GRID SYNCHRONIZATION AND OPERATION OF PARALLEL INVERTERS

Abstract

With rapid industrialization in developing nations, the power demand from the end consumer has exponentially grown. To support the conventional power generation sources (based on fossil fuels) in meeting the rising power demand, power generation based on Distributed Energy Resources (DER’s) enacting as small power sources are gaining popularity. The microgrid architecture with local energy generating sources like photovoltaic, wind, battery systems have opened the pathway for small scale smart distribution grids interfacing the sustainable and clean energy sources with the grid through a voltage / current controlled voltage source inverter. Interfacing a voltage sourced inverter with the grid additionally allows the inverter to support the power quality of grid during grid sag and swell condition. Further based on the power demand at the consumer end the number of interfacing inverters can be added or reduced allowing a scale up or scale down approach in the connected network. The response and dynamics of the microgrid are governed by three major factors – 1) The dynamics of the renewable energy source at the DC side, 2) the synchronization dynamics of the interfaced inverter, 3) Control technique of the interfacing inverter and 4) Effect of high penetration of the low inertial inverters with high switching frequency. The interfacing voltage source inverter is vulnerable to disturbance from both grid side and the renewable energy source or input side, with common intermittencies like partial shading of the photovoltaic panel, different wind speeds, low battery voltages etc. The disturbances at the grid side can be controlled through the control technique of the interfacing inverter or a synchronization algorithm with fast tracking capability. On the DC side the conventional practice is to use a non-isolated boost or buck converter based upon the series parallel combination of the installed solar panels. Conventional converters like buck converter or boost converter struggle with the operating duty requirement at time of partial shading, with the operating duty reaching the nonlinear region, thereby compromising on the operating efficiency. Also, addition of the natural output impedance network through the interfacing vii converter will help in mitigating the spike and transients in the DC link and the control signals during intermittencies at either grid side or input side. This would result in smooth running of the inverter. A VSI interfaced with the renewable energy source typically has low inertia due to lack of moving parts unlike a synchronous generator. As a result, the system remains sensitive towards grid frequency variations. With multiple injection of VSI in the system, the inertia of the system would not increase making the system vulnerable unless and until a fast control on frequency change is not incorporated or virtual inertia is embedded, even though such arrangement provides better reliability. With paralleled VSI having different situations on the connected nodes the response to the change of voltage or frequency often lead to issues of circulating current and large variations during transients due to lack of inertia or damping system. For parallel combination of inverters aligned with the stringent grid codes, synchronization algorithm plays a crucial role. Noncompliance of the code would result in outage of the inverter. The stability of synchronization algorithm dictates the capability dictates the capability of the parallel operation of the inverter especially during off grid mode of operation in master slave configuration. The synchronization algorithms like zero crossing detection (ZCD), voltage unit template-based algorithms and SRF – PLL provide the requisite tracking but suffer immensely during grid disturbances. SOGI adaptive filter based PLL with filtering capability has a superior tracking and filtering capability when compared to conventional PLL. Moreover, the capability of the SOGI filter to generate orthogonal signals eliminates the requirement of Clarke’s transformation reducing the complexity and computation of the algorithm. The work proposes impedance based continuous input current converters for photovoltaic and battery charging applications. The same have been modelled; simulated and experimentally validated under various test conditions. The impedance network allows auxiliary boost supporting the system under severe conditions like partial shading, irradiance change and battery deep discharge. Additionally, the impedance network installed at the input side of the inverter limits the flow of circulating current within the system. viii Synchronization algorithm based on Second Order Generalized Integrator (SOGI) resonating at 100Hz is proposed and experimentally validated for severe grid disturbances. The synchronization algorithm enhances the fault bearing capability of the inverter allowing it to remain synchronized with the grid with minimum transients. A resynchronization algorithm embedded in the Second Order Generalized Integrator Phase Locked Loop (SOGI – PLL) is proposed in the work working with adaptive droop filter. The resynchronization algorithm seamlessly transfers the inverter from grid forming mode to grid connected mode and vice versa while limiting the circulating current among the inverters. The resynchronization algorithm limits the transitional spikes besides administering fast dynamics with prominent signal filtering. The development of algorithm and hardware including development of various control and interface cards have been indigenously done. The simulation and hardware results are presented which show good agreement with the theoretical modeling and analysis.