Minghua Zhu

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.

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.

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.

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.

A Quantitative Evaluation and Comparison of Harmonic Elimination Algorithms Based on Moving Average Filter and Delayed Signal Cancellation in Phase Synchronization Applications

The harmonic components of grid voltage result in oscillations of the calculated phase obtained via phase synchronization. This affects the security and stability of grid-connected converters. Moving average filter, delayed signal cancellation and their related harmonic elimination algorithms are major methods for such issues. However, all of the existing methods have their limitations in dealing with multiple harmonics issues. Furthermore, few studies have focused on a comparison and evaluation of these algorithms to achieve optimal algorithm selections in specific applications. In this paper, these algorithms are quantitatively analyzed based on the derived mathematical models. Moreover, an enhanced moving average filter and enhanced delayed signal cancellation algorithms, which are applicable for eliminating a group of selective harmonics with only one calculation block, are proposed. On this basis, both a comprehensive comparison and a quantitative evaluation of all of the aforementioned algorithms are made from several aspects, including response speed, required data storage size, sensitivity to sampling frequency, and elimination of random noise and harmonics. With the conclusions derived in this paper, better overall performance in terms of harmonic elimination can be achieved. In addition, experimental results under different conditions demonstrate the validity of this study.

A fast and accurate detection method of instantaneous reactive current in single-phase power system

Fast and accurate detection of instantaneous reactive current in single-phase power system is a pre-requisite for the precise control of STATCOM (Static Synchronous Compensator). A major solution is based on the construction of virtual orthogonal signal in rotational coordinate transformation; however, conventional methods show weaknesses in the speed of detection and the ability of noise and harmonic suppression. In this paper, a novel method of instantaneous reactive current detection in single-phase system, with fast and accurate property as well as harmonic and noise susceptibility, is proposed. Firstly, a novel virtual orthogonal signal generation algorithm is deduced, remarkably improving the immediacy and precision of detection in synchronous reference frame. Then an enhanced moving average filter (EMAF) is utilized in cascade and its design principle is proposed, sufficiently eliminating the effects of noise and harmonics. Finally, experiments reveal that the proposed detection scheme can achieve the satisfactory control goals.

Static synchronous generator model for grid-tied PWM inverters of renewable energy generation

Three-phase PWM inverters function as energy converters and grid-connected power generators in renewable energy generation systems, and its dynamic and stability characteristics affect the security and reliability of the entire generator sets and even the connected grid. The three-phase PWM inverters based on the VSG (Virtual Synchronous Generator) technology, which is referred to as the Static synchronous generator (SSG) in this article, possess the operation characteristics of Rotational Synchronous Generator (RSG), and thus enables a compatible connection of the new energy to the conventional power grid. This paper analyzes the inherent unity of SSG and RSG by the mathematical model and physical mechanisms, and applies the concept, tools and methods of the stability analysis in RSG to the SSG system. The equivalent inertia, damping and synchronizing coefficients are introduced for the dynamic analysis of SSG and the small-disturbance stability is analyzed accordingly.