Geng Yang

Bio: Geng Yang is an academic researcher from Tsinghua University. The author has contributed to research in topics: Wind power & Electric power system. The author has an hindex of 25, co-authored 139 publications receiving 2833 citations. Previous affiliations of Geng Yang include Sophia University.

Adaptive-speed identification scheme for a vector-controlled speed sensorless inverter-induction motor drive

Abstract: The authors describe a model reference adaptive control system for speed control of the vector-controlled inverter induction motor drive without a speed sensor. The rotor speed is estimated with a full-order adaptive observer and is used as the feedback signal for the vector and speed controls. In order to estimate the speed accurately at low speeds, the motor stator resistance is also identified at the same time to correct the mismatched resistance value used in the observer. The resulting system is verified to be hyperstable. As a result, the drive can be operated in a wide range of speed, and especially, it can be operated stably at the state of zero speed. Experiments also show that the scheme is effective. >

An LVRT Control Strategy Based on Flux Linkage Tracking for DFIG-Based WECS

Abstract: For doubly fed induction generator (DFIG)-based wind energy conversion systems (WECSs), large electromotive force will be induced in the rotor circuit during grid faults. Without proper protection scheme, the rotor side of DFIG will suffer from overcurrents, which may even destroy the rotor-side converter (RSC). To mitigate this problem, a new flux-linkage-tracking-based low-voltage ride-through (LVRT) control strategy is proposed to suppress the short-circuit rotor current. Under the proposed control strategy, the rotor flux linkage is controlled to track a reduced fraction of the changing stator flux linkage by switching the control algorithm of RSC during grid faults. To validate the proposed control strategy, a case study of a typical 1.5-MW DFIG-based WECS is carried out by simulation using the full-order model in SIMULINK/SimPowerSystems. In the case study, a comparison with a typical LVRT method based on RSC control is given, and the effect of the control parameter on the control performance is also investigated. Finally, the validity of the proposed method is further verified by means of laboratory experiments with a scaled-size DFIG system.

LVRT Capability of DFIG-Based WECS Under Asymmetrical Grid Fault Condition

Abstract: In this paper, the low-voltage ride-through (LVRT) capability of the doubly fed induction generator (DFIG)-based wind energy conversion system in the asymmetrical grid fault situation is analyzed, and the control scheme for the system is proposed to follow the requirements defined by the grid codes. As analyzed in the paper, the control efforts of the negative-sequence current are much higher than that of the positive-sequence current for the DFIG. As a result, the control capability of the DFIG restrained by the dc-link voltage will degenerate for the fault type with higher negative-sequence voltage component and 2φ fault turns out to be the most serious scenario for the LVRT problem. When the fault location is close to the grid connection point, the DFIG may be out of control resulting in non-ride-through zones. In the worst circumstance when LVRT can succeed, the maximal positive-sequence reactive current supplied by the DFIG is around 0.4 pu, which coordinates with the present grid code. Increasing the power rating of the rotor-side converter can improve the LVRT capability of the DFIG but induce additional costs. Based on the analysis, an LVRT scheme for the DFIG is also proposed by taking account of the code requirements and the control capability of the converters. As verified by the simulation and experimental results, the scheme can promise the DFIG to supply the defined positive-sequence reactive current to support the power grid and mitigate the oscillations in the generator torque and dc-link voltage, which improves the reliability of the wind farm and the power system.

Grid-Synchronization Stability Improvement of Large Scale Wind Farm During Severe Grid Fault

Abstract: Loss of synchronization between wind farm and power grid during severe grid faults would cause wind farm tripping. In this paper, the mechanism of grid-synchronization is uncovered, described as motion of an autonomous nonlinear differential equation with specific initial states. The revealed mechanism indicates that even though steady-state working point exists, improper initial states and poor system dynamic properties could lead to synchronization instability. In order to keep wind farm synchronous with the power grid during severe grid faults, special requirements on system dynamic properties are stated. Moreover, to satisfy all the requirements, a current injecting method is proposed. By adjusting active and reactive output currents of the wind farm, the proposed method could ensure system synchronization stability during severe grid faults. Implementation of the proposed method on PMSG- and DFIG-based wind farm is illustrated. Simulation results validate the analysis and the control method.

Finite-Time-Convergent Differentiator Based on Singular Perturbation Technique

Abstract: A finite-time-convergent differentiator is presented that is based on singular perturbation technique. The merits of this differentiator exist in three aspects: rapidly finite-time convergence compared with other typical differentiators; no chattering phenomenon; and besides the derivatives of the derivable signals, the generalized derivatives of some classes of signals can be obtained-for example, the generalized derivative of a triangular wave is square wave, etc. The theoretical results are confirmed by computer simulations.

Ultracapacitors: Why, How, and Where is the Technology

Abstract: The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Overview of Multi-MW Wind Turbines and Wind Parks

Abstract: Multimegawatt wind-turbine systems, often organized in a wind park, are the backbone of the power generation based on renewable-energy systems. This paper reviews the most-adopted wind-turbine systems, the adopted generators, the topologies of the converters, the generator control and grid connection issues, as well as their arrangement in wind parks.

Transducerless position and velocity estimation in induction and salient AC machines

Abstract: This paper presents a viable transducerless rotor position and velocity estimation scheme for PWM inverter driven induction, synchronous, and reluctance machines with the capability of providing robust and accurate dynamic estimation independent of operating point, including zero and very high speeds, light and heavy loading. The injection of a balanced three-phase high frequency signal (500 to 2 kHz) generated by the inverter, followed by appropriate signal demodulation and processing combined with a closed-loop observer, enable the tracking of rotor magnetic saliencies from the machine terminals. Although rotor magnetic saliency is inherent within reluctance machines, and most synchronous machines, saliency in the induction machine is introduced via a modulation of the rotor slot leakage with minimal detrimental effects on the machine performance. Experimental verification for the induction machine is included. >

High-Power Wind Energy Conversion Systems: State-of-the-Art and Emerging Technologies

Abstract: This paper presents a comprehensive study on the state-of-the-art and emerging wind energy technologies from the electrical engineering perspective. In an attempt to decrease cost of energy, increase the wind energy conversion efficiency, reliability, power density, and comply with the stringent grid codes, the electric generators and power electronic converters have emerged in a rigorous manner. From the market based survey, the most successful generator-converter configurations are addressed along with few promising topologies available in the literature. The back-to-back connected converters, passive generator-side converters, converters for multiphase generators, and converters without intermediate dc-link are investigated for high-power wind energy conversion systems (WECS), and presented in low and medium voltage category. The onshore and offshore wind farm configurations are analyzed with respect to the series/parallel connection of wind turbine ac/dc output terminals, and high voltage ac/dc transmission. The fault-ride through compliance methods used in the induction and synchronous generator based WECS are also discussed. The past, present and future trends in megawatt WECS are reviewed in terms of mechanical and electrical technologies, integration to power systems, and control theory. The important survey results, and technical merits and demerits of various WECS electrical systems are summarized by tables. The list of current and future wind turbines are also provided along with technical details.

Sensorless control of AC machines at low speed and standstill based on the "INFORM" method

Abstract: The increasing use of AC machines compared to DC motors in electrical drive applications has several reasons. A very important advantage of AC machines is their simple construction. However, AC drives often need mechanical sensors (tachometers, position encoders) for field orientation. In many applications these sensors reduce robustness and increase costs of a drive considerably. The main objective of this paper is to present the "INFORM" method based on real-time inductance measurements for sensorless control of AC drives.