Study On Economic Scheduling And Load Transfer Strategy Of Active Distribution Network With Flexible Multi-state Switch

With the development of economy,more and more loads are connected to the distribution network.Therefore,the consumption of electricity is getting heavier and heavier.However,there are several unreasonable problems in the distribution network,such as construction lag,poor structure,limited control method,which restricts the flexible operation control of distribution network.On the other hand,more and more Distributed Generations(DG)successively connect to the distribution network.Due to the instability of natural conditions,the output of the distributed generations fluctuates constantly,which leads to the unstable distribution network operation.Under this circumstance,Flexible Multi-state Switch(FMS)comes into being.FMS can transform through alternating current(AC)and direct current(DC)mutually.Thus,distribution networks with different voltages and phase angles are connected by FMS,which greatly contributes to reducing harmonious waves effectively and improving operation stability.Meanwhile,FMS can improve DG supply penetration and achieve the sustainable development.However,due to the high cost of construction,there is increasing needs to reduce the operation cost of distribution network and develop more FMS's functions.As a result,the author makes corresponding research and exploration.Firstly,this article summarizes the control mode and switching strategy of FMS.There are mainly three kinds of control mode,PQ,VdcQ and Vf,which is controlled by FMS control center to insure one of the port is controlled by VdcQ to maintain the stability of DC bus.On this basis,the operation state of FMS under fault condition and some extreme condition is analyzed.When a fault occurs on a feeder connected to the port in PQ control mode,FMS can successfully traverse faults.While a fault occurs on a feeder connected to the port in VdcQ control mode,FMS and the whole distribution network cannot operate stably.However,while the switching strategy is operated,FMS can successfully traverse AC faults.While a FMS port is in Vf control mode and the available power or port capacity cannot meet the large loads,the distribution network will operate at reduced voltage to realize the balance of active power and stable operation of distribution network.Finally,the corresponding model is built in PSCAD to simulate and verify the above situation.In order to reduce the operating economic loss of the distribution network connected to FMS,in this paper,a method for calculating the quantity,position and actual running port of FMS is presented.Firstly,the constraint conditions for normal and safe operation of distribution network are established.Secondly,objective function that contains electricity cost,installation charge of FMS and maintenance of energy storage battery is established.Aiming that the traditional power flow calculation method converges and iterate slowly,the structure and constraints of the distribution network are transformed linearly through the second-order cone programming algorithm in this paper.Finally,Cplex solver is used to solve the above problems in MATLAB and fetch the best quantity,location and running constant value.Due to the frequent faults occurring in the distribution network,a puzzle that how to restore the downstream power supply failure quickly rises.In this article,a strategy to ensure the safe operation of distribution network and restore power supply rapidly is proposed.During the fault,the linear relationship between the power of the non-fault port and the node voltage is established.After the fault is removed,the downstream load of fault is supplied by switching control mode rapidly.Under the constraint condition of safe operation of distribution network,the objective function with minimum voltage change before and after load switching is solved.Meanwhile the load cutting amount is calculated repeatedly by binary method.Besides,aiming accomplishing load transfer calculation quickly,calculating constraint time and load removal accuracy are set as stopping criteria in this paper.Finally,the feasibility of this strategy is verified by PSCAD-MATLAB joint simulation.