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Can somebody explain to me how robust the high voltage grid systems and railway grid systems are against shorting? For example, if a thinner bare wire or cable falls on two wires of the HV cable, will it cause the grid to trip or get damaged? Or if the two HV wires of the grid carrying two different phases somehow come in contact with each other momentarily, will the grid get damaged?

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    \$\begingroup\$ not generally.. grid is designed to absorb a lot worse. \$\endgroup\$ – Trevor_G Mar 10 '17 at 4:37
  • \$\begingroup\$ The grid here is designed for 700V 8000A, which will convert a "thin" wire to plasma very fast. \$\endgroup\$ – Christian Mar 10 '17 at 11:42
  • \$\begingroup\$ Voters: This doesn't deserve the VTC's "Unclear what you're asking". This is an excellent power transmission / distribution question. \$\endgroup\$ – Li-aung Yip Mar 11 '17 at 6:40
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Generally no. This is the sort of expected hazard the grid is designed to shrug off. Remember that it doesn't need a piece of wire to short circuit conductors either, an arc is effectively a short circuit, and you can get one of those provoked by a lightning strike, or tracking across a fallen branch.

It derives its resilience from a combination of current limiting, and over current disconnection. The generators and transformers feeding any section of line will have a designed impedance, such that the current resulting from a phase-to-phase, or a phase-to-ground fault will be ...

a) small enough that thermal effects and Lorentz forces will not damage in the time before disconnection, and

b) big enough to operate over-current fuses or other types of circuit breaker fast enough to protect it

Bearing in mind a high voltage fault will often be an arc, and if you remove power from an arc it goes out, the first 'fault clearing' method tried by automatic switchgear is to interrupt the current for a short period, then reclose automatically. When you experience a 'brownout' in your home, that's the result of an adjacent link experiencing a short, and dragging your voltage down for the duration of the fault-clearing episode.

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We expect that overhead lines are going to have short-circuit (and open circuit!) faults.

We install protection systems that minimise the damage.

Phase to phase faults

For example if a thinner naked wire or cable falls on two wires of the HT cable, will it cause the grid to trip or get damaged.

Suppose that someone throws a metal chain over the transmission line. (Thieves do this to cut the conductors down, and steal them for scrap metal.)

The metal chain will create a phase-to-phase fault.

If the fault current is large enough, and flows for enough time, a protection relay will operate. The protection relay trips a circuit breaker, interrupting the fault.

The amount of damage done depends on how fast the protection operates. Faster protection tends to be more expensive. (See "Protection Schemes" below.)

Reclosing

Or if the two HT wires of the grid carrying two different phases somehow come in contact with each other momentarily, will the grid get damaged.

When two phases of an overhead line come into contact with each other, that's called clashing. The phases will flash over (arc) and a fault current will flow.

The fault current is detected by a protection relay. The protection relay opens a circuit breaker, clearing the fault. This is the same as above.

If the fault was due to something temporary - conductor clashing, a tree branch, or wildlife - then the circuit can be turned on again. Therefore, some protection relays include an auto-reclosing function. This waits a short period after opening the breaker, then closes it again, restoring the power.

Auto-reclosers are usually set to allow a certain number of re-close attempts, i.e. "three shots in 60 seconds". If the fault hasn't cleared after three attempts, it's due to something permanent. The auto-recloser locks out, and signals a crew to come out and look for the fault.

Protection schemes for overhead lines

All overhead lines are equipped with protection systems.

The national transmission grid, i.e. 132kV, 220kV, is generally equipped with very fast, very sensitive protection. This is required because a fault on the transmission grid can destabilise generators, causing a blackout, unless the fault is cleared quickly. The "critical fault clearance time" is often less than 500ms. i.e. if a fault persists for longer than 500ms, there will be a blackout.

Distribution networks, i.e. 33kV, 11kV, do not have such good protection.

In order of cost - from cheapest to most expensive:

  • Fuses.

    Not very sensitive.

  • Overcurrent relay.

    Looks for some amount of current, flowing for some period of time. The heavier the fault, the faster it operates.

  • Earth fault relay.

    Like an overcurrent relay, but looking for large currents flowing to earth. Can distinguish between load current and fault current. Disadvantage: shares the same sensors (CT's) as overcurrent relay, which limits sensitivity. I.e. can't detect the small current from a downed conductor lying on the road.

  • Sensitive earth fault relay.

    Uses a dedicated "core balance" CT to detect small earth fault currents.

  • Differential relay

    Uses a pair of relays, one at each end of the line. Detects faults in the zone between the relays. Very sensitive. Very fast. Disadvantage: Need to install and maintain a communications line (pilot wire, or telecom, or optical fibre) between the relays.

  • Distance relay

    Measures the impedance (current ÷ voltage) of the power flow onto the line. Operates when the impedance drops below the normal value. Fast, sensitive, and doesn't need a communications channel. Somewhat tricky to apply, and can't be applied to short lines.

    Often applied as a backup to a differential scheme, in case the differential communications channel fails.

Summary

Can somebody apprise me as to how robust are the HT grid systems and Railway Grid systems against shorting.

  • We expect short circuit faults to happen.
  • We install protection systems that limit the damage.
  • Important assets get expensive protection systems. (Fast, sensitive.)
  • Protection costs money. We can't put the best protection on everything. We have to design protection schemes which are appropriate to the importance of the asset they are protecting.
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