Island power system in a facility with 2 X 2MW 400V, 50Hz, 1000RPM Gas Engine Driven generators A & B, each capable of supplying the plant total load. The problem is, Gen A works perfectly by itself but Gen B can only work if paralleled with Gen A. If Gen B is working alone (for 2-22 hours), the voltage suddenly drops to about 200V and trips the plant. We have checked all wiring, rotating rectifier, AVR and governor settings, everything looks OK but the issue still persists. Please, does anyone have any ideas on what else to check?

 

Gen Man do you think it is possible that there is a problem with the field? Have you megged it? Perhaps read total ohms resistance and made comparisons between the two units?

Thanks
Mark Allen

 

Gen Man... if your machines are the brushless type, I would be more inclined to look at Pilot-Exciter circuit!

Regards, Phil Corso

 

Do the generators share a lead/lag control system?

It seems odd that they are fine while operating together.

 

Thanks for your response, Phil Corso, the machines are the brushless type but without pilot exciter. We have checked the circuit and nothing looks strange for now. We are going to change the PT fuses for the sensing lines just in case they are fatigued and are causing errors in the AVR.

Thanks Curiousone for your question, the generators have independent lead/lag controls. We noticed something strange recently, they no longer work for more than 24Hours in parallel. They start exchanging the total load between each other and make the entire system unstable.

 

Although they have independent controls, they must have some communication about the status of each other.

Perhaps, they are designed to switch lead every day but GenB is receiving a signal that GenA is on-line and well and switches to lag and unloads.

 

As I read my own comment, I suddenly realize information I should of included.

As a programmer, I would not have used a real time clock (unreliable) for the lead/lag sequence. I would of used a 24 hour timer.

Basically for example; I command something to happen and this happens for an hour and reset the timer. Therefore the command will execute an hour later than the day before.

GenA senses that GenB cannot carry the load and takes over i.e. load fighting. Timer reset! Back and Forth.

 

Hi Mark Allen,

I just realized that my initial response to you hasn't been posted. Thank you for you comment. We are also thinking that the issue may be with the field circuit, we have meggered the stator and rotor windings of the exciter and also checked the rotating rectifier for damage. Everything looks ok.

Thanks once again, Curiousone, that is a very interesting perspective, I will definitely take a look at the clock settings on both machines. Maybe that would resolve the load swapping issues, but there is still the issue of the voltage drop on Gen B when it is running alone.

Thanks a lot guys for the ideas, please keep them coming.

Cheers.

 

I think that one clue is that it is happening after a long period of time. If the problem is not with the exciter windings, there maybe a problem with old rectifier diodes or the varistor where they fail after operating at a certain temperature for a period of time.

Another problem would be to check the AVR power supply, is it powered by aPMG, if so, check the health of those windings.

I would check the terminations of the power supply to the AVR and the excitation output from the AVR to the alternator for damage or loose terminations.

Lastly, how old is the AVR, there is a chance that it could be faulty...

 

Thank you very much, Peterhj, for your suggestions. We changed the complete rectifier assembly earlier, but the issue still persists. The AVR takes its power from the bus through a PT and this has also been changed out. The AVR is about 5 years old, it was also swapped with the other unit's and that still didn't solve the problem.

 

I cannot think of any other specific reason off hand as to why the alternator would be dropping the voltage with the information you have provided so far, so I would do the following tests.

Firstly, I would determine what the alternator field winding resistance is, and what the alternator and AVR manufacturers advertised maximum excitation voltage and current level are. Also check whether the AVR has any inbuilt protection settings (under/over excitation, volts/hertz relationship and where the knee point is).

I would not discount the engine causing the problems, I have recently come across a CAT 3512B engine shutting down in an under voltage which was caused by an internal alarm which was driving the speed demand in the ECM to zero. While this alarm activate, it was not logged nor connected to any shutdown output on the ECM itself. The under voltage alarm was setup such that it activated before any under frequency alarm. As the alarm is internal to the CAT system, they are currently fault finding their software as we speak.

When the generator is online, I would trend the following items:

- Alternator voltage
- Alternator frequency / engine speed
- Alternator kVA
- Alternator power factor
- Harmonic levels for the current (and voltage if possible)

It is important to know exactly what load the alternator is supplying and whether there is any transient load shifts near the time the problem occurs. This will provide you important clues as to whether there is a problem with the alternator not operating within it's allowable capability. Depending on the % of load with harminics (VSD etc), this could seriously de-rate the capability of the AVR operation and also send a large amount of current through the neutral connection back to the alternator. In some cases, it is not unusual for the current level in the neutral conductor to be high than the current levels in the phases.

- Under voltage alarm, or whichever alarm causes the generator package to shut down

While this is important, it may not be necessarily the initial item in the sequence of event shutting down the package.

- Excitation Voltage
- Excitation Current
- AVR Supply Voltage
- AVR Supply Current
- AVR alarm output if available

This will give you a clue as to whether the AVR is 1) operating near it's upper or lower limits, 2) the output to the alternator is steady when the load is at steady state conditions. This give you a clue as to 1) whether the alternator is supplying a load not allowable in it's operating capability chart or has not be selected correctly or 2) if it is not steady, the control loop may not be set correctly, a problem with harmonic levels in the load or a problem with the any other part of the excitation system.

- Engine speed demand

- If the engine has an electronic controller (ECM), use the manufacturers software to trend any other internal alarms that may activate

- If the engine uses a non ECM governor, but does provide an alarm output, monitor this as well.

This should be checked as from previous experience, I would not discount the engine causing problems.

Hopefully, this should give you a clue as to what the sequence of events for the alarms is...

 

I would try checking the droop settings first. sometimes if one of the generators is set up for synchronizing it will start drooping the voltage when its run as a stand alone. so if it is an electronic system i will try copying the configurations on both units and going from there.