Fang Zhuo

Simplified Feedback Linearization Control of Three-Phase Photovoltaic Inverter With an LCL Filter

The conventional grid-connected photovoltaic (PV) inverter is controlled by a dual-loop control strategy in synchronous reference frame, and the controllers are designed for steady-state operating point based on the small signal model by neglecting the high-order and coupling terms. However, in an LCL filter, the coupling terms are complicated due to the dq transformation which will affect the dynamic performance. In this paper, an innovative simplified feedback linearization (SFL) control strategy is proposed for the PV inverter with the LCL filter, which offers satisfactory performance, particularly, in decoupling the control system, improving the dynamic performance, and enhancing the adaptability. Furthermore, the SFL controllers are simpler than the high-order tracking controllers used in conventional feedback linearization control. The detailed simplification process and accurate transfer functions for SFL control strategy have been presented, and the performance comparisons between the proposed SFL control strategy and the classical dual-loop method are carried out to show the characteristics of the proposed control algorithm. Finally, a laboratory prototype of a 150-kW PV inverter with the LCL filter has been implemented to test the feasibility and effectiveness of the proposed strategy. The proposed SFL control strategy can also be applied to a higher order system or other power converters.

A novel time-domain current-detection algorithm for shunt active power filters

A novel current-detection algorithm based on the time-domain approach for three-phase shunt active power filters (APFs) to eliminate harmonics, and/or correct power factor, and/or balance asymmetrical loads is analyzed in this paper. A basic overview and evaluation of the performance of existing current-detection algorithms for active power filters are presented. According to different complicated power quality issues and various compensation purposes, a novel current-detection algorithm is then proposed. Comparing with existing algorithms, this algorithm has shorter response time delay and clearer physical meaning. Different compensating current references can, thus, be accurately and easily obtained by adopting the proposed algorithm. It ensures that the shunt APF can very well achieve different compensation purposes. Moreover, it is very easy to implement this algorithm in a digital signal processor (DSP). Simulation results obtained with MATLAB and testing results on an experimental shunt APF are presented to validate the proposed algorithm.

Modeling and Control of Dual-Stage High-Power Multifunctional PV System in d - Q -

A high-power photovoltaic (PV) system based on dual-stage topology of boost converter plus paralleled four-leg inverter is presented, which can perform as both grid-connected inverter and active power filter. The system not only can allow a wide range of input voltage from PV arrays, but also can compensate the nonlinear and unbalanced load current. This paper first presents the small-signal model of the presented inverter with LCL filter in d -q-o coordinate; then, the unified control strategy which combines PV power injection and distortional current compensation is demonstrated; finally, a method to control the zero-sequence circulating current between paralleled inverters is proposed. Simulations and experimental results are carried out on a 100-kVA prototype to validate the practicability of the whole system.

o

Photovoltaic (PV) systems frequently suffer disproportionate impacts on energy production due to mismatch cases. To remedy this, academia proposed a distributed max power point tracking (MPPT) solution and has been implemented commercially. Taking the trend of the “distributed MPPT” concept a step further, this paper discusses and analyzes an MPPT converter that connects to each PV cell string, called a subpanel MPPT converter (SPMC), to better address the real-world mismatch issues. The SPMC system with a unified output MPPT control structure is also proposed in order to reduce the cost and simplify the distributed MPPT system. The proposal saves A/D units, current sensors, and MPPT controllers on the premise of guaranteeing that the SPMC is working on its optimal maximum power point regardless of the mismatch case. This is favorable for the further integration and makes the whole SPMC system less expensive and easier to realize. Finally, the effectiveness of the proposal is confirmed experimentally.

Coordinate

A shunt active power filter (APF) with current detection at the source side is considered as a closed-loop system from the view of the whole power distribution system, which is expected with better harmonics filtering performance compared with the one with load current detection. The conventional source current detection control scheme can be mainly grouped into two types: 1) current-source-based (CS) scheme and 2) power-balance-based (PB) scheme. As introduced in this paper, the CS scheme has good dc-voltage regulation, but it bears inevitable conflict between filtering performance and system stability, whereas the PB scheme has excellent stable filtering performance, but it bears great influence on the dc voltage in the transient state. In this paper, a novel source current detection control scheme is proposed, which reserves the advantages of conventional schemes and, meanwhile, avoids their limitations. The proposed scheme employs a vector resonant controller, with which the shunt APF performs as an equivalent multiband rejection filter that series between nonlinear loads and the grid source, blocking selected harmonics components from the load side flowing into the source side. The proposed scheme is simple in structure, is needless of a harmonics extraction algorithm, and shows good stable and transient performance. Finally, the conclusions obtained in this paper are validated through experiments on the laboratory prototype.

Analysis of Unified Output MPPT Control in Subpanel PV Converter System

A renewable energy-based dc micro-grid with hybrid energy storage, consisting of battery and ultracapacitor, is investigated. To achieve high penetration depth of renewable sources into the utility grid, a novel system operation strategy and the corresponding energy management method is proposed. In the operation strategy, the ultracapacitor unit works as the sole voltage source of the micro-grid to support the dc link in both connected and islanding mode. The micro-grid is controlled to deliver/absorb predefined amount of power to/from the utility grid during connected mode and zero during islanding mode. This design will certainly simplify the power dispatching algorithm of the power system and increase the possibility of including large quantities of micro-grids into the utility grid. The energy management method is dedicated to conducting the net power of the micro-grid effectively. The net power is separated into high- and low-frequency components. The high-frequency power is suppressed by the ultracapacitor automatically and the low frequency power is shared by the battery and an adjustment unit. A small-scale dc micro-grid structure with a single dc link is considered for investigation. MATLAB/Simulink simulation results are presented to validate the proposed system operation strategy and the energy management method.

A Source-Current-Detected Shunt Active Power Filter Control Scheme Based on Vector Resonant Controller

This paper proposes a novel three-phase phase-locked loop (PLL) algorithm, which focuses on the reforming of the primary signals before grid synchronization rather than improving the phase estimation methodologies. The unbalanced signals are reformed to balanced ones without damage to the phase angle, through which the negative sequence of the grid voltages is removed. This eliminates the estimation errors of conventional synchronous reference frame PLL and enhances its response speed with a higher bandwidth. The reforming process is supposed to be carried out at every zero-crossing point of the three-phase voltages and choose one phase as reference to balance the other two. Coefficients for the signal reforming are calculated at one zero-crossing point and updated until the next comes. In implementation, a certain phase is chosen as the reference all along and the reforming process will be suspended when it just crosses the zero line. This PLL algorithm has a fast and precise character to reform the three-phase grid voltages and is flexible for application. Under heavily distorted grid conditions, it can still perform effectively even with multiple zero-crossings. Comprehensive experimental results from a digital signal processor-based laboratory prototype are provided to validate the performance of this PLL algorithm.

System Operation and Energy Management of a Renewable Energy-Based DC Micro-Grid for High Penetration Depth Application

Unlike the typical microgrid with a common ac bus, networked microgrid always suffers more serious reactive power sharing issues due to its complex inner structure. In such case, the system reactive power sharing error cannot be easily evaluated and eliminated. So, this paper proposes a wireless control strategy that employs optimized virtual impedance controllers and local load measurements for the reactive power sharing in networked microgrid. First, from the modeling of microgrid network, an estimation method for network reactive power sharing error is derived. Through the estimation method-based network feature analyses, corresponding design for virtual impedance controller is presented. Then, by introducing genetic algorithm, virtual impedance controller parameters of each distributed generation unit are optimized, which aims to minimize the microgrid global reactive power sharing error. The parameter optimization process is performed offline in microgrid configuration stage. By using these optimized virtual impedance controllers, the reactive power sharing performance of a networked microgird can be greatly improved. Finally, the accuracy of the estimation method is validated by MATLAB simulation results, and the feasibility of the proposed virtual impedance optimization method is verified through real power experiments.

A Three-Phase PLL Algorithm Based on Signal Reforming Under Distorted Grid Conditions

With increasing penetration of the renewable energy, the grid-tied PWM inverters need to take corresponding responsibilities for the security and stability of future grid, behaving like conventional rotational synchronous generator (RSG). Therefore, recognizing the inherent relationship and intrinsic differences between inverters and RSGs is essential for such target. By modeling the typical electromechanical transient of grid-tied PWM inverters, this paper first proves that PWM inverters and RSGs are similar in physical mechanism and equivalent in mathematical model, and the concept of static synchronous generator (SSG) is thereby developed. Furthermore, the comprehensive comparison between RSG and SSG is carried out in detail, and their inherent relation is built. Based on these findings, the rationality and feasibility of migrating the concepts, tools, and methods of RSG stability analysis to investigate the dynamic behaviors and stability issues of SSG is therefore confirmed. Taking stability issues as an example, the criteria of small signal and transient stability of a typical grid-tied PWM inverter is put forward to demonstrate the significance of the developed SSG model (including synchronizing coefficient, damping coefficient, inertia constant, and power-angle curve), providing clear physical interpretation on the dynamic characteristics and stability issues. The developed SSG model promotes grid-friendly integration of renewable energy to future grid and stimulates interdisciplinary research between power electronics and power system.

A Virtual Impedance Optimization Method for Reactive Power Sharing in Networked Microgrid

To solve the reactive power sharing issue in droop control application, many solutions have been developed based on the basic wireless manner. However, existing wireless methods cannot eliminate reactive power sharing errors effectively, especially in load change situations. In this paper, a wireless reactive power sharing method that employs feeder current sensing and adaptive virtual impedance control is proposed for islanded microgrid. To improve reactive power sharing accuracy of virtual impedance method, an equivalent feeder concept is introduced, which can reflect the mismatch in connecting circuits equivalently. Through fast feeder current sensing, distributed generation units can calculate their respective equivalent feeders in real time. With the cooperation between real-time calculation and virtual impedance control, the proposed method achieves both accurate and fast performance in reactive power sharing. Compared with communication-based methods, the proposed wireless control provides the same high accuracy in reactive power sharing; meanwhile, its response speed to load change is much faster than that of other methods. Moreover, the proposed method can work as normal even though load changes during the transient process. Matlab simulation and real-time digital simulator test are used to validate the feasibility of this method.