PRIMARY SHORT CIRCUIT TESTS

[xxjohnh]

Does any one still do primary current testing using the actual large generator? It seems to be seldom done anymore.
I witnessed some line impedance measurements and short circuit relay tests using a 300 MW Hydro unit. The generator voltage was increased slowly till trip values are reached.

 

[GTstartup]

European OEMs are still doing this as a matter of course. I don't know about hydro but it's done on gas turbines all the time. As you say, place a short circuit on the bus, set the AVR minimum to zero, close field breaker and slowly increase field current until you get the desired stator current.

 

[slavag]

Hi.
I don't know about hydro units too, but for steam and gas turbine ALWAYS provided few primary short circuit tests.

3-ph and 1-pf. Before GSU and after GSU.
Tested generator and block generator-transformer differential protection stability. Underimpedance protection, unbalnce protection, 100/95% stator earth fault protection and GSU high side earth fault protection.

Good Luck
Slava

 

[rcw retired EE]

European generator suppliers and power plant designers usually perform the primary current tests on generators. It is not a typical ANSI/NEMA/North American practice in my experience.

Primary testing does verify more of the complete system and allow heat runs on the equipment. If the shorting mechanisms are not designed into the system it can be very difficult to set up the tests. The individual components of the system are already tested (protection relays, CT circuits, bus duct) so performing an additional test on the complete system is difficult to sell to start up managers.

The biggest reason for not doing seems to be financial. It takes a few more days at the end of a project to do this test, more if the design did not anticipate doing it. If the plant is late getting on line, the liquidated damage penalites for each day of delay can be large.

 

[slavag]

rcwilson, you are absolutly right.
It's take .....up to 2-3 days.
test with generators with big steam turbine..not simple things, you need down to turngear, startup again....
AVR guys request this tests too.

 

[ScottyUK]

One useful trick is to use the HV earthing switches on the GSU transformer circuit to force current through windings which you want current to flow in. With a bit of creativity it's possible to simulate quite a few conditions on the LV side, and the currents are much more manageable than trying to short out maybe 15-20kA.


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If we learn from our mistakes I'm getting a great education!

 

[rcw retired EE]

Scotty -That's my favorite way of doing the test.

Another method that works well is to jumper across the generator windings in the HV termination enclosure and use a 3-phase portable generator to inject currents from the High Voltage switch yard. Sometimes the construction power can be used to backfeed from the 230 kV yard to the generator neutral.

The major impedance in the circuit is the GSU transformer so a low voltage generator can push enough current to verify differential circuits, CT polarities, and some protection functions. The test can be carried out in sequence, moving the shorting location around the plant to test many circuits.

A big advantage is that the turbine does not need to run, which saves the cost of fuel and avoids other scheduling issues. Usually the electrical work is far enough ahead of mechanical that this test does not impact the over all schedule.

Disadvantages are the current is low (0.005 tp 0.25) per unit and there is no test of the excitation system.

 

[ScottyUK]

Used that one as well.

The rental people usually get uncomfortable (or question your sanity) if you tell them you have a 3000A single phase load, or if you mention disconnecting the AVR and controlling the field manually. I like to let them sleep easy at night, and usually forget to tell them.



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If we learn from our mistakes I'm getting a great education!

 

[xxjohnh]

Primary injection using the generator and the AVR turned down seems rare now. Most plants are late and a few days means lots of money. Unless the owner has it in the original specification it seldom gets done. Even when in the spec some vendors refuse. Most of the newer start up people have never done it. Thanks for the input we live in changing times.

 

[slavag]

Hi.
Im not sure about most of the newer start up people.

I was worked last time with GE, Siemens(twice) and Alstom ( gas/gas(steam)/steam turbine 70MW,70MW/12MW, 250MW). We was done primary test.
But with short procedure, you are right, it's lot of money.

I have also expirience with commissioning w/o primary test.
With newer relays you can see lot of parameters on-line and correct them on-line, but not all ( not stator 100%/95% earth fault, not GSU earth fault, not REF protection).

OK, all depend on, who take responsible for cancelled primary test.

Best Regards.
Slava

 

[odlanor]

In my utility we always make these tests in Hydrogenerators(40,10,200,300 up to 400MVA)

 

[odlanor]

Does any one knows how to make primary current testing for distance relay perfomance?
It seems to me that the method to increase slowly field current until you get the shrot circuit is not effective.
You have to impinge a short circuit with rated voltage .