On Control Strategies For VSC-HVDC

Voltage Source Converter based HVDC (VSC-HVDC) is a relatively innovative technology and has many advantages over the classic HVDC in many aspects. With the advances in semiconductors and the cost reduction for installing and operating, VSC-HVDC is expected to find increased use both at transmission and distribution level for its high controllability, environmental benefits and easy expandability in near future. It enables fast control of active and reactive power independently and can delivery power to the weak or passive network on the islands and oil platforms. The rating and voltage level of VSC-HVDC are increasing gradually. The design and installations of VSC-HVDC are in the ascendant.The dissertation is organized as follows.(1) A general discussion about the hierarchy of VSC-HVDC control system, namely the system control, the converter control and the fire control, is presented. The dissertation focuses on the converter control. Based on the properly selected base values, a per unit model in the dq synchronous rotating frames under balanced and unbalanced operating conditions for VSC-HVDC is addressed.(2) The design of dc capacitors and interfacing reactors is an important part for the design of a VSC-HVDC. The sizing of the dc capacitors and interfacing reactors is generally investigated.(3) To enhance the operating performance and the ability to survive the faults, two refined converter control strategies for VSC-HVDC have been introduced and investigated:(i) a novel Active Power Controller (APC) is designed as a Generalized DC Voltage Controller (GDCVC), which takes over the function of DC Voltage Controller (DCVC) automatically and smoothly when the DC link voltage can not be maintained by DCVC for some reasons.(ii) a DC Voltage Dependent Current Order Limiter (VDCOL) is included between the outer active power control loop and the inner current control loop. VDCOL changes the inner active current orders according to the DC link voltage. When the DC voltage is abnormal, VDCOL rapidly modifies the current orders, which are derived from the outer control loops, before been send to the inner current control loops according to pre-set characteristics. With the two proposed converter control strategies, the active power unbalance between the ac and dc system can be both reduced during transients. Therefore the DC undervoltage and overvoltage can be effectively suppressed and the risk of tripping the whole system is reduced.(4)There are uncertainties in the practical VSC-HVDC system, such as the interfacing reactors and dc capacitors may deviate from their rated values for many reasons. These uncertainties may deteriorate the performance without dedicatedly designed controllers. To deal with these uncertainties and meet the performance specifications, a design methodology for robust control of VSC-HVDC using quantitative feedback theory (QFT) is proposed. In order to obtain a high performance, an inner-outer cascaded control scheme of quantitative feedback theory is utilized to derive the inner current loop controllers and outer power loop controllers.(5) In order to reduce the undesired impacts and damages on the VSC-HVDC system and the connected AC system during the startup and re-startup procedures, a two-stage self startup procedure is proposed for two-level and diode-clamped three-level VSC-HVDC system. During the first stage, the resistors are inserted to limit the inrush current during the charging. If the DC voltage can not reach the preset value after predetermined time, the startup procedure switches to the second stage and the capacitors will be further charged with a DC voltage controller. The startup procedure completes till the DC link voltage reaches the preset value and then the converters are ready to change from the startup mode to the normal operating mode.Digital simulation has been used to demonstrate the feasibility of the proposed control strategies and algorithms.