Power control strategies during voltage sags according to Spanish grid code

In order to connect any power converters into the grid there are some grid requirements to insure the safe operation of the grid. So, the control of the converters especially during abnormal condition e.g. during voltage sags is a very important key to guarantee the good behavior of the distributed generation system. In this paper four control strategies, will be stated in the literature, are discussed in order to ensure their ability to match the grid requirements when unsymmetrical voltage sags are produced in the network. The Spanish grid code did not give any information about the negative sequence, and it only represents the positive sequence components. Therefore, the main contribution of this paper is to verify the grid code with not only the positive sequence but also with the negative sequence. Moreover, the system is tested by simulation to show that the results cope well with the analytical equations.Postprint (published version

Enabling grid-feeding converters with a dissonant-resonant controller for negative-sequence voltage elimination

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting /republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other worksThe mitigation of the adverse effects of voltage unbalance in equipment and power quality can be performed by the power electronic converters that interface distributed generators to the grid. Inspired in a resonant controller, this article presents a dissonant-resonant controller for negative-sequence voltage elimination for a grid-feeding converter connected to the grid. The controller eliminates the negative-sequence voltage at the converter output with a regulable precision, it does not require knowing the grid impedance for successful operation, and it can be a good candidate for parallel operation because it operates not like an integrator, but like an “untuned” integrator. Using the stationary aß frame, a closed-loop model is developed in a complex space vector built from the complexification of the stationary components. This allows extracting stability conditions for safe closed-loop operation as well as deriving design guidelines for the controller parameters. Numerical and experimental results show the ability of the proposed controller to meet its design goals, thus, corroborating the theoretical approachPeer ReviewedPostprint (author's final draft

PI-based controller for low-power distributed inverters to maximise reactive current injection while avoiding over voltage during voltage sags

This paper is a postprint of a paper submitted to and accepted for publication in IET Power Electronics and is subject to Institution of Engineering and Technology Copyright. The copy of record is available at the IET Digital Library.In the recently deregulated power system scenario, the growing number of distributed generation sources should be considered as an opportunity to improve stability and power quality along the grid. To make progress in this direction, this work proposes a reactive current injection control scheme for distributed inverters under voltage sags. During the sag, the inverter injects, at least, the minimum amount of reactive current required by the grid code. The flexible reactive power injection ensures that one phase current is maintained at its maximum rated value, providing maximum support to the most faulted phase voltage. In addition, active power curtailment occurs only to satisfy the grid code reactive current requirements. As well as, a voltage control loop is implemented to avoid overvoltage in non-faulty phases, which otherwise would probably occur due to the injection of reactive current into an inductive grid. The controller is proposed for low-power rating distributed inverters where conventional voltage support provided by large power plants is not available. The implementation of the controller provides a low computational burden because conventional PI-based control loops may apply. Selected experimental results are reported in order to validate the effectiveness of the proposed control scheme.Peer ReviewedPostprint (updated version

Reactive current injection protocol for low-power rating distributed generation sources under voltage sags

Voltage sags are one of the main problems in transmission and distribution networks. This study proposes a voltage support control scheme for grid-connected low-power rating inverters under voltage sags. Voltage support capability is provided thanks to reactive current injection. The main objective is to inject the maximum rated reactive current during the voltage sag. Second, to raise the higher phase voltage to a predefined maximum boundary, thus preventing over-voltage. Moreover, with this strategy the phase voltages can be equalised. The first objective can be always accomplished during voltage sags. Achieving the second objective depends on the grid characteristics, the sag profile and the power rating of the inverter. Selected experimental results are reported to validate the effectiveness of the proposed control.Preprin

Receding-horizon model predictive control for a three-phase VSI with an LCL filter

© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.This paper presents a Continuous Control Set Model Predictive Control with receding horizon for a threephase voltage source inverter with LCL filter, using a reduced model of the converter. The main advantages of using this reduced model is that an active damping can be achieved while the computational burden is reduced. Besides, in order to eliminate the model uncertainties, and also to achieve a zero steady state error, the proposed converter model includes an embedded integrator. Regarding the control scheme, a Kalman filter is used in order to estimate the three-phase currents without oscillation. The objective is to find the control signals vector that minimizes the error between the current and its reference. It is important to remark that the control signals obtained fromthe cost function can be used directly in a space vector modulator, without the use of additional controllers such as proportionalintegral or proportional-resonant. Compared with the Finite Control Set Model Predictive Control, the proposedmethod operates at fixed switching frequency without using any restriction in the cost function. Simulation and experimental results show that this proposalworks correctly even in case of grid harmonics and voltage sags.Peer ReviewedPostprint (author's final draft

Imbalance-voltage mitigation in an inverter-based distributed generation system using a minimum current-based control strategy

©2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Voltage imbalances are one of the most severe challenges in electrical networks, which negatively affect their loads and other connected equipment. This paper proposes a voltage support control strategy to mitigate the voltage imbalance in inverter-based low voltage distribution networks. The control scheme is derived taking in mind the following control objectives: a) to increase the positive sequence voltage as much as possible, b) to decrease the negative sequence voltage as much as possible, c) to inject the power generated by the primary source, and d) to minimize the output current of the inverter. The innovative contribution of the proposed solution is based on the design of a control algorithm that meets the aforementioned objectives without resorting to communications with other grid components. The theoretical results are experimentally validated by selected tests on a laboratory setup with X/R ratio close to one.This work was supported in part by the Ministry of Economy and Competitiveness of Spain and in part by the European Regional Development Fund under Project RTI2018-100732-B-C22.Peer ReviewedPostprint (author's final draft