Infinite Bus

Definition: The bus whose voltage and frequency remains constant even after the variation in the load is known as the infinite bus. The alternators operating in parallel in a power system is the example of the infinite bus. The on and off of any of the alternator will not affect the working of the power system.

The capacity of a parallel operating system is enormous. Their voltage and frequency remain constant even after the disturbance of the load. The connection and disconnection of any of the machine will not affect the magnitude and phase of voltage and frequency of an infinite bus. In an infinite bus system

  • The voltage and frequency always remain constant.
  • The synchronous impedance of the bus is low because of parallel operations of the machine.

Synchronous machine on Infinite Bus

The performance of the synchronous machine varies on the infinite bus. When the synchronous machine operates independently,  variation in their excitation causes the changes in their terminal voltage. The power factor of the synchronous machine depends only on their load. But when the synchronous machines are operating in parallel, the change in their excitation changes the power factor of the load.

Obtaining an Infinite Bus

Consider generators G1, G2, G3……Gn connected to an infinite bus as shown in the figure below.

INFINITE-BUS-FIGProof of Voltage Remaining Constant

Let,

  • V be the terminal voltage of the bus
  • E be the induced emf of each generator
  • ZS is the synchronous impedance of each generator
  • n is the number of generators in parallel

infinite-bus-eq-1

When n is very large ZSeq → 0 and, therefore, I ZSeq → 0

Therefore, V = E (constant)

If the number of alternators operating in parallel is infinite only then ZS = 0

Proof of Frequency Remaining Constant

Let,

  • J be the moment of inertia of each alternator

Total moment of inertia of all n alternators is given as

infinite-bus-eq-2If the value of n is very large, nJ is also very large.

Therefore, acceleration → 0 and the speed is constant.

The above equation shows that the constant voltage and frequency of the bus depend on the number of machines operating parallel.

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