Effectiveness of Grounding Cable Over Trench for Transient Voltage Protection 1

[cuky2000]

Does anyone know how effective is a set of Grounding Cable over a substation trench next to a 230 kV 200 Mvar shunt capacitor bank to mitigate transient voltage protection of shielded control and power circuits?
The picture below shows a propose ground cable installation. Please advise.

 

[marks1080]

It's very effective. Without out you'll blow up a lot of relays taking in feeds from CVTs during disconnect switch operations. I would even suggest that the grounding conductors should be located more centrally in the trench, as opposed to being strapped to the side. You want any cables in that trench as close to the grounding conductor as possible. The shielding properties of the cable shields on the cables running in this trench are directly proportional to the physical area between the shields and grounding conductors.

I've studied this from the point of view of transient suppression from the arc's of HV disconnects opening. That really is your worst source of 'noise' in a switchyard. For simply switching caps, my gut feeling is the transients wouldn't be on the same level, but I would still want my copper ground run. In general it's very good practice, and I wouldn't signoff on commissioning unless all my cable trenches had this type of grounding.

 

[Power0020]

Would that be a good point to run these control cables in a cable tray?perforated with a lid? a tray can run inside the trench or above ground with frequent connections to grounding grid. It will provide a very high degree of shielding for such cables at a cost comparable to extra copper runs.

any thoughts?

 

[cuky2000]

Does synchronous on the IPO circuit breaker and surge arrester will help to reduce the induced electromagnetic interference?

 

[waross]

Not much will reduce electro-magnetic interference.
One mitigating measure is aluminum shield plates.
Much of the energy of the electro-magnetic field is dissipated by eddy currents in the aluminum.
The aluminum must be thick enough to facilitate eddy currents.
In proximity to EHV lines and equipment electro-capacitive coupling is a concern. A grounded conductor or plate forms a ground potential electrode of the capacitor and provides protection.
A thought for comment: Grounding the re-bar of concrete trench lids for safety and shielding?
Does anyone do this?

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

 

[marks1080]

Remember that this cable trench ground run is just one component of your transient suppression. The ground runs are entirely meaningless if the cable shield grounds are not properly applied. The single biggest factor (imho) in bad grounding, when all the proper materials were bought and all the extra engineering was done, is the physical construction of the cable shield bonding to ground connection; this is where most mistakes are made. Best practice (imho) is to find a very good cable gland that can be used as the bonding 'conductor' to ground. If you are using a grounding conductor to bond the cable shield to ground make sure the length is as short as possible - NO GOOF LOOPS IN THE BACK OF THE MECH BOX!

 

[cuky2000]

Hi Bill & Mark,

Intuitively I do concur with Mark1080 that the ground conductor will be used as a mitigation technique to reduce the impact of EMI interference in control and LV power cable in substation application. The control cables shield are grounded for the utility grounding practice. The biggest concern is for control circuits associated with a neighboring larger generator that will be relocated in the next year but desirable to fully operate ASAP.

Waros, please let us know the bases of your concern

Below is a recommendation from the IEEE Std 99 regarding the use of ground cable on the trench particularly for Cap Bank peninsula grounding application.

 

[marks1080]

Cuky2000 - i believe waross is actually using the reference of EMI interference correctly. Think about geo-magnetically induced currents from solar storms, really the only way to protect against those is to install in-line caps on ALL transformer neutrals. Not really a practical solution. You are more concerned about the capacitivley induced voltage transients from the capacitor switching being inductively coupled on to your control cable.... or something like that. I may have messed up a word or two in that statement

 

[cuky2000]

Mark1080,
Sorry for the confusion regarding the area of interest presented on this post.
We in search of practical mitigation options to add protection to the existing shielded control and LV power cables located in the vicinity of a new single mechanical switched cap bank by a 230 kV IPO circuit breaker equipped with POW synchronous control that we hope not only to withstand the 80 kA and TRV but also to control the transient spike overvoltage that could be induced on those cables.
I am taking the liberty to use an excerpt from the IEEE Std 525 (Guide for the Design and Installation of Cable System in Substations) to indicate the benefits suggested.
Based on the enclosed IEEE recommendations, it is our opinion that installing grounded parallel conductors along the trench and the use of breaker POW for zero-crossing (Synchronous closing Control) will add extra safety assurance to mitigate the effect of transient switching induced overvoltage on the existing shielding grounded conductors.
We welcome further comments in this matter to determine if this is a valuable mitigating option to keep the induced transient overvoltage to a reasonable level.