Transformer inrush current problem at wind farm 3

[ctnie]

I am currently working within the wind farm sector (which is a new area for me and my company) can anyone help with the following problem;

Our companies normal arrangement for the connection of wind farm generators to a utilities network is via a daisy change set-up (ie Transformers 33/.690kV, 1.5MVA linked together with 33kv cables on one continuous circuit without 33kv RMU’s.). This form of connection has caused a problem at one site; when the transformers are energised the inrush current from the string of transformers causes a voltage dip on the utilities network that is outside their licence standard.

Is there any other solution to this problem other that fitting 33kv RMU’s at each transformer, along with a supervisory control scheme that allows each transformer to be energised individually? This would no doubt solve the problem but won’t help the bank balance.

 

[rcw retired EE]

What is an RMU? I am not familiar with this designation.

Could you replace the HV circuit breaker with one that has pre-insertion resistors or with a single phase operating mechanism and use a point-of-wave relay to initiate closing? Either method will limit the inrush current.

 

[ctnie]

RMU = Ring main unit, apologises probably not the correct terminology used. What I mean is the three-panel 33kv switchboard (two switches, one circuit breaker) close coupled to each transformer. This is the proposed solution to the problem, but will be an expensive one, compared to the cabling set-up.

 

[jghrist]

You might consider US style padmounted transformers which can be purchased with integral oil switches and internal or bayonet (withdrawable) primary fuses. This probably would be much more economical than separate RMUs.

You could also consider US style deadfront padmounts with two loadbreak elbows per phase (in and out). This would not provide a switch for each transformer, but you could selectively connect portions of the transformers using the loadbreak elbows.

 

[ctnie]

Thanks for the comments,

Rcwilson,

‘pre-insertion resistors’ I managed to get some information on this, and I’m beginning to understand the concept. Within the information I have received it makes comment on ‘significant maintenance requirements’ have you any idea of what these requirements might be? As it doesn’t say within the documents I’ve received.

‘single phase operating mechanism’ I’ve had less success with this, can you explain what is meant by this? (Does the CB switch in one phase at a time?)


jghrist,

I’ve seen one wind farm site which has US style padmounted transformers with internal withdrawable fuses, I wasn’t aware that this type of transformer style can also come with integral oil switches. However, I’m not sure how this would help with the inrush current problem when energising a radial circuit. The idea behind the RMU CB set-up with supervisory control was so that each transformer could be switched in individually from one central point back at the main switch house.

‘US style deadfront padmounts with two loadbreak elbows’ I’m not sure what you mean here, can you explain? (Is this a MV cable ‘piggyback’ arrangement, which allows the MV cables to be disconnected from the transformer, but remain part of the ‘live’ circuit?)

Thanks again

 

[rcw retired EE]

High voltage circuit breakers can be supplied with three individual poles and three operating mechanisms - one pole and one operating mechanism for each phase. This allows the poles to be opened or closed individually. Areva's Point-of-Wave relay monitors the voltage across the breaker and closes each pole at the peak of the voltage wave to minimize inrush curents. It is usually used on long transmission lines or on large transformers fed from weak systems. It would work to energize your complete system.

The load break elbows are Medium Voltage versions of an extension cord with a plug and receptacle. The cabel to the Xfmr HV bushing is terminated in an elbow that can be plugged and unplugged hot using a hot stick. It is a manual operation.

 

[windie]

You might not need RMUs at each transformer. You could work out how many transformers you can safely energize without causing an unacceptable voltage dip on the utilities network. You could then install RMUs or another switching device to divide your feeders into the appropriate numbers of transformers.

For example you may find that you can energize half the transformers on a feeder at a time without causing a problem to the network. In that case you could install a switching device halfway down each feeder, so that only half the transformers on the feeder are energized simultaneously.

Although load break elbows are theoretically good for this purpose, I've not met any people on wind farms who like using them for this purpose in practice. I would prefer a proper switch like a RMU or a sectionalizer.

Windie

 

[alehman]

Padmount switchgear with remote control capability are available from several manufacturers.

http://www.sandc.com/products/padmountedgear/pmh_rs.asp

http://www.sandc.com/products/windenergy.asp

 

[jghrist]

Loadbreak elbows are like banana plugs for MV systems. Loop-feed padmount transformers will have two loadbreak elbows per phase connecting to the transformer primary. One is the source and the other is connected to the next transformer in the loop.

Let's say you have 10 generators (on transformers numbered 1-10) daisy chained together and you figure you can only handle the inrush of two transformers. If you disconnect the the load side elbow of every other transformer, then energize transformers 1&2 - connect the elbow on transformer 2 to energize transformers 3&4 - connect the elbow on transformer 4 to energize transformers 5&6 - and so on until all 10 are energized two at a time. Maybe pretty time consuming, but you eliminate the need for any switches.

 

[davidbeach]

jghrist, I like your "banana plugs for MV systems", but your idea of using them to energize a bunch of unloaded transformers gives me visions of all sorts of ferroresonant problems. Single phase switching of unloaded transformers connected with underground cables seems to be the classic recipe for trouble.

 

[hvcad]

You might find a paper I have written on this topic useful. This will not help with your present problem, but the type of analysis described in this paper would have identified your voltage drop problem at the design stage.

http://www.ipst.org/TechPapers/2005/IPST05_Paper026.pdf

Ken Smith

Dr K S Smith
Mott-MacDonald, Power Systems Division
Glasgow, Scotland.

 

[jghrist]

davidbeach,

I agree with you on using the loadbreak elbows to switch 3Ø transformers. Better get the under oil 3Ø switches.

 

[ctnie]

Once again thanks for the comments gents

Thanks for the link to your paper hvcad (very useful)