Jinli Zhao

Coordinated Control Method of Voltage and Reactive Power for Active Distribution Networks Based on Soft Open Point

The increasing penetration of distributed generators (DGs) exacerbates the risk of voltage violations in active distribution networks (ADNs). The conventional voltage regulation devices limited by the physical constraints are difficult to meet the requirement of real-time voltage and VAR control (VVC) with high precision when DGs fluctuate frequently. However, soft open point (SOP), a flexible power electronic device, can be used as the continuous reactive power source to realize the fast voltage regulation. Considering the cooperation of SOP and multiple regulation devices, this paper proposes a coordinated VVC method based on SOP for ADNs. First, a time-series model of coordinated VVC is developed to minimize operation costs and eliminate voltage violations of ADNs. Then, by applying the linearization and conic relaxation, the original nonconvex mixed-integer nonlinear optimization model is converted into a mixed-integer second-order cone programming model which can be efficiently solved to meet the requirement of voltage regulation rapidity. Case studies are carried out on the IEEE 33-node system and IEEE 123-node system to illustrate the effectiveness of the proposed method.

Study on power system extended small signal stability region (DE-SSSR) in time delay space

In power system, coordination controllers are usually designed based on the data from PMU and WAMS. Time delays in the measurement data are significant so that they can not be simply ignored. So, it is important to evaluate the impact of time delays on power system stability analysis and controller design. In this paper, an extended small signal stability region in time delay space (DE-SSSR) is presented so that it can consider the low frequency oscillation phenomena captured in the power system operations. Definition of DE-SSSR is first given, and the structure of its boundaries is discussed. Then a practical method is given to trace the boundaries of DE-SSSR. Finally, WSCC 3-generator-9-bus system is employed to validate the effectiveness of the presented method. Work of this paper is helpful to power system stability assessment and controller design with considering time delays in wide-area environment.

An interactive operation strategy for microgrid cooperated with distribution system based on demand response

Microgrid is an autonomous system which can achieve self-control, self-protection and self-management. It will not only comprehensively exploit the advantages of renewable energy sources (RES) in environment protection and energy conservation, but also overcome its intermittency and volatility, reduce the impact to distribution system when the distributed energy resources (DERs) are connected to the distribution network as microgrids. Deployment of microgrids can facilitate efficient demand side management and integration of RESs at the distribution level. This paper proposed an interactive operation strategy for microgrid cooperated with distribution system based on demand response. In the present paper, a day-ahead operation optimization model of microgrid on account of electricity purchasing cost, electricity selling benefits, generation cost of distributed generations (DGs) is proposed. And then an interaction model in which the microgrid responds to the interaction demand through adjusting the scheduling plan aiming at dealing with the excessive peak load of distribution system is presented. We analyzed the relationship between interaction capacity and interaction cost via sensitivity analysis and finally achieved the interaction between the microgrid and the distribution system. The simulation results in a microgrid, which has a wind turbine (WT), photovoltaic (PV) system, energy storage system (ESS) and loads, demonstrate the effectiveness of the proposed method.

Optimal operation of soft open points in active distribution networks under three-phase unbalanced conditions

The asymmetric integration of distributed generators (DGs) exacerbates the three-phase unbalanced condition in distribution systems. The serious unbalanced operation causes inefficient utilization of network assets and security risks in the system. Soft open point (SOP) is a flexible power electronic device which can achieve accurate active and reactive power flow control to balance the power flow among phases. This paper proposes an SOPs-based operation strategy for unbalanced active distribution networks. By regulating the operation of SOPs, the strategy can reduce power losses and simultaneously mitigate the three-phase unbalance of the upper-level grid. Semidefinite programming (SDP) relaxation is advocated to convert the original non-convex, nonlinear optimization model into an SDP formulation, which can be efficiently solved to meet the requirement of rapid adjustment. Case studies are conducted on the modified IEEE 33-node and IEEE 123-node distribution system to verify the effectiveness and efficiency of the proposed strategy.

Application of conic programming for optimal distributed generation allocation in distribution network

The widely application of distributed generation (DG) technologies poses new challenges to the operation and planning of the distribution network. Firstly, based on the basic DG allocation problem, a conic transformation model is proposed through variable substitution to realize the required format conversion, and then the conic programming(CP) algorithm is employed to acquire the optimal solution accurately, which also realizes the solving of power flow simultaneously. Finally, the optimal DG capacity obtained under different penetration levels and power factors are analyzed on the IEEE 33-node test feeder. Meanwhile, the validity of conic model as well as the CP algorithm is also verified.

A fast modeling method of distribution system reconfiguration based on mixed-integer second-order cone programming

Network reconfiguration is a primary means of changing the operation mode of the distribution system with distributed generation. A model for the distribution system reconfiguration problem based on mixed-integer second-order cone programming is presented in this paper. Firstly, the network reconfiguration model is built to minimize the active power losses of distribution system with distributed generation. Then, some equivalent conversion and relaxation are presented to cast the initial mixed-integer nonlinear model above into a mixed-integer second-order cone model, which can be reliably and efficiently solved to global optimality by using the available commercial software. Finally, case studies on IEEE 33-node test feeder are conducted and the results demonstrate the validity as well as effectiveness of the proposed model.

Adaptive DAE solving algorithm for power system transient simulation via matrix exponential operator

The ever-increasing scale of system interconnection and diverse dynamic time constants of power equipment pose challenges to power system transient simulation algorithms. In this paper, an adaptive numerical integration method for DAE systems based on matrix exponential operator is presented. Krylov subspace technique is utilized to effectively reduce the problem dimension. A novel step adaption scheme is established to enable the method adjusting the computation step size according to error budget while maintaining the output step size unchanged. Acceleration is accomplished without perceived degradation in the simulation results. Numerical performance and computation efficiency is demonstrated through numerical case studies.

An equivalent A-Stable dynamic simulation algorithm of active distribution networks based on the implicit projective method

The dynamic characteristics of active distribution networks (ADNs) need to be concerned if a large number of distributed generators are connected. A highly efficient and reliable simulation algorithm with good numerical stability is therefore essential for the stability analysis of ADNs. This paper proposes a novel dynamic simulation algorithm of ADNs based on the implicit projective method, which is a second order integration algorithm. The proposed method is an equivalent A-Stable method and the calculation efficiency is increased significantly compared with the traditional integration algorithms. It is especially suitable for the dynamic simulation and stability analysis of the ADNs with a large number of DGs. Case studies based on the IEEE 123-node test feeder show the feasibility and effectiveness of the proposed method, which is verified through the comparison with the commercial simulation tool DIgSI-LENT/PowerFactory and the traditional trapezoidal method.

A fast optimization method for the distribution system with energy storage based on conic programming

Application of energy storage system (ESS) has been a chance and challenge to distribution system operation and integration of renewable energy sources (RESs). A conic programming (CP) based method is proposed for the optimization of distribution system with ESS in this paper. Firstly, the operation optimization model considering the state of charge (SOC) of ESS is built to minimize the active power losses of distribution system with distributed generation (DG). Equivalent CP model is conversed based on the initial model as format required, which can be solved by some mature optimization tools. Finally, case studies show the feasibility as well as effectiveness of the proposed method.

A projective based dynamic simulation algorithm of active distribution networks

The dynamic characteristics of active distribution networks (ADNs) need to be concerned if a large number of distributed generators are connected. A highly efficient and reliable dynamic simulation algorithm is therefore essential for the stability analysis of ADNs. This paper proposes a novel dynamic simulation algorithm of ADNs based on the projective integration method, which is a second-order explicit integration algorithm. The proposed method greatly improves the numerical stability of the traditional explicit algorithms and increases the efficiency significantly at the same time. It is especially suitable for the dynamic simulation and stability analysis of the ADNs with a large number of DGs. Case studies based on the IEEE 123-node test feeder show the effectiveness and efficiency of the proposed method, which is verified through the comparison with the commercial simulation tool DIgSILENT/PowerFactory.