current splitting @ parallel paths thru switchyard - effect on hotspot
current splitting @ parallel paths thru switchyard - effect on hotspot
(OP)
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:
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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? ****
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:
h
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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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
1 - What is your gut feel about the effect of high resistance disconnect connection on current sharing in this circuit (two parallel paths of aluminum bus). Is it significant or negligible.
2 - Assuming no abnormal resistances, would the impedance of this type of bus be primarily resistive or inductive?
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
2. Inductive, I would guess. Bus material resistance is available from the manufacturers. X depends on the bus dimensions and spacing. It can be calculated from texts such as Stevenson.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
I agree with #2. The bigger the conductor, the higher X/R.
#2 would seem to make #1 less likely (less likely that varying R in inductive circuit significantly affects current). I admit I have not provided enough info for an answer (unless someone has a feel for inferring resistance from temperature rise). I'll try to gather more data on Monday. In particular I will try to determine current split by looking at other portions of the circuit. In the meantime, any more thoughts or guesses?
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
Properly installed equipment in a yard WITHOUT parallel paths do not ususally create hotspots. When they develop on single-path systems, the result is invariably a loose connection, heavy corrosion, or badly seated switch blade, all of which lead to a relatively high impedance and heating due to I^2*Z effects.
If you are having hotspot problems in one of two parallel paths, then there must still be enough current in your 'hot' path to create it. This implies that the presumably high impedance at the hot point is still 'low enough' compared to the alternate path that appreciable amounts of current still will flow that way.
Are you sure that your other path is good and healthy? Typical impedances for buswork and closed switches in good condition should be down in the microOhm range. A nearly-open condition might NOT show up via thermography if the alternative path allowed near-zero current to flow in your 'good' path.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
We should keep in mind that 11/2 breaker switchyard is expected to be designed to operate at full capacity with one bus open. This mean that all components including switches, breakers, instrument transformer, meter, etc should be capable to carry the system full load continuous at rated capacity.
How much effect do we effect the resistance of that degraded connection have on the splitting of current through parallel paths? I expect the split difference to be marginal different since the reactance is the dominant factor by far (X/R~100). The small unequal split have inconsequential effect in the operation of the system and the revenue metering system other than additional small losses do to the Joule effect.
Check the enclose relations with rough assumed data:
Split current ratio: I1/I2=Z2/Z1=(R2+ΔR2+jX2)/(R1+ΔR1+jX1)
For large X/R, ==> I1/I2~X2/X1
Where:
1 & 2 indicates parallel path loops
I = current
ΔR =Increase in resistance do to hot-spot
R = resistance
X = inductive reactance.
A quick estimation of X/R ratio is presented assuming a typical 345 kV substation bus clearance of 13 ft with 4” Schedule 80, 6061-T2 Aluminum busbar:
- Resistance at 70oC = 5.284x10-6 Ohm/ft
- Reactance @ 1ft spacing = 3.96x10-5 Ohm/ft
- X/R=13*3.96*10-5/5.284*10-6.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
http://cuky2000.250free.com/Switch.pdf
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
1 - all three phases are balanced for both paths.
2 - There is more power flowing out of our generator through the breaker without the hot disconnect than the breaker with the hot disconnect. Since both locations appear balanced among phases we rule out the connection as a cause. The cause of this power split would seem to depend on the physical layout of the switchyard and where the lines tap in, or possibly also depend on changes in conditions external to the plant... power flowing thru the switchyard.
Cuky - you comments confirm my suspiciouns that we don't affect contact resistance to affect current splitting unless the contact were extremely bad (probably to the point we might see visible evidence of past overheating with binoculars... we don't).
Stevenal - I'm not sure if your comments were serious, but since the disconnect is on the generator side of the breaker, we would have to take 1250MW off-line to work it. Would like to understand risks associated with various options to make informed decisions. One option would be to measure current at CT secondaries inputting to our relay schemes. At this point we elect not to even take that risk because we feel pretty good that this disconnect cannot cause any transient as long as we don't take the parallel path out of service.
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
I'm waiting for approval, so I cannot see your pictures yet. Sometimes simply opening and closing a disconnect can fix the problem. This can be done hot if a parallel path is available. If the parallel path is present for redundancy, relying on its presence continuously defeats this purpose. In my experience, the planned outages are more desirable than the unplanned ones. If it stays cool enough, maybe you can delay repair until the next scheduled outage.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
Our plan is to continue to monitor. If we should reach 100C rise, then we will open the breaker, cycle the disconnect a few times and reclose, as Steve suggested.
Summer peak is already here in Texas. Any switching carries some risk. No-one seems inclined to cycle the disconnect unless really necessary, knowing that we do have a reliable parallel path.
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
Was there only one reading showing high temp? If so, would it be reasonable to suspect a bad measurement? IR can be tricky. Reflections, sunlight, etc. can mess up readings.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
The initial thermographer was qualified and double-checked his result. This shot was shot toward the east in the morning when sun reflection would not be a factor. No other sources of reflection hot enough that I know of.
On the day of the third reading (yesterday - Monday), I used the camera myself. Still 10C rise. Comparing the buswork itself, all three phase temperatures (and I assume currents) are balanced in all paths. However the three-phase path which includes the disconnect appears to have less load than the parallel three phase path. Clear as mud?
It did rain between the first and 2nd reading, prompting speculation by others that the connection was washed. I don't rule out the possibility that even though our generator is constant load, the change in line loading at different points in the switchyard changes the current split. It could be that the three-phase path with the hot connection was carrying the most load at the time of the initial 64C reading. (If the condition starts rising again, that would tend to confirm the latter explanation.... will be getting readings every week)
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
Remember that reactance increases with separation. The preferred return path will be through the local adjacent phases, rather than through the parallel path. A little bit of resistance on one phase may not upset the current balance too much because of this.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
I attribute this to ratio X/R only considering self-reactace X.
"Remember that reactance increases with separation. The preferred return path will be through the local adjacent phases, rather than through the parallel path"
I don't understand what you are saying. I guess I am picturing coupling between phases (mutual reactance) to have a fairly insignificant impact on the division of current because of the large spacing that you mention. Is it reasonable to neglect this?
Or are you saying that if my hotspot high resistance shifts current to the paralllel path, the other phases on the same disconnect will also experience reduced current? If that's what you're saying I would like to understand better.
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
I guess if I thought it was insignificant, I wouldn't have mentioned it. There is a mutual coupling between supply and return conductors that favors the closer lower impedance return path. Conductor tables generally list the reactance for 1 foot spacing. This value is then adjusted according to the actual GMD, and goes up as the GMD increases. Without crunching numbers, my guess is that that a small bit of extra R in one phase has two effects: Do to the extra X in the longer path, return current favors the short path even through the extra R tending to keep the current in that phase high. The other effect would be the one you mentioned, considering the two good phases as the return paths all three phases will have a tendency to drop together. The two effects together would put the current somewhere between normal and where it would be if all three blades had the same extra R. Unbalance would depend on the actual R and geometry.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
1 - "Due to the extra X in the longer path, return current favors the short path even through the extra R tending to keep the current in that phase high."
I read this sentence 5 times and still don't understand it. I think you're assuming high resistance in short connection (like I assume). Then that extra R to the extent it effects impedance (very small) will tend to decrease current in the short path and increase in the long path. So if the extra R (in long path) increase current in the short path (same phase), what is the meaning of the phrase "even though". I may be overanalysing your comment. If I replace "even though" with "and" it makes sense to m.
2 - "The other effect would be the one you mentioned, considering the two good phases as the return paths all three phases will have a tendency to drop together."
The effect I understand but again at maybe 10 or 15 feet spacing I have a hard time imagining a difference. Self inductance must be orders of magnitude higher. Now if the high resistance were out on one phase of a transmission line and the parallel path went miles through another transmission line, I could see the cuumulative effect might be significant. But here the parallel path is only within the switchyard. I haven't done any calcs but my gut says it is not significant. If it is I'd like to know/understand because it changes the way I look at the problem. Steve and others - do you have any more comments on whether mutual inductance is an important part of the problem?
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
should be changed to:
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
To illustrate this case, consider the power flow in the bay with a generator and transmission line as shown in the enclose sketch.
The generator will input current to the bay and the line will deliver out the power. The current in each node under normal conditions do not have to be equal.
The effect in each bay of the mutual impedance may not be significantly different to change the current flow do to the physical symmetry of the substation with respect to the center circuit breaker.
Please notice that the Std C37.30 allows to the switch to be at 70 oC and other individual components can safely capable to handle temperatures from 75 oC up to 125 oC depending in the material (Cu, Al, silver & alloys) and the parts such as mechanical joints, contacts, connectors, etc.
I under the impression that you still are operating the disconnect switch within the allowable temperature.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
The word I used was not "though" it was "through." I may have been unclear what I meant by "long" path. I wasn't referring to extra conductor length, but greater separation.
I'll rephrase. We have parallel paths, but phase B of path number one has a somewhat resistive connection. Current flowing in phases A & C of path 1 normally will return on B phase of path 1. Due to the added resistance, one would expect current returning on phase B to be diverted through path 2. But GMD comprising the A & C phases of path 1 and B phase of path 2 is much higher than the GMD of a single path. So current returning on phase B sees added resistance on path 1 and added reactance on path 2 and divides accordingly.
But phase B is not in isolation, the added impedance also affects phases A & C, which also have path 2 available. A & C currents diverted to path 2 along with B see the lower GMD and reactance of a single path.
Overall effect is less unbalance than one might expect if resistance only is considered.
A similar effect is seen during ground faults. The whole world is available, so one might expect the current to spread out and use the whole cross sectional area of the planet. But current straying too far from the supplying conductor sees increased reactance. Unless there are some nice metallic pipelines nearby, most of the current will return in the ground directly beneath the faulted line even when the pole line zigs and zags.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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.
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
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
Kind of weird considering the ln() function wants a unitless argument.
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
http:
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RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
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.
RE: current splitting @ parallel paths thru switchyard - effect on hotspot
The disconnect is one of two disconnects for a circuit breaker. It happens to be on the same side of the breaker as our generator, so even with the breaker open, at least a portion of the switch would remain energized whenever gnerator is online.
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