Manel Jebali-Ben Ghorbal

Direct Virtual Torque Control for Doubly Fed Induction Generator Grid Connection

This paper presents a grid-connection control strategy of doubly fed induction generator wind system based on the direct control of both a virtual torque and rotor flux of the generator. This control is achieved with no proportional-integral regulator and requires the measurement of only grid voltages, rotor currents, and rotor position. The same switching table is used for grid synchronization and for running process. A field-programmable-gate-array-based design of the proposed control is developed and tested on a 4-kW experimental prototype. Experimental results are provided to show the effectiveness of the fast and soft grid-connection method developed.

A novel grid impedance estimation technique based on adaptive virtual resistance control loop applied to distributed generation inverters

The penetration of the distributed power generation systems (DPGSs) based on renewable sources (PV, WT) is strongly dependent on the quality of the power injected to the utility grid. However, the grid impedance variation, mainly caused by grid faults somewhere in the electric network, can degrade the power quality and even damage some sensitive loads connected at the point of the common coupling (PCC). This paper presents detection-estimation method of the grid impedance variation. This estimation tehnique aims to improve the dynamic of the distributed generation (DG) interfacing inverter control and to take the decision of either keep the DG connected, or disconnect it from the utility grid. The proposed method is based on a fast and easy grid fault detection method. A virtual damping resistance is used to drive the system to the resonance in order to extract the grid impedance parameters, both resistive and inductive parts, using resonant filter frequency determination. Theoretical analysis and simulation results are presented to validate the proposed method.

Online detection and estimation of grid impedance variation for Distributed Power Generation

A better knowledge of the grid impedance is essential in order to improve power quality and control of the Distributed Power Generation Systems (DPGS) and also for a safe connection or reconnection to the utility grid. An LCL-filter associated to a Voltage Source Inverter (VSI) is usually used in DPGS based on renewable energy. The VSI controller is used to determine the grid impedance in both healthy operation and faulty modes. When grid faults occurence, the grid impedance variations have a considerable influence on the control and the stability of the system, even on the decision to disconnect the DPG systems of the network. This work presents a rapid and simple technique to detect the grid impedance variation and to determine the grid impedance before and after grid faults accurs. Implementation on FPGA control board, simulations and experimental results are presented to validate the described method.

Passive islanding detection method based on virtual PCC voltage phase-shift in microgrid

This paper presents a fast passive islanding detection method based on virtual phase-shift between the real voltage seen at the point of common coupling and its created image in a Microgrid. The proposed algorithm consists of extracting the positive and negative sequences of the voltage at this point using a second order generalized integrator in two-way; to synchronize properly the system with the utility grid and to differentiate the islanding condition from other grid disturbance. Then, calculate the phase-shift between the real positive voltage and its image. In one way, once a grid fault occurs, hence leading to additional grid fault impedance emerging, the phase-shift varies. According to this variation duration and level, Microgrid can take the decision of disconnecting from the utility grid to operate autonomously or staying in connected-mode. In the other way, under unintentional islanding condition Microgrid can detect the phase-shift occurrence even in non-detection zone. A simple and efficient phase-shift calculator is proposed. Theoretical analysis, simulation and experimental results are presented to validate the proposed method.

Fast islanding detection of PV grid connected inverters

This paper proposes a new method for fast detection of the point of common coupling (PCC) voltage variations for a grid connected PV inverter. The main feature of this method is the rapidity of the algorithm in such a way that the detection can be carried out even when protection devices are not yet activated. The method uses only voltage sensors at the PCC and the algorithm is based on temporal redundancies obtained with previous stored measurements. The method consists in obtaining a residual, which is generated using simplifications introduced by the real-time constraints. The operation principle is as follows: in healthy mode, the residual is lower than a certain threshold. However, if there is PCC voltage variation, the residual value will increase, allowing a fast detection of this change. The theoretical analysis is performed in order to establish threshold analytic expressions in both cases, with and without PCC voltage variations, and to explain the residual waveform during faults. The proposed method is easy to implement, fast and does not require system parameters knowledge. Simulations and experimental results are presented to demonstrate the effectiveness of the proposed method.