Grounding Switches in HV Substations

[Engineer1916]

Eng Tips Family,
I hope all of you are doing good. I have a question about grounding switches. We have a 230kV switching station with (1) 230kV double circuit double bundle line coming in. It is about 16 miles long. I am trying to figure out whether the disconnect switch intercepting this line will need a grounding switch to discharge any charging current or not? Any help on how to determine that will be highly appreciated.
Thank you!

 

[crshears]

I can't answer the technical side of the question, but within my utility a permanently installed grounding switch is required on such circuits in order to prove isolation for Work Protection [read: personnel protection] purposes.

How to rate that switch will depend on a number of factors; length, voltage, number and spacing of parallel circuits, conductor transpositions, and so on come to mind.

I recall one instance where the permanently installed ground switch was closed in with no issues in order to de-energize the line / prove isolation for work...but at the conclusion of the outage, when all working grounds had been removed and the only grounding of the circuit was via that single handwheel-driven ground switch and the operator cranked it open, it drew an arc that never broke; the switch was fully open and the arc just buzzed and snapped away steadily...

The operators were left with no choice but to close that ground switch back in, then go through the process of what we call "ungrounding through a breaker" to get that ground switch open. In a breaker and a half switchyard, the sequence is: [1] open the two diameter breakers for said circuit, [2] open the outside disconnects for both, [3] have Station Maintenance apply a set of working grounds between one of the breakers and its outside disconnect, [4] close the line disconnect, [5] close the breaker with the working grounds on its outside to create a parallel path to ground, [6] open, check open and lock the permanently installed ground switch, [7] open the closed breaker to unground the circuit, [8] have temporary grounds removed, [9] return all equipment to service in the normal manner.

Moral of the story: don't under-rate the permanently installed ground switch.

And 1916, I don't know if this is possible, but perhaps you could modify your OP so it has a subject line; you might get more responses that way.

 

[Engineer1916]

crshears,
Thank you for sharing your experience. I greatly appreciate it.
Also thank you for pointing out the error in my subject line.

 

[crshears]

You're very welcome, 1916.

Frankly I'm quite surprised there have been no other posts to your query; then again, all of ET seems unusually quiet today...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[cuky2000]

(..230kV, double circuit,16 miles long.[/i](]

A good substation design practice at this voltage will provide a grounding switch no because of the charging current but to provide grounding for inspection and facilitate safely maintenance, repair, or replacement of other substation equipment.

The estimated charging current produced by the line described is around 16 amps (Considering ~1A/mi per the enclosed graph).

Most of the line capacitive current will be discharged as an arching during the breaker and main switch disconnection process. The grounding switch will operate later usually interlocked to avoid misoperation.

[sub]Grounding Switch Rating: It should be noted that the traditional momentary (asymmetrical) current rating is now covered in the standard by the peak current rating. To obtain the 3-sec ratings, divide the momentary current rating by 1.6. The ratio of the peak withstand current to short-time (symmetrical) withstand current is 2.6. Rated duration of the short-time (symmetrical) withstand current is three (3) seconds with allowable I2 t variations given in IEEE C37.34. Rated duration of the peak withstand current test is 167 msec (10 Cycles).[/sub]

 

[thermionic1]

Grounding switches must be a regional preference. I have seen at 500kV, where the distance would be too great to hang traditional grounds. I don't know many maintenance crews that would trust a grounding switch only to work on a breaker or other apparatus. In our region, we typically don't use grounding switches at 138 or 345kV.

A few years ago I was at a 500kV station, overseeing construction. There was a grounding switch installed in conjunction with the disconnect. At the time there was only a mechanical interlock. Since both the GS and DS were electrically operated, questions were asked about why not an electrical interlock. We were told it was not necessary. After the interlock was accidentally destroyed during testing, the few wires required to electrically interlock the two switches were installed.

 

[cuky2000]

As far as I know, the best safety practice in the power industry is to use both personal grounding system and grounding switch to eliminate any residual charges that may harm the personnel going for maintenance.

It is interesting to know about the diversity of safety practices. The utilities that I worked with require the use of a safe grounding switch to provides extra safety to the working personnel. O&M personnel also uses a personal grounding system.

Be aware that the personal grounding system also has a downside.

 

[bacon4life]

Another think to keep in mind is that grounding switches at the substations may need to be opened to prevent excess circulating currents when crews apply grounds at their worksite.

 

[crshears]

What bacon4life said; within my utility we have several heavily loaded corridors where minutely detailed instructions have been written to specify where and in what sequence permanently installed ground switches [PIGSs] are to be opened and closed, the idea being to break the circulating current on the switch most capable of it; in the simpler applications the truth tables generally use "first on, last off" and "last on, first off" phraseology, whereas in circuits with PIGSs installed in three or more possible locations the truth tables can get very convoluted, sometimes so much so it seems to take a Toronto lawyer to figure them out. One of the considerations in these twisted instances is where the line crews will be working and where they need to apply their temporary grounds.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]