Step Distance for Busbar Protection

Are you using protection relays? Have you had a look at the application notes for these to get some ideas?

A protection relay manufacturer may be able to advise the best combination of protection relays to achieve what you are after.

Yes, SEL relays.

Nothing about this scheme is mentioned and don't expect it to be- most aren't. The manuals just cover the basics and how to set them.

If the manual and guides are not clear enough you may need to employ a protection relay consultant (or similar) to assist in designing your protection scheme. This is what we have had to do in the past.

I'd like to learn this for myself though.

I would suggest having a look online for a technical book on the subject to assist in the self-learning process. Someone here may have some suggestions.

Its a safe bet you've never worked with transmission relaying answering the questions along the lines of "just connect brown with brown, blue with blue and green with green. This will make it light. Consult you local home center on guidance for further instruction. They have books with step by step instructions for further detail"

When an electrician knows home center employees posses just enough knowledge to burn down a home with the picture posted having 5 code violations that need to be addressed first along with installing a fan rated box to hold that paddle fan.

I get our payed consultants to do the design based on the customer spec. I may design and build with grid connected systems incorporating SEL and MiCOM relays myself but am not an expert on their protection settings and schemes, I leave that to others.

If I wanted to get more clued up myself I would go out and find some literature and examples to assist me, hence my response. Then I would get our payed consultants to check and revise it, again because I am not the expert.\

Your question on this forum is beyond what you should realistically expect a detailed answer on how you should do what you are asking. You will likely need to hire professional help for a detailed answer.

I can't find any literature on the subject. I'm sure its out there, but can't find it.

I'm not looking for a detailed step by step of what ohms and angles I should use, but rather general guidance and if this is even doable to begin with.

If I am smart enough to think this up myself, then I am certainly smart enough understand the pointers. At this point this thread is predominantly disclaimers as though I'm some clueless DIY looking to change out my consumer unit 'what is the green yellow wire for and why does it connect to this button thingy?'

Maybe you would like to learn this for yourself, but the cost of failure if you do it wrong is quite steep (400kV/132kV transformers are not bought at your local hardware store!), so you still should get professional assistance. When you hire the consultant to do the settings, stipulate that they provide a detailed explanation of how the settings were reached, so that you can learn from that how it was done.

There are some things where you can just "jump in and try to swim," and there are others where a failure will endanger others and prove very expensive. I think this is one of the second kind.

In other words you wouldn't know how to do it yourself. A senior protective relaying engineer would be able to say if this will be able work or not from the start and to what degree.

Not sure why you would use transmission line relays for transformer protection. Internal and through fault protection is done with differential and straight over current relays, nothing complicated. The distance relay settings are adjusted to maintain system stability, or prevent burning the line through.

As a backup form of busbar protection. The SEL-487Es don't have any step distance elements within them. Unless directional over current could work somehow.

Back up protection for bus bars is usually implied with line conductor protection, as the bus bars are (usually) sized to exceed the line conductor thermal ratings. Also, the bus bars are implicitly protected by transformer through fault protection, which would be your backup for your line protection relays or breakers failing. If your busbars are a weak link, somehow, a set of 3 phase over current relays is a common solution to keep I2R damage to the copper within limits.

Not sure which bus bars are of concern, HV or LV, and what breakers are available to interrupt a fault. Trying to use distance relays to discriminate and protect a relatively short length of conductor will be difficult at best. Perhaps a small sketch of your double bus substation will make clear your concerns, as those words could be interpreted several ways.

BTW, your distance relays are directional over current relays, fancier version. You need the polarizing source usually from voltage transformers.

Both the HV and LV busbars.

The idea is that is a fault takes place on any one bus, only that bus will clear and not the other.

Identical scheme:

https://www.youtube.com/watch?v=nfOSlqqzuQk

The only way I can see to do this quickly and no mistakes is with bus differentials, where the sum of all generation matches the sum of all loads, fast clearing is assured, and system stability is maintained. You need CTs on every branch.

Distance relays or simpler, as you suggest, directional overcurrent relays on every generation source breaker, (4 sources x 2 buses) could discriminate which bus is carrying the fault current, your fault power factor would normally be very low, so close in to your relaying, so distance relays would be unnecessary complexity.

You need to match the (very inverse?) time characteristic of your load line protection, and stack your new protection above them, so you don’t shut off your buses for a through fault. So you save some CT wiring for your load breakers, but now you have a complex protection scheme that requires lots of modeled power system information to set up, will require careful examination of all cases where power system configuration will affect fault values and time-current settings, and still some fault outside of your protective zone can make a false trip, Murphy’s Law. So that the poor electrical engineer after you can understand the protection system, a simple foolproof bus differential protection will give you the best chance you have of surviving a really bad event, major flashover on the heart of your equipment, keeping the power on, by getting the bad equipment off the system in the shortest time possible with today’s technology. Your future protege will appreciate it, at the very least.

This is the replies I am looking for.

Ideally numerical busbar protection would be ideal, but in older stations, especially those in the 3rd world, such protection is a pipe dream. However with EM relays being replaced by SELs there is in theory the option to make use of what already exists.

Each bay including the bus coupler has CTs.

I'll think about directional over current- but- in my eyes step distance would be the most secure and the most predictable.

You really don’t have any distance to work with( I’m assuming) so the advantage of protection that makes use of reactance and resistance of your protected equipment (simple open bus bars?) will be applying your distance relays at the end limit of their setting. Maybe confusing complexity for next person in your job to analyze.

However, if you set them up as an extension of your line protection distance relays, by physical location of the relays and wiring, it might be obvious the method to coordinate them, which will be your greatest challenge to provide a robust system that will survive longer than your career. I have both types of projects in protective relays, even in the same plant. Complex protection well documented on single line diagram similar to your application where bus differential not possible, pilot wire equipment that could not stand up to local lightning, lasted 2 years, directional current backup for pilot wire working fine since 1978.

The key to applications like this is to go through how each relay will react to various faults on the system, and make sure it will react as desired. A tedious job on a system like yours, but that’s how we earn our pay.

Open busbar, plus 10% of the shortest outgoing lines.

I mean if a bus fault can take out 50% of the circuits vs 100% of the circuits then its worth it.

That sounds reasonable, open bars are exposed to significant fault potential due to wind blown debris and insulator failure.

The trick is to make sure your protection does not operate for faults outside of the protection zone, though in many cases of incorrect operation, the penalties are not obvious to your potentially most vocal critics. Often, the mis-operation is accepted as normal or unavoidable.

I know, but I'm sure how the step distance at the transformer terminals will function or coordinate.

I have the lines and bus coupler set ok I think.

Sounds like you’re all set, then.

Far from it, will it even work for the transformers? If the secondary is feeding radial load, I'm not sure much current will pass through the transformers during the fault.

Also, should I have my MHO reach through the transformer into the faulted busbar or look into the faulted busbar via the breaker? Reason I ask is if the fault is coming from a burst CCVT, the latter may not work. However the prior I'm not sure about coordination.

I think modeling the transformer impedance in a relay designed for line protection will be difficult. The typical transformer is X/R of 12, so your transformer will look like miles of odd characteristic line, I think better not to try to look through it.

Your short circuit study will have the various fault currents through each transformer covering the faults at any location. You can use this information to figure out how each of your relays will respond for each case. I’d expect that you need to analyze fault current for at least 8 cases, 3 sub-cases for each; line to ground, double line to ground & 3 phase bolted fault, and if you have high and low generation available, 48 cases, fault in each transformer, bus faults either side, and remote transmission line faults at various distances up to and including downstream breakers with relaying you need to evaluate the response, especially if that line has lots of your generation & fault contribution.

Once you’ve gone through this, then you will know if your planned relays will actually coordinate and sectionalize your power system properly. You need to consider the saturation characteristics of your current transformers, and adjust your pickup settings accordingly. For instance, a residually connected element, though mathematically set to operate a 1/2 ampere of unbalanced current, can require 5 amperes worth of primary current to operate reliably, due to CT saturation characteristics, this observed and measured during field tests with induction disk type relays. I.e. rule of thumb, 2000/5 CTs will need to see 2000 amperes of primary current on one phase to reliably operate a residual relay set to pick up at 0.5 amperes. Solid state relays could be different, as the nature of the CT burden has a significant affect on the CT ratio accuracy.

Once you’ve done one of these, the next ones will go much faster, you’ll figure out which cases don’t need individual solution, but until you’ve gone through it once, check them all out.

I don't want to look through it either, but do you think a backup if you will be a good idea? I ask because while looking directly into the bus is ideal, what if the bus fault is from a failed CCVT where as the bus on the other side will have healthy CCVTs.

BTW, great reply, thank you! :)

I think a faulted Capacitively Coupled Voltage Transformer is likely to be self clearing, it seems like this system will have plenty of fault capacity to explode & burn the coupler before anything but bus differentials could clear it. I don’t see how the CCVT would be much different than a sheet of metal roofing or an animal bridging your bus insulators.

Also, your relays will be using the CCVTs for phase angle determination, I assume, so that loss will change how your relays react, not sure how, though.

i think if you try to look through the transformer, you’ll need to stack these relays above all of your feeder relays, so you don’t get false trips, so uncertain if there is any value associated with the extra complexity?

A failed CCVT will cause loss of 3 phase voltage to the step distance elements.

Also, what do think about decreasing zone 2 and having a busbar fault clear through zone 2? Only thing holding me back is the reclosing. What would happen if 6 lines all reclosed at once into a faulted bus?

Not knowing the particulars of your install, if you can adjust the relay to do this, it should work fine. I was thinking you would be out of range due to X or R.

I don’t think the reclosers will make much difference to the relays, unless they are reclosing faster than the reset time of the relays. Old style induction disc relays have mechanical inertia, so you need a certain Off Time in order to maintain the original timing, could be several seconds to bring the disk back to start location.

I have no practical experience with recloser applications, BTW, large/heavy industrials, the size of small utilities.

You make a good point... though I know of applications where line relays are deliberately set to reach through a distant transformer.

Oh yes, if you have all of the information, it should be good. A distant transformer may be easier than one in right front of your face, a lot of delay to let the local protection sectionalize. You won't really know if you can set the transformer distance elements until you get the SC study results and do the coordination, but I would assume that you can. If not, it is probably not unusual to disable them, especially with 4 paralleled autotransformers.

Typically you don't even have distance elements on the trafo bays, but in this case I've got SEL-487E and 311Cs in the control panel.

I have to say that no reaching through the transfomer is perhaps the best advice I've gotten so far- but- devils advocate- if you could reach through it would you do it? I'm still freaking out over a failed CCVT trigger a bus fault and dropping 3 phase voltage from the relay.

I assume you have separate voltage sources for each load bus, maybe none for each transformer? If you do lose the polarizing voltage, how do the relays react?

i would assume they might back down to straight over current relays, or turn off, assuming there are already backup relays.

You’d lose selectivity, but prevent a larger disaster...

Each bus has there own CCVTs.

Reverting to over current for loss of voltage might be a good idea preventing a bigger outage.

I’d assume that would be built into your distance relays, being microprocessor based, an easy option.

It is.