Laurentiu D. Marinovici

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.

Synchronization Algorithms for Co-simulation of Power Grid and Communication Networks

The ongoing modernization of power grids consists of integrating them with communication networks in order to achieve robust and resilient control of grid operations. To understand the operation of the new smart grid, one approach is to use simulation software. Unfortunately, current power grid simulators at best utilize inadequate approximations to simulate communication networks, if at all. Cooperative simulation of specialized power grid and communication network simulators promises to more accurately reproduce the interactions of real smart grid deployments. However, co-simulation is a challenging problem. A co-simulation must manage the exchange of information, including the synchronization of simulator clocks, between all simulators while maintaining adequate computational performance. This paper describes two new conservative algorithms for reducing the overhead of time synchronization, namely Active Set Conservative and Reactive Conservative. We provide a detailed analysis of their performance characteristics with respect to the current state of the art including both conservative and optimistic synchronization algorithms. In addition, we provide guidelines for selecting the appropriate synchronization algorithm based on the requirements of the co-simulation. The newly proposed algorithms are shown to achieve as much as 14% and 63% improvement in performance, respectively, over the existing conservative algorithm.

Distributed Smart Grid Asset Control Strategies for Providing Ancillary Services

With large-scale plans to integrate renewable generation driven mainly by state-level renewable portfolio requirements, more resources will be needed to compensate for the uncertainty and variability associated with intermittent generation resources. Distributed assets can be used to mitigate the concerns associated with renewable energy resources and to keep costs down. Under such conditions, performing primary frequency control using only supply-side resources becomes not only prohibitively expensive but also technically difficult. It is therefore important to explore how a sufficient proportion of the loads could assume a routine role in primary frequency control to maintain the stability of the system at an acceptable cost. The main objective of this project is to develop a novel hierarchical distributed framework for frequency based load control. The framework involves two decision layers. The top decision layer determines the optimal gain for aggregated loads for each load bus. The gains are computed using decentralized robust control methods, and will be broadcast to the corresponding participating loads every control period. The second layer consists of a large number of heterogeneous devices, which switch probabilistically during contingencies so that aggregated power change matches the desired amount according to the most recently received gains. The simulation results show great potential to enable systematic design of demand-side primary frequency control with stability guarantees on the overall power system. The proposed design systematically accounts for the interactions between the total load response and bulk power system frequency dynamics. It also guarantees frequency stability under a wide range of time varying operating conditions. The local device-level load response rules fully respect the device constraints (such as temperature setpoint, compressor time delays of HVACs, or arrival and departure of the deferrable loads), which are crucial for implementing real load control programs. The promise of autonomous, Grid Friendly™ response by smart appliances in the form of under-frequency load shedding was demonstrated in the GridWise Olympic Peninsula Demonstration in 2006. Each controller monitored the power grid voltage signal and requested that electrical load be shed by its appliance whenever electric power-grid frequency fell below 59.95 Hz. The controllers and their appliances responded reliably to each shallow under-frequency event, which was an average of one event per day and shed their loads for the durations of these events. Another objective of this project was to perform extensive simulation studies to investigate the impact of a population of Grid Friendly™ Appliances (GFAs) on the bulk power system frequency stability. The GFAs considered in this report are represented as demonstration units with water heaters individually modeled.

Distributed hierarchical control of multi-area power systems with improved primary frequency regulation

The conventional distributed hierarchical control architecture for multi-area power systems is revisited. In this paper, a new distributed hierarchical control architecture is proposed. In the proposed architecture, pilot generators are selected in each area to be equipped with decentralized robust control as a supplementary to the conventional droop speed control. With the improved primary frequency control, the system frequency can be restored to the nominal value without the help of secondary frequency control, which reduces the burden of the automatic generation control for frequency restoration. Moreover, the low frequency inter-area electromechanical oscillations can also be effectively damped. The effectiveness of the proposed distributed hierarchical control architecture is validated through detailed simulations.

Control and coordination of frequency responsive residential water heaters

Demand-side frequency control can complement traditional generator controls to maintain the stability of large electric systems in the face of rising uncertainty and variability associated with renewable energy resources. This paper presents a hierarchical frequency-based load control strategy that uses a supervisor to flexibly adjust control gains that a population of end-use loads respond to in a decentralized manner to help meet the NERC BAL-003-1 frequency response standard at both the area level and interconnection level. The load model is calibrated and used to model populations of frequency-responsive water heaters in a PowerWorld simulation of the U.S. Western Interconnection (WECC). The proposed design is implemented and demonstrated on physical water heaters in a laboratory setting. A significant fraction of the required frequency response in the WECC could be supplied by electric water heaters alone at penetration levels of less than 15%, while contributing to NERC requirements at the interconnection and area levels.

Hierarchical decentralized control strategy for demand-side primary frequency response

The Grid Friendly™ Appliance (GFA) controller was proposed to autonomously switch off appliances by detecting under-frequency events. In this paper, a new frequency responsive load (FRL) controller is proposed by extending the functionality of the existing GFA controller. The proposed FRL controller can autonomously switch on (or off) end-use loads by detecting over-frequency (or under-frequency) events through local frequency measurement. Using the proposed FRL controller, a hierarchical decentralized control strategy is also developed for engaging the end-use loads to provide primary frequency response. The developed control strategy consists of two decision making layers including supervisory and device layers, and has several desirable features for primary frequency control. The FRL controller at the device layer preserves the autonomous operation of the enduse loads. The coordinator at the supervisory layer coordinates autonomous response to overcome the stability issue associated with high penetration of FRLs. The simulation results illustrate the effectiveness of the developed hierarchical decentralized control strategy in providing primary frequency response with the proposed FRL controller.

Improved controller design of Grid Friendly™ Appliances for primary frequency response

The Grid Friendly™ Appliance (GFA) controller, developed at Pacific Northwest National Laboratory, was originally designed to autonomously switch off appliances by detecting under-frequency events. In this paper, the feasibility of using the GFA controller to provide primary frequency response is investigated. In particular, the impacts of an important design parameter, i.e., curtailing frequency threshold, on the primary frequency response are carefully analyzed for different situations. In the normal situation, the current method of selecting curtailing frequency thresholds for GFAs is found to be insufficient to guarantee the desired performance especially when the frequency deviation is shallow. In the extreme situations, the power reduction of online GFAs could be so excessive that it can even impact the system frequency negatively. As the first step towards the efforts to make GFAs suitable for providing primary frequency response, the existing controller design is improved by modifying the strategy of selecting curtailing frequency thresholds to ensure the effectiveness of GFAs in the normal situation.

On resilience analysis and quantification for wide-area control of power systems

Wide-area control is an effective mean to reduce inter-area oscillations of the bulk power system. Its dependence on communication of remote measurement signals makes the closed-loop system vulnerable to cyber attacks. This paper develops a framework to analyze and quantify resilience of a given wide-area controller under disruptive attacks on certain communication links. Resilience of a given controller is measured in terms of closed-loop eigenvalues under the worst possible attack strategy. The computation of such a resilience metric is challenging especially for large-scale power systems due to the discrete nature of attack strategies. In this paper, we propose an optimization-based formulation and a convex relaxation approach to facilitate the computation. Furthermore, we develop an efficient algorithm for the relaxed problem with guaranteed convergence to identify structural vulnerabilities of the system. Simulations are performed on the IEEE 39-bus system to illustrate the proposed resilience analysis and computation framework.

Distributed hierarchical control architecture for transient dynamics improvement in power systems

Summary form only given. 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 scillations 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.

Mitigation of remedial action schemes by decentralized robust governor control

Remedial action schemes (RAS) in power systems are designed to maintain stability and avoid undesired system conditions by rapidly switching equipment and/or changing operating points according to predetermined rules. The acceleration trend relay currently in use in the US western interconnection is an example of RAS that trips generators to maintain transient stability. Recently, a new distributed hierarchical control (DHC) architecture was proposed for multi-area power systems. In this architecture, local decentralized governor controllers were developed to improve the frequency stability. In this paper, the integration of RAS to the newly proposed DHC is investigated. Specifically, the interactions of RAS with local decentralized robust governor control are analyzed. The influence of the decentralized robust governor control on the design of RAS is studied. Benefits of combining these two schemes are increasing power transfer capability and mitigation of RAS generator tripping actions; the latter benefit is shown through simulations.