Development Of Efficient Control Strategies For Single Phase Grid Integrated Inverters For Photovoltaic Applications

Abstract

Diminution of fossil fuel reserves and increased concern about environmental pollution has amplified the demand of renewable energy sources (RES) for power generation. Penetration of renewable energy based power plants into the conventional distribution system has increased the use of power electronics converters (PEC). The PECs are used to convert the power generated by the RES based distributed generation(DG) plants into a form of power which is compatible with the distribution grid. The PECs integrating the DG plants with the grid have stringent control requirements, which are specified in the standards such as IEEE 1547 and IEC 61727.
Solar energy utilization is the fastest growing sector among all forms of renewable energy, with net increase in solar power generation by an average of 8.3 % per year as per
International Energy Outlook (IEO), 2016 report. Single phase voltage source inverter (VSI) is used to interface photovoltaic (PV) based DG plants with the single phase grid in residential areas. Generally, there are two controllers associated with grid connected PV systems. One is the input side maximum power point tracking (MPPT) controller and the
other is the grid side controller. The grid side controller has to perform multiples tasks, which include DC-link voltage control, grid synchronization, regulation of active and reactive power exchange between the DG plant and the grid and injection of high quality power into the grid. The grid side controller incorporates two control loops: the outer voltage control loop and the inner current control loop. The inner current control loop is responsible for injecting good quality current into the grid, power flow control and grid synchronization. Researchers have proposed several current control techniques for single phase grid tied inverters in recent years. Several drawbacks have been identified with the conventional control techniques. Hence, there is a need to develop control strategies with
intuitive design methodology, to satisfy the control requirements of grid tied DG inverter under both steady state and transient operating conditions.
In this thesis, various current control strategies for single phase grid integrated inverters have been reviewed with their pros and cons. The significant control strategies
namely current hysteresis control (CHC), proportional integral (PI) control, proportional resonant (PR) control and dead beat control (DBC) have been designed and analyzed for a grid tied single phase VSI. A digital model predictive control (MPC) based current controller is introduced for single phase grid tied inverter. A comparative performance
assessment of the proposed current control strategy with the conventional controllers is also performed. The model predictive current controller (MPCC) is found to outperform
the other existing current controllers in steady state and transient state operating conditions. The design and implementation of the MPCC scheme is simple. The proposed
control strategy is implemented on a hardware prototype using TMS320F2812, digital signal processor (DSP). A delay compensation technique is also proposed for the MPCC
to compensate for the delay that is introduced when the control scheme is implemented on digital platform. The delay compensated model predictive current controller (DC-MPCC)
is implemented for a dual stage single phase grid integrated PV system.
A grid voltage sensorless control algorithm is also introduced for single phase grid tied VSI. In this control algorithm, a damped resonant compensator (DRC) is used to
estimate the voltage reference signal and the grid voltage is estimated using some simple mathematical calculations. A phase locked loop (PLL) less synchronization scheme is
proposed to synchronize the inverter output current with the grid voltage. The DC-MPCC is employed for current control. The voltage sensorless control technique does not interfere with the current controller performance. Decoupling control of active and reactive power is achieved through the proposed scheme. When the grid demands reactive power, the same can be provided by the VSI, by controlling the reactive current component which is in quadrature with the active current component. Elimination of voltage sensor reduces the overall cost of the system and improves the system reliability. The efficacy of the proposed grid voltage sensorless control scheme is validated by experimental implementation using dSPACE 1104 real time controller.