Interruption of line charging current 3

[kjd]

Why is it that a circuit breaker can interrupt thousands of amps of fault current, but not a few dozen amps of charging current? I understand that charging current - which is capacitive and more or less 90 degrees out of phase with load current - can arc across circuit breaker contacts. But the current is sooooooo much lower than load or fault current, that I just can't see how 50 amps or so can damage a circuit breaker which is capable of interrupting 20,000 amps of fault current at 69 or 115 kV.

Can anybody enlighten me?

Thanks much!
Kevin

 

[electrageek]

kjd

What application are you talking about? Do you have an application that the breaker is failing while trying to open? Is it an SF6 breaker, Air, Oil? What kind of damage are you getting? Is the breaker tripping on some kind of fault relay? Are you talking about a molded case breaker?

 

[kjd]

We have a number of older oil breakers at subtransmission voltages (69 and 115 kV) which are being systematically replaced due to their being unrated for charging current interruption. We also re-evaluate line charging current whenever a portion of an overhead subtrans line gets converted to underground. I'm the engineer who makes the call if a breaker needs to be replaced. I do this by using an in-house spreadsheet to calculate the increase in charging current as a result of the UG conversion, and comparing it to the nameplate on the breaker.

Recently, someone asked me to explain why breakers have problems interrupting charging current, when its magnitude is so much less than load or fault current. I was not able to explain, and have been trying to educate myself on the subject. I suspect that it has something to do with the fact that charging current is capacitive, and is therefore 90 degrees out of phase with resistive load current (a little more than 90 if the power factor < 1.0). But that's as far as I could go.

Can anyone provide a tutorial, or point me to a publication that can give me a good background on the subject?

Thanks, Kevin

 

[dpc]

You might want to take a closer look at your spreadsheet. I don't have a copy of ANSI C37 in front of me, but I'm not sure that the de-rating is necessarily that severe for circuit breakers.

If you're talking about circuit switchers, that may be a different story.

From the viewpoint of the breaker designer, the worst duty is capacitor switching. Line charging interrupting is a similar situation.

You're right - the basic problem is the phase angle difference between the voltage and the current. A HV breaker attempts to extinguish the arc at a zero current crossing, after the contacts open. If the voltage is too high, the arc is not extinguished. For capacitor switching, the voltage will be at a maximum when current is zero. After arc is extinquished, the system imposes a transient recovery voltage across the contacts of the breaker. If the dielectric strength of the medium between the contacts is sufficient, the arc will remain extinquished. If it isn't the arc will re-strike and the breaker must try again at the next current zero. With capacitor switching the voltage is immediately at a maximum value leaving the dielectric no time to cool off and re-build dielectric strength.

For so-called &quot;back-to-back&quot; cap bank switching, it is *closing* the breaker that creates very high currents and surge voltages on the system, due to the exchange of energy between the cap banks.

Having said all that, I don't think a circuit breaker should have a problem interrupting cable charging current for any practical circuit for which it can handle normal fault duty. Unless I'm missing something - or wrong again.

If it is a load-break switch or circuit switcher, then it might be a problem.

Hope that helps.

 

[electrageek]

kjd

I agree with dpc on the capacitive effect. Have you had breakers blowup? Have you been noticing a degrading of the oil in the breaker? Are the contacts wearing out? Maybe you have not had any failures you are just doing what the breaker manufacturer recommends. Their design might not be adequate for your application. But most oil circuit breakers are capable of handling line charging currents.

 

Please provide the electrical schematics for these two line connections. Any switching transients on or off will load the neutral. All capacitive charge depletion/discharge will also occur as Zero Sequence current hitting the ground as triplins. Normally switching off for the unloaded lines is done by first going through a grounding contact and then open the line. Since I do not know how you are opening the isolated unloaded line, I believe that the PT ground will take the hit and MCB is opening up. Discharge current has lot of stored energy and all of it is going through neutral to ground of the circuit breakers of the connected PT's, on the line. Choice may be to have a small resistive ground on PT breakers to temporarily solve the problem. I believe it shall not affect the system performance in any way, but will prevent the breakers from opening with do to line capacitive discharge current, as it's magnitude will be considerably reduced. If it works please let me know at kapila.r@sympatico.ca. It is a typical problem in power quality based protection design. Eventually you might need a proper line circuit breakers with become ON or OFF any after going through a proper grounding contacts. Good luck Rakesh Kapila P.Eng

 

[kjd]

First of all, thanks to everyone for your advice and expertise!

I have only been with my company for three years, so I don't know a lot of the history behind our breaker replacement program. It is my understanding that there have been some problems blamed on charging current interruption, but I don't know if these have been catastrophic (such as a CB exploding).

Why is it that some breakers are not rated for charging current (mostly older ones, pre-1960s), some are rated for 20 amps, and the newer gas breakers from ABB are rated 755 amps? Is it simply the material used as a dielectric, or the extinguishing medium (oil vs. SF6), or what?

So, just to make sure I understand correctly, the problem with interrupting charging current is that the CB attempts to trip (or open) when the fault (or load) current is at a zero crossing - but since the capacitive current is 90 degrees out of phase, that's when the capacitive current is at a maximum. Is this correct?

Last question - what is this transient recovery voltage that dpc mentioned, and how does the system impose it on the breaker contacts?

Thanks! Kevin

 

[GRPatel]

Let us try to understand the phenomenon of short circuit clearing and the capacitor current switching.
In case of the short circuit current there is lot of thermal energy input and the gas gets ionised (in oil Cbs it is hydrogen). This ionised gas has a damping effect after curent zero.It also requires certain minimum contact gap before the arc gets interrupted. So the concept of minimum arcing time exists for interrupting the larger currents. The arcing time can not be reduced less than say 7 to 10 ms (50 Hz system). This means there is a definite minimum contact gap when the arcing ceases at current zero.After initial time of thermal energy balance in the contact gap, the dielectric race between the contact gap and the system takes place.
In case of capacitive current the arc energy is practiclly nil. The arcing time can be as low as zero ms. The voltage will try to build up instantaneously. The dielectric race between the cotact gap (which is increasing due to speed of the contact) and the system takes palce.
The difference between the two currents is that in case of short circuit current breaking there is definite minimum contact gap, that means minimum dielectric strength of the gap when the voltage starts building up. Where as in case of capacitor current the dielectric strength of the contact gap is zero when current zero is passed. The dielectric strength build up is proportional to the velocity of the contact.
If you win the dielectric strength race, you have a successful performance in case of capacitive current interruption. You can increase rate of dielectric build up across the contacts by increasing the contact velocity.

The short circuit performance can be achieved at lower contact speeds, where as you need higher contact velocity for capacitor current breaking. May be earlier designs of the Cbs were designed for short circuit breaking and did not have enough contact velocity.

 

[redtrumpet]

Kevin - you may want to see Schneider Electric Cahier Technique No. 193, MV Breaking Techniques. I believe this can be downloaded from

http://www.schneider-electric.com

It covers breaking principle, breaking of load currents, breaking of fault currents, and breaking medium (air, oil, vacuum, SF6) with comparisons.

 

[stevenal]

I've been looking at C37.06 1996. It would seem that converting overhead to UG puts you into another category. For example, a general purpose 72kV 20kA outdoor breaker built to this standard should interrupt 20A overhead line charging current or 250A of cable charging current. I don't the purpose of this distinction, or if older standards were similar. Anyone know?

 

[dpc]

I looked this up in ANSI C37.06 and I see what stevenal is talking about. The ratings for switching of overhead line charging current are really low. I don't quite understand why they would be this low.

They are probably concerned about a situation where the breaker at the remote end of the line has just opened leaving the line charged. If the second breaker opens immediately after that, voltage across the contacts could become quite high causing restrike, especially since the arc will initially be extinquished very quickly, since the current is so low. Then the stored energy in the line tries to dump across the breaker contacts.

Maybe someone more familiar with the ANSI HV breaker standards can offer a better explanation.

dpc

 

[dpc]

This might help, but not based on ANSI:

http://www.ipst.org/TechPapers/2001/IPST01Paper117.pdf

 

[kjd]

Dear Colleagues:

Thanks again for all the terrific advice!

dpc, I had already found that paper on the Internet, and maybe I just don't have the background to understand it, but it didn't seem to help too much. But thank you for trying to locate info for me!

redtrumpet - the Schneider site is going into my bag of tricks. I am printing out the CT 193 paper as I type, and will read it on the way in to work tomorrow (don't worry, I ride a train).

stevenal, GRPatel, electrageek, and others - thanks for your time and expertise. I am sitting down with our technical guru on circuit breakers on Wednesday to pick his brain, and I will have completely read this thread and the Schneider paper by then.

Thanks again!!!
Kevin

 

[GRPatel]

kjd,
Hope you has some more inputs from your Guru.
There are different switching conditions which a circuit breaker can see. If you refer IEC standard 62271-100 (Old IEC60065), it gives different possible overvoltages the breaker will see depending on the switching conditions (for capacitor current switching). One should know as what is the application he has. Now you can decide if the breaker is suitable for performing that switching duty.

 

[jbartos]

Suggestion: Reference:
1. C. L. Wadhwa (Professor and Dean in Delhi, India) &quot;Electrical Power Systems,&quot; Second Edition, John Wiley & Sons, 1991, Section 12.6 Capacitive Switching (on page 289)
&quot;The switching of a capacitance such a disconnecting a line or a cable or a bank of capacitors poses serious problems in power systems in terms of high voltages across the circuit breaker contacts.&quot; The voltage can be 2V, which can be dangerous to the circuit breaker and cause a restrike. This is equivalent to closing the switch suddenly, which can result in oscillations in the circuit at the natural frequency f=1/[2 pi sqrt(LC)]
The voltage across circuit breaker can build up in 2V increments, which may result in an external flashover or the failure in the capacitor. The circuit breaker is not capable to provide sufficient dielectric strength to contacts of avoid restrikes after they are opened first. The capacitive switching problems can be resolved by using air blast breakers or multibreak breakers.

 

[GRPatel]

Now that we are on capacitor current switching, I would like to emphasise that making (closing of CB or Switch) of the current is also important. It creates high frequency inrush current, particularly for the capacitor banks. This high frequency current has tendency to create noise which can create interference with the modern electronic controls.
For the closing of the lines, it creates ascillations and can result into high over voltages (particularly very long lines).
To avoid such conditions, a synchronous switching of the capacitors is being done. The switch is closed at a point which is most favourable.

 

[Bung]

I think the importance of the line and system inductance is being forgotten in this discussion. A couple of people have touched on it, and it magnifies the problem as so nicely explained by GRPatel because you get this nasty high frequency LC ringing.

Don't forget that inductive current is also 90deg away from the voltage, just lagging not leading, and is also a maximum at current zero.

WE have in the past let the smoke out of quite a few voltage sensing relays due to switching short line stubs which had CVTs connected - the transients in the secondary systems have to be seen to be believed! (GE's old &quot;Art and Science of Protective relaying discusses this paticular problem).

Bung

 

[GRPatel]

I did not touch various applications of synchronous switching as we are discussing capacitor switching.
You can refer

http://www.abb.com,

they have such relays and they outline all possible application.
You can even use such relays for reducing short circuit current breaking severity by reducing the arcing time to your advantage!

 

[stevenal]

I may have answered my March 25 question above. C37.09 indicates that tests for switching of open line charging current takes into account high speed reclosing and the probability of trapped charges. Cables and capacitors are apparently assumed to have time to discharge. Maybe that's what DPC was getting at. Still seems like a severe derating.

 

[jbartos]

Suggestion: Reading Professor and Dean Wadhwa book carefully, the possible oscillation and potential voltage resonance should not be ruled out. It appears to be a concern more on the higer voltage and low power side rather than on high current and rated voltage side.