Longfu Luo

Wide-Area Robust Coordination Approach of HVDC and FACTS Controllers for Damping Multiple Interarea Oscillations

A robust coordination approach for the controller design of multiple high-voltage direct-current (HVDC) and flexible ac transmission systems (FACTS) wide-area controls (WACs) is presented in this paper and has the aim of stabilizing multiple interarea oscillation modes in large-scale power systems. The suitable wide-area control signals, which are given to HVDC and FACTS wide-area controllers, respectively, are chosen from a large number of candidate items. Then, a sequential robust design approach is planned for the wide-area controller coordination of HVDC and FACTS devices. This approach is based on the robust control theory and is formulated as a standard problem of multiobjective mixed H2/H∞ output-feedback control with regional pole placement constraints. The linear matrix inequality (LMI) theory is applied to solve such a robust control problem. A case study on the 16-machine 5-area system, which is modified with one HVDC interconnected transmission, one shunt-FACTS device (SVC), and one series-FACTS device (TCSC), is performed to validate the robust performance in terms of multiple oscillations damping under various operating conditions.

A New Converter Transformer and a Corresponding Inductive Filtering Method for HVDC Transmission System

A new converter transformer and an inductive filtering method are presented to solve the existing problems of the traditional converter transformer and the passive filtering method of the high-voltage direct current (HVDC) system. It adopts the ampere-turn balance of the transformer as the filtering mechanism. A tap at the linking point of the prolonged winding and the common winding of the secondary windings is connected with the LC resonance circuit. It can realize the goal that once theharmonic current flowsinto the prolonged winding, the common winding will induct the opposite harmonic current to balance it by the zero impedance design of the common winding and the proper configuration of LC parameters, so there will be no inductive harmonic current in the primary winding. Moreover, the reactive power that the converter needs can be partly compensated in the secondary winding. Simulation results have verified the correctness of the theoretical analysis. The new converter transformer can greatly reduce the harmonic content in the primary winding, loss, and noise generated by harmonics in the transformer, and the difficulty of the transformers insulation design.

A Multi-Purpose Balanced Transformer for Railway Traction Applications

A multi-purpose balanced (MPB) transformer is proposed in this paper for electric railway applications. The MPB transformer is developed from impedance-matching (IM) transformers widely used in Chinas railway systems. The proposed MPB transformer has three main functions: (1) it provides a symmetrical two-phase voltage of 27.5 kV for railway overhead lines to power electric trains; (2) it supplies a three-phase balanced voltage of 10.5 kV for railway substations; and (3) it can accommodate a three-phase harmonic filter with improved power factor and reduced manufacturing cost. In this paper, the operating principles of the MPB transformer are elaborated, the design of the harmonic filters is discussed, and the results of computer simulations are provided. A 16-MVA MPB transformer was designed and commissioned, and real-time field measurements are provided.

Assessment and Choice of Input Signals for Multiple HVDC and FACTS Wide-Area Damping Controllers

This paper proposes a hybrid method to assess and select suitable control inputs for multiple HVDC and FACTS supplementary wide-area damping controllers (WADCs) of large-scale interconnected power system. This method can not only reduce the interaction among different WADCs, but also get good performance of damping multiple inter-area oscillations. The residue analysis method based on observability/controllability index is used to preselect the input signal candidates for each WADC. Then, the relative gain array (RGA) analysis, including the steady-state values and the dynamic behavior, is carried out to evaluate and determine the suitable input signals for multiple WADCs. Since the undesired interaction among multiple WADCs is reduced significantly by the proposed method, we can design each WADC independently without considering the coordination of controllers, which can reduce the design difficulty of multiple WADCs. A detailed case study validates the proposed method, and indicates that the multiple WADCs adopting the selected wide-area signals can effectively damp multiple inter-area oscillations at different operating scenarios (e.g., line fault, line outage, and load shedding).

Study on Characteristic Parameters of a New Converter Transformer for HVDC Systems

Different from the traditional converter transformer and ac passive filtering method, this paper presents a new converter transformer and a corresponding inductive filtering method for HVDC systems. Based on this, characteristic parameters of the new converter transformer are studied in detail by theoretical analysis, simulation, calculation, and experimental verification. By deducing voltage relation, the constraint condition of the turn ratio is obtained, which can satisfy the demand of a 12-pulse converter in HVDC systems. By deducing current relations, the characteristics of the fundamental and the harmonic currents in the winding are obtained. According to the established operation equations, the simple but practical calculation methods for the short-circuit impedance and the rated capacity are proposed. Then, the change characteristic of the short-circuit impedance is analyzed and calculated. The detailed simulation and test results verified the correctness of the aforementioned theoretical analysis and calculation.

An Industrial DC Power Supply System Based on an Inductive Filtering Method

This paper proposes a new direct current (dc) power supply system based on an inductive filtering method for one industrial dc load. The operating mechanism of the new inductive filtering method is analyzed, and then, the harmonic model and the equivalent model are established. Moreover, the impedance conditions for the implementation of the inductive filtering method are presented, and the influence of the filtering loops impedance on the filtering performance is clarified. Both the simulation and the experimental study on one practical inductive-filtering-based industrial dc supply system for chemical electrolysis are used to validate the theoretical analysis. The research results show that the inductive filtering method not only greatly reduces the harmonic magnetic flux in the rectifier transformer but also prevents harmonic currents from flowing into the primary (grid) winding of the rectifier transformer. It presents good filtering and reactive power compensating performances to public networks and also increases the operating efficiency of the industrial dc power supply system itself.

Study on Steady- and Transient-State Characteristics of a New HVDC Transmission System Based on an Inductive Filtering Method

Different from the main circuit topology of the traditional current source converter (CSC)-based HVdc transmission system, this paper proposes a new CSC-HVdc transmission system based on a new inductive filtering method. Such new HVdc system characters as a new main circuit topology and mainly contains a new converter transformer and related full-tuned (FT) branches. With regard to the implementation conditions of the new inductive filtering method, the basic main circuit parameter design for the new converter transformer, the related FT branches, and the HVdc control system are presented, respectively. Based on these designed main circuit parameters and control system, the new CSC-HVdc system is modeled in the PSCAD/EMTDC environment, and correspondingly, through comparative study with the International Council on Large Electric Systems (CIGRE) HVdc benchmark model, the steady- and the transient-state operating characteristics of the new CSC-HVdc system are studied in detail. The research results show that the new CSC-HVdc system can widen the stable operating range, increase the dc transmission capacity, and meanwhile, improve the commutating performance of the CSC-HVdc converter. Moreover, it maintains the stable operating performance and represents the good fault-recovery ability when suffering the typical faults (e.g., single-phase or three-phase faults at the rectifier and the inverter side of the HVdc transmission system).

A New Integrated Hybrid Power Quality Control System for Electrical Railway

This paper presents an integrated hybrid railway power quality control system (HRPQC) to deal with the power quality problems in the electrical railway supply system. The prominent advantage of HRPQC is that, in the premise of adopting a hybrid compensating scheme for better performance, it can integrate the active system with the main transformer without other expensive low-frequency high-power auxiliary transformers. HRPQCs mathematical model, control idea, and transforming relationship in harmonic frequency are first presented. Second, the harmonic-suppressing mechanism and the resonance characteristics are analyzed in detail. Finally, simulation and experimental results verify the effectiveness of the proposed compensating system.

Simulation of the Electromagnetic Response Characteristic of an Inductively Filtered HVDC Converter Transformer Using Field-Circuit Coupling

A field-circuit coupling computation method is proposed in this paper to reveal the special electromagnetic characteristic of a novel inductive filtering (IF) method applied in high-voltage direct current (HVDC) converter station. The new converter transformer is modeled by using 3-D finite-element method, and the surrounding fully tuned branches, for creating precondition of implementing the IF method, are modeled by using the coupled circuits. The harmonic characteristic of the load-side current is modeled by using mixed harmonic current sources. The parameter design of the coupled circuit is also performed for the coupling model. Following this, based on the established coupling model, the multiphysical characteristics (e.g., electric field, magnetic field, excitation characteristic, and core losses) are analyzed in detail. The system simulation and the experimental results validate the modeling method. All of the research results indicate that the IF method can greatly reduce the harmonic currents in the grid winding, can improve the excitation performance, can improve the electromagnetic environment, and can reduce the harmonic losses of the HVDC converter transformer.

A Virtual Impedance Comprehensive Control Strategy for the Controllably Inductive Power Filtering System

In this letter, a virtual impedance comprehensive control (VICC) strategy is proposed for the controllably inductive power filtering (CIPF) system with a new filtering mechanism. This control strategy aims to satisfy the zero-impedance design precondition of the inductive power filtering system, and can dampen the harmonic resonance at the grid side. By the proposed zero-impedance control, the quality factor of the passive power device can be adjustable, and the single-tuned filter can be multituned. First, the main circuit topology for implementing the VICC-based CIPF is presented. Then, on the basis of the multipurpose control, the VICC strategy is designed. Furthermore, by means of the established equivalent circuit model and the corresponding mathematical model, the principles of the harmonic damping and the zero-impedance realization are revealed. Finally, the experimental results verify that the proposed control strategy can weaken the harmonic amplification effectively, and improve the filtering performance significantly.