synchronization of 4-generators in parallel 1

[odlanor]

A typical transformer unit of a power plant comprises:
- A step-up transformer with 3 windings 230 kV (star grounding) -13, 8 kV (delta) -13, 8 kV (delta)
- Two bus generation directly connected to 13.8 kV low voltage side of step-up transformer
- Each two hidrogenerators are connected to the bus generation through circuit breaker.
The high side of transformer is connected to SF6-substation through circuit breaker.

The ideal procedure for synchronizing the power plant to the 230kV system would be:
- Synchronize the four generators at the LV transformer by bus generation.
- Synchronize HV transformer to SF6-substation by controlling the set of four generators.

People of regulation and governor do not agree , because of circulating current between generators. Generators should be connected one by one.
Someone could explain the problem?

 

[davidbeach]

It is very difficult to have four unloaded generators operating in parallel without some circulating current.

As far as I'm concerned, the ideal order would be energize the transformer from the 230kV side and then parallel each generator one at a time across its own 13.8kV breaker.

Obviously that wouldn't work for black start if that is a concern.

 

[odlanor]

my problem is black start!

 

[davidbeach]

Then bring up one generator, add some load. Bring up a second generator, add more load. Repeat as necessary.

 

[odlanor]

I am understanding that for 230kV enrgization will be:
...from the 230kV side and then parallel and add load each generator one at a time across its own 13.8kV breaker.

 

[waross]

If you connect the transformer to the first generator before the generator is started and ramp the transformer up with the generator it may be the friendliest method of energizing the transformer. Then synchronize with the substation or grid. The pick up some load and proceed to sync and load the other generators one by one.
You could energize the transformer from the 230 kV side but bringing it up with the first generator avoids inrush issues.

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

 

[odlanor]

correction:

...Then synchronize(at 230kV) with the substation or grid. The pick up some load and proceed to sync(at 13,8kV) and load the other generators one by one.

 

[rhatcher]

davidbeach has good (lps) answers

 

[odlanor]

I think this would be the sequence of switching:

1- normal synchronization
the ideal order would be energize the transformer from the 230kV side and then synchronize and add load each generator one at a time across its own 13.8kV breaker.

2- black start:
-connect the transformer to the first generator before the generator is started and ramp the transformer up with the generator and connect to substation 230kV ( previously desenergized)
-Connect 230kV transmission line up to remote busbar substation.
- Add some load.
- For other generators synchronize and add load each generator one at a time across its own 13.8kV breaker.

My concern is the inrush current of transformer in black-start over generator:- If we assume a value of 10 times rated current transformer, this will be 40 times rated current generator.

 

[davidbeach]

There is no transformer inrush when you bring the transformer up with the generator. That's the whole point of starting the first generator with the transformer already connected.

 

[waross]

Further to David's last post. Bringing the transformer up with a generator will avoid inrush issues on the grid as opposed to energizing the transformer from the 230 kV side.

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

 

[odlanor]

....Bringing the transformer up with a generator will avoid inrush issues on the grid..


At another power plant with a similar case, they should modify the ramp of automatic starting of AVR and other precautions.
I do not know how the energization sequence of the this power plant.
In what case , could that happened?

 

[waross]

There are two methods for transformers.
The frequency and the voltage may be ramped up together to maintain the Volts/Hertz ratio.
The generator may be brought up to speed and then the voltage may be ramped up.
The second method is not suitable for starting relatively large motors.

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

 

[odlanor]

Transformer energization of a single generator could be done without causing any inrush current with manual starting;
- could increase tension in the generator very slowly to the rated value.

Manual starting is only for maintenance.

The automatic starting by regulators is the operating procedure.
In this case there is the risk of inrush current.

 

[davidbeach]

Start the generator with the transformer connected. Ramp to speed. Let the regulator bring the voltage up. No inrush.