Liansong Xiong

Static Synchronous Generator Model: A New Perspective to Investigate Dynamic Characteristics and Stability Issues of Grid-Tied PWM Inverter

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 Fast Orthogonal Signal-Generation Algorithm Characterized by Noise Immunity and High Accuracy for Single-Phase Grid

Conventional orthogonal signal-generation methods for single-phase grid suffer from long response time, approximation errors, and/or noise amplification. To remedy this problem, a novel orthogonal signal-generation algorithm, which is characterized by remarkable response speed, high accuracy, noise immunity, and easy implementation, is developed in this paper. The developed algorithm can precisely calculate the orthogonal signal within about 2.5 ms when the amplitude/phase of the grid signal changes suddenly, under the premise of not significantly increasing the computational burden compared with the conventional methods. Far more important is that the high accuracy of the proposed method is independent of the system sampling frequency; hence, effectively avoiding the approximation errors in a wide range of sampling frequencies. In addition, it is shown how the noise amplification issue caused by the developed algorithm, which is nearly equal to that of the first-order differentiating method, can be effectively solved by proper time-span scaling and restricted into an acceptable range. Finally, the validity and advancement of the developed algorithm are verified by experimental results.

A novel method for modeling of DC micro-grid based on characteristic parameter

The characteristic parameters of micro-grid determine its transient response and stability. Due to uncertain load changes as well as large-scale intermittent energy accesses, the micro-grid systems are always subjected to large number of random disturbances which results in micro-grid structure parameters change. To solve this problem, this paper presents a novel model for characteristic parameters calculation of DC micro-grid. The derived model is based on real-time measurement of the transient current which quickly and accurately calculates the characteristic parameters change. A controller is designed using the derived model and simulation is carried out to verify the derived model.

A novel real-time and on-line computation algorithm for characteristic parameters of micro-grids

The characteristic parameters of the micro-grids determine its transient response and the stability. The generation, the grid structure and the electric loads of all the whole micro-grids are always changing randomly due to the random load changes and the integration of large-scale batch energy, indicating that the characteristic parameters of the micro-grids also changes. A novel real-time and on-line computation algorithm for the characteristic parameters of the micro-grids is presented in this paper. This new method can calculate the key characteristic parameters of the micro-grids according to the measured transient currents. And hence the response character and the stability of the transient, the important information for the regulation and dispatching of the micro-grid system, can be pre-estimated quickly and accurately.

Chopper Controller Based DC Voltage Control Strategy for Cascaded Multilevel STATCOM

The superiority of CMI (Cascaded Multilevel Inverter) is unparalleled in high power and high voltage STATCOM (Static Synchronous Compensator). However, the parameters and operating conditions of each individual power unit composing the cascaded STATCOM differ from unit to unit, causing unit voltage disequilibrium on the DC side. This phenomenon seriously impairs the operation performance of STATCOM, and thus maintaining the DC voltage balance and stability becomes critical for cascaded STATCOM. This paper analyzes the case of voltage disequilibrium, combines the operation characteristics of the cascaded STATCOM, and proposes a new DC voltage control scheme with the advantages of good control performance and stability. This hierarchical control method uses software to achieve the total active power control and also uses chopper controllers to enable that the imbalance power can flow among the capacitors in order to keep DC capacitor voltages balance. The operating principle of the chopper controllers is analyzed and the implementation is presented. The major advantages of the proposed control strategy are that the number of PI regulators has been decreased remarkably and accordingly the blindness of system design and debugging also reduces obviously. The simulation reveals that the proposed control scheme can achieve the satisfactory control goals.

Study on the compound cascaded STATCOM and compensating for 3-phase unbalancd loads

The existing 3-phase cascaded STATCOMs have two connection types, Y connection and Δ connection, which are very different from each other in the structure, functionality, reliability, economy and technical realization. Based on the deep comparative analysis of the merits and demerits of these two connection types, this paper puts forward a compound cascaded STATCOM. The internal relations and distinctions among the Y-connected and the Δ-connected and the compound cascaded STATCOM are revealed from two aspects, mathematical model and physical significance. The implementation method of compensating 3-phase unbalanced loads by the compound cascaded STATCOM is presented and the simulation results have proved the effectiveness and superiority of the new topology. Research results show that the compound cascaded STATCOM with comparative advantage in technology and economy has inherited the superiorities and overcame the shortcomings of the Y connection and Δ connection, which is worth popularizing in the field of high power and high voltage 3-phase unbalanced system.

Optimal design of moving average filter and its application in distorted grid synchronization

This paper focuses on the optimal design of Moving Average Filter (MAF). The necessary and sufficient condition for MAF to filter out a group of harmonics with specified orders simultaneously is deduced based on the presented model. Accordingly, the enhanced MAF (EMAF) algorithm is proposed and used for the fast open-loop phase locking of the distorted grid. Compared with the Cascaded Delayed Signal Cancellation (CDSC) algorithm, the superiority of EMAF is highlighted from the aspects of dynamic response time, transient oscillation amplitude, required data storage size, thorough harmonics elimination and random noise suppression ability. The experiments validate the advantage of the proposed EMAF algorithm.

A Novel Fast Open-loop Phase Locking Scheme Based on Synchronous Reference Frame for Three-phase Non-ideal Power Grids

Rapid and accurate phase synchronization is critical for the reliable control of grid-tied inverters. However, the commonly used software phase-locked loop methods do not always satisfy the need for high-speed and accurate phase synchronization under severe grid imbalance conditions. To address this problem, this study develops a novel open-loop phase locking scheme based on a synchronous reference frame. The proposed scheme is characterized by remarkable response speed, high accuracy, and easy implementation. It comprises three functional cascaded blocks: fast orthogonal signal generation block, fast fundamental-frequency positive sequence component construction block, and fast phase calculation block. The developed virtual orthogonal signal generation method in the first block, which is characterized by noise immunity and high accuracy, can effectively avoid approximation errors and noise amplification in a wide range of sampling frequencies. In the second block, which is the foundation for achieving fast phase synchronization within 3 ms, the fundamental-frequency positive sequence components of unsymmetrical grid voltages can be achieved with the developed orthogonal signal construction strategy and the symmetrical component method. The real-time grid phase can be consequently obtained in the third block, which is free from self-tuning closed-loop control and thus improves the dynamic performance of the proposed scheme. The proposed scheme is adaptive to severe unsymmetrical grid voltages with sudden changes in magnitude, phase, and/or frequency. Moreover, this scheme is able to eliminate phase errors induced by harmonics and random noise. The validity and utility of the proposed scheme are verified by the experimental results.

Development of fast simulation models for photovoltaic generation system based on Simulink

In this paper, a detailed mathematical presentation of Photovoltaic (PV) generation system, as well as its simulation model based on MATLAB / Simulink is established first. Its inverter-based simplified models, i.e. the switching-function equivalent model and average model, are simulated and compared quantitatively from different aspects. Simulation results reveal that the switching-function equivalent model, with high accuracy in representing steady-state and transient behaviors, is suitable for the Hardware-in-the-loop (HiL) applications. A novel phasor simulation model of PV system, which exploits the phasor presentation of three-phase power systems and thus significantly improves the simulation speed, is proposed. The validity and utility of the proposed phasor model are assessed by comparing the steady-state performance with the detailed model. Finally, it is shown how this model can be effectively used in the fast analysis of power flow and electromagnetic transient behavior of PV system.

Research on fast open-loop phase locking scheme for three-phase unbalanced grid

Fast and accurate grid synchronization is essential for grid-connected converter control. The existing phase locking schemes are divided into two categories: closed-loop phase locking (CLPL) and open-loop phase locking (OLPL). OLPL methods show good steady-state performance and strong robustness to the variations and imbalance of the grid, however, the dynamic response is not fast enough due to their PID-based controller. Usually, more than one grid cycle is required for CLPL methods to lock completely the new phase. On the contrary, OLPL methods are characterized by fast grid synchronization, which has attracted a lot of attention in recent years. This paper proposes a novel fast open-loop phase locking scheme based on the synchronous reference frame (SRF-OLPL) to synchronize the true phase angle of the grid, which possesses well adaptive disturbance elimination properties and remarkable performance of dynamic responses at the same time.