Current Unbalance during start up
Current Unbalance during start up
(OP)
Hi all,
I have a client with some unbalance current problem during start up of their HV motors.
They have three Siemens 6.6KV induction motors having a current unbalance @ starting of 15 - 30% for 1-1.5 seconds. They run a 6.6KV HV switchboard with 2 x 5 Mwatt steam turbines as base load and 5 x 3.8 Mwatt gas turbines as feed to the HV board. The board has an A & B sides, which is spilt by a fault limiter. From this they run 3 Compressors 6.6KV motors. 2 x Export compressor motors @ 757Amps FLC and 1 x Recycle compressor motor @ 426Amps FLC, Siemens rate these motors @ 3.5 times FLC at starting.
All 3 compressor motors run all day every day. The facility has been up and running since nine years ago and the motors have been in place since then with no previous current unbalance issues, but lately one of the compressors motor tripped during starting on high current unbalance of more than 20%.
This scenario happened to the other two motors with the smaller one having a less unbalance around 15%.
Some chart recordings from the secondaries of the protection CT's on motors were carried out which showed a very unusual waveform on Red Phase during start up.
Readings from the VT's on either side of the bus indicate that there is no voltage drop issue during normal operation of the plant.
Another fact is that once the compressor motors are up and running they run at a 1 - 2% unbalance indicating that the unbalance issue is only during high starting currents.
Since there are no major single phase loads in the system (unevenly distributed loads= voltage unbalance), and motors are connected to the board without any transformer being in between (which eliminates the possibility of having different taps = voltage unbalance), I am focusing on resistances in the system which may cause this sort of unbalance, as the three motors are behaving in the same manner and they started this from a specific time.
Have you guys experienced similar problems before and what is the line of your thinking in this regard? Thanks for your input.
I have a client with some unbalance current problem during start up of their HV motors.
They have three Siemens 6.6KV induction motors having a current unbalance @ starting of 15 - 30% for 1-1.5 seconds. They run a 6.6KV HV switchboard with 2 x 5 Mwatt steam turbines as base load and 5 x 3.8 Mwatt gas turbines as feed to the HV board. The board has an A & B sides, which is spilt by a fault limiter. From this they run 3 Compressors 6.6KV motors. 2 x Export compressor motors @ 757Amps FLC and 1 x Recycle compressor motor @ 426Amps FLC, Siemens rate these motors @ 3.5 times FLC at starting.
All 3 compressor motors run all day every day. The facility has been up and running since nine years ago and the motors have been in place since then with no previous current unbalance issues, but lately one of the compressors motor tripped during starting on high current unbalance of more than 20%.
This scenario happened to the other two motors with the smaller one having a less unbalance around 15%.
Some chart recordings from the secondaries of the protection CT's on motors were carried out which showed a very unusual waveform on Red Phase during start up.
Readings from the VT's on either side of the bus indicate that there is no voltage drop issue during normal operation of the plant.
Another fact is that once the compressor motors are up and running they run at a 1 - 2% unbalance indicating that the unbalance issue is only during high starting currents.
Since there are no major single phase loads in the system (unevenly distributed loads= voltage unbalance), and motors are connected to the board without any transformer being in between (which eliminates the possibility of having different taps = voltage unbalance), I am focusing on resistances in the system which may cause this sort of unbalance, as the three motors are behaving in the same manner and they started this from a specific time.
Have you guys experienced similar problems before and what is the line of your thinking in this regard? Thanks for your input.






RE: Current Unbalance during start up
RE: Current Unbalance during start up
A rule of thumb for motor current imbalance is equal to voltage unbalance multiplied by the ratio of locked-rotor current over full-load current.
Because of the relatively short startup time, thermal imaging may not be revealing.
As to the weird voltage waveform, any chance the bus has any VF or SSRV drives with an open or shorted semiconductor?
RE: Current Unbalance during start up
RE: Current Unbalance during start up
Busbar_ There are no VF or SSRV drives in the system. The other connected load to 6.6Kv busbar are 4 X 6.6KV/415V transformers feeding LV switchboards.
Jbartos_The site has conducted low resistance tests on the 6.6KV breakers (from one breaker to the other, which checks the contact resistances on the connections as well). All the values seem well balanced between three phases and seem to be within limits The maximum being less than 7000 Micro Ohm.
RE: Current Unbalance during start up
RE: Current Unbalance during start up
Since the three motors are behaving in a sme manner, would you still think they have a common problem (inter-turn shorts), which started almost at the same time?
RE: Current Unbalance during start up
might recommend having someone in to do a thermography scan
of your distrabution system. If there is a high resistance
joint you will be able to find it with out shutting down.
RE: Current Unbalance during start up
RE: Current Unbalance during start up
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RE: Current Unbalance during start up
A last-ditch oddball experiment may be considered. Although primarily intended for single low-voltage motors on dedicated transformers, there is this novel “current-matching/-equalization” procedure. Because of busing or limited cable flexibility at terminations, it may not be possible with larger motors. §12.A of www.pge.com/docs/pdfs/customer_service/new_construction_services/greenbook/service_requirements/058087.pdf
RE: Current Unbalance during start up
RE: Current Unbalance during start up
RE: Current Unbalance during start up
RE: Current Unbalance during start up
I have seen this in other locations at startup for MV motors (even including, I believe, an 'unusual waveform' on one phase) , but it was not serious enough to investigate further(no nuisance trips; no running inbalances).
RE: Current Unbalance during start up
Jbartos,
Since the three motors are behaving in a sme manner, would you still think they have a common problem (inter-turn shorts), which started almost at the same time?
///Common mode (cause) failure can happen; especially, if all motors are by the same manufacturer. E.g. a spike in one phase can cause insulation damage in all three motor windings, perhaps, the mentioned interturn shorts.\\\
RE: Current Unbalance during start up
If so,the ground fault relay trip from unnequal stauration of three CTs,CT red phase saturated during high starting current.This means the burden of CT is higher than CT burden.
Have you checked whether CT for red phase was changed before problem occured or not ? the CT might be changed by replacing the new one have not enough burden compared with the setting current of ground fault relay.
If problem is like that mentioned above,just set time delay more than 1.5 sec can solve the problem.However make sure that the system grounding is resistance grounded.
RE: Current Unbalance during start up
RE: Current Unbalance during start up
Regarding Ground Fault, it must be mentioned that the protective relay trips due to U/B, so there is no G/F issue present.
RE: Current Unbalance during start up
RE: Current Unbalance during start up
RE: Current Unbalance during start up
RE: Current Unbalance during start up