Generalized Proportional-Integral Control for Voltage-Sag Compensation in Dynamic Voltage Restorers

The Dynamic Voltage Restorer (DVR) is a custom power device used to protect sensitive loads in power distribution systems from the most frequent voltage disturbances, such as sags, swells, imbalances and harmonics. This paper presents a control strategy for voltage-sag compensation in DVR systems which offers an accurate tracking of the voltage compensation reference and a very fast transient response. The proposed control scheme is based on the technique known as Generalized Proportional-Integral (GPI) control. This type of control is closely related to the use of the classical compensation network controllers and consists in the estimation of certain state variables by means of integral reconstructors avoiding the use of derivative operators. Furthermore, the control scheme is completed by adding a feedforward term in which an approximation of the inverse dynamics of the system is used to improve and quicken the transient response. The paper includes a brief theoretical introduction to GPI control design and simulation results to corroborate the suitability of the GPI controller for voltage-sag compensation in DVR systems. The presence of disturbances in the electrical grid, causing poor power quality, is becoming an important issue due to their financial impact and negative effects on the end-user satisfaction. Voltage sags, swells and harmonics are among the most frequent disturbances nowadays (1). These disturbances are caused by short-circuits, the connection and disconnection of large loads and the presence of non-linear loads. The negative impact of these disturbances has motivated the development of a variety of custom power devices to mitigate the effects they produce. Voltage sags are particularly frequent and can produce an important damage on sensitive loads. They are characterized as a short interruption, typically of several cycles of the fundamental frequency, in which the voltage amplitude is between 10% and 90% of its nominal value. These conditions are sufficient to cause tripping and an unaffordable damage on certain sensitive loads and industries such as semiconductor, paper and textile manufacturing. The compensation of voltage sags to protect sensitive loads re- quires a custom power device able to provide a very fast and stable response, being the DVR the current solution that better fulfils these requirements. Furthermore, DVRs can additionally compensate voltage imbalances in three-phase systems and the voltage harmonic distortion caused by non-linear loads. The first DVR was built in the U.S. by Westinghouse for the Electric Power Research Institute (EPRI), and it was installed in 1996 on the Duke Power Company grid system to protect an automated yarn manufacturing and weaving factory (2). Several topologies for the DVR implementation have been proposed, having all of them in common the use of a Voltage Source Converter (VSC) connected in series with the supply system. The series connection is usually done by means of a transformer, although transformerless configurations are also possible. A detailed comparison of several topologies depending on the energy storage employed is carried out in (3). Furthermore DVR systems in which the VSC DC-Link is supplied from one distribution grid to compensate disturbances in a different distribution grid are also proposed in literature. The control of the DVR has to be designed to quickly apply the voltage in series with the grid line that compensates the sag and other disturbances such as imbalances and harmonics. The simplest control solution consists in a feedforward action based on the error between the reference voltage that has to be supplied to the load and the voltage supplied by the grid during the sag. This solution however is not able to completely cancel errors while tracking the reference value. Moreover, it provides undesired voltage oscillations due to the poor damping of the resonant frequency introduced by the output LC filter, which is used to attenuate the switching frequency components of the VSC output voltage. These drawbacks have been addressed by introducing a feedback control loop to improve the tracking of the references and provide active damping of the filter resonant frequency. Several solutions can be found in literature regarding the feedback control implementation including: State-Feedback control (4), Proportional-Integral control in a synchronous reference frame (5), robust H1 control (6), Proportional-Resonant control (7), Repetitive control (8) and Predictive control (9). This paper presents a control strategy to design the feedfor- ward compensation and the feedback control loop by means of an approximation of the dynamic inversion and a GPI control (10) respectively. The proposed method provides a very fast response and an accurate tracking of the voltage