Fusheng Liu

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.

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.

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).

Realization of Reactive Power Compensation Near the LCC-HVDC Converter Bridges by Means of an Inductive Filtering Method

In this paper, a reactive power (Q) balance strategy is proposed to enhance the stability of the HVDC inverter connected to weak grids. The main circuit topology of a new line-commutated-converter (LCC)-based HVDC system is presented, where a new converter transformer and related fully tuned (FT) branches are included in the converter subsystem for implementing an inductive filtering method. The phasor analysis is used to reveal a special Q-compensation performance of the new LCC-HVDC system. Then, an equivalent impedance representation, which takes the Q-compensation degree at the ac valve side of the converter into consideration, is established by a mathematical modeling. Based on these, the impact of the inductive filtering on the Q-compensation characteristic of the converter is investigated under different HVDC control modes (e.g., constant dc-current and dc-voltage controls at the rectifier and the inverter sides, respectively). Finally, both the transient simulation and experimental results validate the theoretical analysis, and further demonstrate that the proposed Q -balance strategy can compensate reactive power near the converter bridges (the Q-absorber), widen the operating range of the converter, and enhance the stability of the LCC-HVDC connected with weak grids.

Harmonic Transfer Characteristics of a New HVDC System Based on an Inductive Filtering Method

This paper presents a new HVDC system improved by an inductive filtering method, and afterward proposes a unified equivalent circuit model and a corresponding mathematical model for the calculation of the harmonic transfer characteristics of the new HVDC system. First, according to the special topology of the new HVDC converter station, an equivalent circuit model described by the fundamental and the harmonic impedances is established. Second, via the mathematical modeling of this new HVDC system, a harmonic transfer model is also established, which can be used to express the effect of inductive filtering method on the harmonic distribution characteristics. Moreover, by the theoretical analysis on the commutation process of the HVDC converter, a time-domain expression of the valve current is obtained to represent the harmonic characteristics of the converter. Finally, an experimental study validates the established analytical model and illustrates the harmonic transfer characteristics of the new HVDC system.

Harmonic characteristics of new HVDC transmission system based on new converter transformer

The study on the harmonic characteristics of HVDC transmission system is important for analyzing the effect of harmonics on the operation of the electrical installation and the system. The new HVDC transmission system adopts the new converter transformer and the corresponding inductive filtering method, and it is a new electrical topological structure. As for a new system, the harmonic characteristics of the new HVDC transmission system are completely different from that of the traditional one. This paper introduces the main circuit topology of the new HVDC transmission system, and then analyzes the connection of the new converter transformer and the filtering mechanism of the inductive filtering method. Based on these, the mathematic model has been established to calculate the harmonic characteristics of the new HVDC transmission system. Furthermore, the amplitude and phase angle of the main characteristic harmonics of the new HVDC transmission system have been calculated by the established mathematic model and then simulated by MATLAB/SIMULINK. Tested results have verified the correctness of above analysis and calculated results. The research findings are useful for the design of the characteristic parameters of the new HVDC transmission system, the selection of the wire cross-section of the new converter transformer, and the design of the inductive filtering installations.

A Novel Method of Harmonic Suppression in the AC/DC Transmission System Based on Novel Converter Transformer

In the traditional AC/DC transmission system, to reduce the effect of harmonics to the AC system produced by the converter, filters and reactive power compensating equipments are installed in the primary side. This article presents a novel converter transformer and harmonic suppression scheme- filter commutated converter (FCC), which can greatly restrain the harmonic current from flowing into the AC system. The article also introduces the connection mode of windings, calculation of voltage offset angle, determination of windings turn ratio and request of windings equivalent impedance; and it analyzes the harmonic suppression principle, too. Finally, it establishes the digital simulation models of DC transmission system based on the novel converter transformer and traditional converter transformer. The simulating results of frequency spectrum of current wave indicate that the 5th , 7th , 11th and 13th harmonic components can be effectively suppressed in the valve side with FCC. Compared with the results of the traditional harmonic suppression, the reliability and feasibility of the novel converter transformer and the scheme of harmonic suppression (FCC) are testified.

Transient response characteristics of new HVDC transmission system based on new converter transformer

The new HVDC transmission system adopts the new converter transformer and the corresponding inductive filtering method, and its electrical topology structure is completely different from that of the existential HVDC transmission system. For the change of the controlled system, the transient response characteristics of the new HVDC transmission system will be accordingly changed when it adopts the standard control model that the traditional HVDC transmission system adopts. Referring to the parameters of the main circuit of the OGRE HVDC first benchmark model, this paper designs a similar benchmark model of the new HVDC transmission system based on the technical characteristics of the new converter transformer and the corresponding inductive filtering method, which mainly contains the rated parameters of the new converter transformer and the inductive filtering device. Then, the typical transient response characteristics of the new HVDC transmission system have been analyzed and simulated by PSCAD/EMTDC. The results express that the new HVDC transmission system, which adopts the standard control model that the HVDC transmission engineering adopts, has a good transient response characteristic, and can operate stably when subjecting to various typical disturbance.

Transient Simulation of the AC/DC System Based on the New-type Converter Transformer

For announcing the electromagnetic transient course of the auto-compensator and harmonic shielding converter transformer, simulating the new-type converter transformers voltage, currents input and output characteristics in the HVDC system and the interaction of the AC/DC system, This paper establishes the new-type converter transformers transient simulation mathematic model using the synthetic companion method and the controlled source theory, and the simulation experiment is carried on by applying this model into the HVDC transient simulation. The experimental results show that the new-type converter transformer optimizes the systems structure in the certain degree. Through the zero impedance design of the third winding and the filter parameters reasonable setting, it not only can greatly reduce the harmonics in the AC side and the loss to the converter transformer, but also improve the converter bridge AC sides voltage and current wave, which is advantageous to the converters normally moves.

Influence analysis of Compensation Factor at the valve side on HVDC transmission system based on filter commutated converter

In the paper, a novel filter commutated converter (FCC) is proposed, taking its special configuration into account, the reactive power compensation factor at the valve side (compensation factor in short) is defined. Its indeed necessary to research Compensation Factor how to influence on the commutated reactance and the HVDC transmission systems line voltage of the valve side without load, DC voltage, commutated angle and power factor. Based on the mathematical model of FCC, the relationships between the above parameters with compensation factor are analyzed respectively. Depending on the testing platform of DC transmission system adopting FCC, through comparing the theoretic results with the testing ones, the results verified the above analytical conclusions reliable. The results indicate that the commutated reactance and system stability both must be taken into account when selecting rational compensation factor, a bigger compensation factor does not necessarily be the better.