Transmission and Distribution MCQ [Free PDF] – Objective Question Answer for Transmission and Distribution Quiz

Explanation:

• Three core cables are generally used for voltages upto 33 Kv but beyond that weight and size of the three core cables is not economical and nor practical.
• Extra high tension cable can be of 2 types
  1. Oil-filled cables
  2. Gas pressure cables

Explanation:

Methods of Laying underground Cables

• Generally, there are three methods of laying an underground cable
  1. Direct laying
  2. Solid system
  3. Draw-in-system

Direct laying Method

• In this method, a trench of 1.5 meters is dug and it is covered with a layer of fine sand to restrict the entry of moisture from the ground.
• The cable is laid over the sand-belt after that cable is covered with another layer of sand.
• Then the trench is covered with bricks and stones to protect the cables from mechanical injury.
• This method of laying underground cables is clean, safe and very cheap in comparison with the other two methods
• The main disadvantages of this method are high initial cost and the alternation of the cable network is difficult.

Draw-in System Method

• In congested areas where excavation is expensive and inconvenient this system of the laying of underground cables is often adopted.
• This method of cable laying is most suitable for short length cable routes such as in workshops, railway bridge crossing, road crossing where frequent digging is costlier or impossible.

Solid system Method

• This method of laying underground cables is rarely used because of its high cost.
• In this system, the cable is laid in open pipes or troughs dug out in the earth along the cable route.
• The troughing is of cast iron or treated wood and it is filled with bituminous after cables are laid.
• The advantage of this method is that the cables are protected mechanically and from chemical reactions due to impurities in the soil.
• The disadvantages of this method are
  1. laying and repair require more time
  2. laying and repair cannot be carried out in the rainy season.

Explanation:

Cable Losses

• In a cable, if the rate of heat generation is more than the rate of heat dissipation than the temperature of cable increases.
• The three main sources of heat generation in a cable are:
  1. Dielectric losses in cable insulation
  2. Conductor losses
  3. Sheath loses

Dielectric losses

• Dielectric losses consist of losses due to leakage through cable insulation and losses caused by dielectric polarization under AC stresses.
• These losses depend upon the voltage, frequency, and permittivity of the material.
• The dielectric losses are more profound in the HV and EHV transmission cables.

Conductor losses

Conductor losses I₂R loses depends upon the RMS current I and effective AC resistance of the cable conductor R_ac

Sheath Losses

When the alternating current flows in the cable it produces a pulsating magnetic field. This pulsating field links with the lead sheath and induced current in it. This sheath current produces sheath losses.

Cables should not operate too hot for the following reasons

1. Voids may be created in the insulation due to unequal expansion; this will lead to ionization.
2. Owing to the rapid increase of dielectric losses with temperature, thermal instability may arise.
3. The sheath may burst due to the expansion of oil.
4. The oil may lose its viscosity and may start draining off from higher levels.

• With the increase in voltage levels and the interconnection of power grids, power systems became much more complicated. It was necessary to find an accurate way to calculate the active power and reactive power that flows in the lines
• The Network Model of interconnected power systems in the power flow study includes the representation of generators as complex power sources, loads as complex power demands, and transmission lines as a Π network consisting of series admittance and line charging admittances.
• As the AC power system is a nonlinear system, the simple Ohm’s Law is not suitable. Using Kirchhoff’s s Law, electrical engineers wrote active power and reactive power balancing equations for all nodes of the system. They are power balancing equations, also called power flow equations. The equations are able to mathematically represent the power network.
• The Mathematical Model for the study is a set of nonlinear simultaneous algebraic equations. Network equations in the study can be formulated by using either the Z_bus or Y_bus matrices. However, Y_bus is preferred, as the matrix has more number of zero elements or has more ‘sparsity’. This enables fast solutions using only the non-zero entries.
• Either the Gauss-Seidel or the Newton—Raphson iterative method is used to solve non-linear algebraic equations. While the former method is used for small-sized power systems, the latter finds application in the study of large-sized systems. Newton Raphson’s method is one of the effective ways to solve equations. As the study is conducted under steady-state conditions of the system an only a single phase-based positive sequence network is considered and all numerical values are given as per unit values.
• Thevenin’s and superposition’s theorems are very useful in the calculation of currents and voltages due to short-circuit faults in power system networks.

Explanation:

• A booster is a series-wound generator that is inserted into the circuit to increase the DC voltage level so that the excessive voltage drop in the feeder can be compensated.
• A booster may be defined as a dynamo-electric machine, the armature of which is connected in series with a circuit. its generated e.m.f. being added to or subtracted from that of the circuit pending upon the polarity of its excitation.
• Boosters may be driven by any form of a prime mover but are generally direct-connected to a motor taking current from constant-potential means.
• The primary use of a booster is to raise the voltage of a generator or of a section of the bus bars of a central station by an amount sufficient to compensate the ohmic drop in a feeder supplying the distant load. in case the load is of such character as to require the same voltage as receiving devices at or near the source of supply. Since the line drop is directly proportional to the current, the voltage of the booster should also be proportional to the current.

Explanation:

Radial System

• A radial system is an electric transmission system that supplies power from the main generating station to the various sub-station.
• From the substation, the power line is connected to the distribution transformer where the voltage is step-down to the value required by the customers.

Advantages of Radial System

1. The initial cost is low.
2. Useful when the generating capacity is low.
3. Preferred when the station is located in the center of the load.

Disadvantages of Radial System

1. When the load on the distributor changes, the consumers at the distant end of the distributor face serious voltage fluctuations.
2. This system is least reliable because there is no guarantee of continuous service because no backup distribution system is provided.

Most of the distribution and part transmission of electrical power is nowadays carried out through underground cables because of several advantages over the over-head system. Many times locating a fault becomes a difficult task because the cable is hurried under the ground and is not accessible. The faults which are most likely to occur are :

1. Ground fault: A breakdown of the insulation of the cable which allows current to flow from core to earth or to cable sheath.
2. Short circuit fault: A cross or short circuit between two cables or between two cores of a multicore cable.
3. Open circuit: Where the conductor becomes broken or a joint pulls-out. The method for locating an open circuit fault differs from those used for the other two faults. Amongst various methods used for localizing cable faults. Murray loop test is very common and is described here.

Explanation:

• In India, a single-phase supply is a 230V supply through two wires and In North America, a typical three-phase system will have 208 volts between the phases and 120 volts between phase and neutral. 

Explanation:

A four-wire system allows you to have phase voltages or line-to-ground voltages. In a 3 phase 4 wire system, neutral wire carries return current in case any fault occurs in the line there is more flexibility to keep the system running since it may only affect one of the phases.