Chunying Gong

New Overall Power Control Strategy for Variable-Speed Fixed-Pitch Wind Turbines Within the Whole Wind Velocity Range

Variable-speed fixed-pitch (VSFP) wind turbines have good prospects in small-to-medium-scale wind power markets due to their simple structure, low cost, and high reliability. One difficulty with VSFP concept wind turbines is to prevent overspeeding and overloading problems at excessive wind velocities, which has rarely been reported in literatures until now. This paper first proposes a sensorless overall power control strategy for a commonly used permanent-magnet-synchronous-generator-based VSFP concept wind power system, with which maximum power point tracking operation, constant speed stalling operation, and constant power soft-stalling operation are all realized. The proposed control scheme has a special advantage of simple structure, i.e., only two regulators are used to realize the three operational modes and also the natural transition between them. An aerodynamic power observer is adopted in the proposed scheme to fasten the MPPT speed. In addition, to enhance system robustness to parameter variations and optimize dynamic and static speed-control performance, an adaptive PI-like fuzzy logic controller is proposed and used as speed regulator in the overall power control scheme. The proposed strategy is verified by simulation and experimental results performed by a 1.2-kW VSFP concept wind turbine prototype.

Dual-Buck Half-Bridge Voltage Balancer

Micro-dc grid is a novel power system focused on the development of renewable resources. However, two-wire transmitting power mode is generally accepted in a micro-dc grid, which is usually not suitable for the requirements of the input voltage levels of different power converters and loads. In order to meet the requirements, a half-bridge voltage balancer was introduced in a micro-dc grid, which can convert a two-wire mode into a three-wire mode in a micro-dc grid via a neutral line. However, the shoot-through problem existing in bridge-type converters degrades the reliability of the voltage balancer. In this paper, a dual-buck half-bridge voltage balancer and a control strategy are proposed, which can avoid the shoot-through problem. The small-signal model of the voltage balancer is derived for designing the control parameters and the current relationships of the inductors; the capacitors and the unbalanced loads are analyzed particularly. Finally, a prototype, which can deal with 2-kW unbalance ability, is built to verify that the proposed voltage balancer may have a good ability of balancing the voltage by building a neutral line.

Constant-Bandwidth Maximum Power Point Tracking Strategy for Variable-Speed Wind Turbines and Its Design Details

Implementation of maximum power point tracking (MPPT) control to variable-speed wind turbines below the rated wind velocity helps the turbine improve its efficiency, thereby increasing its annual energy yield. However, for the most commonly used optimum power (OP) MPPT method, it is proved to have the drawback of low tracking speed. Moreover, what is worse, the tracking bandwidth will get narrower when the turbine is operating under low wind velocities, which results in the reduction of annual energy yield. To overcome this drawback, an improved MPPT method, which is improved from the conventional OP control method, is designed in this paper to broaden the MPPT bandwidth. The design details on how to realize the improved MPPT method and the principle of choosing a proper system dynamics are both pointed out after analyzing the system dynamics in frequency domain. The theoretic analysis is verified by simulation and experimental results performed by a 1.2-kW variable-speed wind turbine.

On Optimizing the Aerodynamic Load Acting on the Turbine Shaft of PMSG-Based Direct-Drive Wind Energy Conversion System

The blades rotational sampling to the spatial distributed wind velocities will induce 3P oscillating aerodynamic torque during the wind energy generation process. This causes the turbine drive train bare high aerodynamic load because the generator is driven by this aerodynamic torque through it. Moreover, the systems inherent resonant mode will be also induced by the aerodynamic load, causing fatal damage to the whole system. To damp the serious aerodynamic load of the permanent-magnet-synchronous-generator-based direct-drive wind energy conversion system (WECS), a new power control strategy with damping injection is proposed in this paper. The proposed method is realized by adding a compensation torque, which is proportional to the small-signal value of the generator speed, into the system torque control loop. Both the aerodynamic load and the systems inherent resonant mode could be well damped if the proposed method were adopted. Theoretic analysis is verified by experimental results performed by a 10-kW WECS established in the laboratory.

Impedance-Phased Dynamic Control Method for Grid-Connected Inverters in a Weak Grid

A power distribution grid exhibits the characteristics of a weak grid owing to the existence of scattered high-power distributed power-generation devices. The grid impedance affects the robust stability of grid-connected inverters, leading to harmonic resonance, or even instability in the system. Therefore, a study of the stability of grid-connected inverters with high grid impedance based on impedance analysis is presented in this paper. The output impedance modeling of an LCL-type single-phase grid-connected inverter is derived, where the effects of the PLL loop and the digital control delays on the output impedance characteristics have been taken into account. To enhance the stability of grid-connected inverters with different grid impedance, a novel impedance-phased compensation control strategy is proposed by increasing the phase margin of the grid-connected inverters. Specifically, a detailed implementation and parameter design of the impedance-phased compensation control method is depicted. Finally, an impedance-phased dynamic control scheme combined with online grid impedance measurement is introduced and also verified by the experiment results.

On Optimizing the Transient Load of Variable-Speed Wind Energy Conversion System During the MPP Tracking Process

In order to increase the annual energy yield of a variable-speed wind energy conversion system (WECS), maximum power point tracking (MPPT) control is required at below the rated wind speed. However, the only aspect that has been paid attention to by most of the researchers is the way to accelerate the MPPT speed. In fact, the research of this paper finds out that the faster the MPPT speed is, the larger transient load the turbine shaft bears. Thus, attempts are first made to find out the relationship between the MPPT speed and the transient load through quantity analysis. Then, a principle to choose a proper bandwidth of the MPPT system is given out to optimize the transient load. Furthermore, to ensure that the system can track the varying winds at the designed bandwidth, the design details on how to make the MPPT bandwidth constant are also proposed in this paper. Finally, a 1.2-kW variable-speed WECS, which is based on a permanent-magnet synchronous generator, is established in the laboratory. Theoretical analysis is verified by experimental results performed by it.

Design of LCL filter for wind power inverter

An LCL filter is more and more widely used in wind power inverters. However, it may easily result in instability due to resonance, which makes it difficult for stability control without damping. A voltage oriented control strategy under the dq synchronization reference frame is adopted based on the mathematical model of the three-phase inverter for wind power, and then the characteristics and performance of an LCL filter compared with the L filter are investigated in detail. At the same time, the design of the LCL filter and the influence of system stability with different damping are given. Experimental results have effectively verified the design.

A novel technique of low frequency input current ripple reduction in two-stage DC-AC inverter

The input current of the two-stage DC-AC inverter contains low frequency ripple whose frequency is twice that of the inverters output voltage. The proportion of the low frequency current ripples amplitude to the average currents can achieve as high as more than 30%. The stability of dc power system and life span of the input voltage source such as battery or fuel cell could be influenced seriously. The reason of low frequency input current ripple production is analyzed, and the new control strategy of the first stage DC/DC converter is proposed on this paper. Compared with the traditional control strategy, the resonant controller is introduced. The proposed new control strategy is effective in reducing the current ripple in the output filter inductor of the DC/DC converter. Moreover, the input current ripple of the two-stage DC-AC inverter can be decreased. The design of the resonant controller and the system transfer function are given in the paper. The design principles of system stability are also discussed. At last, the effectiveness of proposed new control strategy to reduce the input current ripple that exists in the two-stage DC-AC inverter is verified by simulation and experimental results.

Design and analysis of dynamic wind turbine simulator for wind energy conversion system

Static and dynamic simulation methods are two commonly adopted techniques in constructing wind turbine simulator (WTS). To better understand the difference between static simulation method and actual wind turbine, their frequency-domain models which have never been discussed before are elaborated in this paper, and the reason why conventional static simulation method can not reproduce the dynamic behavior of a wind turbine and why dynamic simulation method should be adopted are carefully discussed. Then, the relationship between inertia of wind turbine and time constant of LFP are discussed based on the simplified model of dynamic WTS proposed in the paper, which reveals the ways to simulate different wind turbines with one hardware specification. An experimental platform which can accurately simulate both the static and dynamic characteristic of real wind turbines is implemented. The theoretical analysis and proposed strategy is verified by simulation and experimental results performed by a 7.5KW laboratory prototype.

A New Control Strategy of Single-Stage Flyback Inverter

This paper presents a new control strategy for a single-stage flyback inverter, which consists of two flyback converters. The two converters alternately work in the proposed control scheme, which eliminate about 30% additional freewheeling power compared with a conventional differential control scheme. The control scheme is analyzed in detail. The 100-VA prototype with improved efficiency verifies the analysis.