Inrush current to the transformer 1

[asdqwert]

11Kv SYSTEM with only generator as power source, 2x2.2MW+3x4.375, the short circuit proctection setting for 2,2MW - I>1.2In, 10s, I>>2.5In; 4.375MW, I>1.2In, 5s, I>>1.5In, 0.5s.

We put one 10MVA transformer in, all of Genset shunt down due to inrush current of closing.

My question is how to set the correction setting; if the seeting is changed to standway from inrush current, the small genset could be damgaged or not.

 

[rbulsara]

How does it shut down? Does a breaker trip? Does a relay operate? What flag/function?

At minimum, the INST shall be set to accommodate 10 to 12X the rated current of the transformer for 0.1 second.

Assuming there is no fault on the system, get someone experienced to look at the protective settings and overall protection scheme. If the differential is tripping, it could be related to harmonics.



Rafiq Bulsara

http://www.srengineersct.com

 

[asdqwert]

The trip is due to low voltage. we believe it is due to over current, but why it is trip of low voltage.

can we block current protection for this case, I think differical protection is main one, after we finish start up , we can setting over current.

 

[rbulsara]

Check the time delays.

Are all generators on line (connected to the bus) before energizing the transformer or the first gen getting on the bus energized the transformer?

Make sure all gens are online before closing the transformer CB.

You need someone experienced on the site.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

Connect the transformer to the generator before starting the generator if possible.


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

 

[asdqwert]

2,2MW - I>1.2In, 10s, I>>2.5In; Un=75%, 5s
4.375MW, I>1.2In, 5s, I>>1.5In, 0.5s. Un=75, 5s.

Why Connect the transformer to the generator before starting the generator if possible could be ok or not, please interpret more.

 

[rbulsara]

It than acts as a soft starter as the voltage gradually builds up. It generally works for single generator-transformer unit. But not for multiple generator system, where the first generator coming to speed (and voltage) closes to the bus first. There the transformer will still see a sudden application of voltage.

Your settings are not of much use, unless translated in amps. Also you did not answer my previous question. How many gen trip? If more than gen is on line, the amps will be shared between them.

You may have other issues, I am not sure what testing was done so far and what else you may have in the system. You should have someone experienced on the site.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

If one generator is able to supply the no-load current of the transformer, then the easiest way to energize is to let the generator bring the transformer up with the generator. That avoids saturation and the rusulting current surges. Syncronise on the secondary side of the transformer.
Given the ratio between no-load current and full load current, it is difficult to imagine an installation where one generator is unable to energize a direct connected transformer. With no load current estimated at 2% of FLC, and allowing some overload capacity in the generators, it may take an installation with 50 or more paralleled similar sized generators before one generator may be too small to energize the transformer with a direct connection.
There are two methods;
If the generator is started with the field excited the Under Frequency Roll Off function of the Automatic Voltage Regulator will ramp the voltage up as the frequency ramps up. This is good for both motors and transformers. This method emulates a Variable Frequency Drive by keeping the Volts per Hertz ratio almost constant.
If the generator is up to speed and the AVR is then turned on, the voltage will ramp up as the field excitation ramps up. This emulates a Soft Start as the frequency is constant while the current is ramped up. If your set has a Permanent Magnet Generator to supply the field, use this scheme with caution the first few times. The voltage may come up very fast, but as long as you avoid saturation you should have no problems.
The second scheme is not so good for starting large motors as it does not maintain the V/Hz ratio.

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