Transformer energisation restriction 2

[jimmy2times]

I’ve picked up an operating procedure implemented some years ago that recommends a period of 12hrs before a transformer is re-energised after in has been shut down. Nobody knows why this restriction is in place, I contacted manufacturer also they didn’t have any restrictions on how often and just referred to thermal red protection that will take transformer offline if temperature too hot in core/winding. Someone suggested it may have been frequently shutdown and energised previously, 7 or 8 times per day, and this was perhaps a way of limiting number transformer energisation to 2 /day. Anyone come across this before?

 

[jimmy2times]

Surge arresters are fitted at MV switchboard feeder, in cable compartment

 

[prc]

1) I am sure inrush will not heat the winding any significant way. In rush current is not RMS current. Peak will be there only for microseconds. Heating depends on current value alone. It depends on time and specific heat of material also.There is no need to check temperature rise. You will not find any increase by RTD.

2) If you ask for frequent energization there are several mitigation measures. That IEEE I mentioned contain several of them. LA alone will not help. Extra Snubbers is another solution.

3) Special designs have special surge diverters across the windings. Such transformers can also be offered in case of frequent energization.

4) Please go through pre-mag literature. This time lag may be only to reduce residual magnetism in core, may be to some extent.

 

[jimmy2times]

Hi prc,

I've now obtained copy of C.57-142 2010.

I see it states "Users of switching devices switching transformers that are inductively loaded (that is reactor loads, or
stalled or accelerating motors) or those operated on a frequent basis (daily or weekly) should strongly
consider transient mitigation means (see [B39], [B43]– [B45],and [B48])

B39 being a paper for surge limiters for VCB.

I don't have that paper, but as mentioned in earlier post we do have surge arrester at the MV switchboard.

I do note the point of the author's though that internal transformer insulation may well be overstressed when oscillatory voltage is applied at the winding terminal. And that they recommend also to protect transformer with dedicated surge arrester as IEEE Std 62.22

Your point you make about always using the pre-mag circuit we have , so we reduce the probability of interruption of higher inrush currents that would lead to these oscillatory voltages correct??

So if i have understood generally (as i have not digested it fully), the point about regularly energisation of your dry type transformer is not so much the energisation itself but rather the interruption on load (or no load) when taking transformer offline and also possibility of VCB contact bounce/contact gap transients on closing when bringing transformer back on line, and in each case the possibility to set up oscillatory voltages internally in transformer.


The pre-mag circuit itself only being of some benefit if a VCB trip condition were to occur during energisation, the pre-mag circuit is not a mitigation itself to this phenomena, only some form of safe guard each time transformer is energised.

Only real safe guard against this failure when considering need for frequency energisation would be use of RC snubber at transformer or otherwise pre-insertion resistors to damp the transient wave. One would not specify pre-mag transformer as a solution to frequent energisation.

Have i understood that correct.

 

[prc]

Yes your understanding is correct. This is a feature of VCB. Of course lot of improvements happened in VCB. Still for dry type transformers it is a threat.Could you go through pre-mag literature? Is there a need for reducing residual magnetism for effective working of it?

 

[jimmy2times]

There is next to no information on the pre-mag transformer, I’m still trying to search if anything exists. I will get back in touch with supplier next week to see if they have history on it

 

[jrostron]

It is new to me that there is a limit to the number of time or the time required between energizations. If done correctly when using a soft switching technique, there is no limitation and inrush currents are controlled hence does not cause transformer damage. Conventional vacuum breakers will not do this soft switching needed however. The inrush current is likely the reason for this limitation as it can exceed the fault current rating of the transformer and hence can be very damaging.

Estimating inrush and voltage sags or RVC can be very complex but there are some simple rules of thumb that will assist. One such technique is in the link below.

https://www.southernstatesllc.com/applications/connecting-solar-to-the-grid

All switching of transformers leaves residual magnetism or remnant flux. Switching to reenergize transformers at maximum flux exaggerates the inrush while switching at a reverse flux minimizes it. Determining the polarity of the flux is critical to determining when to close. One technique exists that eliminates the need to do this. It is the use of pre-insertion resistors. This technique requires no special sensors or controllers to implement. This technique can be designed to limit inrush currents to less than half of load current. I see not reason to require a minimum time between transformer energizations when the inrush current is limited to below load currents.