Interview Question On Power System Part 5

Q. What are the different types of transmission lines based on capacity?

A.

1. Short transmission lines (up to 50 Km) (<20 KV)
2. Medium transmission lines (50-150 Km) (>20 KV & <100 KV)
3. Long transmission lines (>150 Km) (>100 KV)

Q. What is per unit value?

A. Per unit of any quantity is defined as the ratio actual to base value expressed in decimal.

Q. What are the advantages of per unit system?

A.    1. The pu impedances of machines of same type and widely different ratings lie within a narrow range.
2. Per unit values referred to either side of the transformer remains same. Pu impedance of a 3 phase transformer is independent of type of winding
3. Computational efforts are reduced by a great extent.
4. The chance of confusion between line and phase quantities in a three phase balanced system is greatly reduced.
5. Manufactures usually provide impedance values in pu.

Q. What is the need for base values?

A. The various components of a power system may operate at different voltage and power levels. So, it will be convenient if voltage, power, current and impedance ratings of these components are expressed with reference to a common value called base value.

Q. What is a one line / Single line diagram?

A. A Single or One line diagram is a diagrammatic representation of a power system, in which the components are represented by their standard symbols and interconnection between them is shown by a straight line.

Q. What is an impedance diagram and what are the approximations made while drawing it?

A. The impedance diagram is the equivalent circuit of a power system in which various components are represented by their approximate or simplified equivalent circuits.
Approximations:
1. The neutral impedance is neglected.
2. All static loads are represented as impedances.
3. Transmission line is represented by its  equivalent.

Q. What is a reactance diagram?

A. The reactance diagram is drawn omitting all static loads, neutral impedance, resistances, magnetizing components of each transformer and the capacitance of the transmission line from the impedance diagram.

Q. What are positive sequence components?

A. The positive sequence components of a three phase unbalanced vectors consists of three Vectors of equal magnitude, displaced from each other by 120 degrees in phase and having the Same phase sequence as the original vectors.

Q. What are negative sequence components?

A. The negative sequence components of a three phase unbalanced vectors consists of three Vectors of equal magnitude displaced from each other by 120 degree in phase and having the phase sequence opposite to that of the original vectors.

Q. What are zero sequence components?

A. The zero sequence components of a three phase unbalanced vectors consists of 3 vectors of equal magnitude and with zero phase displacement from each other.

Q. When do you call a fault symmetrical and unsymmetrical?

A. A fault is called symmetrical fault if the fault current is equal in all the phases. The fault is unsymmetrical fault if the fault current is not equal in all the phases.

Q. What is a PQ bus?

A. A bus is called PQ bus when real and reactive components of power are specified for the bus. In a load bus the voltage is allowed to vary within permissible limits.

Q. What is swing bus?

A. A bus is called swing bus when the magnitude and phase of the bus voltage are specified for it. The swing bus is the reference bus for load flow solution and it is required for accounting line losses. Usually one of the generator bus is selected as swing bus.

Q. What is the need for slack bus?

A. The slack bus is needed to account for transmission line losses. In a power system the total power generated will be equal to sum of power consumed by loads and losses. In a power system only the generated power and load power are specified for buses. The slack bus is assumed to generate the power required for losses. Since the losses are unknown the real and reactive power are not specified for slack bus.

Q. What is a bus admittance matrix?

A. The matrix consisting of the self and mutual admittances of the network of a power System is called bus admittance matrix.

Q. What is a bus impedance matrix?

A. The matrix consisting of driving point impedances and transfer impedances of the Network of a power system is called bus impedance matrix.

Q. What are ACSR conductors and their advantages?

A. ACSR conductors are composite stranded conductors. The outer strands have high conductivity and low tensile strength (Aluminum). The central strands have low conductivity and high tensile strength (steel). With ACSR conductors skin affect can be minimized.

Q. What is Ferranti effect?

A. When the transmission line is operating at no load or light load condition, the receiving end voltage is more than the sending voltage. This phenomena is called Ferranti effect. It is prominent is long transmission lines.

Q. What are the major components of a distribution system?

A. Feeder: A line or conductor that connects the major station to the distributor is known as feeder.
Distributor: A line or conductor to which various consumers are connected through service mains is known as distributor.
Service Main: A line which connects the consumer to the distributor is known as service main.