Parallel Switching with Disconnect Switches
Parallel Switching with Disconnect Switches
(OP)
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.
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.






RE: Parallel Switching with Disconnect Switches
what you refer is done especially in double busbar stations to switch the breaker from one bus to the other without interruption.
RE: Parallel Switching with Disconnect Switches
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, “
RE: Parallel Switching with Disconnect Switches
RE: Parallel Switching with Disconnect Switches
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.
RE: Parallel Switching with Disconnect Switches
RE: Parallel Switching with Disconnect Switches
for both formulas.
See also http://ww
RE: Parallel Switching with Disconnect Switches
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?
RE: Parallel Switching with Disconnect Switches
RE: Parallel Switching with Disconnect Switches
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:
Ra=28710·L÷I1.4 where
Ra = 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.
RE: Parallel Switching with Disconnect Switches
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.
RE: Parallel Switching with Disconnect Switches
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
RE: Parallel Switching with Disconnect Switches
Sounds like you get the honor of the first operation.
RE: Parallel Switching with Disconnect Switches
RE: Parallel Switching with Disconnect Switches
It is not a safe thing to do unless the breaker operation can be prevented during bypass isolator operation.
RE: Parallel Switching with Disconnect Switches
RE: Parallel Switching with Disconnect Switches
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.
RE: Parallel Switching with Disconnect Switches
RE: Parallel Switching with Disconnect Switches
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
RE: Parallel Switching with Disconnect Switches