HomeApplicationVFD controller of 500KV HVDC terminal valve cooling

VFD controller of 500KV HVDC terminal valve cooling

A problem on VFD Controller of valve Cooling Fan of 500 KV HVDC Terminal. Problem is that when station auxiliary supply goes off then all station load transfer to DG set then VFD controller perform perfectly (for a certain time its showing under voltage alarm and reset automatically after DG start) but when auxiliary supply comes back and we switch on the breaker to transfer all load to auxiliary from DG set. And then 1st VFD try to reset (under voltage alarm) ones but it was not getting success all time sometimes it's does not reset automatically (or sometimes reset 3/4 VFD out of 12 VFD) then we have to do it manually very fast cause temperature rises very fast otherwise it can trip the HVDC line within 2 min.

For find out the root cause we checked
1) supply voltage which was ok.
2) DC control voltage also ok.
3) contact pick up - ok.
4) PLC perform ok.

If the DG can be made to synchronise back to the normal power, then there will be no 2nd interruption. Depends on how the 500kV HVDC Station is being manned, the DG can be designed for automatic synchronise-back or manual synchronisation.

If you cannot implement the DG synchronisation solution, probably there is another way around it. For the first blackout of the station auxiliary supply, the DG would take 10-15seconds to start-up and to re-energize the station auxiliary. For switchover from DG to normal power supply it would be very fast, i.e. open DG breaker and then close in the normal supply breaker is occurring sequentially with no time gap. The problem lies in the correct timing of the VFD internal control scheme, eg the variable frequency drive internal control power may not die off quickly that it doesn't sense a severe voltage dip, or it could be due to other reasons. Therefore it is the duration of blackout that can influence the re-starting of VFDs.

Suggest to provide a time delay between opening of DG breaker and closing of normal supply breaker.
VFD may be built with certain logic to avoid re-starting the motor when the motor winding still carries some residual voltage.

The motor may be immediately restarted on the same speed if the supply failure last for a few cycles only because the residual voltage in the motor may still be in synchronism with the inverter output. Beyond that initial few cycles, the inverter may need to wait until the motor winding is voltage free.

I am not sure how the variable speed drives re-starting logic is being designed. However, I suggest to provide a 10seconds delay time between change-over of the two circuit breakers.

I would be interested to know whether you would try this time-delay approach and let us know whether you are successful in solving the non-restart problem.

A 10second duration of blackout in this normalization is not a pleasant thing to see. It would be good you are able to fine tune it by reducing the time delay. May be by some trials you can find an optimum setting. Factors that affect the time delay setting could be the time taken for the cooling fans to coarse/slow down. The faster it slow down, the shorter time you set the time delay.

Alternatively an in depth study into the VFD control system and see how the re-start parameters are being setup. May be it is possible to adjust the delay time for a restart. In this case, you don't need to set the 10s delay time for the circuit breakers.

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