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Topic Title: What is the advantage and the disadvantage of pilot cable in power system protection?
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Created On: 11 March 2014 01:58 PM
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 11 March 2014 01:58 PM
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garychan1993

Posts: 2
Joined: 11 March 2014

Hello everyone,

I am conducting a research on pilot cable fault repair as my final year project. Everyone know that power company will use pilot cable to control protection relay between two substation. What is the advantage and the disadvantage of pilot cable in power system protection? Concerning frequent pilot cable fault, what technical tool is better than using pilot cable?

Thanks,
Gary
 11 March 2014 02:18 PM
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neilmcd84

Posts: 56
Joined: 02 June 2003

Gary,

In my experience the big downside is cost of installation and difficulty locating and repairing faults when they occur - pilot cables have to be laid in expensive excavations, and there have to be big holes dug to allow them to be repaired when they fault. This all costs money.
On the plus side, historically the alternative to pilot cables at 33kV was a pretty brutal solution - the fault thrower. This 'communicated' between the Primary s/s and the GSP by forcing an earth fault onto the system. For example if an internal transformer fault was identified, the relay at the primary would tell the fault thrower to close, connecting one phase (usually red) to earth. The GSP saw this earth fault and the CB at the GSP would open. This has the obvious downside of stressing all apparatus, generating the need for post-fault maintenance of the CB at the GSP etc. The pilot cable - although more expensive to install - avoided these issues.
Someone might come along that can give statistics on the reliability of a pilot cable compared to modern alternatives eg radio wave - type technologies.
 11 March 2014 03:27 PM
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ArthurHall

Posts: 737
Joined: 25 July 2008

I agree with every thing Neil says. Some pilot cables were strung on wood pole lines so were a bit easier to repair but just as hard to locate. Rented circuits from BT were also popular for a while but are almost obsolete now as BT use so many fibre circuits. Transmission circuits used voice frequency on the power lines but the cost of line traps insulated to transmission voltage was high.
Fibre optics are the new thing, either wrapped around the earth wire on tower circuits, buried direct or incorporated in the power cable.
Wind farms use fibres in the cables for SCADA, and comms as well as protection. There is even a system called DTS which can locate hot spots in a power cable, a fibre is built into the cable and is monitored from both ends, any heat in the cable changes the refraction index of the fibre and this point can be located as a distance down the cable.
 11 March 2014 04:16 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

The decision to use any protection is a balance of the cost of installing and running the proteciton scheme versus the cost of dealing with an unprotected fault.


Advantage - unit protection discriminates in-zone faults. Length of outages due to faults reduced (no "fault-bashing" required). Very useful for closed rings, or where the cost of an outage due to a fault is onerous.

Disadvantage - cost of installation, maintenance and repair.

On secondary networks (11 kV in the UK) this disadvantage is the detemining factor. You rarely see unit protection on DNO 11 kV feeders, which tend to be open rings with OC/EF protection at the feeder end and OC protection on the secondary transformers (often, fuses)

On such networks, smart metering will enable the monitoring of load at along the network and allow faults to be sectioned and isolated.


If this is for a project, I suggest you get hold of the books, "Power Systems Protection" McDonald, London - its an oldie but a goodie.

Edited: 11 March 2014 at 10:28 PM by Zuiko
 11 March 2014 09:26 PM
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neilmcd84

Posts: 56
Joined: 02 June 2003

As Arthur mentions, there has been a recent move towards fibre optic cables, esp for windfarms. However these have caused me significant difficulties recently - one job in particular required the diversion of such a cable for a new roundabout. The protection people told me that it couldn't just be diverted locally - instead they had to pull in new cable all the way back to the intermediate joint boxes (or whatever the correct term is). The costs associated with this were crazy. My costs to divert the 500 (1) Cu cables around the roundabout were dwarfed by the fibre optic diversion charge.

- although of course all costs were passed straight through to the developer, so I didn't actually mind all that much!
 11 March 2014 11:26 PM
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garychan1993

Posts: 2
Joined: 11 March 2014

Thank you for all replies.

I think so many people to support that using fibers to replace with pilot cable. However, is the fibers more reliable than pilot cable?
In addition, I have a idea on pilot cable fault checking. My supervisor tell me that major cause of pilot cable fault is some groundwater infiltrate to the jointing box. Can I design a water/humidity sensor to check it?

Thanks
 12 March 2014 06:51 PM
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Zuiko

Posts: 521
Joined: 14 September 2010

You could do I suppose, but then you are protecting the protection and will need some mechanism to get the sensor output back to the control room where alarms can be monitored which may introduce more problems than you are trying to solve. And then do you need to protect this protection?


A decent joint *should* be robust when underwater (often they are encased in heat-shrink wrap then resin), so you will get lots of nuisance alarms telling you there is water when you do not have a problem. You are not going to investigate every alarm if it means digging up roads!



Instead, many networks will test the pilots peridocially (simple continuity and insulation resistance). This period could be quite short if the price of failure is high. At a longer period the maintenance may include a primary current injection both to check stability of the local and remote relays under normal conditions; and then checks under simulated fault conditions to ensure the relays trip the breakers.

Edited: 12 March 2014 at 06:58 PM by Zuiko
 12 March 2014 08:37 PM
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statter

Posts: 126
Joined: 06 February 2013

Unit protection is generally faster too. It doesn't need to discriminate between fault and load current. Lots of unit protection originally fitted to 6.6kV and 11kV circuits was removed in the 1980s to save the costs of maintaining the system - mostly relay and cable testing. No fundamental reason why pilot cables should be unreliable if properly installed in my view.
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