Feed fault testing on AC generator? 1

[Baswm]

A 40 KVA AC Generator, 400 Hz, 3 phase 115V, 6000 rpm.

Reading the General Specification sheet, Under Feeder Fault, it says THE SYSTEM SHALL PROVIDE FOR THE GENERATOR TO BE DEENERGIZED AND DISCONNECTED FROM THE LOAD BUS WHEN THE FAULT CURRENT ON ANY PHASE EXCEEDS 25 AMPERES.

I took that to mean if I have no load on phase A or B, and I put a variable load on phase C and adjust it so the current increases. When it exceeds 25 Amps, the GCU should take the generator off line. I measured up to 50 amps on phase C and nothing on Phase A or B and the Generator was still going strong.

If I short two phases together or any phase to neutral, the Generator goes off line after 6 seconds.

How do I check for feeder fault?

 

[waross]

Really we need to know a little more about your system.
Define fault current. Phase to phase, phase to ground, phase to neutral?
Is the system delta or wye? Does it have a neutral and/or any type of grounding?
From the information given, you would need differential protection on the feeders to be sure of detecting a feeder fault. (I fearfully assume that a feeder fault is a fault in the feeder cables, not a fault in the generator or the connected load.
We sometimes use a test set to check the protection settings so as to avoid abusing the generator with intentional faults.


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

 

[rbulsara]

Baswm:

1. Do not repeat, ever, what you did to check for the "fault" with any electrical system.

2. In order to check for fault, first you calculate the available fault currents and then select the protective device/settings accordingly. Then you bench test the protective device using test sets to verify that they operate as intended.

You do not crash a car to the a light pole to check whether it air bag system works! Although it may prove it for once!

Let the experienced and qualified people test the system.

You are lucky in a way to be able to ask this question here after what you did. Firstly, do not short any of the phase conductor to ground or to other phase and run a machine or turn on a switch. You could have injured or killed your self and even damaged the machine. Luckily the overcurrent device seem to have operated in time to limit the damage. The saving grace was that this was a generator and having the phases shorted out prior to starting (I presume) helped the fact that it may not have developed full voltage and hence took 6 seconds to trip.


By the way the machine is rated for 115.9 A full load so in normal load conditions it will not trip until it reaches beyond that limit, if the protective settings are correct.

The 25A fault current limit is not making much sense. It could be a typo or referring to a particular type of protection system such as a ground fault.

Rafiq Bulsara

http://www.srengineersct.com

 

[Baswm]

1. Do not repeat, ever, what you did to check for the "fault" with any electrical system.

ACTUALLY WHAT I DID WAS FINE. BETWEEN THE DRIVE STAND AND THE CONTROL STATION, I HAVE THE INTERFACE BOARD THAT ALLOWS TESTING OF THE GENERATOR BY THE GCU. THE GENERATOR CONTROL UNIT PROTECTION FUNCTIONS ARE BEING TESTED TO ENSURE IT WORKS BEFORE I ALLOW IT TO GO ON THE AIRCRAFT. I HAVE TESTED THE PHASE TO PHASE SHORTS AND THEN THE PHASE TO NEUTRAL SHORTS AND THE GCU UNIT REMOVED THE LINE VOLTAGE AS DESIGNED WITHIN THE SPECIFIED TIME. THIS UNIT HAS BEEN FLYING SINCE 1994 ON FOREIGN AIRCRAFT SO I AM VERIFYING IT WORKS.

2. You do not crash a car to the a light pole to check whether it air bag system works! AMEN TO THAT.

Let the experienced and qualified people test the system.
THEY ARE THE ONES WHO SET THE SYSTEM UP FOR ME.

You are lucky in a way to be able to ask this question here after what you did. IF YOU COULD SEE THE SET UP, YOU WOULD SEE IT WAS SAFE AND PROTECTION DEVICES SET.

Firstly, do not short any of the phase conductor to ground or to other phase and run a machine or turn on a switch. You could have injured or killed your self and even damaged the machine. Luckily the overcurrent device seem to have operated in time to limit the damage. The saving grace was that this was a generator and having the phases shorted out prior to starting (I presume) NOPE. I WAS RUNNING IT AT 6000 RPM NOMINAL AND THEN RECORDING THE POR, PMG, CONTROL FIELD VOLTAGE AND CURRENT WHEN WE TESTED THE SHORTS. WE ARE SAFELY OUT OF THE DRIVE AREA AT THE CONTROL CONSOLE AS WE PROGRAM THE SHORT. NO WORRY OF LIFE THREATING EXPLOSION.


helped the fact that it may not have developed full voltage and hence took 6 seconds to trip.By the way the machine is rated for 115.9 A full load so in normal load conditions it will not trip until it reaches beyond that limit, if the protective settings are correct.The 25A fault current limit is not making much sense. NOT A TYPO. I FOUND OUT WHAT IT MEANT. IT IS TESTING THE DIFFERENTIAL CURRENT. THE MANUAL JUST CALLS IT FEEDER FAULT. THE CURRENT GOING INTO ONE CT MUST EQUAL CURRENT COMING OUT. I NEED TO ADD IN A SLIDE RESISTOR TO GRADUALLY ALLOW CURRENT TO SLOWLY GO TO THE RESISTOR AND SEE AT WHAT CURRENT THE GCU WILL TRIP. SHOULD BE BY 25 AMPS DIFFERENTIAL.

WHAT I WAS DOING WAS FINE, JUST AN UNBALANCED LOAD BY 25 AMPS ON THE ONE PHASE. I READ THE REQUIREMENT AS UNBALANCED CURRENT DRAW ON THE PHASE WHICH DID NOT MAKE MUCH SINCE AND EXPLAINS WHY IT DID NOT WORK. NOW I KNOW TO TEST FOR DIFFERENTIAL. THE TEST GROUP WILL INSTALL THE SLIDE RESISTOR FOR ME TOMORROW.

rbulsara, Thanks for thinking of my safety.

 

[rbulsara]

Well, I stand corrected. I think you are very capable. Thank you for enlightening me. I had no idea of your expertize!



Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

Baswm

It's obvious you & your "test group" did not understand what that "fault current" meant and how it would affect the generator. So any test setup by you & them is inherently suspect. I would listen to Rafiq if I were you.

And stop "SHOUTING".

 

[brutus1955]

i have little to add, but a couple of questions

it appears your alternator exhibits behavior very much in keeping with that of an automotive alternator of the lundell design, in that it is heavily reactance limited.

that would explain the 50amps across one phase with no damage, due to huge voltage drop across the reactance of the machine.

anyway, my question

can you give me a link to your machine, make, model
anything? is it a clawpole design/type of rotor?

physically about how large is it? guessing a bit larger
than a 5 gallon bucket, ok maybe half again bigger?

very interested to see a pic or anything about your unit.

thanks
bob g

 

[Baswm]

I did not mean to SHOUT, I was using all caps to seperate an answer to the question. I did not see the italics code otherwise I would have used that instead.

I have 20 years experience in space flight instruments, infrared systems, and was working on a retena prothesis device before accepting a job closer to home with a nice raise. I am no power expert and do not have the "hands on" experience as alot of you do with power systems so this is why I posted my question. I was hung up on some terminology.

As far as testing safety goes, the test division hooks up everything to do the test. My main question was with feeder faults. The way I understood the defination, i approached it as a current imbalance on the phases, which to me did not make sense, since there will be variable loads on the aircraft depending on what system is on (3 phase or single phase). Hence, the GCU was fine with it as I assumed.

When I wrote the question, I did not know it was referring to differential current protection.

I very much appreciated Rafiq Bulsara post and understood where he was coming from. I am letting the experts set up the system and install / run the equipment while I take the data and see that the measurements are with in the specified ranges.

This GCU I am testing is fielded, and getting high reliability. I am qualifying it for our use since all the qualifying data was tossed out by the army when they changed headquarters back in the 90's.

Bob,

The AC generator is a brushless generator designed to provide three phase output at 400 Hz with voltages at 120 vac phase-to-neutral in a wye configuration, or 208 vac phase-to-phase to the electrical system.

The machine is your typical drivestand like this avtron K938 Test Stand.

http://www.avtron.com/electrical_test_equipment.htm

We have the AC Generator bolted onto the drive stand, then the inhouse test box that interfaces the generator to the test stand.