VOLTAGE CONTROLLER FOR STAND-ALONE INDUCTION GENERATOR USING INSTANTANEOUS POWER CONTROL

G.V.Jayaramaiah Energy Systems Engineering,

Indian Institute of Technology Bombay, Powai, Mumbai - 400 076, INDIA.

vj ayaram@iitb.ac.in

Abstract-This paper presents the voltage controller for induction generator (IG), to work with variable speeds and load, based on instantaneous power control (IPC). It is based on the concept of controlling the instantaneous real and imaginary power into the machine. This paper outlines the basic algorithm of IPC and presents simulation results of IG performance using Matlab I Simulink.

Kqwords-Instanfaneous power control; Induction Generator; Voliage source inverter; Imaginary power

I . INTRODUCTION The Induction Generator using conventional squirrel

cage induction machine has number of advantages such as ruggedness, brushless cage rotor construction, absence of a separate dc source, better transient performance and better inherent overfoad protection These advantages justify the use of IG for stand-alone applications. Despite all these advantage, the fimdamental problem with the 'Self-excited Induction Generator (SEIG) is inability to control the frequency and i t s terminal voltage under varying load and speed conditions. SElG requires external reactive power compensation to maintain the excitation. In a conventional SEIG with 3 Phase capacitor bank connected across the machine terminals has been recognized for many years. The utility of this mode of operation is limited because it does not provide a constant voltage and frequency regulation under variable loads and variable speed operation. Another problem of this technique i s that the machine can only achieve and maintain excitation under certain speed and load conditions. To overcome the poor voltage regulation of the SEIG, a number of schemes have been proposed in [1]-[4]. The scheme based on switched capacitors [11 finds limited application because it regulates the terminal voltage in discrete steps.

To improve the IG operation, various control strategies using PWM control of the 1G have been proposed in [S, 6 andlo], but all of them have some limitations. Some of these proposals use current controlled voltage source inverter with capacitor bank on the stator terminals 151 and field oriented controlled inverter [6] to control the terminal voltage of the IG under variable speed and load. Field oriented technique requires costly and unreliable mechanical position

B.G.Fernandes Department of Electrical Engineering,

Indian Institute of Technology Bombay, Powai, Mumbai ~ 400 076, INDIA.

bgf@ee.iitb.ac.in

sensing systems such as encoders or resolvers. There are other proposals supplying rcactive current to the IG by using a static reactive power compensator [5]-[7j and shunt connected PWM voltage source inverters [SI. In all these schemes 3 phase capacitor bank is connected on the stator side. The value of the capacitor required is inversely proportional to the square of the prime mover speed. This increases the cost and complexity of the Hardware because the undesired influence to the control structure. Another limitation is that for a given capacitor valuc, self-excitation can only be achieved and maintained for certain load and speed conditions.

This paper proposes instantaneous power control for controlling the terminal voltage of the IG under varying load and speed without connecting 3 phase capacitor bank on stator side of IG. Instantaneous power control is used to control the inverter current references. It is based on control of instantaneous real and imaginary power into the machine. It is motivated by the fact that real power controls the capacitor voltage and the imaginary power controls the flux of the IG. This control the reactive power required by the IG as we11 as load. TPC excitation techniques allow a better frequency control and better voltage regulation than capacitive self-excitation technique under varying speed and load conditions

11, INSTANTANEOUS POWER CONTROL ALGORITHAM

The proposed control strategy is based on the instantaneous power control of the IG. Therefore, some definitions, that will be used later, are outlined in the present section. The complex power expression for a machine in space vector notation can be written as:

s = iv* (1) It can be written in two phase stationary frame as:

where:

(3)

Expanding equation (2) the expressions for the instantaneous real and imaginary power as defined in [91:

Therefore:

p = v,i, + vBip

q = v,i, - vaip

When the instantaneous powers and voltages are known, thc corresponding currents can be obtained as fo I lows.

- "p P k]=-[; V , ] [ J (7)

It is important to emphasize that the value of p and q are independent of the defined reference frame.

The reference frame which is useful to analyze the instantaneous real and imaginary power is stationary reference frame. Using this reference frame, the direct and quadrature voltages are:

Vd = o vq = [VI = J-

Therefore:

* P 1, =-

a 4 l p =-

I4

IVI

111. MATHEMATICAL MODELING OF 3-4 SELF EXCITED INDUCTION GENERATOR

The d-q axes equivalent circuits of an induction generator (IG) in stationary reference frame are given in Fig. 1 . The dynamic model of the IG in this frame is described by the following equations [8, 101:

d dt d dt

-'qx = Vqr + Ts'yJ + B5'qr

-Ads = 'ds -k ' BsAdr (13)

(12)

d dt d dt

-Aqr = vqr +TAq7 +BrAq,T - ( o - ~ r P d r

- A d r = vd, + TA& f Br& + (0 - U, )Aqr (1.9

(14)

(16) 3P 4

T, =-(i qr A dr - Rqridr)

where (8)

(9)

Where i,, is active current this transmit only active power to the load side and ip, is the reactive component.

Combining equations (9) and (10) with ( 7 ) , a transformation that relates active and reactive currents to the currents at arbitrary reference kame.

D=L,L , -L ;

L, = Llr + L ,

Vdrl I 'ds

(b)

Figure I . d-q model of an 1G (a) d-axis (b) q-aus

For a squirrel cage induction generator, Vqr and Vdr in equation (1 4) and (1 5) are set to zero and select w=O in equations (12) - (1 5) .

The Parameters of 3 4 Self-Excited Induction Generator [I21 used for simulation study are given in Appendix .

A. MATHEMA TiCAL MODELLING OF 3- 4 CC - vsi

The capacitor voltage equation is governed by:

U1

where Vdc is the voltage across the capacitor and id, is the current passing through it, as shown in Fig. 2. The set point of Vdc must be greater than the peak value of the generated phase voltage. Total dc current idc can be expressed in terms of inverter switching function as (suf ix e identifies the compensator phase currents)

The three switching functions take the value of 1 if the upper switch in the given inverter leg is on and the lower switch is off. Tt is 0 if the lower switch in the same inverter leg is on, while upper switch is off.

B. MODEL OF THE VOLTAGE SOURCE INVERTER (VSI)

Phase voltages generated by the inverter are expressed in terms of switching functions as:

V 3

Ybn = d'(ZS, - s, - S , )

127)

Figure 2 . Overall block diagram

IV. SYSTEM DESCRIPTION OF INDUCTION GENERATOR AND ITS CONTROLLER

The schematic of the proposed system is shown in the Fig. 2. It comprises of Induction Generator, load, and Voltage source inverter with external battery of 12 V. Initially the external battery charges the capacitor to 12 V. The minimum initial capacitor voltage required for self excitation process of 1G depends on the parameters of IG and voltage drop across the devices of the inverter. Self-excitation is the result of the interaction between the voltage provided by the CGVSI and the residual flux. The remnant flux is insufficient to promote the system start-up, an auxiliary DC battery must be connected to the capacitor to provide the buil& up energy. When capacitor voltage reaches more than 12 V and flux reaches a desired level the battery is disconnected and the 1G supplies itself the necessary energy to control the capacitor voltage,

It is well known that the IG terminal voltage can be controlled by controlling the magnetizing current. A CC-VSI has to provide the excitation to the 1G.

Fig.2 shows the block diagram of the proposed controller in which only two PI controllers and one hysteresis controller are used. Input to the PI-AC regulator is the difference between the magnitude of stator voltage and the ac reference voltage. Its output is defined as inverter reactive current reference. This regulator maintains a constant voltage across IG terminals even when the load is applied and also supply desired reactive power to the IG.

A capacitor voltage regulator has been implemented using PI-DC regulator to achieve a high enough capacitor voltage for proper current controlled inverter operation. Input to this regulator is the difference between the actual capacitor voltage and the reference voltage. The output of the regulator has been defined as inverter active current reference. This reference current represents the flow of active power necessary to keep the capacitor voltage constant.

The inverter instantaneous active and reactive current references are calculated using equation (1 1).

These reference currents are compared with actual currents. The proposed controller uses a hysteresis current controlled, voltage source inverter to supply the desired reactive power for induction generator and active power to the load, if needed during transient condition.

V. SIMULATION RESULTS Self-excitation is started at P O sec by connecting

the static compensator to the IG terminal. The capacitor is selected as 1000 micro farads and it is charged initially to 12V through the battery. IG provides the required energy to charge the capacitor.

u . 4 / *U 2 4 6 8 10 I2

l'ims IF1

Figure 3. Variation of (a) Capacitor voltage (b) Air-gap flux

Application of step load (150+i340WA at t = 4.5 sec

The appIication of load at the stator terminal causes the capacitor voltage to decrease as shown in the Fig. 3(a) due to insufficient active power produced by the IG to meet the active power demand by the load. Hence, the deficient active power required by the load is provided by the inverter. The capacitor voltage must be kept constant in-order to force the desired currents into the system. The flux in the machine also decreases because reactive power produced by the inverter is not sufficient to meet the sudden Ioad demand as shown in the Fig. 3(b). Therefore the terminal voltage of the IG decreases as shown in the Fig. 4. To overcome the decrease in terminal voltage, a closed loop control has been implemented using IPC scheme. This scheme makes the capacitor voltage, flux in the machine and terminal voltage constant and is shown in the Figs 3(a), 3(b) and Fig. 4(a) respectively. By the fast action of the closed loop CC-VSI controller using IPC, the terminal voltage is maintained within a fraction of a second. Its zoomed view is shown in the Fig. 4(b). The induction generator is normally used in applications such as wind or micro-hydro energy generation. The IPC controller is capable of regulating the generator voltage within specified variation of rotor speed. The speed of the IG, frequency, and

electromagnetic torque are shown in the Figs. 6(a), (b) and (c) respectively. The generator speed drops from 1900 rpm to 1880 rpm due to increase in load. The frequency also decreases fiom 62 Hz to 60 Hz. Due to Closed loop CC-VSI controller action using IPC scheme, the speed and frequency are regulated to normal value.

Removal o f Load from 1150+i340) VA to j50+i210)VA at t=8 sec

The capacitor voltage increases fiom 200 V to 280 V during t = 8 sec to t = 10 seconds due to excessive power generated by the IG. This excess power is not absorbed by the load. Hence, the capacitor voltage and the air-gap flux increases. The closed loop control regulates the voltage back to the reference voltage of 200 V. The effectiveness of thc proposed voltage regulator using IPC in controlling the terminal voltage of a three-phase SEIG is evaluated by sudden application and removal of Ioad.

151

7.1 7.10s 1.11 7116 '1.11 7.125 7.13 7.135 714 Time IS1

-m '

Figure 4. Variation of (a) Building up generator terminal voltage (b) Zoomed voltage and current of IG

7.8 7 6 5 7.V 7.95 8 8.a 8.1 1.15 8 2 8.2G ilmrls)

Figure 5. Variation of Load current

An overshoot of terminal voltage of the 1G is observed when the load is suddenly decreased. The

zoomed waveform of terminal voltage and current of IG for few cycles is also shown in Fig. 4(b). The Load current waveform are shown in Fig 5 due to sudden application and removal of load at e4.5sec and t=8 sec respectively.

Figure 6 . Variation of (a) Speed (b) Frequency (c) Electromagnetic Torque

APPENDIX

1

Stator resistance (U,) 1.51 (0 Rotor resistance ( rr) 0,564 (Ir) Leakage stator inductance (L , J 0.0096 (H, Leakage rotor inductance ( L d 0.0096 (H) Rated power 2 HP Number of Poles 4 Rotor inertia (4 0.02664 (kg - m2)

VI. CONCLUSION

A voltage controller is proposed to excite the induction generator under varying load and speeds based on the concept of IPC scheme. This scheme does not require computations and information regarding rotor speed for calculating the reactive power. This minimizes the cost of the controller. It was shown,

through simulation results that the system can operate over a wide range of variations in load and speed with good terminal voltage regulation. This increases the overall system reliability.

191

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