Parallel Switching with Disconnect Switches 1

[dcdtn]

I am looking for guidelines for the maximum current that a disconnect switch can break when the parallel path has a near-zero impedance (such as in the same station). The voltages I am looking at are 46kV through 161kV.

Specifically I am wanting to open the bypass switch for a breaker after putting it back in service without opening the breaker and dropping load.

I know that the voltage across the switch before and after opening is practically zero. But the current, obviously, is not. Is it possible for the voltage to be low enough that current does not matter?

Thanks in advance.

 

[mersin33]

the switch has to be rated for parallel operation-you need to talk to the switch manufacturer for confirmation.

what you refer is done especially in double busbar stations to switch the breaker from one bus to the other without interruption.

 

[cuky2000]

Bypassing interrupting devices is a common practice in main and transfer bus and similar type of substation configuration. In contrast, breakers in ring bus or breaker 1 ½ substations arrangement are isolated opennig two disconnect switches in each side of the breaker.

Bypassing is OK provided that an acceptable proven safe operating practice is in place to assure that not interrupting current is allow across the switch. Often the targeted bypass breaker is precluded to open during this process and a backup protective device is also online.

Beware that high voltage disconnect switches do not have break current rating. I see some of the manufacturer tech info that limit the current to ~ 2 Amps.

If the current to be interrupted is not negligible, an arching horn or an interrupter device in the switch should be considered.

For interrupting capability and unusual application double check with the switch manufacturer and investigate further in the applicable standards and technical references.

Here is a list of applicable standards for disconnect switches in the US:

· ANSI Std. C37.32, “Standard for Switchgear—High-Voltage Air Switches, Bus Supports, and Switch Accessories
· ANSI/IEEE Std. C37.100, “IEEE Standard Definitions for Power Switchgear”
· IEEE Std. C37.34, “Test Code for High-Voltage Air Switches”
· IEEE Std. C37.35, “Guide for the Application, Installation, Operation and Maintenance of High-Voltage Air Disconnecting and Load-Interrupter Switches”
· NEMA Std. SG-6, “

 

[stevenal]

You need voltage to maintain an arc. The amount of current doesn't matter as long as the parallel path can handle it. The proccess described is common, and is why the station was built that way. The caution is to make sure the parallel path is in place. Check and double check breaker indicators and ammeters prior to switching.

 

[dcdtn]

cuky2000 -
Thanks for the info and standard references. I plan to look through them.

stevenal -
Actually the station is not built yet. I am just trying to determine if a plain switch is adequate or if I need to spec something better. Most stations have spare breakers so this issue doesn't come up too much.

How much voltage is needed to maintain an arc? There would be a slight voltage drop across the breaker and buswork.

 

[jghrist]

Also see IEEE/ANSI C37.36b, IEEE Guide to Current Interruption with Horn-Gap Air Switches. It won't help much, though, because the current interrupted is indeterminant. There will be some arcing, so use a switch with horn-gaps. The current will go to practically zero as soon as there is a small amount of arc resistance.

 

[stevenal]

440 V/ft acording to Blackburn. Warrington's formula is a little more complex. See

http://www.oriontechserv.com/arc_resistance_calculator.htm

for both formulas.

See also

http://www.usda.gov/rus/electric/pubs/1724e300/1724e300.pdf

section 7.7.2.1 for the type of switch needed for bypassing.

 

[dcdtn]

I have looked through all the references mentioned (except NEMA), and they seem to be silent when it comes to breaking parallel current with near-zero impedance.

In "ANSI/IEEE Std. C37.100" for the definition of disconnect switch, there is a note that states it must be able to "open or close circuits either when current is negligible or when no significant change in the voltage across the terminals of each of the switch poles occurs." But it does not define "significant voltage".

In Annex A of IEEE C37.36b there are formulas to calculate arc length for resistive and capacitive current interruption given voltage and current. I believe these formulas were probably intended for high voltage and low current. If they work for the low voltage and high current conditions seen when breaking parallel, then maybe I could calculate a maximum parallel breaking current by setting the expected arc length equal to the arc length the switch was designed for? Does this make sense?

 

[stevenal]

It's not high current after t=0, see jghrist's post above. I think you are over thinking this. Put the switch up and use it. Do infrared scans yearly, and micro-ohmmeter the breaker contacts when the switch spits more than usual. The only incidents I've heard of involved an open parallel path, and even those caused no injury.

 

[jghrist]

dcdtn,

The problems with your approach to C37.36b are:

1. What arc length is the switch designed for?
2. You could assume some volts/foot using stevenal's value or Warrington's formula:

R_a=28710·L÷I^1.4 where

R_a = arc resistance (ohms)
L = length of arc (meters)
I = current in the arc (amps)

and maybe calculate a maximum paralleling current, but how much of the current that goes through the arc?

I'd ask the switch manufacturer. He's likely to say that as long as there are arcing horns to prevent damage to the main contacts, it's OK. I have observed that when breaking paralleling current with a 12 kV hookstick (without arcing horns), there is a little arcing, but not nearly enough to worry about the arc blowing into anything.

 

[dcdtn]

Problem is that dispatching will refuse to operate the switch unless current is less than 50A, which it is not. I am trying to find some way to prove that it is okay.

I have found an IEEE paper that specifically recommends against switching (with disconnect switch) loop currents at 69-kV and above, but it might be referring to situations where the loop impedance is much greater.

 

[jghrist]

The following paper described a method to evaluate loop switching in a 765 kV breaker-and-one-half station. Calculations of loop impedance are made to determine the energy that is interrupted. AEP uses loop switching up to 600A, based on this method. In your situation, the loop impedance is much less because your loop is only through the switches and breaker instead of through another portion of the bus. I think you could use this to justify opening the bypass switches when the breaker is closed.

American Electric Power experience with loop switching in EHV stations using air-break disconnect switches
Keane, J.J. Andrei, R.G. Halley, B.R.
American Electr. Power Service Corp., Columbus, OH , USA;
This paper appears in: Power Delivery, IEEE Transactions on

Publication Date: Oct. 1991
On page(s): 1476 - 1483
Volume: 6 , Issue: 4
ISSN: 0885-8977
Reference Cited: 1
CODEN: ITPDE5
Inspec Accession Number: 4064129

Abstract:
The authors focus on the loop switching operation inside a substation, which is called local loop switching. Within the local loop there are circuit breakers and disconnect switches that can be used to perform a loop switching operation. The interrupting duty of the air-break disconnect switch is calculated theoretically as a function of the station bus physical arrangement and current flow pattern inside the station. The loop switching capability is experimentally determined for two types of 765 kV air-break disconnect switches. The benefits of loop switching performed with air-break disconnect switches inside electrical substations are illustrated by three examples of actual field experience. The switching procedure involved in these examples is discussed

 

[stevenal]

dcdtn,

Sounds like you get the honor of the first operation.

 

[jghrist]

I'd worry more about what would happen if there was a fault while the breaker was bypassed.

 

[RRaghunath]

You are very right jghrist, the occurrence of fault and breaker receiving a trip command is a very definite possibility to be considered during bypass isolator closing.

It is not a safe thing to do unless the breaker operation can be prevented during bypass isolator operation.

 

[stevenal]

If there is a fault while the breaker is bypassed, backup protection will operate. But a fault while opening the bypass? Switch blades are barely apart when breaker operates causing switch to restrike as the blades continue to open? Better play the lottery on the day that happens. Bypassing usually takes place during breaker maintenance which is not generally performed during wind storms. Is opening the trip circuit part of anyone's usual switching procedures?

 

[dcdtn]

jghrist - That AEP paper was interesting. They calculated the equivalent loop interrupting duty for their 765-kV switches to be 600 Amps with 200 Volt voltage difference across them. They tested them in a lab at this interrupting duty (120 kVA) and noticed that contacts were burning and have to be replaced after 10 operations.

120 kVA equates to less than 0.1A at 765kV, so either the 765kV switches are not rated to interrupt very much or their is a big difference between interrupting the same kVA with low or high voltage.

 

[jghrist]

The 120kVA in the AEP paper is not based on the system voltage but on the required 200 volt recovery voltage and the required 600A interrupting (120V·600A = 120000VA or 120kVA). The 600A is the total current in the two parallel paths. It isn't clear how much current flows in the switch while it's opening. Figure 5 shows the current decreasing to a small value quickly after opening. It also isn't clear from the pictures if there were arcing horns on the switch.

 

[raisinbran]

dcdtn,
As someone suggested earlier, I would disable the circuit breaker trip when I operated the disconnect. With the circuit breaker closed and the disconnect closed (or closing), there are two paths for current to flow in each phase. The chances of the current split in phase A equalling the current spilt in phase B equalling the current split in phase C is fairly small. Any difference from the perfect world could show up as a ground fault in the circuit breaker zero sequence relaying and trip the circuit breaker while the disconnect is closed, or worse, while the disconnect is in operation.
Raisinbran

 

[dcdtn]

jghrist - I realize that the 120kVA was not on system voltage. Was just asking whether it is possible to compare kVA interrupting duties (i.e. 765kV @ 0.1 Amps vs. 220V @ 600 Amps). I know EHV switches are not usually rated to break charging current. But HV switches typically are, but at small amounts (<2.5 Amps). If kVA was calculated based on that rated amount could it be used to establish a "loop flow" kVA limit?