Increasing system impedance 1

[sparky1976]

Dear all,

I have island system with 150 kV bus. From this bus wehave 150/33 kV step down xfmr and and 150/11 kV xfmr. the 11 kV xfmr has NGR 200 amps 10 sec. the 33kV xfmr is solidly grounded.

Several times if we had fault on 33 kV the voltage dip on 33 kV side untill 3 kV and the voltage on 11 kV side also dip around 75% and trips the drives for big motors on 11kV. Some one told me thats because our system inpedance is low.

What do I need to do to increase system impedance ?
If I change my xfmr from 6.5 % Z to maybe 15 % Z is it going to be a significant change to system impedance ?

I understand that more Z to the system more electrical loss.

Adding NGR to 33 kV is not an option because the fault mostly phase to phase.

regards
Ilyas

 

[mpparent]

Also,(to add a bit),

If you increase your transformer impedance, there will be a couple of things. One, is that it will cost more, since it would be non-standard, and two, increasing the impedance would lower the available fault current(as mentioned above), but also, there could be a possible problem with voltage drop/regulation.

Mike

 

[Bung]

A short circuit on the 33 side will mean the voltage collapse will be passed on to all points downstream (assuming a radial system) - including the lv side of any 33/11 transformers connected to the faulted 33 system. Changing the system impedances won't alter this fundamental fact. Bung
Life is non-linear...

 

[sparky1976]

All the trips on 33kV are set at instantenous,it takes 4 cyles (50hz) for 33 kV breaker to open.

The under voltage occur during that 4 cyles, the under voltage protection for motor drives in 11 kV are trips, the under voltage setting is 85 % rating, 30 ms delay.

I cannot change the setting for this protection for drives, the drive vendor says BIG NO, since the additinal delay will cause burns to some component on the drives.

The cost per event is 75000 USD of loss of production.

Any one got any idea how I can reduce the volatege dip during that 4 cycles.

regards

 

[jbartos]

Suggestion: It is very unusual that you have the 33kV voltage system solidly grounded. The industry standards use the solidly grounded neutral for low voltage systems (up to 1000V, or 600V for all practical purposes).

 

[busbar]

Would it be feasible to furnish some sort of uninterruptible power for the drive controls?

 

[sparky1976]

Jbartos,
could you give me recommended practice or reference of this solidly ground system or high impedance ground.

Busbar,
The drives is for motor 13.5 MW and 7.5 MW. so it is hard to find feasible uninterupted power system.

Thanks

 

[jbartos]

Suggestion: The increasing Utility side Zsys would make the volage dips even bigger since
Vdip=Vsyssource x Zsys/(Zsys + Zdownstream), in Volts

 

[busbar]

yamin--I meant to suggest the UPS on the control power only.

 

[RAgrawal]

I think every one in the world is suffering from tripping of Drives due to voltage dip. The suggestion to put control supply of drives through UPS by 'busbar' is good sggestion. Aletrnatively you can ask to provide 5 cycle voltage ride over inyour drives (since your fault is getting cleared in 4 cycles only).

 

[RRaghunath]

Yamin,

The common point for the 11kV system and the 33kV system is 150kV bus bar. If the impedance of the 150/33kV transformer is increased, to that extent the drop in 150kV bus bar voltage would be lower (for faults in 3kV system).

The drop in 11kV system (derived from 150kV source) voltage will also be lower, benefitting the drives. Hence, I think this line of action is practicable.

Two things in the message I didn't quite follow:

1) 150kV island system - then, what is the power source

2) 33kV faults are phase to phase faults - this is a bit strange as statistically 80 to 85% of all faults in electrical systems are supposed to be phase to earth faults.

I would appreciate if you could throw some light on these.

Thanks.

 

[stevenal]

The UPS is overkill. No reason to supply your controls during an outage. Use a constant voltage transformer. No batteries to maintain. It will ride through the longest of sags since no energy storage is involved.

 

[sparky1976]

rraghunath,

The 33 kV is over head line and in my area theres alot of bats.

The power source is 4 turbine 16 km away.

regards
Ilyas

 

[babu67]

There is practically nothing that can be done to avoid the drop in voltage for few cycles for a ph-ph fault, if you don't want any costly re-designs. Motors are designed for undervoltages of few 100msec and if drive control is the problem, go for seperate constant voltage source for the same like UPS. The cost of one drive trip will compensate your investment in independant drive control supply.

 

[jbartos]

Suggestion to yamin (Electrical) Nov 26, 2002 marked ///\\Jbartos,
could you give me recommended practice or reference of this solidly ground system or high impedance ground.
///Reference: IEEE Std 143-1991 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, (Green Book),
Page 25 Par. 1.4.3. Resistance Grounding. A system properly grounded by resistance is not subject to destructive transient overvoltages.
Page 27 Par. 1.4.5. Ground-Fault Neutralizer (Resonant Grounding). This method of grounding is used primarily on systems above 15kV (i.e. good for 33kV system), consisting largely of overhead transmission or distribution line.
Page 27 Par. 1.4.6. Solid Grounding. To ensure the benefits of solid grounding, it is necessary to determine the degree of grounding provided in the system. Effectively grounded systems will have a line-to-ground short circuit current of at least 60% of the three phase short circuit value. Effective grounding of a system is accomplished only when Ro<=X1 and Xo<=3X1 and such relationships exists at any point in the system.\\

 

[RRaghunath]

Ilyas,

So, the phase - phase fault caused by Bats is truly an issue.

While you go ahead with ordering the 150/33kV transformer with higher impedance, you can think of devices that can be mounted on the 33kV OHL poles to scare away the bats and thus reduce the fault incidence (I know there are suppliers who can provide devices to scare away the birds from perching on top of the OHLs).

Having instantaneous protection on the OHL feeders that will isolate the faulty one quickest also may be considered. It should be possible to isolate the feeder in less than 100ms with modern fast acting relays and circuit breakers available. The large fault current availabel due to the fault being a phase - phase one is also an advantage in ensuring the speed of operation of protective relays.

If the transformer is being replaced, it is necessary to study the impact of higher transformer impedance on the 33kV system (voltage regulation etc.) before taking a final decision.

These are some of the thoughts. Hope you find them useful.

 

[sparky1976]

rraghunath,

Thanks for the advise, I'm also think to buy breakers faster than 30 ms.

Many thanks to all that contibute to this threads.

 

[jbartos]

Small correction (I beg your pardon). Reference should read:
IEEE Std 142-1991 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, (Green Book)