Current Unbalance during start up 4

[Deansharafi]

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.

 

[edison123]

Looks like high resistance connection (loose cable / bus duct terminations) to the 6.6 KV bus at the input side. Suggest measure dc resistances across all termination points including supply breakers. Require a micro-ohmmeter though.

 

[busbar]

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?

 

[jbartos]

Comment: There may be a higher resistance connection due to loose nuts, etc. Do you have a proper maintenance of the power distribution system that would include checking tightness of connection, contact pitting removals, etc.?

 

[Deansharafi]

Thanks for the comments.
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.

 

[jbartos]

Comment: The motor windings might be tested. There might be inter-turn shorts.

 

[Deansharafi]

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?

 

[AFab]

As with all the previous posts I agree there is a high resistance problem. It is also common to all 3 motors so I
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.

 

[Deansharafi]

The unbalance starting current has risen up and have caused more trips on start-up. The value of start-up shown on motor protection relays show a trend of rising U/B current from 20% (a few weeks ago) to 30% (this week).

 

[electricpete]

If I understand you correctly the power for the motors is provided by several gas turbines in paralle. Can you monitor the currents in each gas turbine generator to see if just one of them or all of them is supplying unbalanced currents during the motor start?

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[busbar]

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

http://www.pge.com/docs/pdfs/custom...ces/greenbook/service_requirements/058087.pdf

 

[alehman]

Do the motor loads pass through the fault limiter? What does the fault limiter consist of?

 

[Deansharafi]

alehman_ from the system arrangement, some of the current can pass through fault limitor. I will investigate and post the information on fault limitor. My collection is that there might be a high resistance on one of the phases of the fault limitor, but I want to be open to other possibilities as well.

 

[Arevaengineer]

Maybe Iam a bit less knowledgeable than all of the above but 7000microOhms is a lot of resistance on that particular section.As compared to values I have noted on new switchgear installations from 3.3kV to 11kV they were never in access of 150microOhms for a sigle joint.

 

[DanDel]

Check for voltage imbalance in two locations at the same time during a start; at the starting motor, and as close to the source as you can get. Compare the values and you will see if there is a voltage drop across one phase of your system during the start. If the voltage drops are similar at both locations during start, then the problem is either in all the motors(unlikely) or further upstream closer to the source.
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).

 

[jbartos]

Comment on Deansharafi (Electrical) Apr 19, 2004 marked ///\\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.\\\

 

[Stefa]

Does the protection trip come from ground fault relay ?,Do the CT connected with ground fault relay by three CTs of each phase as residual connected ? This kind of problem is common for ground fault connection from three phase CTs.The unbalance occur around 1.0 to 1.5 sec. as you experienced.
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.

 

[jbartos]

Suggestion: Ground fault protective devices are set by the system grounding method type. E.g. Medium resistance system grounding usually trips within 10seconds. The high-resistance system grounding method may allow a ground fault for an indefinite period of time if the cable and transformer insulation level is 173%. This allows a plenty of time for finding the ground fault location.

 

[Deansharafi]

Jbartos_ Since the unbalance is only during the start up and in normal running the U/B value drops to 1%, there is no possibility of interturn faults. However "common Cause Failure" you mentioned is a very interesting issue.
Regarding Ground Fault, it must be mentioned that the protective relay trips due to U/B, so there is no G/F issue present.

 

[jbartos]

Comment on the interturn shorts. If the unbalance under the normal operation is only 1-2%, this does not mean that the unbalance under starting conditions can be much higher due to interturn shorts since it depends how those interturn shorts are formed. The interturn short may create an extra shorted turn loop that produces electromagnetic field interfering with the rest of winding such a way that the unbalance increases from 1-2% to higher values.