L. Wang

Bio: L. Wang is an academic researcher from National Taiwan University. The author has contributed to research in topics: Control theory & Static VAR compensator. The author has an hindex of 2, co-authored 2 publications receiving 114 citations.

Damping of subsynchronous resonance using excitation controllers and static VAr compensations: a comparative study

Abstract: The results of a comparative study on the application of two countermeasures, i.e. the excitation controller and the static VAr compensator (SVC), for damping of subsynchronous resonance (SSR) are presented. To stabilize all the SSR modes, a unified approach based on modal control theory is proposed for the design of the excitation controller and the SVC, which are essentially dynamic output feedback compensators. The two damping schemes differ in the way they modulate the reactive power flow in the system to damp out the subsynchronous oscillations. To demonstrate the effectiveness of the proposed damping schemes under disturbance conditions, time-domain simulations based on a nonlinear system model are also performed. The relative merits of the two countermeasures are compared with respect to their validities under various loading conditions and different degrees of series compensations and their capabilities to expand the stable region on the real-capacitive reactance plane. >

Damping of a parallel AC-DC power system using PID power system stabilizers and rectifier current regulators

Abstract: A scheme for improving the dynamic stability of a parallel AC-DC power system is presented It uses a proportional-integral-derivative (PID) power-system stabilizer and a PID rectifier current regulator to enhance the damping for the electromechanical mode of the system The parameters of the proposed PID controllers are determined using a unified approach based on modal control theory Eigenvalue analyses are performed for the system under various operating conditions in order to compare the damping effects provided by the two different control schemes The effectiveness of the proposed damping schemes under disturbance conditions is demonstrated by computer-simulated dynamic-response tests based on a nonlinear system model >

Design of fuzzy power system stabilisers for multimachine power systems

Abstract: A new type of power system stabiliser based on fuzzy set theory is proposed to improve the dynamic performance of a multimachine power system. To have good damping character istic over a wide range of operating conditions, speed deviation (δω) and acceleration (δω) of a machine are chosen as the input signals to the fuzzy stabiliser on that particular machine. These input signals are first characterised by a set of linguistic variables using fuzzy set notations. The fuzzy relation matrix, which gives the relationship between stabiliser inputs and stabiliser output, allows a set of fuzzy logic operations that are per formed on stabiliser inputs to obtain the desired stabiliser output. Since only local measurements are required by the fuzzy stabiliser on each generating unit, the proposed stabiliser is of decentralised output feedback form and is easy for practical implementation. The effectiveness of the proposed fuzzy stabiliser is demonstrated by a multimachine system example.

Dynamic Stability Analysis of a DFIG-Based Offshore Wind Farm Connected to a Power Grid Through an HVDC Link

Abstract: This paper presents the dynamic-stability analyzed results of an 80-MW offshore wind farm (OWF) connected to a power grid through a line-commutated high-voltage direct-current (HVDC) link. The studied OWF is simulated by an equivalent 80-MW doubly-fed induction generator (DFIG) driven by an equivalent wind turbine through an equivalent gearbox using an aggregation method. The damping controller of the rectifier current regulator (RCR) of the proposed HVDC link is designed by using modal control theory to contribute adequate damping to the studied OWF under various wind speeds and different disturbance conditions. A systematic analysis using a frequency-domain approach based on eigenvalue calculations and a time-domain scheme based on nonlinear model simulations is performed. The eigenvalue analysis is performed to validate the effectiveness of the designed damping controller under different wind speeds. The nonlinear model simulations are carried out to demonstrate the effectiveness of the designed damping controller under various disturbance conditions. It can be concluded from the simulation results that the proposed line-commutated HVDC link joined with the designed damping controller not only can render adequate damping characteristics to the studied DFIG-based OWF under various wind speeds but also effectively mitigate the fluctuations of the OWF under disturbance conditions.

Power-Flow Control and Stability Enhancement of Four Parallel-Operated Offshore Wind Farms Using a Line-Commutated HVDC Link

Abstract: This paper presents an effective control scheme using a line-commutated high-voltage direct-current (HVDC) link with a designed rectifier current regulator (RCR) to simultaneously perform both power-fluctuation mitigation and damping improvement of four parallel-operated 80-MW offshore wind farms delivering generated power to a large utility grid. The proposed RCR of the HVDC link is designed by using modal control theory to contribute adequate damping to the studied four offshore wind farms under various wind speeds. A systematic analysis using a frequency-domain approach based on eigenvalue analysis and a time-domain scheme based on nonlinear model simulations is performed to demonstrate the effectiveness of the proposed control scheme. It can be concluded from the simulation results that the proposed HVDC link combined with the designed RCR can not only render adequate damping characteristics to the studied offshore wind farms under various wind speeds but also effectively mitigate power fluctuations of the offshore wind farms under wind-speed disturbance conditions.

Mitigation of Multimodal SSR Using SEDC in the Shangdu Series-Compensated Power System

Abstract: The Shangdu power plant has four 600-MW turbine-generators connected to the North-China Grid through two 500-kV transmissions, which are compensated with 45% fixed series capacitors. Extensive studies conducted on the system model indicate that the system suffers from multimodal subsynchronous resonance (SSR). To solve the problem, a countermeasure is developed by combining the supplementary excitation damping control (SEDC) and the torsional stress relay (TSR). In this paper, the characteristics of the SSR problem are investigated. Then the developed SEDC is presented. To validate the effectiveness of the proposed SEDC as well as the results of model studies, field tests were conducted under various operating conditions. The tests fully exposed the realistic threat of SSR in the system. Meanwhile, it is demonstrated that the developed SEDC can improve torsional damping significantly, and thus solve the multimodal SSR problem effectively. This is the first time in China that practical SEDCs have been developed and their ability to mitigate multimodal SSR has been verified in a real series-compensated system.

A Simple Approach to Damp SSR in Series-Compensated Systems via Reshaping the Output Admittance of a Nearby VSC-Based System

Abstract: This paper presents a new technique to damp subsynchronous torsional oscillations in a series-compensated system with multimass synchronous generator by reshaping the output impedance of an electrically nearby voltage-source converter (VSC; e.g., interfacing a high-voltage dc transmission system or wind farm). The incremental output admittance of a VSC that is constructed by the circuit components and control parameters of a VSC is the key element for VSC-grid interactions. The proposed subsynchronous resonance damping controller is based on reshaping the virtual output admittance of the interfacing VSC-based system to ensure positive damping in the vicinity of torsional modes. The proposed reshaping technique uses the voltage at the point of common coupling in cascaded compensators to magnify the positive resistance without using generator or turbine speed and/or torque perturbations. Time-domain simulation results and laboratory-scale experimental results are presented to validate the theoretical analysis and show the effectiveness of the proposed approach.