Huanhai Xin

A Self-Organizing Strategy for Power Flow Control of Photovoltaic Generators in a Distribution Network

The focus of this paper is to develop a distributed control algorithm that will regulate the power output of multiple photovoltaic generators (PVs) in a distribution network. To this end, the cooperative control methodology from network control theory is used to make a group of PV generators converge and operate at certain (or the same) ratio of available power, which is determined by the status of the distribution network and the PV generators. The proposed control only requires asynchronous information intermittently from neighboring PV generators, making a communication network among the PV units both simple and necessary. The minimum requirement on communication topologies is also prescribed for the proposed control. It is shown that the proposed analysis and design methodology has the advantages that the corresponding communication networks are local, their topology can be time varying, and their bandwidth may be limited. These features enable PV generators to have both self-organizing and adaptive coordination properties even under adverse conditions. The proposed method is simulated using the IEEE standard 34-bus distribution network.

A New Frequency Regulation Strategy for Photovoltaic Systems Without Energy Storage

To maximize the revenue from selling energy, photovoltaic systems (PVs) in general operate in the so-called maximum power point tracking mode. However, the increasing penetration of renewable energy sources in power systems has motivated the design of innovative control to provide ancillary services. The focus of this paper is to develop a new control strategy that enables PVs to adjust the active power outputs and provide frequency regulation to power systems. In this strategy, two different modes are designed: 1) the frequency droop control mode for PVs to provide primary frequency support to power systems, and 2) the emergency control mode to prevent system frequency collapse and, therefore, to prevent too much generation tripping after fault. Based on a detailed PV dynamic model, simulation results show the effectiveness of the proposed control strategy in improving the frequency stability.

A Center-Free Control Strategy for the Coordination of Multiple Photovoltaic Generators

Coordinated regulation for the outputs from renewable energy sources is an appealing issue in future smart grid. This paper presents a distributed control strategy for multiple photovoltaic generators (PVs), which makes all the PVs have the same reserve ratio with respect to their maximum available power, but also makes their aggregated output support power network by providing power regulation service within the power limit. In addition, an estimation method is proposed to get the maximum available power of PV. The proposed control strategy is nearly center-free, i.e., there is no centralized station which collects the output of each PV and sends the power command to each PV, and only uses local communication networks (CNs) to avoid expensive sometimes unreliable, long distance communications. Simulation results based on the IEEE standard 123-bus distribution system are presented and discussed, illustrating the effectiveness of the proposed control strategy.

Applications of Stability-Constrained Optimal Power Flow in the East China System

Stability-constrained optimal power flow (SOPF) models for reactive power reinforcement and HVDC control tuning are formulated, several derivative-free solution algorithms are presented and implemented in a prototype, and the behavior of the objective function value as injection space parameters vary is studied using a qualitative approach. Results of the East China 3296-bus system are provided.

Control of Island AC Microgrids Using a Fully Distributed Approach

A fully distributed control scheme of island ac microgrids that can perform the primary, secondary, and tertiary control locally in distributed generators (DGs) is proposed. In the control scheme, no central controller or external information exchange is needed, while the final frequency can be controlled within the allowable range and the DGs can share loads according to their increment costs. The low-pass filters are designed to decouple the dynamics of the microgrid and to improve the system performance. Simulation studies verify the effectiveness of the control scheme.

Distributionally Robust Solution to the Reserve Scheduling Problem With Partial Information of Wind Power

A distributionally robust optimization (DRO) model is presented for the reserve schedule decision-making problem with partial information of wind power, aiming to find a robust solution to the uncertainty of wind power probability distribution. The stochastic problem can be converted into an equivalent deterministic bilinear matrix inequality (BMI) problem. Numerical results verify the effectiveness of the proposed method.

Distributed Control and Generation Estimation Method for Integrating High-Density Photovoltaic Systems

The presence of distributed generators (DGs) such as photovoltaic systems (PVs) is increasing significantly in distribution networks, and in order to accommodate a higher penetration of DGs, technical issues arising from fluctuation and unpredictability of their power output must be addressed. It is beneficial if DGs of high penetration can be dispatched when necessary. To this end, a distributed control and generation estimation approach is developed to dispatch multiple DGs, each of which consists of a PV and a controllable load. A strongly connected digraph with a row stochastic adjacency matrix is a sufficient requirement for the communication topology. A distributed weights adjustment algorithm adaptively makes the adjacency matrix doubly stochastic so that the aggregated power generation capacity can be estimated. Then, the expected consensus operational point of the DGs is calculated by those DGs that can obtain power dispatch command from the supervisory control and data acquisition system and is propagated to the rest of the DGs with a consensus algorithm. With this method, all the DGs operate at the same ratio of available power, while their aggregated power meets the power dispatch command. Simulations in the IEEE standard 34-bus distribution network verify the effectiveness of the proposed approach.

Risk-Based Coordination of Generation Rescheduling and Load Shedding for Transient Stability Enhancement

This paper presents a risk-based method for coordinating generation rescheduling and load shedding to enhance overall transient stability of power systems. Firstly, a generation rescheduling model that can consider the risk-based dynamic security constraint is proposed, in which some acceptable risk level should be respected and can be adjusted by a risk coordination parameter. For the purpose of considering the potential corrective control costs at the stage of preventive control design, a bi-level coordination optimization model is developed, in which on the upper level the total coordination cost is minimized by the risk coordination parameter adjustment; and on the lower level the generation rescheduling and the load shedding are performed successively to enhance the risk-based dynamic security and the transient stability performance, respectively. Finally, a hybrid method that can combine the Golden Section Search and the Successive Linear Programming is developed to solve the bi-level optimization model. The effectiveness of the proposed method is demonstrated by using the New England test system and a real power system.

Synchronous Instability Mechanism of P-f Droop-Controlled Voltage Source Converter Caused by Current Saturation

Under large disturbances, P-f droop-controlled voltage source converters may lose synchronous stability, just like synchronous generator. Moreover, the current limitation leads to a more complicated instability mechanism and decreases stability margin greatly. This letter describes a virtual power angle instability problem and analyzes its mechanism from the virtual power angle characteristic curves of a converter with respect to an infinite bus. Simulation verifies the rationality of the analysis.

A Real-Time Power Allocation Algorithm and its Communication Optimization for Geographically Dispersed Energy Storage Systems

The paper presents a distributed algorithm that regulates the power outputs of multiple dispersed energy storage systems (DESS), which can be used to provide desirable services for power systems, such as renewable generation output smoothing and secondary control. The algorithm is based on the cooperative control principle of network control theory and it satisfies both the power balance requirement of power systems and the fair utilization among the DESS. The most distinct feature of the algorithm is that each DESS only requires the information from its neighbors through some local communication networks (CN), utilizing a virtual leader embedded in one or several DESS to receive a power command signal. In addition, to improve the robustness of the CN among DESS, two optimization models are formulated to design the so-called “ N-1” redundant network, while considering both economic issues and convergence rate. These features enable the DESS under the algorithm to have both self-organizing and adaptive coordination properties under some adverse conditions. Simulation on the IEEE 123-bus distribution system validates the effectiveness of the proposed algorithm.