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 transformer's insulation design.

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

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high-efficiency dc–dc converters with high voltage step-up ratio to interconnect medium-voltage (MV) dc distribution grids and high-voltage (HV) dc transmission grids. This paper presents an isolated bidirectional soft-switching dc–dc converter combining two-level converters in parallel on the MV side and a modular multilevel converter (MMC) on the HV side. A dedicated control method of the proposed converter is presented. By the proposed method, a certain reactive current is injected into the MV side by the MMC to ensure soft-switching on the MV side. The proposed converter presents low power-semiconductor total device rating and low semiconductor losses over a wide power range at variable input and output voltages. Simulation of a 50 kV/400 kV, 400 MW converter is conducted to evaluate semiconductor losses and verify the validity of this work.

A High Step-Up Ratio Soft-Switching DC–DC Converter for Interconnection of MVDC and HVDC Grids

An HVDC transmission system has a converter transformer as one of its main components. The failure of the converter transformer is one of the major concerns for electric power utilities all over the world. Invariably, the top portions of the secondary windings of the converter transformers fail whereas the primaries are left unaffected. In this paper, an effort has been made to analyze the causes for these failures by means of modeling a practical HVDC system existing in India which ties up Talcher and Kolar and has a length of 1368 km. The modeling and analysis have been carried out in the MATLAB/SIMULINK environment. Based on the analysis, possible solutions for this problem have been suggested, such as providing passive filters on the secondary windings of the converter transformer, connecting a parallel capacitor on the dc side of the converter and R-C snubbers across the secondary windings. The suggested solutions have been compared to bring out their relative merits and demerits.

Analysis of Converter Transformer Failure in HVDC Systems and Possible Solutions

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.

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

This paper proposes an inductively active filtering (IAF) method to comprehensively improve the power quality (PQ) for not only the distribution network (public grid) but also the power-supply system (nonlinear load) connected to the network. At first, a new main-circuit topology for implementing the IAF method is presented, which consists of an inductively filtered transformer and a fully tuned branch controlled by an inverter. Its operating principle and technical features are introduced by comparing with the traditional active filtering method. Based on this, the equivalent circuit is established and by means of mathematical modeling, the unique filtering mechanism of the IAF method is revealed in theory. Further, the control strategy of the FT branch and the impedance coordination for the inductively filtered transformer are designed based on theoretical analysis. A case study is investigated in detail to illustrate the operating characteristics of the IAF method. Both the theoretical and the case studies show that the IAF method can effectively prevent harmonic components from flowing into the primary (grid) winding of the transformer. Since the harmonic components are suppressed near the harmonic source, it is good for the power-supply system and especially good for the converter transformer. Besides, since the harmonic flow is limited to near the harmonic source, the PQ of the public network can be guaranteed completely.

An Inductively Active Filtering Method for Power-Quality Improvement of Distribution Networks With Nonlinear Loads

Harmonic frequency leakage flux can be a limiting factor in three-phase, three-winding HVDC converter transformers. Investigation of a three-phase, three-winding 240-MVA converter transformer failure indicated the failure was caused by harmonic fluxes. Calculations indicated that the magnitudes of these harmonic fluxes to be approximately 45% of the power frequency leakage flux for the transformer, and are little affected by the transformer impedance or the converter firing angle. A study of the failed transformer loading during its life was made and a calculation made of the hot spot temperature considering various insulation half-life factors. Based on published information on insulation half-life factors it was estimated that at full load the hot spot temperature of the transformer was about 159/spl deg/C. From examination of the insulation in the hot spot area, this estimate of hot spot temperature was considered reasonable.

J.A.C. Forrest

Papers

Thermal problems caused by harmonic frequency leakage fluxes in three-phase, three-winding converter transformers