Short Term Course On“Hydropower Development Engineering (Electrical)” for

Teachers of Polytechnics in Uttarakhand

( July 14-18, 2007)

Lecture on

L42-1

By

S.N.SinghSenior Scientific officer

ALTERNATE HYDRO ENERGY CENTRE INDIAN INSTITUTE OF TECHNOLOGY, ROORKEE

ROORKEE-247 667

INDUCTION GENERATOR

Induction generator is also called as Asynchronous generator or induction motor used as induction generator (IMAG).

An induction motor is called an induction generator when it is sued with negative slip i.e. speed of the rotating magnetic field is less than the rotor speed.

CONSTRUCTION

An induction generator consists essentially of two main parts asshown in Fig. 1.

Stator:The stator of induction generator is same as that of synchronous generator. The stator carries a 3- phase winding. Windings are wound for a definite numbers of magnetic poles.

Rotor:The rotor of induction generator is different from that a Synchronous generator. The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors (heavy bars of Copper or Aluminum). The rotor conductors are electrically welded or bolted and short circuited at each end by Copper or Aluminum rings called end rings.

Fig. 1: Main part of induction generator

WORKING PRINCIPLE

Supply (Grid) connected induction motor operations:

When three phase stator windings are fed 3-Phase supply then a magnetic flux (stator field) of constant magnitude but rotating at synchronous speed is sets up. The synchronous speed of the field is given by

ns = 120 f/pWhere,

f = Electric Supply frequency p = no. of stator poles

The rotating magnetic field created by the stator, cuts the rotor bars (rotor at standstill) and induces voltage into rotor bars. When rotor is at standstill, the frequency of this induce voltage is same as the rotating magnetic field frequency. Due to short-circuited rotor bars (by means of the end rings) the induced current, will flow through the rotor. This current produces a rotating flux wave similar to one of the stator. Fig:2 below presents a simple induction machine with only one phase shown. The stator and rotor fields interfere with each other and the resulting field creates a force couple or torque on the rotor bars. The torque on the rotor is directed insuch a way that the rotor will turn in the direction of the rotating stator field. Figuratively, we can say that the rotor follows the main field in order to catch up to it.

Sφ

Rφ

Sφ

However, there must always be a relative motion between the rotating stator field and the rotor bars; if the rotor bars rotate at the same speed as the rotating field, i.e. at synchronous speed n-s, the rotor conductors would no longer be cut by the flux lines. As a consequence, there would be no induced current and no corresponding rotor field and hence no torque. That is why the induction machine is also called asynchronous machine as the rotor must rotate below (or above) synchronous speed to produce electro-magnetic torque. Even if the motor is run with the load uncoupled, a small torque is required to overcome friction and other losses of the machine and the rotor will rotate slightly below synchronous speed.

Sφ

Rφ

Fig. 2 Development of torque in an induction motor (schematically for a single rotor coil)

SLIP SPEED

The difference between the synchronous speed ns and the rotor speed nr is called slip speed ng and represents the speed of the rotating field viewed from the rotor. Relating ng to the synchronous speed, we obtain the so-called slip s:

ng = ns – nr (1)

(2)

Slip s may be negative, i.e. the rotor speed nr is above synchronous speed; the machine operates as a generator driven bya turbine or a diesel engine and generates electric current in the stator windings.

s

rs

nnns −

=

Supply (grid) connected induction generator operation:

If the same supply connected induction motor is now driven (with the help of hydro turbine or pump as turbine i.e. PAT) at above synchronous speed, so that slip becomes negative i.e.

ns- nr = Negative (i.e. nr > ns), a torque is supplied to the rotor rather than by the rotor and the induction motor acts as an induction generator, supplying power to the network (grid).

However, it still takes its magnetizing current (I sin φ) from the grid in order to create the rotating field.

The Induction generator operates only at a super -synchronous speed, a speed above synchronous speed corresponding to supply frequency and number of poles of the machine. The generator voltage & frequency are controlled by electrical system with which it is connected. Since the voltage and frequency are independent of generator, automatic voltage regulating equipments and speed governor are not usually required. The induction generator draws its excitation current (reactive power) from the electrical system with which it is connected. It is essential that electrical system must be strong supply lagging kVA. Therefore this type of generator cannot operate in isolation. The working of induction motor as well as induction generator is shown in Fig. 3& 4 respectively.

Fig. 3: Working of Induction Motor

Fig. 4: Working of Induction Generator

INDUCTION MACHINE CHARACTERISTICS

The typical torque - speed characteristics of induction machine is shown in fig. 5. The machine will work as motor when rotor speed is below synchronous speed and generator when speed is above synchronous speed.

Fig. 5: Typical torque-speed characteristics of a squirrel-cage induction machine

SELF EXCITED INDUCTION GENERATOR OR STAND ALONG GENERATOR

In stand –alone operation, a battery of capacitors must be connected in parallel to the stator winding which will supply necessary reactive power to the induction generator. It will then work as a self- excited generator. Figure 6 (a) and (b) shows a schematic circuit diagram of a self –excited asynchronous generator.

Fig. 6 : (a)Self excited 3 phase induction generator with star connected capacitors.

Fig. 6: (b) Self excited 3 phase induction generator with delta connected capacitors.

If the capacitors are connected in star than three times as much capacitance is required than for delta connection. An induction motor of a given rating cannot deliver the same electrical power in generator mode as it would absorb from a grid in motor operation since the losses (copper, iron, friction and windage) reduce the output. Theoretically, the mechanical input power could be increased to make up for the losses and arrive at overload the machine, i.e. overheating and finally burn the stator winding. Therefore, generator operating conditions are determined by the stator current which must not exceed the rated motor current which the windings are designed for.

POWER

There are three types of electrical power as listed below:

Real power or Active power: This power is used for doing actual work and this power is given as P =

Reactive Power : This is a wattless power and use for magnetizing purposes. This power is given as Q =

Apparent power: The apparent power is the ratio of real power tothe power factor i.e. apparent power = Real power/power factor.

Where, VL, IL and cos are line voltage, line current and power factor respectively.

φcos 3 LL IV

φsin 3 LL IV

φ

POWER FACTOR

Cosine of the angle between phase current and phase voltage is known as power factor.

Power factor is the ration of real power (kW) to apparent power (kVA) component consider fig. 7.

kWA

kVARkVA

O900

Fig. 7: Power triangle

OA = kW component or real powerAB = kVAR component or apparent power

Power factor =

kVAR = kVA

Leading power factor: The power factor will be leading if the phase current is leading the phase voltage.

Lagging power factor: The power factor will be lagging if the phase current is lagging the phase voltage.

kVAkWCos =φ

φSin

EFFECT OF LOW POWER FACTORConsider an alternator (single phase) having full load rated capacity of 1000A at 500V.

∴Rating of the alternator =

If the alternator is operating :

At unity power factorkW = kVA x Cosφ

=500 x 1 = 500

At 0.8 power factor kW = kVA x Cosφ = 500 x 0.8 = 400

kVAVI 5001000

50010001000

=×

==

Thus alternator is supplying only 80% of its full load capacity (at unity power factor) although the alternator is fully loaded i.e. developing maximum current 1000 A at 500 V. So in order to supply 500 kW at 0.8 power factor alternator must be overloaded. In this case current carrying capacity of the alternator winding will be

i.e. the alternator would be overloaded and the conductors connecting the alternator to load would be made of much larger cross section to carry the overload current. Thus the size of a given type of alternator is decided by on the basis of kVA output and not by kW basis.

ACosVPI 1250

8.05001000500

=××

==φ

For 500 kW output the kVA rating of alternator will be:

At 0.6 p.f. kVA = 500/0.6 = 833.33 large sizeAt 0.8 p.f. kVA = 500/0.8 = 635.00 small size

Since kVA = C0D2LnWhere,

Co = out put coefficientL = lenth of rotor of alternator D =diameter of rotor of alternator

Hence, for a given power if power factor is lower then cost of generation and transmission will be more. Due to this reason supply under taking is always stress upon the consumers to increase their power factor.

METHODS OF IMPROVING (CORRECTION) POWER FACTOR Following are the methods of improving the power factor.

With the use of capacitors: They are connected in parallel with the supply main and take current leaching by 900 from the mains (voltage) which neutralizes the reactive logging component of the load current.

With the help of a synchronous condenser:- The synchronous condenser is also called as synchronous motor. This is the only motor which can worked at leaching power factor at the same time. This can supply mechanical power. It is preferable to use synchronous motor in comparison with Induction motors where ever possible.

Phase Advancers: These are special commutator machines which improve the power factor of induction motor.

ECONOMICS OF POWER FACTOR CORRECTION OR IMPROVEMENT

The low P.F. means larger size of generating plant and transmission & distribution equipment. Hence electric supply under takings penalize industrial consumer for his low power factor by charging increased tariff for kVA maximum demand basis in addition to useful kWh charge. For the same kW demand if the P.F. is improved, kVA demand will reduce and the consumer will pay less for his kVA demand charge. On the other hand, he will have to pay some amount for P.F. improvement apparent (static capacitors or synchronous capacitors). As such he would choose to work his installation at such a power factor that the total charge has to pay is a minimum.

The most economical power factor for a consumer can be found as kW demand constant basis or constant kW assumption:

Suppose a consumer is taking power of P kW at a power factor of , as shown in Fig.8. 1cosφ

O

kVA90

kVAR

kWA

021

2

kVA1B

C

Fig 8: The most economical power factor for a consumer on constant kW assumption basis.

Then kVA demand is OC = kVA1 = P/Cos and

kVAR is AC = P tan

Let by installing static capacitors or synchronous capacitors the improves this p.f. to Cos (his power consumption P remains same)

New kVA demand = OB = kVA2 = P/ Cos

Reduction in kVA demand= kVA1 – kVA2= OC – OB

1φ

1φ

2φ

)1(21

−−=φφ CosP

CosP

2φ

If Rs. x is the demand charge per kVA /annum then saving in demand charge

= Rs. x×

= Rs. Px

Against this saving, the kVAR to be provided by power factor improvement apparatus (synchronous capacitors) BA = CA – CB

= =

If Rs. y is the annual working cost (i.e. interest and depreciation etc on capital investment) per KVAR of improvement plant, then its cost per annum

−

21 φφ CosP

CosP

)2(11

21

−

−

φφ CosCos

21 tantan φφ PP −

( )21 tantan φφ PP −

= Rs y - (3)

Net annual saving from equations (2) & (3)

S = Px

This net saving is maximum when

( )21 tantan φφ PP −

( )2121

tan(tan11 φφφφ

PyCosCos

−

−

02

=φdds

( )

−

−=∴ 21

2122

tantan11 φφφφφφ

PyCosCos

xPdd

dds

0tan 22

22 =+− φφφ SecPySecPx

22tan φφ Secyxor =

xySinor =2φ

Thus the sine of most economical angle is equal to ratio of annual cost of phase advancing plant to annual tariff per kVA of max demand.

The most economical

( )22

2

2

22

2

1

1

1..

yxx

xy

SinCosfp

−=

−=

−= φφ

REFERENCES

Manual on Induction motors used as generators by J.M. Chapallaz, J.Dos Ghali, P.Eichenberger, G. Ficher

Electrical power by Dr. S.L. Uppal

Handbood of electrical Engineering by SL Bhatia

Basic Electrical Engineering by IJ Nagrath