Search

The increasing penetration of grid-connected RES systems, advanced control algorithms have been developed to operate under grid faults and fulfill strict requirements of the grid codes. In order to overcome this, the current controller performance is critical considering it as the inner control loop of any grid-connected RES system. Based on the resonant control concept, this paper presents a modified structure for this controller which results advantageous when implemented on RES systems, as it permits better performance during the dynamic state of the controller. This paper also deals with the analysis of the decoupling terms in the aß reference frame, as well as the capability to generate a decoupled control of the positive and the negative sequence. The proposed controller will be analyzed, discussed and finally validated by means of simulation analysis.

This paper discusses the implementation of proportional resonant (PR) current controllers for a Voltage Source Converter (VSC) with LCL filter which is synchronized to the grid by virtual flux (VF) estimation with inherent sequence separation. Even though there is an extensive amount of literature and studies on the PR current controller for tracking the current reference of a VSC in the stationary reference frame, there is no discussion taking into account voltage sensor-less operation based on virtual flux estimation with an LCL-filter. Separate estimation of the positive and negative sequence virtual flux components at the grid-side of the LCL-filter, as well as current sequence separation, using the Second Order Generalized Integrator-Frequency Locked Loop (SOGI-FLL) is presented as part of a proposed method. The LCL filter is characterized in order to reduce the parameter deviation that might affect the virtual flux estimation. The stability of the proposed method is analyzed in the frequency domain while the operation and performance of the proposed system is verified by simulation studies.

A three-level neutral-point-c1amped inverter needs a controller for the neutral-point voltage. Typically, the controller design is based on a dynamic model. The dynamic model of the neutral-point voltage depends on the pulse width modulation technique used for the inverter. A pulse width modulation strategy which allows modeling the neutral-point voltage as a first-order continuous function is proposed in this paper. The new model shows that the neutral-point current is proportional to the reactive power drawn from the inverter. Since the model is a first-order model, a simple proportional controller is used in this paper to control the neutral-point voltage. Experimental results verifying the proposed modulation strategy and the controller are also presented in this paper.

Larger percentages of wind power penetration translate to more demanding requirements from grid codes. Recently, voltage support at the point of connection has been introduced by several grid codes from around the world, thus, making it important to analyze this control when applied to wind power plants.This paper addresses the analysis of two different voltage control strategies for a wind power plant, i.e. decentralized and centralized voltage control schemes.The analysis has been performed using the equivalent and simplified transfer functions of the system. Using this representation, it is possible to investigate the influence of the plant control gain, short circuit ratio, and time delays on thesystem stability, as well as the fulfillment of the design requirements.The implemented plant voltage control is based on a slope voltage controller, which calculates the references to be sent to the wind turbines, according to the slope gain and the difference between the reference and measured voltage at the point of connection.The results show that for a system where the time delay between the central control and the actuators is not negligible, the performance of a decentralized voltage control is better than the centralized one in terms of time reaction, disturbance rejection, and short-circuit ratio (SCR) dependency. However,for a system with reduced time delay (10 ms) the centralized scheme can have a performance similar to the decentralized one, in the SCR range of interest.