Distance protection operation on lightning strike 5

[RRaghunath]

Is it normal for distance protection to operate in the 132kV Overhead lines (even other voltage levels for that matter) for lightning strikes!
The lightning strike is confirmed from the surge arrester counter advance.
Appreciate sharing your experiences.

 

[rasevskii]

Yes, if a single phase to earth flashover occurred over an insulator string, the distance protection should trip that phase breaker pole and reclose it in one second. If further faults occur within a time period on that phase or other phases, the protection shold trip all 3 phases and lockout without reclosing. That is or was a common programming sequence on older protections, in any case.

rasevskii

 

[DTR2011]

Mr. Rasevskii,

I agree with your statement, however not all utilities utilize single pole tripping at this voltage. I assume from the 132kV that this is in the IEC world where this is very likely the case, but in the ANSI/IEEE world, our 115/138kV Distance protection / breakers would generally be 3 phase tripping, with a similar reclose delay / scheme.

I am not disagreeing, rather respectfully pointing out the differences.

 

[RRaghunath]

rasevskii & smallgreek,
Thanks for the response.
It is true the power system in IEC world.
The utility doesn't practice auto reclose as on date (may be in near future).
In my post, my concern is about trip associated with lightning discharge through Surge Arrester (and not an insulator flashover or any genuine fault) during rain and associated lightning activity.

Appreciate sharing your experiences.

 

[veritas]

Hi

I think an important clarification is required here. The distance relay does not operate for the actual lightning strike - indeed we are talking of a phenomenon which lasts in the order of microseconds (way to fast for any distance relay to respond).

The lightning strike causes a potential rise of the tower when it discharges to earth. This potential rise of the tower results in a "backflash" of tower to conductor across the insulator. This ensuing follow up powerarc is now a proper earthfault and this is what the distance relay detects and if necessary operates for.

And yes, whether autoreclose philosophy is applied and how it is applied is entirely up to local practices and requirements. Singlepole tripping still allows for about 70% of power transfer during the single-pole deadtime but it is a more complicated scheme.

Hope this helps.

Regards.

 

[RRaghunath]

veritas,
Thanks for touching on that point of flashover due to potential rise at the time of lightning strike.
But, should a power system designed with BIL of 650kVp allow a secondary flash over to take place due to lightning strike!
OR
Is this a problem of earthing system (long leads / resistive path from surge arrester to earth or earth mat resistance being higher etc.!
Appreciate sharing your knowledge / experiences.

 

[odlanor]

when there are multiple discharges in line with several rays on the same channel precursor, the first beam can cause a short circuit on the line and, consequently, opening their circuit breakers by protection relays. The second ray, or any other subsequent to fall in line or cable guard wire, with the chain still ionized insulators, could reach the substation finding circuit breaker still open.
In coordination insulation studies , the performance analysis of substation for this configuration (line with the breaker open), will be made to the maximum value that the subsequent discharge can get, say 30 kA with wavefront of 1.2 ms.

 

[jghrist]

Backflash can occur if:

1. The lighting stroke current in higher than normal.
2. The tower grounding is too high of a resistance.
3. The insulation BIL is too low.

It's normal to design for about 1 lightning outage per 100 miles per year. Designing for no outages would be cost prohibitive.

 

[RRaghunath]

jghrist,
Thanks for the response.
"..1 lightning outage per 100miles per year" - can you elaborate on this please.
You meant one lightning strike per year or one lightning outage (if it is outage, isn't it too much)!!
We have long 132kV OHLs strung on wooden poles. There is no shield wire on top and the surge arresters are located at either end at the entry to the substations as per normal practice. The line lengths in some cases are of the order of 130km.
Under the above conditions, if the lightning strikes some where in the middle of the OHL (far away from the substations / Surge arrester locations), would the lightning reach the surge arrester or is it likely to cause flashover along the way!
Appreciate you sharing your knowledge / experience.

 

[odlanor]

ragun,
1- insulation coordination covers the studies of direct discharge (shielding failure) and indirect discharge("backflashover")
its transmission line has no guard wire!. The situation is worse than I thought!
This means that the line is fully capable of receiving direct rays, besides the blackflashovers.
This reinforces the need to analyze from the point of view of Multiple discharge currents.
In the hypothesis with the Multiple discharge the first stroke can cause a short circuit on the line and, consequently, opening their circuit breakers by protection relays.The circuit breaker opens and then comes a secondary discharge.

2-The protection of a substation or line against lightning stroke is by shielding all line (guard wire).
The purpose of the surge arresters at the substation is to protect equipments against voltage traveling waves from the line.

3 - if the discharge current in the middle of the line is very weak however enough to break the isolation to carry short circuit, the traveling wave voltage reflected toward the substation may suffer attenuation along the way and getting on with values ​​below the voltage disruption or discharge voltage (OZn-arresters), which in consequence will NOT operate.

 

[RRaghunath]

Thanks Odlanor for the insight.

 

[jghrist]

"..1 lightning outage per 100miles per year" - can you elaborate on this please.You meant one lightning strike per year or one lightning outage (if it is outage, isn't it too much)!!We have long 132kV OHLs strung on wooden poles. There is no shield wire on top and the surge arresters are located at either end at the entry to the substations as per normal practice. The line lengths in some cases are of the order of 130km.

I mean one flashover per 100 miles per year. This would normally result in a momentary outage with reclosing.

If you have no shield wires, then the lightning will flashover the insulators and cause an outage before the surge gets to the substations. The average surge is 20 kA and typical surge impedance is 300 ohms, so average voltage will be 3000 kV if half of the surge travels each direction from the strike point, a little higher than the typical line BIL.

 

[odlanor]

Of course 3000kV surge voltage will not arrive at substation!
It will flashover the insulators of other towers up to limit of line BIL.

 

[RRaghunath]

Thanks jghrist for the explanation. So, if shield wire is there, it would have conducted most of energy to ground through the nearest earth connection (every other tower will have shield wire connected to earth grid as per standard practice, if I remember right). Thus, the chances of voltage building up on the phase conductors beyond the rated BIL are eliminated by use of shield wire, I guess.

 

[odlanor]

raghun,
now I think you're exaggerating! I find it hard to eliminate the resistance of grounding. It's hard resistance of a tower-foot below 20 ohms at 138 kV.
To eliminate short circuits on the line from lightning, I know the solution is to install surge arresters in all the towers in three phases of condutor cables.

 

[jghrist]

With shield wires located to intercept all strokes, then the voltage won't build up on the conductors but when there will be a voltage built up on the tower from the impedance of the tower and groud. If the voltage is high enough, it will flash over to the conductor (a back flash). There will be more voltage on the shield wire if the stroke is in between ground points because of the surge impedance of the shield wire. If spans are long or if the shield is not grounded at every tower, there may be a flashover from the shield wire to the conductor at midspan.

 

[odlanor]

if tower-foot resistance were zero there would be no flashover.

In practice, every time occurs flashover line will be short circuit to ground on lin.

 

[cuky2000]

Is not unusual in region with low ground flash density such as in the arctic or desert areas to design HV transmission lines without shield wires.

If any improvement in lightning performance is desired, the following low cost mitigation strategies are suggested:

• Provide a shield wire a few poles close to the substations.
• Install surge arrester along the T. Line & substations entrance.
• Implement reclosing scheme.

 

[RRaghunath]

Thanks cuky2000!
I think that was the background for the 132kV transmission line design minus shield wire in our case too. The remedial measures you suggested match with our line of thinking.
Great to hear.