Diesel generator and transformer inrush

[Alex68]

A client of mine asked me to make a list of the requirements of a diesel generator which must be able to withstand the inrush of one transformer.

Generator data:
Rated voltage: 6.6 kV
Rated Freq: 60Hz
Rated power: 2.1 MW
power factor: 0.8

Transformer:
Vltg ratio: 6.6 / 0.45 kV
Rated power: 1.6 MVA
Inrush current: < 8*Irated

Could you suggest any requirements of the generator or the transfomer?

 

[Alex68]

RAVWARD,

Point 1. I can't be sure that the transformers are de-energised for a long time before a new inrush. I must guarantee reliability in any condition above all in case of emergency.

Point 2. Connected loads dump inrush currents because they add impedences in parallel to the magnetising impedance of the transformer.

But the operative requirements you suggested can't be respected in case of emergency in a ship.

 

[davidbeach]

I guess I don't have a full picture of how the system is arranged. You have four generators and four transformers; are there any connections between generators/transformers other than just one generator to one transformer? If there is not a means of paralleling generators ahead of the transformers, it would seem that a control scheme could be worked out that would eliminate transformer inrush by energizing the transformer as the generator builds up voltage.

 

[Alex68]

I tried to depict the scheme of the system:

| | | |
( ) ( ) ( ) ( )
( ) ( ) ( ) ( )
| | | |
X X X X
| | | |
--------------------X------------------
| | | |
X X X X
| | | |
(G) (G) (G) (G)

-Two busbars (one at the forward end of the ship and one at after end) with a coupler,
-two transformers and two generators for each busbar
- each components has a circuit breaker

Any configuration is possible

 

[Alex68]

Sorry for the previous figure

| | | |
( ) ( ) ( ) ( )
( ) ( ) ( ) ( )
| | | |
X X X X
| | | |
--------------X------------------
| | | |
X X X X
| | | |
(G) (G) (G) (G)

 

[davidbeach]

Well, in that case you are probably stuck having to deal with transformer inrush currents.

On a side note, when you want to do ASCII graphics, put them in a code box and you will get fixed pitch spacing as:

    |    |    |    |
   ( )  ( )  ( )  ( )
   ( )  ( )  ( )  ( )
    |    |    |    |
    X    X    X    X
    |    |    |    |
------------X------------------
    |    |    |    |
    X    X    X    X
    |    |    |    |
   (G)  (G)  (G)  (G)

That is done using the [ignore]

 and

delimiters.[/ignore]

 

[rbulsara]

ravward:

I do not think any one of your two points matter. The system shall still be designed for the worst case.

For all practical purposes tranformer inrush is independent of load. (inrush is in the range of 8-12 times the full load and the full load is 1 at the max, plus they are not in phase so it really does not matter).

As for residual flux, unless you are using a static transfer switch to switch sources, 10-15 cycles will be more than enough to dampen most inrush transient. 30 cycles would be the most.