Control Analysis of Integrated Fuel Cell Systems with Energy Recuperation Devices.

This work is focused on control-oriented analysis of integrated fuel cell systems that incorporate energy recuperation mechanisms. The high complexity of such fuel cell systems calls for precise control and regulation of multiple inputs. The need for robust and efficient steady state and transient operation imposes the need for intelligent control schemes. The models of two fuel cell systems are developed in this work and used for the design of feedback controllers. It is shown, through simulation, that the proposed controllers enhance the performance and meet the operating constraints. The two plants considered in this dissertation are (i) a catalytic partial oxidation fuel processor system (FPS) coupled with a proton exchange fuel cell and a catalytic burner (CB) and (ii) a hybrid solid oxide fuel cell and gas turbine (SOFC/GT) system. Both systems rely on energy recuperation devices (ERDs), such as a catalytic burner or a gas turbine, for achieving high fuel efficiency. Through model-based open loop analysis the FPS is shown to exhibit fuel cell H2 starvation and reactor overheating while the SOFC/GT system is prone to shutdown during load transitions without proper feedback in place. It is identified that the transient issues can be resolved through reactant ratio control and load filtering for the FPS and the SOFC/GT systems, respectively. Using the insights from the open loop analysis, feedback control schemes are designed to address the transient issues. For the FPS, an observer-based linear controller, that utilizes temperature measurements to control the air and fuel flows into the reformer and maintain proper reactant ratios, is proposed. For the SOFC/GT system, a reference governor control scheme is developed to filter the application of the load in order to avoid GT shutdown. For both systems, the designed control schemes utilize measurements from the ERDs, such as shaft speed or catalytic burner temperature and manage to mitigate the transient operating difficulties. Thus, the ERDs, besides increasing the steady state efficiency of the system by reducing the energy losses, also provide vital measurements for feedback control.Ph.D.Naval Architecture & Marine EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/57700/2/djvas_1.pd

Fuel governor augmented control of recompression HCCI combustion during large load transients

Abstract-A control strategy designed to track desired combustion phasing for a homogeneous charge compression ignition (HCCI) engine during large load transitions is presented in this work. Three inputs are controlled, namely valve timings, fuel injection amount and fuel injection timing. The valve and fuel injection timings are manipulated to track combustion phasing using a mid-ranging control strategy. A fuel governor is then added on to the compensated system to modify the fuel injection amount by enforcing pointwise-in-time actuator constraints. The fuel governor is shown to improve the transient response of combustion phasing and load during large load transitions, when the possibility of future constraint violations exists. The use of the fuel governor during large load reductions can prevent engine misfire. Moreover, the fuel governor strategy simplifies the overall controller design by decoupling the phasing controller from the constraint enforcing mechanism. System complexity is reduced by approximating the nonlinear fuel governor as a set of linear algebraic expressions. This is solved with very little computational overhead and without incurring a significant loss in performance, as presented in simulations

Optimal Control of Hybrid Systems and Renewable Energies

This book is a collection of papers covering various aspects of the optimal control of power and energy production from renewable resources (wind, PV, biomass, hydrogen, etc.). In particular, attention is focused both on the optimal control of new technologies and on their integration in buildings, microgrids, and energy markets. The examples presented in this book are among the most promising technologies for satisfying an increasing share of thermal and electrical demands with renewable sources: from solar cooling plants to offshore wind generation; hybrid plants, combining traditional and renewable sources, are also considered, as well as traditional and innovative storage systems. Innovative solutions for transportation systems are also explored for both railway infrastructures and advanced light rail vehicles. The optimization and control of new solutions for the power network are addressed in detail: specifically, special attention is paid to microgrids as new paradigms for distribution networks, but also in other applications (e.g., shipboards). Finally, optimization and simulation models within SCADA and energy management systems are considered. This book is intended for engineers, researchers, and practitioners that work in the field of energy, smart grid, renewable resources, and their optimization and control

Optimal Control System of Under Frequency Load Shedding in Microgrid System with Renewable Energy Resources

Book ChapterNowadays many of the power systems are facing serious problems because of the lack of know-how to utilize the available renewable energy resources (RER) so as to balance between the power supply and demand sides. As the consequence of the power unbalancing into their distribution networks, under frequency load shedding (UFLS) which leads to life span reduction of various expensive equipment and deteriorating production in general are of much concerns. Thus, proper control system for the load flow in a system like microgrids (MG) with RER in general is the first thing to carry out the assessment with the aim to solve the power balancing problem within the power system networks. Actually, the major problems which many utilities are facing all over the world are how to utilize the available and future energy resource reserves in order to balance between the supply and demand sides within their power distribution networks. Moreover, because of the quick, improvised and unforeseen increasing number of consumers’ power demands and lack of additional macro energy resources plants which can favorably respond to the instantaneous consumer requirements, optimal control strategy (OCS) is inevitable. The OCS is required to maintain the steady-state operations and ensure the reliability of the entire distribution system over a long period. For that case, the OCS is required to principally stabilize parameters such as voltage, frequency, and limit the injection of reactive power into the MG system under stress. Therefore, in this chapter, the OCS is proposed as an approach to be applied in an intelligent way to solve the UFLS and blackout problems (BP) in a typical MG with RER. The proposed control solution is analyzed using emergency power supply reserves integrated with RER. These typical energy resources can be wind and photovoltaic (solar PV) systems associated with the battery energy storage system (BESS), hydro pump storage, biomass power plant and fuel cell systems

Monitoring and control requirement definition study for Dispersed Storage and Generation (DSG). Volume 2, appendix A: Selected DSG technologies and their general control requirements

A consistent approach was sought for both hardware and software which will handle the monitoring and control necessary to integrate a number of different DSG technologies into a common distribution dispatch network. It appears that the control of each of the DSG technologies is compatible with a supervisory control method of operation that lends itself to remote control from a distribution dispatch center

Integrated SOFC/GT Systems with Improved Dynamic Capabilities for Mobile Applications.

This work is focused on developing control and system integration solutions to achieve rapid and reliable load following operation of solid oxide fuel cell/gas turbine (SOFC/GT) systems for mobile applications. Both the traditional recuperating-SOFC/GT system and the newly proposed sprinter-SOFC/GT system are studied through model-based methodologies. It is shown that solutions developed in this research could enhance system performance and meet operating objectives. For the recuperating system, the generator/motor (G/M) dual mode operation and its implications are investigated. Active shaft load control is used to manage transients by: (a) pre-conditioning of G/M power for load step-up transients; and (b) absorbing the excessive power through motoring operation for load step-down transients. Feedback and optimization algorithms are developed. By taking advantage of the dual operating G/M, better trade-offs between power tracking and thermal safety can be achieved, the battery requirements can be reduced and system performance can be enhanced. The sprinter-SOFC/GT system, which has far superior load following capability than traditional systems, is proposed in this research. In the system, the SOFC operated at constant temperature provides only the baseline power with high efficiency while the GT-generator’s transient capability will be fully explored for fast dynamic load following. System design and control framework suited for the proposed system are investigated. An SOFC operational strategy is derived to keep fairly constant SOFC power and temperature over the entire load range. A design procedure is also developed to determine various component sizes. The “actual” operational envelope is determined by integrating the SOFC power/temperature constraints with safety factors. An optimization problem is proposed to determine the optimal feed-forward operation map. Control analysis and feedback design are presented for the sprinter system. The stability of steady-state operation is studied through numerical simulations and linearized analysis of a simplified “2-state” model. Open-loop instability was identified for the low and medium airflow regions. Open-loop analysis and relative gain array (RGA) technique are used to gain insights on system operation and input-output interactions. Feedback control design is performed to address transient issues. The sprinter system achieves far superior performance than its recuperating counterpart for fast and safe load following.PhDNaval Architecture and Marine EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/110400/1/zhenzjia_1.pd

Enhanced frequency regulation functionality of grid-connected PV system

Thesis (MEng (Electrical Engineering))--Cape Peninsula University of Technology, 2019Electric utilities are confronted with challenges like rising fuel costs, aging equipment, increasing energy demand, frequency regulation and the difficulty to integrate renewable energy resources into the grid. The presence of photovoltaic (PV) penetration on the utility grid is also increasing significantly in recent years. With the recent rise in PV penetration and the advancement of the global PV industry, there is an urgent and a necessary need to introduce features in PV systems that will make them respond smartly. However, much of these can be addressed without negatively affecting the total performance and power quality of the grid. Hence, engaging smart Grid technologies, and leveraging the benefits of the distributed nature of PV, new prospects to unearth value can be created. Through the implementation of progressive energy storage techniques, efficient two-way communications, a grid-tied PV system can create significant value, mostly through improved PV contribution in grid support functions like frequency regulation. An enhanced frequency regulation functioning scheme for a grid-connected photovoltaic (PV) system is modelled in MATLAB/Simulink software environment. The system is designed to operate in grid ancillary services precisely, frequency regulation function. The model consists of a Photovoltaic (PV) plant with a battery connected to the grid through a three-phase inverter. A bi-directional DC-DC converter between the grid and the battery system is included. The model has a battery storage system that provide steady and regular active/reactive powers available while the grid transmit specific amounts of power needed for a specific duration. According to the design, either the grid or the PV system depending on the dominant energy situation charges the battery. The battery is designed to discharge only when the grid demands energy from the PV and if the PV system fails to meet the demanded active power or reactive power. The PV system and the battery storage is integrated with the grid with the aid of dc-ac inverter in such a manner that bi-directional flow of active and reactive power is achieved. A 1 MW PV system is connected to the utility grid through a three-phase voltage source inverter system. The grid nominal frequency is set at 50 Hz under normal operation. However, the frequency decreased when the PV was not producing required power hence, the battery responded almost instantaneously and returned the frequency to the nominal frequency. The effectiveness of battery storage system for utility grid frequency regulation was substantiated from the simulation results attained

Enhanced active power control of photovotaic systems

The share of electrical power generation from renewable energy sources is increasing and is expected to keep on increasing as various countries intensify their efforts to reduce CO2 emissions. Differences in the nature and characteristics of some renewable generation affect the ability of power systems to maintain frequency stability. This is because some renewable generation sources do not have inertia or are converter connected and decoupled from grid frequency. Solar Photovoltaic system do not have any stored inertia and usually operate at maximum power. There is need for control method for solar PV systems to contribute to frequency stability. This thesis proposes operation methodologies for photovoltaic (PV) systems to carry out active power control functions - including frequency control and proposes a framework for comparing frequency support ability of different generation sources. First, a modification to the conventional Perturb and Observe (P&O) maximum power point tracking (MPPT) algorithm is proposed to avoid leftward and rightward from maximum power. Results presented show that PV systems employing P&O with the proposed modification avoid both leftward and rightward drift when subjected to rapidly increasing irradiance, sinusoidal irradiance, and real irradiance. This drift-free P&O enables the PV system to participate in active power control function at rapidly increasing irradiance. An offline MPPT that uses the characteristics of PV modules to determine the maximum power point offline and reduce online computation is proposed for frequency support. Two methods for achieving de-loaded operation of a PV system using the offline MPPT are presented and compared for accuracy. The ability of offline MPPT and the P&O with proposed modification to maintain a power reserve under different irradiance conditions are compared. Second, this thesis examines the ability of a PV system to contribute to frequency support. Different methods for frequency support from a PV power plant under different penetration levels are examined. Results show with the appropriate amount of reserve and support parameters, PV systems can contribute to frequency support. The results also show that PV power plants with the proper support parameters can adequately compensate for the loss of inertia with regards to its effect on the nadir of frequency response. A variable droop control method for frequency support is proposed to reduce the amount of reserve required for frequency support. The effect of MPPT choice on frequency support is evaluated by comparing responses from PV systems with the offline MPPT, P&O with proposed modification, and the constant voltage MPPT. Lastly, this thesis proposes a framework for comparing the frequency support ability of different generating units based on their response speed and support parameters. Different response speeds are emulated by changing one the time constant of a Second-order system. The effect of response speed, support method, and support parameters on the nadir of frequency response and maximum power increase are evaluated for different response speeds. A method for comparing support ability by considering the economic cost and benefit for support is also presented.The share of electrical power generation from renewable energy sources is increasing and is expected to keep on increasing as various countries intensify their efforts to reduce CO2 emissions. Differences in the nature and characteristics of some renewable generation affect the ability of power systems to maintain frequency stability. This is because some renewable generation sources do not have inertia or are converter connected and decoupled from grid frequency. Solar Photovoltaic system do not have any stored inertia and usually operate at maximum power. There is need for control method for solar PV systems to contribute to frequency stability. This thesis proposes operation methodologies for photovoltaic (PV) systems to carry out active power control functions - including frequency control and proposes a framework for comparing frequency support ability of different generation sources. First, a modification to the conventional Perturb and Observe (P&O) maximum power point tracking (MPPT) algorithm is proposed to avoid leftward and rightward from maximum power. Results presented show that PV systems employing P&O with the proposed modification avoid both leftward and rightward drift when subjected to rapidly increasing irradiance, sinusoidal irradiance, and real irradiance. This drift-free P&O enables the PV system to participate in active power control function at rapidly increasing irradiance. An offline MPPT that uses the characteristics of PV modules to determine the maximum power point offline and reduce online computation is proposed for frequency support. Two methods for achieving de-loaded operation of a PV system using the offline MPPT are presented and compared for accuracy. The ability of offline MPPT and the P&O with proposed modification to maintain a power reserve under different irradiance conditions are compared. Second, this thesis examines the ability of a PV system to contribute to frequency support. Different methods for frequency support from a PV power plant under different penetration levels are examined. Results show with the appropriate amount of reserve and support parameters, PV systems can contribute to frequency support. The results also show that PV power plants with the proper support parameters can adequately compensate for the loss of inertia with regards to its effect on the nadir of frequency response. A variable droop control method for frequency support is proposed to reduce the amount of reserve required for frequency support. The effect of MPPT choice on frequency support is evaluated by comparing responses from PV systems with the offline MPPT, P&O with proposed modification, and the constant voltage MPPT. Lastly, this thesis proposes a framework for comparing the frequency support ability of different generating units based on their response speed and support parameters. Different response speeds are emulated by changing one the time constant of a Second-order system. The effect of response speed, support method, and support parameters on the nadir of frequency response and maximum power increase are evaluated for different response speeds. A method for comparing support ability by considering the economic cost and benefit for support is also presented