With more renewable energy based distributed generation (DG) units connected to utility power grids, deterioration of power quality at the point of common coupling (PCC) becomes a major concern. There are two types of harmonics associated with DG units, and together they may cause excessive harmonic distortion at the PCC. The first type of harmonics is generated by power electronic devices in DG units such as photovoltaic systems, which contains high-frequency harmonic components at multiples of the carrier frequency of the DG interfacing inverter. Such harmonics are first reviewed in this paper, and the potential operational effect at the system level due to LCL or LC filters installed at the inverter output to mitigate such harmonics are discussed. The second type of harmonics is generated by other nonlinear local, PCC, and utility loads in the system, which are common type of harmonics at multiples of the power grid frequency, 50/60 Hz. Harmonic mitigation for such harmonics achieved through advanced control of the DG interfacing inverter operated as a power quality conditioner are reviewed and summarized. This systematic review can facilitate better understanding of harmonics associated with renewable energy based DG units and provide guidelines on advanced control schemes to realize ancillary harmonic compensation service through DG interfacing inverters.

Harmonics and Mitigation Techniques Through Advanced Control in Grid-Connected Renewable Energy Sources: A Review

High performance grid-tied inverters have stringent control requirements both under steady-state and under transient conditions. Many different control systems have been applied to grid-tied inverters. However, there are few publications reviewing the literature on these control systems and their classification, particularly with regard to recent developments in this area. In this paper, a review of solutions for the control of grid-tied inverters is carried out. These control systems are compared and classified as implementation platform, reference frame, output filter of inverter, control strategy, modulation method, and controller. The major advantages and disadvantages of these parameters are highlighted and compared. Then, the most important characteristics of these parameters have been presented in a table to show which parameters can be used in various control systems for grid-tied inverters.

Review and Classification of Control Systems in Grid-tied Inverters

This paper presents a comparison study of a typical grid-tie photovoltaic system between two types of topologies: the two stages (boost circuit adds inverter) and the single stage (inverter alone adds a step-up transformer) Some essential models built in this paper include PV array, Boost chopper and grid-tie inverter The basic characteristics and descriptions of these two topologies are described Perturb and observe and incremental conductance methods are introduced for MPPT (maximum power point tracking) As key-point, the out loop controller design details for the grid-tie inverter are investigated The PV array, two system topologies and two MPPT strategies are implemented in PSCAD/EMTDC using its user- defined facility Simulation results show that the output power of PV array can achieve a stable and accurate MPPT function under ambient conditions change At the same time, the inverter can maintain a unit power factor by controlling the transferred instantaneous reactive power to zero in the synchronous d-q frame The simulating results are compared with that of the two stages case in aspects including energy converting efficiency, power quality, and MPPT accuracy Through this study work, engineers and researchers can obtain an overall comprehension about advantages and disadvantages of these two topologies in designing a PV system

Comparative study of two stages and single stage topologies for grid-tie photovoltaic generation by PSCAD/EMTDC

This paper proposes a grid interfaced solar photovoltaic (SPV) power generating system consisting of a SPV, DC-DC boost converter, voltage source converter (VSC), interfacing inductors, ripple filter and a three phase grid feeding variety of linear and nonlinear loads. The reference grid currents are estimated by using power balance theory (PBT) to control the three-leg voltage source converter (VSC). The proposed SPV power generating system provides unity power factor, reactive power compensation for voltage regulation in a three phase distribution system. In this two stage SPV power generating system, the DC link voltage of VSC is regulated through a DC voltage PI (Proportional integral) controller and the maximum power point tracking (MPPT) is achieved using a DC-DC boost converter. The grid interfaced SPV power generating system is modeled and simulated in the MATLAB using the Simulink and the sim power system (SPS) toolboxes.

Power balance theory based control of grid interfaced solar photovoltaic power generating system with improved power quality

Despite the vast benefits of integrating renewable energy sources (RES) with the utility grid, they pose stability and power quality problems when interconnected with the existing power system. This is due to the production of high voltages and current overshoots/undershoots during their injection or disconnection into/from the power system. In addition, the high harmonic distortion in the output voltage and current waveforms may also be observed due to the excessive inverter switching frequencies used for controlling distributed generator’s (DG) power output. Hence, the development of a robust and intelligent controller for the grid-connected microgrid (MG) is the need of the hour. As such, this paper aims to develop a robust and intelligent optimal power flow controller using a grasshopper optimization algorithm (GOA) to optimize the dynamic response and power quality of the grid-connected MG while sharing the desired amount of power with the grid. To validate the effectiveness of proposed GOA-based controller, its performance in achieving the desired power sharing ratio with optimal dynamic response and power quality is compared with that of its precedent particle swarm optimization (PSO)-based controller under MG injection and abrupt load change conditions. The proposed controller provides tremendous system’s dynamic response with minimum current harmonic distortion even at higher DG penetration levels.

Optimal power flow controller for grid-connected microgrids using grasshopper optimization algorithm

Effective control strategies for reliable photovoltaic (PV) grid-connected systems are needed to efficiently use solar energy, an abundant and clean renewable energy source. A space vector pulse width modulation (SVPWM) has been widely applied in the current control of three-phase voltage source inverters (VSIs). The control strategy combines a constant voltage tracking method for the variable photovoltaic power with SVPWM-based proptional-intergral (PI) current controller in a single stage three-phase PV grid-connected system. The controller mimics deadbeat control in the synchronous d-q reference frame, and is very simple and robust to implement. With the necessary grid voltage detection in Photovoltaic systems for protection, grid harmonics disturbance is effectively suppressed through feed-forward compensation. The simulation results show that the current controller makes the PV grid-connected VSI output current to be in phase with the grid voltage and has an excellent steady-state response as well as an extremely fast dynamic response.

Three-phase grid-connected photovoltaic system with SVPWM current controller

Electric Power Steering (EPS) has important significance in improving the auto's dynamic and static performances This paper deals with the mathematical model and characteristic curves of EPS System First, the principle and structure of EPS are discussed Then the characteristics of EPS system are analyzed, including the typical power curves which deal the relationship of the assist torque and the steering wheel torque At last the EPS system's dynamic model and the computerized simulation model have been established in MATLAB/Simulink in teams of three aspects which are the mechanical steering without power system, EPS system and EPS system with PID control The simulation result that the actual current of the motor follows precisely the target current validates the designed assist characteristic

Modeling and characteristic curves of electric power steering system

With the development of power system, the disadvantages of power system stability appeared. Because of the rapid development of power electronic technology, people have paid more close attention to distributed generation (DG). The intermittence and unpredictable nature of the wind or solar power make the successful integration of the DG schemes, which based on green/clean energy, a challenging task. As an energy storage element, superconducting magnetic energy storage (SMES) plays a very important role in improving operating stability of the whole system, which is made of the DG and the power system. SMES is coupled with the DG system through a power electronic converter. This paper constitutes a combination of the DG system and SMES. Take distributed by photovoltaic generation system as an example, a structure of the DG system with SMES is designed. An exact simulation model of DG system with SMES equipment is set up. The results of simulation demonstrate that the SMES equipment can substantially improve operating stability of the DG system and the power system. The experimental prototype with the capacity of 10 kVA is completed.

Research on superconducting magnetic energy storage in the photovoltaic distributed generation

Electric Power Steering (EPS) systems have many advantages over traditional hydraulic power steering systems in engine efficiency, space efficiency, and environmental compatibility. A brief introduction of the structure and principle of EPS system is presented and the control scheme is proposed. Based on the principle of Brushless DC (BLDC) Motor, a novel method for modeling and simulation of BLDC control system is developed in Matlab/Simulink. Organically combining the blocks, a test was performed on a timing system with current hysteresis control. The plot of the motor assisted torque versus the armature current was demonstrated. The feasibility, high reliability of the system and the validity of the control method are proved by the results. And a new kind of test bench model is offered for designing and verifying EPS system.

Research on Electric Power Steering with BLDC drive system

Photovoltaic generation system is made an overall and systematic introduction from several aspects. In this paper, the attention is devoted to research the maximum power point tracking (MPPT) based on analyze the output character of photovoltaic array, and mainly study the CVT method, increase conductance (IncCond) method, variable step MPPT method, and variable step IncCond method based improved starting characteristic which combine both the advantage of CVT and variable step IncCond method. More importantly, this paper make a detailed analysis about the implementation method of MPPT based two circuit topology, boost circuit and push-pull circuit, and resistance load and battery load of dc-dc circuit are considered particularly. Finally, contrast simulation results by PSIM software indicated that the variable step IncCond method improved starting characteristic method have both fine starting characteristic and tracking effect compare to above methods.

Jing Ren

Papers

Three-phase grid-connected photovoltaic system with SVPWM current controller

Modeling and characteristic curves of electric power steering system

Research on superconducting magnetic energy storage in the photovoltaic distributed generation

Research on Electric Power Steering with BLDC drive system

Research on MPPT control and implementation method for photovoltaic generation system and its simulation