Generator Behavior To 3-Phase Short Circuit

By definition, there is no voltage or frequency coming out of the machine during a 3-phase short circuit fault.

http://cr4.globalspec.com/thread/12082/Generator-behavior-to-3-phase-short-circuit

PWSlack

for your information: during the fault the speed of the engine go down due to the high torque so the frequency go down as well as the voltage, then after the fault the voltage and frequency start to recover to go back to normal values.

but the curve can show you the recovery period and this is very important for the protection settings.

That would be the answer then.

You had better re-consider your statement that the speed goes down during a 3 phase short.

Remember the prime mover input horsepower remains the same, and the KW is the horsepower out, but the kilowatt's (by definition) is the Voltage times the amps (times pf).

In this case you have bucoe amps but the voltage is zero (short circuit)

Bucoe amps times zero is zero, so the horsepower out is zero.

You are still putting the same horsepower into the machine, There is no horsepower out and there is no voltage to hold synchronization, the machine will speed up until the governor grabs it.

This is for real, Many people do not understand this.

Also keep in mind that the short circuit currents are almost all reactive (watt less) power. The load is a coil on a piece of iron,

Snakers

Whatever the French word is for BANG!

This can be sustained only by bypassing all protection. The AVR on the generator will try to correct the voltage as voltage decays rapidly in the process trying to increase the excitation trying to sustain the fault current..So the rotating rectifier assembly on alternator can blow,AVR can Fail and generator also may burn out.The engine will try to deliver the torque but likely to stall when it is overloaded.

If you really want to go around blowing up generators please take pictures so we can all see the results. Big generators make bigger bangs

The response curves should be available from the diesel generator manufacturer. They have all of the required information. The voltage and frequency curves are a mathematical outgrowth of the current curve in your original post.

To generate new curves you would need a great deal of information, such as the inertial moment of the rotating members, the response characteristics of the governor and voltage regulator, and the transient and sub-transient reactances of the machine, to name just a few. I have had to create these curves for new-design generators. The process is very tedious and time consuming. If the manufacturer is still in business, I highly recommend contacting them.

Sir. If you have a short circuit at the terminals of the machine. the voltage is ZERO

The current is limited by the internal inductive reactance of the machine and the winding resistance.

Your Graff does not reflect this.

A short is zero resistance There can be NO VOLTAGE at the terminals.

Your Graff suggest that the fault is down stream and not at the machine terminals and there is some impedance between the fault and the machine terminals.

But this is not compliant with the question.

Please explain your comment further....

Snakers

In a theoretical ideal circuit, voltage would indeed go to zero instantly. In the real world it is not possible to ignore those pesky "negligible" values. If the physical response to a bolted fault actually occurred as Snakers says, you could throw a steel pipe across the generator output bushings, and no arc would occur. I can personally affirm that a high power arc and accompanying serious damage will indeed occur.

There is indeed a small amount of impedance in the remaining circuit due a variety of factors, including the internal leads from the winding ends to the bushings, connection resistance, internal impedance of the phase coils in series with each other, residual magnetics in the core iron, etc.

The total circuit impedance however, will drop to near zero as soon as the fault connection is made. The time difference between impedance change and voltage collapse causes the current surge.

On the other hand, I agree with Snakers that the generator would not be able to sustain the fault current for the time shown on the graph. If you're lucky, the torque on the shaft would probably tear the coupling apart before you burn up the windings.

The only one who got a correct answer for the limited information supplied was of course:-

pwr2thepeople

Many thanks for your input Pwr2thepeople, the rest of you MAY be right, but with such poor infos, that is not 100% possible say either way.....

The way a generator reacts to short circuits is "built-in" to the generator.......by the manufacturer.It is possible for example to have a "Drooping", or "flat" or "rising" characteristic when more current is drawn....

"Drooping" is usual with some sort of automatic excitation control......

If the short circuit is at the generator terminals (upstream from the current transformers. Then the speed droop will see either zero current (or reverse current if there is another machine running in parallel.)

I have seen speed droop on Hydro machines with Woodward governors but not on steam or DG machines. Is it common on DG machines.

Snakers

Maybe or maybe not!!

What do you mean maybe or maybe not..

Are you challenging the Electrical Engineering Handbooks or are Gods laws different in Germany?

If you read my earlier posts, you will find out (to your advantage) that the way an AC generator reacts to extra load and overload etc is a function of its physical design and of its controller (if it actually has one!! Not all have, but you may have been wrongly informed otherwise to this point).

I am sure God understands these laws far better than you do, so you might need to pop into a Church, get on your knees and pray for forgiveness for your most unfriendly and ignorant post!!!

If you had majored in Electrical Design, you might not have made such a blunder, if you did, then you did not learn enough!!

Generally speaking for the usual applications (I am sure that someone else will now jump down my throat for these generalizations!!)AC Generators are designed with a 45% (or close) droop characteristic and have some sort of excitation controller. Only then will it possibly react in the way you have fixed in your mind!!

Try and think laterally, take off the blinkers, it will get you far in this world.....

I trust that you have managed to learn with my help two things today, one about good manners and the other about AC Generators and their design parameters....

Have a great weekend.

Andy.. I just got home from Playing Pipe organ for church services.

Your comments have opened the door for me to testify as to the Lords Work. You are very correct that God understands his laws better that I do, And why not, He made them.

He also allowed me learn and apply many of them, so that I could better serve him as a good Engineer. God has blessed me with 80 years and I thank and praise him daily.

By God's grace, I have had the opportunity to work on many Hydro generators, from small 50 KW, a 12 MW Kaplin, 3 Pelton, a 30 MW Allis Chambers, 2 40 MW Westinghouse Vertical machines.

I give God the credit that I have been allowed to work on installation, testing and operation of many machines including two Topping turbines, 9 steam machines from 70 MW up to 210 MW, and my most exciting experience was being the engineer in charge of conducting Singe and 3 phase short circuit test, (Low field excitation) On a NUCLEAR 470 MW Westinghouse machine to obtain and plot the short circuit shaft torque harmonic characteristic's for the US Atomic Energy Commission. (we were investigating L3 turbine blade failures.)

I have co-authored 2 accepted papers with EIEE and have served on several power study committees.

Andy, As professionals, It does not behove either of us to get engaged in a Hissing contest. My comments were common to all basic generators, regardless of core, coil or air gap design. They do not apply to any field or governor control functions.

They were based on Gods Laws and supported from the handbooks, and demonstrated in my testing experience.

You have made some good engineering comments, Lets put this to bed..

Oh and I did have a good weekend, Thank you Lord , A'men.

Don

To copy your last line in full agreement:-

Oh and I did have a good weekend, Thank you Lord , A'men.

Andy

It depends on the type of generator, Permanent magnet, self excited or self excited with current boost, I've seen self excited with short circuit faults just collapse the field and go to no output, others the engine bogs down until the fault clears or something burns up, the PMG and current boost are for fault clearing, I just I would weigh in.