Power Flow Model for Medium-voltage Distribution Systems Considering Measurement and Structure Characteristics

Medium-voltage distribution systems(MVDSs)mainly consist of a feeder head,lines,distribution transformers,and the equivalent load or power supply interfaced with the distribution transformers.The information of such load or power supply can be measured via the three-wattmeter method(THM)and the two-wattmeter method(TWM).The measurements can be used to perform the control of the power supply and simulate the characteristics of the load,so the models of the load and the power supply need to consider the measurement characteristics.Existing research works on three-phase power flow(PF)just consider the measurement characteristics of THM.Hence,the PF equation of the bus measured via TWM is firstly built.Based on conventional measurements,an accurate and general model of the grounded and ungrounded slack bus is proposed.Furthermore,the influence arising from the connection type and angle shift of distribution transformers on the admittance matrix is considered,and thus a general three-phase transformer model is summarized,which is applicable for all the transformers mentioned herein.Finally,Newton's method is adopted to solve the PF calculation,and the performance of the proposed PF model is demonstrated through designed tests.

Multi-time Scale Optimal Power Flow Strategy for Medium-voltage DC Power Grid Considering Different Operation Modes

Direct current(DC)power grids based on flexible high-voltage DC technology have become a common solution of facilitating the large-scale integration of distributed energy resources(DERs)and the construction of advanced urban power grids.In this study,a typical topology analysis is performed for an advanced urban medium-voltage DC(MVDC)distribution network with DERs,including wind,photovoltaic,and electrical energy storage elements.Then,a multi-time scale optimal power flow(OPF)strategy is proposed for the MVDC network in different operation modes,including utility grid-connected and off-grid operation modes.In the utility grid-connected operation mode,the day-ahead optimization objective minimizes both the DER power curtailment and the network power loss.In addition,in the off-grid operation mode,the day-ahead optimization objective prioritizes the satisfaction of loads,and the DER power curtailment and the network power loss are minimized.A dynamic weighting method is employed to transform the multi-objective optimization problem into a quadratically constrained quadratic programming(QCQP)problem,which is solvable via standard methods.During intraday scheduling,the optimization objective gives priority to ensure minimum deviation between the actual and predicted values of the state of charge of the battery,and then seeks to minimize the DER power curtailment and the network power loss.Model predictive control(MPC)is used to correct deviations according to the results of ultra short-term load forecasting.Furthermore,an improved particle swarm optimization(PSO)algorithm is applied for global intraday optimization,which effectively increases the convergence rate to obtain solutions.MATLAB simulation results indicate that the proposed optimization strategy is effective and efficient.

Dynamic State Estimation of Medium-voltage DC Integrated Power System with Pulse Load

The dynamic characteristic evaluation is an important prerequisite for safe and reliable operation of the mediumvoltage DC integrated power system(MIPS),and the dynamic state estimation is an essential technical approach to the evaluation.Unlike the electromechanical transient process in a traditional power system,periodic change in pulse load of the MIPS is an electromagnetic transient process.As the system state suddenly changes in the range of a smaller time constant,it is difficult to estimate the dynamic state due to periodic disturbance.This paper presents a dynamic mathematical model of the MIPS according to the network structure and control strategy,thereby overcoming the restrictions of algebraic variables on the estimation and developing a dynamic state estimation method based on the extended Kalman filter.Using the method of adding fictitious process noise,it is possible to solve the problem that the linearized algorithm of the MIPS model is less reliable when an abrupt change occurs in the pulse load.Therefore,the accuracy of the dynamic state estimation and the stability of the filter can be improved under the periodic disturbance of pulse load.The simulation and experimental results confirm that the proposed model and method are feasible and effective.

中压全调节大功率电源系统在北斗三号卫星上的应用

China’s BeiDou Navigation Satellite System(BDS)construction has been completed and the system has been formally commissioned.Most of the Electric Power Systems(EPSs)for MEO satellites were developed by the Shanghai Institute of Space Power-sources.The 42 V medium-voltage fully-regulated high-power EPS has been adopted for the first time in medium Earth orbit,with an output power reaching about 3 kW.Compared with the 42 V medium-voltage semi-regulated bus power system used in the Regional Navigation BDS-2 satellite,the EPS of the BDS-3 MEO satellites has increased power by about 80%,adopting many newly developed products such as high-efficient triple junction GaAs solar cells,high-energy-density lithium ion batteries and a high-efficient autonomous power control unit(PCU).Based on the studies on the medium-voltage fully-regulated and high-power EPS technical principles,and the adaptability and reliability of various working modes,the test verifications for the EPS were conducted both on the ground and in orbit.Compared with other global navigation satellite systems such as GPS,Galileo and GLONASS,the EPS of the BDS-3 MEO satellite has a long design life time which is equivalent to that of the GPS and Galileo,but with a larger power supply capability and power ratio,distinguishing its advancement in the field of satellite power technology.

Adaptive droop control for better current-sharing in VSC-based MVDC railway electrification system

The share of voltage source converter(VSC)technology is increasing in conventional power systems,and it is penetrating into specific transportation systems such as electric vehicles,railways,and ships.Researchers are identifying feasible methods to improve the performance of railway electrification systems(RESs)by utilizing VSC-based medium-voltage direct current(MVDC)railways.The continuous motion of electric trains makes the catenary resistance a variable quantity,as compared to the traction substation(TSS),and affects the currentsharing behavior of the system.A modified droop control technique is proposed in this paper for VSC-based MVDC RES to provide more effective current-sharing while maintaining catenary voltages above the minimum allowable limit.The droop coefficient is selected through an exponential function based on the ratio between the concerned TSS current and the system average current.This enables small adjustments of droop values in less concerning marginal current deviations,and provides higher droop adjustments for large current deviations.Meanwhile,the catenary voltages are regulated by considering the voltage data at the midpoint between two TSSs,which experiences the lowest voltages owing to the larger distance from the TSSs.The proposed techniques are validated via simulations and experiments.

Coordinated control for medium voltage DC distribution centers with flexibly interlinked multiple microgrids

From flexible interconnection among feeders to hybrid alternating current(AC) and direct current(DC)distribution structures of future smart distribution systems medium-voltage DC distribution centers with flexibly interlinked multiple microgrids(MGs) will have wide applications on the demand side. A generic coordinated control framework based on a distributed cooperation scheme is proposed for such DC centers, as opposed to centralized control structures. A novel unified control only using local measurements is proposed for these interlinking converters. During normal power disturbances, automatic coordinated power control and mutual support among sub-systems can be realized, thereby improving DC voltage and AC frequency stability to enable multiple MGs to be treated as a real unified cluster. Moreover, with this method, interlinking converters can realize seamless transition in power dispatching mode, common DC bus voltage control mode, and MG support mode without communication and control system switching. A simplified dynamic model has been developed to verify the proposed control strategy. This work is expected to provide a new solution for flexible interconnection and operational control of large-scale MG clusters.

Integration of PV and Battery Storage for Catenary Voltage Regulation and Stray Current Mitigation in MVDC Railways

Innovative advancement in power electronics is reshaping the conventional high-voltage transmission systems and has also opened a new paradigm for researchers to consider its benefits in the railway electrification system(RES).In this regard,the medium-voltage direct current RES(MVDC-RES)is a key area of interest nowadays.In this paper,a secondary energy source(SES)consisting of renewable energies(REs)and energy storage systems(ESSs)is proposed to solve the issues of catenary voltage regulation,rail potential,and stray current in the MVDC-RES.Some of the major integration topologies of the SES are analyzed for MVDC-RES and the most effective one is proposed and implemented.The voltage at the point of connection(PoC)of the SES is used as a reference for controlling different operation modes of REs and ESSs.Moreover,feedforward control is used at the ESS converter to attain the quick response from the batteries for the desired operation.The proposed scheme improves the catenary voltage,and reduces the rail potential and stray current.Besides,the scheme provides higher energy density and reduces line losses.Simulation results are provided to validate the operation modes and advantages of the proposed scheme.