CHAPTER 3 MODELLING OF PV SOLAR FARM AS STATCOM

Transcription

1 47 CHAPTER 3 MODELLING OF PV SOLAR FARM AS STATCOM 3.1 INTRODUCTION Today, we are mostly dependent on non renewable energy that have been and will continue to be a major cause of pollution and other environmental degradation. Finding the sustainable alternative is becoming increasingly urgent because of these problems and the dwindling supply of petroleum. Perhaps, the greatest challenge is in devising a sustainable future, which relies on integration and control of renewable energy sources in grid distributed generation. This chapter presents the modelling of current and voltage module of PV arrays and their characteristics. The formation of PV array using V PV module is also explained. Also the basics of STATCOM along with the controller are also presented. The modelling of PV array as STATCOM along with the MPPT algorithm for constant and variable insolation values are carried out using MATLAB SIMULINK. 3.2 PV CELL PV cell is very similar to that of the classical diode with a PN junction. In figure 3.1, when the junction absorbs light, the energy of absorbed photons is transferred to the electron proton system of the material, creating charge carriers that are separated at the junction. The charge carriers may be electron ion pairs in a liquid electrolyte or electron hole pairs in a solid semiconducting material. The charge carriers in the junction region create a potential gradient, get accelerated under the electric field and circulate as current through an external circuit. The square of the current multiplied by the resistance of the circuit is the power converted into electricity. The remaining

2 48 power of the photon elevates the temperature of the cell and dissipates into the surroundings. Figure 3.1 PV effect converts the photon energy into voltage across the PN junction PV CELL TECHNOLOGIES In comparing alternative power generation technologies, the most important measure is the energy cost per kilowatt hour delivered. In PV power, this cost primarily depends on two parameters: the PV energy conversion efficiency and the capital cost per watt capacity. Together, these two parameters indicate the economic competitiveness of the PV electricity [33]. The conversion efficiency of the PV cell is defined as follows electrical power output η= solar power impinging cell The primary goals of PV cell research and development are to improve the conversion efficiency and other performance parameters to reduce the cost of commercial solar cells and modules. The secondary goal is to significantly

3 49 improve manufacturing yields while reducing the energy consumption and manufacturing costs and reducing the impurities and defects. This is achieved by improving our fundamental understanding of the basic physics of PV cells. The continuing development efforts to produce more efficient low-cost cells have resulted in various types of PV technologies available in the market today in terms of the conversion efficiency and the module cost. Types of PV Cells Single crystalline silicon Polycrystalline and semi crystalline silicon Thin film cell Amorphous silicon Spherical cell Concentrator cell Multi junction cell MODULE AND ARRAY The solar cell described in the preceding subsection is the basic building block of the PV power system. Typically, it is a few square inches in size and produces about 1 W of power. To obtain high power, numerous such cells are connected in series and parallel circuits on a panel (module) area of several square feet. The solar array or panel is defined as a group of several modules electrically connected in a series parallel combination to generate the required current and voltage as shown in figure 3.2 [34].

4 50 Figure 3.2 PV cells, Module and Array 3.3 ELECTRICAL EQUIVALENT CIRCUIT PV cell can be represented by the equivalent electrical circuit shown in figure 3.3. [41]. The circuit parameters are as follows. R S - Internal series resistance R P - Shunt resistance of the diode R L Load Resistance I SC Source current I D Current through Diode V D Voltage across Diode I PV output current of PV cell V PV Output voltage of PV cell

5 51 Figure 3.3 Equivalent circuit of a solar cell Applying KCL, (3.1) The voltage of the cell V PV is given by the following by applying KVL, (3.2) The diode current is given by the expression (3.3) 3.4 MODELLING OF PV ARRAY The two models of PV module are current input (I PV module) and voltage input module (V PV module). The I PV module is well suited for the case when modules are connected in series and share the same current and V PV module is well suited for the case when modules are connected in parallel and share the same voltage.

6 I PV Module Figure 3.4 I PV Module The MATLAB SIMULINK circuit of the I PV module is given by Figure 3.5 Circuits inside the Current I PV Model V PV Module Figure 3.6 V PV Module

7 53 1 Vpv 2 Insolation Ipv Vpv Insolation Ppv PV module (I) Solve f (z) z f(z) = 0 Algebraic Constraint 1 Ipv 2 Ppv Figure 3.7 Circuit inside the Voltage V PV Module I-V AND P-V CURVES The electrical characteristic of the PV cell is generally represented by the current vs. voltage curve. The MATLAB SIMULINK Simulation diagram for obtaining the electrical characteristics of PV cell is shown in figure 3.8. Figure 3.9 shows the I-V characteristic of a PV module under different values of Insolation. (200, 400, 600, 800, 1000 W/m 2 ) Vpv Vpv Vpv PV power Ipv I-V characteristic PV module (V) Vpv Ipv Insolation Ppv Insolation PV1 Figure 3.8 SIMULINK Circuit for PV Module characteristics

8 54 Figure 3.9 Current vs. Voltage characteristic of the PV module for different insolation levels The power output of the panel is the product of the voltage and current outputs. Figure 3.10, shows the P-V characteristic of a PV module under different values of Insolation. (200, 400, 600, 800, 1000 W/m 2 ) Figure 3.10 Power vs. Voltage characteristic of the PV module

9 MAXIMUM POWER POINT TRACKING A controller that tracks the maximum power point locus of the PV array is known as the Maximum Power Point Tracking (MPPT). The points of maximum array power form a curve is termed as the maximum power locus. Due to high cost of solar cells, it is necessary to operate the PV array at its maximum power point. Several MPPT algorithms have been proposed from time-to-time. Some of the popular schemes are the hill climbing method, incremental conductance method, constant voltage method, modified hill climbing method, β method, system oscillation method and the ripple correlation method, perturb and observe method, open and short circuit method, fuzzy logic and artificial neural network [40] Perturb and Observe Method The perturb and observe method, also known as perturbation method, which is the most commonly used MPPT algorithm in commercial PV products. This is essentially a trial and error method. The PV controller increases the reference for the inverter output power by a small amount and then detects the actual output power. If the output power is indeed increased, it will increase again until the output power starts to decrease, at which the controller decreases the reference to avoid collapse of the PV output due to the highly non-linear PV characteristic [39].

10 56 Figure Perturb and Observe Method flow chart Figure 3.11 shows the flow chart of the P&O method. The present power P(k) is calculated with the present values of PV voltage V(k) and current I(k), and is compared with the previous power P(k-1). If the incremented power increases, keep the next voltage change in the same direction as the previous change. Otherwise, change the voltage in the opposite direction as the previous one. [40] Incremental Conductance Algorithm In the incremental conductance method, the MPP is tracked by matching the PV array impedance with the effective impedance of the converter reflected across the array terminals. The latter is tuned by suitably increasing or decreasing the value of M. [39].

11 57 Figure Incremental Conductance Algorithm flow chart The main task of the incremental conductance algorithm is to find the derivative of PV output power with respect to its output voltage, which is dp/dv. The maximum PV output power can be achieved when its dp/dv approaches zero. The controller calculates dp/dv based on measured PV incremental output power and voltage. If dp/dv is not close zero, the controller will adjust the PV voltage step by step until dp/dv approaches zero, at which the PV array reaches its maximum output. The main advantage of this algorithm over the P&O method is its fast power tracking process. However, it has the disadvantage of possible output instability due to the use of derivative algorithm. Also the differentiation

12 58 process under low levels of insolation becomes difficult and results are unsatisfactory. [42] β method The other method is based on β tracking which has the advantage of both fast and accurate tracking. It is observed that the value of β remains within a narrow band as the array operating point approaches the MPP. Therefore by tracking β, the operating point can be quickly driven to close proximity of the MPP using large iterative steps. Subsequently, small steps (i.e. conventional MPPT techniques) can be employed to achieve the exact MPP. Thus, β method approximates the MPP while conventional MPPT technique is used to track the exact MPP. Flow chart for the β method algorithm is given in figure [42] Figure β method flow chart

13 Open and Short circuit Method The open and short circuit current method for MPPT control is based on measured terminal voltage and current of PV arrays. By measuring the open-circuit voltage or short-circuit current in real-time, the maximum power point of the PV array can be estimated with the predefined PV current-voltage curves. This method features a relatively fast response and do not cause oscillations in steady state. However, this method cannot always produce the maximum power available from PV arrays due to the use of the predefined PV curves that often cannot effectively reflect the real-time situation due to PV nonlinear characteristics and weather conditions. Also, the online measurement of open-circuit voltage or short-circuit current causes a reduction in output. [40] Fuzzy Logic and Other Algorithms Since the PV array exhibits a non-linear current-voltage or powervoltage characteristic, its maximum power point varies with the insolation and temperature. Some algorithms such as fuzzy logic or artificial neural network control with nonlinear and adaptive in nature fit the PV control. By knowledge based fuzzy rules, fuzzy control can track maximum power point. A neural network control operates like a black box model, requiring no detailed information about the PV system. After learning relation between maximum power point voltage and open circuit voltage or insolation and temperature, the neural network control can track the maximum powerpoint online. [40]. In the proposed MPPT algorithm, the following conditions are considered It is assumed that the Boost output voltage V out = V DC is constant I ref is used as the control variable for the Boost DC-DC converter

14 60 PV array current ideally tracks the Boost input current reference: I PV =I ref The Perturb and observe algorithm is applied in this method where the value of I pv i s adjusted to I ref to operate at MPP. The flowchart for the perturb and observe method applied is given in figure Figure 3.14 Flowchart for Perturb and Observe Algorithm MPP Tracking Operation For a six module PV of 85 Watts each connected in series at full sun the maximum power of W is achieved as shown in the figure 3.15 and the MPP tracking results are shown in the figure.3.16.

15 61 Figure 3.15 MPP for Six 85W module connected in series Figure 3.16 MPP tracking operation

16 FORMATION OF PV ARRAY The PV array is formed by connecting six I PV modules, in series as shown in the figure Figure 3.17 Formation of PV Array 3.7 PRINCIPLE AND OPERATION OF STATCOM STATCOM is a shunt-connected reactive-power compensation device that is capable of generating and absorbing reactive power and in which the output can be varied to control the specific parameters of an electric power system. It is in general a solid-state switching converter capable of generating or absorbing independently controllable real and reactive power at its output terminals when it is fed from an energy source or energy-storage device at its input terminals. Specifically, the STATCOM considered in this chapter is a voltage-source converter that, from a given input of dc voltage, produces a set of 3-phase ac-output voltages, each in phase with and coupled to the

17 63 corresponding ac system voltage through a relatively small reactance (which is provided by either an interface reactor or the leakage inductance of a coupling transformer) [7]. To summarize, a STATCOM controller provides voltage support by generating or absorbing reactive power at the point of common coupling without the need of large external reactors or capacitor banks The Principle of Operation A STATCOM is a controlled reactive-power source. It provides the desired reactive-power generation and absorption entirely by means of electronic processing of the voltage and current waveforms in a voltagesource converter (VSC). A single-line STATCOM power circuit is shown in figure (a), where a VSC is connected to a utility bus through magnetic coupling. In figure3.18 (b), a STATCOM is seen as an adjustable voltage source behind a reactance meaning that capacitor banks and shunt reactors are not needed for reactive-power generation and absorption, thereby giving a STATCOM a compact design, or small footprint, as well as low noise and low magnetic impact. The exchange of reactive power between the converter and the ac system can be controlled by varying the amplitude of the 3-phase output voltage, Es, of the converter, as illustrated in figure3.18 (c). That is, if the amplitude of the output voltage is increased above that of the utility bus voltage, Et, then a current flows through the reactance from the converter to the ac system and the converter generates capacitive-reactive power for the ac system. If the amplitude of the output voltage is decreased below the utility bus voltage, then the current flows from the ac system to the converter and the converter absorbs inductive-reactive power from the ac system. If the output voltage equals the ac system voltage, the reactive-power exchange becomes zero, in which case the STATCOM is said to be in a floating state. Adjusting the phase shift between the converter-output voltage and the ac system

18 64 voltage can similarly control real-power exchange between the converter and the ac system. Figure 3.18 The STATCOM principle diagram: (a) a power circuit; (b) an equivalent circuit; and (c) a power exchange The reactive- and real-power exchange between the STATCOM and the ac system can be controlled independently of each other. Any combination of real power generation or absorption with var generation or absorption is achievable if the STATCOM is equipped with an energy-storage device of suitable capacity, as depicted in figure With this capability, extremely effective control strategies for the modulation of reactive and real output power can be devised to improve the transient and dynamic system stability limits.

19 65 A STATCOM can improve power-system performance in the following areas: 1. dynamic voltage control in transmission and distribution systems 2. power-oscillation damping in power-transmission systems 3. transient stability 4. voltage flicker control 5. control of not only reactive power but also (if needed) active power in the connected line, requiring a dc energy source. Figure 3.19 The power exchange between the STATCOM and the ac system Figure 3.20 An elementary 6-pulse VSC STATCOM

20 66 An elementary 6-pulse VSC STATCOM is shown in figure 3.20, consisting of six self-commutated semiconductor switches (IGBT, IGCT, or GTO) with antiparallel diodes. In this converter configuration, IGBTs constitute the switching devices. With a dc-voltage source (which may be a charged capacitor), the converter can produce a balanced set of three quasisquare voltage waveforms of a given frequency by connecting the dc source sequentially to the three output terminals via the appropriate converter switches. 3.8 CONVENTIONAL STATCOM AND SOLAR FARM PV ARRAY BASED STATCOM There has been many compensating devices performing reactive power compensation, voltage regulation, etc. but device that has the structural advantage is necessary for the system assumed. STATCOM proves to have the structural advantage to act as the compensating device for the assumed system. PV array and the inverter setup are analogous to the design of conventional STATCOM. From figure 3.22 it is clear to understand how PV array setup can be utilized as STATCOM. Figure 3.21 conventional STATCOM VS PV based STATCOM

21 67 A structural advantage that helps in utilization of PV array as STATCOM is PV array output is dc voltage which is used as capacitor as in conventional STATCOM. Also, inverter design is made to operate as converter in the PV array arrangement. So PV array along with inverter is been conveniently used as STATCOM for the assumed system. 3.9 MODES OF OPERATION OF PV BASED STATCOM The operation of the proposed STATCOM has been divided into three modes. The modes are (i) day time excess power mode, (ii) day time mode, (iii) night time mode. i. Day time excess power mode: In this mode, the output voltage of the PV array drives the boost converter based STATCOM for compensating the source as well as charges the battery. ii. Day time mode: When continuous compensation is required, if the PV output voltage is equal to the requirement of the boost converter input, the PV array can directly connect to the boost converter so as to stepup the voltage and match the dc link voltage of the three-leg VSC. In this mode, the battery is not charged. iii. Night time mode: In this mode, PV output is absent and only the battery supplies the boost converter for providing compensation at the night time CONTROL OF DC LINK VOLTAGE WITH BOOST CONVERTER The boost converter is used to step up the input voltage to obtain a desired output voltage. The circuit operation is divided into two modes. In mode 1, when the switch is in on condition, the input current supplies energy

22 68 to the inductor for a period T on. Similarly in mode 2, when the switch is off, the inductor voltage adds to the source voltage and current is forced to flow through diode and the load for a period T off. The PV or battery voltage is fed to the boost converter and the output voltage of the boost converter is obtained to maintain the dc link voltage of the three-leg voltage source converter. The output voltage, V out is greater than the input Voltage V in and the output equation is shown in the following equation. (3.4) Where V out =V dc, V in =V (3.5) where V is the PV or battery voltage, D is the duty cycle, T on is the ON time and T off is the OFF time CONTROL OF PV BASED STATCOM There are many control algorithms available for the generation of reference source currents for the control of proposed STATCOM in the literature such as, synchronous reference frame theory, instantaneous reactive power theory (p q theory), power balance theory etc.[44],[47],[71]. The synchronous reference frame theory is found suitable for the control of VSC. A block diagram of the controlling algorithm is shown in figure The feedback signals are sensed from the load currents, PCC voltages and dc bus voltages of STATCOM. The load currents from the a b c frame are first converted to a b 0 frame and then to d q 0 frame using the following equation,

23 69 (3.6) A three phase PLL (phase locked loop) is used to synchronize these signals with the PCC voltage. The dc component of i d and i q are obtained by passing a d q 0 current component through the low pass filter. The input of first PI (Proportional Integral) controller is the error between the reference dc bus voltage (V dc *) and the sensed dc bus voltage (V dc ) of STATCOM. The output of PI controller is the loss component of the current (i loss ). (3.7) where V de (n) is the error between reference and sensed dc voltage at the n th sampling instant. K pd and K id are the proportional and integral gains of the DC bus voltage PI controller. Therefore the reference source current is, (3.8) Similarly, the amplitude of actual PCC voltage and its reference value are fed to another PI controller for regulating the PCC voltage. The output of the PI controller is added to the dc component of i q because this is a quadrature component of current required for regulating the ac voltage. (3.9) where V de (n) is the error between reference (V s *) and sensed supply voltage (V s (n)) amplitude at the n th sampling instant. The proportional and integral gains of the PCC voltage PI controller are K pq and K iq. The reference supply quadrature axis current is,

24 70 (3.10) By using reverse Park s transformation, the resultant d q 0 currents are again converted back to reference source currents. The reference currents in all the three phases (i sa *, i sb *,i sc *) are used for generating the gate pulses for three-leg VSC based STACOM. A PWM current controller is used for generating the gating signals for the IGBT s in VSC by using the reference and sensed source currents. Figure 3.22 Control Algorithm of STATCOM

25 SIMULATION OF PV ARRAY SYSTEM In this case, the model parameters are the standard PV module datasheet parameters: short-circuit current I sc open-circuit voltage V oc rated current I R at maximum power point (MPP) rated voltage V R at MPP Under standard test conditions (1kW/m 2, 1.5 AM, 25 o C). A bypass diode (a single diode across the entire module) can be included. Temperature effects are not modelled. PV array consisting of 6 PV modules connected in series. PV array is operated at the maximum power point (MPP) under all conditions. V pv, I pv is set as operating point for MPP. 6-module (85 W each) PV array with full sun (1,000 W/m 2 insolation). PV array operates at MPP: P pv = 6 * 85 W = 510 W. The DC output is then fed to boost converter and then to DC-AC converter for obtaining the required AC output voltage V inv

26 PV Array Simulation for Constant Insolation Figure 3.23 PV array system combined with DC-DC boost convertor and DC-AC Inverter for constant Insolation Figure 3.24 Simulink Block of Constant insolation

27 73 The PV array combined with DC-DC boost convertor and DC-AC inverter for a constant insolation value of 1000 kw/m 2 is simulated and the output voltage, output current, input and output power of inverter is obtained. The output voltage from the inverter gives a constant value of 158 W for a constant insolation as shown in the figure Figure 3.25 Output Voltage of Inverter for constant insolation Figure 3.26 Output current from inverter for constant insolation

28 74 The output current is also a constant value of 5.8 A for a constant insolation value as shown in the figure Similarly the various outputs such as Inverter duty and input and output power of a PV array is also a constant value for the constant insolation as shown in figure 3.27 and 3.28 respectively. Figure 3.27 Inverter Duty cycle for constant insolation Figure 3.28 Input and Output power for constant insolation

29 PV Array Simulation for Variable Insolation Figure 3.29 PV array system combined with DC-DC boost convertor and DC-AC Inverter for variable Insolation The PV array combined with DC-DC boost convertor and DC-AC inverter for a variable insolation value as shown in figure 3.29 is simulated and the output voltage, output current, input and output power of inverter is obtained. The insolation values chosen here are 0, 400, 850, 950, 1000, 950, 850, 400, 0 kw/m 2 for a time duration 0, 60, 120, 180, 240, 300, 360, 420, 480 seconds respectively as shown in figure 3.30.

30 76 The simulation outputs of voltage, current, input and output power is shown in the figure 3.31, 3.32, 3.33, 3.34 and 3.35 respectively. The values of all the parameters are varying with respect to the insolation values. Figure 3.30 Simulink block for variable insolation Figure 3.31Output Voltage of Inverter for Variable insolation

31 77 Figure 3.32Output current of Inverter for Variable insolation Figure 3.33 Efficiency of Inverter for Variable insolation

32 78 Figure 3.34 Duty cycle of Inverter for Variable insolation Figure 3.35 Ideal, input and output power of Inverter for Variable insolation

33 79 Thus the simulation results shows that the PV array responds to the different insoltaion values by giving the corresponding variations in the outputs. Hence this model can be used for different insolation values SUMMARY A new concept of using a PV solar power plant as STATCOM is introduced here. A MATLAB/SIMULINK based model of PV array is discussed in this chapter. The Utilization of the PV array and the converter as a STATCOM device is explained with simulations. Various MPPT algorithms are introduced and perturb and observe method is utilized. The simulation results show the various outputs of PV array for different insolation levels. This newly developed system thus can act as a FACTS device providing a flexible control over both active and reactive power on a transmission line. The PV based STATCOM can be implemented during night hours when PV solar plant produces no real power. The configuration can possibly be realized during daytime hours too. The PV based STATCOM can be used to regulate the transmission/distribution line voltages, to support inductive load VAR requirements, to improve the system performance during dynamic disturbances and to suppress harmonics.