| Three-phase power flow(TPF)calculation of distribution network(DN)is a basic electrical calculation and plays an important role in DN planning and operation.Be based on the TPF,three-phase state estimation is an important part of the distribution management system and the source of data for other advanced application software.Due to the lack of phase angle measurement of the low-voltage distribution network,the existing power flow calculation methods assume that the three-phase angle of the slack bus is symmetrical.The medium-voltage feeder network(take 10 k V as an example)is composed of 10 k V lines and 10 k V transformers.10 k V lines are connected in a way that the neutral point of the three-phase three-wire is not grounded.10 k V transformers include "dedicated transformer" and "public transformer".In the distribution network management,the medium-and low-voltage distribution network is divided into two parts: the medium-voltage feeder network and the low-voltage distribution network.Therefore,it is of great significance to study the TPF and three-phase state estimation for them respectively.However,most of the existing asymmetric models do not fully consider the measurement and control characteristics of medium-and low-voltage networks(MLN),so they cannot be fully applied in practice.In this paper,the following researches are carried out for the low-voltage distribution network and medium-voltage feeder network:(1)Propose a three-phase power flow model of low-voltage distribution network considering phase asymmetry and ungrounded photovoltaicIn practice,the low-voltage side of the actual distribution transformer is often asymmetric.Based on the actual smart meter measurement,the PV model and PQ model of the two-phase nodes of B and C in the slack bus are established.Considering the actual access mode and control mode of the ungrounded photovoltaic inverter power supply connected to the low-voltage distribution network,the independent control models of voltage and power of three phases relative to the neutral point are established.On this basis,the three-phase power flow model of low-voltage distribution network considering phase asymmetry and models photovoltaic is further proposed.The validity of the model built in this paper is verified on the constructed IEEE-13 node correction system.The simulation results show that the model built in this paper can accurately calculate the three-phase power flow of the low-voltage distribution network.(2)Propose a three-phase power flow model of medium-voltage feeder network considering measurement and control characteristics of MLNFor the medium-voltage feeder network,the transformers in the medium-voltage feeder network are divided into dedicated transformers and public transformers.Considering the measurement and control characteristics of MLN and the constraint that the zero-sequence current is zero,two improved power flow models of PV bus and PQ bus at the high-voltage side of dedicated transformer are established.Based on this,combined with the line model and the improved general model of the transformer,a three-phase power flow model for the medium-voltage feeder network considering measurement and control characteristics of MLN is proposed and which is solved by using the current injection Newton method.The simulation results based on the constructed 15-buses medium-voltage feeder system show that the method proposed in this paper can reflect the measurement and control characteristics of MLN and effectively calculate the three-phase power flow of the medium-voltage feeder network.(3)Propose a three-phase state estimation model of medium-voltage feeder network considering measurement characteristics of dedicated and public transformerOn the high-voltage side of the dedicated transformer in the medium-voltage network,the smart meter uses the two-meter method for measurement.On the low-voltage side of the public transformer in the low-voltage network,the smart meter uses the three-meter method for measurement.Base on the three-phase power flow model constructed in(2),a three-phase state estimation model of the medium-voltage feeder network considering measurement characteristics of dedicated and public transformer is established according to the actual measurement types.Using the Lagrange multiplier to transform the optimization problem with zero-sequence current and zero injection constraint into an unconstrained optimization problem,the three-phase state estimation method is solved by the Newton method.The simulation results of the 15-buses medium-voltage feeder system constructed in(2)show that the method proposed in this paper can effectively perform the three-phase state estimation calculation of the medium-voltage feeder network. |
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