Research On Extended Technologies Of DC-DC Autotransformer

Power transmission using alternating current(AC)and direct current(DC)hybrid technologies has become one of the main features of the modern power system,which leads to the inter-connected AC grids and DC grids in the future power system.In such a hybrid AC-DC power system,the high-capacity DC/DC converter used to interconnect the DC grids at different voltage levels is an essential equipment in the future power system.Among the various topologies of high-voltage and high-capacity DC/DC converters,the DC/DC.Autotransformer(DC AUTO)is an emerging technology with distinct economic advantage.However,the original topology of DC AUTO is not able to isolate the DC faults at the primary side and secondary side.So it cannot be directly applied to real-world DC grids.Meanwhile,there are many practical issues related to the application of DC AUTO.This thesis will focus on the research of key technologies of DC AUTO including the con-troller design,fault ride through strategy,development of low-cost topology and so on.It is desired that the results can make guidance for the engineering application of DC AUTO.Firstly,the operating principle of DC AUTO is analyzed in this thesis,together with its original topology,and the power balance relationship.The controllability of the internal common AC bus voltage and the control method for normal operation mode are interpreted.And the fault characteristics during DC fault at the primary side and secondary side are concluded in detail.Based on these analyses,the improved DC AUTO topology with bi-directional DC fault isolation capability is proposed.The detailed design of the convert-ers and the current limiting reactors in the proposed DC AUTO are presented.The effe c-tiveness of the designed topology and the fault isolating capability of proposed DC AUTO are validated by simulation in PSCAD/EMTDC.The conversion losses and the cost of the proposed DC AUTO are also evaluated.In order to achieve the fault ride through function of DC AUTO,it is necessary to let the converter operate under wide DC voltage range.Therefore,taking the hybrid MMC based on full bridge sub-module and half bridge sub-module into consideration,this paper comprehensively studied the hybrid MMC operating under reduced and even negative DC voltage.And the control scheme system under the fault conditions is interpreted in detail.From the control perspective,the control schemes with a DC current dynamic limiter at the inner DC current control loop and with an additional controller at the inner AC current con-trol loop are proposed to avoid the abnormal sub-module capacitor voltage.To improve the response speed of the control system,a feed forward DC voltage loop is added in the inner DC current control loop.To advance the traditional submodule voltage balance algorithms with power loss reduction function,a novel PING-PONG submodule voltage balance algo-rithm with power loss reduction function implemented by introducing the voltage hysteresis is proposed.Additionally,a universal equivalent electromagnetic simulation accelerated model based on dual-diode and dual controlled voltage source is proposed.The proposed equivalent accelerated model can well represent the dynamics of the detailed model under all operating scenarios.Finally,the effectiveness of the proposed methods is demonstrated in PSCAD/EMTDC.In this thesis,the DC AUTO with the fault riding through capability is studied in detail.By analyzing the target and the requirements to DC fault ride through,the detailed design method and control scheme are presented.The proposed fault ride through strategy is well applicable to different grounding configurations at the MMC2 valve side.Once a pole to ground fault happens,the fault current can be effectively eliminated and the system can transmit a certain amount of active power.Under extensive simulation in PSCAD/EMTDC,the ride through capability of DC faults at the primary side and secondary side is validated.In addition,to reduce the cost of DC AUTO,a unidirectional step-up DC AUTO(UUDAT)and a unidirectional step-down DC AUTO(DUDAT)with DC fault isolation ca-pability are proposed.The design method of the topologies,the element parameters and the controllers for these two unidirectional DC AUTOs are given.And the fault isolation mechanism is analyzed.The effectiveness of these two unidirectional DC AUTOs is veri-fied by simulation in PSCAD/EMTDC.To further verify the theoretical results,the laboratory prototypes of the proposed DC AUTOs,the hybrid MMC and the related test systems are established.A series of dynamic experiments have been carried out.The main tests include power flow validation of the bi-directional DC AUTO,unidirectional step-up DC AUTO and unidirectional step-down DC AUTO;reduced DC voltage and negative DC voltage operation of the hybrid MMC;pole to pole DC short circuit fault riding through validation of the hybrid MMC.The con-figuration,components,controller design,main circuit design of the test system are dis-closed.The experimental results validate the correctness and effectiveness of the proposed DC AUTO and the hybrid MMC.