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# DTT for small distribution generator

## DTT for small distribution generator

(OP)
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.

### RE: DTT for small distribution generator

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

### RE: DTT for small distribution generator

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.

### RE: DTT for small distribution generator

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.

### RE: DTT for small distribution generator

@QBplanner,

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

### RE: DTT for small distribution generator

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

### RE: DTT for small distribution generator

@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.

### RE: DTT for small distribution generator

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

### RE: DTT for small distribution generator

@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.

### RE: DTT for small distribution generator

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.

### RE: DTT for small distribution generator

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)

### RE: DTT for small distribution generator

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

### RE: DTT for small distribution generator

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.

### RE: DTT for small distribution generator

@Sn00ze
here is what happenedin the utility I work. For most of D-IPPs without DTTs. for any force outages, to allow crew to start work on the faulted feeder. control center will call IPP to go off line and open entrance disconnect switches. After that, PSSP worker will be sent to virtually confirm if the IPP disconnect switch is open. then crew will start to work if there are no other sources on that feeder.
For any scheduled outages, 5 days in advance notice to D-IPPs.

@waross
"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."
maybe in Canada 30 years ago when people were still caring about life and customers. and the utility I work, a few years ago one P&C tech. was killed during the work in a indoor sub.and his accompany even left him alone and went out for sth else. Nothing happened to the CEO. nothing happened to anybody. I appreciate your old school staff but what I learned here is completely different than what you experienced.
Give your a example, Here is what happening in the utility now, last year, a sub is supplied by 2 X 230kV lines, one line is under maintenance so the whole 230kV sub was supplied by one 230kV line and P&C tech wanted to wire checking on one of the two CBs for the remaining 230kV line. The work even got approved and during the work sth happened and mistakenly tripped the remaining 230kV line and lost 60000 customers. the number is correct, 60000 customers. what happened next is even more funny, the utility claimed that is trying hard to restore the power so the customer should appreciate it.
@davidbeach
P&C guy like you always think you save the world. I have no doubt about that the first person gets call when sth goes wrong will be P&C guys in any utility. Guess what in the utility I work when P&C guys could not figure it out what had happened because they checked everything out and nothing went wrong they either asked help from system planners to figure out from broader aspects other than the narrow P&C perspectives. or they start to blame system design.

When the system I designed energized successfully, I got a call from control center manager said thank you for the system you designed it works beautifully. I am not aware that P&C guy got the similar phone call to show appreciation. DO I need to mention the reason?
BTW,
I am not like you said only knows N-0,N-1,N-2. These are just some dummy new generation planners who work in a utility for a few years and want a manager position and who only know following NERC AND WECC standards. I was trained in the country I came from by the old timers and dealing with much more complicated network issues than here in North America. I came here was trying to learn from the best such as experts in BPA and the utility I work. What I learned quickly the facts is these experts I called they are either planning the system for God or retired long time ago. The people around me from manager to the manager's manager to the VP have no clues what the hell they are doing and they are not even electrical engineers. what a pity.

### RE: DTT for small distribution generator

Although I do find QBPlanner's negativity and cynicism somewhat disheartening, I cannot completely disagree with him, either; ime [in my experience] utilities, and sometimes even the disparate geographic regions of a given utility, have varying levels of dysfunctionality. The eventual outcomes of court proceedings, criminal prosecutions, etc. can also be all over the map.

Within my utility, freewave communication media are becoming the comm path of choice; relatively inexpensive and generally quite reliable, although not universally so...and the instances where DG is tripped off due to loss of comm tends to replicate in specific schemes, iow there is some sort of problem with that particular comm path not being as line-of-sight as designed or constructed, vegetation behaviours not being taken into account, specific types of atmospheric conditions rendering that specific scheme's function sub-optimal at times, etc., etc., and where these situations occur the developers/investors/owners may not have deep enough pockets to afford the changes needed to correct the issues, threatening the viability of the entire project.

Hope this helps.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

### RE: DTT for small distribution generator

Where I am at, P&C is at a loggerhead with planners. It is difficult to get good models for fault current. We have to check the models and sometimes are told why we don't create the model additions. I don't know why it is this way. It probably won't get fixed until an expensive mistake happens even though people have been complaining about it for awhile.

### RE: DTT for small distribution generator

What I've found is that the planners are likely to have a model that is wider in extent, more of the relevant interconnection, but far shallower than the protection models. From what I've seen, planners live in a dull and boring world of positive sequence. The fault models allow entry in the vastly more interesting world where things also happen in the negative and zero sequences. The vast majority of faults cannot be looked at, with any real meaning, in just the positive sequence. Ground faults and zero sequence mutual coupling - that's where the fun can be found.

I once had a planner ask me what the impedance was for a ground fault at a certain bus. Seemed an odd question, as it obviously could anything one might want it to be from zero up, particularly the resistive component. No, that's not what he meant, he wanted the zero and negative sequence source impedances associated with that fault so that he could use them as the fault impedance to get the right positive sequence values. Yep, the positive sequence values were correct, but for what? No ground fault I've ever been aware of produces a balanced set of phase currents nor a balanced set of phase voltages. If your world is just the positive sequence it is apparently good enough, but seems to me to miss the entire point of running the fault in the first place. Most of the other planners seem to be much more in touch with how the system actually works.

### RE: DTT for small distribution generator

Maybe in the utility you r in that planners only know the positive sequence network. In the utility I am in we calculated SLG impedance by hand using positive, negative and zero sequence network impedances. Also when performing EMTP study, we have much complicated model than P&C planning has in Aspen. you may disagree.
NTW, positive, negative and zero just a mathematical models just like ABC to DQ0, there is nothing fancy and so simple.
Try double fourier transformation if you want to understand what is complicated.

### RE: DTT for small distribution generator

Modeling lines probably has to only be withing 10% for P&C with the margins we use. We still have a hard time getting them to put anything in the model because they don't have all the commitments or know all the construction details or something. I don't understand the whys completely. Per/mile values would probably be enough for all or most of the line modeling and that includes negative and zero sequence. Screwing up modeling a transformer can easily swing around the fault current by way more than 10% and that occasionally happens. We have mutual coupling modeled everywhere down to the 0.00001 PU. I think CYA engineering was made the priority on that one. 99% of our mutual coupling data serves no purpose other than someone can say everything is modeled.

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