Analysis and mitigation of dead time harmonics in the single-phase full-bridge PWM converters with repetitive controllers

In order to prevent the power switching devices (e.g., the Insulated-Gate-Bipolar-Transistor, IGBT) from shoot through in voltage source converters during a switching period, the dead time is added either in the hardware driver circuits of the IGBTs or implemented in software in Pulse-Width Modulation (PWM) schemes. Both solutions will contribute to a degradation of the injected current quality. As a consequence, the harmonics induced by the dead time (referred to as "dead time harmonics" hereafter) have to be compensated in order to achieve a satisfactory current quality as required by standards. In this paper, the emission mechanism of dead time harmonics in single-phase PWM inverters is thus presented considering the modulation schemes in details. More importantly, a repetitive controller has been adopted to eliminate the dead time effect in single-phase grid-connected PWM converters. The repetitive controller has been plugged into a proportional resonant-based fundamental current controller so as to mitigate the dead time harmonics and also maintain the control of the fundamental frequency grid current in terms of dynamics. Simulations and experiments are provided, which confirm that the repetitive controller can effectively compensate the dead time harmonics and other low-order distortions, and also it is a simple method without hardware modifications

Exploitation of Digital Filters to Advance the Single-Phase T/4 Delay PLL System

With the development of digital signal processing technologies, control and monitoring of power electronics conversion systems have been evolving to become fully digital. As the basic element in the design and analysis phases of digital controllers or filters, a number of unit delays (z-1) have been employed, e.g., in a cascaded structure. Practically, the number of unit delays is designed as an integer, which is related to the sampling frequency as well as the ac signal fundamental frequency (e.g., 50 Hz). More common, the sampling frequency is fixed during operation for simplicity and design. Hence, any disturbance in the ac signal will violate this design rule and it can become a major challenge for digital controllers. To deal with the above issue, this paper first exploits a virtual unit delay (zv-1) to emulate the variable sampling behavior in practical digital signal processors with a fixed sampling rate. This exploitation is demonstrated on a T/4 Delay Phase Locked Loop (PLL) system for a single-phase grid-connected inverter. The T/4 Delay PLL requires to cascade 50 unit delays when implemented (for a 50-Hz system with 10 kHz sampling frequency). Furthermore, digital frequency adaptive comb filters are adopted to enhance the performance of the T/4 Delay PLL when the grid suffers from harmonics. Experimental results have confirmed the effectiveness of the digital filters for advanced control systems

VAR controller for STATCOM solar inverter

In the past five years over 1.2 million PV installations were made in Australia which has had a detrimental effect on the overall power quality of the system. Utilities now provide a higher level of system harmonicas as well as having to cope with unintentional DC current injection into the grid. At the same time, these PV installations have not helped the utilities at peak periods, or for localized loads such as starting of induction machines. Thus there is clearly a need for a more dynamic inverter system which can help improve system efficiency and maintain power quality standards. In order to help improve the power quality of the grid, the existing STATCOM inverter would be adapted to monitor grid conditions with a smart energy meter and interfaced to a PLC to either bring voltage within limits or improve power factor to a desired level. The PLC will utilize both power factor information and voltage and current information from the smart energy meter to allocated an active power and a reactive power of either a capacitive or inductive reactive power in order to maintain power factor levels. Weak networks suffer from a range of problems depending on conditions. Typically when they are heavily loaded, they require additional capacitive reactive power to be injected into the grid. On SWER networks, due to the high X on R ration they also require at peak times the injection of active power. When lightly loaded by comparison, they require the injection of reactive power in order to bring the voltage within limits due to the Ferranti Effect. In order to accommodate all of these functions, the power factor and the voltage conditions from the smart energy meter were read into the PLC where the decision was made as to what level of active, capacitive reactive or inductive reactive power was required to be injected into the grid. A set of SIMULINK models were developed to analyse a range of switching strategies in order to minimize injected harmonics and maximize STATCOM efficiency. The key outcome of this project was that the implementation of a PLC based system which provided active and reactive power support and was able to react to localized load changes rapidly. The broad application of this technology to existing PV systems will ultimately allow in system power quality and efficiency improvements throughout the state

Control Based Soft Switching Three-phase Micro-inverter: Efficiency And Power Density Optimization

In the field of renewable energy, solar photovoltaic is growing exponentially. Grid-tied PV micro-inverters have become the trend for future PV system development because of their remarkable advantages such as enhanced energy production due to MPPT implementation for each PV panel, high reliability due to redundant and distributed system architecture, and simple design, installation, and management due to its plug-and-play feature. Conventional approaches for the PV micro-inverters are mainly in the form of single-phase grid connected and they aim at the residential and commercial rooftop applications. It would be advantageous to extend the micro-inverter concept to large size PV installations such as MW-class solar farms where threephase AC connections are used. The relatively high cost of the three-phase micro-inverter is the biggest barrier to its large scale deployment. Increasing the switching frequency may be the best way to reduce cost by shrinking the size of reactive components and heat-sink. However, this approach could cause conversion efficiency to drop dramatically without employing soft switching techniques or using costly new devices. This dissertation presents a new zero voltage switching control method that is suitable for low power applications such as three-phase micro-inverters. The proposed hybrid boundary conduction mode (BCM) current control method increases the efficiency and power density of the micro-inverters and features both reduced number of components and easy digital implementation. Zero voltage switching is achieved by controlling the inductor current bidirectional in every switching cycle and results in lower switching losses, higher operating frequency, and reduced size and cost of passive components, especially magnetic cores. Some iv practical aspects of hybrid control implementation such as dead-time insertion can degrade the performance of the micro-inverter. A dead-time compensation method that improves the performance of hybrid BCM current control by decreasing the output current THD and reducing the zero crossing distortion is presented. Different BCM ZVS current control modulation schemes are compared based on power losses breakdown, switching frequency range, and current quality. Compared to continuous conduction mode (CCM) current control, BCM ZVS control decreases MOSFET switching losses and filter inductor conduction losses but increases MOSFET conduction losses and inductor core losses. Based on the loss analysis, a dual-mode current modulation method combining ZVS and zero current switching (ZCS) schemes is proposed to improve the efficiency of the micro-inverter. Finally, a method of maintaining high power conversion efficiency across the entire load range of the three-phase micro-inverter is proposed. The proposed control method substantially increases the conversion efficiency at light loads by minimizing switching losses of semiconductor devices as well as core losses of magnetic components. This is accomplished by entering a phase skipping operating mode wherein two phases of an inverter are disabled and three inverters are combined to form a new three-phase system with minimal grid imbalance. A 400W prototype of a three-phase micro-inverter and its hybrid control system have been designed and tested under different conditions to verify the effectiveness of the proposed controller, current modulation scheme, and light load efficiency enhancement method

Study and evaluation of distributed power electronic converters in photovoltaic generation applications

This research project has proposed a new modulation technique called “Local Carrier Pulse Width Modulation” (LC-PWM) for MMCs with different cell voltages, taking into account the measured cell voltages to generate switching sequences with more accurate timing. It also adapts the modulator sampling period to improve the transitions from level to level, an important issue to reduce noise at the internal circulating currents. As a result, the new modulation LC-PWM technique reduces the output distortion in a wider range of voltage situations. Furthermore, it effectively eliminates unnecessary AC components of circulating currents, resulting in lower power losses and higher MMC efficiency.Departamento de Tecnología ElectrónicaDoctorado en Ingeniería Industria

Real Time Testing and Validation of a Novel Short Circuit Current (SCC) Controller for a Photovoltaic Inverter

About 45% applications from PV solar farm developers seeking connections to the distribution grids in Ontario were denied in 2011-13 as the short circuit current (SCC) capacity of several distribution substations had already been reached. PV solar system inverters typically contribute 1.2 p.u. to 1.8 p.u. fault current which was not considered acceptable by utility companies due to the need for very expensive protective breaker upgrades. Since then, this cause has become a major impediment in the growth of PV based renewable systems in Ontario. A novel predictive technique has been patented in our research group for management of short circuit current contribution from PV inverters to ensure effective deployment of solar farms. This thesis deals with the real time testing and validation of a short circuit current (SCC) controller based on the above technique. With this SCC controller, the PV inverter can be shut off within 1-2 milliseconds from the initiation of any fault in the grid that can cause the short circuit current to exceed the rated current of the inverter. Therefore, the power system does not see any short circuit current contribution from the PV inverter and no expensive upgrades in protective breakers are required in the system. The performance of the PV solar system with the short circuit current controller is simulated and tested using (i) industry grade electromagnetic transients software PSCAD/EMTDC (ii) real time simulation studies on the Real Time Digital Simulator (RTDS) (iii) physical implementation on dSPACE board to generate firing pulses for the inverter. The validation of controller is done on dSPACE board with actual PV inverter short circuit waveforms obtained from Southern California Edison Short Circuit Testing Lab. This novel technology is planned to be showcased on a physical 10 kW PV solar system in Bluewater Power Distribution Corporation, Sarnia, Ontario. This proposed technology is expected to remove the technical hurdles which caused the denials of connectivity to several PV solar farms, and effectively lead to greater connections of PV solar farms in Ontario and in similar jurisdictions, worldwide

Single-Phase Photovoltaic-Inverter Operation Characteristic in Distributed Generation System

Single-phase grid tied inverter is one among types of inverters widely used in photovoltaic (PV) generation system due to the advantages they offer. This chapter describes model and simulation of such inverter in operation as distributed generation in electrical power system. Power characteristics including power quality, grid interaction behavior and load sharing that are important aspects in their operation as grid connected inverter will be simulated and analyzed. The role of current or voltage control and associated mechanism in photovoltaic inverter such as photovoltaic I-V characteristic, maximum power point tracker (MPPT), and other mechanism that involves in power flow and load sharing control are described. Further, some observation and measurement from a 5-kWp laboratory scale grid interconnected photovoltaic plant that employ single phase photovoltaic inverter will be presented. The load sharing behavior between photovoltaic plant and utility grid during supplying both linear and non linear load that connected on their point of common coupling. In addition, observation and measurement results of power quality parameter behavior during photovoltaic inverter operation along extremely density variation of photovoltaic produced energy that comes from the atmospheric condition will be presented. Keywords: single phase PV Inverter, distributed generatio

Non-PLL Direct Power Control for a Single-Phase Grid-Connected Three-Level Inverter

The growing demand for clean, reliable renewable energy generation has led to the widespread adoption of solar energy as a source of electricity. Technological advancement aiding to reduce the cost of solar photovoltaic (PV) panels, as well as improvement in power electronics and control strategies for solar PV systems have also contributed to the growing popularity. For grid-connected solar systems to adequately meet future demand and grid requirements, the system must be reliable, and not affected by instability or distortions on the power grid. In this thesis, a control strategy for single-phase grid-connected inverters that can synchronize to the grid without a phase lock loop (PLL) is proposed. The PLL is an important device that is relied on for the synchronization of solar PV systems to the electrical grid. However, the PLL has an inherently complex design and its performance is often negatively affected if the grid voltage has poor quality. In addition, eliminating the use of PLL for synchronization can avoid the issue of slow dynamic response, higher harmonics, and increased computation complexity. The real and reactive power of the single-phase, three-level neutral point clamped (NPC) inverter is controlled by using a direct power control (DPC) strategy. A novel method of computing the power components of the single-phase inverter is proposed and this technique further improves the precision of the power components calculated by compensating the frequency and phase deviation compensation. Finally, simulations are carried out by using MATLAB/Simulink to demonstrate the effectiveness of the proposed methodology