Shielded Cable for CT & PT Wiring 2

[Fritzy92]

Is it recommended to use shielded cable for CT and/or PT secondary wiring?

I'm working on a design for a small substation that will have MV cable and CT/PT/control wiring in the same concrete encased duct bank. Obviously the MV will be in separate conduits from the LV wiring. What's the minimum separation between MV and LV conduits you would be comfortable with and would you use shielded cable for the PT/CT secondaries?

 

[marks1080]

Avoid grounding the CT cct more than once to prevent circulating current, which can directly effect relay operation.

Ground the CT cable shield at both ends (with proper techniques) for transient suppression. This is more important when running cables in a swithcyard where you will have exposure to a lot of different types of transient noise.

 

[DTR2011]

@marks1080-

I am a contractor commissioning engineer. Many of the stations in my area have changed ownership several times over the years. There is a new owner that is trying to upgrade the system, over time. In the case of the 27 conductor unshielded cable, we are changing out OCB's for GCB's and part of that is new shielded cables for CT's, with each CT set having its own dedicated cable & shield. The old cable was originally connected to EM relays + a SEL 221.

I've been in larger 345/138/25kV stations with similar 27 conductor cables and if a feeder breaker is open, the amount of induced signal is crazy / scary and can lead to all kinds of erroneous readings.

I do not disagree with what you have said, I was only pointing out that these things do exist in older stations.

 

[marks1080]

Thanks DTR2011,

I completely agree, and would go as far to say that these problems exist way more in older stations. I believe partly because of brown field work over the years, perhaps being done to different standards. Partly to changes in work practices. Partly to the fact that the older EM relays were much more resilient to transients. So it's not necessarily that the problem wasn't always there, just that the consequence was not nearly as bad.

Most of my experience has been with older HV stations, which I prefer. They're more interesting work environments than the brand new installations. I've seen some EM relays get toasted, but not nearly on the scale that we've seen the newer IEDs get toasted. I've seen quite a few CVT boxes damaged as well. The particular CVTs we use (talking 230/500kV) are unusually high in capacitance, which can create even more problems with transients.

In my experience, proper grounding (and more importantly - proper grounding techniques) are not given the priority they deserve. I've seen teams of people chase gremlins in a system when the entire problem was a grounding conductor too long, or some cable was not properly shielded.