Effect of Voltage drop by MV motor

[yk052052]

In our plant, electric system consists of 35kV/6kV/400V. (Current voltages are approximately 35kV/6.2kV/413V)

We ran the MV motor (6kV, 3100kw) connected to soft starter. It took around 14Sec until bypass mode and once current reached at the starting current(around 1540A), it stayed at the starting current until bypass mode(10-11 Sec.

We measured the voltage dip at each system and found as below.

35kV -> 33kV
6.2kV -> 5.2kV (Upstream TR capacity is 10MVA)
413V -> 345V (Upstream TR capacity is 2.5MVA)

We're concerned about voltage dip that is larger than our expect.

Our 27 is set to 80% of each rated voltage so protection didn't operate. But I'm not sure this voltage dip at this duration is ok for loads at downstream(especially motor loads).

Do you think this situation is fine? If not, what can we try to solve this issue?

 

[unclebob]

Not sure about your 14 sec and 10 sec meaning.

What is the load? A fan? a conveyor? a compressor?

The more you limit the current, the less the voltage dip you'll have but acceleration will be slower.

You can limit the current more, but you want to keep enough torque so the motor don't stall or trips on overload.

 

[yk052052]

The load is compressor and 14 sec means starting time of compressor.

Starting current is around 1540A and the current stays at 1540A for 10-11Sec (the rest of 3-4 Sec is the time until the current reaches the peak current from start). After 14 Sec, current drops to normal current (around 300A depending on load condition).

We discussed regarding this issue with soft starter vendor if there is any option we can minimize voltage dip and your suggestion(to lower the limit of starting current) is the one. Currently current limit is set to 460% and we tried it with 450% of FLC(335A). The voltage became higher (20V more), but not enough.

We're keeping trying to adjust limit of current now.

 

[7anoter4]

In my opinion, the System is weak [about 160 MVA short-circuit power],[ long overhead -or cable-35 kV line, small conductor cross section area, conductor material and distance between phases has to be improved].
Transformer low rated power [10 MVA] and [probably] high short-circuit impedance[10% it could be].
Soft start mode not good-has to be current limiting mode[?].
The load torque to high-try to start in no-load mode-the start time could decrease to about 7 sec.-in my opinion.