This paper discusses a control strategy for the integration of wind turbine generators (WTGs) with fuel cells (FCs), diesel generator (DG) and electrolyzer systems for the regulation of frequency in a microgrid. By incorporating a FC system and electrolyser, the issues related to the fluctuating wind power can be minimized up to certain extend. When the wind power output is high, the surplus energy is stored as hydrogen gas after water electrolysis and during low wind speeds; the stored hydrogen is used for generating electric power using fuel cells. The DG is responsible for the overall frequency balance in the system, which is assisted by WTGs and FCs. The effectiveness of the control strategy was verified through simulations by using detailed models of WTG, FC, electrolyser, hydrogen compressor and storage tank.
Frequency control rebalances supply and demand while maintaining the network state within operational margins. It is implemented using fast ramping reserves that are expensive and wasteful, and which are expected to grow with the increasing penetration of renewables. The most promising solution to this problem is the use of demand response, i.e. load participation in frequency control. Yet it is still unclear how to efficiently integrate load participation without introducing instabilities and violating operational constraints. In this paper we present a comprehensive load-side frequency control mechanism that can maintain the grid within operational constraints. Our controllers can rebalance supply and demand after disturbances, restore the frequency to its nominal value and preserve inter-area power flows. Furthermore, our controllers are distributed (unlike generation-side), can allocate load updates optimally, and can maintain line flows within thermal limits. We prove that such a distributed load-side control is globally asymptotically stable and illustrate its convergence with simulation.
This paper is the first part of a two-part series in which we present results from one of the first worldwide experimental demonstrations of frequency regulation in a commercial building test facility. We demonstrate that commercial buildings can track a frequency regulation signal with high accuracy and minimal occupant discomfort in a realistic environment. In addition, we show that buildings can determine the reserve capacity and baseline power a priori , and identify the optimal tradeoff between frequency regulation and energy efficiency. In part I, we introduce the test facility and develop relevant building models. Furthermore, we design a hierarchical controller for the heating, ventilation, and air conditioning system that consists of three levels: 1) a reserve scheduler; 2) a building climate controller; and 3) a fan speed controller for frequency regulation. We formulate the reserve scheduler as a robust optimization problem and introduce several approximations to reduce its complexity. The building climate controller is comprised of a robust model predictive controller and a Kalman filter. The frequency regulation controller consists of a feedback and a feedforward loop, provides fast responses, and is stable. Part I presents building model identification and controller tuning results. Specifically, we find out that with an appropriate formulation of the model identification problem, a two-state model is accurate enough for use in a reserve scheduler that runs day-ahead. In part II, we report results from the operation of the hierarchical controller under frequency regulation.
This paper studies the problem of frequency regulation in power grids under unknown and possible time-varying load changes, while minimizing the generation costs. We formulate this problem as an output agreement problem for distribution networks and address it using incremental passivity and distributed internal-model-based controllers. Incremental passivity enables a systematic approach to study convergence to the steady state with zero frequency deviation and to design the controller in the presence of time-varying voltages, whereas the internal-model principle is applied to tackle the uncertain nature of the loads.
•The VSWT contribution to frequency regulation in an isolated system is evaluated.•Three controls strategies for the VSWT using the kinetic energy stored are analysed.•A modal analysis of the joint regulation provided by VSWTs and a PSHP is conducted.•Different settings for the VSWT and the PSHP control loops gains is proposed.•Hybrid wind–hydro system performance is within grid code requirements. The wind energy penetration rate is being increased in majority of European countries. However, a high penetration rate could endanger the stability of power systems, particularly in small islands. Hydropower plays an important role in the regulation and control of isolated power systems with renewable sources, but it may not be able to maintain the frequency within grid requirements. This is the case of El Hierro power system (Canary archipelago), where a hybrid wind–pumped storage hydropower plant (PSHP) was committed to reduce the use of fossil fuels. Currently, frequency regulation is only provided by the PSHP and diesel generators. Therefore, it is proposed that variable-speed wind turbines (VSWTs) contribute to frequency regulation, thereby minimizing the need for fossil fuels. This study aims to conduct an analysis on the effect of the VSWT contribution to frequency regulation in the power system of El Hierro. It is based on classical control tools from a linearized mathematical model considering different VSWT regulation strategies. The eigenvalues, damping ratio, and participation factors of the state variables have been obtained. The more significant oscillation modes in the dynamic response of the system have been identified. According to this modal analysis, a methodology for the adjustment of the PSHP and VSWT controller gains is proposed. An improvement in the quality of frequency regulation while maintaining the El Hierro system frequency within grid requirements has been proved based on simulating different events related to wind speed or variations in the power demand, using a nonlinear model of the combined wind–hydro power plant.
We present a method to design distributed generation and demand control schemes for primary frequency regulation in power networks that guarantee asymptotic stability and ensure fairness of allocation. We impose a passivity condition on net power supply variables and provide explicit steady-state conditions on a general class of generation and demand control dynamics that ensure convergence of solutions to equilibria that solve an appropriately constructed network optimization problem. We also show that the inclusion of controllable demand results in a drop in steady-state frequency deviations. We discuss how various classes of dynamics used in recent studies fit within our framework and show that this allows for less conservative stability and optimality conditions. We illustrate our results with simulations on the IEEE 68-bus transmission system and the IEEE 37-bus distribution system with static and dynamic demand response schemes.
This paper addresses a robust decentralized proportional-integral (PI) control design for power system load-frequency regulation with communication delays. In the proposed methodology, the PI-based load-frequency control (LFC) problem is reduced to a static output feedback control synthesis for a multiple-delay system. The proposed control method gives a suboptimal solution using a developed iterative linear matrix inequalities algorithm via the mixed H 2 / H infin control technique. The control strategy is suitable for LFC applications that usually employ the PI control. To demonstrate the efficiency of the proposed control strategy, an experimental study has been performed at the Research Laboratory, Kyushu Electric Power Company, Japan.
This paper is the second part of a two-part series presenting the results from an experimental demonstration of frequency regulation in a commercial building test facility. In part I, we developed relevant building models and designed a hierarchical controller for reserve scheduling, building climate control, and frequency regulation. In part II, we introduce the communication architecture and experiment settings, and present extensive experimental results under frequency regulation. More specifically, we compute the day-ahead reserve capacity of the test facility under different assumptions and conditions. Furthermore, we demonstrate the ability of model predictive control to satisfy comfort constraints under frequency regulation, and show that fan speed control can track the fast-moving RegD signal of the Pennsylvania, Jersey, and Maryland power market very accurately. In addition, we discuss potential effects of frequency regulation on building operation (e.g., increase in energy consumption, oscillations in supply air temperature, and effect on chiller cycling), and provide suggestions for real-world implementation projects. Our results show that hierarchical control is appropriate for frequency regulation from commercial buildings.
This study presents a control strategy for the frequency regulation in a wind–diesel powered microgrid. With wind as a major energy resource, ensuring reliability and quality of power supplied in the system is a great challenge. To reduce the adverse effects caused by wind's variability, intermittency and uncertainty on the system frequency and improve the performance of diesel generator (DG), a solution is explored that involves the use of two different energy storage technologies. A test system is proposed consisting of a wind farm and a DG, supplemented by hydrogen storage with fuel cell (FC) as a long-term and a flywheel (FW) as a short-term energy storage. During low demand or high wind periods, the surplus energy generated is stored as kinetic energy in the FW and as hydrogen gas after water electrolysis. During periods of low wind speed or increased demand, the FW supplies energy by shedding its rotor speed and hydrogen is converted into electricity through the FC. The effectiveness of adding a short-term and a long-term energy storage in enhancing the robustness of wind–diesel system is demonstrated in this study.
We consider the problem of distributed generation and demand control for primary frequency regulation in power networks, such that stability and optimality of the power allocation can be guaranteed. It was shown in Part I of this work, that by imposing an input strict passivity condition on the net supply dynamics at each bus, combined with a decentralized condition on their steady-state behavior, convergence to optimality can be guaranteed for broad classes of generation and demand control dynamics in a general network. In this paper, we show that by taking into account additional local information, the input strict passivity condition can be relaxed to less restrictive decentralized conditions. These conditions extend the classes of generation and load dynamics for which convergence to optimality can be guaranteed beyond the class of passive systems, thus, allowing to reduce the conservatism in the analysis and feedback design.