experimental verification of a utility interactive wind energy conversion scheme [WECS] with an asynchronous link comprised of a diode bridge rectifier and a ...
IEEE Transactions on Energy Conversion, Vol. 9, No. 3, September 1994
558
A UTILITY INTERACTIVE WIND ENERGY CONVERSION SCHEME WITH USING A SUPPLEMENTARY CONTROL LOOP
AN ASYNCHRONOUS DC LINK
R. M. HILLOOWALA Member, JEEE
A. M. SHARAF Senior Member, IEEE
University of New Brunswick Dept. of Electrical Engineering Fredericton, N.B. Canada E3B 5A3
ABSTRACT: The paper presents modelling, simulation and experimental verification of a utility interactive wind energy conversion scheme [WECS] with an asynchronous link comprised of a diode bridge rectifier and a line commutated inverter. The control objective is to track and extract maximum power from the wind energy system [WES] and transfer this power to the utility. This is achieved by controlling the firing delay angle of the inverter. Since the diode bridge rectifier has no control on the dc link voltage, a supplementary control loop is used to limit the voltage within a preset voltage threshold. The proposed scheme for regulating the flow of power through the dc link ensures reduced reactive power burden on the self-excitation capacitor banks and better utilisation of available wind energy, while limiting the dc link voltage within a preset voltage threshold. The simulated results are experimentally verified and found to give good power tracking performance.
I. INTRODUCTION Wind energy conversion schemes generally use self-excited induction generator on account of its simplicity, ruggedness, ease of implementation and low cost [I-41. Due to intermittent and stochastic nature of wind energy, these systems (WES) are generally used with some form of energy backup to provide continuous supply of electrical power even with fluctuating available wind energy [5]. In this paper, the utility interactive WES is proposed wherein the utility acts as the energy reservoir to supply the deficit amount of energy or absorb excess energy. Directly interfacing the WES to the utility gives rise to problems of voltage fluctuations, flickering and generation of subharmonics/harmonics associated with the pulsating torque, characteristic of the vertical axis wind turbine (prime mover) driving the induction generator [6]. Hence, to overcome these difficulties an asynchronous (AC-DC-AC) link is used to interconnect the WES to the utility. Generally, the asynchronous
link consists of a phase controlled rectifier and a line commutated inverter [5] as shown in Fig.1. Since, the reactive power consumed by a controlled rectifier is a function of the firing delay angle cc, (increases sinusoidally with a,), there is a limit on the maximum value of cq+ This limits the reactive power burden on the selfexcitation capacitor bank which has to supply VARs required for self-excitation and that demanded by the converter. This constraint will limit the operating speed range of the induction generator driven by the wind turbine, since, once cc, reaches its upper limit there is no control on the dc link voltage. Hence, the input voltage to the rectifier has to be restricted by limiting the maximum speed of the wind turbine mechanically. This will prevent complete utilization of available wind energy. In the proposed scheme this constraint is overcome by using an uncontrolled (diode bridge) rectifier and a line commutated inverter to form the asynchronous dc link. This arrangement helps in reducing the reactive power burden on the self-excitation capacitor bank since the power factor at the uncontrolled rectifier input is almost unity (neglecting the losses in the converter). This helps in reducing the installed capacity of the self-excitation capacitor bank, reducing the overall cost of the system. Also, better utilization of available wind energy is ensured by increasing the operating speed range of the wind turbine driven induction generator (since there is no restriction imposed by a, as in the case of a controlled rectifier). The maximum power tracking controller modulates the firing delay angle a,of the inverter to ensure complete utilization of available wind energy. Since, the diode bridge has no control on the dc link voltage, a supplementary control loop is used to limit the dc link voltage to a preset voltage threshold. A model of the WES with an asynchronous link is developed, simulated and the results are experimentally verified.
Induction
Controlled
Generator
9 4 WM 1 0 2 - 4 EC A paper recommended and approved by the IEEE Electric Machinery Committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1 9 9 4 Winter Meeting, New York, New York, January 30 - February 3, 1994. Manuscript submitted July 1 2 , 1 9 9 3 ; made available for printing December 6, 1993.
Turbine
Capacitors
Figure.1
0885-8969/94/$04.00 0 1994 IEEE
Block schematic diagram of the generalised wind energy conversion scheme interconnected to the utility through an asynchronous link (controlled rectifier and line commutated inverter).
559 11. SYSTEM CONFIGURATION
pi, = KZr&
The block diagram schematic of the proposed WECS is shown in Fig.2. It consists of a vertical axis wind turbine driving a self-excited induction generator interfaced to the utility through an asynchronous link (diode bridge and line commutated inverter). The asynchronous link virtually decouples the two systems (WES and the utility) and allows each to operate at its own voltage and frequency. The variable frequency, variable magnitude ac voltage available at the induction generators terminals is fust rectified using a diode bridge and the dc power is then transferred to the utility at its own voltage and frequency using a line commutated inverter. A brief description of the system subsections is as follows.
A. VERTICAL AXIS WIND TURBINE The vertical axis wind turbine acts as a prime-mover to drive the self-excited induction generator. The average torque output T, of the wind turbine is given by [7]
TT = 1/2pRAGV;
(1)
where C, is the average torque conversion coefficient which is a nonlinear function of the tip speed ratio h = w,R/V, [7]. In practice, the wind turbine is coupled to the generator shaft through a step-up gear-box (l:ngca-bx)$so that the generator runs at a higher speed w, in spite of the low speed of the wind turbine wT. The average torque T, at the generator shaft is T, = TT/ngear.bx.
+ L,K,w,i, +
pi, = - L,K,w,i,
(L,K,w,
- (rz + LmKZrJL& - iqJcvd&
+ K2rliL - (L,K,w, - iqs/Cv,)iq (5)
+ (r2 + LK2rz)LzL + K2V,
pv, = (I/C)[ib - (2d3/n)iD,l
(6)
pw, = - (O/J)w, - (3N;LJ3J)[iqi,
- i,iqr]
+ (NJ2J)T,
It is essentially an induction machine driven by a primemover (wind turbine) and having a source of reactive power (selfexcitation capacitor bank). The dynamics of the self-excited induction generator can be represented by the following electromechanical equations derived in the synchronously rotating reference frame [5,8]
where, K, = V(L,L, - L,’) and K, = LJ(LILz - L,’) The above equations were derived assuming that the initial orientation of the q-d synchronously rotating reference frame is such that the d-axis is aligned with the stator terminal voltage phasor (i.e. vqs= 0 and v, = Vs).
C . ASYNCHRONOUS AC-DC-AC LINK It consists of a diode bridge rectifier, a dc choke and a line commutated inverter. The diode bridge rectifier converts the variable magnitude, variable frequency voltage at the induction generator terminals to dc. The dc voltage at its output can be expressed in terms of the peak phase voltage of the induction generator vds = V, (by choice of axis orientation) and the input transformer’s turns ratio l:ni V,
=
(8)
(3d2/n)(d3vJd2).ni
p
iDC
= (lbC)[vR -
-
RDC iDC
1
pi, = (iJcY,
+ KzL,w,)iq, + K&,
- K,r&
- K,v,
(9)
where V, is the input dc voltage to the inverter and is related to its output phase voltage V,, (peak) and firing delay angle a,by VI = - (3d2/n)(d3VinJd2) COS(CI,)
+ KzLmwm)i, + Kzrziqr- K,L,w,i,
(7)
The dc choke reduces the ripple content in the dc link current iDc, which is govemed by the following differential equation
B . SELF-EXCITED INDUCTION GENERATOR
pi,, = - Klrliqs- (i,/cY,
(4)
(10)
(2)
+ K,L,w,iq, (3)
D. ISOLATING TRANSFORMERS The input and output transformers at the extreme ends of the asynchronous link provide isolation and adjust the level of voltage and current to ensure proper operation of the system.
Figure.2 Block schematic diagram of the proposed utility interactive wind energy conversion scheme.
III. CONTROLLER FOR WIND ENERGY SYSTEM
B . SUPPLEMENTARY CONTROL LOOP In the proposed scheme a diode bridge rectifier is used in the asynchronous link, to reduce the reactive power burden on the self-excitation capacitor bank. This helps to reduce the installed capacity of the self-excitation capacitors which now have to supply VARs required for self-excitation only. However, with increase in wind velocity, the rotor speed and terminal voltage of the induction generator will increase. Since, the diode bridge rectifier has no control, the dc link voltage can also increase to an unacceptable value (from voltage rating point of view). This is overcome by using a supplementary control loop (as shown in Fig.2) which continuously monitors the dc link voltage. If the dc link voltage tends to exceed the preset voltage threshold, the supplementary control loop slightly reduces the firing delay angle a, of the inverter as decided by the maximum power tracking controller. This increases the power output of the inverter and also the load on the induction generator whose terminal voltage drops, automatically limiting the dc link voltage to the preset voltage limit. This can be represented by the following rule
A . MAXIMUM POWER TRACKING CONTROLLER The control objective is to track and extract maximum power from the WES and to transfer this power to the utility grid. To achieve this a maximum power tracking controller is implemented using a proportional and integral [Pa controller as shown in Fig.2. The power transferred over the dc link (output power of the rectifier Po)can be related to the maximum power output P,, of the WES by the conversion efficiency of the generator (qG) and the rectifier (qd.The maximum power output of the W E S is a function of the wind velocity V, and the tip speed ratio h. The maximum power output of the induction generator at different V, is computed. The data obtained is used to relate P,, to V, using polynomial curve fit as shown below P,,
= -3.0
+ 1.08Vw- 0.125V$ + 0.842V:
(11)
Since the control objective is to track and extract maximum power from the W E S and to transfer this power over the dc link to the utility, the reference power at the rectifier output PWRconsidering the generator and rectifier efficiencies (qGand qR)is taken as 'ref
= qGUR'mu
(12)
where Aa varies as a function of the difference (VR - V-) Fd has a maximum value Aa- = 2 O (found by trial and error to give good voltage regulation and transient response).
By making the output power of the rectifier track the reference power, maximum utilization of the available wind energy is ensured. The actual power output Pois compared with the reference power and any mismatch is used to change the firing delay angle a,of the inverter as follows
IV. SIMULATION RESULTS The model of the proposed WES is represented by the set of differential equations (2)-(7), and (9). Using the model and the software simulation package: TUTSIM, the closed loop system response to variation in wind velocity is studied. Simulation results are depicted in Fig.3. It is seen that with increasing wind velocity, the mechanical power input (represented by the average torque
Thus, by controlling a,,the power transferred over the dc link and also the power extracted from the W E S is controlled to ensure complete utilization of the available wind energy. lo, '
.
W I N D VELOCITY I
,
d-axis. STATOR YOLTAGE
,
155,
FIRIriG DELAY ANGLE
,
I35
6
dIq time
3251
time
(s)
GIBERATOR SPEBD
I
;F,
140
I
time
(s)
IO (s)
5
15
RECTIFIER O/P VOLTAGE
INPUT TORQUE
-MI
time
10 (s)
time
(8)
I5
0
.p
2
I - 013
0
,
5 time
IO (s)
d-axis STATOR CURRENT
I' 0
IS
I 5
IO time
l.9
I
5
15
time
(8)
q-axis STATOR CUBRWT
- m
IO (s)
(s)
time
time
(8)
INYERTER O/P POWER
RECTIFIER O/P POWER
10 time
I5
(s)
- m
IO
I5 time
Figure.3 Simulation results depicting variation in system variables with step change in wind velocity.
-
-
(9)
15
561
input T, ) to the system increases. This causes the rotor to accelerate to a higher speed increasing the terminal voltage and the synchronous electrical frequency w, of the induction generator. Also, the slip speed (difference between synchronous speed w, and the rotor speed w,) which is negative for generator operation increases, indicating that the generator is capable of delivering more power output. Thus, the maximum power tracking controller comes into operation and reduces the f i n g delay angle (q) of the inverter. This increases the power output of the inverter and also the power drawn from the WES, thereby ensuring complete utilization of the available wind energy. It is also seen that with increasing rotor speed, both the terminal and the dc link voltage increase. This continues till the dc link voltage reaches the preset voltage threshold V,-, when the supplementary control loop comes into operation and reduces a, (as decided by the maximum power tracking controller) by a small value A a . As a result, both the dc link current i, and the power output of the inverter Pmv increase. This acts as an additional load on the induction generator whose rotor speed w, and terminal voltage as represented by vds become constant (no longer increase) which automatically limits the dc link voltage to the preset voltage threshold. Thus, the simulation results indicate that by using the maximum power tracking controller and the supplementary control loop it is possible to track and extract maximum power from the WES and at the same time regulate the dc link voltage even with a diode bridge rectifier.
V. EXPERIMENTAL RESULTS A laboratory prototype of the proposed WECS was developed. The characteristics of the vertical axis wind turbine was emulated using a separately excited dc machine controlled by a power amplifier and an analog computer. A dc machine can be represented by the following set of equations
T = K$Ia V, = K$w
+ RJ,
where T is the average developed torque, K is the machine constant, $ is the magnetic flux per pole, 1, is the armature current, V, is the applied armature voltage, w is the rotational speed in rads/s. If the dc motor is separately excited, the flux (I becomes a constant, and from (15) it is seen that the torque is directly proportional to the armature current I,. Since it is desired to emulate the output torque speed characteristics of the wind turbine by that of the dc motor, equations (l), (15) are combined to give
By controlling the applied armature voltage V, of the separately excited dc motor, such that the actual armature current I, tracks the reference current Iaref (given by the above equation), it is possible to emulate the torque speed characteristics of the vertical axis wind turbine. This is achieved by experimentally implementing the block schematic shown in Fig.4 (a). The nonlinear functional block relating h and C,is implemented using operational amplifiers as shown in Fig.4 (b). The wind turbine simulator with 1 kW rating is implemented using a 2 kW dc machine. The wind turbine simulator is used to drive a 2 kW induction generator loaded appropriately. Load test is carried out at different wind velocities and the results are presented in FigS. It is seen that there is good agreement between the calculated torque speed characteristics and the actual measured values. The difference in the peak torque values can be attributed to the friction and windage losses which were assumed to be constant over the operating speed range. The maximum power tracking controller and the supplementary control loop is implemented using operational amplifiers as shown in Fig.6. Experimental results obtained using the laboratory prototype of the proposed WES are shown in Fig.7. The supplementary control loop is enabled/inhibited to show the relative effect of such a loop on the system's response. It is seen that when the supplementary control loop is inhibited, with increase in wind velocity V,, the maximum power tracking controller reduces the firing delay angle of the inverter. This increases the power output of the rectifier and also the power drawn from the induction generator. Also, the dc link voltage increases with Vw since there is no control exercised by the diode bridge rectifier.
Current Transducer
PI
Controller
Power Amplifier
(a)
Divider
b
cT
Figure.4 (a) Block diagram of the wind turbine simulator. (b) Operational amplifier circuit to implement A --- C, nonlinear characteristic.
562
ROTOR SPEED
Figure.5
Nm
(rpm)
Torque vs. speed characteristics of the wind turbine simulator at different wind velocities. (All variables referred to the high speed (generator) side)
However, when the supplementary loop is enabled, it is seen that the firing delay angle of the inverter, as decided by the maximum power tracking controller (main loop), is further reduced. This increases the output power of the inverter and also the load on the induction generator whose rotor speed and terminal voltage reduce and automatically limit the dc link voltage to the preset voltage limit. Thus, by using the supplementary control loop in conjunction with the maximum power tracking controller, it is possible to track and extract maximum power from the WES and at the same time limit the dc link voltage to the preset voltage threshold even when using a diode bridge rectifier.
VR
( 0 -)
123
.
+ with
w w/o+
123
130
13s
123
S u p p l e m e n t a r y Control
v i t h o u t S u p p l e c c n t a r y Control v
u
ulo
vi0
w
Supp 1 e c e n t a r y Signs1 t
Figure.7
(r)
b54--10+1O--rl,5Jc10-.(
Experimental results depicting effects of the supplementary control loop on the response of the proposed wind energy conversion scheme.
VI. CONCLUSIONS The paper presents modelling, simulation and experimental results of the utility interactive wind energy conversion scheme. The WES is interconnected to the utility through an asynchronous link consisting of a diode bridge rectifier and a line commutated inverter. This helps to reduce the reactive power burden on the self-excitation capacitor bank whose installed capacity can be reduced. Also, there is no limitation on the operating speed range as imposed by f ~ delay g angle of the controlled rectifier. All these factors tend to reduce the installation cost and enable better utilization of available wind energy. A maximum power tracking controller is implemented to control the firing delay angle cl, of the inverter. This ensures complete utilization of available wind energy.
Since, the diode bridge rectifier has no control on the dc link voltage, a supplementary control loop is used to modify (increase by Aa) the firing delay angle a, (as decided by the maximum power tracking controller), whenever the dc link voltage increases beyond the preset voltage threshold. This increases the power output of the inverter and the load on the induction generator, reducing its terminal voltage and automatically limiting the dc link voltage within the preset voltage threshold. The simulated results are experimentally verified and found to give good power tracking performance while limiting the dc link voltage to the preset voltage threshold.
uc,
,,,,
Figure.6 Circuit schematic diagram of the maximum power tracking controller and the supplementary control loop.
nv
563
REFERENCES D.Watson, J. Anilaga and T. Densen, "Controlled dc power supply from wind driven self excited induction machines", IEE ROC. Vol. 126, NO. 12, pp. 1245-48, 1979. N. Mohan and A. Riaz, "Wind driven capacitor excited induction generators for residential electric heating", IEEE PES Winter Meeting, New York, Jan. 1978. J. Arrilaga and D. Watson, "Static power conversion from self excited induction generators", IEE Proc. Vol. 125, pp. 743-746, 1978. G. Raina and O.P. Malik, "Wind energy conversion using a self-excited induction generator", IEEE Trans. PAS, Vol. 102, pp. 3933-36, 1983. K. Natarajan, A. Sharaf, S. Sivakumar and S. Nagathan, " Modelling and control design for wind energy power conversion scheme using self excited induction generator", IEEE Trans. EC, Vol. 2, No. 3, pp. 506-512, Sept. 1987. R.M. Hilloowala, A.M. Sharaf and M. Lodge, "Wind energy conversion schemes and electric power supply quality", h o c . European Wind Energy Conference, Madrid, Sept. 1990. G.L. Johnson, "Wind Energy Systems", Prentice Hall Inc., Englewood Cliffs, New Jercy, 1985. R.M. Hilloowala and A.M. Sharaf, "Modelling, simulation and analysis of variable speed constant frequency wind energy conversion scheme using self-excited induction generator", Proc. South-Eastem Symposium on System Theory, South Carolina, pp. 248-253, March 1991.
NOMENCLATURE peak phase voltage of induction generator dc voltage at rectifier output dc voltage at inverter input dc link current rectifier output power maximum output power of WES efficiency of induction generator efficiency of rectifier rotor speed of induction generator synchronous speed of induction generator d-axis component of stator current q-axis component of stator current d-axis component of stator voltage number of poles of induction machine moment of inertia of the system frictional coefficient firing delay angle of inverter as decided by maximum power tracking controller change in firing delay angle to limit dc link voltage firing delay angle of inverter peak phase voltage at inverter output peak phase current of inverter wind velocity radius and swept area of wind turbine tip speed ratio average torque conversion coefficient average torque output of wind turbine input transformer's tums ratio (1:0.58) output transformer's tums ratio
APPENDIX Parameters of vertical axis wind turbine: Rating: 1 kW at 450 rpm with V, = 12 m / s Height 4 m, Equator radius R = 1 m Constants: Swept area A = 4 m2, Gear ratio = 1 : 4.28 Parameters of dc motor: Rating: 2 kW, 120 V dc, 21 A, 4 pole, 1750 rpm Constants: R, = 0.7 0,R, = 67 Q, K = 0.632 "/A2 Parameters of induction machine (wound rotor): Rating: 2 kW, 120 V, 10 A, 4 pole, 1740 rpm Constants: rl = 0.62 Q, rz = 0.566 Q, X,, = X, = 1.57 !2, X, = 20.35 0, J = 0.0622 kg.m2, B = 0.00366 "/rads.
Rohin M. Hilloowala (M'82) was bom in Aden. He received the following degrees: B.E. (Elect. Eng.) from South Gujarat University, India, in 1980, M.Tech (Elect. Eng.) from Indian Institute of Technology (Bombay), India, in 1982 and Ph.D (Elect. Eng.) from the University of New Brunswick, N.B. Canada, in 1993. He was employed in the Research and Development Division of National Radio and Electronics Co. Ltd. from 1982-1988, serving as a Design Engineer and later as the Deputy Development manager. His responsibilities included the design of Control and Switching Power Circuits for lnverters used in UPS and AC Drives. His research interests are in the areas of Power Electronics, Energy Conversion and the use of Intelligent Controllers in Power Engineering. Dr. A. M. Sharaf obtained his B.Sc in Electrical Engineering from Cairo University, Egypt in 1971. M.Sc and Ph.D degrees from the University of Manitoba, Winnipeg, Canada in 1976, 1979 respectively. He is currently a professor of Electrical Engineering at the University of New Brunswick. Dr. Sharaf authored and co-authored 120 papers and technical reports. He did consulting and collaborative work with Asea-Brown Boveri, ABB, Canadian Electrical Association (CEA), Japanese Central Research Institute of the power industry, and Electric Utilities in Canada and abroad. Dr. Sharaf is a senior member of IEEE and a registered professional engineer in the province of New Brunswick. He is president of "Sharaf Energy Systems" an R & D Engineering consulting company incorporated in the province of New Brunswick, Canada.
