Power Systems - short answer questions from AMIE exam (Winter 2020)

Discuss all the terms in brief (2 marks each):

Swing equation

The Swing Equation of the generator describes the relative motion between the rotor axis and the synchronously rotating stator filed axis with respect to time. This equation is very helpful in analysing the stability of connected machines (machine here means generator).
It is given by
Ps – Pe = Md2δ/dt2 
Pa = rotor accelerating power
Ps = shaft power 
δ = Load Angle or Torque Angle
M = angular momentum of generator rotor  

Capital and running cost

  • Capital cost. It includes the cost of the plant, land acquisition (unless a rent is paid, in which case this is a running cost), grid connection (although in some European states, the utility bears the cost), and initial financing costs (not repayment costs).
  • Running cost. It is the cost which depends only upon the number of units generated. The running cost is on account of annual cost of fuel, lubricating oil, maintenance, repairs and salaries of operating staff.

Surge impedance loading

The surge impedance loading (SIL) of a transmission line is the power (MW) loading of a transmission line when the line is lossless.

Intersheath grading

Providing metallic intersheath between successive layers of the same dielectric materials and maintaining appropriate potential level at the intersheath is called “intersheath grading”.

Advantages of underground transmission

  • The underground cable is not affected by lightning, thunderstorms and other severe weather conditions. So, there are no interruptions in service to consumers.
  • Accidents caused by the breaking of the conductors will be reduced.
  • The maintenance cost of underground system is very low because of less chance of fault occurrence.
  • Because of less spacing between the conductors in an underground system, inductance is very low as compared to overhead lines; therefore, the voltage drop is less.
  • In an underground system, surge effect is smoothened down as the sheath absorbs surge energy.

Diversity factor

Diversity factor is defined as the ratio of the sum of individual maximum demand to the simultaneous maximum demand of the whole group of consumers. 

Diversity Factor = Sum of individual maximum demands/Simultaneous maximum demand of the consumers

Propagation constant

The propagation constant is defined as the natural logarithm of the ratio of the input to the output current, and is usually denoted by 𝛾.
γ = loge(IS/IR)

Function of Surge tank in Hydro Power Station

The surge tank relieves water hammer effect in the conduit or penstock and acts as a storage reservoir which stores water during low load and quickly supplies extra water during high load on the turbine. Surge tanks are required for high and medium head plants, where water is taken to the powerhouse through tunnels and long penstocks.

Function of FD (forced draught) and ID (induced draught)

  • Forced Draught Fans (FD Fan) The combustion air is supplied through the FD fan and air ducts.  Normally. Radial/centrifugal fans are deployed for FD fan service, which would offer more discharge pressure and flow, as is expected. The FD fans supply the total air or most of the combustion air required, which is divided into many categories such as fuel air, auxiliary air, over fire air. PA (if suction taken from FD), etc. 
  • Induced Draught Fans (ID) After burning the fuel in the furnace, the product of combustion with the residual heat is guided to pass through the economizer to preheat the feed water and then through the air heater to preheat the air to make it suitable for combustion. Furnaces usually have a negative pressure for various reasons, and automatic control devices are part of this system.

Load forecasting

The estimation of the active load at various load buses in advance is commonly known as load forecasting. Load forecasting plays an important role in power system planning, operation, and control.
Demand Forecasting
This is used to determine the capacity of the generation, transmission, and distribution system additions. Future demand can be predicted on the basis of a fast rate of growth of demand from past history and government policy. This will give the expected rate of growth of load.

Energy Forecasting
This is used to determine the type of facilities required, i.e., future fuel requirements. 


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