Voltage dipped on primary side of xfmr ? 2

[sparky1976]

Recently we have fault on 33kV line phase to phase BC and sucked the B voltage to 13 kV for 100ms. And it was cascading the trip to two 11 kV motor,trip on this motor was undervoltage B phase(different bus and xfmr). Here's the on line diagram illustration: We have two bus 150kV that always tied, in each of bus we have 2 xfmr, one to 33kV and 11kV.
so we have two 33/150kV and two 11/150kV.
Is it possible the fault in 33 kV will also cascade to 11 kV bus ?, the motor on 11 kV is huge 13,5 MW each we use cycloconverter,and the U/V relay is build in relay inside the converter.

 

[SooryaShrestha]

yamin,
I think with the severe phase to phase fault the voltage dipped to low value. Because of the high fault current the voltage drop on the primary side is also very high. So the primary voltage also dipped to very low value. This in turn gave the lower secondary voltage to the 11 kV side of the the other transformers. Because the transformers were also loaded to appreciable value, it further aggravated the drop. If U/V relay is built in it naturally operated.
In this case the 150 kV side did not operate as infinite bus, as usual, though normally planners calculate in such system as infinite bus.
The 13 kV you noticed must be from the disturbance recorder on the 33 kV side. If you would have checked the voltage profile during the fault on 150 kV side, you would have noticed it very clearly.

 

[sparky1976]

Soorya,
The problem is I donot have any record on 150 kV side, the only record that I got for 150 kV side the sampling rate too slow 1 sample per second. The record for 33 kV was 16 smple per cyle.
I still looking for the setpoints for U/V relay pick up and delay time.

 

[jbartos]

Suggestions:
1. Considering the one-line and nature of the power distribution and loads, would not it be prudent to have fast static transfer switches for voltage dips lasting more then 4 or so milliseconds?
2. The 100 millisecond voltage dip is too long for nowadays power electonics. In some instances undervoltage relay (27) could be set to not to trip for times and voltage dips in excess to which you stated.

 

[peterb]

You need to do a comprehensive fault study of the bus arrangement that you described, in order to evaluate possible solutions. Sounds as though the utility supply to the plant has a very low short circuit capacity (which is basically the same thing that Soorya is saying above).
What is the size and impedance of each of the transformers? This, together with a knowledge of the source impedance of the utility, will give an idea of the order of magnitude of the voltage drop involved.
Based on the results of a thorough study, you may want to consider operating the 150 kV bus with the tie open, so that faults on one side will not cascade to the other - this could be of value if your process supplies can be effectively split on this basis.
As far as the operating time is concerned, 100 mS (5 cycles on 50 Hz)is quite normal for standard circuit breakers operated by instantaneous protection. It would be great if someone did manufacture a high speed static transfer switch rated to switch a 13.5 MW, 11 kV motor circuit, but I'm not aware of anyone who does at this time. You should definitely explore with the equipment manufacturer the possibility of modifying the undervoltage relay trip time.