IMPROVED POWER CONVERTER FOR DISTRIBUTED PHOTOVOLTAIC GENERATION SYSTEM

Abstract

Solar power is one of the most popular form of renewable energy which can be easily used in the distributed generation (DG) system through the use of photovoltaic (PV) module. A Photovoltaic Distributed Generation (PVDG) system is well suited for clean power generation. However, the power generated by the PV module is still considered to be expensive and the spread of the PVDG system is subject to pace of its cost reduction. A lot of research work has been carried out to obtain an efficient and cheaper PV module based on new cell materials and manufacturing technologies. Moreover, an alternative way to reduce the overall cost of PVDG system is by increasing the effectiveness of the whole system using an improved power conditioning system and also by introducing an improved power quality features in its control system. The primary goal of a power conditioning unit in PVDG system is to increase the energy injection to the grid by optimizing the energy extraction process from the PV module by keeping track of maximum power point (MPP), by operating at a lower switching frequency thereby reducing the switching losses. In addition to that the price of the power converter also plays a vital role in developing a cost effective PVDG system. The main focus of this research is to optimally control the power conditioning unit to improve the overall performance of the single phase PVDG system. These power conditioning units are used to regulate the voltage and current for the power flow during gridconnected operation, and also for the maximum power extraction. In order to analyze the power converters for PVDG system, it is essential to model a PV module attached to the converter. PV module presents a nonlinear I V characteristic with several parameters to be adjusted according to experimental data obtained from the datasheet of a practical device. For the dynamic analysis of the converters, and maximum power point tracking (MPPT) algorithms mathematical modelling of the PV module is useful. The first part of this research work presents the modelling and simulation of PV module. However, the major challenges in the modelling a PV module is that, all the parameters which are required for the modelling purpose are not available on the manufacturer’s datasheet. Here the determination of parameters of the PV module is performed by using Gauss Seidel iteration Method and the modelling is carried out using all the known and the determined parameters.. At present the most commonly used converter topology for PVDG system is the two level multistring inverter. This inverter consists of several PV strings that are connected with DCDC converters to form a common DC-bus. This topology has advantages of independent tracking of the MPP of each string. However, the major challenges in such type of system are, low efficiency of PV module and high implementation cost, thus it becomes necessary to use these systems more optimally. For this, a fuzzy-logic based MPPT controller is proposed. ii Fuzzy-logic based MPPT technique offers fast and accurate converging to the MPP during steady-state and varying weather conditions compared to the conventional perturb and observe based MPPT method. The synchronization of PV inverter with the grid is made with the help of a phase locked loop (PLL). The main task of the PLL is to provide a unity power factor operation which includes synchronization of the inverter output current with the grid voltage. There has been an increasing interest in PLL topologies for DG system. It is a grid voltage phase detection structure which requires an orthogonal voltage system. In the single-phase PLL, accurate and fast phase estimation can be obtained by processing a signal in phase with the grid voltage (original signal) and another one which is 900 phase shifted from it. The PLL that generates the orthogonal signal by delaying the original signal is called a transport-delay based PLL(TDPLL). This type of PLL is simple and its transient response is fast and smooth among all available PLL methods. The other methods for generating orthogonal voltage are Hilbert transformation, Park transformation, etc. All these methods have some shortcoming such as high complexity, nonlinearity and have slower response than TDPLL. The main drawback of a conventional TDPLL is its sensitivity to the grid frequency changes, since the delay is determined assuming constant frequency. Here, a modified TDPLL is presented which uses two delay blocks to make TDPLL robust against frequency variation. In a PVDG system, the demand of local load is fulfilled by the combined action of PV inverter and grid, based on the atmospheric and load conditions. During favourable atmospheric condition (or light load condition), the local load demand is supplied by the PV inverter and surplus power is fed to the grid. Similarly, during the unfavourable atmospheric condition or overload condition, both PV inverter and the grid jointly meet the local load demand. Thus, in both the aforementioned conditions the quality of the grid current is decided by the load. In present day distribution systems, major power consumption has been in reactive/non-linear loads, such as motor drives, fans, pumps, and power electronic converters. These types of loads draw non-sinusoidal currents from the generation system. The non-sinusoidal current is comprised of reactive and harmonic components in addition to the active component. Excessive reactive component of current results into low power factor, poor voltage regulation and reduction in active power capability of the distribution system. Therefore, the electric power quality (PQ) has become an issue of concern and extensive research is being done to improve the power quality. On the other hand, the PV distributed system produces an amount of active power and injects it into the distribution system. This active power is influenced by atmospheric condition, which causes voltage fluctuations at PCC, because it changes independently of load power demand. This makes voltage regulation in the distribution lines more difficult. On the other hand, harmonic regulations or guidelines such as IEEE 1547 and IEC 61727 are applied to limit the current and voltage harmonic levels. To meet these requirements, these harmonics must be mitigated by using iii harmonic filters. Active and passive filters are used either together to form hybrid filters or separately to mitigate harmonics. Recently, many researchers have devoted efforts to develop a PV inverter with real power injection with improved PQ features such as compensation of reactive power and local load current harmonics. One of the possible ways of the PQ enhancement scheme in a DG system is by interfacing an external shunt active power filter (APF) at the PCC. Here the PV inverter is only used for the active power flow in PVDG system and shunt APF is responsible for the PQ enhancement in PVDG system. However, the main drawback of such PVDG system is the high cost and underutilization of hardware circuits. Therefore, to eliminate the complexity and underutilization of the above mentioned topology of PVDG system, an enhanced power quality based PVDG system with an integrated shunt APF mechanism is presented. The main advantage of this topology is that it doesn’t need any additional hardware circuit for reactive power and load current harmonic compensation. With this approach, the PV inverter regulates the active power flow between the PV module and the grid. In addition to this, the system also carries out the compensation of reactive power and load current harmonics thereby making the grid current sinusoidal. Moreover, one of the major drawbacks of the conventional grid-tie PV inverter is that, it generates real power only during the daytime, with completely in idle state during night-time. This leads to further underutilization of such an expensive system in the nighttime. To address the issue of underutilization in PVDG system, the PV inverter is utilized as a shunt APF during night time. The main goal of this research work is to formulate and implement the 1-ϕ instantaneous reactive power theory in the PVDG system for the enhancement of PQ to obtain the following objectives, • To transmit the maximum possible real power from the PV module by using the MPPT controller • To meet the real power demanded by the local load • To compensate the reactive and harmonic components of load current at PCC • To utilize the PV inverter as a shunt APF during night time. Thus, with an adequate control of PV inverter, all the above-mentioned objectives have been achieved either separately or simultaneously. Hence, in this research work the PQ requirements as per the utility standards at the PCC are accomplished without requiring any additional hardware circuits. For the further improvement of PQ and also to increase the overall power transmission capacity of the two-stage PVDG system a multilevel inverter topology is introduced in place of the two-level inverter. In recent years multilevel inverter topologies have become more attractive for researchers as they offer improved output waveform as compared to the conventional two level inverter. With the improvement in the output waveform, the harmonic content and hence the size of the filter can be reduced. Among all these available MLI topologies, the cascaded H-bridge MLI (CHBMLI) requires separate DC sources and hence constitutes a promising alternative, providing a modular iv design that can be extended to allow a transformer less operation in grid connected PV system. The power flow control in such type of PVDG system requires two control loops. The inner current control loop is used to modulate the output current of the PV CHBMLI to meet magnitude and phases of waveform whereas the outer voltage control loop regulates the output power of the PV inverter according to the MPP of PV modules. These two control loops are realized by two stages of power conversion. One is a DC to DC converter with MPPT control and the other is a DC to AC inverter. But two stage operation may lead to more power loss than that of a single-stage conversion. In a single stage PVDG system, both the control loops are realized simultaneously in one power conversion stage, thus simplifying the system topology and hence decreases the overall power loss in the system. However, there is always a chance of imbalance in the input DC-link voltages of CHBMLI when fed from PV module. This is due to the non-ideal conditions in PV modules. The most common non-ideal conditions of a PV module include partial shading, dust collection and PV ageing. Hence the balancing of the DC voltages is one of the important issues in the control of CHBMLI when used in PV application. If this voltage balance is not perfectly accomplished, the modulation methods create errors in the modulated output voltage. This leads to the distortion in the output voltages and currents of the CHBMLI. To overcome the DC-link voltage error of CHBMLI, a single phase space vector modulation (SVM) scheme for the CHBMLI is proposed and implemented. In order to verify the proposed control approach for CHBMLI based two-stage and single stage enhanced PQ based PVDG system an extensive simulation is carried out using MATLAB/Simulink environment. Both two-stage and single-stage PVDG system is actively controlled to achieve sinusoidal grid current at unity displacement factor in spite of highly non-linear load connected at PCC under varying atmospheric and load conditions. The MPPT control of the two-stage PVDG system is accomplished by DC-DC converter, whereas in single-stage PVDG system, it is achieved by each H-bridge cell of CHBMLI. Finally, for the power flow control in PVDG system, the reference compensating current is derived from the DC-link voltage controller and the PQ enhancement controller. A non-linear load consisting of an uncontrolled rectifier and a RL element on the DC-side, have been taken for the analysis of the system. The system is validated for different modes of controller action, varying atmospheric and load conditions. Finally, based on the simulation results a comparative analysis is made between two-level and five level CHBMLI based PVDG system with singlestage CHBMLI based topology. In single-stage topology, the need of DC-DC converter for MPPT is eliminated. This leads to more power injection to the grid as compared to two-stage system. To verify the simulations of 1-ϕ, 230 V, two-stage and single stage PVDG system, the following prototypes have been developed in the laboratory: (i) Enhanced PQ based Single-phase two-stage 2-level inverter in PVDG System v (ii) Enhanced PQ based Single-phase two-stage 5-level CHBMLI in PVDG System (iii) Enhanced PQ based Single-phase single-stage CHBMLI in PVDG System For the hardware development, two 128 W PV modules manufactured by Maharishi Solar, India are used. A 1-ɸ downscaled topologies has been designed and constructed to realize the above mentioned PVDG topologies. In case of two-stage topology, the outputs of two DC-DC converters are connected in a cascaded manner to have a single DC-link for the twolevel inverter. However, in case of 5-level CHBMLI, each DC-DC converter output is directly connected across two individual H-bridge cells. Finally, the developed PV inverter output is connected to the PCC with a series connected coupling inductor. A 35.8V (50 V peak), 50 Hz grid is developed in the laboratory by using a step down transformer and is interfaced with the PV inverter through an isolation transformer. All the rquired controllers for the PVDG system are implemented in dSPACE. The performance of the above mentioned single phase two stage enhanced PQ based PVDG system is investigated for following modes of operations (i) under different modes of power quality enhancement controller (PQEC) (ii) under different modes of MPPT Controller (iii) under varying load condition. With the integrated PQ enhancement scheme, in PV inverter, the grid current has been observed to be sinusoidal and its corresponding THDs have also been found to be well within the limits of IEEE 1547 and IEC 61727 recommended value of 5%. A smooth control of DC voltages ensures the effectiveness of the DC voltage controller. Further, the experimental results of the capacitor voltage balancing of the H-bridge cells in CHBMLI have been studied. The experimental results have been found to be in good agreement with the simulation results.To verify the viability and effectiveness of the enhanced PQ based single stage PVDG system for power flow operation, harmonic elimination and reactive compensation, experimental investigations have been conducted with non-linear loads. Here, the maximum power extraction from each PV module is accomplished by PV CHBMLI itself. As in case of singlestage CHBMLI based PVDG system, with the PV module directly connected across the Hbridge cell, there is always a possibility of an input DC-link unbalancing in CHBMLI. This voltage unbalance in CHBMLI leads to distortion in the output voltage and current of the multilevel inverter. Therefore, to obtain an optimal output from CHBMLI fed from PV module, the single stage PVDG system is operated with SVM controller. The experimental validation of single stage CHBMLI based PVDG system is divided into two stages. In section-1, the 1-ɸ SVM scheme for the CHBMLI under both balanced and unbalanced DC-link voltage conditions are experimentally validated and in section-2, the experimental results of enhanced PQ based single stage CHBMLI based PVDG system with SVM scheme under varying load conditions is presented.