or generators, a reactive current will circulate.
This circulating current does not add to the loading except as a minor second order effect due to the increased I√R caused by the greater current.
KiloWatt output will never be greater than kiloWatt input and is controlled by the governor. If the governor opens too far both the kW input and output will increase (for parallel operation).
As long as the generator is in parallel and has less capacity than the load on the plant, the frequency will be controlled by the healthy sets.
If the governor starts to back off, the prime mover power will be less and less. As the prime mover drops below synchronous speed, the reverse power relay will trip the generator off-line and without the other sets to "lock" the frequency, the frequency will drop.

The AVR will have an effect on the current but no effect on the kW sharing.
The information given points to a faulty governor. The AVR may have issues but it will not effect frequency or kW load sharing.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

If the governor starts to back off, the prime mover power will be less and less. As the prime mover drops below synchronous speed, the reverse power relay will trip the generator off-line and without the other sets to "lock" the frequency, the frequency will drop.

Hi Bill, ummm...how can a prime mover "drop below synchronous speed" if it is mechanically connected to an electrical machine that is synchronized to a power system? Only when the unit breaker has opened can the machine operate at any speed other than that of the system to which it is othersise tied... only underexcitation by reduction or loss of field would allow a synchronous machine to either slip a pole or go out of step, no?



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

I guess I should have said as the governor setting drops below synchronous speed, the reverse power relay will trip the generator off-line

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Agreed.

As to the query in the OP, are governor and AVR functions ever integrated into one P[A]LC in such a way that both can be jeopardized by a common problem? I'm wondering if what may appear to be an AVR issue actually has a source deeper into the circuitry or software that could be adversely affecting both functions without any governor-type alarms being received...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[BK32]

Hi waross,

point taken regarding use of the "load" terminology.

From the responses provided, if I have understood correctly, it appears as though defects in AVR circuits would not normally have an effect on the governor, wrt kW sharing, unless there is common hardware/software?

crshears,

in this case the governor is a Woodward UG8 mechanical-hydraulic governor which mechanical linkage to the diesel fuel racks. The AVR is located in a seperate cubicle and is primarily made up of circuit card assemblies, however, will look into the control circuits within the switchboard regarding paralleling gen sets, there might be some common components/wiring, or atleast components located adjacent to each other that may be effecting the operation of both the AVR and governor.

Also, after looking at the troubleshooting section of the system manual for the prime mover (in which is contained a dedicated chapter on the governor), it states therein, that in the event the engine hunts or surges, a possible root cause is that the voltage regulator is not operating properly. Based on the advice received in this thread til now, this appears unlikely? I have attached the section of the troubleshooting guide which states this.

Am I reading it wrong, or is there more to it than first appears? Thanks for all the replies so far.

 

[waross]

Here's a good starting point from the instruction manual;
"5. The source of most troubles in any hydraulic governor stems from dirty oil. Grit and other
Impurities can be introduced into the governor with the oil, or form when the oil begins to
breakdown (oxidize) or become sludgy. The internal moving parts are continually lubricated
by the oil within the unit. Valves, pistons and plungers will stick and even freeze' in their
bores, due to grit and impurities in the oil. If this is the case erratic operation and poor
response can be corrected (if wear is not excessive) by flushing the unit with fuel oil. The use
of commercial solvents is not recommended as they may damage seals of gaskets. Change the
oil and flush the governor twice a year if possible. Remove the drain plug and drain out the old
oil. Flush the governor by filling it with fuel oil, and with the prime mover running at low
speed, cycle the governor by opening the needle valve two or three turns. Let the governor
hunt for a minute or two, then stop the engine and drain all the governor. Flush the governor
once again. Refill the governor with oil. Restart the engine and reset the compensation
adjustment and needle valve."

As for AVR problems causing speed variations, a lot of the characteristics of generator control change between parallel operation and islanded operation. When islanded, an increase in voltage will cause an increase in kW. For most loads more Volts means more Amps. The frequency may dip until the governor can respond and correct the speed back to set point.
The last time that I saw a set hunting I checked both the fuel filters and the AVR. The AVR was the problem (no stability control) but I don't think that this is an AVR problem. AVR problems will not cause an unloaded set to drop below synchronous speed.
In parallel operation AVR variations causes reactive currents to flow with very little increased load.
Another issue may be fuel supply. Badly plugged fuel filters or an other restriction in the flow of fuel could be the problem. When the engine is slowing down and the frequency is dropping look at the governor actuator. If the problem is the governor the governor rod will be moving towards the no fuel position as the engine slows down.
If the governor rod is pinned at the maximum fuel position when the engine is slowing down, check your fuel supply.
Hope that it is plugged filters. Other possible causes may be a failing engine. Fuel filters are a lot cheaper than an overhaul.
Even if the problem is fuel restriction this may be a good opportunity to flush the governor and refill it with new oil.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[IBRCAN]

when not under load "frequency continues to reduce".

When the engine is slowing down and the frequency is dropping look at the governor actuator. If the problem is the governor the governor rod will be moving towards the no fuel position as the engine slows down.
If the governor rod is pinned at the maximum fuel position when the engine is slowing down, check your fuel supply.

A genset will never operate properly in parallel if it doesn't work right on its own. Definitely sounds like an engine problem as opposed to an AVR problem.

Follow Waross' recommendation. I'm guessing this may be intermittent. Instruct the operators to watch the governor output lever when the problem is occurring. If there is a gauge, have them monitor fuel pressure also.

 

[flhouck]

Just a little note no one has mentioned here, a lot of today's engine are electronic in nature. If there is a problem with an engine sensor or fuelthe engine's computer wil start derating horse power which will be seen as a reduction in both power and speed.

general_genrator