An adaptive observer-based approach is presented to estimate single-phase grid voltage parameters under harmonic disturbances. By treating the grid voltage as a dynamic system with unknown parameters related to the system frequencies, we build a parameterized dynamic model and then develop adaptive observers for the estimation of the grid voltage parameters including frequency, amplitude and phase angle with zero steady state error. The contributions of the paper are two-folds: 1) when the grid involves only fundamental voltage, a third-order adaptive estimator is derived by observer design combined with the notion of passivity. It is shown that the third-order adaptive observer can estimate the grid voltage parameters without steady state error; 2) when the grid voltage involves harmonic disturbances, we design a higher-order adaptive observer to estimate the grid voltage parameters. Passivity and Lyapunov-based arguments ensure that the proposed method also achieves zero steady state estimation error. Simulations and experiments are given to validate the effectiveness of our estimation strategies.Index Terms-Estimation of grid voltage parameters, dynamic model, adaptive observer, zero steady state error, harmonic voltage disturbances 0885-8993 (c)

As an important ancillary service for smart grid, voltage support has the capability to improve the grid condition when grid voltage unbalanced fault occurs. Several solutions, taking into account not only the root mean square voltage, but also the shape of the voltage fault, have been proposed. In particular, the control scheme proposed in this study introduces two contributions: a novel reference generator for voltage support and a new current tracking controller. Taking into account of the shape of the grid voltage, this reference generator has the capacity to increase the positive sequence fundamental voltage magnitude and decrease the negative sequence fundamental voltage magnitude, which improves the grid condition when unbalanced fault and voltage sags occur. Compared with other works, phase locked loop is eliminated in this generator which simplifies the structure of the controller. On the basis of output regulation theory, a simple-structure current tracking controller is proposed, which has a zero steady error under arbitrary order (or their combination) grid voltage harmonic disturbance. Simulations and experiments have verified the proposed control scheme.

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There is increased worldwide wind power generation, a large percentage of which is grid connected. The doubly fed induction generator (DFIG) wind energy conversion system (WECS) has many merits and, as a result, large numbers have been installed to date. The DFIG WECS operation, under both steady state and fault conditions, is of great interest since it impacts on grid performance. This review paper presents a condensed look at the various applied solutions to the challenges of the DFIG WECS including maximum power point tracking, common mode voltages, subsynchronous resonance, losses, modulation, power quality, and faults both internal and from the grid. It also looks at approaches used to meet the increasingly stringent grid codes requirements for the DFIG WECS to not only ride through faults but also provide voltage support. These are aspects of the DFIG WECS that are critical for system operators and prospective investors and can also serve as an introduction for new entrants into this area of study.

The increase in wind power penetration, at 456 GW as of June 2016, has resulted in more stringent grid codes which specify that the wind energy conversion systems (WECS) must remain connected to the system during and after a grid fault and, furthermore, must offer grid support by providing reactive currents. The doubly fed induction generator (DFIG) WECS is a well-proven technology, having been in use in wind power generation for many years and having a large world market share due to its many merits. Newer technologies such as the direct drive gearless permanent magnet synchronous generator have come up to challenge its market share, but the large number of installed machines ensures that it remains of interest in the wind industry. This paper presents a concise introduction of the DFIG WECS covering its construction, operation, merits, demerits, modelling, control types, levels and strategies, faults and their proposed solutions, and, finally, simulation. Qualities for the optimal control strategy are then proposed. The paper is intended to cover major issues related to the DFIG WECS that are a must for an overview of the system and hence serve as an introduction especially for new entrants into this area of study.

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