ARRESTER AND CAPACITOR 3

[powerjunx]

Anyone knows to diagnose or any comments:

1. we routinely (annual) conduct maintenance to our switchyard..and we found out everytime we do this after we energized the 69kv feeder, one of the lightning arrester connected after our SF6 breaker blown out.. my associate tells me that it indeed blown out whenever it comes from hot operation, then cold when subject to maintenance, later when energized it blows itself..

then on, i conclude about TEMPERATURE VARIATION affects the catastropy..but my boss want us to justify the case, and i myself never satisfied of the CAUSE..please help me know all about this problem.


2. we found out a hot spot on ligthning and capacitor cables (rated 13.8KV line-to-line) of our switchgear panel, as traces of cable ties and insulator pads burn out..after a week of the findings.. one of the bus bar of the 13.8kv main line burn out its insulator where the capacitor and arrester connected.my question, is there a problem on our arrester and capacitor? is there a surge induced in hte sytem?

please provide me with your comments, and also provide me a preventive and predictive maintenance program about this matter.


thanks and regards..

 

[jbartos]

Suggestion: The arrester is supposed to have appropriate parameters for its application, namely:
1. Voltage rating in kV
2. etc.
3. Compliance with industry standards, e.g.:
3a. ANSI/IEEE C62.1
3b. ANSI/IEEE C62.2
See reference:
Donald G. Fink, H. Wayne Beaty "Standard Handbook for Electrical Engineers," 13th Edition, McGraw-Hill, Inc., 1993,
Section 27 Lightning and Overvoltage Protection
by A.P. Sakis Meliopoulos

 

[alehman]

I think item 1 in the previous posting may need particular attention. Arrestor voltage ratings must be selected based on the manufacturer's recommendation. Selection depends on the type of arrestor and method of system grounding. If the votlage rating is too low, pre-mature failure will result.

In item 2 you mention a capacitor. Are the arrestors at the capacitor? There is a slight possibility you may some type of harmonic resonance leadign to overvoltage condition causing failure of the arretor.

 

[busbar]

Review of §5 and §6 of IEEE Std C62.22-1997 …Application of Metal-Oxide Surge Arresters for Alternating-Current Systems may rule out basic problems.

 

[powerjunx]

item one is all complies the requirements as barts mention.. but i think the failure is due to thermal aging of the equipment? that is why it fails?

alehman, regarding to item 2.. the capacitor is in parallel with the arrester connected directly to the ground. it is installed after the incoming circuit breaker of the generator (rated 13.8KV) to the common bus of the parelling generator.

aleh, you mention about harmonics, correct me if im wrong with my views that is it something to do with import and exportation of load to our system?

thanks busbar..


regards..

 

[busbar]

Long Shot —

http://www.google.com/search?hl=en&q=surge+arrester+moisture+ingress

 

[alehman]

Arrestors don't normally operate at elevated temperature. If it is, there is something wrong (like application at incorrect voltage). It's not surprising that it didn't fail until being denergized and re-energized. If it was overstreesed for whatever reason, it probably physically failed after it cooled, which became evident when it was re-energized. I've seen that happen before.

I think the harmonic resonance idea is a longshot. You would need a scope or harmonic analyzer to verify. Be sure to check various operating modes. I don't think it would have anything to do with import or export of power.

Is the capacitor switched frequently or under automatic control? Switching surges could damage arrestors. Again a scope would be needed to verify.

Also consider how your system is grounded. A fault or failing capacitor on a high-impedance grounded system will cause overvoltages on the other phases. Capacitors on an impedance-grounded system should not be connected to ground.

 

[rbulsara]

I am not clear on something, you mention 69 kV feeder and then 13.8kV cable, LA and cap and generator.

You must have a transformer in between.

Please describe the set up (0neline) properly. You must be replacing the arrestor after it 'blows' out. If so repeated operation means there is in fact over voltages caused.

I suspect, along the line of alehman suggests, the capacitor/cable/transformer set up is causing ferroresonance conditions causing overvoltages and operating the LA. This happens when all three phases do not close exactly at the same time on a system consiting of just 'right' indcutance (cable, xfmer) and capactiotrs to cause resonance. Ferroresoance also more likely to occur when the transformer being energized is lightly loaded.

Please do a keyword search on "ferroresonace" in eng-tip and you will find quite a bit of discussion and links.

 

[powerjunx]

alehman, i got your points so with rbulsara.

but i want some more expanded views as you want to closely clarify the set up..

for 13.8KV set up:

generator (with neutral ground resistor) -- vacuum CB -- Arrester and Capacitor -- primary wdg connection (X'mer).

..a neutral resistor, arrester & capacitor, primary wdg (X0)of X'mer were both commonly grounded.

for 69KV set up:

bus tie only;

SF6 CB -- arrester -- disconnect switch -- Load.

i think rbulsara is quite close to the problem as what he pointed out about "Ferroresonance" but can you suggest a device in order to protect our system against this cause?


i do really appreciate most if youy can give me a product for this..

regards.

 

[rbulsara]

Some considerations:

Is the capacitor switched? That is connected through a breaker?

If so you will do well to disconnect the capacitor. After the maintenance, energize the transformer without the capacitor. Close the capacitor once the system is stabilized and somewhat loaded.

In fact why not keep capacitor disconnected, (unless it has any real critical use) and see what happens.

I would also test the SF6 breaker to see if all if its contacts of all three poles close at the same time.

Also I am not sure why do you ground the transformers X0 (generator side) when generator is grounded? Not that it has anything to do with the issue you have on hand.


Other thought:

Look at your switching procedures; I would think you would keep the transformer disconnected if the generator is not in service. While bringing the generator on line you would start the generator-transformer as a unit, that is your 13.8kV VCB remaining closed and 69kV SF6 CB open?

Or you start the generator, close VCB to energize the xfmr and then close 69kV CB?

Trying to close SF6 breakers to energize the unloaded transformer/capacitor combination is like inviting ferroresonance conditions.

A long shot: Do you close the SF6 breaker to energize the transformer or the isolator switch? I have known some people using isolator switch to energize the transformer, keeping the breaker closed. If so this is even more dangerous, as all blades of the isolator almost never close simultaneously! Single phasing the transformer with critical capacitance and inductance with no load is a perfect recipe for over voltages.

The Standard hand book for electrical engineers has some discussion to this effect.

Good luck

 

[alehman]

When you open the generator circuit breaker leaving the capacitor connected to the transformer, you undoubtedly cause a significant ring wave transient. If the SF6 breaker is open when you operate the generator, there is very little resistance in the circuit to absorb the stored energy. This could easily develop voltage high enough to damage any of the components. I concur with rbulsara's recommendation to test it with the capacitor disconnected if possible.

I don't think a ferroresonance condition could develop in the sub-cycle time lag between closing of poles of a circuit breaker or gang-operated switch. Ferroresonance could certainly be a concern if your generator bus became ungrounded for any reason, or on the transmission side of that system was ungrounded.

http://www.cadickcorp.com/download/TB004a_Ferroresonance.pdf

http://www.dstar.org/Public/Ferroresonance_gallery.htm

 

[powerjunx]

thank you rbulz and ahlemz..

i think i should have to review our swiching operation of the system and other electrical aspect ..and check all our equipments connection maybe thermal scanning, etc.

thank again.

 

[busbar]

IEEE 1036: Grounded-wye, ungrounded-wye, and delta-connected capacitors may be subject to ferroresonant overvoltages if they are switched together with transformer banks of certain winding connections with single-pole switching devices or if a stuck pole should occur on a three-phase device. For the ungrounded capacitor, if the transformer has a grounded neutral or even if it consists of many single-phase transformers applied about equally along the feeder, a potentially ferroresonant circuit exists if single-phase switching devices are operated upstream. Both transformers and surge arresters have failed under these conditions.

 

[jbartos]

Comment on alehman (Electrical) Apr 3, 2004 marked ///\\\
When you open the generator circuit breaker leaving the capacitor connected to the transformer, you undoubtedly cause a significant ring wave transient.
///Yes, true, since there is a L-C circuit created that tend to oscillate if enough energy is stored in the circuit; especially, if the capacitance stores enough energy.\\ If the SF6 breaker is open when you operate the generator, there is very little resistance in the circuit to absorb the stored energy.
///Correct.\\ This could easily develop voltage high enough to damage any of the components.
///Yes, due to resonance in L-C circuit.\\ I concur with rbulsara's recommendation to test it with the capacitor disconnected if possible.
I don't think a ferroresonance condition could develop in the sub-cycle time lag between closing of poles of a circuit breaker or gang-operated switch.
///There will be some transient time first. The resonance depends on the circuit parameters that form the steady state for the resonance. The resonance is a steady state phenomenon.\\ Ferroresonance could certainly be a concern if your generator bus became ungrounded for any reason, or on the transmission side of that system was ungrounded.

http://www.cadickcorp.com/download/TB004a_Ferroresonance.pdf

http://www.dstar.org/Public/Ferroresonance_gallery.htm

///The ferroresonance is caused by the hysteresis, associated with the nonlinear B-H characteristics. It can be mathematically obtained as a solution of a differential equation that has nonlinear coefficients due to nonlinear characteristics of the inductance with hysteresis.
Reference:
Soumitro Banerjee, George C. Verghese "Nonlinear Phenomena in Power Electronics," IEEE Press, 2001, Section 7.1 Modeling a Nonlinear Inductor Circuit, page 292\\