Research On Key Technologies Of AC-to-DC Transmission Lines Conversion

With the rapid growth of power load demand, the relatively lagging behind of transmission and distribution infrastructure investment makes the transmission capacity of existing AC lines difficult to meet the requirements of development, and parts of transmission lines’ capacity have been close to saturation. It means that these lines need to be upgraded. However, time, cost, land source constrains and other issues increase the the difficulty and infeasibility of the construction of a new transmission line corridor. Therefore, it is necessary to select other effective ways to reconstruct the existing AC line to upgrade its transmission capacity. Conversion of AC lines to DC lines (AC-to-DC line conversion) can increase the transmission capacity significantly, also has the ability of fast control and network segmentation, which contributes to alleviating some problems in AC systems effectively and has good application prospect. Nevertheles, at present, there are only a few of literatures involve the technique of AC-to-DC line conversion, so it has engineering significance to study it deeply and systematically. This thesis focuses on several key issues of AC-to-DC line conversion, and the main works are organized as follows:(1) The topology structure of the tripole structure based HVDC (TPS-HVDC) which is suitable for overhead lines conversion and its operating characteristics are studied. The operation requirements of converters and their combination method are analyzed, and two kinds of typical TPS-HVDC schemes are derived. The topological structure characteristics and current modulation mode of TPS-HVDC are studied, and the maximum possible transmission power is calculated. The characteristics of TPS-HVDC are analyzed from the aspects of power transfer and loss, overload capacity, economy and application etc.. According to different pole’s converter requirements in TPS-HVDC, several feasible combination modes of converters are presented. Two kinds of typical TPS-HVDC schemes are put forward:1) the line commutated converter (LCC) based TPS-HVDC; 2) the modular multilevel converter (MMC) based TPS-HVDC. Based on the different characteristics of these two kinds of schemes, coordinated control methods and additional control strategies are proposed to satisfy the smooth transition of voltage and current in the transiton process, and maintain the stability of the DC transmission power in this process.(2) A three-wire bipole structure based HVDC (TWBS-HVDC) system which is suitable for overhead lines conversion is proposed, the specific control sequence of the transition process is designed, and the operating characteristics and topology structure of the current regulation controller are analyzed and put forward. The topology structure and operating principle of TWBS-HVDC are analyzed in detail, and its maximum possible transmission power is calculated. Based on the analysis of the operating characteristics of the equivalent circuit, detailed action sequence of each switch in the transition phase is given. Current regulating controller is an indispensable link in the stable operation of TWBS-HVDC, and it is analyzed and calculated from three aspects:operating range, power fluctuation, and location. The realization form of two kinds of feasible topologies is compared and analyzed, and its overcurrent protection and control strategy is proposed.(3) A square wave AC based HVDC (SWAC-HVDC) system which is suitable for AC cable conversion is proposed, and its asymmetric modulation and symmetrical modulation methods of voltage and current are given, what’s more, the maximum possible transmission power under two modulaiton mothods are calculated respectively. Based on the analysis of modulation method, two feasible topology structures of SWAC-HVDC are given, and the operating principle and action sequence of switches in the transition process for the three-phase bipole type SWAC-HVDC (TPBT-SWAC-HVDC) are analyzed. Two kinds of feasible schemes for current distribution on parallel lines in TPBT-SWAC-HVDC are compared, and the advantage of the variable resistor is given. Resistance distribution and the realization form of the variable resistor are designed.(4) Based on TPS-HVDC and TWBS-HVDC, two kinds of topological structure schemes for conversion from AC lines to multi-terminal HVDC (MTDC) system are put forward, which provides technical supports for the power transmission in larger scale, and contributes to further improving the transmission capacity of networks and relieving the transmission congestion. Through the comparison of simulation results, the difference of communication requirements, control coordination and the influence scope of transition processes between the TPS-HVDC based MTDC and the TWBS-HVDC based MTDC is studied. And it is pointed out that the TWBS-HVDC technology has an advantage in application of conversion from AC lines to MTDC. An AC/DC simulation model with the TWBS-MTDC is established, with the New-England 10 machine 39 bus system seen as the converted object. For AC/DC faults, the operating stability of the system is studied and analyzed.(5) A hybrid HVDC transmission system based on LCC and full-bridge sub-module (FBSM), half-bridge sub-module (HBSM) based hybrid MMC (FH-MMC), which is used for TWBS-HVDC, monopole HVDC and bipole HVDC, is proposed. A detailed description of the structure characteristics of FH-MMC is presented, and operation characteristics of FH-MMC in steady state and DC side fault conditions are analyzed. Combined with the characteristic analysis of voltage drop under the AC system fault at the sending end, a configuration method for the arm sub-module number is derived, which can achieve the effect of less investment and low loss under the premise that FH-MMC meets all operation requirements. The control characteristic of the hybrid HVDC system is studied. The two-layer control method including the converter station layer and valve layer of FH-MMC is put forward, and the sub-module capacitor voltage balancing strategy and the sub-module selection process are studied prominently to maintain the normal operation of FH-MMC in transient and steady state and reduce switch losses.(6) The power flow calculation model of the DC grid with power flow controllers is proposed. Based on the comparation and analysis of the advantage and disadvantage of existing power flow controllers, a new current flow controller (CFC) is proposed. The topology structure of CFC and its basic operating characteristics are introduced. The operation principle of CFC is studied from three kinds of branch current relationship, and nine kinds of operating states are summarized. What’s more, the control strategy of CFC is given and the relationship of each state variable is deduced. The economy of CFC is analyzed simply, which indicates that the investment cost and operation losses of CFC are both low, and CFC has good application prospect.