Cat Genset Excitation Problem

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

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

what kind of info?

Model numbers of the gen and avr might help..

http://www.partsfortechs.com/asapcart/cat-vr6-replacement-avr-for-caterpillar-vr3-and-vr4-p-1137.html

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/

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.

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?

no load.

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?

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

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.

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.

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?

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.

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

Think of it as lightening protection of a type.