Pengwei Du

Sizing Energy Storage to Accommodate High Penetration of Variable Energy Resources

The variability and nondispatchable nature of wind and solar energy production presents substantial challenges for maintaining system balance. Depending on the economic considerations, energy storage can be a viable solution to balance energy production with consumption. This paper proposes to use discrete Fourier transform to decompose the required balancing power into different time-varying periodic components, i.e., intraweek, intraday, intrahour, and real-time. Each component can be used to quantify the maximum energy storage requirement for different types of energy storage. This requirement is the physical limit that could be theoretically accommodated by a power system. The actual energy storage capacity can be further quantified within this limit by the cost-benefit analysis (future work). The proposed approach has been successfully used in a study conducted for the 2030 Western Electricity Coordinating Council system model. Some results of this study are provided in this paper.

PMU-Based Wide-Area Security Assessment: Concept, Method, and Implementation

This paper presents a concept, method, and implementation of utilizing phasor measurement unit (PMU) information to monitor the wide-area security of a power system. The close dependency of major transmission paths requires an approach that takes that interaction into account while establishing operational transfer capability, and evaluates grid reliability and security on a system-wide basis. Thus, the concept of wide-area security region, which considers all essential constraints, including thermal, voltage stability, transient stability, and small signal stability, is proposed. This approach expands the idea of traditional transmission system nomograms to a multidimensional case, involving multiple system limits and parameters such as transmission path constraints, zonal generation or load, etc., considered concurrently. In this paper, the security region boundary is represented using piecewise approximation with the help of linear inequalities (so called hyperplanes) in a multidimensional space, consisting of system parameters that are critical for security analysis. The goal of this approximation is to find a minimum set of hyperplanes that describe the boundary with a given accuracy. Offline computer simulations are conducted to build the security region and the hyperplanes can be applied in real time with phasor information for on-line security assessment. Numerical simulations have been performed for the full size Western Electricity Coordinating Council (WECC) system model, which comprises 15 126 buses and 3034 generators. Simulation results demonstrated the feasibility and effectiveness of this approach, and proved that the proposed approach can significantly enhance the wide-area situation awareness for a bulk power system like WECC.

Distributed Hierarchical Control Architecture for Transient Dynamics Improvement in Power Systems

In this paper, a novel distributed hierarchical control architecture is proposed for large-scale power systems. The newly proposed architecture facilitates faster and more accurate frequency restoration during primary frequency control, by providing decentralized robust control to several selected pilot generators in each area. At the local level, these decentralized robust controllers are designed to quickly damp oscillations and restore frequency after large faults and disturbances in the system. Incorporating this supplementary governor control helps the system reach the nominal frequency without necessarily requiring secondary frequency control. Thus, at the area level, automatic generation control (AGC) actions are alleviated in terms of conducting frequency restoration. Moreover, at the area level, AGC coordinates with the decentralized robust controllers to successfully perform tie-line power balancing, while efficiently damping low-frequency inter-area oscillations. The interaction of local and area controllers is validated through detailed simulations.

Optimal size of energy storage to accommodate high penetration of renewable resources in WECC system

The variability and intermittence of wind power will cause the large imbalance power that demands more expensive ancillary service. Energy storage, fast response but costly, is a viable solution to suppress the fluctuation of wind power. However, the determination of energy storage is a great challenge given the load demand and wind power uncertainties This paper proposes to use discrete fourier transform (DFT) to decompose the imbalance power into different time-varying components, i.e., intra-week, intra-day, intra-hour and real-time. Therefore, the imbalance power to be compensated by energy storage can be quantified. By compensating the fast-changing imbalance power (the slowly changing power is provided by the conventional generators), energy storage can be optimized to accommodate integration of high penetration wind power. The simulation results on the 2030 Western Electricity Coordinating Council (WECC) system demonstrate effectiveness and efficiency of this approach.

Smart meter data analysis

This paper presents a thorough analysis of 15-minute residential meter data sets to identify possible value propositions of smart meter measurements. Meter measurements of 50 houses were used to derive a few key data signatures for several target applications such as identifying demand response potentials, detecting abnormal load behaviors, and fault diagnosis. Results showed that for different applications, the communication needs from meters to control centers, data storage capabilities, and the complexity of data processing intelligence varies significantly. Therefore, it is important to build a dynamic data signature database and optimize the distribution of data processing capability between local devices and control centers to avoid communication congestion and to identify problems early. This paper also demonstrates that highresolution smart meter data can make distribution power grids more economical, reliable, and resilient.

Two-step solution to optimal load shedding in a micro-grid

Intentional islanding (referred to a micro-grid) holds promise to improve power system reliability and power quality due to its more flexible control. To operate intentional islanding successfully, more efficient and robust control methods are required. A new concept is developed as an enhancement to the application of optimal load shedding for corrective control to support an islanded power system. The new proposal is comprised of two major stages in which one nonlinear optimization problem and one linear integer optimization problem are formulated. Numerical simulations conducted on a 15-bus system have demonstrated the performance of the new algorithm.

Distributed dynamic state estimation with extended Kalman filter

Increasing complexity associated with large-scale renewable resources and novel smart-grid technologies necessitates real-time monitoring and control. Our previous work applied the extended Kalman filter (EKF) with the use of phasor measurement data (PMU) for dynamic state estimation. However, high computation complexity creates significant challenges for real-time applications. In this paper, the problem of distributed dynamic state estimation is investigated. One domain decomposition method is proposed to utilize decentralized computing resources. The performance of distributed dynamic state estimation is tested on a 16-machine, 68-bus test system.

The potential of thermostatically controlled appliances for intra-hour energy storage applications

This paper investigates the potential of providing a variety of energy storage services by directly control the thermostatically controlled appliances (TCAs) from a centralized controller. Dispatch algorithms for the controller to arrange the turn-on and turn-off time and duration of individual TCAs are presented. The control goal is to operate each TCA within the customer-desired temperature range and maintain the TCA load diversity, and make the aggregated TCA load at the target load level. Methods to minimize the communication needs by reducing the monitoring and control data flows between the central controller and the end devices are also discussed. A thousand space heating units are modeled to demonstrate the control algorithms to provide load shifting and load balancing services for a period of 24 hours. The results demonstrate that the energy and ancillary services provided by the TCA loads meet the performance requirements and can become a major source of revenue for load-serving entities where the two-way communication smart grid infrastructure enables direct load control over the TCA loads.

A multi-layer, hierarchical information management system for the smart grid

This paper presents the modeling approach, methodologies, and initial results of setting up a multi-layer, hierarchical information management system (IMS) for the smart grid. The IMS allows its users to analyze the data collected by multiple control and communication networks to characterize the states of the smart grid. Abnormal, corrupted, or erroneous measurement data and outliers are detected and analyzed to identify whether they are caused by random equipment failures, human error, or tampering. Data collected from different information networks are crosschecked for data integrity based on redundancy, dependency, correlation, or cross-correlations, which reveal the interdependency between data sets. A hierarchically structured reasoning mechanism is used to rank possible causes of an event to enable system operators to proactively respond or provide mitigation recommendations to remove or neutralize the threats. The model satisfactorily identifies the cause of an event and significantly reduces the need to process myriads of data.

Application of ensemble Kalman filter in power system state tracking and sensitivity analysis

An ensemble Kalman filter (EnKF) method is proposed to track dynamic states of generators. The algorithm of the EnKF and its application to generator state tracking are presented in detail. The accuracy and sensitivity of the method are analyzed with respect to initial state errors, measurement noise, unknown fault locations, time steps and parameter errors. It is demonstrated through simulation studies that even with some errors in the parameters, the developed EnKF method can still effectively track generator dynamic states.