01․ Most commonly used material for insulator is
The most commonly used material for insulators of overhead line is porcelain but glass, steatite and special composition materials are also used to a limited extent. Porcelain is produced by firing at a high temperature a mixture of kaolin, feldspar and quartz. It is stronger mechanically than glass, gives less trouble from leakage and is less affected by changes of temperature.
02․ If the breakdown voltage of the insulator is high, the thickness of the insulator is
The dielectric strength of a material is expressed as Volts/meter and gives the maximum electric field strength that it can withstand intrinsically without breaking down. Therefore, the thicker the layer of insulator, the higher is the breakdown voltage. The dielectric constant varies slightly with increase in thickness and temperature, but is generally assumed to be constant for simplicity.
03․ Operating time of pumped storage plant is
Advantages of pumped storage plants:
1. Most economical as a peak load supply plant
2. Used as load frequency control
3. If it is combined with thermal plant of an area load factor is increased
4. Pollution free
5. Easily adaptable for remote operation
6. Operating time is minimum
04․ Advantages of per unit system is/are
Per unit of any quantity is defined as the ration of actual value of quantity to the base value of quantity in same units.
Per unit of any quantity = Actual value of quantity /Base value of quantity in same units
Advantages of per unit system:
1. Only one equation is required
2. Calculation time is less
3. No units for any quantity
4. No need of transformation ratio
5. Memory required is less
05․ Sparse matrix is matrix which contain
A matrix is said to be a sparse matrix, when it contains more number of zero elements. Y bus matrix is a sparse matrix, containing more number of zero elements. So that faster calculation is possible.
06․ How to find the total number of strands (N) when number of layers (x) are given
Total number of strands (N) when number of layers (x) are given is
Total number of strands N = 3x² - 3x + 1
Where, x = Number of layers
07․ Reactance relays are used for the protection of
1. Reactance relays ----- Short transmission lines
2. Impedance relays ------ Medium transmission lines
3. Mho relays ------------- Long transmission lines
In short transmission line LG faults are more frequent which produces arcing grounds. This will causes the impedance relay operating point shifts towards the blocking region. So that relay may not be operated for the faults within the arcing ground. To overcome this problem reactance relay is used, it is independent of arcing ground and it is used for protection of the short transmission lines.
08․ The main objectives of power system is/are
Objectives of power system are
1. Cost of electrical energy per KWh is to be minimum
2. Rated voltage and frequency has to be supplied to the consumers
3. Reliable power has to be available
4. Effective protection system has to be used for isolating the faulty section and keeping other sections healthy.
5. More stable generators are to be used, so that it should not lose synchronism under faulty condition.
6. Flexible power has to be available.
09․ If L and C are 4 mH and 0.0001 μF respectively a current chop of magnitude 50 Amp would induced a voltage
10․ If the percentage reactance of an element is 20 % and the full load current is 50 Amp, the short circuit current will be
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