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Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Automatic Power Generation Control of Grid Connected 3-Ø Asynchronous Generator Using Electronic Controller in the Rotor Circuit 1

K. Subramanian* and K. Hari Prasad2

*Power Electronics Division, School of Electrical Engineering VIT University Vellore, Tamil Nadu, India, 632 014 1 Email id: ksubramanian@vit.ac.in ; 2Email id: hari_sekhar18@yahoo.co.in generator over a wide range of wind speeds is proposed in this paper. Constant stator power output control involves keeping the shaft torque constant as the wind speed changes. Torque control is achieved by electronically varying the rotor resistance of the induction generator. The control system monitors the difference between the actual and the desired stator power output and adjusts the external resistance in the generator rotor circuit so as to reduce the differential to zero.

Abstract— This paper explains the power generation control of wind driven grid connected 3-Ø self-excited asynchronous generator. Power semiconductor switch based Electronic controller has been used to change the rotor resistance value. The changes in the resistance could be change the rotor power which is fed to the grid in double fed induction generators. The proposed control technique is either addition or deletion (remove) the external resistance to the rotor circuit. Wind turbine characteristics are derived and simulated. Simulation of the proposed system has been done using power system tools box in MATLAB / Simulink. Results are presented.

The proposed scheme consists of wind driven grid connected self-excited asynchronous generator (SEASG), rectifier and Electronic controller shown in Fig.1. In this configuration wind turbine and rotor of the generator is mechanically coupled. The external resistance (effective resistance) is to be connected to the rotor circuit using the electronic controller.

Index Terms—grid connected asynchronous generator, electronic controller wind characteristic

I. INTRODUCTION The increasing rate of depletion of conventional energy resources has revived worldwide interest in wind turbine generators. For small and isolated communities where electricity is generated using diesel fuel, this interest is largely due to the economic benefits associated with the reduction in diesel fuel consumption. However, integration of wind power with a small isolated power grid is not straightforward because the system voltage, frequency, and performance of other diesel generators connected to the grid depend on the amount of the wind power penetration [1]. During gust periods, a wind turbine is subjected to rapid changes in wind velocity and hence violent fluctuations in input power. By adjusting the turbine blades, it is possible to regulate the energy that a wind turbine is extracting from the wind at a given instant. But mechanical regulation of power involves considerable capital cost and usually complicates the system at the expense of reliability. The simplicity and flexibility exhibited by the induction machine in providing electromechanical energy conversion make it the most favored choice for wind powered systems operating in parallel with an existing power grid [2]. Although not as common as the squirrelcage induction machine, the wound-rotor type has several attractive features in providing constant-frequency ac power when driven from a variable-speed source such as a wind turbine.A scheme for scheduling a desired output power from a wind-driven wound-rotor induction

Fig.1 Schematic arrangement of proposed wind energy conversion scheme

Since the stator terminals of the slip ring induction motor are connected to the grid, if the wind velocity is sufficient to drive the rotor above the synchronous speed it generates power. The amount of power generation depends on the wind velocity and system efficiency. This scheme describes the constant power generation irrespective of wind velocity.

SYSTEM DISCRIBTION

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

In constant power generation, the motor shaft torque should maintain constant irrespective of wind power extraction in such way that the extra power is wasted in the external resistance of the rotor circuit. The power extraction is also depends on the cut in and cut off speed of the wind velocity.

Td =

The wind turbine charatcerisitcs based on the mechanical power Pw and the shaft torque Tw developed by a wind turbine of blade radius R rotating in a wind stream of velocity V are can be ralted [00]

Tw =

1 C t ρπR 2 V 3 = K T C τ V 2 2

III. CONTROL STRATEGY It is well known that power generated by the grid connected asynchronous generator depends on the wind velocity. The frequency and voltage of the generator is equal to the grid voltage and frequency. The design of the proposed controller is based on the maximum wind power utility and also should satisfy the condition such that the stator power output is constant irrespective of the wind velocity. The power induced in the stator winding from the rotor winding is depend on the rotor circuit resistance. The proposed control technique is to change the effective rotor resistance value according to the wind velocity with constant stator power output operation. Hence the control action has been implemented in the rotor circuit of the generator, i.e. either adding or removing the effective external resistance of 370 ohm. This resistance has been

(1)

(2)

Where, K w = ρπR 3 The power coefficient C p and the torque coefficient C τ are function of the ratio of the shaft speed ( ω ) to

wind speed ( V ), expressed as the tip-speed β =

and C τ are related as C τ =

ωR Cp V

. The variation of C p β and C τ with β for the horizontal wind turbine is shown in Fig.1. The tip-speed ratio ( β max ) at which the power coefficient maximum possible to determine a family of curves ( C p − β curves) describing the output shaft power

divided into two resistors R1 and Rex with a value of R1 = 800 Ω and R2 = 700 Ω respectively. Resistance R1 is

permanently connected the rotor circuit and Resistance R 2 is connected using electronically controlled switch to the rotor circuit.

versus shaft speed at a number of given wind speeds. 1 Pw _ max = C p _ max ρπR 2 V 3 And (3) 2 C p _ max ; Equation (3) can be alternatively Cτ(β max ) = β max expressed in terms of ω as

A.. Controller operation The schematic arrangement of proposed control scheme is shown in Fig.2. The absolute values of voltage and currents have been taken from the stator windings of the generator and fed to the power calculator as a feed back signals. The calculated power is compared with the desired power and the error signal is fed to the proportional integral (PI) controller. The output of the proportional controller fed to the pulse width modulation (PWM) signal generator. The signal generated by the PWM to change the state (ON/OFF) of the Thyristor which concludes that the external resistance R 2 is includes in the rotor circuit or not. In this way the rotor resistance is controlled. So that the stator power output is constant irrespective of wind velocity/wind power available.

1 ρπR 5 ω 3 (4) C τ(β max) 2 2 β max C τ max is the maximum torque coefficient developed by the turbine at a tip-speed ratio β r max .C τ max is less than Pw _ max =

C τ max . The turbine can operate without being stalled

for β < β r a fixed pitch turbine; the variation of C τ in the region β r ≤ β ≤ β e may be approximated as C τ (β) = C1 − C 2 β

(5)

B. Wound Rotor Generator The torque equation for a wound induction generator with negligible stator impedance is given by

(6)

Where Vt the Thevenin is equivalent source voltage; R 2 and X 2 are the rotor resistance and rotor leakage reactance referred to the stator winding. Depend up on the rotor resistance (R 2 + R ext ) we have to control the rotor power so that we can adjust the stator power.

A. Wind Turbine characteristics

1 Pw = C p ρπR 2V 3 2

sVt2 (R 2 + R ext ) 3 ω s (R 2 + R ext ) 2 + (sX 2)2

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

RESULTS

The simulation results of various stages are presented. Fig .5 shows the wind characteristics curve. The Fig.6 shows the stator power with out controller where we can observe that the stator power is changing along with the wind speed. It understood that there is no constant power output .Where as in Fig. 8 the stator power is constant along with changing wind speed. Fig.7 shows the pulses for Thyristor Th . 8

Wind Turbine Characterstics for wind speed 4 m/s

x 10

2.5

P ower

1.5

Fig.2 Rotor resistance control scheme with controller blocks

IV. SIMULATION

0.5

The overall simulation circuit is shown in appendix-A. The simulation was done with a 1HP, 415 volts, 1.3 A, 1440 rpm, wound type induction machine parameters are r1 =10 .7 Ω , r2 = 0.55 Ω , X s = 393.941 Ω ,

-0.5 0

30 Speed

Fig. 5 wind characteristics curve

X r = 0.82 Ω and X m = 328.22 Ω over a periods of 4seconds using the ode23 stiff solver in MATLAB / Simulink power system tools. The external resistance is connected in the simulation is shown in Fig. 3. The gating signal generation for the switch is shown in Fig. 4.

2 b

3 c

2 G1

-60

+ g

Machine power

1 m

1 a

-40

-80

-20

Machine power wind speed for 4,5,6 m/s 0

-100 R2

NOT 3 G2 G3

NOT

-120

0.5

1.5

2.5

3.5

Time

T4 2

6m/s

5m/s

4m/s

Fig. 6 Machine output power without controller

R3 R4

pulses for switch 2

Fig. 3 simulation of rotor resistance control

1.5

[Pm ]

K-

0.5

PID

< 71

-0.5

[G1]

-1

Fig.4 Pulse generation for T1

0.1

0.2

0.3

0.4

0.5 Time

0.6

Fig. 7 Pulses for Th

0.7

0.8

0.9

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Machine power with controller 0

[3]

-10

-20

[4]

M ac hine power

-30

-40

-50

[5]

-60

-70

[6]

-80

-90

0.5

1.5

2 Time

2.5

3.5

Fig. 8 Machine output power with controller

VI.

K. Subramanian He received B.E degree in Electrical and Electronics Engineering and M.E degree in Power System from National Institute of Technology (Formerly Regional Engineering College), Thiruchirappalli-15 in 1994 and 1998. His research interest is Induction generator, Electrical Machines, Modeling & Simulation, Power Electronics applications in Reactive Power Control.

CONCLUSION

This paper demonstrates the feasibility of a closed loop system for controlling the stator power output from a wind driven grid-connected induction-generator. The salient features of the controller are the following: i) It is simple, automatic, and has no moving parts. Mechanical control systems such as hydrostatic or hydrodynamic transmission systems, electro mechanical torque control systems, and blade position systems can be dispensed with, thus saving on capital and maintenance costs. ii) The external rotor power which appears as heat can be used to supplement grid power in a variety of applications such as hot water heating, space heating, farmhouse heating, and crop drying. The overall efficiency of the system can thus be quite high.

K. Hari Prasad He received B.Tech degree in Electronics and Instrumentation Engineering from Sreenivasa Institute of Technology and Management Studies, Andhra Pradesh in 2007. He is currently pursuing Master of Technology in Power Electronics and Drives at Vellore Institute of Technology University, Vellore, India. His area of interest is Induction generator, Power Electronics

ACKNOWLEDGMENT The authors acknowledge the management of Vellore Institute of Technology University, Vellore, India, 632014 for their support and keen interest in promoting the research and development in the Power Electronics Division by providing all the required facilities and resources. REFERENCES [1] VelayudhanNayar C. and Bundell, J.H., “A new automatic generation controller for a wind-driven slip-ring induction generator” Proceeding of the IEEE, 72, No. 9, 1984, pp 1226-1229. [2] C.VelayudhanNayar, J.H. Bundell, “Output Power Controller for a Wind-Driven Induction Generator” IEEE

Transactions on Aerospace and Electronic systems vol.AES-23, No.. 3, May 1987, pp.388-401. Bhim Singh, S. S. Murthy and Sushma Gupta, “Analysis and Design of Electronic Load Controller for Self-Excited Induction Generators” IEEE Transactions on Energy Conversion, Vol. 21, No. 1, March 2006, pp 285 – 293. Bhim Singh and Gaurav Kasal “An Improved Electronic Load Controller For an Isolated Asynchronous Generator Feeding 3-Phase 4-Wire Loads” IETE Journal of Research Vol-54 Issue Jul-Aug 2008, pp. 244-254 Nyein Soe, Thet Han Yee, and Soe Sandar Aung, “Dynamic Modeling and Simulation of Three phase Small Power Induction Motor” World Academy of Science, Engineering and Technology 42 2008. J. R. P. Gupta, Bhim Singh, And B. P. Singh, “A ClosedLoop Rotor Resistance Control Method for Improved DC Dynamic Braking of Wound Rotor Induction Motor” IEEE Transactions On Industry Applications, Vol. Ia-21, No. 1, January/February 1985.pp. 235-240

Proc. of Int. Conf. on Control, Communication and Power Engineering 2010

Appendix-A

Continuous

powergui

[w]

Clock 0

Beta

[N]

[w]

Wind turbine characteristics simulator [G1]

[G3]

Is_d

[Id ]

Vs_q

[Vq ]

Vs_d

[Vd ]

[w]

In 1

Is_a

[Ia ]

Is_b

[Ib ]

Is_c

[Ic]

[N]

3ph ,415 v,1.3A,Slipring Induction generator

[Te ]

Machine measurement

Load with measurement system

[Pm ]

V_abc

Vabc

[Ia ]

Ia 1

Qm1

[Ib ]

Ql1

Qg1

Grid performance calculator

[Ic]

Load performane calculator [Pm ]

K-

Machine performance calculator PID

I_abc < 71 VL_abc [G1] IL _abc

348 ÂŠ 2009 ACEEE

Pm1

Pl1

Pg1

[G2]

[G3]

G1 G2

Grid with measurement block

V_abc

[Iq ]

[G2]

Is_q