current splitting @ parallel paths thru switchyard - effect on hotspot 4

[electricpete]

Our powerplant has a breaker and half scheme in our 345kv switchyard. Our generator feeds 2200A at into one of the bays and splits in two directions through two generator-position breakers.

We have a hotspot identified by thermography on a disconnect for one of the generator position breaker disconnects.

First reading was 64C rise under 7.5mph wind conditions.
Two days later we had only 10C rise under 2.5mph wind conditions (normally expect decreased wind to cause temperature to go up). Thermographic images are here if anyone is interested:

http://maintenanceforums.com/evefil.../ubb.x/a/ga/ul/8071076041/Y601_Disconnect.ppt

I haven't been able to determine current in these two branches. Power flow through various circuits is monitored with revenue metering but due to deregulation, it is a highly protected secret.

I haven't drawn any conclusions as to the cause of the decrease in temeprature yet. It is possible the connection begins to heal itself or the current has changed either due to other external loading conditions or remotely possible the current has changed due to the resistance of the connection. I rule out the possibility that the OTHER parallel path same phase has a high resistance connection forcing higher current through this phase based on adjacent connection points... this phase is not higher than it's sister phases at the adjacent connection points. Again I am still gathering data and not drawing any conclusions but I have a question:

*** My questions are: how much effect do we effect the resistance of that degraded connection have on the splitting of current through parallel paths. To get to the same point, the two parallel paths have to go perhaps 80 yards distance through the shortest path (with multiple other parallel paths). It is tubular aluminum bus. I am suspecting that the current sharing in a normal circuit would be determined primarily by inductance and resistance would be much lower. If that were true, I wouldn't think the resistance of the hot connection would affect current sharing unless the resistance was extremely high. Any thoughts on this question? What would be the rough X/R ratio of this type of bus? ****

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[jghrist]

I think I follow Stevenal. The point is, I think, that not only is there an increase in the one of the parallel path impedances because of the high resistance connection, there is also an imbalance in the phase impedances. Normally, we use symmetrical analysis where the mutual impedance terms Z₀₁, Z₀₂, Z₁₂, Z₁₀, Z₂₁, and Z₂₀ are zero. If the ØA, ØB, and ØC resistances are unequal, this is not true. But unless the buses are transposed, it is not true anyway because the mutual reactances between phases are not equal.

 

[electricpete]

I think I understand the effect of mutual coupling and Steve's comments make sense qualitatively.

With two paths, 1 and 2, the current division is described by
(Ia*Za + Ib*Mba + Ic*Mcc)₁ = (Ia*Za + Ib*Mba + Ic*Mca)₂
where Za is self-impedance of A phase and Mba and Mca are the mutual impedance ot other phases.
Similar equations for other two phases. Also Ia1+Ia2 = Ia_generator (known) and same for other two phases. Gives 6 equations in 6 unknowns Ia1, Ia2, Ib1 etc

I'm still thinking about how large those M terms are compared to Z terms. I think our spacing may be closer to 30 feet than 10-15 feet that I said previously.

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[cuky2000]

Correction to Previous Post:
The average X/R~22 also adjusted to 30 ft spacing without considering the effect of the increase in resistance do to hot spot.

There is still enough room to considering the reactance the dominant factor even if there is increase in localized resistance in the switch.

I also believe that the effect of mutual impedance is not large enough to produce significant imbalance in the current flow in the external phase.

 

[electricpete]

Are you saying X/R is 100 with 13 foot spacing and 22 with 30 foot spacing? The only variable that changed was spacing?


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[electricpete]

If so, that would imply a much bigger interaction among phases than I had thought.

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[jghrist]

The reactance X increases with phase spacing. The resistance R stays the same. Increasing phase spacing would increase X/R. How does this jive with X/R being 100 with 13 foot spacing and 22 with 30 foot spacing?

 

[electricpete]

You're right, that's backwards. There must be some other change.

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[cuky2000]

How does this give with X/R being 100 with 13 foot spacing and 22 with 30 foot spacing?

There is an error in the previous post. The correction should be:

- Resistance at 70oC r= 5.284x10-6 Ohm/ft
- Reactance @ 1ft spacing xa= 3.96x10-5 Ohm/ft
- Geometric Mean Distance. GMD=(Dab.Dbc.Dca)^1/3=1.26xDab
- Reactance Spacing Factor xd= 5.292 x10-5.log(GMD)Ohm/ft
= 5.292[0.1+log(Dab)]. x10-5 Ohm/ft
- Total Reactance xt=xa+xd:
o For 13 ft: xt=0.104 Ohm/1000ft
o For 30 ft: xt=0.123 Ohm/1000ft

- Average X/R ratio = xt/r:
o For 13 ft: X/R=18.1
o For 30 ft: X/R=21.5

For additional Details see the enclose file containing a graph for 4,5 & 6 inches busses nominal diameter and phase-to-phase clearances from 13 to 33 ft.

I hope this contribute to clarify this issue

http://cuky2000.250free.com/XoverR.pdf

 

[mc5w]

A modern electronic revenue meter can be programmed to record amperage in each phase as well as the neutral. If you do not have access to this data then each metering point should have 2 revenue meters, one for use as the cash register and the other to record engineering data. Since the burden of a modern meter is very low and switchboard revenue meters have separate terminals for the power supply there should not be any change in accuracy if you install your own meter next to the cash register.

You should also be able to connect ammeters with remote readouts to the circuit protection current transformers in the circuit breakers. If you can put all of the remote amperage readouts onto a single dedicated board you would be able to read off the current division.

I do not think that your problem with getting the data that you need is a trade secrecy problem but rather that the accounting people are being territorial and political. The fear that someone else will screw up their database when reading it and so forth is a common fear and is often given for not hiring people. The fear is that someone who does not have EXTENSIVE experience with something will damage something or hurt themselves or take too long to do something or otherwise be disruptive. This illogic applies even to something which is rather easy such as 15 KV power cable terminations and splices - nobody does wiped lead joints anymore unless it is an underriver crossing.

In other news I did have an instance of a dirty fuse clip that caused a 240 volt motor to run partially single phase causing it to burn up. After rewinding this motor refused to come up to speed when idling. Some amp and voltage checks revealed the problem and cleaning and greasing all 3 fuse clips fixed the problem. However, in your case putting a voltmeter across a connection joint is rather impractical even if the you stick the voltmeter up there with stuff deenergized and the voltmeter uses a radio link to send information.

At any rate, if you have hotspots then you are overdue for maintenance. Thanks to catalytic converters metal that is out of doors corrodes a little faster than it did in the olden days. There is a reason why there are 2 antioxidant compounds that are formulated for use only with copper wire and copper alloy conduit threads. You will need lots of #220 silicon carbide abrasive paper - tests that Dr. Jesse Aronstein ran showed that a wire brush is 100% INEFFECTIVE at getting rid of aluminum oxide.

 

[electricpete]

mc52 - under deregulation (at least in our state), power producers (competitive) cannot have access to certain transmission type information (regulated) because it could provide an unfair advantage. So there are laws restricting access to certain revenue metering info.

It is not entirely relevant - It looks to me like the revenue metering captures the plant output but not the split going both directions.. The only point for accessing the split is the protective relaying. As discussed above at this point the mood is that we shouldn't clamp on there without good reason. I have to agree there is no good reason right now with the parallel path but I'm still a little bit uneasy. The reason the plant is so jumpy about clamping on to protective CT's.... we had a trip more than a decade ago due to vibration of the HCB electromechanical pilot wire relays which are notoriously sensitive. Everyone remembers that. What they don't remember is that since then we have gone to Fiber Optic pilot wire system which doesn't have the vibration sensitive relays. So, not an option at this point.

Cuky - that is some great info as usual. What program is that? Is alpha beta epsilon a company or something?

I was looking at the formula and trying to convert it to my formula. It seems like if I could find the reactance at 30 feet and reactance at infinity, the difference between would be equal to something like 2*M (M being mutual inductance at 30 feet... and taking some liberties to neglect that fact that M depends on which phases we are comparing). That sounded like a good plan. Then I see that the formula makes reactance go to infinity (slowly... log dependency) as spacing goes to infinity. Nothing is ever as simple as it should be. I looked in Bergen Power Systems analysis and I see their treatment has a lot of infinite length line assumptions which makes my head hurt. With infinite length line it perhaps makes a little more sense that reactance per length continues to increase without bound? I'm not sure.

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[electricpete]

should have been ".... to increase without bound as spacing increases"

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[cuky2000]

Cuky - that is some great info as usual.


Hi Pete,

This is nothing fancy. This program is MathCAD interfacing with excel and pasting a Power point sketch for the bus configuration. In my opinion, this is a great engineering tool very easy to use since do not require complex programming codes. I highly recommend it to produce elegant and fast engineering reports and calculation templates compatible with excel. This type of program made the life of power engineers a little bid more enjoyable since we do not have to concern with diversity of units, graphics and perform matrix with complex operations.

Is alpha beta epsilon a company or something? This is a personal logo that represent my nickname.

I was looking at the formula and trying to convert it to my formula. You mentioned looking in Bergen Power Systems analysis, check eq. 3.33 for inductance formula:
l=2x10^-7.Ln(Dm/Rb) [H/m]
_{Where : Dm = GMD & Rb=GMR
I made the following modification to the original formula to easily express X/R = f(D) and take advantage that the bus table provides the xa at 1 ft spacing.
Xt=w.l=2pi.f[2x10^-7.Ln(GMD/GMR)] = k.Ln(1/GMR) + k.Ln(GMD) = xa +k1.Ln(GMD).
xa= value provided in the table as reactance of 1 ft spacing
For flat configuration with equal phase spacing:
GMD = (Dab.Dbc.Dca)^1/3 = (D.D.2D)^1/3 = 1.26D
K1.Ln(GMD)= k1.Ln(1.26) +k1. Ln(D) = k2+k1.Ln(D)
NOTE: K1 & k2 varies depending of the units or weather using Ln or Log.
Check also Stevenson and other power system references}


Nothing is ever as simple as it should be. I looked in Bergen Power Systems analysis and I see their treatment has a lot of infinite length line assumptions which makes my head hurt. Often modeling systems require approximations to get close to ideal conditions particularly if this is use for teaching purposes. In real world, engineers have to compromise between accuracy vs. simplicity.
_{Please notice that for the case of finite short bus, the flux will be distorted at the ends creating some inaccuracy in the model. Also the overhead shield wires, apparatus and discontinuity will create additional challengers to determine the system parameter via simple calculations within reasonable level of precision.}

It seems like if I could find the reactance at 30 feet and reactance at infinity…Then I see that the formula makes reactance go to infinity (slowly... log dependency) as spacing goes to infinity. ……With infinite length line it perhaps makes a little more sense that reactance per length continues to increase without bound? I'm not sure. For practical application, the rate of reactance increase is reducing dramatically. Considering doubling the bus spacing only gain less than 15% in additional reactance.

... taking some liberties to neglect that fact that M depends on which phases we are comparing). The good news is the mutual reactance will decrease with the phase spacing and separation from other adjacent energized busses. The mutual reactance approach to zero for large distance.

<sub>The mathematical model to consider the untransposed condition creating a mutual coupling. This effect may be considered using a matrix approach with the off diagonal and diagonal elements estimated by the bellow generic relations.
Self: Zii=ri + j.k.Ln(1/GMRi) = ri + j.xa (1)
Mutual: Zij= j.k.Ln(1/Dij) (2)

Notice that (1) is a function of the conductor diameter, while (2) is a function of the distance among conductors. In practical application, Zii>Zij</sub>

 

[electricpete]

Thanks cuky. I saw that one in Bergen too. Suff like ln(1/D) and ln(1/GMD)

Kind of weird considering the ln() function wants a unitless argument.



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[mc5w]

I too have had an instance of a circuit breaker that wnet open circuit and cycling it fixed the problem. This as a mere 50 amp 2 pole 120/240 volt molded case unit that built up nonconducting siver sulfide in the contacts. Cycling the circuit breaker created enough heat to convert the silver sulfide into conductive silver oxide. Sounds stupid but it worked.

Are you sure that your newer relays that use fiber optic links cannot also function as the ammeters that you need?

Tell the PUC(K)O that you need the amperage data to know if something is overloaded or is otherwise a reliability problem. If you are maintaining a switchyard that is connected to 2 transmission systems you do have a "need to know" as to whether there is a maintenance or reliability problem. Amperage data will not tell you that much about what is outside of the plant because 345 KV systems have enough voltage variation that you also need voltage data.

Some of these laws that compartmentalize things do not account for actual system design. Redesigning your switchyard so that there is a part that is strictly under transmission system ownership is impractical.

You can also tell these 2 transmission systems that if they will not allow you to diagnose switchyard problems you will not take responsibility for an unplanned outage.

You could also wait for an unplanned outage and tell the media that state law would not allow you to diagnose or fix a known problem beyond that you had it.

Your transmission yard was never designed for this application otherwise the revenue meters would be in a more correct place and there would be provision for system monitoring. The telephone companies had to redesign local networks with access tandems that allowed multiple long distance carriers a place to tap in not that telephone switch technology underwent at least 2 upgrades between 1970 and 2000.

 

[mc5w]

Also, consider that a volt of drop in a 1600 amp wiring joint will liberate quite a bit of heat, about 1600 watts. If you have say 5 volts of reactive drop in your yard when all of the load is on 1 bus, that volt of excess drop would only push the current distribution 40% at the most when both buses are in use.

Also, metal oxides and sulfides in wiring joints are NONLINEAR resistors meaning that Ohm's law goes Out-The-Window. This means that the wiring joint could be quite sensitive to small changes in load or just a particular threshold of load.

 

[electricpete]

The revenue metering checks the power flow at every bay position in the breaker-and-a-half scehem (every generator line and every transmission line). If you add them all up the incoming should equal the outgoing because the switchyard uses now power.

We know how much power leaves our generator and if we convinced the accountants to break the law we would know how much power goes down each line. That is still not enough to determine which part of our generator power "turns left" and which part "turns right" when it hits the switchyard. Draw a picture of a breaker and a half scheme.... Put numbers in or out on each bay position. Still not enough to know what is flowing on any given point of the rungs of the ladder.

Our fiber optic relay is ABB FCB-1. I will double check but I'm pretty sure no capability. Not like an SEL where you can read all the inputs.

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[electricpete]

Here is the same powerpoint - no passwords required here. It's about 415 kbytes

http://www.brazosport.edu/~pschimpf/Y601_Disconnect.ppt

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[cuky2000]

Nice pictures Pete,

If the switch contacts jaws are silver plated or silver alloys, still the max. IR reading temperature of 94 oC is within the maximum allowable temperature of 105 oC. Other materials such as cooper or other copper alloys may not be a good new for the substation center-break disconnect switch.

SUGGESTION:
Check the switch O&M manual for info or consult the switch manufacturer.

_{NOTE: Beware that many of the large global names such as ABB & Siemens, sold their manufacturing facilities to other smaller companies in the USA.}

 

[RalphChristie]

Pete

Probably way off, but can you confirm that the different parts on each switch is from the same material? Something like that two switches use the same type of bolt (one kind of material) and that the other switch use another type of bolt? (other kind of material)

Everyone is looking to different contact resistances (and it is most probably the problem) but keep in mind (although it sounds stupid) that different materials absorb and reflect thermal infra-red at different wavelengths depending on the composition of each material.

Other than that the best would be to monitor the spot (like you are already doing) and if it heat up again, to isolate it, and to inspect/repair it physically.

 

[mc5w]

Looks like the problem is in a isolation switch - you should be able to open 1 circuit breaker at a time to force all of the current for a particular generator or transmission line to flow through the other circuit breaker and isolation switches.

I know that this sounds primitive but the idea behind a breaker and a half switchyard is to be able to do that. Just pretend that you are doing oil or sulfur hexafluoride sampling in the circuit breakers or something like that.

One of the bigger pains in the rear end is doing maintenance on isolation switches such as cleaning and replating contacts. You may have to get out a clean dry fiberglass ladder and some chain mail suits so that you can do this live while each switch is open. In the case of a bus you can do this with the isolation contacts dead but only on the 1 side of a circuit breaker. For the other isolation switch contacts you have to shut down a generator or a transmission line to do this deenergized. Getting the down time could be hard to do - about the only times that you could do this are overnight or in the spring or the fall.

There are some states such a Pennsylvania where you are not allowed to barehand a live transfmission circuit even if you are wearing a chain mail suit.