Two Marks Questions with Answers

Two Marks Questions with Answers

 

Q.1 Define voltage regulation of transmission lines.

Ans. : Voltage regulation of a transmission line is defined as the difference in voltage at the receiving end of the transmission line at the no load and full load expressed as a percentage of the voltage at the receiving end with supply frequency and voltage at sending end remaining unchanged. Mathematically it is given as,

 % Voltage Regulation = V_NoLoad - V_FullLoad  / V_FullLoad × 100

 

Q.2 How transmission lines are classified ?

Ans. : Based on the line length and voltage, the overhead transmission lines are classified as

i) Short transmission line

ii) Medium transmission line

iii) Long transmission line

 

Q.3 What do you mean by short transmission line ?

Ans. : If the transmission line length is about 50 km and the line voltage is low i.e. about 20 kV or less then that line is treated as short transmission line.

 

Q.4 What do you mean by medium transmission line ?

Ans. : If the transmission line length is lying between 50 km and 150 km with moderately high line voltage in between 20 kV and 100 kV then that line is treated as medium transmission line.

 

Q.5 What do you mean by long transmission line ?

Ans. : If the transmission line length is more than 150 km with very high line voltage above 100 kV then that line is considered as long transmission line.

 

Q.6 What is Ferranti effect ?

Ans. : In case of long transmission line under no load conditions, the voltage at the receiving end is found to be more than that at the sending end because of the effect of line capacitance. This is called Ferranti effect. 

 

Q.7 Write the expression for characteristic impedance and the propagation constant.

Ans. : The expression for characteristic impedance and the propagation constant is given as,

Characteristic impedance, Zc = √Z / Y ; Z is series impedance and Y is shunt admittance of line

Propogation constant, ɤ = √ZY ; Z is series impedance and Y is shunt admittance of line

 

Q.8 What is surge impedance ?

Ans. : The surge impedance is defined as the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in the absence of reflections in the other direction. Its SI unit is Ohm. It is purely real with no reactive component.

 

Q.9 What are units for generalized circuit constants A, B, C and D ?

Ans. : A and D are dimensionless constants. B has unit of impedance i.e. Ohm (Ω) while unit of C is mho which is that of admittance.

 

Q.10 Draw the power angle diagram of a transmission line.

Ans. : The power angle diagram of a transmission line is as shown in the Fig. 2.23.1.

 

Q.11 Mention the approximate value of surge impedance for overhead line.

Ans. : The approximate value of surge impedance for overhead lines is 400 Ω while typically it is in the range of 400 Ω to 600 Ω.

 

Q.12 In long transmission lines and cables receiving end voltage is greater than sending end voltage during light load or no load operation. Why ?

Ans. : In a long transmission lines and cables it is observed that the receiving end voltage is greater than sending end voltage during light load or no load operation due to Ferranti effect. Under this light load condition, the line capacitance generate more reactive power than the reactive power which is absorbed which results in greater voltage at receiving end than that at sending end.

 

Q.13 Define transmission efficiency.

Ans. : The transmission efficiency is defined as the ratio of power at the receiving end to the power at the sending end expressed as a percentage. Mathematically it is given as,

% Transmission efficiency = Power at receiving end (P_R ) / Power at sending end (P_S ) × 100

 

Q.14 For controlling reactive power, what adjustment should be done in transformer present in the system ?

Ans. : The transformers are equipped with taps on the windings to adjust the reactive power flow through the transformer.

 

Q.15 Draw the nominal K representation of a transmission line.

Ans. : The nominal K representation of a transmission line is shown in the Fig. 2.23.2.

 

Q.16 Draw the nominal T representation of a transmission line.

Ans. : The nominal T representation of a transmission line is shown in the Fig. 2.23.3.

 

Q.17 What is power circle diagram ?

Ans. : The real and reactive powers at sending and receiving end can be computed mathematically and the transmission line characteristics can be represented graphically. By taking sending end or receiving end voltage or current as a reference, these characteristics can be plotted which represent circle and the corresponding diagram is called circle diagram. The real power is plotted on X axis while the reactive power is on Y axis.

 

Q.18 What is surge impedance loading ?

Ans. : The Surge Impedance Loading (SIL) of a line is defined as the power delivered by a line to a purely resistive load equal to its surge impedance. The line is assumed to have no resistance. The reactive power is neither produced nor absorbed. Mathematically it is given as,

SIL = V²_R / Z_c

 

Q.19 List out any two reasons for line loss in a transmission line.

Ans. : The losses in line are resistive loss which is due to finite small resistance associated with the line as perfect conductor never exists in practice. The other loss is corona loss which is due to corona effects which is caused due to ionization of air molecules near the transmission line conductors and carries current in the air along the wire causing the loss.

 

Q.20 State the condition for maximum power delivered.

Ans. : The maximum power is delivered when power angle δ is 90°.

 

Q.21 Mention the significance of surge impedance loading.

Ans. : The Surge Impedance Loading (SIL) of a line is defined as the power delivered by a line to a purely resistive load equal to its surge or characteristics impedance. SIL is called natural power of the line. It helps in expressing power transmitted by a line in terms of per unit of SIL which is the ratio of the power transmitted to the surge impedance loading. The permissible loading of a transmission line can be expressed as a fraction of its SIL and it provides a comparison of load carrying capabilities of lines.

 

Q.22 What is shunt compensation ?

Ans. : Compensation of a line with the help of a shunt capacitor across the line in order to improve the power factor and voltage profile as well as to reduce the losses is known as shunt compensation.

 

Q.23 What are the factors which govern the performance of a transmission line ?

Ans. : The transmission line performance is mainly governed by its four parameters - series resistance and inductance, shunt capacitance and conductance where the shunt conductance is often neglected as it is very small. All these parameters are distributed over the length of the line. Based on these parameters the performance measures of transmission lines are the transmission efficiency and voltage regulation both expressed as percentage.

 

Q.24 What is the range of surge impedance in case of underground cables ?

Ans. : The approximate value of surge impedance for underground cables is 40 Ω while typically it is in the range of 40 Ω to 60 Ω.

 

Q.25 Distinguish between attenuation and phase constant.

Ans. :

 

Q.26 What is the use of power circle diagram ?

Ans. : The power circle diagrams are useful in studying various aspects power transmission at sending and receiving end. It aids in determining real or active power P, reactive power Q power angle δ, power factor at given load conditions, also voltage conditions and impedance Z of the line.

 

Q. 27 What are the types of compensation used in case of transmission lines ?

Ans. : The types of compensation used in case of transmission lines are series compensation and shunt compensation.

 

Q.28 What are the advantages of series compensation ?

Ans. : The advantages of series compensation are

i) Increase in power transmission capacity of line

ii) Improvement in system stability

iii) Improved voltage regulation

iv) Load division between parallel circuits

v) Damping of power swings and transients

 

Q.29 What are the disadvantages of series compensation ?

Ans. : The disadvantages of series compensation are

i) Increase in fault current level due to reduced reactance affecting short circuit current rating of the circuit breaker

ii) Change in natural frequency of the transmission system. Gives rise to high torsional losses

iii) Possibility of faulty operation of distance relays

iv) Possibility of hunting increases

v) High voltage may occur across capacitor terminals due to phenomenon called ferroresonance which may damage capacitors

 

Q.30 What are the main constraints in capability of power transfer for transmission line ?

Ans. : The main constraints in capability of power transfer for transmission line are thermal limits, voltage drop limit and transient and steady state stability limits.

 

Q.31 What are the advantages of shunt compensation ?

Ans. : The advantages of shunt compensation are

i) Increase in kW rating of generators, transformers and lines

ii) Reduction in line current

iii) Reduction in losses taking place in power transformers and cables

iv) Avoids overloading of transformers and switchgears

v) Improved voltage is obtained at receiving end

vi) Distinguish between series and shunt capacitors used for compensation.

 

Q.32 What are the effects of shunt compensation ?

Ans. : The effects of shunt compensation are

i) Reduction in line losses due to generation of reactive power.

ii) Reduction in line current.

iii) It improves the power factor of the transmitted power.

iv) It reduces the voltage drop and it is uniformly distributed along the length of the line.

 

Q.33 Why series compensation is used ?

Ans. : The series compensation is used to

i) increase power transmission capacity of line.

ii) improve system stability.

iii) improve voltage regulation.

iv) damp out power swings and transients.

 

Q.34 What do you mean by voltage stability ?

Ans. : The voltage stability corresponds to limit on maximum power transfer through the transmission line beyond which the voltage collapses and stability is lost.

 

Q.35 Distinguish between voltage stability and rotor angle stability.

Ans. :

 

Q.36 Distinguish between series and shunt capacitors used for compensation.

Ans. :