Q.1. What is restriking voltage?

DRex Electronics

Answer: The transient voltage which appears across the breaker contacts at the instant of arc extinction is known as the restriking voltage.

Q.2. How the voltage of a particular bus can be regulated in a power system?

Answer: The voltage of a particular bus is regulated by controlling the reactive power of the bus. If the reactive power generated is greater than consumed, the voltage goes up and vice-versa.

Q.3. What is Ferranti effect?

Answer: When a long line is operating under no load or light load condition, the receiving end voltage is greater than the sending end voltage. This is known as Ferranti effect.

Q.4. What do you mean by swing equation?

Answer: The equation describing the relative motion of rotor (load angle δ) with respect to the stator field as a function of time is known as swing equation.

Q.5. What is the good effect of corona on overhead lines?

Answer: Corona, is helpful in one respect, namely, it reduces the effect of surges and acts as a relief value for them. This is so because the surges are partially dissipated as corona.

Q.6. What do you mean by critical clearing time of a fault in a power system?

Answer: The critical clearing time is the maximum elapsed time from the intimation of the fault until its isolation such that the power system is transiently stable. So, critical clearance time of a fault is related to transient stability limit.

Q.7. For which type of motors, is the equal-area criterion for stability applicable?

Answer: Three-phase synchronous motor.

Q.8. What do you mean by surge impedance loading of transmission line?

Answer: A transmission line may be considered as generating capacitive reactive volt-amperes in its shunt capacitance and consuming inductive volt-amperes in its series inductance. The load at which the inductive and capacitive reactive volt-amperes are equal and opposite is called surge impedance loading (SIL) or natural load of the line.

Q.9. What do you mean by equal area criterion for stability?

Answer: According to equal area criterion, the system is stable if the area under Pa (accelerating power) – δ curve reduces to zero at some value of δ. In other words, the positive (accelerating) area under Pa – δ curve must be equal the negative (decelerating) area.

Q.10. What is the purpose of grading ring?

Answer: Grading ring serves two purposes:

  • Equalization of voltage drop across the units.
  • When used with arcing horn it protects the insulator string from flashover whenever an over voltage appears between the power structure and the power conductor.

Q.11. What is the main objective of load frequency controller in a power system?

Answer: Using load frequency controller, the change in frequency and the tie-line real power are sensed which is a measure of the change in rotor angle.

Q.12. What is low inductive current chopping problem in circuit breakers?

Answer: When low inductive current is being interrupted and the arc quenching force of the circuit breaker is more than necessary to interrupt a low magnitude of current, the current will be interrupted before its natural zero instant. In such a situation, the energy stored in the magnetic field appears in the form of high voltage across the stray capacitance, which will cause restriking of the arc.

Q.13. When relay is said to be “over-reach”?

Answer: A relay is said to be “over-reach” when it operates at a current which is lower than its setting.

Q.14. In which context, the equal area criterion in power systems is used?

Answer: Equal area criterion in power systems is used in the context of stability of a machine connected to infinite bus bar.

  • For one machine system and infinite bus bar this method is employed.

This method is applicable to any two machine system. This method is not applicable to multi machine system directly.

Q.15. What do you mean by recovery voltage?

Answer: The power frequency r.m.s voltage that appears across the breaker contacts after the transient oscillations die out and final excitation of arc has resulted in all the poles is called the recovery voltage.

Q.16. How corona loss in an EHV line is reduced by the use of bundle conductors?

Answer: A bundle conductor is a conductor made up of two or more sub-conductors and is used as one phase conductor. The reactance of the bundle conductors is reduced because the self GMD of the conductors is increased.

By bundling the conductors the self GMD of the conductors is increased thereby; the critical disruptive voltage is increased and hence corona loss is reduced.

Q.17. What do you mean by breaking capacity of a circuit breaker?

Answer: The breaking capacity of a breaker is the product of the breaking current and the recovery voltage.

Q.18. Why is one of the buses taken as slack bus in load flow studies?

Answer: In a power system there are mainly two types of buses: load and generator buses. The power injection is positive for generator buses and is negative for load buses. The losses remain unknown until the load flow solution is complete.

It is for this reason that generally one of the generator buses is made to take the additional real power to supply transmission losses. That is why this type of bus is also known as the slack or swing bus.

Q.19. Which type of circuit breakers is generally used in railway electrification?

Answer: Air blast circuit breaker is generally used in railway electrification.

Q.20. What do you mean by current chopping in air blast circuit breakers?

Answer: It is the phenomenon of current interruption before the natural current zero is reached. This results in the production of high voltage transient across the contacts of the circuit breakers.

Q.21. What is the difference between type tests and routine tests of circuit breakers?

Answer: Routine tests are performed on every piece of circuit breaker in the premises of the manufacturer. The purpose of the routine tests is to confirm the proper functioning of a circuit breaker.

Type tests are performed in a high voltage laboratory, such tests are performed on sample piece of circuit breaker of each type confirm their characteristics and rated capacities according to their design. These tests are not performed on every piece of circuit breaker.

Q.22. What do you mean by proximity effect?

Answer: The alternating magnetic flux in a conductor caused by the current flowing in a neighbouring conductor gives rise to circulating currents which cause an apparent increase in the resistance of a conductor. This phenomenon is called proximity effect.

Q.23. Why it is desirable to have a high power factor of the system in case of HVDC transmission?

Answer: In case of HVDC transmission it is desirable to have a high power factor of the system for the following reasons –

  • For a given current and voltage of the thyristor and transformers, the power rating of the converters is high.
  • The stresses on the thyristor and damping circuits are reduced.
  • For the same power to be transmitted the current rating of the system is reduced and also the copper losses in the ac lines are reduced.
  • In the ac lines the voltage drop is reduced.

Q.24. State Kelvin’s Law.

Answer: It states that, the most economical conductor size is one for which annual cost of energy loss is equal to annual interest and depreciation on the capital investment of the conductor material. This is known as Kelvin’s Law.

Q.25. What is radio interference?

Answer: The corona discharge produces the radiations which may introduce noise signals in the communication lines, carrier signal, radio and television receivers, navigation signals etc. such noise signals which adversely affects the wireless signals, produced by corona is called radio interference.

Q.26. What are the properties of insulators?

Answer: The insulators must have the following properties –

  • The main function of the insulators is to resist any leakage current.Thus the insulators must have very high insulation resistance.
  • The insulators must be free from internal impurities such as holes, cracks, laminations etc. This reduces the permittivity of the insulators.
  • The dielectric strength of the insulators must be very high.
  • To have high dielectric strength, the relative permittivity of the insulating material should be very high.
  • The insulating material should be non-porous.
  • The insulators should not be affected by the changes in the temperature.

Q.27. How dielectric power factor vary with temperature?

Answer: The variation roughly follows a V shape; it decreases with increase in temperature to a minimum value and rises again with increase of temperature. The minimum point lies somewhere between 30 degree and 60 degree Celsius depending upon the type of impregnating compound.

Q.28. What is the use of stringing chart?

Answer: Stringing chart is useful for finding the sag in the conductor.

Q.29. What is “expanded ACSR”?

Answer: “Expanded ACSR” are conductor composed of filler between the inner steel and the outer aluminium strands to increase the overall diameter of the conductor.

Q.30. How sheath losses occur?

Answer: When single core cables are useful for a.c transmissions, the current flowing through the core of the cable gives rise to a pulsating magnetic field which when links the sheath, induces voltage in it. This induced voltage sets up currents under certain conditions in the sheaths and these results in sheath losses.

Q.31. What are the advantages of neutral grounding?

Answer: The advantages of neutral grounding are –

  • Voltages of the phases are limited to phase to ground voltages.
  • The high voltages due to arcing grounds or transient line to ground faults are eliminated.
  • Sensitive protective relays against line to ground faults can be used.
  • The over voltages due to lightning are discharged to ground, otherwise there will be positive reflection at the isolated neutral of the system.

Q.32. What are the methods of neutral grounding?

Answer: There are various methods of grounding the neutral of the system. They are –

  • Solid grounding.
  • Resistance grounding.
  • Reactance grounding.
  • Voltage transformer grounding.
  • Zig-zag transformer grounding.

Q.33. When the Ferranti effect on long overhead lines is experienced?

Answer: In lightly loading condition because in lightly loading condition line capacitance is dominating to load reactive power requirement.

Q.34. Why mostly air blast circuit breakers are susceptible to current chopping?

Answer: Current chopping mainly occurs in air-blast circuit breakers because they retain the same extinguishing power irrespective of the magnitude of the current to be interrupted. When breaking low currents with such breakers, the powerful de-ionizing effect of air-blast causes the current fall abruptly to zero well before the natural current zero is reached.

Q.35. Define power system voltage stability.

Answer: A power system at a given operating state and subject to a given disturbance is voltage stable if voltage near loads approach posts disturbance equilibrium values. The disturbed state is within the region of attraction of the stable post disturbance equilibrium.

Q.36. What are the main insulating materials used in cables?

Answer: The main insulating materials which are in use are –

  • Poly vinyl chloride (PVC).
  • Paper.
  • Cross linked polythene.
  • Vulcanized India rubber (VIR).

Q.37. What is the difference between steady state and transient state stability of power system?

Answer: The steady state stability limit refers to maximum power transfer that is possible with small changes in power flow or gradual disturbance without losing stability.

The transient stability refers to the maximum power transfer that is possible for given amount of sudden or large changes in power disturbance without loss of stability.

Q.38. What is skin effect?

Answer: When direct current flows in the conductor, the current is uniformly distributed across the section of the conductor where as flow of alternating current is non-uniform, with the outer filaments of the conductor carrying more current than the filament closer to the centre. This results in higher resistances to alternating current than to direct current and is commonly known as skin effect.

Q.39. What are the necessary requirements of good distribution system?

Answer: The necessary requirements of good distribution system are –

  • The continuity in the power supply must be ensured .thus system should be reliable.
  • The specific consumer voltage must not vary more than the prescribed limits.
  • The efficiency of the lines must be as high as possible.
  • The system should be safe from consumer point of view.
  • There should not be leakage.
  • The lines should not be overloaded.
  • The system should be economical.

Q.40. What do you understand by infinite line and infinite bus?

Answer: Infinite line – A transmission line of finite length (lossless or lossy) that is terminated at one end with an impedance equal to the characteristic impedance appears to the source like a infinitely long transmission line and produces no reflection. Infinite bus – A system bus of constant voltage and constant frequency regardless of the load is called infinite bus bar system or simply infinite bus.

Q.41. Which relay has the capability of anticipating the possible major fault in a transformer?

Answer: Buchholz relay is the gas detector relay used to protect transformers which operates when the oil level in the conservator pipe of a transformer is lowered by the accumulation of gas caused by a poor connection or by an incipient breakdown of insulation.

Q.42. Why shunt capacitors are preferred over series capacitors for improvement of power factor in distribution system?

Answer: Series capacitor compensation reduces the series impedance of the line which causes voltage drop and is the most important factor in finding the maximum power transmission capability of a line.

For the same voltage boost, the reactive power capacity of a shunt capacitor is greater than that of a series capacitor. The shunt capacitor improves the power factor of the load while the series capacitor has hardly any impact on the power factor.

Q.43. What is RRRV?

Answer: RRRV stands for rate of rise of restriking voltage. It is the slope of steepest tangent to the restriking voltage curve. It is expressed in volts per microsecond. RRRV = peak value of restriking voltage / time taken to reach to peak value.

Q.44. What is the need of transposition of power transmission lines?

Answer: When the power and communication circuits run parallel to each other, interference can be reduced by transposing the conductors of the power line and the communication line. The transposition of power line neutralizes the unbalances in the capacitances of the lines that the electrically induced voltages are balanced out in a complete set of transposition.

Q.45. What are pumped storage plant?

Answer: Pumped storage plants are an special type of power plant which work as ordinary hydro power plants for part of the time and when such plants are not producing power, they can be used as pumping stations which pump water from tail race to the head race. During this time, these plants utilize power available from the grid to run the pumping set.

Q.46. What are the advantages in using bundle conductors?

Answer: The following are the advantages in using bundle conductors –

  • Reduced reactance.
  • Reduced voltage gradient.
  • Reduced corona loss.
  • Reduced radio interference.
  • Reduced surge impedance.

Q.47. What is the major aspect of voltage stability?

Answer: The ability to transfer reactive power from sources to sinks during steady operating conditions is a major aspect of voltage stability.

Q.48. Which is the main relay for protecting up to 90% of the transmission line-length in the forward direction?

Answer: Mho relay is inherently a directional relay as it detects the fault only in the forward direction.

Q.49. What is the difference between symmetrical and asymmetrical breaking current?

Answer: Symmetrical breaking current – It is the rms value of a a.c component of the current in the pole at the instant of contact separation.

Asymmetrical breaking current – It is the rms value of the total current comprising the a.c and d.c components of the current in the pole at the instant of contact separation.

Q.50. What are the factors affecting corona and corona loss?

Answer: The various factors affecting corona and corona loss are –

  • Electrical factors.
  • Line voltage.
  • Atmospheric conditions.
  • Size of the conductor.
  • Surface conditions.
  • Number of conductors per phase.
  • Spacing between conductors.
  • Shape of conductors.
  • Clearance from ground.
  • Effect of load current.

Due to all these factors for the long transmission lines the corona loss per km of line at various points is obtained and net corona loss is obtained by taking average of all the values.