Pole-placement control of 4-leg voltage-source inverters for standalone photovoltaic systems: Considering digital delays

Three leg inverters for photovoltaic systems have a lot of disadvantages, especially when the load is unbalanced. These disadvantages are for example, small utilization of the DC link voltage, the dependency of the modulation factor of the load current and the superposition of a DC component with the output AC voltage. A solution for these problems is the four-leg inverter. Most papers dealing with 4-leg inverters ignore the effect of digital delays in control loop and suggest classic controllers, such as PI controller. However, the transient performance of the closed-loop system does not become, accurately, adjustable and acceptable, in most cases. This paper proposes a novel model, considering digital delays, for 4-leg inverters. Then, it applies pole-placement control strategy, via state feedback, to adjust the transient performance of the closed-loop system to a desired second-order system. Simulation results validate theoretical results and proposed control strategy.     

Adaptive decoupled control of 4-leg voltage-source inverters for standalone photovoltaic systems: Adjusting transient state response

Three leg inverters for photovoltaic systems have a lot of disadvantages, especially when the load is unbalanced. These disadvantages are for example, small utilization of the DC link voltage, the dependency of the modulation factor of the load current and the superposition of a DC component with the output AC voltage. A solution for these problems is the four-leg inverter. Consequently, generation of balanced voltage with sinusoidal waveform is necessary for these inverters. The power stage model of the 4-leg inverter in rotating coordinates dqo is highly coupled. Consequently, controller design procedure is very difficult and complex. On the other hand, adaptive control for 4-leg inverters has not yet been discussed in the literature. This paper proposes the state feedback approach to decouple the system and convert it to a second-order system which has two poles equal to zero. Then, it suggests a compensator to cancel poles of the closed-loop system and to convert the final system to a desired second-order system. Thanks to use of this strategy the transient performance of the system, such as overshoot and speed of response, becomes greatly adjustable. In addition, an STR (Self-Tuner Regulator) is introduced to tune the state feedback matrix and to guarantee the adaptive performance of the system. Simulation results validate that, by using proposed control strategy, the 4-leg inverter generates balanced voltage, with perfect sinusoidal waveform, in spite of the presence RL time-variant loads.     

带恒功率负载多电平Boost变换器的复合非线性控制

针对带恒功率负载的多电平Boost变换器,提出一种将非线性干扰观测器和自适应滑模控制器相结合的复合非线性控制策略。首先应用精确反馈线性化方法将模型转化为布鲁诺夫斯基标准形式。然后在保证大信号稳定的前提下,将自适应控制方法和非线性干扰观测器加入到滑模控制器的设计中,利用李雅普诺夫理论证明整个闭环系统的稳定性。仿真和实验结果表明,与双闭环PI控制方法相比,该控制策略具有更好的动态调节性能和更强的鲁棒性。     

A linear quadratic regulator control of a stand-alone PEM fuel cell power plant

This paper introduces a technique based on linear quadratic regulator (LQR) to control the output voltage at the load point versus load variation from a standalone proton exchange membrane (PEM) fuel cell power plant (FCPP) for a group housing use. The controller modifies the optimal gains k _i by minimizing a cost function, and the phase angle of the AC output voltage to control the active and reactive power output from an FCPP to match the terminal load. The control actions are based on feedback signals from the terminal load, output voltage and fuel cell feedback current. The topology chosen for the simulation consists of a 45 kW proton exchange membrane fuel cell (PEMFC), boost type DC/DC converter, a three-phase DC/AC inverter followed by an LC filter. Simulation results show that the proposed control strategy operated at low commutation frequency (2 kHz) offers good performances versus load variations with low total harmonic distortions (THD), which is very useful for high power applications.     

Laboratory implementation of a neural network trajectory controllerfor a DC motor

The laboratory implementation of a neural network controller for high performance DC drives is described. The objective is to control the rotor speed and/or position to follow an arbitrarily selected trajectory at all times. The control strategy is based on indirect model reference adaptive control (MRAC). The motor characteristics are explicitly identified through a multilayer perceptron type neural network. The output of the trained neural network is used to drive the motor in order to achieve a desired time trajectory of the controlled variable. The neural network controller is assembled in a commercially available PC-based real-time control system shell, using software subroutines. An H-bridge, DC/DC voltage converter is interfaced with the computer to generate the specified terminal voltage sequences for driving the motor. All software and hardware components are off the shelf. The versatility of the motor/controller arrangement is displayed through real-time plots of the controlled states     

An Autonomous System Supplied Only by a Pitch-Controlled Variable-Speed Wind Turbine

This paper presents a control strategy for a variable-speed pitch-controlled wind turbine (WT) generation scheme for the supply of an autonomous system with no energy storage units. The synchronous generator includes two three-phase stator windings displaced by 30deg that are connected to the transformer load through two dc links with voltage source inverters (VSI). Following priority rules, the load is divided into steps. Each load step can be supplied by the WT when the wind speed varies between two predefined speed levels. The first goal of the WT control system is to supply the load with constant real power under constant voltage as the wind speed varies between two levels and the second is to operate smoothly interchanging the load steps when the wind speed breaks through a speed level. There are two controllers: the inverter controller that keeps the load voltage constant and the pitch controller acting on the blade's angle. Using simulation techniques, the operation of the WT system and the efficiency of the proposed control strategy are demonstrated for a wide range of wind speeds.     

SMC-DPC based active and reactive power control of grid-tied three phase inverter for PV systems

In this paper, sliding mode control (SMC) – direct power controller (DPC) based active and reactive power controller for three-phase grid-tied photovoltaic (PV) system is proposed. The proposed system consists of two main controllers: the DC/DC boost converter to track the possible maximum power from the PV panels and the grid-tied three-phase inverter. The Perturb and Observe (P&O) algorithm is used to transfer the maximum power from the PV panels. Control of the active and reactive powers is performed using the SMC-DPC strategy without any rotating coordinate transformations or phase angle tracking of the grid voltage. In addition, extra current control cycles are not used to simplify the system design and to increase transient performance. The fixed switching frequency is obtained by using space vector modulation (SVM). The proposed system provides very good results both in transient and steady states with the simple algorithms of P&O and SMC-DPC methods. Moreover, the results are evaluated by comparing the SMC-DPC method developed for MPPT and the traditional PI control method. The proposed controller method is achieved with TMS320F28335 DSP processor and the system is experimentally tested for 12 kW PV generation systems.     

风光互补发电系统功率输出的自适应控制

针对风光互补发电系统在运行过程中出现的供电功率空缺以及直流母线电压不稳定问题,依据光伏为主力发电源,风为辅助发电源的发电方式,提出了以风力发电的输出功率补偿光伏发电的输出功率的自适应控制方案。鉴于发电系统中固有的非线性特征和运行中的参数变化,对风力发电系统采用自适应控制实现对互补发电系统中所需功率的差额实时补偿,保证直流母线电压的平稳。为实现蓄电池充电稳定性,结合蓄电池非线性充电模型设计了充电电流跟踪控制器,确保对期望充电电流的跟踪。理论分析和仿真结果均表明,在互补发电系统运行中采取的控制策略的可行性。     

Decentralized control strategy for fuel cell/PV/BESS based microgrid using modified fractional order PI controller

The concept of renewable energy based microgrid (MG) and its control has been evolved as key area of research in energy sector. In this paper, a decentralized control strategy based on modified fractional order PI (MFO-PI) and two-degree of freedom PI (2DOF-PI) controllers is proposed for efficient operation of an autonomous MG. The autonomous MG consists of solid oxide fuel cell (SOFC) & photovoltaic (PV) system as distributed generation (DG), battery energy storage system (BESS) as a storage unit and various AC & DC loads. The MFO-PI controller is utilized for controlling voltage source inverters (VSI) and 2DOF-PI is utilized for controlling various DGs and BESS. An evaporation rate-based water cycle algorithm (ERWCA) is employed to optimally tune the proposed controllers. To show the effectiveness of the proposed decentralized control strategy, a comparison of various performance indices such as overshoot, settling time and integral absolute error is made with PI and fractional order PI controllers. The results show that proposed control strategy is efficient in improving the steady state as well as dynamic performance of the system under all operating conditions by effectively regulating the real and reactive power flows among the DGs.     

Model-based power control strategy development of a fuel cell hybrid vehicle

An integrated procedure for math modeling and power control strategy design for a fuel cell hybrid vehicle (FCHV) is presented in this paper. Dynamic math model of the powertrain is constructed firstly, which includes four modules: fuel cell engine, DC/DC inverter, motor-driver, and power battery. Based on the mathematic model, a power control principle is designed, which uses full-states closed-loop feedback algorithm. To implement full-states feedback, a Luenberger state observer is designed to estimate open circuit voltage (OCV) of the battery, which make the control principle not sensitive to the battery SOC (state of charge) estimated error. Full-states feedback controller is then designed through analyzing step responding of the powertrain and test data. At last of the paper, the results of simulation and field test are illustrated. The results show that the power control strategy designed takes into account the performance and economy characteristics of components of the FCHV powertrain and achieves the control object excellently.     

Hydrogen consumption minimization method based on the online identification for multi-stack PEMFCs system

The proton exchange membrane fuel cell (PEMFC) stacks are not widely used in the field of transportation industry, due to their limited power. Thus, the PEMFC stacks usually connected in parallel or series to meet the load demand power in high-power applications. The hydrogen consumption of multi-stack fuel cells (MFCs) system is related to the efficiency and output power. In addition, the efficiency of PEMFC depends on the applied voltage and other parameters. Consequently, the hydrogen consumption of system changes with varying load, because the system parameters are also varying. It makes reducing the fuel consumption of system a challenging assignment. In order to achieve the goal of minimizing fuel consumption of parallel-connected PEMFCs system, this paper proposes a novel power distribution strategy based on forgetting factor recursive least square (FFRLS) online identification. The FFRLS algorithm is based on data-driven and uses real-time data of the system to improve the estimation accuracy of PEMFC system parameters. On the test bench of parallel-connected PEMFCs system consists of two 300 W PEMFC stacks, PEMFC stack controller, DC/DC converters, and DSP controller etc., a multi-index performance test and comparative analysis are carried out. The results showed that, the performance of proposed power allocation strategy has been successfully validated. In addition, compared with the power average and daisy chain algorithms, the proposed online identification power distribution method can get more satisfactory results. Such as, reducing the hydrogen consumption and improving efficiency.     

基于神经网络的最优带宽风电并网自抗扰控制

为了提高风能转换系统在故障穿越期间直流母线电压的暂态特性,解决线性自抗扰控制（LADRC）中固定的带宽带来的观测器响应速度和系统抗干扰性之间的矛盾,在线性自抗扰控制（LADRC）的基础上提出一种基于神经网络的最优带宽线性自抗扰控制（LADRC-OB）策略。首先分析带宽对系统性能的影响,然后根据系统已知模型设计LADRC控制器,并利用BP神经网络算法通过直流母线电压的参考值与实际值之间的偏差调整网络的输出。而神经网络的输出为LADRC的2个重要参数——观测器带宽ω₀和控制器带宽ω_c,这可解决LADRC的参数整定问题。最后,将LADRC-OB应用于1.5 MW的风能转换系统仿真模型中,并与采用双闭环PI时的控制效果进行对比,验证LADRC-OB具有更好的控制特性。此外,还对LADRC-OB的稳定性进行分析。     

A new method for dynamic performance improvement of a hybrid power system by coordination of converter's controller

In this paper, a new strategy for modeling and controlling a hybrid power generation system that contains a fuel cell (FC) and super capacitor (SC) system is proposed. The main drawback of FC systems is its slow dynamic because the FC current slope must be limited in order to prevent fuel starvation problems and to improve its efficiency and lifetime. To overcome this slow dynamic and to improve dynamic performance, a new control strategy is proposed to combine FC system with SC system. The proposed control strategy can be also used for cold starting and different types of FC systems with different dynamics. The control strategy is capable of determining the desired FC power to prolong FC system lifetime and keeps the AC and DC voltages around its nominal value in transient event by supplying propulsion power and recuperating FC energy. The minimum SC system is computed in new method and used to meet the load demand to constraint the DC bus voltage and enhances power regulation under various active and reactive load conditions. Two different case studies are used to obtain the simulation results using MATLAB/SIMULINK to verify the validity of the proposed control strategy.     

Observer-based type-2 fuzzy approach for robust control and energy management strategy of hybrid energy storage systems

In this paper, an observer-based type-2 fuzzy method is proposed for control and energy management strategy (EMS) of the hybrid energy storage system (HESS) which can be composed of the fuel cell (FC), battery (BA), and supercapacitor (SC). The objective and main contribution of the suggested strategy is to provide: 1) Appropriate tracking performance of power sources by an observer-based control method in the presence of noise and signal ripples. 2) An observer-based composite adaptive type-2 fuzzy (OCAT2F) to approximate the voltage of power sources. 3) A dynamical model of DC-bus to guarantee the stability of closed-loop system. 4) An intelligent EMS. To have a high-power supply, the proposed EMS includes two parts; a type-2 fuzzy logic control rule table (T2FLCRT), and an observer-based robust adaptive fuzzy type-2 fuzzy (ORAT2F). Furthermore, stability analyses of the closed-loop system are provided by the input-output linearization (I-OL) approach and based on the Lyapunov theorem. The simulation results of the proposed control scheme under MATLAB/Simulink indicate that the suggested strategy can provide a suitable control performance, and stability of the whole system is achieved.     

集散式光伏逆变系统低电压穿越控制策略

针对集散式光伏逆变系统低电压穿越时稳定性差的问题,提出了一种低电压穿越控制策略。该策略以智能控制器和逆变器相互耦合的直流母线电压为参考量,对智能控制器和逆变器进行协调控制,实现低电压穿越时的功率平衡。并通过负序电流指令补偿,抑制电网电压不平衡时直流母线电压2次纹波。1 MW集散式光伏逆变系统LVRT试验结果表明,该方法具有良好的动态响应和稳态性能,低电压穿越时,直流母线电压控制稳定;低电压穿越恢复时,有功功率恢复平稳快速。研究成果有重要的工程应用价值。     

Controller design for an induction generator driven by a variable-speed wind turbine

This paper presents the modeling, controller design and a steady-state analysis algorithm for a wind-driven induction generator system. An output feedback linear quadratic controller is designed for the static synchronous compensator (STATCOM) and the variable blade pitch in a wind energy conversion system (WECS) in order to reach the voltage and mechanical power control under both grid-connection and islanding conditions. A two-reference-frame model is proposed to decouple the STATCOM real and reactive power control loops for the output feedback controller. To ensure zero steady-state voltage errors for the output feedback controller, the integrals of load bus voltage deviation and dc-capacitor voltage deviation are employed as the additional state variables. Pole-placement technique is used to determine a proper weighting matrix for the linear quadratic controller such that satisfactory damping characteristics can be achieved for the closed-loop system. Effects of various system disturbances on the dynamic performance have been simulated, and the results reveal that the proposed controller is effective in regulating the load voltage and stabilizing the generator rotating speed for the WECS either connected with or disconnected from the power grid. In addition, proper steady-state operating points for an isolated induction generator can be determined by the proposed steady-state analysis algorithm. Constant output frequency control using the derived steady-state characteristics of the isolated induction generator is then demonstrated in this paper.     

An Adaptive Stabilizer for Thyristor Controlled Static Var Compensators for Power Systems

A static VAR systems play an important role as a stability aid for small signal and transient disturbances in an interconnected power system. Further the introduction of an adaptive stabilizer loop to its controller can significantly improve power system performance under a variety of abnormal situations such as 3-phase short-circuits, voltage collapse, load shedding, reactance switching, etc. This paper presents the analytical development of a new controller scheme for a static VAR system and presents detailed simulation results to highlight its effectiveness in comparison with a discrete PI stabilizer under a variety of operating conditions for power transmission systems. The adaptive stabilizer is suitable for microprocessor digital implementation as the numerical steps involved are minimal.     

Adaptive multi-layer self-tuning high performance tracking controlfor DC brushless motor

Adaptive control for high performance drive systems has become an important subject of research. In applications such as robotics, actuation, and manipulation, the rotor of the electric motor should follow a pre-selected track at all time. The tracking accuracy should not be affected by parameter uncertainties, unknown load variations, or sudden external disturbances. In this paper two schemes of adaptive control are developed and tested for a DC brushless motor. The first scheme is a single-layer self-tuning controller based on the generalized minimum variance theory. The second is a multi-layer adaptive controller consisting of a self-tuning control layer and a supervisory control layer. The supervisory controller continuously monitors the status of the system parameters, the structure of the controller, and the motor performance. A laboratory setup is constructed to test the proposed methods. Laboratory results show that the multi-layer controller is capable of achieving the tracking process with a high degree of accuracy, even in the presence of large and sudden disturbances     

Development and implementation of self-tuning tracking controllerfor DC motors

A self-tuning control strategy for tracking applications of DC drives is proposed. By this technique, the rotor speed and/or position of a DC motor can follow any arbitrarily selected track (or trajectory). The developed control strategy is adaptive and robust, i.e. insensitive to system variations, such as load changes, noise, and disturbances. This results in what is known as a high-performance drive system, which is required in applications such as robotics, actuation, and manipulation. The developed drive system was implemented and tested in the laboratory. The system was examined under various uncertainties such as load variations, noisy measurements, and field disturbances. All lab tests confirm the accuracy and the robustness of the proposed tracking controller. It is shown how such a powerful controller can be implemented by off-the-shelf hardware and software instead of by specially designed items that are often time consuming and costly     

Decentralized coordination control of PV generators,storage battery,hydrogen production unit and fuel cell in islanded DC microgrid

Renewable energy sources (RESs) have been limited to connect to main grid because of their inherent disadvantages such as the fluctuation and intermittence of the output power, the inconsistency with load curve and the impact on the relay protection. Hydrogen production unit (HPU) can address the above issues because it can achieve large-capacity and long-term power absorption, and requires not much of the RESs. In this paper, an adaptive coordination control strategy is proposed in the islanded DC microgrid containing PV generators, storage battery, fuel cell and HPU. As for HPU, the energy conversion efficiency from electric energy to hydrogen energy of HPU is derived and it reveals that there exists a peak value in the efficiency curve. Then an efficiency adaptive control is proposed to adjust the power absorption by regulating the efficiency point based on the dc bus voltage. As for storage battery, the state of charge (SoC) and the instantaneous charging and discharging power of the battery are considered, which can avoid the battery being overused or damaged. As for PV generator, the designed PV controller can adaptively regulate its output power from the maximum power point to the reference power point. As for fuel cell, it is designed that the fuel cell starts to supply power in low-SoC condition with constant power control strategy. Finally, the stability of the coordination control strategy is analyzed based on Nyquist stability criterion and the control effectiveness is verified with simulation and experimental results.