Compensation of current harmonics in single phase grid connected inverters with deadtime under distorted grid voltage

Abstract

As the human population in the world increases, the demand for energy, which is the most basic need of human beings, also increases. In order to meet the increasing demand, the energy supply is enlarged with the investments made. While energy investments continue to grow in all areas, investments in renewable energy production have reached serious levels in the last 20 years and its share among other investments is increasing every year. Among these investments, wind and solar energy investments have the highest ratio. The increase in these investments has also increased the development and production of photovoltaic panels and grid-connected inverters and other power electronic devices. In this thesis, the power electronics circuit consists of a two-stage structure. In the first stage, the solar panel voltage is increased to the inverter DC bus voltage by a DC/DC converter. In the second stage, an inverter works in connection with the grid and transfers energy from the solar panel to the grid. Single-phase full-bridge inverter is selected as the inverter topology. The inverter is designed as 1500 VA. The switching frequency is 16 kHz. The full-bridge inverter is connected to the grid with an LCL filter. LCL filter has a 3rd order transfer function and when the resonance frequency is set correctly, the harmonics generated by the switching frequency can be filtered out easily. However, when the frequency response of the transfer function of the LCL filter is examined, it is seen that it peaks at the resonance frequency. Active and passive damping methods have been developed to reduce the gain at the resonant frequency. In passive damping methods, the peak is damped by adding a passive element to the LCL filter structure. Passive damping, which is made by adding resistance to the filter capacity is an easily applied method. The disadvantage of this method is that the capacitor current flowing through the resistor creates a loss. However, since the inverter is designed as low power in this thesis, the loss is less compared to the large power inverters. For this reason, damping with resistance is preferred. In most of the studies on the selection of the resonance frequency, it is suggested that it should be greater than 10 times the grid frequency and less than half of the switching frequency. LCL filter capacitor is selected so that the reactive power loss in the capacitor is not more than 5% of the inverter rated power. The inductance of the filter on the inverter side is selected according to the current ripple ratio. In the studies, it has been suggested to choose the ripple ratio between 20% and 40%. In this study, the ripple ratio was chosen as 20%. It has been suggested that the grid side inductance be the same as the inverter side inductance. In this study, it was chosen to be the same as the inverter side inductance. Grid-connected inverters, as power electronic devices, need to be designed in accordance with international standards. These standards have criteria for both the safety of the device and its operating performance. In this study, the operating performance of grid-connected inverters and the standards related to the grid are examined. The most common of these are IEEE Std 519™-2014 and IEC 61727:2004. The design has been made taking into account these standards. A synchronous reference signal must be given to the controller to work synchronously with the grid. Various methods have been developed for generating the synchronous reference signal such as Phase Locked Loop (PLL). When grid voltage is applied to the input of this control structure, a synchronous reference signal to the grid can be obtained at its output. This structure is usually shown in three parts. These are Phase Detector (PD), Loop Filter (LF), and Voltage Controlled Oscillator (VCO). PD structures with adaptive filters, which have better performance than others have been examined. Secondary Generalized Integrator (SOGI), which was obtained by making some changes in these structures, was examined and the SOGI-PLL structure was implemented. The Bode diagram and step response of the transfer function with variable coefficients are examined. Research and simulation studies have been carried out for the development of the inverter current controller. PI controller and PR controller are used as current controllers of grid-connected inverters. PR controllers are implemented in this thesis. It has been understood that this controller alone is not sufficient. Because grid harmonics and inverter-induced harmonics cause odd harmonics such as 3rd, 5th, 7th, and 9th harmonics. Resonant controllers parallel to the PR controller are used to compensate these harmonics. It has become compliant with standards at full load. In order to compensate the grid voltage harmonics, the grid voltage is added to the controller as a feed-forward path. Harmonic components originating from the grid are greatly attenuated. However, harmonics are still present at low loads. Repetitive controllers (RC) are added in parallel to the current error signal in the control structure, reducing the harmonics both at rated load and light loads. Although the dynamic performance of RC is slow compared to multi-resonance controllers, their steady-state performance is quite beneficial. Controllers with theoretical studies have been tested in a grid-connected inverter study with a simulation model. In the tests, the total harmonic distortion in the grid voltage is 3%, and the dead time of the PWM signals is set to 1.5 µs. When only PR controllers are used, the total harmonic distortion of the grid current at full load is 6.92%. This ratio decreased to 3.71% with multiple resonance controllers. As a result of grid voltage feed-forward, the harmonic distortion was 2.87%. The rate decreased to 1.53% when the repetitive controller was added. The results confirmed the theoretical work. The results were also confirmed by the experimental implementation. In the experiment, the total harmonic distortion was slightly higher with the effect of the disturbances that were not taken into account in the simulation. When only PR controllers are used, the total harmonic distortion of the grid current at full load is 9.84%. This ratio decreased to 6.11% with multiple resonance controllers. As a result of grid voltage feed-forward, the harmonic distortion was 3.93%. Results were similar to the simulation when the disturbances were taken into account. Theoretical studies for harmonic compensation of single-phase grid-connected inverter were verified on simulation and circuit, and the results were compared with each other. With the applied control methods, the total harmonic distortion of the grid current has been successfully reduced.