Loss Evaluation And Reduction Approaches For VSC-HVDC Systems

The relative higher converter loss is one of the main barriers for the application of VSC-HVDC in bulk power transmission systems. Thus to research the loss evaluation and reduction approaches is of evident application value and practical significance. In this dissertation, the loss evaluation and reduction approaches of VSC-HVDC are investigated by means of theorical analysis and the simulation validatins. The main contents and related conclusions of this dissertation are as follows:(1) The loss evaluation of VSC-HVDC is investigated and an innovative generalized loss calculation method based on the curve fitting theory is proposed. By analyzing the switching characteristics of IGBT and the anti-parallel diode, the functions of IGBT losses with voltage, current, junction temperature and dead time are derived. The parameters in the functions can be obtained from the datasheet supplied by the manufacturers. Compared with the physical model and functional model, this proposed method is more suitable for practical engineering. A universal VSC-HVDC loss calculation module is established in PSCAD/EMTDC and it provides a powerful tool for VSC-HVDC loss reduction.(2) The characteristic of the multi-level converter is investigated and an innovative multi-pulse voltage source converter with low loss ratio is proposed. The proposed converter is composed of a 12-puls converter and an auxiliary circuit. The auxiliary circuit injects DC voltage via the mid point of the 12-puls converter. Proper injection ratio and frequency parameters are selected to convert the standard 12-pulse into 60-pulse configuration without using PWM or increasing the number of bridges, thus the voltage and current can fulfill the THD limit without conventional filter. As the main bridges operate under at fundamental frequency, the switching lose is relatively low. This converter may be widely used in the HVDC and FACTS system. The principle and control strategy of the proposed converter are described. Its operational feasibility is also verified by digital simulation using the PSCAD/EMTDC package.(3) The loss characteristics and control performance of the common modulation methods is analyzed and an innovative hybrid modulation method, which can reduce the VSC-HVDC loss and meet its static and dynamic performance requirement, is proposed. The new hybrid modulation method is equipped with two different PWM methods, i.e, SPWM and Minimum Switching Losses PWM. When the VSC-HVDC transmission system is under disturbance or transient states, the SPWM method, which has faster response, will be used; otherwise, in static state, the Minimum Switching Losses PWM method will be used. The system states can be obtained by the "Disturbance Detector", then the system states is used to select the proper modulation method dynamically. The operational principle of the hybrid modulation method is demonstrated in this paper and its operational feasibility is also verified by digital simulation using the PSCAD/EMTDC package. The results show that this hybrid modulation method can fulfill the static and dynamic performance and can be used in the control of VSC-HVDC.(4) The feasibility of loss reduction using Hybrid HVDC is investigated. The limitations of the Hybrid HVDC based on LCC and VSC is analyzed and an innovative Hybrid HVDC topology is proposed. In this new Hybrid HVDC, the traditional LCC is used at rectifier side and the CSC is used at inverter side. The mathematical model, firing scheme and control strategy of this Hybrid HVDC is set up and carefully designed. The system responses following the common faults are analyzed by simulation. The simulation results show that: (1) This Hybrid HVDC can clear DC line faults its converter controllers and can restart quickly after fault clearness; (2) When the AC voltage drops to 0.3 pu because of severe faults both at rectifier and inverter side ac systems, the hybrid HVDC can still transmit certain amount power to inverter side AC system and is helpful for the stability of the inverter side system. When the AC fault has been cleared, the Hybrid HVDC can recover quickly to its steady states before fault. Therefore, this Hybrid HVDC system has a promising potential for the bulk power transmission.