Deqiang Gan

Stability-constrained optimal power flow

Stability is an important constraint in power system operation. Often trial and error heuristics are used that can be costly and imprecise. A new methodology that eliminates the need for repeated simulation to determine a transiently secure operating point is presented. The theoretical development is straight-forward: dynamic equations are converted to numerically equivalent algebraic equations and then integrated into the standard OPF formulation. Implementation issues and simulation results are discussed in the context of a 162-bus system.

Locational Market Power Screening and Congestion Management: Experience and Suggestions

The existing New England electricity market follows a uniform pricing system: infra-marginal generators are paid a uniform clearing price while constrained-on generators are paid bid prices subject to a locational market power screen. In this paper, we first report the operation experience of this approach during the markets first two years. We then briefly review the NEPOOL locational pricing proposal being implemented. Two new approaches for locational market power screening are presented. The first one is based on a zonal network model and the second is based on a nodal transmission model. Both approaches are being evaluated by an independent consultant and are be to recommended to NEPOOL. Test results of both approaches are included.

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.

Multi machine power system excitation control design via theories of feedback linearization control and nonlinear robust control

The dynamics of a large-scale power system are both nonlinear and interconnected. The equilibrium of such a system is typically unknown and uncertain, and the controllers within are also subject to physical limitations. In this paper, a new application of nonlinear robust control is presented for power system control design. It is assumed that the controllers are designed as a part of generator excitation system design. First, a customized exact feedback linearization scheme is developed for the power system under investigation. This new linearization scheme allows one to transform the power system with a single-axis system model into a linear uncertain system with an unknown equilibrium. Based on the latest development of nonlinear robust control theory, a novel control design is then applied to stabilize the resulting linearized uncertain system. Finally, a nonlinear decentralized excitation control is obtained by the inverse transformation. Compared with existing control schemes, the proposed control is free from such common deficiencies of power system nonlinear controllers as network dependence and equilibrium dependence. Detailed stability analysis and engineering judgment in the control design are provided. The results of simulation studies are presented.

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.

Large-scale var optimization and planning by tabu search

Abstract A generalization on the tabu search method, recently developed for combinatorial optimization problems, is described in this paper. The novel version of the tabu search method can be used to solve a larger class of combinatorial optimization problems. Application of this method to var optimization and planning, which is formulated as a nonlinear large-scale mixed integer programming problem with non-differentiable objective function, is demonstrated. Judicious engineering judgment which is essential for a successful application of the proposed tabu search is developed. Simulation results of a real-world power system are included. A simulation comparison is done between the proposed method and the simulated annealing method, which is currently one of the most popular method for combinatorial optimization problem.

Joint Estimation of State and Parameter With Synchrophasors—Part II: Parameter Tracking

Applications of synchrophasors to parameter estimation (and state estimation) in power systems are investigated. Joint state-and-parameter estimation in parameter estimation is reformulated as two loosely-coupled linear subproblems of state tracking and parameter tracking. This paper focuses on the state tracking, which can be used to determine bus voltages in parameter estimation or to track the system state in (dynamic) state estimation. Dynamic behavior of bus voltages under possible abrupt changes is studied, using a novel and accurate prediction model. The measurement model is also improved. An adaptive filter based on optimal tracking with correlated prediction-measurement errors, including the module for abrupt-change detection and estimation, is developed. With the above settings, accurate solutions are obtained. They provide a reliable support for the parameter tracking presented in a companion paper. Simulation results reveal that the proposed approach yields improved state estimation accuracy. It also has a desired convergence property.

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.

Min–max transfer capabilities of transmission interfaces

Abstract Transfer capability of a transmission interface is often regarded as a single number calculated under a pre-assumed generation-load profile. In general, transfer capability lies in an interval; the upper bounds of such intervals, maximum transfer capability, can be calculated using a standard optimal power flow program that is commercially available. In this paper, we study the properties, formulation, and computation of the lower bound of transfer capability. Based on a relatively new concept called bi-level optimization, we introduce a mathematical formulation of the lower bound of transfer capability (we term min–max transfer capability) and outline a standard branch-and-bound algorithm. We investigate the structure of this optimization model of min–max transfer capability and describe an efficient solution algorithm that is suitable for large-scale power systems. Several examples are provided to demonstrate the feasibility and possible applications of the results.

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.