Kato Generator Voltage Issues

[michelin3x]

Good morning,
This is Michel. I have a concern and this is the first time I have heard of such an issue occurring. Any help would be much appreciated.

I have a Kato alternator that is a PMG set. Now since my AVR doesn't support PMG supply and works on the Generator bus voltage, the connections for the PMG are left disconnected. Now this is a brand new Generator that we just commissioned. Below are the technical details.
'The Generator came with an exciter coil, the terminals on the exciter were marked EF1 and EF2. We connected the positive of the Field supply from the AVR to EF1 and the negative to EF2 as per the standards. The first time we started the generator, we had to flash it, it build up voltage about 350V in Idle and 600V in Run. We let it run for a while, shut down the set and started it up again but the voltage wouldn't build again neither on Idle nor on Idle. Had to repeat the flash procedure. This continued about 4 times.
Based on our recommendations of our Senior Engineer, we were asked to reverse the leads to the field and try. This time the voltage did build up on its own but only on Run speed.'
Before I get to my questions, let me give you some insight into the system:
This is Ross Hill system on a rig, we have 4 generators that are self excited and 5th one has a PMG type excitation system, which as mentioned earlier the PMG supply has been ignored and supply has been taken from the Generator Bus for the AVR. On the other four generators, when we go to Idle(900RPM), we get about 350-400V and on Run(1200RPM, we get 600V.

Now here are my questions:
1. Why isn't the voltage building up when the connections are the right way?
2. Why is the voltage building up on Run speed when the connections are the wrong way?
2. Why is the voltage not building up when on Idle speed like the others?
3. Is there a way to confirm the polarity is connected the right way?
4. Would there be a difference in the properties between the PMG set and the other 4 Self Excited sets that's causing this?

Please share your thoughts and suggestions. Thank you very much in advance.

 

[waross]

1&2. Because of residual magnetism left in the brushless exciter. Generally running at full voltage will everse the residual magnetism, but not always. I am guessing that you were testing the generator without load.
2. Voltage buildup is a bootstrap process. The residual magnetism generates a few volts when the generator is turning. It may be less than 10 Volts.
This voltage is taken by the AVR and sent to the brushless exciter. This increases the output voltage slightly. With slightly higher input, the AVR has a slightly higher output. The process continues until rated voltage is reached and the AVR starts controlling.
This initial low voltage is proportional to speed. Apparently your new set does not start to bootstrap at idle speed.
3. It works or it doesn't work.
4. Maybe different iron in the brushless exciter field, maybe greater air gaps in the brushless exciter field. Many sets are designed for self excitation. A PMG may be added as an option. If a designer is designing for PMG excitation only he may not be concerned with bootstrapping performance.
Before Under Frequency Roll Off became a standard feature on AVRs, running at idle would toast an AVR in a few seconds. Those of us who worked on the old sets are in the habit of always running a set at or near full speed.
The AVRs were switched on the old sets and the AVR was not switched on until near full speed was reached. Anyone who idled a running set without first turning the AVR off would see the magic smoke within a few seconds.

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

 

[redlinej]

1.Do you have the spec for the AVR in question?
2. Do you have the connection diagram for the AVR?

 

[michelin3x]

@waross Thank you for your response. From what you have said, I too am in concern with the "bootstrapping performance". I am currently having this checked.

@redlinej
Thank you for your response. Attached is the connection diagram. Please note that this AVR is not designed for PMG operation.

 

[waross]

Have you checked the rotating diodes? This may be a sign of a failed rotating diode.
"But it's brand new!"
There are two kinds of engineers/technicians.
1. Those who have seen failed components on new equipment.
2. Those with less field experience.
I have found that the most difficult issue to overcome when a new piece of equipment has a failed component is the mantra from fellow workers;
"It can't be bad. It's brand new."

To compare this set with one of the other sets:
Disconnect the AVR from the gen-set and start the gen-set. Measure the output voltage of gen set with no excitation.
Do the same with one of the other gen-sets and compare the readings.
If the readings are similar, you may have an issue with the AVR.
Note that with the PMG in use, the set does not depend on bootstrapping to bring up the voltage.
As to the seriousness of this issue;
If this was my set I may never become aware of the issue for two reasons;
1. Historically most operators of my generation avoided running a set at less than full speed. Old habits die hard.
Anecdote on:
" For a number of years I was the system engineer of a small utility. We had about 5000 users and five generators with a combined capacity of 2.2 Mega-Watts. I was more concerned with planning, metering, troubleshooting, additions and changes than with day to day running.
I can't tell you the voltage at idle because I never saw any of the sets running at an idle.
Start-up and cool-down was typically done at about 95% of full speed."
Anecdote off.
2. The greatest number of sets that I installed were on standby duty. These sets would typically go from a cold stop to full load in under 10 seconds.
Life is hard for a standby set, but "That's Life".
Notwithstanding; your issue is interesting. As mentioned, it may be a sign of a failed rotating diode and is worth pursuing.
Please share your findings with us.

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

 

[redlinej]

If the no excitation voltage is within the AVR spec and diode are good; try changing the AVR and see if that solve the problem.

Looking at the AVR circuit, it looks like the AVR use the the genset terminal voltage and rectify it and use that to bulid up the initial voltage through K1 relay then the SCR circuit takes over; So the level of the no excitation is very important at start up for the AVR.

 

[michelin3x]

@waross I have taken into consideration the "its brand new issue" already.
The guy on site found good IR readings on the Exciter Stator and also found the diodes to be good. The system we work on with the self excited setup generates about 350V on Idle and 600V on Run speed. This works fine on the other 4 alternators. However, this one being a separately excited type doesn't take into account the residual magnetism requirement I suppose. I will let you know my findings. Thank you for your support.

@redlinej I already swapped the AVR with a known good AVR, no change to the symptoms.

 

[redlinej]

1.what is non excitation voltage at the genset terminals?
2.Compare the no load excitation field voltage and current(f+ and F-) with a working genset or with spec sheet.

 

[waross]

Try again. 350 Volts at idle. Not what we are looking for.
Try it with the AVR disconnected. That is the voltage readings you want to see. Probably less than 10 Volts.

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