Physics High School

A 69 kV 3-phase power distribution line is suspended from grounded steel towers via insulators with a BIL of 350 kV and protected by a circuit breaker. The neutral of the transmission line is solidly grounded at the transformer, just ahead of the circuit breaker, but the tower has a resistance of 30 2 to ground. (i) Calculate the peak voltage across each insulator under normal conditions. [10%] (ii) Suppose that, during an electrical storm, one of the towers is hit by a bolt of lightning of 20 kA, lasting a few microseconds. Describe the sequence of events during the strike, and its immediate aftermath. [20%] (iii) Strikes of this magnitude are fairly common. What could be used to replace the circuit breaker to ensure the power outage is minimised? [5%] (iv) Give two applications of high voltage d.c. power links in power distribution networks.

Answers

Answer 1

As grids operating at different frequencies or with significant phase differences. HVDC converters can convert the AC power from one grid to DC and then convert it back to AC at the desired frequency and phase for interconnection.

(i) To calculate the peak voltage across each insulator under normal conditions, we need to consider the voltage distribution in the 3-phase power distribution line.

The line voltage is given as 69 kV, which is the phase-to-phase voltage (Vph). The phase-to-neutral voltage (Vpn) can be calculated using the formula Vpn = Vph / √3.

Vpn = 69 kV / √3 ≈ 39.8 kV

Since the line is solidly grounded at the transformer, the neutral voltage is at ground potential. Therefore, the peak voltage across each insulator is equal to Vpn, which is approximately 39.8 kV.

(ii) During the lightning strike, the high current of 20 kA will flow through the tower and the grounding resistance.

This high current can cause a significant voltage drop across the grounding resistance, resulting in a potential rise on the tower with respect to ground. The tower and surrounding area may experience a voltage surge due to the lightning strike.

The immediate aftermath of the lightning strike can include the activation of protective measures, such as the circuit breaker tripping to interrupt the fault current flow.

The lightning strike can also cause damage to the tower or insulators, requiring inspection and potential repairs before restoring power.

(iii) To minimize power outages during lightning strikes, one option is to replace the circuit breaker with a lightning arrester or surge arrester. A surge arrester is designed to divert the excessive voltage caused by lightning strikes to ground, protecting

the equipment downstream from damage. Surge arresters have a high energy absorption capacity and fast response time, making them effective in limiting voltage surges.

(iv) Two applications of high voltage DC power links in power distribution networks are:

Long-distance transmission: High voltage DC (HVDC) transmission is often used for long-distance transmission of power. HVDC systems have lower losses compared to AC transmission over long distances.

HVDC links can efficiently transmit power over hundreds of kilometers, reducing the need for multiple intermediate substations.

Interconnection of asynchronous grids: HVDC links can be used to connect asynchronous grids, such as grids operating at different frequencies or with significant phase differences.

HVDC converters can convert the AC power from one grid to DC and then convert it back to AC at the desired frequency and phase for interconnection. This allows for better control and stability in interconnected power systems.

To know more about converters refer here;

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