Placement of VTs/PTs 3

[Mbrooke]

Is any one aware of, or sees an advantage in placing PTs within breaker bays (between the circuit breaker and disconnect) instead of on the outgoing line positions?

 

[davidbeach]

In the electromechanical days the relays accounted for much of the CT secondary burden, but now the bulk of the burden is the cable between the breaker and the control house.

Opening the current blades of the test switches before removing a relay takes care of keeping the loop intact.

 

[Mbrooke]

How much burden does a modern microprocessor relay add? Id imagine looping through 12 relays adds substantial burden?

 

[davidbeach]

At 15A secondary current, an SEL-487E has 2.51VA of burden. At 8 times tap, the HU that the SEL replaced has a burden in the 50-60 VA range for each restraint; plus operate circuit has a burden of about 1.5 times that much. Three KD line relays would have had a burden of about 6 VA at 5 A secondary vs less than .5 VA for the SEL-411L.

 

[Mbrooke]

Sounds like I could get away without shorting any of the CT inputs when not in use an just tell the relays to ignore breaker 2 current when desired via user interface.


Thank you so much with all your time and help. I just want to say your idea about using an SEL-411L was a life saver and I truly appreciate it. I certainly would have missed it. Looking at the manual it even lets you configure the actual bus bar being used (on screen) so it truly does work for any application, even those outside being advertised in the brochure.

 

[Mbrooke]

For future readers who may stumble upon this same issue or thread I finally found the papers you were referring to btw.

https://conferences.wsu.edu/forms/wprc/2014/Papers/bea2_pap.pdf

https://conferences.wsu.edu/forms/wprc/2014/Presentations/bea2_ppt.pdf

I just want to say that these are worth an IEEE (or equivalent) publication IMO. International recognition is much needed, which I will explain. I can say for a fact there are many engineers/utilities all over the world who encounter this very same problem regarding M&T where this simple solution often gets missed. For example, I've been told in India main and transfer buss is used on nearly all 66 and 132kv with half of 220kv substations holding it as well. The rest of 220kv and half of 400Kv is single breaker-double main with transfer which also holds the same philosophy. An example of such:

http://www.bsptcl.in/FINAL_SLD/Gaurichak.pdf

Single 3 phase CT Electromechanical relays are being replaced with modern microprocessors and new substations receiving the same. Single 3 phase CT relays are chosen simply because engineers are unaware that dual breaker relays can take care of the complexity despite being available from IEC manufactures. To add insult to injury main and transfer substations dominate new construction in developing countries. As load grows necessitating higher reliability, the transfer disconnect is removed and a compact disconnecting circuit breaker takes it place (or a modified GIS solution) similar to what is mention here:

https://www.google.com/url?sa=t&rct...j6tL_0_qrOAhVESSYKHWBdBu8QFggrMAU&url=http://

http://www.cigre.org/content/downlo...Sg8p8NIMjf7PeC0ZzEuvxg&bvm=bv.129391328,d.eWE

The single CT relays are then called upon to run 2 breakers when a dual breaker relay could have been selected in the first place. In fact the added cost of a dual bay relay is easily off set by those dedicated relays which can be eliminated for the bus tie. In fact its things like this sold by manufactures which reinforces the myth dedicated relays or CT switching is needed for M&T:

https://cdn.selinc.com/assets/Liter...ure/Flyers/2126_PF00118.pdf?v=20150812-085228

For reasons I have yet to understand I have not heard a word from manufactures regarding dual breaker relays being applied to M&T applications. While M&T and other transfer variant substations are historically less common and fading away in North America, they make up for what I would say 70% of all substations around the world.

For this reason I believe manufactures should be offering literature on this solution along with options to configure software and relay labeling to reflect it. (like changing line VT to buss VT and buss VT to line VT).

Have you presented any of this to say SEL?

 

[davidbeach]

They are well aware of my feelings about their failure to consider that bus configuration.

 

[Mbrooke]

And oh, the protection issues mentioned with paralleling CTs (such as a station latter going to double breaker double buss) are documented and legit.

 

[Mbrooke]

They are well aware of my feelings about their failure to consider that bus configuration.

Sounds like they are refusing to listen. Most of the guys which I tell them about this are hearing it for the first time.

 

[Mbrooke]

Update... This should perhaps be a new thread but it ties into this. SEL's latest versions of the 421 have 6 voltage inputs (VAY, VBY, VCY & VAZ, VBZ, VCZ) instead of the older VA, VB, VC, VN & VS, NS)Page 22:

https://selinc.cachefly.net/assets/...ets/421-4-5_DS_20160624.pdf?v=20160805-161028

I am just digging into the manual, however would this be any indication that I can swap bus and line voltage inputs on the relay to get the intended displays? Or is there another reason for the change and if so would anyone know?

 

[davidbeach]

Nope; you can tell the relay to base the protection on VY or on VZ. But which ever one you choose, the 3-phase voltage used for protection is by definition (in the relay) the line voltage.

 

[Mbrooke]

Ok, thanks But why do they offer a full 3 phase VZ? And am I correct to assume that on the 311C, you have the option to tell the relay to base VA,VB,VC on the line instead of the bus bar? I ask because the 311C's manual appears to offer such an option (see attached pic).

 

[davidbeach]

If you don't want synch voltages, you can set the relay up to use either of two 3-phase voltages. Say for instance you're doing line protection in a double bus-double breaker station and the only 3-phase voltages on on the buses rather than on the lines. You could have a preferred voltage, say the north bus if the north breaker is closed, but switch to the south bus voltage if the north breaker is open and the south breaker closed. I wouldn't, but maybe somebody else would...

 

[Mbrooke]

Another bizarre relay function that has no bearing to the real world LOL! In the very least, can LOP (loss of potential) be set to switch between those 2 sets of 3 phase inputs or am I being to optimistic?

 

[davidbeach]

If you can define the criteria, you can program the relay based on that criteria. The problem with alternate protection voltages is that you give up on synch check capability. Personally, I'd rather wire the A and B relays to different voltages (at least different secondaries with different fuses), provide LOP alarms, and keep the synch check/voltage supervision capability that have multiple protection voltage sources. We also program overcurrent elements that turned on an LOP condition is declared; obviously these are non-directional overcurrents and may not coordinate well, but some allowance is given for last ditch efforts to keep the line protected. PRC-023, for instance, excludes overcurrents active only during LOP conditions.

 

[Mbrooke]

I will make note of losing sync check on 2, 3 phase inputs (one would think the relay manufacture would not do that). As for the rest it makes sense and pretty much the same here. Straight buss for me always has 2, 3 phase VTs on the buss with each set dedicated to either A or B relays. Less critical stations have one 3 phase set on the buss, but each individual winding is dedicated to either A or B relay group.


If the 311C (or 421) can legitimately be programmed recognize 3 phase sensing on the line in straight buss applications, it has been spoken placing 3 VTs on the line for step distance protection and using the buss only for single phase sync check. That way if an LOP occurred, it would effect the relays only for one bay (circuit) and not every single A or B relay in the station.

When all of A or B are effected, two general options exist: 1. disable all protection for the group under LOP. Works well provided all the remaining groups are in service and none will fail. 2. Have the LOP group revert to over-current only. The risk here is that a fault elsewhere on the system can trip multiple lines due to lack of coordination on one group even though the other group is fully functional. Of course setting LOP overcurrent times delays slower than zone 1 reduces some risk of miscordination, but not all.


Now, when 3 phase current sensing is on the line, an LOP will only cause one bay to become a trade off, not all of them. Is my thinking correct here?

 

[scottf]

Davidbeach-

Your comments reminded me of a situation coming up with more regularity in the IEEE market. We have a lot of customers now requesting MV VTs with 2 secondary windings, where the traditional requirement has been 1 secondary winding for these voltage levels. The feedback we get is that the protection group wants 2 secondary windings for redundancy or the meter group and the protection group want to each have their own winding to separate the voltage source.

Given that VT outputs typically go through a junction/marshalling box where fusing is provided, it seems to me to be much simpler to just provided multiple fused circuits for each phase to provide isolation/redundancy. Reason being is that a VT with 2 secondary windings does not provide true isolation between the 2 windings. If there is a short/fault on one winding, it impacts the voltage on the other secondary windings, since they are both wound about the same core.

I now see a lot of utilities buying larger and more expensive VTs, which in reality, do not provide any significant improvement on redundancy over just adding an additional fused circuit from a single seocndary winding.

 

[davidbeach]

For better or worse, NERC recognizes two VT secondaries as being redundant even if they share the same primary.

 

[scottf]

Interesting, thanks. Is there a specific code/clause you could refer me to?

 

[Mbrooke]

Given that VT outputs typically go through a junction/marshalling box where fusing is provided, it seems to me to be much simpler to just provided multiple fused circuits for each phase to provide isolation/redundancy. Reason being is that a VT with 2 secondary windings does not provide true isolation between the 2 windings. If there is a short/fault on one winding, it impacts the voltage on the other secondary windings, since they are both wound about the same core.

This is exactly why I use individual, dedicated VTs for step distance protection.

For better or worse, NERC recognizes two VT secondaries as being redundant even if they share the same primary.

Perhaps this may not be the place to say this, but NERC is not on my favorites list. Countries that do not deal with such standards are way ahead imo.

 

[davidbeach]

I'm not sure there's anything enforceable at the moment, but a few years make there was a data request survey of the degree of redundancy of components of protection system components, they stated that a VT with independent secondaries would be considered redundant, at least for that purpose. Formal rules requiring redundancy haven't arrived yet. We lose VT fuses much more often than we lose VTs, so separate secondaries are far more redundant than one secondary daisy chained between relays.

Then there's the question of how do you lose a VT primary without tripping the bus or line it is connected to? I guess you might get an open, but anything nasty happening to one VT primary would render any other VT on the bus or line temporarily unavailable. It seems to me that a second, fully redundant, VT adds a lot of expense, space, and complexity for a fairly small gain in redundancy.