Transformer sizing

[gce1991]

Dear All,

I need to size a step down tansformer for 3 PH, 50Hz ,600 H.P ,6600V motor .The Voltage to the primary of the transformer is 11000V .Please assume typical fault level .

Thanks in advance

Regards

Eswaran

ESW,

 

[EEIre]

What type of starter do you intend using?

 

[gce1991]

It will be a DOL starter .

ESW,

 

[jbartos]

Suggestion: The transformer should not be sized out of the power distribution location context, if it is supposed to be properly and accurately sized, which is almost always the case or it is considered prudent.
Therefore, the upstream system voltage and associated system impedance are needed. The motor minimum starting voltage is needed and the motor starting current should not cause the bigger voltage drop to cause the motor have voltage across its terminal lower voltage than the motor minimum starting voltage. Various voltage drops downstream of the transformer beside the transformer impedance shall be included in the calculation, e.g. contact resistances, feeder impedance, etc.

 

[ScottyUK]

jbartos,

You are correct in saying that the system impedance is needed for an accurate result - the upstream voltage is 11,000V as the original post indicates.

However, on a typical 11kV network 600HP is a small drive and, since the 11kV fault levels are 'typical', the 11kV supply will be sufficiently stiff to start a 600HP drive without undue drop in voltage (Calculate the starting MVA for, say, 8x rated current during start as 3.6MVA - not a lot on a typical 11kV network)

The governing factors are likely to be:

Maximum MVA demand of the motor during the start, and the time for which it exists during acceleration. Most transformers will take a heavy overload for a short time because they have fairly long thermal time constants, but frequent starting or long run-up times will equate to a higher average load which will require a larger transformer.

The maximum acceptable transformer impedance will depend upon the minimum voltage you need for the motor to start, during the lowest likely 11kV voltage level. For a really accurate calculation you should take into account the upstream impedances, as indicated by jbartos. This is especially necessary if the 11kV system is weak. The minimum impedance may be restricted by your 6.6kV switchgear's maximum acceptable fault level.

I guess the transformer will be ONAN at this rating, so you need to consider how quickly it will cool down following a start (depends on ambient conditions).

 

[peebee]

Voltage drop will definitely be the key issue, which means your transformer impedance will be of greater concern than than the transformer kVA rating. Selecting the permissible voltage drop is kind of a gray area, but I suspect your worst permissible drop on starting should be limited to 10 or 15% undervoltage. Maybe you could get away with 20%, but I'd check with the motor & starter vendors first.

Cable sizes & lengths will also have a great impact on the voltage drop.

Soft start, reduced voltage start, or VFD's can all be effective in reducing voltage drop too. You might want to consider them.

 

[gce1991]

Thank you all for the sugestion .Initially we decided to proceed with 1000 KVA ,11/6.6KV ,Oil filled tranfomer (assumed 6% impedence) and will review the rating once we get the motor data and cable lengths.

Regards



ESW,

 

[jbartos]

Suggestions:
1. 11kV power distributions are usually stiff around generating stations. When it comes to rural areas, the 11kV power distribution transmission lines have conductors relatively small, often underrated and on rotten poles. In that case, the 600HP motor will be felt there.
2. 1000HP transformer for the motor of 600HP (~497kW with ~EFF=0.9) DOL started appears to be small. There were some postings in this Forum where a multiplier of four of the motor kW was used. This would imply 4 x ~500kW ~ 2000kVA transformer instead of 1000kVA transformer. The power factor is included in 2000kVA transformer size. The motor Locked-Rotor Amperes (LRA) are also a significant factor. The large LRA needs the larger sized transformer.

 

[ScottyUK]

jbartos,

1) I absolutely agree, but esw did suggest a 'typical' fault level. I guess it depends on whether your 'typical' is at the end of a long single-circuit line, or on top of a heavily interlinked city substation, or in a generating site. Can esw expand on 'typical' with an order-of-magnitude estimate?

2) I'm going to guess, because I can't find the thread you refer to, that the 2MVA transformer might be required to keep the volt drop within acceptable limits during the start rather than from a thermal limitation. As peebee said, volt drop during starting will probably the governing factor in the sizing calculation, unless there is another factor such as excessive number of starts or long acceleration time.