K. De Brabandere

A Voltage and Frequency Droop Control Method for Parallel Inverters

In this paper, a new control method for the parallel operation of inverters operating in an island grid or connected to an infinite bus is described. Frequency and voltage control, including mitigation of voltage harmonics, are achieved without the need for any common control circuitry or communication between inverters. Each inverter supplies a current that is the result of the voltage difference between a reference ac voltage source and the grid voltage across a virtual complex impedance. The reference ac voltage source is synchronized with the grid, with a phase shift, depending on the difference between rated and actual grid frequency. A detailed analysis shows that this approach has a superior behavior compared to existing methods, regarding the mitigation of voltage harmonics, short-circuit behavior and the effectiveness of the frequency and voltage control, as it takes the R to X line impedance ratio into account. Experiments show the behavior of the method for an inverter feeding a highly nonlinear load and during the connection of two parallel inverters in operation.

Control of Microgrids

The motivation to develop microgrids, as a particular form of active networks is explained and presented as an effective solution for the control of grids with high levels of distibuted energy resources. The operation, more in particular the voltage and frequency control, is discussed. Control concepts useful with microgrids are detailed and implemented. Besides technical control aspects, also economical ones are developed. Primary, secondary and tertiary control algorithms are designed operating in a completely distributed way. The theoretical concepts are tested in a extensive laboratory experiment implementing a realistic scenario by using a setup of four inverters able to communicate through an Internet connection.

Design and Operation of a Phase-Locked Loop with Kalman Estimator-Based Filter for Single-Phase Applications

This paper describes the design procedure of a phase-locked loop (PLL) preceded by a Kalman estimator-based filter. It provides a highly accurate and fast estimate of the grid frequency and phase angle in grid-connected power electronic applications. Moreover, it enables the transformation to the rotational dq frame for various control purposes. Closed-form equations allow for a performance driven design for both PLL and estimator. By using a variable sample frequency controlled by the PLL, the Kalman filter is always operated around its center frequency, which is the rated grid frequency. The new topology is compared to other published single-phase PLL designs and its operation is verified by both simulations and experiments. The PLL exhibits excellent performance even under severely distorted utility grid voltage conditions. This robustness makes it very suitable for use in systems connected to grids having an important share of non-linear loads

Using a fully digital rapid prototype platform in grid-coupled power electronics applications

This paper describes a rapid development system that was used to speed up implementation and testing of a number of grid-coupled power electronic systems. The system uses a fully software reconfigurable processing core based on a powerful DSP and FPGA and a limited set of well-specified functional hardware blocks. Three examples using the same hardware in diverse areas of power electronic control demonstrate that this technique allows the implementation of demanding state-of-the-art power electronics systems, with the important benefit of a much reduced development time and increased reliability. It also proves to be an excellent tool in power electronics courses.

Development of a measurement system for power quantities in electrical energy distribution systems

The design of an experimental flexible energy measurement system consisting of a DSP, sensor and communication units is treated. This system is intended to be used in modern electricity distribution networks, characterized by multiple suppliers in a deregulated market, bi-directional energy flows due to distributed generation and a diversified demand for the quality of electricity delivery. Various system aspects concerning signal processing, communication and dependability are discussed Examples of the use of such devices are included.

A Three-Phase Voltage and Frequency Droop Control Scheme for Parallel Inverters

As the number of grid-connected distributed generation units continues to increase, it becomes more difficult to exclude these units from participating in the control of the grid voltage magnitude and frequency. Another future development might be the division of the grid in smaller microgrids, able to operate either connected to the utility grid or in island mode. In this paper a three-phase voltage and frequency droop control scheme for parallel connected inverters is described, which can be used for connecting a distributed energy source (a generator or a storage unit) to a microgrid, allowing both grid-connected and islanded operation. Another application is the control of parallel inverters in UPS systems. A detailed description of the control scheme is given. The performance of the proposed droop control scheme is verified by experimental results. Also an outlook to future work is given.

DSP and FPGA based platform for rapid prototyping of power electronic converters and its application to a sampled-data three-phase dual-band hysteresis current controller

A platform for prototyping industry relevant power electronic converters and systems is presented. The use of a digital signal processor (DSP) and programmable logic (field programmable gate array-FPGA-and complex logic device-CPLD) shortens implementation time and reduces electromagnetic interference (EMI) susceptibility. The design concept is demonstrated with an implementation of a modified three-phase dual hysteresis band current controller.

Transmission line effects on motor feed cables: terminator design and analysis in the Laplace-domain

When long motor supply cables are used to connect a motor with a PWM drive, overvoltages may arise at Motor terminals. To study these phenomena, this paper applies transmission line theory. From the results, a perfect line terminator is designed and constructed. To model the interaction between motor and cable, their behavior is studied in the Laplace domain and an equivalent Thevenin model is illustrated with examples.

Agents controlling the electric power infrastructure

The electric power grid is evolving from a centrally controlled grid with only a handful of regulated monopolies to an open, liberalised electricity market. In addition, more and more small-scale dispersed generators are deployed in distribution nets. This puts extra stress on the power grid, in an era when electricity is one of the most important commodities for economic, industrial and everyday activities. Therefore, new control strategies are proposed to maintain the desired degree of availability. This paper proposes a general multi-agent approach to cope with these changes, and presents a scheme for distributed control of dispersed generators in a low-voltage distribution net. Based on a thorough investigation of this scheme, we underline the importance of understanding the interdependencies between the power grid and the Information and Communication Technology (ICT) infrastructure controlling it, which may introduce new or amplify existing vulnerabilities. Some mitigation strategies are presented to avoid or deal with these vulnerabilities.

Evaluation of control algorithms for shunt active filters under unbalanced and nonsinusoidal conditions

The different power theories as described in literature to control shunt active filters are compared with regard to their performance under normal, unbalanced voltage, unbalanced load, and distorted voltage conditions. The theoretical evaluation is done on a three-phase system with diode rectifier. Since the objective is to evaluate the performance of the algorithms, system dynamics are not included in the simulation model.