DC current breaking capabilities of an EHV CB 3

[protkid1]

Hi friends,
I have a 275 kV SF6 CB having the following rating :
Rated short time withstand : 40 kA
Rated peak withstand : 100 kA
Rated making current : 100 kAp
Rated breaking current asymmetrical : 40 kA
Out of phase breaking current: 10 kA
First pole to clear factor : 1.5
Transient recovery voltage : 249
Rated line charging breaking current : 50 A

http://www.eng-tips.com/

Rated cable charging breaking current : 160 A
Rated Back to back capacitor bank breaking current : 400 A

Rated reactor current breaking : Max 315 A/ min 41 A
Max overvoltage factor on any switching duty : 2.3 p.u.

Total break time: 33 ms.

1) I want to know whether the above CB can break a DC current of 500 A ? The application is for switching in a shunt reactor feeding a 100 km long transmission cable. Sywitching studies indicate a 500 A dc persisting for 1.5 sec upon energisation of the shunt reactor.
2) Is it recommended to use the above CB for breaking a purely capacitive current of approx. 250 A ?
Thanks
protkid

 

[odlanor]

CB can break a DC current of 500 A?
ie the HV or HVDC circuit?

HVDC CB are not needed at HVDC scheme because the control of converters permits rapid blocking and unblocking of this scheme,
to enable transient line faults to be cleared and, in conjunction with fast-acting switches isolators, permanent faults can be deal with.
Ultimate protection is provided by CB on the ac side of each terminal.

 

[protkid1]

Sorry , I forgot to mention that it is an HVAC CB. 50 Hz. employed in a 132 kV Transmission system.

 

[waross]

Breaking DC is not the same as breaking an asymmetrical AC wave form caused by a DC component.
The appropriate spec may be
"Rated breaking current asymmetrical : 40 kA"
A DC current has no zero crossing. An asymmetrical AC current does drop to or past zero..

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[protkid1]

Thanks waross,
You got my problem right. The spec mentions 40 kA asymmetrical breaking capacity. Also , most of the sub-transient , transient waveform show current zeroes in the text books. But the problem here is that the switching study waveform displays an asymmetrical waveform with "no current zeroes" for approximately 1.5 seconds. Now what if I switch OFF my CB during those 1.5 seconds ? This is what I want to know.

 

[protkid1]

Also, the above current value is 500 A (peak value around 1000 A) and there is no current zero appearing for 1.5 sec !!

 

[waross]

I was under the impression that the peak to peak symmetrical value of the current was a limit to the asymmetric peak value. I may be wrong.I reread the spec more carefully.
This may be the limiting factor regardless of the current waveform. I don't understand the minimum rating.
"Rated reactor current breaking : Max 315 A/ min 41 A"

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[protkid1]

I will check about the significance of the min value of 41 A given by our supplier. What is the limiting value ? Sorry couldn't get that. Is it that the CB could break the current without the current zeroes if it was only 315 A ( Again question comes to mind whether this 315 A is rm.s or peak ?)

 

[waross]

Normally RMS unless stated otherwise. But....

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[protkid1]

245 kV is the rated power frequency withstand value. The rated voltage is only 145 kV.

So the CB can break an rms value of only 315 A for reactor switching. Quite a remarkable thing when we compare, it can break 40 kA under fault conditions but a switching duty involving a reactor is only 315 A and Rated cable charging breaking current : 160 A
Rated line charging breaking current : 50 A.

Can anybody provide some good reference that explains the above situation. To the extent I know , it is all connected to the Transient recovery voltage which appears at the instant of breaking. And if I am correct, the recovery voltage limits must be exceeded in case the capacitor performs the above switching duties like capacitor /reactor current breaking.

Thanks

 

[FPelec]

Hello protkid1,
standard modern HV SF6-selfblast circuit breakers are not able to interrupt DC currents above a few amps: when the current magnitude is low (i.e. 500 A)the energy of the SF6 blast is reduced
if compared to normal short circuit values (tens of kA). The CB is thus not able to chop the DC current and the persisting arch will probably damage the interruption chamber.
Some tests were performed in Japan, to investigate if a SF6 CB could open the DC current caused by a busbar-connected shunt reactor after a low impedance phase-to-ground short circuit on the busbar itself; this case is however different from line energization, because phase voltage was almost zero after the fault.

http://direct.bl.uk/bld/PlaceOrder.do?UIN=202582320&ETOC=RN&from=searchengine

Proper countermeasures (for instance synchronized switching), should be adopted in order to avoid delayed zero crossing currents after line energization:

http://www.pesicc.org/iccwebsite/subcommittees/subcom_c/Presentations/2012Spring/C-3.pdf

As regards capacitive load switching, after current interruption the capacitances retains an electrical charge which causes higher TRV and potentially, multiple re-ignition of the CB.


Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[protkid1]

Thanks Fpelec and warross . It is nice to get the opinion of industry experts !!

 

[FPelec]

protkid1,
regarding rated reactor current breaking: the max 315 A value refer to a zero crossing current and not to a zero missing current; it is the maximum RMS value of the (zero crossing) reactor current that the CB can open without risk of re-ignition. This does not means that the CB could 315A dc current
Also the minimum value can be important, because too low currents could be chopped, with high risk of re-ignition.
This topic is dealt with in IEC 62271-110 "Inductive load switching"

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[protkid1]

Thanks Fpelec,

So what is the CB spec that I should check to find out whether it can interrupt this 500 A during 1.5 sec ? The scenario is like this:
A submarine cable (120 MVAr,100 km) compensated by 2 shunt reactors 60 MVAr each with variable taps . System voltage is 132 kV. I got the CB rating wrong initially. The rated voltage is 145 kV for CB. Power frequency withstand is 275 kV.
The reactors are switched by dedicated GIS CBs. The initial energisation procedure requires the shunt reactor at sending end to be de-energised and receiving end in service. The main cable GIS CB is switched ON. This procedure ensures there are some current zeroes for the main CB to interrupt. Thereafter the sending end shunt reactor is switched ON. Now the problem starts. The main cable feeder sees a 500 A dc for 1.5 secs at the instant the shunt reactor CB is switched ON. If it is called to interrupt during this period : NO CURRENT ZEROES !!! Pre - insertion resistor and Point of wave switching is suggested as solution. The second one may be not possible since the GIS CB is gangue operated and not single pole. Pre-insertion resistor for GIS , we are trying to find if it is available at 132 kV voltage level ( for 380 kV levels it is available). Any other solution is welcome . One solution that I was going through a while ago says. " Open the shunt reactor CB (whose current alone will have current zeroes) before the main cable CB is opened. It says the current zeroes will appear in the main CB current and you can interrupt the current. The relevant paper is attached. But , I was wondering what about a protection maloperation and the CB opening sequence is not followed religiously. Again the main Cable CB may have to open the 500 A dc which is beyond its rating !!

 

[prc]

ABB last year annonced that they have developed a DC HV circuit breaker (hybrid type)which may revolutionise electric power transmission and may finally prove that Thomas Alva Edison was after all right ! Remember the war of currents.

 

[QBplanner]

"120 MVAr,100 km) compensated by 2 shunt reactors 60 MVAr each with variable taps"
Another way to solve the problem maybe take a second look of the system desgin.
I assume 120Mvar is the total cable charging. If that is correct, I am not sure why 100% cmpensation is required in your case. I would lower the reactor to 35~40 Mvar each to see if they work from system perspectives (60~70% compensation) first. if so, try the switching study to see if the non zero crossing current can be lower and duration can be shorter.

 

[FPelec]

protkid1,
your project is very interesting to me, can we have some further (even geographical) information?
Are you dealing with an offshore windfarm or something line the Mallorca-Ibiza link?
When the compensation degree exceed 50% zero missing currents can occur during line energization. As far as I know 145 kV GIS CB can single-pole operated, so that synchronized switching is the most straightforward solution.
However, let me say that there is something in your energization procedure which in my opinion sounds strange: if you energize the cable with only one shunt reactor connected the no-load capacitive current will probably exceed the rated cable-charging breaking current, which according to IEC 62271-100 is 160 A (approximatively corresponding to 40 Mvar) for 145 kV CBs. Thus, in case the CB is called to open the no-load cable line with only one shunt reactor connected, restrikes are likely to happen.
That is why in very long ac cable lines, the shunt reactors should be solidly connected to the cable line itself, as I have always seen in all the similar projects I've dealt with, like the Sicily-Italy (400 kV- 43 km) and Malta-Sicily (230 kV - 120 km).
As regards the possibility to open the shunt reactor CB during a fault before the line CB, this would lead in a severe overvoltage after fault clearing. My suggestion is to use single-pole CBs, or alternatively, to carefully estimate the zero crossing delay and inhibit line CB opening for a correspondin period after line energization.

You could also check these papers:
F.M. Gatta, A. Geri, S. Lauria, M. Maccioni "Steady-state operating conditions of very long EHVAC cable lines" Electric Power Systems Research Volume 81, Issue 7, July 2011, Pages 1525-1533
F.M. Gatta, S. Lauria - “ Very long EHV cables and mixed overhead-cable lines. Steady-state operation” in proc. IEEE PowerTech 2005 conference
L. Colla, M. Rebolini F. Iliceto – “400 kV AC new submarine cable links between Sicily and the Italian mainland. Outline of project and special electrical studies” CIGRE 2008 conference.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[protkid1]

QBplanner :
Client wants pf at 0.9 to 0.95. So 120 MVar of cable to be compensated with 2 reactors 60 MVar each with variable taps ranging from 30 MVar to 60 MVar. Source end will compensate for p.f. Load end will be reduced and finally switched off in heavy load condition to reduce the current through the cable.
FPelec :
This is a middle east project with 100 MW to be supplied to an island for Oil and gas load.
I will reply the technical aspects in the evening. Heavy work load now !!

Thanks

 

[QBplanner]

pf requried at 0.9 to 0.95 receiving end should be with some limited conditions such as the load on the customer side will not be less than 75% of maximum loading?

If customer has 100MW peak load but only requires 5MW load in initial stage, then it would be very difficult for any utility to meet 0.9 or 0.95pf lagging/leading at point of common coupling point by using a long cable to supply.

Normally, utility sends power to the CCP at rated pf 0.95 or 0.9 means full load or minimum 75% of full load. utility can not assure the pf if the load is 5% as long as the steady -state voltage is below 1.1 p.u. or MCOV which can be tolerated by the equipment.

100% compensation is very aggressive compensation level and will creat some problems such as the one you are facing right now.
If you are still in design stage, it is not too late to change. If you already in the implementation satge, you have have to talk to the CB manafacturers to be very specific about your requirements.

 

[protkid1]

Thanks QBplanner and FPelec,
The island has existing load (oil wells and refineries) supplied by local generation. The project is past engineering phase and is under manufacturing phase. CB manufacturer has confirmed that they have type test reports to prove the CB has breaking capacity upto 650A with a TRV of 390 kV as proved by BC2,CC2 tests mentioned in IEC 62271-100. Well the discussions are on-going with SVC also coming up for consideration. At present , point of wave switching is being considered to change the GIS CB for line and reactor to single pole switching CBs and couple the same to the GIS part that is non-segregated. That is one part of the GIS is phase non-segregated and another part will be phase segregated for enabling single pole switching. If I inhibit the opening of the CBs for 1.5 to 2 sec after energiztion of shunt reactor, how will be the protection duty of the same be taken care of. Most probably it will be passed to the LBB (BF) CB and the same problem of DC interruption may occur. Else, I think under fault condition, current zeroes will appear and the LBB Cb will not be damaged.