Induction Generators

Induction Generators

To run the induction machine as a generator, its slip must be less than zero i.e. negative. The negative slip indicates that the rotor is running at a speed above the synchronous speed. When running as a generator it takes mechanical energy and supplies electrical energy from the stator. As the speed of induction generator is not in synchronism with the line frequency, it is often called asynchronous generator.

Thus when the slip of the induction motor is negative i.e. when the induction motor runs faster than synchronous speed, the induction motor runs as a generator called induction generator.

In the Fig. 5.23.1 the induction motor is shown which is driven by a mover like petrol engine. The motor is supplied with electrical power from 3 phase lines. When the motor speed exceeds the synchronous speed, the active power is delivered by the motor and the corresponding mode of operation of motor is called generating mode.

 The induction generator is not self exciting in the sense that supply must be maintained to act is as a generator. Thus it must be operated with other generators which supplies it exciting current of fixed frequency which is required for the production of rotating magnetic field. Thus it takes reactive power from the line to create the magnetic field.

Let us consider that a 3 phase induction motor is provided with exciting current of say any frequency f. Due to this a rotating magnetic field of speed 120f / P is produced.

Now if the slip is zero, there will not be any e.m.f. or current in the rotor winding. The stator will take only the magnetizing current from the line.

If now the speed of the motor is increased above the synchronous speed slightly, the e.m.f. and current of slip frequency will appear in the rotor winding but with opposite direction as compared to the direction when it is operating below the synchronous speed. The slip frequency current in the rotor produces rotating m.m.f. moving at slip speed relative to the rotor winding but the direction is reversed compared to when it is acting as motor. The rotor m.m.f. moves in the air gap at the same speed as that of rotating magnetic field. Thus relative to the stator the rotor current has line frequency. 

The magnetizing effect of rotor current is balanced by the component of primary current. Thus there is supply of current from stator winding to line. Thus independent of value of negative slip, the primary current will have the frequency that corresponds to speed of rotating magnetic field. The frequency of the current is same as that of the generator connected to the line.

The torque-slip characteristics for motoring and generating action is shown in the Fig. 5.23.2.

The construction of induction generator is same as that of motor with the difference that the direction of rotation of the motor and a generator is opposite for the same current direction.

 

1. Phasor Diagram of Induction Generator

The action of induction machine as a generator can be explained from the phasor diagram.

Consider the phasor diagram of the induction motor on load.

Let us first consider the speed of the induction machine is less than synchronous speed so that machine takes current I₁ from supply. This current I₁is phasor sum of no load current I_o and I_2r which is opposite of I_2r and referred as reflected rotor current in stator. The rotor current can be resolved into two components, one in phase with rotor emf and the other one is quadrature component.

The rotor current I_2r is given by,

Let the real part of above current be denoted by A while the imaginary part of the current be denoted by B. Thus the total rotor current I_2r be assumed as A - jB.

Now let the speed of the induction machine is increased. With increase in speed of prime mover i.e. of rotor of induction motor slip goes on reducing and hence the rotor current also as it depends on it. Thus I_2r decreases.

At synchronous speed, it completely vanishes. Hence its opposite cu rrent I_2r also vanished and the resultant stator current is nothing but the no load current I₀. The core losses are supplied from line whereas friction and windage losses are supplied mechanically.

When the speed is increased further the machines enters in generating region. At zero power factor no power is interchanged between machine and supply lines. But the machine generates power to meet its core losses. When the speed is increased, the current I_2r increases in magnitude but it changes the phase. The current supplied by the generator will be then vector sum of I_o and I_2r which is reversed in phase as indicated in the phasor diagram. 

The rotor current is now given by

It can be seen that the in phase component reverses while the quadrature component remains in the same direction.

The phasor diagram of induction machine as generator is shown in Fig. 5.23.4.

The current I2r leads the voltage - E_2r which is opposite of E_2r. The angle between V₁ and I₁ is more than 90° which shows that electric power of the machine is negative i.e. it is supplying the power. Thus when the rotor is rotated above synchronous speed with the rotating field remaining in the same direction, then the direction of cutting of rotor is in opposite direction which results in reversal of rotor emf, current and torque. The machine is said to be operating in generating mode.

 

2. Externally Excited Induction Generator

The induction generator is either self excited type or externally excited requiring external source for its excitation. In externally excited type, it is always connected to an a.c. supply. Generally, it is operated in parallel with synchronous machines which is shown in Fig. 5.23.5.

Consider an example of a load which requires a lagging current which cannot be supplied by induction generator alone as it supplies leading current. But this current requirement is fulfilled with the help of synchronous generators operating in parallel with induction generator. Consider the following phasor diagram. 

The load current I_L can be resolved into two components one in phase component I_m and the other quadrature component Ie. The speed of the induction generator is adjusted in such a way that it supplies current Ic which is leading one. The induction generator current Is is nothing but vector sum of Ic and I_m.

The synchronous generator which is in parallel with the induction generator must supply the remaining part of load current. For this the induction generator current Is is subtracted vectorially from IL (subtracting vectorially means reversing Is and adding it with IL). This current (generator current) is nothing but algebraic sum of currents Ic and Ie. The synchronous generator supplies no power. The total current supplied by synchronous generator is lagging quadrature current.

If the load requires a leading current then theoretically the quadrature component of current can be supplied entirely by the induction generator. But for satisfactory operation it should be run in parallel with synchronous generator.

 

3. Self Excited Induction Generator

If the bank of delta connected capacitors is operated in parallel with induction generator then the reactive power requirement of induction generator is met by capacitors. This arrangement is shown in Fig. 5.23.7.

The induction generator in this case is said to be isolated induction generator supplying a load. The external voltage source is not required in this case. 

Unlike in synchronous generators, induction generators are not rotating at a definite speed at a given frequency. The speed varies with load as the load is proportional to slip. The frequency of the induction generator is same as the frequency of the line to which it is connected.

In case of self excited induction generators, the bank of delta connected capacitors supply the necessary magnetizing current for exciting the generator. With the load put on the generator, the operating frequency of the stator changes. It depends on rotor speed and is affected by the load. The voltage is primarily decided by the capacitor's capacitive reactance at that operating frequency. The equivalent circuit on per phase basis is as shown in the Fig. 5.23.8

Initially the induction generator is running at synchronous speed. is the magnetizing current in motoring mode. If voltage drop in R₁ and X₁  is neglected then V₁ ~ E'₂. But IJH is the magnetising current supplied by capacitors, so it flows through the capacitors.

.V₁ = I_m • X_c

The simplified equivalent circuit is shown in the Fig. 5.23.9.

The magnetization characteristics is as shown in the Fig. 5.23.10

The intersection of magnetizing characteristics with reactance line gives the no load voltage. The frequency is rated in this case which changes slightly from rated with load which in turn changes V₁.

 

4. Circle Diagram of Induction Generator

Using circle diagram, the induction generators can also be analysed. 

As the inphase component of current reverses, direction of current Ii also changes. It will be below horizontal shown by OE. E is the operating point. As seen from the circle diagram.

AB = Rotor Cu loss

BC = Stator Cu loss

CD = Constant losses

DE = Generator output BE = Rotor input

The slip s is given by,

s = Rotor Cu loss / Rotor input = AB / BE

Similarly other required quantities can be obtained from the circle diagram.

 

5. Comparison of Induction Generator and Synchronous Generator

The distinct features of induction generator compared to synchronous generators are as follows : 

i) It will not require d.c. excitation.

ii) It is not self excited but external a.c. supply of fixed frequency is required.

iii) The frequency of induction generator is decided by the frequency of the excitation voltage which is supplying current to it.

iv) Synchronization of generator is not required as no emf is generated until it is connected to the line.

 

6. Advantages

The following are the advantages of induction generator.

i) Synchronization for induction generator is not required.

ii) The construction is rugged for rotating parts.

iii) Unlike in synchronous machines, there is no danger of hunting or drop out of synchronism for induction generators.

iv) When it is short circuited, it delivers small power as its excitation quickly reduces to zero.

v) Induction generators are more suitable for high speeds.

vi) With the help of excitation supply and frequency, the voltage and frequency of induction generator are controlled.

 

7. Disadvantages

Although induction generators are having above mentioned advantages, it has following disadvantages.

i) It must be run in parallel with the synchronous machine.

ii) The load is not deciding the power factor of induction generator but the power factor depends on slip.

 

8. Applications

Because of distinct superiority of the synchronous generator, induction generators are rarely used to supply commercial power.

One application of induction generator is in railway for braking purposes. When the train is moving down a gradient, the induction generator rims above synchronism. As the torque in this region is negative, the braking action is achieved in the train. In addition to this the energy generated by induction generator is given to the line so that the load on main generating station is somewhat relieved. In this case no complicated control apparatus is required. 

a. Importance of Induction Generators in Wind Mill

The induction generator is extremely important in wind power electricity generation system. It is suitable because the stator frequency depends on that of the paralleled synchronous machines and not on the rotor speed.

Induction generator is most commonly used in wind turbines because of low cost, ruggedness, operates with slip (synchronism not required), availability in many sizes and advance technology available.

Induction generators have outstanding operation as either motor or generator. They have robust construction features. It provides natural protection against short circuits. The abrupt changes in speed are easily absorbed by its solid rotor. Also any surge in the current is damped by the magnetization path of the core, avoiding the possibility of demagnetization which is possible in case of permanent magnet generators.

 

Example for Practice

Example 5.23.1 A 220 V, 3-phase, 50 Hz, 4-pole induction machine is running as generator. Stator resistance per phase - 0.545 ohm. At a particular value of slip, the observed data are as under :

V - 220 V, stator current 12 AJphase. Output - 3910 watts. Slip speed - 72 rpm. Constant losses from no load run - 213 watts of which 65 watts represent friction and windage. Find the efficiency.

[Ans.: ƞ = 85.65 %]

Review Questions

1. Write a brief note on induction generator.

2. Explain the operation of induction machine as a generator with neat diagram.

3. Draw and explain circle diagram of an induction generator.

4. State the advantages and disadvantages of an induction generator.

5. State the applications cf an induction generator.