Isolator or disconnecting Switch: An isolator is a switch that is designed to open a circuit under no-load condition. Its main purpose is to isolate one portion of the circuit from the other and is not allowed to be opened while current is flowing in the line. Such switches are used on both sides of a circuit breaker so that its repair works or replacement could be done.
Note that an isolator is never opened until the circuit breaker in the circuit is opened and it is closed before the circuit breaker is closed. If an isolator is switched OFF when a high current is flowing through the circuit, a heavy spark will be produced. This heavy spark may break the supporting insulator of the isolator which may cause a fatal accident to the operator.
In two wattmeter method the phase angle is tanφ = √3(W1 − W2)/(W1 + W2) tanφ = √3(500 − 500)/(500 + 500) tanφ = 0° φ = tan−10° = 0° Power factor = cosφ PF = cos0° = 1
In transmission lines, a large amount of power is transmitted over a long distance. So, voltage regulation is not important because in some lines, 40% regulation is considered satisfactory. In such cases, only the economy is important. The cost of the conductor is the main part which decides the total cost of the transmission line. Hence, the selection of the proper size of the conductor for a particular line is most important.
The most economical area of the conductor is that for which the total annual cost of the transmission line is minimum. This is known as Kelvin’s law and was given by Lord Kelvin in the year 1881.
It states that the most economical cross-section of a conductor is the value at which the annual cost of the electric energy wasted in the conductor, and annual cost of the interest and depreciation on the capital cost of the conductor are equal. Thus, the total annual charge on an overhead transmission line can be expressed as :
Total annual charge =P1 + P2α or Variable part of the energy charge = Annual cost of energy wasted.
P1 and P2 are constants
α is the area of the X-section of the conductor.
Medium transmission line:- When the length of an overhead transmission line is between 100 km and 250 kin with an operating voltage ranging from 20 kV to 100 kV, it is considered as a medium transmission line.
In medium lines, the series impedance and shunt admittance (pure capacitance) lumped at a few pre-determined locations are considered for calculation. These lines can be analyzed by using load end capacitance, nominal-T, and nominal-π methods.
Short transmission line:- When the length of an overhead transmission line is less than 80 km with an operating voltage upto 20 kV, it is considered a short transmission line. Due to the smaller length and low operating voltage, the charging current is low. So, the effect of capacitance on the performance of short transmission lines is extremely small and therefore, can be neglected.
Long transmission line:- Lengths of more than 250 km are classified as long transmission lines; with an operating voltage of above 100 kV.
Voltage regulation:- When a transmission line is carrying current, there is a voltage drop in the line due to resistance and inductance of the line. The result is that the receiving end voltage (VR) of the line is generally less than the sending end voltage (Vs). This voltage drop (Vs − VR) in the line is expressed as a percentage of receiving end voltage VR and is called voltage regulation.
The difference in voltage at the receiving end of a transmission line between conditions of no-load and full-load is called voltage regulation and is expressed as a percentage of the receiving end voltage. Mathematically,
% Voltage regulation = (Vs − VR) ⁄ VR × 100
Obviously, it is desirable that the voltage regulation of a transmission line should be low i.e., the increase in load current should make very little difference in the receiving end voltage. In the transmission line, the voltage regulation is negative whenever the receiving end voltage VR is Greater than the sending end voltage.
The regulation will depend upon the power factor of the load. If the power factor is lagging, the voltage at the sending end is more than that at the receiving end. Hence, voltage regulation is positive. On the other hand, if the power factor is leading, the voltage at the sending end will be somewhat less than that at the receiving end. In that case, the regulation is negative.
In the capacitor start, single-phase induction motor the capacitor is connected in series with the starting auxiliary winding. In this manner, the current in the starting winding may be made to lead the line voltage. Since the running winding current lags the line voltage, the phase displacement between the two currents is made to approximately 90° on starting.
Placing the capacitor in the auxiliary winding circuit to produce a greater phase difference between the current in the main and the auxiliary windings. Due to greater phase difference capacitor Start motors have very high starting torque for a single-phase AC motor.