Power System Performance - short answer questions from AMIE exams (Winter 2020)

Answer the following in brief (2 marks each)

Why Arcing ground should be avoided?

Arcing ground is the surge, which is produced if the neutral is not connected to the earth. In a three phase line, each phase has a capacitance on earth. When the fault occurs on any of the phases, then the capacitive fault current flows into the ground. If the fault current exceeds 4 – 5 amperes, then it is sufficient to maintain the arc in the ionised path of the fault, even though the fault has cleared itself.

The arc extinguishes and restrikes in a repeated regular manner. The phenomenon of intermitting arcing is called the arcing grounding. Overvoltage thus produced damages the healthy conductor at some other points of the system.

What is the function of d.c. reactor?

The reactor is suitable for reactive power compensation and harmonic control system.It can improve the power factor, filter the harmonics, suppress the voltage waveform distortion, change the quality of power network and ensure the safe operation of power system.

Why is plug setting required?

Plug setting multiplier of relay is referred as ratio of fault current in the relay to its pick up current.

Explain the concept of speed governor.

A governor protects the prime mover from over speed and keeps the prime mover speed at or near the desired revolutions per minute. When a prime mover drives an alternator supplying electrical power at a given frequency, a governor must be used to hold the prime mover at a speed that will yield this frequency.

How is impedance diagram drawn?

The diagram shown below is the balanced 3-phase diagram. It is also called positive sequence diagram. Three separate diagrams are also used for representing the positive, negative and zero sequence networks. The three separate impedance diagrams are used in the short circuit for the studies of unsymmetrical fault. The impedance diagram can further be simplified by making certain assumptions and reduced to simplified reactance. Reactance diagram is drawn by neglecting the effective resistance of generator armature, transformer winding resistance, transmission line resistance line charging and the magnetizing circuit of transformers. 

Why acceleration factor is used in a load flow analysis?

An acceleration factor is a value that can be used to speed up the convergence and reduce the number of required alteration in a Gauss Seidel method of power flow analysis . Very high or very low values may cause the system solution to diverge and slows down convergence.

Why should transmission loss be taken into account in economic dispatch?

The main aim in the economy of operation problem is to minimize the total cost of generating real power at various stations while satisfying the loads and the losses in the transmission links. 

Economic dispatch is the on line economic dispatch where in it is required to distribute the load among the generating units actually paralled with the system in such manner as to minimize the total cost of supplying the minute – to – minute requirements of the system.

Why per unit method is widely used in power system calculation?

  • Circuits are simplified.
  • When expressed in the per-unit, machines parameters usually fall in narrow range regardless of machines size. For example, generator reactances in per unit are similar for both 100 MVA machines and 1000 MVA machines.
  • For circuits connected by the transformers, per- unit system is suitable. By choosing a suitable base kV for the circuits, the per-unit reactance remains the same referred to either side of the transformer.
  • √3 factor in the three phase circuit calculations is eliminated.
  • Per-unit (pu) system is ideal for computer simulations.

Why numerical solution of swing equation is necessary in many cases?

A model can be made for digital computer to solve Swing equation which may give solution in realtime.

What is the speed of Turbo-alternator?

1500 to 3000 rpm.


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