Question 1: State the advantages and disadvantages of using low-voltage transmis

Question

Question 1: State the advantages and disadvantages of using low-voltage transmission lines.

Question 2: State the advantages and disadvantages of using high-voltage transmission lines.

Question 3: Why are transformers used with transmission lines?

Question 4: A two-pole generator is to be connected to a 60 Hz power grid. Compute the speed of its turbine.

Question 5: Why are there different voltage and frequency standards worldwide?

Question 6: How is the lightning energy that strikes power lines dissipated?

Question 7: Why are transformers often blown up when a lightning strike reaches its terminals?

Note: type the answer please

Explanation / Answer

Advantages of high voltage transmission lines

· Due to increase in the transmission voltage size of the conductors is reduced (Cross section of the conductors reduce as current required to carry reduces).

As the reduction in current carrying requirement losses reduces results in better efficiency

Due to low current voltage drop will be less so voltage regulation improves

Disadvantages
Due to the increase in the voltage of transmission, the insulation required between the conductors and the earthed tower increases. This increase the cost of line support

With increase in the voltage of transmission, more clearance is required between conductors and ground. Hence higher towers are required.

With increase in the voltage transmission, more distance is required between the conductors. Therefore cross arms should be long

Why transformers used with transmission lines

High-voltage transmission lines are used to transmit electric power over long distances. Normally, high voltage (HV) transmission power lines are made of high voltage (between 138 and 765 kilovolts) conducting lines of copper and/or aluminum.

Assume the power to be transmitted is P, and the resistance of the transmission line is r.
If the power is transmitted with voltage V, then the current flow through the transmission line is I=P/V.

Since P and r are fixed conditions, less power will be lost if high voltages V are used.

From Ohm's law. if the voltage is increased, the current will increase ,too. Why is the current smaller when high voltage is used to transmit the power. the transmission line needs a transformer to step down the voltage.

Why are there different voltage and frequency standards worldwide

It was a slightly different story in Europe and the United Kingdom – German power company AEG established a major grip on electrical generation across the continent, while electric companies in the UK mostly matched the AEG standard of 110V AC at 60Hz. This continued until World War II where all across Europe a 220V alternating current at 50Hz was adopted.

In the UK this required an Act of Parliament to nationalise (purchase by the State) the many local electricity companies, and the subsequent standardization of current, voltage and frequency has enabled European companies to buy and sell electricity supplies depending upon demand

How is the lightning energy that strikes power lines dissipated?

A direct lightning strike on a conductor of a power line causes extremely high voltage pulses at the strike point, which are propagated as traveling waves in either direction from the point of strike. The crest of the pulse can be calculated as:

V = I × Z

V is the crest voltage
I is the peak lightning current
Z is the impedance seen by the pulse along the direction of travel.

Impedance Z is equal to half the surge impedance of the line when struck at mid-point and can be approximately as much as 150 . Thus for a peak current of 40 kA, the voltage of the pulse can be as high as 6000 kV. Since the basic insulation level of most systems is much lower than this value, it is clear that such a pulse will cause failure of insulating components along the line.It is therefore necessary that no direct strike must be permitted on the overhead power line’s phase conductors.

Why are transformers often blown up when a lightning strike reaches its terminals?

A lightning strike on a transformer can cause a major explosion due to the amount of extra voltage injected into the transformer. Lightning bolts can contain up to 120 million volts – depending on the length and size. When a transformer blows, it can knock out power for hundreds to thousands of people. In addition, collateral damages can occur because any line connected with the blown transformer, is now live and could be carrying thousands of volts. Blown transformers can also set its surroundings on fire, especially in dry environments. A two-pole generator is to be connected to a 60 Hz power grid. Compute the speed of its turbine.

Speed of turbine N = (120*f)/ P

f= frequency in hz = 60hz

P = number of poles = 2

        N = (120*60)/ 2= 3600 rpm

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