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Kato Generator Voltage Issues

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.

Super Contributor

Re: Kato Generator Voltage Issues

So first I'll answer your questions,

 

1. Why isn't the voltage building up when the connections are the right way?

    There may not be enough residual magnetism in the rotor for the AVR in the Ross Hill controls to be able to apply power to the field, have you measured the field output when this occurs?
2. Why is the voltage building up on Run speed when the connections are the wrong way?

     First look at what the field ouput is doing, is it going from zero the a level that supports the no load voltage buildup? Not unheard of for F1 and F2 to be mis-marked. On some generators field polarity makes little or no difference, some generators are polarity sensitive and some generators actually use reverse field voltage to suppress the field-called negative field forcing.
2. Why is the voltage not building up when on Idle speed like the others? Is it a zero volts at idle, or some smaller value?  Again, what if the field output when this is happening?
3. Is there a way to confirm the polarity is connected the right way?

    Best way is to measure field volts at rated speed no load with unit at rated voltage, lowest field output to attain rated volts is the "best" connection.
4. Would there be a difference in the properties between the PMG set and the other 4 Self Excited sets that's causing this?

    Newer generator may have a rotor design that assumes with PMG power it doesn't have to provide enough residual magnetism to produce minimum voltage needed for a SE type AVR

 

Have you just run the generator with fields leads lifted at rated speed (no excitation), do you get ANY residual voltage at the generator terminals?  Try this right after you have run the unit at rated speed and voltas, then stop the unit, lift the field leads and restart (make sure you disable the AVR), if you get no residual voltage you may have a ground in the rotor, or a problem in the excitation system, Understand it aa new machine, but it could have been damaged in transport or installation or just bad from manufacture.  I would do a set of electrical tests on the generator to make sure.

 

Hope that helps, MikeL.

Contributor

Re: Kato Generator Voltage Issues

MikeL Thank you for the detailed explanation, it's the residual magnetism I am concerned with too. 

 

Response to a few of the questions you have asked

1. Voltage at Idle is 0V with the wires lifted to the exciter.

2. The electrical megger checks have been done, they have got readings of >550Megohms. 

 

Yes, we have run the generator at rated volts, shut down the generator and started up again with the exciter leads disconnected. We get 0v. The only difference is: On run speed with the exciter connections reversed, the voltage builds up on its own to 600V without needing to be flashed. 

 

Thank you,

Michel

 

Super Contributor

Re: Kato Generator Voltage Issues

The Ross Hill AVR may have a field flash circuit installed as part of its design, I know many of the older ones did. In that case I don't think the Ross Hill AVR will try to apply field volts until the engine speed is close to rated.  I have seen a cople of differnt Ross Hill AVR versions, so I'm not exactly sure what you have, but if it working with the "leads reversed" and following the field output from the AVR, I'd say things are probably wroking ok. Just may have a set of reverse marked leads.

 

Mike L.