Although virtually every overhead HVDC line is based on LCC technology, hardly any work has been reported on modeling and analysis of the impact of inertia and effective short circuit ratio on control of frequency in weak grids interfacing LCC-HVDC and onshore DFIG-based wind farms This paper develops a comprehensive modeling and stability analysis framework of a weak grid that interfaces an LCC-HVDC station and a DFIG-based wind farm To that end, a nonlinear averaged phasor model of the system is derived, which is then benchmarked against a detailed switched model The averaged model is linearized to design a frequency controller for the LCC-HVDC rectifier station Participation factor and eigenvalue sensitivity measures indicate that the AC system electromechanical mode and the ‘HVDC PLL’ mode are highly sensitive to changes in Effective DC Inertia Constant and Effective Short Circuit Ratio, respectively Root-locus analysis in a progressively weak grid validates these results A systematic design process of the frequency controller parameters reveals a negative interaction between the ‘generator speed-HVDC PLL-frequency controller’ mode and the ‘DFIG-GSC controller’ mode

Impact of inertia and effective short circuit ratio on control of frequency in weak grids interfacing LCC-HVDC and DFIG-based wind farms

The parameter values such as stator resistance, stator inductance, permanent magnet flux and moment of inertia for permanent magnet synchronous motor (PMSM) are of great significance to the controller design. During the operation of PMSM, these parameter values will change due to the influence of temperature and other external factors. In order to obtain these parameter values in real time, this paper put forward an online identification method based on the combination of extended Kalman filter and gradient correction. The simulation results show that the online identification method can obtain PMSM parameter values quickly and accurately. The method has a certain guiding significance for the design controller of PMSM.

Parameter Identification of Permanent Magnet Synchronous Motor Based on Extended Kalman Filter and Gradient Correction

A coordinated adaptive feedback linearization controller (FLC-A) based on a flux observer for DC-based DFIG to track the maximum power point (MPP) is introduced in this paper. The stator and rotor of DFIG are connected to DC grid directly by two voltage source converters. First, an adaptive controller is used to track the MPP on the system which is decoupled by a single-loop feedback linearization strategy. Meanwhile, the proposed controller can estimate and compensate the changes of stator and rotor resistances caused by physical environment in real time, and improve the dynamic performance of the system; second, under the persistent excitation condition (PED), a stator flux observer with online resistance estimation in the stationary reference frame is adopted to replace the traditional flux calculation method or flux meter, which increases system reliability. The stability of adaptive control rate and nonlinear observer is proved by Lyapunov theory. Simulations are carried out to prove the effectiveness and robustness of the proposed control strategy.

MPPT Adaptive Controller of DC-based DFIG in Resistances Uncertainty

This paper proposes a control methodology based on feedback linearization for a doubly fed induction generator (DFIG) incorporating the magnetic saturation. The feedback linearization algebraically converts a nonlinear system model into a linear model, allowing the use of linear control techniques. Feedback linearization control depends on the model of the system and is therefore sensitive to parameter variations. The doubly fed induction generator (DFIG) operating under the magnetic saturation conditions results in the nonlinear variation of magnetizing inductance, which affects the performance of the control algorithm. From this stand point, on the basis of the dynamic model of the doubly fed induction generator considering magnetic saturation, the feedback linearizing control technique has been formulated. The mathematical model of the doubly fed induction generator, integrating the magnetic saturation has been formulated in the stator flux-oriented reference frame with rotor current and stator magnetizing current as state variables. Simulation studies demonstrate that the inclusion of magnetic saturation in the feedback linearization control of the doubly fed induction generator model increases its accuracy and results in a more efficient and reliable synthesis of the control algorithm.

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Mathematical Formulation of Feedback Linearizing Control of Doubly Fed Induction Generator Including Magnetic Saturation Effects

Variable wind speed, electromagnetic induction, heating and aging can subject the Doubly fed induction generator (DFIG) to parameter variation and uncertainties. As such, an integral sliding mode controller (ISMC) has been designed to suppress any disturbance that may arise in the system. ISMCs cause chattering phenomena in systems. A novel power exponential reaching law has been proposed to mitigates the chattering effects and achieve very fast convergence.

Integral Sliding Mode Control With Power Exponential Reaching Law for DFIG

This paper presents a coordinated feedback linearization strategy (CFLS) for DC-based doubly-fed induction generator (DFIG) system to track the maximum power point. The stator and rotor of DFIG are connected to DC grid directly by two voltage source converters. Compared with a traditional DFIG system, the DC-based DFIG system has more system inputs and coupling, which increases the difficulty of vector control strategy. Accordingly, CFLS is proposed to make DFIG operate at the maximum power point (MPP), and two aspects are improved: first a single-loop control is adopted to make DFIG operate steady and accurate under coordinated the control of RSC and SSC. Second system control laws are obtained by the feedback linearization strategy that achieves DC-based DFIG system decoupling fully during the MPPT and system control. Simulations are carried out the comparison between CFLS and conventional vector control (VC), and it shows that the control performance of CFLS is superior.

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Maximum Power Point Tracking of DFIG with DC-Based Converter System Using Coordinated Feedback Linearization Control

Model predictive control (MPC) relies heavily on the accurate mathematical model which is usually difficult to be obtained. Parameters of permanent magnet synchronous motor (PMSM) would be changed in real time and influenced by uncertain factors. For this purpose, this paper proposes a multiparameter identification strategy for PMSM based on MPC. The simulation is carried out and its results show that the accurate model of PMSM for MPC can be obtained and the impact of parameter errors can be eliminated by the proposed control method.

Research on multi-parameter identification of PMSM based on model predictive control

Offshore wind power is the focus of wind power development. The flexible HVDC power transmission system (HVDC-Flexible) will be an important power transmission mode for offshore wind power. Voltage drop in the DC grid will threaten the safe operation of the HVDC-Flexible system of the doubly-fed offshore wind farm (DFOWF-HVDC), so this paper proposes a LVRT scheme for DFOWF-HVDC based on supercapacitor. Utilizing the fast charge and discharge characteristics of supercapacitors, the transient stability of DFOWF-HVDC is guaranteed. The design scheme of the energy storage system based on supercapacitor is described. The control methods of the converter system during the LVRT process are given. The converter system consists of stator-side converter (SSC), rotor-side converter (RSC) of DFIG, and DC grid-side converter (GSC) and energy storage side converter (ESC). The simulation results verify the correctness of the proposed control strategy.

LVRT Control Strategy Based on Supercapacitor for Offshore DFIG Wind Farm with HVDC-Grid

This paper proposed a two-level optimum design for DC and doubly-controlled DFIG system to lower the gearbox ratio and improve the DFIG system economic benefit in the DC grid. Two voltage source converters consisting of a rotor side converter (RSC) and a stator side converter (SSC) interface DFIG with DC grid, which could regulate DFIG system synchronous speed, current and voltage flexibly. Without the limitation of AC grid, a regulation scheme of system current and generator poles is presented by two-level optimum strategy on the basis of the system costs model. The first level optimum strategy is carried out to regulate the generator poles and lower the gearbox ratio. And the second level optimum strategy is adopted to regulate the system current and lower the gearbox ratio. Finally, simulations are carried out to verify the system design effectiveness.

Shaomin Yan

Papers

MPPT Adaptive Controller of DC-based DFIG in Resistances Uncertainty

Maximum Power Point Tracking of DFIG with DC-Based Converter System Using Coordinated Feedback Linearization Control

Research on multi-parameter identification of PMSM based on model predictive control

LVRT Control Strategy Based on Supercapacitor for Offshore DFIG Wind Farm with HVDC-Grid

Study on an Optimum Design for DC and Doubly-Controlled DFIG System