Topology And Key Technologies Of MMC For Overhead Lines Transmission

Recently,due to the serious environmental pollution problems,it is necessary to inte-grate large-scale renewable energy at long distance to optimize energy distribution and achieve the target of carbon emission reduction.The high voltage direct current transmis-sion technology based on modular multilevel converter(MMC-HVDC)has the merits of expandability,modular structure,decoupling control of active and reactive power,being accessible to passive power grid,low power loss and switching frequency.It is well acknowledged that the MMC-HVDC is a promising approach to integrate the bulk power.Since the energy centers are far away from the load centers,power transmission using overhead lines is inevitable.But the overhead lines are prone to DC faults and the half bridge MMC(HB-MMC)which is widely applied in industry is vulnerable to DC faults.To deal with DC faults,the high power DC circuit breaker can be used to isolate the DC fault lines.Nevertheless,the existing high power DC circuit breaker is too expensive and the technology is still not mature for commercial applications.Therefore,it is necessary to ex-ploit the new MMC topologies with DC fault handling capability.Aiming at overhead lines transmission using MMC-HVDC technology,this paper will thoroughly study the key technologies such as new MMC topologies,operation and control strategy and the equiva-lent electromagnetic transient modeling from the perspective of sub-module topology,con-verter topology and HVDC application.The main contents and contributions are summa-rized as follows:(1)This paper firstly introduces the development and application of MMC-HVDC technology.Focused on the overhead lines application,two approaches that use high power DC circuit breaker(DCCB)and new MMC technologies with DC fault handing capability to deal with DC fault are summarized.Then,the sub-module(SM)topologies using the method of blocking IGBTs to block DC fault and the method of riding through DC faults without blocking IGBTs are introduced and compared.Additionally,the combined convert-ers composed of different converter technologies are presented.Based on these new MMC topologies,the research on control strategies and equivalent electromagnetic modeling are concluded.(2)As for the sub-module topologies using the method of blocking IGBTs to isolate DC faults,this paper studied the fault isolation mechanism and disclosed the simultaneous-ly trigging problems for some types of the SMs.To solve the high cost and simultaneously trigging problems,this paper proposed two types of self-blocking sub-modules(SBSM).To further reduce the cost of MMC,the self-blocking MMC(SB-MMC)with mixed SBSMs and HBSMs on each arm is proposed.Compared with traditional HB-MMC,the number of power electronics required by SB-MMC is only increased by 25%and the power loss is only 32%.The effectiveness and correctness of the proposed SBSM and SB-MMC are val-idated in PSCAD/EMTDC.The results show that SB-MMC is well applicable for industry application.(3)To solve the low efficiency of simulating MMC with hundreds of voltage levels,this paper studied the equivalent electromagnetic transient modeling of SB-MMC.A con-trolled voltage source is used to represent the dynamics of all SMs on each arm.The current paths during pre-charging,normal operation,DC fault blocking and recovery are analyzed.The calculation of voltage sources is given.The simulation results show that the equivalent model greatly improves the simulation speed.This modeling method can also be applied to other types of MMC.(4)As for the sub-module topologies using the method of riding through DC faults without blocking IGBTs,this paper studied the operating and fault ride through control of these SMs.Compared with the SMs using the method of blocking IGBTs,they can effec-tively solve the problem of providing reactive power to support the AC system during DC faults.Taking the hybrid MMC consisted of mixed full bridge sub-modules(FBSMs)and half bridge sub-modules HBSMs into account,an independent pole control scheme and a pole to ground fault ride through strategy is proposed.The output voltages of the upper and lower arms can be independently controlled with the proposed controller.Therefore,the hybrid MMC can continually operate under unbalanced DC pole voltages and ride through DC faults with reactive power support.The simulation results validate the effectiveness of the proposed controller.(5)As for the combined converter,this paper studied the operation and control of the hybrid AC cascade converter which is composed of FB-MMC and two-level voltage source converter.The combined converter uses FBSMs to form the wave-shaping circuit while us-es the two-level voltage source converter as director switches to conduct the current.The asymmetric fundamental frequency modulation strategy is adopted for the director switches and the nearest level modulation is adopted for the wave shaping circuit.The dimensioning method of full bridge SMs and the parameter design method of the SM capacitance are de-duced by instantaneous power analysis.The performance and cost of the combined con-verter are analyzed and compared with other topologies.The validity of the proposed con-trol strategy is verified by simulation analysis.(6)From the perspective of HVDC system,the fault protection schemes of DC grid based on the high power hybrid DC circuit breaker and hybrid MMC topology are analyzed respectively.For the DC grid based on HB-MMC and hybrid DCCB,the modeling of hy-brid DCCB is presented as well as the DC fault clearing method and the multiple re-closure sequence.For the DC grid based on hybrid MMC and fast mechanical DCCB,the continu-ous operation of DC grid during DC fault and the multiple re-closure sequence is designed.The simulation results show that the faulty lines can be cut off by the hybrid DC circuit breaker quickly and the un-faulty system can continue to transmit power.But it requires the hybrid DCCB have high operating speed and large capacity.The DC grid based on hybrid MMC can ride through the DC fault without blocking IGBTs and continually provide reac-tive power to support the AC system.