DOI / ISSN IJESC

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1 DOI / ISSN IJESC Research Article June 2015 Issue Statcom Based Controller with PV Source for a Three-Phase Seig Feeding Single Phase Loads Neeraja Ramachandran 1, Kumar R 2 PG Scholar 1, Assistant Professor 2 Department Electrical & Electronics Engineering Maharaja Prithvi Engineering College Avinashi neerajaramkallingal@gmail.com 1 Abstract: This paper presents a single phase power generation using a three-phase self-excited induction generator (SEIG) working in combination with a three-phase static synchronous compensator (STATCOM), with a photovoltaic(pv) panel based dc source and a D-Q frame theory based control algorithm. Injection of wind power into an electric grid affects the power quality. The effect of SEIG in the grid system concerning the power quality measurements are -the active power, reactive power, variation of voltage, flicker, harmonics, and electrical behavior of switching operation. Hence a STATCOM control scheme is employed to compensate the unbalanced currents caused by single-phase loads that are connected across the two terminals of three-phase SEIG. In this proposed scheme STATCOM is connected at the point of common coupling. A single-phase synchronous D-Q frame theory based control algorithm is used to generate gating pulses to three-phase STATCOM. The proposed method of single-phase power generation from the three-phase SEIG is investigated experimentally on a 3.7-kW, 230-V, Y-connected induction machine. The performance of the SEIG STATCOM system is evaluated for both linear and nonlinear single-phase loads. Furthermore, the performance of the SEIG at different terminal voltages is investigated and the terminal voltage corresponding to the maximum power output is identified. Keywords: Self-excited Induction generator (SEIG), Static Synchronous Compensaor(STATCOM), Photovoltaic solar panel(pv,) D-Q frame theory, hysteris control technique. I. INTRODUCTION Recently, renewable wind energy and solar energy is enjoying a rapid growth globally to become an important green electricity source to replace polluting and exhausting fossil fuel. To achieve sustainable growth and social progress, it is necessary to meet the energy need by utilizing the renewable energy resources like wind, solar, biomass, hydro, co-generation, etc. In sustainable energy system, energy conservation and the use of renewable source are the key paradigm. The need to integrate the renewable energy like wind, solar energy into power system is to make it possible to minimize the environmental impact on conventional plant. The power quality is an essential customer- focused measure and is greatly affected by the operation of a distribution and transmission network. When an SEIG is driven by prime movers such as wind energy biomass, biogas, and biodiesel engines, the frequency of the generated voltage is almost constant from no load to full load. But, poor voltage regulation has been the major drawback of an SEIG in its applications. A number of attempts have been made to operate a three-phase SEIG in a singlephase mode. In all these methods, the SEIG cannot be loaded up to its rated power, requires de-rating of the machine. Moreover, the SEIG currents are not balanced and unequal voltages across the generator windings are the major drawbacks of the aforementioned single-phasing methods of a three-phase SEIG Hence, the terminal voltage of an SEIG needs to be regulated during load perturbations. Several voltage regulating schemes have been reported for SEIGbased autonomous power generation systems. STATCOM based voltage regulators exhibit better dynamic performance and their voltage regulation capability would not be affected by nature of the load. The STATCOM injects compensating currents to make the SEIG currents balanced and regulates the system voltage as well. Moreover, this method offers balanced voltages across the generator windings and ensures the sinusoidal winding currents while feeding nonlinear loads. The system consists of an SEIG driven by renewable energy-based prime mover. A two-level, three-leg insulated gate bipolar transistor (IGBT)- based VSI with a self sustaining dc-bus capacitor is used as a STATCOM. The STATCOM is connected at point of common coupling (PCC) through filter inductors. Photovoltaic (PV) panel is used here to provide dc to STATCOM controller structure. This overcomes the implementation of external battery source. Batteries accumulate excess energy created PV system and store it to be used at night or when there is no other energy input. Batteries can discharge rapidly and yield more current that the charging source can produce by itself, hence system can run intermittently. II. SYSTEM CONFIGURATION The system consists of an SEIG driven by renewable energy-based prime mover. A two-level, three-leg insulated gate bipolar transistor (IGBT)-based VSI with a self sustaining dc-bus capacitor is used as a STATCOM. The STATCOM is connected at point of common coupling (PCC) through filter inductors. The STATCOM regulates the system voltage by maintaining equilibrium among the reactive power circulations within the system. Moreover, the STATCOM suppresses harmonics injected by nonlinear loads and provides load balancing while feeding single-phase loads. The unbalanced load currents in a three-phase system can be divided into two sets of balanced currents known as positive sequence components and negative sequence components

2 The system consists of an SEIG driven by renewable energy-based prime mover. The single-phase consumer loads are connected across a and c phases of the SEIG. A two-level, three-leg insulated gate bipolar transistor (IGBT)- based VSI with self sustaining dc-bus capacitor is used as STATCOM. The STATCOM regulates the system voltage by maintaining equilibrium among the reactive power circulations within the system. Moreover, the STATCOM suppresses harmonics injected by nonlinear loads and provides load balancing while feeding single-phase loads. The unbalanced load currents in a three-phase system can be divided into two sets of balanced currents known as positive sequence components and negative sequence components. In order to achieve balanced source currents, the source should be free from the negative sequence components of load currents. Therefore, when the STATCOM is connected across PCC, it supplies the negative sequence currents needed by the unbalanced load or it draws another set of negative sequence currents which are exactly 180 out of phase to those drawn by unbalanced load so as to nullify the effect of negative sequence currents of unbalanced loads. Fig.1.Schematic diagram of the SEIG STATCOM system feeding single-phase loads. A. SELF-EXCITED INDUCTION GENERATOR SOURCE An externally driven squirrel-cage induction machine with its stator terminals connected to a reactive power source (capacitance) is popularly known as a self excited induction generator. Due to the specific induction machine with its stator terminals connected to a reactive power source (capacitance) is popularly known as a self excited induction generator. Due to the specific advantages of the squirrel-cage induction machine over the conventional synchronous machine such as low cost, brush-less construction, ruggedness, cheap, and inherent short-circuit protection, SEIGs are being employed in remote and isolated power generating systems as mechanical energy conversion devices. But poor voltage regulation has been the major drawback of an SEIG in its applications. Hence, the terminal voltage of an SEIG needs to be regulated during load perturbations. Several voltage regulating schemes have been reported for SEIG-based autonomous power generation systems. STATCOM based voltage regulators exhibit better dynamic performance and their voltage regulation capability would not be affected by nature of the load. B. STATCOM BASED CONTROLLER One of the many devices under the FACTS family, a STATCOM is a regulating device which can be used to regulate the flow of reactive power in the system independent of other system parameters. STATCOM has no long term energy support on the dc side and it cannot exchange real power with the ac system. In the transmission systems, STATCOMs primarily handle only fundamental reactive power exchange and provide voltage support to buses by modulating bus voltages during dynamic disturbances in order to provide better transient characteristics, improve the transient stability margins and to damp out the system oscillations due to these disturbances. A STATCOM consists of a three phase inverter (generally a PWM inverter) using SCRs, MOSFETs or IGBTs, a PV panel based battery provides the D.C voltage for the inverter, a link reactor which links the inverter output to the a.c supply side, filter components to filter out the high frequency components due to the PWM inverter. PV based battery is used as dc source. From the d.c. a three phase voltage is generated by the inverter. This is synchronized with the a.c supply. The link inductor links this voltage to the a.c supply side. This is the basic principle of operation of STATCOM III. STATCOM CONTROLLER STRUCTURE The STATCOM control structure is based on a voltage oriented vector control applied to three-phase SEIG feeding single phase loads connected. It is a cascade control structure with inner PI current controllers in a rotating D-Q reference frame theory. Resonant controllers tuned at 100 Hz in the same positive D-Q reference frame are added to realize the negative sequence current control. Note, that the control of the negative sequence currents can also be performed in a negative rotating reference frame with PI controllers, but by using resonant controllers in a positive rotating reference frame there is no need for a sequence separation of the currents. A. PHOTOVOLTAIC POWER SOURCE In this system photovoltaic panel based battery is used as dc source for the STATCOM control structure. Batteries accumulate excess energy created by PV system and store it to be used at night or when there is no other energy input. Batteries can discharge rapidly and yield more current that the charging source can produce by itself, system can run intermittently. The ongoing effort to develop solar photovoltaic (PV) arrays as a viable long-lasting renewableenergy source, the modules themselves, and the silicon PV cells that they comprise, have attracted greatest attention. This is hardly surprising, as they are the visible part of the system, and the one where a great deal of research effort has been directed into continuously improving conversion efficiency. PV cells produce DC, but very few applications employ that DC output directly Some applications require electronic converters to process the electricity from the photovoltaic device. As it has already been mentioned, PV cells are source of direct current electrical energy. In practical applications direct current voltages are mostly up to 1000 V d.c. The values of voltage are given by the number of panels connected in series of individual PV arrays

3 In ideal case the direct current is of a constant value, and does not come through zero as opposed to alternating current. For this reason it is evident that switching off the direct current, especially that of a higher voltage, is more difficult compared to alternating current, and therefore in d.c. applications it is necessary to use special protective and switching devices, which are designed for this purpose. Where, B. D-Q FRAME THEORY BASED CONTROL ALGORITHM It is simple to design a controller for a three-phase system in synchronously rotating D-Q frame because all the timevarying signals of the system become dc quantities and timeinvariant. In case of a three-phase system, initially, the threephase voltages or currents (in abc frame) are transformed to a stationary frame (α β) and then to synchronously rotating D- Q frame. Similarly, to transform an arbitrary signal x(t) of a single-phase system into a synchronously rotating D-Q frame, initially that variable is transformed into a stationary α β frame using the single-phase p-q theory and then to a synchronously rotating D-Q frame. Therefore, to transform a signal into a stationary α β frame, at least two phases are needed. Hence, a pseudo second phase for the arbitrary signal x(t) is created by giving 90 lag to the original signal. The original signal represents the component of α-axis and 90 lag signal is the β-axis component of stationary reference frame. Therefore, an arbitrary periodic signal x(t) with a time period of T can be represented in a stationary α β frame as, x α (t)= x(t) Fig.2. Stationary α β frame and synchronously rotating D-Q frame representation of vector x(t). For a single-phase system, the concept of the stationary α β frame and synchronously rotating D-Q frames relative to an arbitrary periodic signal x(t) is illustrated in Fig. 7. The signal x(t) is represented as vector, and the vector can be decomposed into two components x α and x β. As the vector rotates around the center, its components x α and x β which are the projections on the α β axes vary in time accordingly.now, considering that there are synchronously rotating D- Q coordinates that rotate with the same angular frequency and direction as, then the position of with respect to its components and x Q is same regardless of time. Therefore, it is clear that the x D and x Q do not vary with time and only depend on the magnitude of and its relative phase with respect to the D-Q rotating frame. The angle θ is the rotating angle of the D- Q frame and it is defined as Where ω is the angular frequency of the arbitrary variable.the relationship between stationary and synchronous rotating frames can be derived from Fig. 7. The components of the arbitrary single-phase variable x(t) in the stationary reference frame are transformed into the synchronously rotating D-Q frame using the transformation matrix C as Fig.3. Block diagram STATCOM control algorithm based on single-phase synchronous D-Q theory. C. REFERENCE SOURCE CURRENTS ESTIMATION The main objective of employing a three-phase STATCOM in a three-phase SEIG-based standalone power generating system feeding single-phase consumer loads is to balance the generator currents so that the generator can be loaded to its full capacity without de-rating. The control structure of the STATCOM employs an ac voltage PI controller to regulate system voltage and a dc bus voltage PI controller to maintain the dc bus capacitor voltage constant and greater than the peak value of line voltage of PCC for successful operation of the STATCOM. The PCC voltages (v a, v b, v c ), source currents ( i sa,, i sb, i sc ), load current (i l ), and dc bus voltage (V dc ) are sensed and used as feedback signals. Considering PCC voltages as balanced and sinusoidal, the amplitude of PCC voltage (or system voltage) is estimated as

4 The sinusoidal signal filters based on a second-order generalized integrator or a sinusoidal signal integrator (SSI) can be used for creating β-axis signals which are lagging the original signals. In the present investigation, a filter based on SSI is used. The SSI filters generate quadrature signals using system frequency information. Since the system frequency fluctuates under load perturbations, a PLL is used to constantly estimate the system frequency, and the estimated frequency is fed to SSI filters which makes the proposed control adaptive to frequency fluctuations, thereby avoids the loss of synchronization of the STATCOM. Now consider a synchronously rotating D-Q frame for phase a which is rotating in the same direction as v a (t), and the projections of the load current i l (t) to the D-Q axes give the D and Q components of the load current. Therefore, the D-axis and Q- axis components of the load current in phase a are estimated as where cos θ a and sin θ a are estimated using v aα and v aβ as follows: processed through a PI controller. The dc-bus voltage error of the STATCOM V dcer at k th sampling instant is expressed as where V dcref (k) and V dc (k) are the reference and sensed dcbus voltages of the STATCOM at k th sampling instant, respectively.the output of the PI controller for maintaining the PCC voltage at the reference value in k th sampling instant is expressed as where K pa and K ia are the proportional and integral gain constants of the PI controller, V er (k) and V er (k 1) are the voltage errors at k th and (k 1) th instants, respectively. I Q (k) is the equivalent Q-axis component (or reactive power component) of the current to be supplied by the STATCOM to meet the reactive power requirements of both the load and SEIG, thereby it maintains the PCC voltage at the reference value.using the D-axis and Q-axis components of currents derived in and, the phase a, α- axis and β-axis components of the reference source current can be estimated as I la D represents the active power component of the load current as the signals belong to the same axis are multiplied and added to estimate the D-axis component, whereas I la Q represents the reactive power component of the load current as the orthogonal signals are multiplied and added to derive the Q-axis component. Similarly, the D-axis and Q-axis components of the load current in phase c are estimated as In the above matrix, the α-axis current represents the reference source current of actual phase a, and the β-axis current represents the current that is at π2 phase lag which belongs to the fictitious phase. Therefore, one can have Similarly, reference source currents for phases b and c are estimated as The negative sign of currents in (11) indicates that the load current in phase c is equal to phase a but 180 out of phase. As the single-phase load is connected across the phases a and c, D-axis and Q-axis components for phase b are not estimated. The D-axis components of the load current in phases a and c are added together to obtain an equivalent D-axis current component of total load on the SEIG as Similarly, an equivalent Q-axis current component of total load on the system is estimated as To maintain the dc-bus capacitor voltage of the STATCOM at a reference value, it is sensed and compared with the reference value and then the obtained voltage error is Three-phase reference source currents (,, and are compared with the sensed source currents and the current errors are computed as These current error signals are fed to the current-controlled PWM pulse generator for switching the IGBTs of the STATCOM. Thus, the generated PWM pulses are applied to the STATCOM to achieve sinusoidal and balanced source currents along with desired voltage regulation

5 Fig. 4.Simulation model of proposed system Fig. 5.Simulation model of dq based controller technique IV. EXPERIMENTAL IMPLEMENTATION Fig. 1 shows the STATCOM-compensated SEIG system feeding single-phase loads. A 3.7-kW, 230-V, 50-Hz, Y connected induction generator has been used to investigate the performance while feeding single-phase loads. A -connected 4-kVAR capacitor bank is connected across the SEIG terminals to provide self-excitation. A 3.7-kW, 415-V, 50-Hz, Y-connected induction motor fed from a variable frequency drive is used to realize the prime mover for the SEIG. A three-phase two-level IGBTbased VSI has been used as the STATCOM. The estimation of kva rating of the STATCOM is presented in Appendix A. The STATCOM is connected across the PCC through filter inductors Lf. Both linear and nonlinear loads are considered for testing the system. A single-phase uncontrolled diode bridge rectifier feeding a series R L load is used as a nonlinear load. The Hall effect current sensors (LEM-25 A) are used to sense the source currents of phases a and c. The current in phase b is estimated under the assumption that the sum of instantaneous currents in three phases is zero. Three voltage sensors have been used to sense the phase voltages va, vb, and the dc-bus capacitor voltage of the STATCOM (Vdc). A dspace-based digital controller (DS1104 DSP) has been used to implement the control algorithm and to generate the switching

6 pulses to the STATCOM. A fixed step sampling time of 55 μs has been used for processing the control algorithm. Detailed data of system is given in appendix A. V. SIMULATION AND ANALYSIS A prototype of the proposed SEIG STATCOM system has been developed and tested experimentally at different loads. The experimental results presented demonstrated the performance of the developed system under steady state condition. The simulation outputs are observed and compared to understand the compensation given by the STATCOM controller. A. STEADY-STATE PERFORMANCE OF SEIG- STATCOM SYSTEM FOR LINEAR LOAD Fig.6. shows the performance of the SEIG STATCOM system feeding a single-phase resistive load of 3.35 kw in the absence of STATCOM. But when STATCOM is connected the total harmonic distortion (THD) of generator currents is found less than 5% meeting IEEE-519 standard requirements on current harmonics. The single-phase load current under rated loading.the generated power (Pgen) and the power consumed by singlephase loads (Pload) are connected across the phases a and c. It has been observed that the generator is feeding a single-phase load of 3.35 kw which is almost the rated capacity of a generator without any unbalance in voltages and currents. Fig.6. Steady-state performance of SEIG-STATCOM system for linear load in the absence of STATCOM. (a) wave form of source voltage and current(b) wave form of load voltage and current. B. STEADY-STATE PERFORMANCE OF SEIG- STATCOM SYSTEM FOR NON-LINEAR LOAD The steady-state performance of the SEIG STATCOM system feeding nonlinear loads is shown in Fig.7. The wave form of SEIG source under non-linear load in shown in Fig.7.(a). The wave form at the load is shown in Fig.7(b). The dc-bus voltage and the RMS PCC line voltage are seen to be quite stable in steady state. The compensation currents injected by the STATCOM to balance the source currents are shown in Fig.9., it is observed that the SEIG currents are balanced and sinusoidal which demonstrates the harmonics suppression and load balancing capabilities of the STATCOM. (a) (a) (b) (b) Fig.7. Steady-state performance of SEIG-STATCOM system for nonlinear load. (wave form of source). (b) wave form of load

7 TABLE I PERFORMANCE OF THE SEIG AT DIFFERENT TERMINAL VOLTAGES AND RATED WINDING CURRENT Fig.8. Wave form of STATCOM current in SEIG-STATCOM system for single-phase load. C. PERFORMANCE OF THE SEIG AT DIFFERENT TERMINAL VOLTAGES In Fig.8. it can be observed that the SEIG terminal voltage is maintained at 220 V which is slightly lower than the rated voltage of the SEIG. The reason for maintaining a slightly lower terminal voltage is to operate the SEIG close to optimum voltage point. An optimum voltage is the allowable terminal voltage of the SEIG at which the SEIG is able to generate power more than the rated capacity without exceeding the rated winding current. An experimental set up is arranged and performance of SEIG-STATCOM system at different terminal voltages are analyzed from the this set up. The STATCOM provides the compensation needed for the single-phase load. The output wave form of STATCOM is given in Fig. 8. The details of the experimental set up is given in Appendix A The generator output power and load power at the rated voltage (which is 230 V) are found to be less when compared to those at 220 V and the rated currents incase of linear load. This condition can be explained as follows; the current carried by the SEIG winding at any instant can be divided into reactive power and active power components. The reactive power component of the current is responsible for the generator terminal voltage, and active power component represents the active power delivered. For a constant generator current, if the reactive power component decreases, the active power component can be increased. Any reduction in the reactive power component of the current can be utilized in increasing the active power component keeping the generator current constant. VI. CONCLUSION The proposed method of feeding single-phase loads from a three-phase SEIG and STATCOM combination has been tested, and it has been proved that the SEIG is able to feed single phase loads up to its rated capacity. A single-phase synchronous D-Q frame theory-based control of a three-phase STATCOM has been proposed, discussed, and experimentally implemented for current balancing of the SEIG system.the performance of the SEIG at different voltages has been investigated experimentally to identify the terminal voltage corresponding the maximum power output. It has been observed that when the SEIG is operated at 200 V instead of the rated voltage, the generator is able to deliver 3.91 kw without exceeding the rated winding current. The satisfactory performance demonstrated by the developed the STATCOM SEIG combination promises a potential application for isolated power generation using renewable energy sources in remote areas with improved power quality. APPENDIX A SYSTEM PARAMETERS 1) Parameters of 3.7-kW, 230-V, 50-Hz, Y-Connected, Four- Pole Induction Machine Used as the SEIG Rs = 0.39 Ω, Rr = 0.47 Ω, Lls = H, Llr = H, Lm = H at the rated voltage. 2) STATCOM Parameters: Three-Leg IGBT VSI, Lf = 3 mh, Rf = 0.1Ω, and Cdc = 1650 μf. ac voltage PI controller: Kpa = 0.2, Kia = 0.3 dc voltage PI controller: Kpd = 1, Kid = ) Load Parameters: A single-phase resistive load of a resistance 14.5 Ω connected across phases a and c is used as linear load. Nonlinear loads: Single-phase bridge rectifier feeding R L load are used as nonlinear loads. R = 14Ω, L = 250 mh

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