Research On Coordinated Control Strategy Of MMC Based Composite Active Power Conditioning System In Microgrid

The technology of distributed renewable energy generation is recognized as the significant "weapon" which can can effectively alleviate the shortage crisis of worldwide primary energy and environmental pollution. One of the important realization forms of the large-scale distributed generations access to the grid is microgrid. On one hand, microgrid could complete local consumption of distributed renewable energy under the island operation mode, on the other hand, it could form the two-way energy flow interconnected with the distribution network. The development of the distributed generation and microgrid technology makes the modern power system change from traditional single vertically integrated structure, toward pluralistic decentralized and cooperative complanate structure. However, on the one hand microgird the advantages of the efficient use of energy, the power optimized distribution of power, the increasement of the economic efficiency and the flexibility of power supply, on the other hand, it is a "double-edged sword"-brings the power quality issues, etc. Such as, the large-scale micro-sources have the natural properties of intermittent, volatility, uncertainty and so on; the use of electric arc furnace and the power electronic devices; micro-sources access to distribution network via power electronic converters; the characteristic of the two-way energy flow leads to the power quality issues in microgird spreading to the next higher grid; the switch of the micro-grid operation modes cause the deviation of frequency and voltage, and so on. It is necessary to control the power quality for microgrid comprising a plurality of micro-sources and the distribution network including microgrid, to provide high-quality operating environment for the new generation power system.Composite active power conditioning system is one of the "Custom Power" based on the full controled power electronic devices. It consists of two back to back converters. Compared with the power quality compensation devices whose function is single, composite active power conditioning system is able to compensate voltage and current power quality problems simultaneously, without regard to the coordination and control among multiple devices. The cost is much lower, relatively. Therefore, it is an opportunity for composite active power conditioning system applying to the actual distribution network.In this dissertation, modular multilevel converter (MMC) is adopted as the two converters of composite active power conditioning. MMC could be applied to the high-level voltage and high-capacity environment, meanwhile, the rich practical experience and development of MMC based high-voltage direct current (HVDC) could be used for reference to achieve the engineering of composite active power conditionging system based on MMC.In order to ensure the provision of microgrid electricity more high-quality and the operation more economical, efficient, safe and reliable, MMC based composite active power conditioning system is applied to microgrid containing distributed generations in this dissertation. MMC composite active power conditioning system not only can efficiently improve microgrid power quality with comprehensive compensation, but also is capable of realizing the power transfer between the distribution network and the microgrid, or between microgrid and distributed generation based on microgrid voltage level, microgrid capacity and installation location to achieve the integration of power transfer and power quality compensation. However, in order to achieve above-mentioned functions the series MMC and the shunt MMC should coordinate well. Thus, this dissertation explores the parameters design, the basic control algorithms, the coordinated control strategy, the optimized redistribution control method, the protection approach and the realization of the intergration of power transfer and power quality compensation of MMC based composite active power conditioning system, which provide a theoretical basis for engineering implementation of MMC based composite active power conditioning system.(1) MMC based composite active power conditioning system operation principle and basic control strategyIn order to increase the applicable voltage level and capacity, the MMC is applied to the composite active power conditioning system. The shunt MMC is parallelly connected between the source side and the load side; the series MMC is connected with source side by the coupling transformer. MMC is the basic unit comprising the composite active power conditioning system. The calculation and selection methods of MMC parameters is proposed firstly. Then the mathematical model of MMC is established. Secondly, based on the operation principle of MMC based composite active power conditioning system, the based detection and control method is introduced. Finally, the simulation results of Power Systems Computer Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) simulation software and Real Time Digital Simulator (RTDS) real-time digital-physical closed-loop simulation verify that MMC based composite active power conditioning system could compensate both voltage sag/swell and voltage harmonics, unbalance currents, reactive currents and harmonic currents.(2) Fixed active current limit coordinated control method applied to the MMC based composite active power conditioning systemA large number of distributed renewable energy access to microgrid, which increases the uncertainty of source-side voltage. The voltage sag duration may be more lasting, the frequency of the voltage sag occurrence and voltage sag depth may be more than the traditional power system’s. As an excellent power quality comprehensive compensation devices, MMC based composite active power conditioning system should be able to do its best to compensate voltage in case of severe voltage sags, continuing the strong and stable operation of microgrid. In this dissertation, the voltage sag compensation capability of MMC based composite active power conditioning system is theoretical analyzed. The mathematical expression of the relationship of the voltage sag depth, compensation duration and main circuit parameters is given. A novel fixed active current limit coordinated control approach based on the tranditional DC voltage coordinated control strategy is proposed. It can provide short-term voltage supply in case of severe voltage sags, meanwhile the over current flowing into MMC swithcing devices could be suppressed by limiting the active current of the shunt MMC. The voltage sag compensation capability is increased by using the energy storage of MMC capacitors. The simulation model in PSCAD/EMTDC adopts the fixed active current limit coordinated control method. The simulation results indicate that this method can increase compensated voltage sag depth and compensation duration, ensuring the short-term voltage supply of the source side.(3) Compensation components optimal redistribution control approaches of MMC based composite active power conditioning system considering the economical assessmentThe ideal power system should only provide the fundamental frequency sinusoidal active current, however the current power quality issues become more serious since a large number of non-linear, volatility, and impluse loads are widely used. The existence of harmonics, unbalance, and reactive currents in the power system causes the increasement of losses, heat, insulation aging and reduced life expectancy of generators, transformers and other equipment. The sophisticated electronic equipment can not operate properly. The communication lines and other measuring lines produce radiated emissions. All the above-menditioned harm increases the large economical losses of both uility and consumer. In the case that the capacity for compensation of the load side is greater than the compensation capacity cou’d be supplied by MMC based composite active power conditioning system, technical improvements of existing equipment is easier than replacing a new larger capacity of equipment at the investment of time and the cost. Therefore it is necessary to achieve the redistribution of MMC based composite active power conditioning system capacity through the demand-side guide. Firstly, the quantitative indicator of the average economic loss of power quality events is introduced as a key factor in the decision to optimize the redistribution compensation of the shunt MMC current control. Then a priority assignment control method focusing on priority compensation and a multi-objective optimization distribution control method focusing on the comprehensive compensation is proposed. Finally, a simulation model of MMC based composite active power conditioning system in PSCAD/EMTDC is established. The effectiveness of the proposed two methods are verified by the simulation results.(4) Protection strategy of MMC based composite active power conditioning system for the load side faultWhen a short circuit exists in the load side of the branch installed MMC based composite active power conditioning system, the device should protect its own safety, which is also the requirement of the microgrid. The principle of protection strategy design for MMC based composite active power conditioning system is analyzed and proposed in this dissertation. The anti-parallel thyristors is connected in parallel on the primary side of the coupling transformer, and once the short circuit occurs on the load side it will coordinate with the breakers to isolate MMC based composite active power conditioning system with the fault section. For the load side fault, the series MMC can be in a locked state or the whole sub-modules of upper arms be bypassing while the whole sub-modules of lower arms be locking. The simulation results based on PSCAD/EMTDC software are verified the effectiveness of these two different protective actions and their advantages and disadvantages.(5) The function realization of MMC based composite active power conditioning system for power delivery and power quality compensative compensationIn order to achieve the efficient use of the distributed energy, the high-quality power supply for local loads in micrgrid as well as the power distribution between microgrid and main grid, the topology and the function of the composite active power conditioning system are expanded. The operation mode of MMC based three-terminal composite active power conditioning system in microgrid is analyzed and the coordinated control strategies is designed. The function to realize the power quality compensative compensation as well as the controllable power delivery between microgrid and main grid is verified by the simulation in PSCAD/EMTDC.