Test: Power Systems- 2 - Electrical Engineering (EE) MCQ

Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases with greater depths in the conductor.

The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.

The alternating flux in a conductor is caused by the current of the other nearby conductor. This flux produces a circulating current or eddy current in the conductor which results an apparent increase in the resistance of the wire. Thus, more power losses in the windings. This phenomenon is called proximity effect.

The main considerations for operating transmission line at high voltage are:

1) To increase efficiency only

2) To reduce power loss only

3) To increase power transmission capability

High voltage is used in long distance power transmission in order to

1) Reduce the time of transmission

2) Reduce the transmission losses

3) Make the system reliable

Skin effect is the tendency of an alternating electric current to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases with greater depths in the conductor.

The electric current flows mainly at the skin of the conductor, between the outer surface and a level called the skin depth. The skin effect causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus reducing the effective cross-section of the conductor.

Hence resistance in AC is greater than resistance in DC

The transmission lines which have length less than 50 km are generally referred as short transmission lines.

For short length, the shunt capacitance of this type of line is neglected and other parameters like electrical resistance and inductance are lumped

Aluminium is used for a whole range of busbar applications and to make transmission line conductors due to its light weight and durability.

Different types of insulators used in overhead transmission line are:

1) Pin type

2) Suspension type

3) Strain type

4) Stay insulator

5) Shackle insulator

Rivet type is not a type of insulator used in overhead transmission line

Sag is defined as the difference in level between points of supports and the lowest point on the conductor.

Sag of conductor’s between two poles can be determined by

Where, S is the sag of conductors

W is the weight per unit length of the conductor

L is the length of span

T is the tension in the conductor

Cables in power transmission line are provided with inter sheaths to provide proper distribution of stress.

Radial Distribution System: This system is used only when substation is located at the center of the consumers. In this system, different feeders radiate from a substation and feed the distributors at one end. Thus, the main characteristic of a radial distribution system is that the power flow is in only one direction. It is the simplest system and has the lowest initial cost but it is not highly reliable.

A major drawback of a radial distribution system is, a fault in the feeder will result in supply failure to associated consumers as there won't be any alternative feeder to feed distributors.

Ring main system: In this system, each distribution transformer is fed with two feeders but in different paths. The feeders in this system form a loop which starts from the substation bus-bars, runs through the load area feeding distribution transformers and returns to the substation bus-bars.

Ring main distribution system is the most preferred due to its following advantages:

1) There are fewer voltage fluctuations at consumer's terminal.

2) The system is very reliable as each distribution transformer is fed with two feeders. That means, in the event of a fault in any section of the feeder, the continuity of the supply is ensured from the alternative path.

Interconnected distribution system: When a ring main feeder is energized by two or more substations, it is called as an interconnected distribution system. This system ensures reliability in an event of transmission failure. Also, any area fed from one generating station during peak load hours can be fed from the other generating station or substation for meeting power requirements from increased load.

Power factor can be improved by

1) Synchronous machine

2) STATCOM

3) Static VAR Compensator

In the following two faults, fault occurs in all the three phases of power system.

1) LLL fault (Line-Line-Line fault)

2) LLLG fault (Line-Line-Line-Ground fault)

These two faults are symmetrical faults.

Oil switches: As the name implies, the contacts of switches are opened under oil. Usually transformer oil is used whose purpose is to cool and quench the arc formed on account of opening the contacts. These switches are used for circuits of high voltage and large current carrying capacities.

Air break switches are generally used outdoors for circuits of medium capacity such as lines supplying an industrial load from a main transmission line or feeder

Rewirable fuses are most commonly used in housing wiring and small current circuits. It is also known as a kit-kat fuse. It has a porcelain base which is carrying the fixed contact through which the live wires are connected. The fuse carrier is the independent part which is easily removed or stuck in the base.

The fuse element is made of lead, tin, copper or alloy of tin-lead. The current required for melting the fusing element is twice the value of normal operating current.

The rewirable fuse has the advantages of replacement of fuse element without any risk. The cost of the replacement is also very less.

Current chopping in circuit breaker is defined as a phenomenon in which current is forcibly interrupted before the natural current zero. Current Chopping is mainly observed in vacuum circuit breaker and air blast circuit breaker. There is no such phenomena in oil circuit breaker. Current chopping is predominant while switching shunt reactor or unloaded transformer.

Generally the arc extinction in a circuit breaker take place at natural current zero. But this is true if the capacity of the breaker to extinguish the arc varies with the level of fault current. This means that, the arc extinction capability of breaker will always ensure that arc extinction is taking place at natural current zero.

IDMT relay stands for Inverse definite minimum time lag relay. It is protection relay. It is used on transmission lines to see that the line current doesn't exceed safe values and if it does, triggers the circuit breaker. It decreases the tripping time for a higher fault.

Differential protection for feeders consist of

1) Voltage balance differential relay

2) Current balance differential relay

If maximum demand is more than installed capacity, station output will be more than the installed plant capacity.

Hence utilization factor will be more than 100%

A load curve is a plot of the load demand with respect to the time in the chronological order

From the load curve, the following information can be obtained

1) The variation of load on the power system during different hours of the day

2) Number of units generated

3) The highest point in the curve indicates the maximum demand on the power station

4) The area of the curve divided by time gives the average load on the power station

By observing the above information, it helps in deciding

1. The total installed capacity of the plant

2. The size of the generating units

3. The operating schedule of the generating units

Consumer motor load = 250 kW

Maximum demand = 250/0.85 = 294.11

Unit consumed per year = maximum demand × load factor × hours in a year

= 250 × 0.7 × 8760 = 1533000 kWh

Annual bill = Max demand charges + energy charges

= (294.11 × 50) + (1533000 × 0.1) = Rs. 168005.5