DTT for small distribution generator 1

[JensenDrive]

I have a case of a small distribution generator that is sized larger then the minimum load on the line, so the generator has the ability to carry line load if the utility trips. The utility is saying a direct transfer trip (DTT) scheme will be needed to ensure the gen trips if the utility trips. So far the utility is not saying what technology is going to be needed. I have someone telling me unlicensed radio transceivers would be the way to go (SEL has a couple items), but I question whether the utility is going to buy using unlicensed radio for DTT. Might swing as this is a low risk application. Before I talk to the utility engineer, does anyone have any experience in low budget DTT? Running fiber from the substation to the customer of course is a nice idea, but it seems a major investment.

 

[waross]

If you are running the generator in droop mode you may be able to look at the frequency shift when you lose the grid reference. This won't work if the generator output is exactly equal to the line load.
With the radios, this may be acceptable if the generator trips on the LOSS of radio signal.

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

 

[ScottyUK]

In the UK embedded machines are provided with protection meeting the requirements of the ENA's G.59 performance specification. Typically this is implemented using a rate-of-change-of-frequency (ROCOF) or vector shift relay, or occasionally both functions are implemented within a single relay.

 

[FreddyNurk]

As waross suggests, I'd be looking at some sort of protection scheme for dropping the generator off, these are becoming a lot more common with all the renewable generation on the networks these days. Its really up to the utility, but its possible they would accept a scheme that includes frequency rate of change (similar to what waross is suggesting) and likely reverse power instead. The protection scheme will likely end up being cheaper than running fibre.

I've seen fibre trip units, but haven't seen radio link ones, although if they're made to protection relay standards (e.g IEC60255) and operate the way waross suggests then the utility might accept them. Your client may not if the radio link keeps going down though.

EDMS Australia

 

[QBplanner]

You should ask the utility why the DTT is required. I don't know the other utilities, but the utility I am working has some kind of problems with the P&C department. every time a IPP is coming in (D/T) P&C will ask for DTT for transfer trip. The common reason they give is to avoid O/V or other BS reasons and ripe off the customer which I think it is not a P&C issue. Because the P&C scheme does not cost much but the telecom does. In my utility , P&C decides the telecom requirement. In a latest application with a T IPP, P&C asked DTT and fibre for telecom upgrade which costs about $15 million for the customer. I was questioning why fibre is needed other than leased telephone line, it is not even a BES (above 100kV here in North America) system. They have no answers. It is a bully culture.

 

[Sn00ze]

@QBplanner:

In my province, Leased lines are no longer offered by the telecom company so we have to use either radio or f/o

 

[QBplanner]

I have not seen any utilities proving that the O/V does exist when with the the D-IPPs.

 

[marks1080]

If you need a signal for the generators breaker status (open or closed) than you'll need communications. If you need communications, radio may be a good option. When our telecom folks look at this option I believe they have to survey the area to make sure the geography (and whatever other variables they consider) will work for radio.

If you do not need to monitor anything regarding the generators set up, the suggestion from waross is a good one, but depending on your area it may not be good enough for your regulators/asset planners. Some (most??) utilities won't want to rely on the generator's protection doing the work, rather they will want to explicitly take the generator off line. Most larger utilities will have anti-islanding policies that will dictate how you are to do this.

QBPlanner: your comments seem to be biased. Listen to your P&C technical experts. Ignoring them will almost always cost you more money in the end. There are many technically sound (as well as policy sound) reasons to explicitly trip an embedded generator. Keep in mind, it's typically the utility that guarantees power quality to its customers. If you were in that situation would you want to be responsible for guaranteeing the power quality to your customers when their power is coming from a third party? If I were tasked with providing ammunition of why a utility should enforce anti-islanding, overvoltage may not make my top ten list of reasons why you want anti-islanding. I can see why you're frustrated if this is the reason always given to you, but there are much better reasons people should be giving you when you ask these questions.

 

[GroovyGuy]

Howdy,
I have used the radio link solution in previous projects, including the Utility's SCADA system. This seems to work quite well and the Utilities tend to like it since it is so simple.

Many years ago I installed a rather large inverter on a relatively weak 25kV rural distribution feeder. In this case it was possible for the inverter to carry the entire load if all of the stars were in the right alignment. If I recall correctly the inverter was equipped with an anti-islanding algorithm (IEE-1547). This also worked quite well although I don't know if this spec can be applied to a DG. Comments?
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[QBplanner]

@marks1080
I don't think P&C experts will contribute anything useful to the DTT because if a DTT is required or not is not even a P&C issue.
islanding operation is not a P&C issue and a system issue. P&C is always a second line following by the system design In my humble opinion.

For any system, system planner comes in first before P&C guys say anything.
power quality again is not a P&C issue either. These are system issues.

BTW, the utility I work does not guarantee power quality to its customers unless sth burned.

For Jensondrive's question, there are two issues, first of all, is DTT a must? second, what facilitate the DTT?
Most of the utilities as you said only use policy to guard it without proving the need of the DTT.
In the utility I work, P&C guy jumps out claims that a DTT is required for islanding purposes then throw a expensive Fibre upgrade to the customers.
I respect P&C expert my father worked on P&C for 45 years. But the utility I work is quite different. P&C people always bully the system people.

 

[JensenDrive]

Thanks to all for the input. Will contact the utiity and SEL and get their thoughts. I sort of hesitate to recommend the gen dropping out if there is a loss of signal. I do not know how often and how long a dropout occurs. I suspect this DTT matter will kill the project. We are in the estimating stage and if I add 5 miles of fiber hung on distribution lines or tall antennes (and repeaters?), or leased line to the sub (getting a phone line into a sub is no small task), well, this may be a deal killer. It is only a 500kW solar site, so it does not have a lot of $$ to play with. Odd the utility is afraid of solar cell islanding and carrying the line load.

 

[Sn00ze]

@QBplanner,

So if leased lines are not an option, What is your solution other than DTT?

 

[marks1080]

QBplanner I would agree if i'd ever met a good planner ok. i got my punch in TOO. don't derail the thread.

 

[QBplanner]

@Sn00ze
Before we get into the telecom solution for the DTT, we should first ask why the DTT is required by the utility.
As I said before most of the utilities now a days asking for a DTT with no sound justifications.

For the 500KW solar one, why a DTT is required is a question to me. I would assume that this 500KW solar connecting to a feeder radially with some loads on it and larger than the minimum load on that feeder as Jenson pointed out.

Solar is not a dependable type of generator so the generation output is not constant all the time especially during the night when the load is light. So if the generation is not constant and load is not constant, how can the utility draw the conclusion that a DTT is required.
I will question the utility under what scenario that the DTT is required. what standards or criteria the utility is following to require the DTT and should share them and the study results with the customer. I will not buy in that with a minimum feeder load and maximum solar generation then there will be a O/V voltage or over frequency happening and the power quality is not acceptable. Deterministic planning only applies to the generation with DGC but not these D-IPPs with almost no fixed patterns.

If the utility can demonstrate a DTT is a must which I don't think so. then we can discuss the ways to resolve generation tripping issue such as what Waross suggested.

 

[QBplanner]

@marks1080
I will not derail the thread I think people deserve different angles of a problem other than do whatever the utility asked for.

 

[Sn00ze]

@QBplanner,

Correct me if i'm wrong but, if it is an IPP, then for maintenance crew or during a fault - for the safety of personnel. You would have to have anti-islanding. How are you going to achieve this without DTT? At the very least, it would seem to me, to be a very valid precaution. Whether the load is intermittent or not is irrelevant, since the maintenance crew will not/should not assume it is not powering up lines/equipment.

 

[FreddyNurk]

DTT is definitely not the only means of solving the problem, although it does make it easier for the utility to ensure that they have the only ground reference when feeding into their network. There are a number of protection relays available that have the capability to detect an islanding condition and disconnect the plant, indeed, the utilities here have been quite busy developing connection standards for this exact problem.

Unfortunately the extent of the anti-islanding protection often depends on how the connected network is configured (i.e. generation capacity greater than a certain amount, with the site on its own distribution transformer, as compared to LV only feeding into a sub-board) and the utility will likely require more protection as a result.

At 1/2MW of Solar, if there's a chance it can feed back into an MV network, then the fibre option may not look so expensive compared to other requirements after all. SEL certainly has some capable products for this purpose, as does the MiCOM series of relays. ComAp even makes a couple of lower cost options that have been quite popular for this purpose. The real argument is convincing the utility that you have all the bases covered. Here is a link to one of the Australian utility's standards for connection of such equipment, whilst it won't be useful to follow for your utility, it may provide some guidance as to what options might be available to discuss with the utility.

EDMS Australia

 

[GroovyGuy]

Howdy J-Drive;
Yes I agree with you that 5km of FO cable is not cheap, and may kill the project.
What about a (spread-spectrum) radio link between the Utility and your inverter? A distance of 5km is not an issue for these units, as long as you can maintain line-of-sight between the two radio units. (I have done 25kM on previous projects). These units are inexpensive and reliable.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[waross]

If your anti islanding scheme fails and a backfeed into a de-energized line causes an injury to a worker, the consequences may be more than the utility wants to be exposed to.
More and more jurisdictions are deciding that it is NOT okay to kill or injure workers.
In Canada the CEO of a company may be charged with a criminal offence in the event of the injury or death of a worker if it can be shown that accepted safety practices were not followed.
If the utility wants a trip that is under their direct control, (manually or automatically) then the issue is;
What is the most economical way to meet the requirements of the utility?
I am sure that there are available radio systems that can do the job.
As an example;
In the 70's I was driving a service truck with a two way radio.
The frequency was shared by a number of users.
There was a mute feature on the radios. We didn't hear calls to any other user, however when someone in our company made a call, all of our companies sets would be unmuted.
Remember the pagers that we used to carry before cell phones?
I am sure that there are many dependable reliable radio solutions.
There may be leased frequencies or systems, there will be self owned systems and there will be systems available on a monthly basis.
The issue is to find a solution that you can afford and that the utility will accept.

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

 

[davidbeach]

500kW seems small to need transfer trip, but I can't see the broader picture.

So, QB looks at the planner's N-1, N-2, or N-1-1 world and says all is well, no problems with this installation. The P&C guy knows that when the N-2-1-2-3 event occurs everybody is going to be after his arse demanding to know why it didn't stop at N-2-1. At that point nobody's going to care that the first N-2 didn't cause any particular concern. The P&C guy (or gal) isn't really chicken little, he knows that the sky has fallen and he's carrying it around on his back trying to keep it from making it all the way to the ground.

The planner in his N-x world considers a fault to be a contingency; the P&C guy knows better. The fault is a given; it's there just waiting for the worst possible time and place. Givens are not contingencies; contingencies are things that either happen prior to the fault (line out, transformer out, etc.) or contingencies are the results of things not going according to Hoyle post fault.

Planners and P&C folk can get along well or they can be ever at logger heads; each just needs to understand the other's world. They're not the same world by any means. I've never known a situation where the planner gets the first call from the control center to find out what the he!! just went wrong; no, that call goes to the protection engineer. I take those calls, I make the evaluations, and now and then I recommend planning criteria changes to the planners. And, yes, I get along very well with the planners and transmission ops engineers I work with. Planners deal with the distant future, the transmission operations engineers deal with the immediate future, and the protection operations engineer carries around the pooper scooper. That protection operations experience turns around and drives the protection planning side of the Protection Operations and Planning Engineer. Know how sh!t happens and try to make Murphy figure out a different tact next time. But don't dis Murphy too much, after all he sets the table every night as well as keeping a roof over my head and money in my bank account.

Bottom line, neither planners nor the P&C folk have a lock on "truth", each is right and they need to work together and recognize the differences in how they look at the world as it was, or as it will be.