power point tracking (MPPT) scheme, and simultaneously ... A hybrid wind diesel power system is one of the alternative .... The nominal power of each set.
Maximum Power Point Tracking and Frequency Control for Hybrid Wind Diesel System Supplying an Isolated Load 2 i 3 4 Vigneshwaran Rajasekaran , Adel Merabee, Hussein Ibrahim , Rachid Beguenane , Jogendra Thongam l Division of Engineering, Saint Mary's University, Halifax, NS, Canada 2 Wind Energy Techno-Centre, Gaspe, QC, Canada 3 Department of Electrical and Computer Engineering, Royal Military College of Canada, Kingston, ON, Canada 4 PowerEnerSys Inc., Chicoutimi, QC, Canada
Abstract- A control strategy developed for hybrid wind-diesel system (HWDS) is presented in this paper. The proposed system consists of the induction generator based wind turbine generation (WTG) system, IGBT based power electronics converters, diesel generator (DG) unit and dump load (DL) system. Developed control strategy shares the required power generation between the WTG and DG unit. The control system operates the wind turbine to achieve maximum power extraction using maximum power
point
tracking
(MPPT)
scheme,
and
simultaneously
controls the dump load to absorb excessive power generation, in turn regulates the frequency of power supply. The DG unit is controlled for supplementing the generated power from WTG to supply the complete load power demand. The complete system is modeled and simulated using SimPowerSystems toolbox from Matlab-Simulink software package.
Keywords- wind-diesel system, maxinmm power point tracking, speed control,frequency control, isolated load, damp load
I.
INTRODUCTION
Depletion of fossil fuels and environmental pollution caused by consumption of these fuels are two predominant energy problem faced in contemporary world. But still many communities around the world, living in remote location that lacks access to regular electric power grid supply, utilize the diesel generators for day-to-day power consumption. Since diesel generators are highly expensive option and contribute to very high carbon emission [I], an alternative energy solution is required to reduce diesel fuel consumption by integrating and operating a clean energy source in parallel. A hybrid wind diesel power system is one of the alternative solutions to resolve this problem. Since the wind power generation is estimated to produce 12% of world's electricity requirement by 2020 [2], HWDS are economic reality to reduce the diesel fuel consumption. A HWDS is hybrid combination of a WTG system and DG unit to supply the maximum portion of the load requirements
978-1-4673-2421-2/12/$31.00 ©2012 IEEE
from the intermittent source of wind, while supplying quality electrical power [3]. However, highly complex controls involved in ensuring the proper sharing of intermittent wind energy and controllable DG unit forms the basis for successful integration of WTG systems with DG units to meet the power demand. Since wind is highly varying in nature, HWDS requires control system to improve power generation performance and provide economical solution. Also there is a need to utilize the intermittent wind resource effectively. For any given wind speed above the cut-in speed, the wind turbine should extract the maximum power from available wind speed. For which a maximum power extraction control system needs to be employed. Also an isolated load, not connected to the regular electric grid, tends to have fluctuations in voltage and frequency of power supply and needs to be regulated for ensuring good quality in power supply. In order to maintain the power supply quality with regulated voltage and constant frequency, variable speed wind turbine generation (VSWTG) systems using power electronics converters are considered as good option, fully-rated VSWTG are also employed to avoid power output fluctuations [4] and reduce the overall cost. In a WTG system the squirrel cage induction generators are commonly used generators because of: compactness, robustness, light weight and economical. In spite of abundant wind availability, wind speed fluctuates throughout the day. Maximum power extraction at a particular wind speed depends upon the optimum value of tip speed ratio, which in turn depends upon shaft rotational speed (Of' With MPPT control, an effective usage of wind turbine and wind energy resource is ensured [5] [6]. The slip power of the induction generator controlled by PWM based power electronic converter has been used as a strategy for maximum power extraction in [7] In this paper, a complex control strategy is developed to control and operate HWDS in wind-diesel mode of operation, where both WTG and DG unit together will supply the load. WTG system is controlled to achieve MPPT using vector
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control scheme [8] with IGBT based converter, and the DG unit includes an automatic governor control system to control the power generation, it also supplies the reactive power requirement for the power system. A variable dump load (DL) with DL control is used to handle excessive power generation, and in turn regulates the frequency of the power system. An LC filter has been used at the terminals of the power converters to smooth the power supply and reduce the high frequency power fluctuations occurring from WTG operation. II. WIND TURBINE MODEL
(6)
(7)
(8) where,
CfJqs LJqs +Lllli�r CfJds =LJds +Lmi�r CfJ�r I,i�r +Lmiqs CfJ�r =L>�r +Lmids Ls L,s + LIII Ir =I'r +LIII =
The model is developed by considering the steady state power characteristic of the wind turbine. Where it is assumed that: 1) stiffness factor of the drive train is infinite and 2) friction factor and inertia of the wind turbine is combined with the generator coupled to the wind turbine. The mechanical power output of the turbine is expressed as pm
= Cp (A ' /3)
=
=
pAv� 2
(1)
The equations of the mechanical system are d 1 -OJm = (Te -FOJm -Tm) dt 2H
where, Cp is power coefficient of the turbine, p is the air density (kg/m\ A is turbine swept area (m\ VIV is wind speed (m/s), fJ is blade pitch angle CO) and A is tip speed ratio given by the equation
-
-8m =OJm d t
(3) with
0.035 A + 0.08/3 /33 + 1 1
Ai
(4)
III. INDUCTION MACHINE MODEL
Standard induction machine block from Simulink SimPowerSystem has been used, where fourth-order and second-order state space model is used for representing the electrical and mechanical system of the induction machine respectively. All the variables and parameters of the electrical system are referred to the stator side, and are indicated by the prime symbol ( ) in the machine equations, which are given as follows '
(5)
(10)
The electromagnetic torque of the machine is calculated as
Vw
(2) where, Wr is the rotor shaft speed and R is radius of the rotor blade. Cp of the wind turbine is modelled using a generic form of equation. Turbine characteristic modelling from [8] forms the basis for the following expression
(9)
d
(11) where, R" R'" L,,, L"r are stator and rotor resistance and leakage inductance. Lm is magnetizing inductance. L" L Ir are total stator and rotor inductances. Vq" iqs are q-axis stator voltage and current. V'q" i'qr q-axis rotor voltage and current. Vds. ids d-axis stator voltage and current. V'd" i'dr are d-axis rotor voltage and current. CPds. cPqs stator d and q axis fluxes. CP'd" cp'qr rotor d and q axis fluxes. Wm and 8m are mechanical rotor angular velocity and position. Wr is electrical angular velocity. Te and Tm are electromagnetic and mechanical shaft torques. J is combined rotor and load inertia coefficient. H is combined rotor and load inertia. F is combined rotor and load viscous friction. IV. DIESEL GENERATOR MODEL
The diesel generator consists of a diesel engine with the governor system and a synchronous machine driven by the diesel engine. 1.
Diesel engine
The diesel engine unit is developed with standard third order controller and actuator [9]. The governor control system included in the diesel engine unit regulates the engine speed. The existence of the non-linear, time-varying, dead time between the fuel injection and production of mechanical torque Tmech [10] makes the diesel engine a non-linear system.
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The considered diesel engine model [11] consists of: 1) a controller to check the steady-state error in speed and 2) an actuator with gain K, time constant T, and integrator altogether to control the fuel rack position. Finally the production of Tmech is represented by conversion of fuel-flow to torque after a time-delay TD. The equation used for the conversion is expressed by Tmech ( S )
= e
-s TD ¢J(S )
(12)
2. Synchronous generator
Standard synchronous machine block from Simulink SimPowerSystem has been used, where sixth-order state space model is used in representing the electrical part of the machine. The electrical system for each phase consists of a voltage source in series with RL impedance, which implements the internal impedance of the machine. Dynamics of the stator, field and damper winding are taken into consideration in the model, and the equivalent circuit is represented in the rotor reference frame (d-q frame). The equations of electrical part are given as follows
(13)
V. VECTOR CONTROL STRATEGY
1.
Maximum Power Point Tracking Control
In order to achieve maximum power extraction with variable speed operation, vector control scheme [8] is employed to control the IGBT based generator-side power converter, this in turn controls the rotational speed of the induction machine. To achieve output power control, the rotor currents are controlled in order to control the speed and torque of the IG. Keeping the reference frame as stationary, the speed control of the IG is realized on a rotating frame, where a simple PI controller with rotational speed error as input is used as the speed controller to produce q-axis stator current 1q . The d-q current components are calculated as per below equation. *
(17)
(18) The d-q current components 1/ and 1q• are passed through the park transformation block (dq to abc conversion) to get the reference current lobe for the current regulator. The output of the current regulator is used as the firing angle gate pulse signal for the converter. Power coefficient of wind turbine Cp and the generated power reaches maximum at a particular value of tip-speed ratio called optimum tip-speed ratio Aopl' Cp is a function of tip-speed ratio and pitch angle as mentioned in equation (3). In order to extract maximum power from the wind, the rotational speed of the wind turbine is controlled by tracking optimum speed of rotation, so that the wind turbine is always operated at Aopl' The controller uses OJre! as the reference speed profile calculated from at AOPI for maximum power extraction using the following equation. *
(14) Where,
The subscripts used are: 1) s for rotor and stator parameters 2) I, m for leakage and magnetizing inductance 3) f, k: field and damper winding quantity respectively. The equations for the mechanical system are expressed as
(19)
(15) m et )
=
Ll m (t ) + mo
(16)
Where, H is Inertia constant, Tm is mechanical torque, Te is electromagnetic torque, Kd is damping factor representing effect of damper windings, OJ is rotor speed and OJo is speed of operation.
Thus the controller uses the power converters to track and regulate the rotor shaft speed, so that the wind turbine is operated at maximum Cp, which in turn means achieving maximum power extraction from the available wind. Hence maximum value of Cp is achieved and maximum power is generated from the wind.
2.
DC-link and load voltage control
DC-link voltage is the voltage across the DC-link capacitor shown in Figure l. An IGBT based three phase load-Side converter included in this system plays a major role in maintaining the DC-link voltage Vdc and in turn the load voltage at constant value. A vector control scheme [8] is
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employed to regulate dc-link voltage Vde and load voltage. The input parameters to the control system are Vde, load voltage, load current and reference dc-link voltage V* de' Output of the control system is used as the input for the PWM signal generator. Generated PWM signal is used as the fIring signal for the IGBT based power electronics converter. For each sample time the Vde is measured and compared with V* dc, and the voltage controller reduces the difference between Vde and V* de till the voltage across the load is regulated to the desired voltage. VI. FREQUENCY CONTROL The power supply frequency of the HWDS supplying the isolated load is controlled using dump loads [12]. Dump loads are variable loads controlled by power electronic switches with high frequency switching action. Dump loads are modeled using eight sets of three-phase resistors connected in series with GTO thyristor switches. The nominal power of each set follows a binary progression so that the load can be varied from 0 to 446.25 kW by steps of 1.75kW. GTOs are simulated by ideal switches. The frequency control is completely modeled using Matlab/Simulink. The frequency is regulated by the discrete frequency regulator control shown in fIgure 1. A standard three
Freql
phase phase locked loop (PLL) system block is employed to measure the frequency of power system. The measured frequency is compared to the reference frequency (60 Hz) to obtain the frequency error. A phase error is obtained by integration of frequency error. This phase error is then used as the input to proportional-derivative (PD) controller to produce an output signal representing the required dump load power. This signal is converted to an 8-bit digital signal using sampling system block shown in fIgure 1 and used for controlling the switching of the eight sets three-phase dump loads. Switching action of these dump loads are performed at zero crossing of voltage to reduce fluctuations in the voltage. The reference frequency input for the control has been considered as 60 Hz. For each sample time, frequency of the system is measured and compared with reference frequency. Any difference in the frequency will actuate the required dump loads to minimize the error between the reference frequency and system frequency. The process continuous throughout the simulation to ensure the system frequency is maintained at 60 Hz. The overall hybrid wind-diesel, control strategies and isolated loads are shown in Figure 2.
l-----+-----� Samplin, gl system Pulses
VaDe
Decod'er
Refere,",ce
D i screte
Frequency
3"phase Pll
Fig.l. Simulink model of discrete frequency regulator control
TUI'bine
--- ------ ,
Load Side Converter
Generator side Converter ide
I I
:
I- - - - - - - - J l T t Tm (f)njt
ic Induction "'-....... Generator
��
-l�,; J-I----L---- IL�.) .....
,-
-:
_
WT
I
3-phase circuit breaker
............
LC Filter
- -----, Voltage : Controller I
01._-----_.1
VECTOR CO::-'-rROL
Fig.2. Schematic of hybrid wind-diesel system supplying isolated load
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ADDm oNAL LOAD
��LOAD
VII.
explained above. It is observed that the performance of the governor control system in control the power generation required is good. The DG unit responded immediately to addition of the new load close to 12 seconds. The fourth graph in the figure 3 shows the load power consumption in the dump load. The dump load is a variable load operated by controlled GTO switches. The load varies between 0 and 446.25kW. It can be observed that the dump load control switches ON the required dump load whenever the power generation is excessive. It is observed that the dump load power consumption is fast responsive to any excessive power fluctuations. This in turn regulates the system frequency. Overall the observed performance of the dump load system is good and highly responsive. Figure 4 shows the regulation of the power system frequency. The complete power system is operated at 60Hz frequency. This value is used as the reference frequency. It is observed that throughout the simulation the frequency is regulated and maintained at constant 60Hz. The performance of discrete frequency regulator control i.e. the dump load control is good. A small lag in start and near 12 seconds is attributed to the transient explained above and addition of 25 kW load respectively Figure 5 shows the speed tracking performance of MPPT control scheme. Two step changes in the reference speeds are shown in the graph and it can be observed that the MPPT control system performance is good.
SIMULATION RESULTS AND DISCUSSION
Simulation is carried out to validate the performance of the control system developed for the HWDS. The parameters used for the simulation are mentioned in the appendix. Figure 3 shows the complete power flow involved in the power system. The first graph shows the generated power output from the WTG. The power fluctuation is due to the varying wind speed and operation of MPPT scheme. The high frequency power output fluctuation from the load side power converter is smoothened by the LC filters. Although the power output from the power converters are smoothened, slight power swing distortion is observed. The second graph in Figure 3 shows the load power consumption in main load and additional. The main load is 250 kW and the additional is 25kW. The transient observed in the start of simulation is due to operation of WTG and DG units together. As in real-time HWDS, both WTG and DG are never started supplying simultaneously. Generally, either of the source supplies the load initially and depending on requirement other source is switched ON. Since the paper is focused on wind-diesel mode of operation, both the sources were assumed to in operation simultaneously, leading to a transient curve in the start of simulation. An additional load is added close to 12 seconds is shown in the same graph. The addition of this load is simulated by an automatically operated three phase circuit breaker connected to a 25kW load. The third graph in the figure 3 shows the generated power output curve of the DG unit. The transient behaviour observed in the start of the simulation is
j�bd·".· ••·• "+� ·I · · ·� j I ,,--I -1 t t" � ; i j:� EE-----I �--i-·.. ·1 + 1 j�[·· · · ·d • • • • • •;;•;;; • ! •••••�zt;;]· · �."-r== o
2
4
6
8
10
12
14
16
18
20
� :..:...!-a-�-��-� � - -_ -----.:.:.::-��!..=.:..: a _ ed_ � ___,L;------ ---------------,L- - ----- ----- ---_----__,L__---L-, � _ _ A_d d---L _ o n_a ll_ oad_ --L -1 ..- ----__ '_ _---1._ L o 2 4 6 8 10 12 18 14 16 =__}20 � o
2
4
6
o
2
4
6
_m
8
8
m _.
"
10
12
14
16
18
20
10 ti me (s)
12
14
16
18
20
Fig.3. Power generated by WTG and DG units with power consumption in main and dump loads
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1 00 ,--------,--------,---------,--------,---------,--------,---------,--,------------------------,
1-- Regulated system frequen cy
!
8
N
� c ID c.>
t
6 0 4 0 2
----------------- ------------------,----------------- ----------------- ----------------- -----------------+------------------i ----------------
o1----------------
0
h--�----�--� ''
,
,
,
,
,
,
,
,
i
i
i
i
i
i
i
i
i
, , , , , , , ----------(----------------i ------------------1,------------------t---------------1------------------r---------------------------------1-----------------1------------------j-------1-----------------i------------------1------------------;------------------;-------------------i -----------------1------------------;------------------;-------------------i ---------------'
° L-------�2 --------�4---------6 L-------� O--------�21 ---------1L --------�2 8---------1L 4 --------� 1 6--------�18 0 0
time (5)
Fig.4, Frequency regulation of the power system
- -Actual speed nCe speed �e
200 ,-------,--------,-------,--------,--------,-------,--------,--------,,--------------.
! ! ! ! ! ! r 50 1-----------------i!------------------j------------------!-----------------! ------------------:------------------j------------------! ------------------!! --I ----- Refe
I:: Fc r------------ -l.............. ; 0 0
i
i
2
'" - ,............... i
4
6
, ..............· · ·
i
i
· · · ····· · · · ···
i
time(s)
8
10
, ····· · · · ··· · · · · , · · · ···· · · · ···· · · · · , .............. " .............. -
i
12
i
14
i
16
i
18
20
Fig.5. Speed tracking performance using the MPPT control scheme
VIII.
CONCLUSIONS
[I]
The proposed control strategy operates the HWDS in wind diesel mode of operation. The maximum power point tracking scheme for tracking the optimum turbine speed has been simulated to show maximum power extraction performance for the wind turbine. Usage of dump loads with high frequency switching GTOs proved to be a good option to regulate the power frequency. Simulation results for overall control system for varying wind speed shows good performance. The power generation required was shared between the two sources and the power consumptions were equally balanced between the main load and dump loads. Pitch angle control can be added in the future work for more realistic above rated wind speed regime in handling excessive power generation.
[2] [3] [4]
[5]
[6]
[7]
APPENDIX = 0.5176, C2 = 116, C3 = 0.4, C4 = 5, 3 Cs = 21 and C6 = 0.0068. Aop{=8.1, p = l.l4 kg/m , R=3 m, Gear-ratio = 1:3
[8]
Synchronous Machine:
[10]
Turbine parameters:
Cl
Nominal Power = 300 KVA, line-to line voltage 480V, Reactances (in pu): X,F3.23, X/=0.21, X/'=0.15, Xq=2.79, Xq"=0.37, XL=0.09, Stator Resistance = 0.07 pu, inertia coefficient= 1, pole pairs =2 Induction Machine:
Nominal power = 25 kW, voltage (line line) =480V, Rs=0.087 ohm, L,/=0.8 mH, Rr =0.228, L,/= 0.8mH, Lm=34.7 mH, J = 1.662, F = 0.1, Pole pairs= 2
Main Load =250 kW, Dump Load (variable) =0 to 446.25kW Additional Load= 25kW
[9]
[II]
[12]
[13]
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