| Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) is playing an more and more important role in the power system of China, and it has become the development trend of the Voltage Source Converter based HVDC (VSC-HVDC), due to its special technical advantages. In the Electromeganetic Transient (EMT) programs, such as PSCAD/EMTDC, the conventional2or3-level VSC-HVDC uses single semiconductor to represent the whole multi-valve. However, the ultral-high voltage and power ratings MMC-HVDC has to simulate the large number of sub-modules (SMs) in each multi-valve individually, which makes the EMT-type simulations extremely slow and the circuit parameters selection and optimization of control systems extremely difficult. Furthermore, it constitutes enormous obstacles for the research of ultral-large MMC-HVDC systems and their engineering applications. Therefore, it is of great theoretical and engineering values to analyze the operational characteristics of MMC-HVDC and propose the EMT-type efficient modelling methods of MMC with the simulation accuracy preserved, which is investigated fundamentally and comprehensively in this dissertation.1) Research on the topology mechanisms and operational characteristics of MMC-HVDCFor the purpose of investigating the EMT-type efficient modelling methods of MMC, the universal topology of MMC and the working modes of the three common sub-module implementations, including Half-Bridge Sub-module (HBSM), Clamp-Double Sub-module (CDSM) and Full-Bridge Sub-module (FBSM) are analyzed. This paper theoretically demonstrated that the traditional dq axis based decoupled control strategies are applicable to the three sub-modules based MMCs, and further deduced the specific control formulas for MMC. Then the Nearest Level Control (NLC) and Carrier Phase Shifted Sinusoidal Pulse Width Modulation (CPS-SPWM) based MMC internal modulation and capacitor voltage balancing algorithms are presented.Based on the above MMC internal control algorithms, the different sub-modules based MMCs can be constructed to closed-loop control MMC-HVDC systems. The essential metrics of the three sub-modules are the capabilities to suppress the short circuit currents under DC pole to pole short circuit fault. Hence the DC Faults Ride-through Capability Index (DFRTI) is proposed to evaluate such capabilities precisely. Simulations show that under the same system configuration, DFRTI(FBSM)> DFRTI(CDSM)> DFRTI(HBSM). Taking the FBSM based3-terminal MMC-HVDC to exemplify, the DC faults location, isolation and fast power recovery schemes for the multi-terminal MMC-HVDC (MMC-MTDC) which is capable of riding through the severe DC faults are proposed and simulations in PSCAD/EMTDC valitated its correctness and effectiveness.2) Research on the controlled source based universal modelling method of MMCWith the ultral-large number of semeconductor components in the large scale MMC switch on and off alternatively, the EMT-type programs has to inverse the large order admittance matrix constantly, this is why it takes extremely long time to simulate ultral-large MMC-HVDC system on EMT-type programs. Therefore, all the accelerated MMC models have to reduce the matrix order of the MMC converters with the simulation accuracy preserved.Based on the Node Voltage Analysis method, the controlled source based EMT-type MMC accelerated model is proposed. The essence of the new model is the partition of one large-scale admittance matrix into substantial small-scale matrices which provide the equivalent solutions; hence it guarantees the accuracy of the MMC external and internal behaviros. Meanwhile, the computation burden of MMC is significantly reduced, as compared to the full detailed model. The proposed MMC model uses the existing building blocks in the user library of the EMT-type program, no need for user-defined modules. Therefore, the new model is not only suitable for the three types sub-modules based MMC systems on PSCAD/EMTDC, but also other multi-level converters on other EMT-type programs. Finally, the HBSM based MMC models on PSCAD/EMTDC valitates the accuracy and speed-up factor of the proposed model.3) Research on the Thevenin's equivalent based overall modelling of MMCThere is no need for user-defined module in the above controlled source based MMC model; hence the building blocks as well as the modulation and capacitor voltage balancing algorithms are the same as the full detailed model. However, one of the remarkable features that MMC-HVDC compared to2or3-level VSC-HVDC is the complex voltage balancing algorithms, and simulations have shown that it does have great impact on the computation efficiency of MMC simulating on EMT-type programs. To solve the problem, this paper proposes the user-difined Thevenin's equivalent MMC model and the highly efficient sorting based balancing algorithm to overall model the MMC system.Taking the HBSM and FBSM based MMC models to exemplify, using the Backward Euler Method to discretize the capacitor in each sub-module to the series connection of the Thevenin's equivalent voltage source and resistance, and assuming the switches to be ideal (i.e. infinite resistance) if they are switched off, hence the computation complexity of the converter is significantly reduced. Meanwhile, as Backward Euler Method is applied to the capacitors, the instantaneous capacitor voltage of each sub-module is determined by only one time step history knowledge of its switching state. Then an efficient MMC capacitor voltage sortinig algorithm whose computation burden increases linearly with the number of the MMC sub-modules is proposed, and the blocking mode implementation of the proposed MMC model is introduced. The CPU efficiency of the overall MMC model is in direct proportion to the simulation scale. Also the MMC model which is composed of the improved converter and the tranditional CPS-SPWM control algorithm is also implemented and validated. Simulations on PSCAD/EMTDC show that the proposed MMC models are comparable with corresponding Averaged-value Model (AVM) in CPU efficiency for the case of MMC model with more than one thoudand sub-modules per arm. Finally, the proposed overall modeling method is extended to full-bridge MMC and its accuracy is validated.4) Research on the use of the averaged-value model of MMC in DC gridIn the above proposed controlled source and Thevenin's equivalent based models, the MMC internal capacitor charging and discharging characteristics are precisely simulated, hence their external behaviors are identical to the full detailed MMC models. However, the averaged-value MMC model uses controlled voltage sources and current sources to represent the AC and DC sides of the Detailed Model (DM). The AVM applies averaging technique and has only one equivalent capacitor, thus it cannot simulate the MMC internal charging and discharging characteristics of each capacitor in DM and quite suitable for efficient system-level simulation of multi-terminal DC grid. However, due to the strong nonlinearity and complex operational characteristics of the DM, the applicability of the AVM needs strict proof to avoid misuse.The HBSM based AVM is benchmarked by comparison with a4-terminal DM based MMC-HVDC grid. Analysis results show that the AVM is just effective when the capacitors are large enough to maintain nearly constant voltage across each MMC sub-module. The type of MMC internal modulation and voltage balancing scheme does not appear to have significant impact on the validity of the model, but the speed-up afforded by the AVM is more when a more complex voltage scheme is in effect. Due to the topological reasons, the previously developed AVM cannot simulate dc faults as well as MMC blocking behaviors properly, as would be required for multi-terminal DC grid simulations. To address this issue, the paper proposed the improved AVM which is shown to work well simulating all DC grid operating conditions. The applicability analysis method and improvements for the HBSM based AVM is with an important reference for CDSM and FBSM based AVMs. |
PDF Full Text Request