Requirement of Transfer Trip 1

[sharmaces]

I shall appreciate if someone can highlight the conditions which govern, the requirement of transfer trip in interconnection of embedded generators in transmission or distribution utility network.

 

[davidbeach]

It depends on many factors and the requirements will vary from utility to utility. In many cases, transfer trip will be required for a range of generator sizes compared to circuit loading min and max. If the generator is in the range of more than 1/3 of min circuit load to 3 times max circuit load it is likely that transfer trip will be required. Smaller generators will be swamped by the stranded load and larger generators will rapidly overspeed on loss of utility connection, but in that 1/3x to 3x range the generator output may not immediately become far enough out of spec to trip the protection on loss of utility.

 

[dpc]

It's really up to the serving utility. If they say it's required, you will have to provide it -- and pay for it, of course.

If you complain or question the need, they will say it is safety-related, and that will pretty much be the end of the discussion.

In some cases, I have been given a choice between transfer trip and installing distance relay that will be set to overreach the utility substation. In theory, the distance relays will cause more unnecessary trips than the transfer trip, but if the line is short and exposure low, the cost savings might be worth it.

There are situations where TT is truly a necessity to make sure the generators get off-line before reclosing or to avoid an islanding situation.

 

[sharmaces]

Thanks Davidbeach & dpc.
However my concerns is for smaller generators to the tune of < 500 kW. Is there any protection available which operates fast enough to ensure that the generator is out of the circuit before the re-closer operates.

Next, what is the minimum operating time of 80-R relay (rate of change of frequency).I understand that these relays are normally not supplied with small capacity generators but I am not sure whether there shall be enough rate of change of frequency to initiate these relays when the network load approximately matches the generator capacity.
I shall appreciate if someone can provide, information regarding sites/reading material etc.

 

[davidbeach]

What is the recloser operate time? If it is in seconds, no problems; if in cycles then it may range from no problems to very difficult. How much load would the generator be stranded with? How fast will voltage or frequency decay?

 

[dpc]

In many cases, an undervoltage relay is the best method to detect utility side faults and get the generator off line quickly.

But as David said, high speed reclosing is normally NOT used on circuits with generation. Normally a delay of at least a second or two is used to give the generator a chance to trip off.

 

[sharmaces]

I understand that if the re-closer operating time is in seconds, then, it shall be enough time to be sensed by protection but if the re-closer operating time is around 0.5 to 0.8 sec, is there any protection to sense and operate with in this duration. I hope one can read between the lines that I am looking for the alternative protection so as to avoid costly transfer trip requirement and its add-on costs.

Davidbeach: I have mentioned earlier that the load imposed on generator will be more or less equal to its name plate rating. I am looking for the relationship between: rate of decay of voltage and frequency with the size of generator at different loads. I am not searching a conceptual relationship and I will appreciate if someone can suggest sites or reading materials in this regard.
Is there any means to trip a 500 KW generator with in re-closing time of 0.5 to 0.8 sec.

dpc: Re-closer can not be removed or re-located for the sake of power quality commited by utility to existing customer. Is the additional intentional delay at the re-closer justifed?, because it will be serving more to profit making embedded generation than to existing customers.

 

[davidbeach]

Sounds to me like a lot of questions that need to be worked out with the serving utility; there are no general answers, only answers for that specific location and we can't provide them. A 500kW generator that could be stranded with 500kW of load will require transfer trip.

One thing that you might research and discuss with utility is vector jump sensing. I'm not sure I would accept it, nor am I convinced that it is thoroughly developed, but there is at least one relay, made in Germany, with a vector jump algorithm that is supposed to detect loss of utility with any mismatch between generation and load.

 

[dpc]

Operation of a recloser will always result in a temporary loss of power, no matter how fast it recloses. So adding a few additional seconds of delay isn't a major issue in most cases.

Some utilities never use high speed reclosing. It's isn't like God handed down the rules for reclosing.

As David said, this needs to be worked out with the local utility. They are all different, but the issues are generally the same.

 

[Borti]

I agree that transfer trip is the safest way to isolate the faulted feeder on both ends but it is costly. I know instances where the distributed generation was installed without the transfer trip capabilities. Since your load does not exceed the generator rating it makes sense to use sensitive directional overcurrent and under/over voltage protection at the point of common coupling and island the system during a fault. If a fault occurs on the utility feeder, the re-closer will operate but so your protection islanding the generator. The next step is to set auto synchronizer to allow the breaker close once the healthy voltages are present on the utility side.
I know that some manufactures have some articles on the distributed gnerator protection. Try

http://www.selinc.com

and

http://www.geindustrial.com/multilin/

 

[alehman]

I recently worked on a project where the utility was concerned that their feeder breaker may open for reasons other than a fault. Our plant was by far the largest load and only generator on the circuit. The utility's engineer came up with a scheme wherein the generator voltage regulator was set to export some reactive power when connected. Then when the utility's breaker opened, the generator voltage would slowly increase to the point where a separate overvoltage relay was set to trip the interconnect breaker. Seems like kind of a kludge, but it worked. We had undervoltage for line faults as well, without transfer trip.

 

[ScottyUK]

Almost all embedded generators connected to a UK utility network require a protection relay to ENA guideline G59. Usually this is implemented as a rate-of-change-of-frequency funtion or a vector shift detection. These relays provide protection to the generator by detecting loss of a grid connection and tripping the set before it is damaged. They operate very rapidly - almost certainly fast enough for an MV recloser to pose no risk. There are numerous types on the market - the type from SEG linked below is typical:

http://doc.newage-avkseg.com/doku.pdf/bl/bn1_400/bn1_1_e.pdf

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Sometimes I only open my mouth to swap feet...

 

[davidbeach]

ScottyUK, how secure are the vector shift relays? Will they allow the generator to stay on-line through all sorts of power swings and system events short of separation from the utility? Or do they trip off for recoverable events?

 

[ScottyUK]

Hi David,

I think that is a loaded question: yeah, they can be a pig to set up! Usually the settings are a compromise between reliably detecting the loss of grid connection and ensuring the relay doesn't operate under recoverable conditions. They work well when the generator is 'small' compared to the utility source. When the generator is comparable in size to, or larger than, the utility source then they become problematic. The newer relays use a combination of both ROCOF and vector shift which supposedly improves security.

If the switchgear configuration is amenable, tripping the utility tie breaker while leaving the generator connected to the load at least avoids dropping the load. This can be a demanding requirement for the generator governor, especially if the export to the utility is large relative to the local load. The load will almost certainly see a frequency excursion. The same applies to use of the intertrip.

Merry Christmas!

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Sometimes I only open my mouth to swap feet...

 

[Runsor]

ROCOF or df/dt is as incapable as other measuring methods, when the load of grid doesn't include a major load change.

In Denmark they use positive sequence under-voltage U1< to detect loss of grid, sometimes together with ROCOF. But there are problems. Generators have been disconnected, when they should have been supporting the voltage during remote faults.
See page 33 "2-phase fault in the 150 kV network".

http://www.energia.fi/attachment.asp?Section=4522&Item=10796

47 CHP plants greater than 2 MW were disconnected at distance of up to 150 km from the fault. The number of disconnected small wind turbines is unknown.

 

[bacon4life]

Runsor

That is a really interesting article on how distributed generation and wind power has affected Denmark. I sure hadn't realized anywhere had 55% DG.

 

[sharmaces]

Thanks ScottyUK for the SEG stuff. However, as far as I understand the subject and the prevailing detection techniquies, all of the Passive or Active schemes have Non Detection Zone. I have come across a scheme (also referred by alehman), in which the reactive power change is sensed at AVR to address the loss of grid. Unfortunately I don't have any details. I shall appreciate if you or any one else can provide some detail on reactive power sensing at AVR.