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Cat Genset Excitation Problem

07/03/2017 8:42 PM

Hello guys and gals

I got a call today about a 113kva cat genset that is not generating. Of course the first thing i did was check all resistance values of all windings and everything was fine. Next thing i checked the avr ( very old type with huge capacitors, one bulky transformer and lots of big thyristors). I flashed the excitor stator with a 12 volt battery and i got a reading of 110 between phases and 90 between phases and neutral ( the generator is 400v 50hz and the reading between the phases without flashing is 2.8v). I have one universal avr that i use for testing ( it puts out 100v max at a 240v sensing voltage with a load current of 8A excitation current and the generator has an excitation voltage of 39 and current 7.6A on its nameplate). So i connected my avr and started the engine and the output voltage risen up to 390v and dropped to zero immediatly, after that, i couldnt get any reading in the main winding even if i flash with battery! I suspect either the diodes or the varistor so i will check those tomorrow but what do you think happened? The main winding voltage is still at 2.8v and doesnt change at all when i apply the battery voltage, and theres a current of almost 2A running from the battery to the excitor stator winding so that means the windings are ok but there not generating any power on the excitor rotor so that means either the diodes or the varistor right?

another question, when the value of excitation voltage and current on a name plate are given, are those value at full load? And when an avr has a maximum volatge of say 90vdc how does it adjust to the full load excitation voltage of the generator as in the nameplate?

for example, avr says max output voltage 90vdc @240vac

generator says 39v (im sure with full load at also 240v)

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#1

Re: Cat genset excitation problem

07/03/2017 9:41 PM

Need a lot more info for any help I think....

https://www.smokstak.com/forum/showthread.php?t=161837

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#3
In reply to #1

Re: Cat genset excitation problem

07/04/2017 5:04 AM

what kind of info?

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#6
In reply to #3

Re: Cat genset excitation problem

07/04/2017 7:44 AM
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#2

Re: Cat genset excitation problem

07/03/2017 10:01 PM

I'm assuming that DC current in the exciter field produces AC in the exciter armature, which is rectified and fed to the alternator armature which in turn produces the AC output from the alternator field windings. It would seem that the likely fault is in the rectifying circuitry. The 2.8 volts is probably due to residual magnetism in the alternator armature.

Something changed when you connected your universal AVR, maybe coincidence, maybe not. Maybe it wasn't sensing the output voltage correctly and increased the excitation until the diodes burned out. (Just guessing).

"Brushless Exciter

Another method is the brushless system. In this system the armature of the exciter is on the rotor shaft itself. The DC output of this armature, after rectification by solid-state devices, goes to the rotor coils. Since the armature and rotor are on the same rotating shaft, this eliminates the need for slip rings. Hence it reduces maintenance and operational requirements and thus improving reliability."

http://www.brighthubengineering.com/power-plants/45650-power-plant-generators-what-is-excitation/http://www.brighthubengineering.com/power-plants/45650-power-plant-generators-what-is-excitation/

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#4
In reply to #2

Re: Cat genset excitation problem

07/04/2017 6:18 AM

Something changed when you connected your universal AVR, maybe coincidence, maybe not. Maybe it wasn't sensing the output voltage correctly and increased the excitation until the diodes burned out. (Just guessing).

Thats what i also think happened.

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#8
In reply to #4

Re: Cat genset excitation problem

07/04/2017 11:13 AM

I think somebody made a boo-boo! Ouch! That is going to cost.

Was there any dummy load connected to the genset when fired up with the avr connected?

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#9
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Re: Cat genset excitation problem

07/05/2017 4:41 AM

no load.

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#5

Re: Cat genset excitation problem

07/04/2017 7:10 AM

so i have been reading about minimum field winding resistance for avr. I dont know what the minimum field resistance for my avr but the resistance of the generator field is about 7 ohms. and i tested my avr now by connecting a 7 ohm resistor at a sensing voltage of 240V, the dc output of the avr was 90VDC with 13A. so i think what happened was that the generator was overexcited by the 90v avr output, the generator has an excitation volatge of 39V. this created high enough voltage to damage either the diodes or the varistor.

my question here is, when i look in the datasheet for most avrs, espicially cat avr, they all list an output voltage of about 80 to 120vdc max at 240V. and they generator excitation voltage is 39V. how do you selected a suitable avr? what does the excitation information (current and voltage) do for me? do i have to look for an avr that has an output voltage of 39V?

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#10
In reply to #5

Re: Cat genset excitation problem

07/05/2017 8:23 PM

Your voltage regulator from your description is a controlled excitation supply, a combination generator voltage regulator and exciter. It needs to supply the field at minimum with the current or voltage as stated on the exciter nameplate. It's current that builds the field, so your field will be at full design strength at the 7.6A current, likely 39 volts when the machine is warm. The machine should be spinning with that much current on it, so it doesn't overheat the field windings.

Your SCR field supply will phase back once the set line voltage is achieved on the generator main terminals, which will be something less than 7.6 amperes/39 volts. Using your spare regulator should have worked. There should be nothing on the generator like diodes unless you have a brushless exciter.

However, you mention using a battery with a 2 amp flow to the exciter stator winding, which might lead us to think you have a brushless exciter, tho seems a little overkill for a 113kVA generator. You need to be spinning the generator with the engine to generate voltage at the leads, with your excitation applied. The spinning magnetic field of the rotor passing through the stator conductors is what generates the 400V at the stator terminals

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#7

Re: Cat genset excitation problem

07/04/2017 10:28 AM

Although the way an AVR works may be "automatic", it needs to be set up "manually" to match the excitation system type and generator that it will be controlling. If you know nothing about the genset, then you must set all the controls on the AVR to their manual minimum settings.

When the machine is up to speed, slowly increase the gain/setpoint upward until the generator's rated voltage is reached. If that doesn't happen reset the AVR to minimum then flash the field with your battery, but make sure there is a current limiting resistor in series with the battery. Don't leave the battery connected to the field circuit or you could easily raise the output voltage way above its limits.

As a general rule, any time you work blindly without knowledge about the equipment you must start low and slowly work your way up to rated voltage/current, otherwise you risk finding yourself in the position that you are in now.

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#11

Re: Cat Genset Excitation Problem

07/11/2017 6:58 AM

so I fixed the generator. turns out that my avr is damaged and gives maximum power no matter how you set the voltage potentiometer. the diodes where ok but the surge diode is damaged because of the high voltage of the avr I persume. it is a Selenium surge suppression diode. so i removed it and i also found a resistor connected between the positive plate and the ground (the shaft of the rotor) it was all rusty and leads broken so I just left it as it is. the problem where I live is that there are no spare parts available so I have to work with what I got. so surge suppression is out and ground resistor is out.

a new universal avr was connected and adjusted and the generator now works fine. I have been reading about the purpose of the surge suppression and its used to protect the diodes from high transient voltages, the generator can work without it but the diode would be vulnerable. my question is, what cause the high transient voltage? are they common? and does it affect also the winding of either the exciter or the rotor?

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#12
In reply to #11

Re: Cat Genset Excitation Problem

07/11/2017 8:58 AM

Selenium is not as robust as silicon, the last selenium rectifier stacks I remember seeing were from the 1960s. Each junction only has a PIV rating of about 30 volts. Plus they exhibit an aging phenomenon, with an even higher than an already high forward drop, and higher reverse leakage current.

See this..

http://w3hwj.com/index_files/RBSelenium2.pdf

Any time the power gets turned off a large inductor, you can get a high voltage (whether it appears across the diode depends on the circuit design). My guess is the rectifiers failed only due to old age.

The diodes you are talking about are in the regulator itself, not mounted to the generator shaft in the case of a brushless or pilot permanent magnet exciter? There was some confusion when you mentioned the exciter rotor, which implies a second generator on the shaft supplying the excitation (DC) for the main AC generator.

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#14
In reply to #12

Re: Cat Genset Excitation Problem

07/11/2017 3:53 PM

The diodes you are talking about are in the regulator itself

what do you mean by "the regulator". I'm taking about the rotating exciter rectifier diodes. the generator is separately excited but not PMG. Maybe I got the names wrong but there are exciter stator where the avr is connected, exciter rotor which is connected to a rotating rectifier, and the main rotor. Am I right?

The article talks about replacing selenium rectifiers with silicon one, but I understood that this procedure is done when the selenium is used as a regular rectifier. is it that same when the rectifier is used as a surge suppression? is it a different type of rectifiers?

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#15
In reply to #14

Re: Cat Genset Excitation Problem

07/11/2017 6:40 PM

Yes, you are right, I wouldn't have thought that a 113kVA generator would have a rotating exciter, my ignorance.

You should try to replace any selenium rectifier stacks with silicon, it's the same principle for surge suppression as power conversion, though ratings & construction may differ somewhat. Certainly silicon will outperform selenium in any application, other diodes designed for switching service and specialized applications requiring low forward drop or quick turn-off will use other materials, but usually not built to be robust enough for surge suppression.

The original circuit designer or service organization will be the best source to evaluate whether the selenium diodes higher forward drop was important in some circuits. They should have been through this within the past 2 decades. You can always count up the plates and estimate the PIV required.

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#16
In reply to #15

Re: Cat Genset Excitation Problem

07/12/2017 9:28 AM

Then there is new stuff transistors etc. based on GaN, not silicon. Supposedly these diodes and transistors are superior in power electronics applications.

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#13
In reply to #11

Re: Cat Genset Excitation Problem

07/11/2017 9:22 AM

Think of it as lightening protection of a type.

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