Tripping of Compressor on energizing transformer on same common bus

[tfquote]

We have a cotton weaving unit with 6 gas generators at 380V output. All 6 generators are connected to 6 step up transformers (connected star-delta i.e. output 11kV side is delta and no neutral is present!). Out of all 6 transformers is connected in parallel to a common 11kV busbar. From the busbar, an induction motor compressor (11kV, 3-phase, 50HZ, 1300kW, make: Cooper) is supplied. From the same busbar, a step down transformer of 2500kVA (11kV delta / 415V star) is connected for other loads of the factory. Now, whenever we energize this 2500kVA unit, the Cooper induction motor trips! This is problem of the factory from last 6 years (i just joined the factory and guys there say its a "built-in" problem. they wait for sometime, and after 2500kVA inrush dies away, they re-start the compressor... wierd).
Why does this happen. Is it because there is no neutral present (from the generators transformers upto load)? Will a zigzag transformer solve the issue?

 

[waross]

What is the motor tripping on? Under frequency?, Low voltage?, residual or ground fault current?, Unbalanced voltages?.
A first guess would be unbalanced voltages. The transformer will take a very high inrush current that will not be balanced, phase to phase. That coupled with the fairly high impedance of generators (compared to transformers) may cause enough voltage unbalance to cause a voltage trip.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[tfquote]

Yes waross. Its the voltage relay that causes tripping. It may be due to either unbalance voltage or under-voltage (coz relay doesn't have any third function ). I will check this tomorrow morning. Another important thing to tell here is that the old generators we has were furnace oil powered. Now we don't use these since gas is much cheaper. BUT, that old gensets have unit step up transformers that are delta-star (i.e. delta on gen side, and star on load (11kV) side). With this configuration (where neutral WAS present), no tripping occurs when the same 2500kVA transformer is switched on line.
Then, when the factory procured gas gensets, someone made a mistake to procure these star-delta transformers (delta on load side - 11kV), and then problem started occurring!

 

[slavag]

Hi.
We need more information, settings of protective relays.
Not bad and help us some disturbance recorder data from protective relays.

 

[musaUKPN]

I will ask a simple question: is the undervoltage relay at motor have any time delay settings? I have seen similar tripping at an office building while the chiller motors started.

what I understand that if you add a time switch and/or replace this undervoltage with a modern having time delay function could solve the problem.On other hand if if time switch is already there; try to increase the time delay and/or voltage settings.

As waross indicated the generators & transformers impednace could also be an issue; that is higher impednace mean higher volt drop when inrush currents flows through them.

More data regarding impednaces and relay settings is required.

musa UKPN

 

[tfquote]

The tripping comes on the Undervoltage relay. The relay currently doesn't have a time delay module. But as musaUKPN indicated, i ordered a time delay unit to add with the relay. Lets see if it works!

 

[slavag]

undervoltage relay w/o time delay???
its news....