Finding DC current breaking Capability of Circuit Breaker 7

[kssschsekhar]

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....

 

[davidbeach]

If a dc interrupting rating is not listed on the data sheet, the dc interrupting rating is really easy to calculate. Take the ac rating and multiply by zero.

 

[cranky108]

I sort of agree with david, although i would not have stated it like that.

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.

 

[davidbeach]

One of the other factors is that the breaker has to have a thermal-magnetic trip unit. Any kind of solid state trip unit and it is blind to dc and will never initiate the trip in the first place.

 

[ScottyUK]

David,

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!

 

[davidbeach]

Didn't know that about the VD4.

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.

 

[sibeen]

One of the other factors is that the breaker has to have a thermal-magnetic trip unit.

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.

 

[kssschsekhar]

Thanks for all the responses...
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!!!

 

[ScottyUK]

kssschsekhar,

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!

 

[LionelHutz]

So, it's not really straight DC, but rather the DC fault component you are concerned about. I would contact ABB and discuss the X/R and expected fault currents with them. They should have someone on staff that can help. The breaker will have been tested under some standard assumed "worst case" condition, likely a rather low power factor.

 

[alehman]

Yes, DC component for a fault on an AC system is a very different question than straight DC. Circuit breakers usually have an asymmetrical rating, which if for this purpose.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[Kiribanda]

kssschsekher,
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.

 

[Marmite]

The attached spreadsheet may be of assistance. It's not mine so I'm unable to vouch for its accuracy.
Regards
Marmite

 

[kssschsekhar]

Scotty: Breakers are connected to Induction Motors. Manual Short circuit studies were carried in the year 2003. Now we are adding additional loads and repeating SC calculations with additional loads using ETAP.

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

 

[kssschsekhar]

Kiribanda: Does the given calculation give the circuit breaker asymmetric current breaking capability? The calculation gives the value of % of DC component at various breaking times. As stated by other techies, circuit breaker tested value must be required for considering its capability.

 

[Moltim]

You appear to misundestand IEC. its referring to interrrupting an asymmetric current which consists of a dc and an ac component. I total + Sq Rt of i dc Sqrd = i ac sqrd
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.

 

[Kiribanda]

kssschsekhar,
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.