Qinglai Guo

Emission-Concerned Wind-EV Coordination on the Transmission Grid Side With Network Constraints: Concept and Case Study

This paper presents a study of emission-concerned wind-electric vehicle (EV) coordination on the transmission grid side. An aggregator model representing a cluster of controllable EVs is proposed, and is coordinated with large-scale wind power on the transmission side. Using these EV aggregators, a conceptual framework is developed for wind-EV coordination, based on a three-level hierarchy. At the top level, the control center determines the optimal plant outputs and EV charging strategies from the proposed wind-EV dispatch model, where emission costs, thermal plant generation, controllable EVs, and CO2 capture power plants are taken into account and set as multi-objectives. The time-varying number of EV connections, plant ramp rate limits, and network constraints, including interface and branch transmission capacities, are also considered in the model. The model is used to verify the benefits of wind-EV coordination in an IEEE 14-bus system. Compared with uncoordinated charging, wind curtailment, emissions, total optimized cost, and EV charging fees are all greatly reduced by coordination. Moreover, interface transmission capacity and discharging price are found to be important factors in coordination, and the impacts are investigated.

An Adaptive Zone-Division-Based Automatic Voltage Control System With Applications in China

Summary form only given. The power industry in China has grown significantly over the past decade, spurring the adoption of system-wide automatic voltage control (AVC) technology to meet stricter requirements for security and economical power system operation. To cope with the rapidly developing and frequently changing Chinese power grid, an AVC scheme based on adaptive zone division is introduced in this paper. Logically, this type of system has a three-level hierarchical structure, but here, both secondary voltage control (SVC) and tertiary voltage control (TVC) are implemented via software at the same control center. The control zones are no longer fixed but are reconfigured online and updated in accordance with variations in the grid structure. The technical details of these procedures are presented here. The implementation of TVC (which is based on an online reactive optimal power flow) and SVC are also discussed, together with some technical details on improvements to their reliability and robustness. Some results obtained from field-site applications based on similar-days testing rather than simulations are employed to evaluate the performance and improvements of the AVC system. This system has been installed at over 20 control centers in China.

Dynamic Economic Dispatch Using Lagrangian Relaxation With Multiplier Updates Based on a Quasi-Newton Method

To accommodate large-scale integration of renewable generation, look-ahead power dispatch, which is essentially a type of dynamic economic dispatch (DED), is widely used in power system operation. DED can be reduced to a two-stage dual problem via Lagrangian relaxation. The major challenge to solving this dual problem robustly and efficiently is the multiplier updating strategy. To solve the dual problems of quadratic DEDs, this paper proposes multiplier updating based on a quasi-Newton method, together with an initialization strategy for the multipliers and the approximation matrix. Comparative numerical tests are carried out to verify the performance of this technique in terms of convergence, computational efficiency, and robustness.

Master–Slave-Splitting Based Distributed Global Power Flow Method for Integrated Transmission and Distribution Analysis

With the recent rapid development of smart grid technology, the distribution grids become more active, and the interaction between transmission and distribution grids becomes more significant. However, in traditional power flow calculations, transmission and distribution grids are separated, which is not suitable for such future smart grids. To achieve a global unified power flow solution to support an integrated analysis for both transmission and distribution grids, we propose a global power flow (GPF) method that considers transmission and distribution grids as a whole in this paper. We construct GPF equations, and develop a master-slave-splitting (MSS) iterative method with convergence guarantee to alleviate boundary mismatches between the transmission and distribution grids. In our method, the GPF problem is split into a transmission power flow and a number of distribution power flow sub-problems, which supports on-line geographically distributed computation. Each sub-problem can be solved using a different power flow algorithm to capture the different features of transmission and distribution grids. An equivalent method is proposed to improve the convergence of the MSS-based GPF calculation for distribution grids that include loops. Numerical simulations validate the effectiveness of the proposed method, in particular when the distribution grid has loops or distributed generators.

Development and applications of system-wide automatic voltage control system in China

Traditionally voltage control is done in a decentralized way at the power plant or substation level in China. Such a traditional mode, lacking of automatic coordination in a system level, is no longer fit for the higher requirements of security and economy of modern large power system operation. In recent years, development and field site applications of close-looped system-wide automatic voltage control (AVC) technique were actively done in China. In the paper, a novel control architecture based on online adaptive network zone division is proposed. Logically such a system is in a three level hierarchical structure, however, both secondary voltage control and tertiary voltage control here are implemented with software in a control center, and the control zones are no longer fixed according to variation in power systems, which is more suitable for the fast developing Chinese power grids. Some key techniques implemented in the AVC system are introduced, such as adaptive zone division, strategies of Tertiary Voltage Control (TVC), strategies of secondary voltage control (SVC), etc. Finally some field experiences, especially some results not from simulation tool but from field site applications, are reported. The excellent performance of the AVC system and significant improvements on power system security and economy are validated. The developed AVC system has been applied to more than a dozen control centers in China, such as the North China Power Grid with generating capacity of 119GW. Evaluation of the potential profits of the proposed AVC system on PJM system in USA is ongoing.

Distributed Model Predictive Control of a Wind Farm for Optimal Active Power ControlPart II: Implementation With Clustering-Based Piece-Wise Affine Wind Turbine Model

This paper presents distributed model predictive control (D-MPC) of a wind farm for optimal active power control using the fast gradient method via dual decomposition. The objectives of the D-MPC control of the wind farm are power reference tracking from the system operator and wind turbine mechanical load minimization. The optimization of the active power control of the wind farm is distributed to the local wind turbine controllers. The D-MPC developed was implemented using the clustering-based piece-wise affine wind turbine model. With the fast gradient method, the convergence rate of the D-MPC has been significantly improved, which reduces the iteration numbers. Accordingly, the communication burden is reduced. A wind farm with ten wind turbines was used as the test system. Case studies were conducted and analyzed, which include the operation of the wind farm with the D-MPC under low and high wind conditions, and the dynamic performance with a wind turbine out of service. The robustness of the D-MPC to errors and uncertainties was tested by case studies with consideration of the errors of system parameters.

Cyber-Physical Modeling and Cyber-Contingency Assessment of Hierarchical Control Systems

Online closed-loop hierarchical control systems (HCSs) are widely used in power-system operation. Like typical cyber-physical systems, the contingencies on the cyber side of an HCS may lead to inappropriate control commands, which will influence the physical power system. To evaluate the degree to which these inappropriate control commands influence the power system, we propose a cyber-physical equivalent model for HCSs. In this model, the HCS cyber network is abstracted to a directed graph consisting of data nodes and directed branches, and connectivity is described by using a node-branch incidence matrix. Using this strategy, we can describe the general information flow in an HCS using mathematical equations on the basis of which quantitative evaluation can be carried out. Furthermore, by using existing operation records, several kinds of typical cyber-contingencies are also modeled on the basis of which cyber-contingency assessment (cyber-CA) can be implemented by using a model-based approach. Considering the computational efficiency, such an approach keeps only key characteristics of the information flow rather than all features of the cyber network. In the case study, a coordinated secondary-voltage control system is studied as an example. The physical impacts of various cyber-contingencies on different data transmission and processing modules are compared. The results show that the model-based method provides improved efficiency compared with conventional simulation-based methods while maintaining accuracy.

Interval Power Flow Analysis Using Linear Relaxation and Optimality-Based Bounds Tightening (OBBT) Methods

With increasingly large scale of intermittent and non-dispatchable resources being integrated into power systems, the power flow problem presents greater uncertainty. In order to obtain the upper and lower bounds of power flow solutions including voltage magnitudes, voltage angles and line flows, Cartesian coordinates-based power flow is utilized in this paper. A quadratically constrained quadratic programming (QCQP) model is then established to formulate the interval power flow problem. This non-convex QCQP model is relaxed to linear programming problem by introducing convex and concave enclosures of the original feasible region. To improve the solutions bounds while still encompassing the true interval solution, optimality-based bounds tightening (OBBT) method is employed to find a better outer hull of the feasible region. Numerical results on IEEE 9-bus, 30-bus, 57-bus, and 118-bus test systems validate the effectiveness of the proposed method.

Distributed Model Predictive Control of a Wind Farm for Optimal Active Power ControlPart I: Clustering-Based Wind Turbine Model Linearization

This paper presents a dynamic discrete-time piece-wise affine (PWA) model of a wind turbine for the optimal active power control of a wind farm. The control objectives include both the power reference tracking from the system operator and the wind turbine mechanical load minimization. Instead of partial linearization of the wind turbine model at selected operating points, the nonlinearities of the wind turbine model are represented by a piece-wise static function based on the wind turbine system inputs and state variables. The nonlinearity identification is based on the clustering-based algorithm, which combines the clustering, linear identification, and pattern recognition techniques. The developed model, consisting of 47 affine dynamics, is verified by the comparison with a widely used nonlinear wind turbine model. It can be used as a predictive model for the model predictive control (MPC) or other advanced optimal control applications of a wind farm.

Optimal Voltage Control of PJM Smart Transmission Grid: Study, Implementation, and Evaluation

PJM Interconnection operates the largest synchronized transmission system in North America, where one of the great challenges is being able to meet the system-wide voltage control performance requirements both pre- and post-contingency. Based on a characteristic analysis of the contingencies distribution, a practical method using an iteration scheme between the optimal power flow (OPF) and a contingency assessment (CA) is proposed to develop a control strategy, so that the calculated optimal voltage schedule solutions are better not only for pre-contingency system conditions, also for the post-contingency (N-1) system conditions. At step T, a CA considering all contingencies is carried out and those that may lead to the maximized voltage violations are saved as active contingencies for step T+1. At the same time, another CA, considering only the current active contingencies, is computed in parallel using a trial OPF result as input. According to the post-contingency indices, the new voltage constraints will be compressed and the final OPF is called again to calculate the optimal strategies. The detailed architecture of the PJM optimal voltage control (OVC) system is presented in this paper. The system has completed four-months of online operation, and the results have been evaluated using third-party software. It has been proven that the OVC system presented here will greatly improve both the pre- and post-contingency performance. The PJM OVC is a typical application of a smart transmission grid. Phase III of this study is currently under way.