Finding DC current breaking Capability of Circuit Breaker
Finding DC current breaking Capability of Circuit Breaker
(OP)
Dear Folks,
Q1. If a circuit breaker is tested for a time constant of 45 milli sec, I"k=40kA, how can calculate DC current breaking capability of circuit breaker? From IEC 62271-100 standard, Figure-9, as percentage of DC component varies according to time of contact separation of the circuit breaker, what percentage of DC component is to be selected to know the circuit breaker DC current breaking capability?
Q2. I am looking for ABB VD/4 12.12.40 circuit breaker DC current breaking capability. This value is not available in breaker data sheets.
Advanced Thanks....
Q1. If a circuit breaker is tested for a time constant of 45 milli sec, I"k=40kA, how can calculate DC current breaking capability of circuit breaker? From IEC 62271-100 standard, Figure-9, as percentage of DC component varies according to time of contact separation of the circuit breaker, what percentage of DC component is to be selected to know the circuit breaker DC current breaking capability?
Q2. I am looking for ABB VD/4 12.12.40 circuit breaker DC current breaking capability. This value is not available in breaker data sheets.
Advanced Thanks....






RE: Finding DC current breaking Capability of Circuit Breaker
RE: Finding DC current breaking Capability of Circuit Breaker
If it wasen't designd, or tested for DC, then it may not have a dc rating, for several reasons. This may be a contact speed issue, or developed arc length, or other factors.
RE: Finding DC current breaking Capability of Circuit Breaker
RE: Finding DC current breaking Capability of Circuit Breaker
The VD4 is a medium voltage vacuum circuit breaker.
kssschsekhar,
VCBs usually have a poor DC rating, presumably because of the short gap between contacts in the open state and the lack of zero crossings to assist arc extinction. Your best course of action would be to ask ABB since they built it.
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If we learn from our mistakes I'm getting a great education!
RE: Finding DC current breaking Capability of Circuit Breaker
In general, the DC rating of any vacuum interrupter is 0. The vacuum interrupter is solely dependent on zero crossings and has no arc elongation component for interruption. No zero crossings for DC means no interruption, but severe damage to the contact surfaces in the vacuum bottle.
Like I said, if there isn't a dc rating listed, there isn't one.
RE: Finding DC current breaking Capability of Circuit Breaker
Very good point, David. Many years ago I was working for a manufacturer of fairly large UPS systems. We used a circuit breaker in the battery connection which had a magnetic trip unit and for many years this worked without any problems. The circuit breaker manufacturer then decided to change their trip unit to a solid state version and no one in our engineering department picked up the change, or if they did notice it didn't flag any warning bells.
A year or two later we had a short develop in the inverter of one of our units. The rectifier and upstream protection all worked to spec but as far as the battery circuit breaker was concerned there was nary a problem.
One very, very pissed client as the resultant fire caused some rather extensive damage to his facility and the smell of burning batteries is something not to be forgotten.
This happened at the end of the 80s and I was a fairly junior engineer at the time. It is one lesson that has stuck with me and if I'm ever designing a circuit with large DC currents I will only use a circuit breaker with published DC interrupting ratings.
RE: Finding DC current breaking Capability of Circuit Breaker
Yes. DC interrupting rating is not listed in ABB VD4 circuit breaker data sheet. In fact, the testing of the CB with the time constant of 45 milli sec is also my assumption. All these vacuum CB are used for MV voltage (11kV)distribution and predominately for motor loads. Surpraisingly DC component and assymetrical components are not considered in short-circuit calculations submitted during engineering stage of the project. In our present calculations with additional loads, fault current of DC component is approx 24%.
As per IEC 61227-100, circuit breaker DC interrupting capability should be above the calculated DC current. Then how can we use a circuit breaker without listing the DC component!!!
RE: Finding DC current breaking Capability of Circuit Breaker
Most breaker ratings are quoted at standard conditions. Is the DC component implicitly specified in the form of a maximum X/R ratio? You have stumbled upon one of the common errors made when using distribution class vacuum circuit breakers in generator service where the high X/R ratio and the implicit long time to first current zero means that the breaker is operating outside its rating even though at first glance it may be ok.
As for the calculations... were they done by hand or using one of the many simulation tools?
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If we learn from our mistakes I'm getting a great education!
RE: Finding DC current breaking Capability of Circuit Breaker
RE: Finding DC current breaking Capability of Circuit Breaker
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Finding DC current breaking Capability of Circuit Breaker
Please see the attached sheet which depicts how to calculate the DC componenet.Both IEC & ANSI Bkrs are rated for X/R=45 ms only. If your breaker is made for ANSI/IEEE C37.010 then the then (Top+Tr) = CONATCT PARTING TIME.
Any comments please.
RE: Finding DC current breaking Capability of Circuit Breaker
Regards
Marmite
RE: Finding DC current breaking Capability of Circuit Breaker
Alehman: Thanks for the calculation.
We have received Asymmetrical current ratings of VD4 breaker from ABB: Iasym:47.5kA and % of DC: 35.
Once again thanks for all your contribution
RE: Finding DC current breaking Capability of Circuit Breaker
RE: Finding DC current breaking Capability of Circuit Breaker
Due to the dc-component, the short-circuit current will be asymmetrical for a certain time after the occurrence of the short-circuit. On the one hand this has to be considered when designing circuit-breakers which will be called upon to interrupt such currents. On the other hand this is important for all the components in the network, since the mechanical stresses due to the currents will be most severe at the maximum peak of the asymmetrical short-circuit current.
The highest possible current peak will occur if the short-circuit is initiated at the zero passage of the voltage. The probability of this happening in the network is low, but has to be considered.
RE: Finding DC current breaking Capability of Circuit Breaker
The DC component is decaying exponentially with time.Per ANSI C37 the name plate of all general purpose Bkrs should show the % DC component it can break.For IEC Bkrs it is optional.From the timing test results indicated in the fctory test report you can get the opening time and then calculate the allowable DC% or the manufacturer should print it on the name plate if you request to do so.In all ANSI & IEC Bkrs the DC time constant is 45 ms.