Generation and Distribution

[apprenteng]

We have a client with a generator of rating 500 Kva generating at 415 volts. There are 3 transformers hooked to the 415 bus of ratings 630 Kva, 630 Kva and 1 MVA, 415/11kV, 415/11kV and 415/33kV respecively. The 33kV line is 40 km long. Any suggestions on how to protect the system adequately is highly apprecciated. Is the system configuration feasible? Consider the low fault currents the generator is able to generate especially at the end of the lines. The 11 kV lines are about 30 km long each.

A drawing is attached to illustrate further

 

[odlanor]

It is better to analise the system firstly:
1-Can 500kva generator support 33kV line rejection?
2-Can 500kva generator support 11kV line rejection?
3- Are you sure your 33kV system could drain only 1MVA during a loss of load?
- Equipment should be dimensioned to support adversity(anormality?) conditions on system.
- Protection should be provided to avoid that adversity condition on system.

 

[ScottyUK]

What are you trying to protect - the generator from the system, or the system from the generator? Is the utility feed present or is the generator on its own? If the latter I would say 'no' the configuration is not feasible, or not to any practical extent. If the utility feed is present then it is a pretty typical embedded generator.


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If we learn from our mistakes I'm getting a great education!

 

[cranky108]

As long as the generator size is small compaired to the feeder load, where is the problem. Some utilities use a factor of generator being less than 10% of the feeder load as a limit.
On a fault the feeder should trip as normal, and the generator should trip on under frequency as it can't sustain the load.

Also you want to look at your reclosing times.

At least that's how I've seen it done here (Not at those voltages exactly).

 

[odlanor]

It is difficult for a distibution utility to permit reclosing for distributed generator with this power.
Its better to consult utility

 

[cranky108]

If you consiter 33kv a system, where the outage rate is less than a 11kv system, then you can look at a 3 second first reclosing time.

The settings for the generation should see the fault or loss of system and it is the generation owner's concern to meet your coordination requirments. This might include instalation of PT's to since zero sequence voltage.

Frequently the generation owner isen't serious enough to want to pay for the proper protection, and you have to send them packing.

 

[ScottyUK]

cranky,

Just slightly off-topic but might be of interest: over here the utilities require a "G59" relay which is a relay which implements the recommendation of Guidance Note G59 published by the Energy Networks Association, which deals with detecting a sudden loss of utility infeed and the subsequent islanding of the generator.

Two methods are used: ROCOF, or rate of change of frequency, which detects the sudden slowing of the generator as it tries to support a system load much greater than its capability; and vector shift which detects the rapid change in machine load angle as it tries to supply a load greater than its capability. Both technologies cause rapid islanding of the generator from the system.


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If we learn from our mistakes I'm getting a great education!

 

[cranky108]

So you use a rate-of-decline relay, rather than a under-frequency relay. Different, but they accomplish the same thing.
Would some, generator designs (actually prime movers)require a low frequency cutoff anyway to protect the unit?
(I say some as not all prime movers are sensitive to low frequencys).

Bottom line is adding generation to distribution circuits is possible, if it is within reasonable limits for the target circuit. Above those limits and the cost can increase quite quickly.

 

[waross]

Considering that a fault on any line will probably stall a diesel engine, any cheap and dirty low frequency trip should work.
I found out that the fuse links in the fused cutouts were about 1000% of the recommended size when I manually closed a fuse cutout into a bolted single phase fault. (The boys made a little mistake on the transformer bank connections.)
The generator was 600 kW (750 KVA) at 460 Volts.
I was closing at 13,200/7620 Volts about 250 feet from the power plant.
You know the joke; "Why do bees hum? They don't know the words!"
I guess there were no words to describe this because our whole world was filled with a loud HUMMMMMMMMM.
We could hear the V 12 diesel next door losing speed.
I pulled the cutout back out and that was an interesting light show.
Too late, the engine stalled.
No breakers tripped and no fuses blew out.
The breaker on the genset was properly sized and it did not trip.
I'm getting older and try to avoid these adventures now. Maybe it's creeping maturity.

Bill
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"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Cranky / Bill,

The main reason for using the ROCOF or vector shift relays instead of a thrreshold relay is the speed of response. If you use an under-frequency relay the speed has to fall below the relay setting before the breaker open command is sent, whereas the ROCOF and vector shift relays the command is sent very soon after the grid infeed is lost. It makes the difference whether the generator stays on the bars with its islanded load or whether it trips and has to restart. Definitely a bigger problem for a gas turbine prime mover than a diesel because GTs have a real tough time maintaining output as frequency drops.

From the utility's perspective they rely on these relays to island embedded generation off the system if their system loses its infeed. The last thing they want is a third party generator keeping their equipment live when it should be in a dead condition. perhaps one of the distribution guys can comment a bit further from their point of view?


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If we learn from our mistakes I'm getting a great education!

 

[apprenteng]

The system is islanded i.e not connected to the grid and total load is about 300 Kva presently. The client is planning to add another generator of the same capacity as demand increases.