Operation And Control Of Active Distribution Network Based On Power Electronic Compensators

The distribution network,connecting the transmission network and power load,is the public service facilities of the national economy and social development.The optimized operation methods of the existing distribution network mainly include transformers' economic operation,network reconfiguration and capacitor banks,which are mainly based on switch operation.Moreover,the switch operation has the disadvantages of high operating cost,slow response and large transient impact.Recently,the penetration of the distributed generator(DG)is increasing year by year in Chinese medium-voltage and low-voltage distribution network.The DG output power has the characteristics of volatility,intermittency and randomness,which aggravates the fluctuation of power flow and voltage of the distribution network and highlights the deficiencies of optimized operation and management of the distribution network.For the active distribution network with high permeability of distributed power supply,traditional management technology is difficult to adapt to its complex and variable operating conditions.The power electronics compensation equipment includes Unified Power Flow Controller(UPFC),Soft Open Point(SOP)and Static Synchronous Compensator(STATCOM),which can flexibly regulate the power flow of distribution network,provide flexible and controllable reactive power compensation,and realize multi-objectivecontrol of node voltage and distribution network loss.The power electronics compensation equipment could optimize and manage the active distribution network in real time and have the advantages of fast response,better regulation performance and output power quality,which would be the important device for the active distribution network.This paper further studies the operation and regulation mechanism of UPFC,back-to-back SOP and STATCOM,and proposed some new compensation technologies with the advantages of low cost,multi-function and high performance.The main research contents could be summarized as follows:(1)Research on UPFC-Based economic operation of distribution network.The traditional transformers' economic operation and network reconfiguration technology have the limited operation times,which would restrain the capabilityof network loss optimization.UPFC could flexibly regulate the power flow between the feeders and optimize the distributed power flow in real time.However,the feeders have direct electrical connection after the regulation of UPFC,which would affect the protection and the power supply reliability of distribution network.This paper proposes a novel technology of flexible network reconfiguration and flexible transformer economic operation technology based on UPFC.The UPFC can accurately regulate the power flow through the transformers,distribution lines and help realize the zero current turning on/off of the breakers and transformers.The system can flexibly switch transformers' operation modes and optimize the network reconfiguration in real time.The economic operation of distribution network based on the UPFC is hybrid control and is the combination of UPFC control and mechanical switch operation.The novel method breaks through the strict restrictions of limits on operation times of network reconfiguration and transformers' economic operation adjustment,which could deeply excavate the capability of loss reduction for the distribution network.The economic operation of distribution network based on UPFC has been proved effectively and feasibly in the simulation tests.(2)Research on hybrid Back-to-Back Soft Open Point.The soft open point(SOP)using the back to back power converter can flexibly,fast and accurately regulate the power flow among the adjacent feeders by implementing the multi-targets control including nodes voltages and network losses and also has the capability to isolate the fault and recover the power supply.However,the capacity of full-scale SOP should be set to handle the maximum transferred load power,which results in the disadvantage of largesize and high cost.This paper proposes a hybrid soft open point system with a partial scale back to back converter and a load switch parallel connected.The power flow and voltage distribution could be regulated by the back to back converter as usual.And the load switch could realize power recovery for all the customer interruption under the fault condition.Moreover,the back to back converter can flexibly regulate the power flow through the load switch and help realize the zero current turning off of the load switch,which can effectively extend the working life of the load switch.Furthermore,the performance of the proposed hybrid SOP is verified by the simulation results.(3)Research on flexible third harmonic control of cascaded H-Bridge STATCOM with low capacitance.The traditional static synchronous compensator(STATCOM)using the cascaded H-bridge(CHB)converter will unavoidably suffer the low order harmonic oscillation of the dc voltage per sub-module and therefore the large dc capacitance should be assumed to prevent the overvoltage operation condition.This paper proposes a novel control method to effectively reduce the dc capacitance of CHB-based STATCOMs and meanwhile keep the identical inductive and capacitive reactive power compensation capability.By flexibly injecting the third harmonic voltage and controlling the averaged capacitor voltage simultaneously,the maximum capacitor voltage could be regulated in the allowable range and the dc capacitance can be reduced to half of traditional STATCOM's capacitance.Moreover,the capacitor peak voltage could be optimized in the low region in real time.Doing so,the switching losses and capacitor voltage rating can be obviously decreased.Furthermore,MATLAB simulations and experimental prototype verified the performance of the proposed control strategy.