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Capicitor Application Issues

Capacitors must be built to tolerate voltages and currents in excess of their ratings according to standards. The applicable standard for power capacitors is IEEE Std 18-2002, IEEE Standard for Shunt Power Capacitors.

Heat as one of most common cause of motor failure

This slide speaks about that how motor operation fails due to heat. how heat affect motors?

Saturday, 24 May 2014

Discuss the criterion for choice of voltage for transmission and distribution

Criterion For Choice Of Voltage For Transmission And Distribution.

We know that the power W =VI , so for the same power if voltage is increased then current will decrease. If suppose power required is 500 W and voltage is 250 V then current I will be equal to 500/250 =2A.

Now if voltage is increased from 250V to 500V and the power required is same then I = 500/500 =1 A

Now, if the voltage is still increased to say 1000V for the same power then I = 500/1000 =0.5 A

From the above examples, we see that if the voltage is doubled, the current will be halved and when it is quadrupled then current is reduced to 1/4th of its initial value. Thus we conclude that if voltage is increased to n times then the current will be reduced to 1/nth times for the same power.With the reduction of the current to 1/nth times, the conductor area will also be reduced to 1/nth times of its original area for the same current density, Hence less material is required when the voltage is increased.

We also know that when the current passes through any conductor, there is loss of power in that particular conductor according to the relation, I2 R. As the loss is proportional to the square of the current. So if the current is reduced to ½ value , then the loss will be reduced to 1/4th its original value. Hence the efficiency of the transmission line and all others equipments associated with the line will increase and more power will be available for use.

When current is passing through a conductor there will be a voltage drop according to the relation V=IR. So, when the current is reduced the drop of the voltage is less in the line, of course with the same cross sectional area of the conductor.

With the reduction of cross sectional area, considered the main advantage of transmitting electrical energy at very high voltage viz 132kV , 220kV or even 400kV.

But in case of distribution system such high voltage is dangerous, so distribution voltage is generally 400/230V.

What are the advantages of high voltage transmission? Give its limitations also.

Ans: High voltage transmission is subdivided into HVAC and HVDC transmission systems.

(i) HVAC transmission: Advantages of HVAC transmission are as follows: As the voltage is increased, the current carried by the conductors decreases. The i2R losses correspondingly get reduced. However the cost of transmission towers, transformers, switches and circuit breakers rapidly increases with increase in voltage, in the upper ranges of a.c. transmission voltages.

(ii) HVDC transmission:
Advantages They (HVDC lines) are economical for bulk power transmission. The voltage regulation problem is much less in DC since only IR drop is involved. There is easy reversibality and controllability of power flow through a DC link. Also there is considerable insulation economy.

Limitations: The systems are costly since installation of complicated converters and DC switchgear is expensive. The converters require considerable reactive power. Lack of HVDC circuit breakers hampers network operation. Moreover there is nothing like DC transformer; voltage transformation has to be provided on the a.c. sides of the system.

Understanding Electric Utilities and De-Reguration 2ed - Lor




Electronics Fundametals Slides 7




Electronics Fundamentals Slides 6