Zhenhua Jiang

Modeling and control of an integrated wind power generation and energy storage system

Wind energy is gaining the most interest among a variety of renewable energy resources, but the disadvantage is that wind power generation is intermittent, depending on weather conditions. Energy storage is necessary to get a smooth output from a wind turbine. This paper presents a new integrated power generation and energy storage system for doubly-fed induction generator based wind turbine systems. A battery energy storage system is connected to the DC link of the back-to-back power converters of the doubly-fed induction generator through a bi-directional DC/DC power converter. The energy storage device is controlled so as to smooth out the total output power as the wind speed varies. Control algorithms are developed for the grid-side converter, rotor-side converter and battery converter, and are tested on a simulation model developed in MATLAB/Simulink. The model contains a DFIG wind turbine, three PWM power converters and associated controllers, a DC-link capacitor, a battery, and an equivalent power grid. Simulation studies are carried out on a 2MW DFIG wind turbine and the results suggest that the integrated power generation and energy storage system can supply steady output power as the wind speed changes.

A vision of smart transmission grids

Modern power grid is required to become smarter in order to provide an affordable, reliable, and sustainable supply of electricity. Under such circumstances, considerable activities have been carried out in the U.S. and Europe to formulate and promote a vision for the development of the future smart power grids. However, the majority of these activities only placed emphasis on the distribution grid and demand side; while the big picture of the transmission grid in the context of smart grids is still unclear. This paper presents a unique vision for the future smart transmission grids in which the major features that these grids must have are clearly identified. In this vision, each smart transmission grid is regarded as an integrated system that functionally consists of three interactive, smart components, i.e., smart control centers, smart transmission networks, and smart substations. The features and functions of each of the three functional components as well as the enabling technologies to achieve these features and functions are discussed in detail in the paper.

Active power — Voltage control scheme for islanding operation of inverter-interfaced microgrids

Efficient and effective use of renewable energy, distributed generation and energy storage can potentially solve global problems such as energy crisis and climate change. A promising solution to interconnecting these distributed energy resources with the grid is the microgrid paradigm. A microgrid may comprise a variety of inverter-interfaced distributed energy resources such as photovoltaic arrays, wind turbines, fuel cells, microturbines, energy storage devices (such as batteries, super-capacitors) and controllable loads. The microgrid paradigm provides considerable control flexibility: it can be connected with the utility grid, or it can operate isolated from the main grid in case of disturbances or faults. A key issue is how to control the parallel inverters in islanding operation so as to achieve high performance of power and voltage regulations in the microgrid. This paper presents a new active power and voltage control scheme for inverters and a droop control method for the power sharing among the parallel inverter-interfaced distributed energy resources. The proposed control method is tested in two scenarios: (1) a single inverter operated in the islanding mode, (2) two parallel inverters operated in the islanding mode. Simulation results suggest that this control method can control the active power and voltage magnitude of the single inverter very well and regulate the parallel-connected inverters as well.

Design, modeling and simulation of a green building energy system

Traditional buildings consume more of the energy resources than necessary and generate a variety of emissions and waste. The solution to overcoming these problems will be to build them green and smart. One of the significant components in the concept of smart green buildings is using renewable energy. Solar energy and wind energy are intermittent sources of energy, so these sources have to be combined with other sources of energy or storage devices. While batteries and/or supercapacitors are an ideal choice for short-term energy storage, regenerative hydrogen-oxygen fuel cells are a promising candidate for long-term energy storage. This paper is to design and test a green building energy system that consists of renewable energy, energy storage, and energy management. The paper presents the architecture of the proposed green building energy system and a simulation model that allows for the study of advanced control strategies for the green building energy system. An example green building energy system is tested and simulation results show that the variety of energy source and storage devices can be managed very well.

Indirect field-oriented control of induction machines based on synergetic control theory

Field-oriented control is one of the most significant control methods for high performance AC electric machines and drives. In particular, for induction machines, indirect field oriented control is a simple and highly reliable scheme which has essentially become an industry standard. This paper synthesizes and develops an indirect field-oriented speed control for induction motors based on synergetic control theory. Compared with the conventional PI control approach, our results show the speed controller based on synergetic control is more stable, robust and insensitive to system parameter variations. Effects of controller parameter variations on the system performance have also been studied.

Small-signal modeling and analysis of parallel-connected voltage source inverters

Numerous benefits promote the proliferation of renewable and distributed energy resources in power grids. Some recent research has focused on the concept of microgrids, in which a number of renewable energy sources can be connected together to supply power to customers as a complement to traditional power sources or as main power sources. In a microgrid, multiple distributed energy sources are operated in parallel through power converters with several control loops of different control objectives. Thus the dynamic stability problem is increasingly important in microgrids due to the existence of a large number of power converters, and has not yet been widely studied. This paper presents a small-signal model of parallel operated voltage source inverters, in which the variations in d-q axis currents and output d-q axis voltages are taken as state variables, the variations in the power inverter duty cycles are used as the control input, and the variations in the DC power source voltages are considered as external disturbances. Frequency-domain characteristics of the open-loop and closed loop systems are analyzed. Control strategies are designed for two parallel connected inverters. Time-domain simulation studies are carried out to verify the small-signal model and controller design.

Small-signal modeling and analysis of battery-supercapacitor hybrid energy storage systems

The battery/supercapacitor hybrid energy storage system actively combines two energy storage devices to achieve better power and energy performances. This paper presents a detailed small-signal mathematical model that can represent the dynamics of the converter-interfaced energy storage system around the steady-state operating point. This model takes into account the effects on the currents of a variety of factors such as the voltage-current characteristics of individual energy storage devices, power converter and filter parameters, and controller parameters. The proposed model considers the variations in the battery current, supercapacitor current and DC bus voltage as state variables, the variations in the power converter duty cycles as the control input, and the variations in the battery voltage, supercapacitor voltage and load current as external disturbances. Frequency-domain model and control strategies for the power sharing between the battery and supercapacitor are developed based on the small-signal model of the hybrid energy system. Frequency-domain characteristics of the open-loop and closed-loop systems are analyzed. Time-domain simulation is used to verify the system operation. The effects of system and controller parameters on the system performance are also studied.

Multidisciplinary modeling and simulation of a fuel cell/gas turbine hybrid power system

The fuel cell/gas-turbine hybrid power systems can utilize exhaust fuel and waste heat from fuel cells to increase the system efficiency. This paper considers an internally reforming solid oxide fuel cell/gas turbine (SOFC/GT) hybrid system, where the anode exhaust, which contains the remainder of the fuel, is mixed with the cathode exhaust as well as an additional supply of fuel and compressed air and then burned in a catalytic oxidizer. The hot oxidizer exhaust is expanded through the turbine section, driving an electric generator. After leaving the gas turbine, the oxidizer exhaust passes through a heat recovery unit in which it preheats the compressed air that is to be supplied to the fuel cell and the oxidizer. This paper concentrates on multidisciplinary modeling and simulation of the fuel cell/gas turbine hybrid power system. Transient response of the hybrid energy system is studied through time-domain simulation.

Dynamic modeling and control design of microturbine distributed generation systems

Distributed generation draws increasing attention because of the energy shortage and environmental protection considerations. Furthermore, distribution power generation can improve the power system stability and reliability, providing the local power supply economically. Microturbine based power generation has many applications in distributed generation. In this paper, a mathematical model of a microturbine distributed generation system connected to the utility grid is presented. Two back-to-back power converters are connecting the synchronous generator to the grid. The models of microturbine, synchronous generator, and back-to-back power converters are presented and the corresponding control methods are analyzed and designed. The model is developed in MATLAB and simulation results are shown to verify the system performance.

Power sharing control of fuel cell/gas-turbine hybrid power systems

The solid oxide fuel cell/gas turbine (SOFC/GT) hybrid power systems can utilize exhaust fuel and waste heat from fuel cells in gas turbines to increase system efficiency. The control system plays a critical role in achieving the synergistic operation of various subsystems, improving the reliability of operation, and reducing the maintenance frequency and costs. This paper aims to investigate different control strategies for the power sharing between the subsystems. An internally-reforming SOFC/MT hybrid power system is considered for development of advanced control algorithms, where the anode exhaust, which contains the remainder of the fuel, is mixed with the cathode exhaust in a catalytic oxidizer in which oxidation of fuel is completed. The hot oxidizer exhaust is expanded through the turbine, driving an electric generator. The power electronics interfaces and controls for the hybrid power system are discussed. Two different power sharing strategies are studied and compared. Simulation results are presented and analyzed.