Parallel Operations of Alternators - GATE EE Electrical Engineering SSC

In synchronous generators or alternator, generally salient-pole type of rotors is driven by hydro-turbines and cylindrical type rotors are driven by steam turbines 

Because cylindrical rotors are used in high speed alternators having speed between 1500 rpm to 3000 rpm and salient pole rotors are used in low speed devices, from 100 rpm to 1500 rpm.

Cylindrical type rotors are used in turbo alternators and these are made up of solid steel forging to get high mechanical stress.

Synchronization of alternator means connecting an alternator into the grid in parallel with many other alternators, which is in a live system of constant voltage and constant frequency.

Before connecting an incoming alternator with an already operating alternator, the following conditions must be satisfied:

1. Equal voltage: The terminal voltage of incoming alternator must be equal to the bus-bar voltage.
2. Same frequency: The frequency of generated voltage must be equal to the frequency of the bus-bar voltage.
3. Phase sequence: The phase sequence of the three phases of alternator must be similar to that of the grid or busbars.
4. Phase angle: The phase angle between the generated voltage and the voltage of the grid must be zero.

Necessary condition for parallel operation of two alternators:

• The phase sequence of the busbar voltages and the incoming machine voltage must be the same
• The busbar voltages and the incoming machine terminal voltage must be in phase.
• The terminal voltage of the incoming machine and the alternator which is to be connected in parallel or with the Busbar voltage should be equal.
• The frequency of the generated voltage of the incoming machine and the frequency of the voltage of the busbar should be equal.
• The phase difference between the alternator and infinite voltages must be zero.

In case of failure of the prime mover of the generator and the excitation is present then the generator draws power from the other parallel generator and starts working as a synchronous motor with the same direction and for this motor, the turbine is working as a load.

Because of the huge mass of the turbine the motor draws a huge amount of current which damages the winding. It will absorb power from the bus bar.

Advantages of Parallel Operating Alternators:

• When there is maintenance or an inspection, one machine can be taken out from service and the other alternators can keep up with the continuity of supply.
• Load supply can be increased.
• During light loads, more than one alternator can be shut down while the other will operate in a nearly full load.
• High efficiency.
• The operating cost is reduced.
• Ensures the protection of supply and enables cost-effective generation.
• The generation cost is reduced.
• Breaking down of a generator does not cause any interruption in the supply.
• Reliability of the whole power system increases.

Concept:

The speed regulation of the alternator is given by:

where, SR = Speed regulation

N_NL = No-load speed

N_FL = Full load speed

Calculation:

Given, P₁ = 300 kW

P₂ = 200 kW

Let the load shared by two machines be x and y kW.

8x = 9y

Machine 2 produces less power than 1, so it will work at its rated condition.

8x = 9 × 200

x = 225 kW

P₁ + P₂ = 225 + 200

P_T = 425 kW

Concept:

• Synchroscopes are instruments that show the relative frequency (speed) difference and the phase angle between the machine to be synchronized and the system voltage.

• The synchroscope works based on AC motor in principle. It has two poles connected across any two phases (say red and yellow) of the incoming machine. The armature windings are supplied from the similar two phases (red and yellow) in the switch board bus - bars.
• Synchronization of alternator means connecting an alternator into the grid in parallel with many other alternators, that is in a live system of constant voltage and constant frequency.
• Before connecting an alternator into the grid, the following conditions must be satisfied:
• Equal voltage: The terminal voltage of incoming alternator must be equal to the bus-bar voltage.
• Same frequency: The frequency of generated voltage must be equal to the frequency of the bus-bar voltage.
• Phase sequence: The phase sequence of the three phases of alternator must be similar to that of the grid or busbars. Phase angle: The phase angle between the generated voltage and the voltage of the grid must be zero.
• The first condition of voltage equality can be satisfied by a voltmeter. To satisfy the conditions of equal frequency and identical phases, one of the following two methods can be used: Synchronization using incandescent lamp Synchronization using synchroscope

Effect of Change in Excitation of Alternator:

• A change in the excitation of an alternator running in parallel with others affects only its KVA output it does not affect the KW output.
• A change in the excitation, thus, affects only the power factor of its output.

Explanation:

• Let the initial operating conditions of the two parallel alternators be identical i.e. each alternator supplies equal kW and kVAr of load, thus the operating power factors being equal to the load power factor.
• Now, let the excitation of alternator No. 1 be increased so that excitation emf of alternator 1 (E₁) becomes greater than E2.
• The difference between the two EMFs sets up a circulating current (I_C)
  

• As seen, I_C is vectorially added to the load current of alternator No. 1 and subtracted from that of No. 2.
• The two machines now deliver load currents I₁ and I₂ at respective power factors of cos φ₁ and cos φ₂.
• These changes in load currents lead to changes in power factors, such that cos φ₁ is reduced, whereas cos φ₂ is increased.
• However, the effect on the kW loading of the two alternators is negligible, but kVAR₁ supplied by alternator No. 1 is increased, whereas kVAR₂ supplied by alternator No. 2 is correspondingly decreased, as shown by the kVA triangles.

Concept:

Active power is directly proportional to the load angle i.e. P ∝ δ

Reactive power is directly proportional to the voltage ∝ V

Application:

Machine A is supplying an active power to Machine B i.e.

⇒ δ_A > δ_B

Machine B is supplying reactive power to Machine A

⇒ |V_a| < |V_b|

Concept:

• Synchroscopes are instruments that show the relative frequency (speed) difference and the phase angle between the machine to be synchronized and the system voltage.

• The synchroscope works based on AC motor in principle. It has two poles connected across any two phases (say red and yellow) of the incoming machine. The armature windings are supplied from the similar two phases (red and yellow) in the switch board bus - bars.
• Synchronization of alternator means connecting an alternator into the grid in parallel with many other alternators, that is in a live system of constant voltage and constant frequency.
• Before connecting an alternator into the grid, the following conditions must be satisfied:
• Equal voltage: The terminal voltage of incoming alternator must be equal to the bus-bar voltage.
• Same frequency: The frequency of generated voltage must be equal to the frequency of the bus-bar voltage.
• Phase sequence: The phase sequence of the three phases of alternator must be similar to that of the grid or busbars. Phase angle: The phase angle between the generated voltage and the voltage of the grid must be zero.
• The first condition of voltage equality can be satisfied by a voltmeter. To satisfy the conditions of equal frequency and identical phases, one of the following two methods can be used: Synchronization using incandescent lamp Synchronization using synchroscope