Power Quality Monitoring in a System with Distributed and Renewable Energy Sources

Abstract

The chapter deals with three issues concerning power quality monitoring in power grids with distributed energy sources. The structure of the grid has been described with pointing out the sources of voltage disturbances and the disturbance susceptibility of the grid components. Conclusions have been drawn at what nodes it is necessary to employ power quality monitoring. The technical solutions needed to integrate power quality analysis functions into protection relay have been described. New types of voltage and current transducers for use in primary circuits of power station have been presented. The growing share of unconventional energy sources in the total energy balance of distribution companies carries with itself a necessity to provide adequate energy quality and energy safety to the final user. The importance of this issue has been underlined by many documents accepted by the governments of the individual European countries and by the European Commission itself. The successful integration of various energy sources and consumers in the grid with the general diagram presented in Figure 1 requires meeting the demands of energy sellers who want to sell electricity and consumers who want to use electricity in an economically efficient way. The grid has to be balanced and the energy supplied to the customer has to meet quality standards. The need to supply consumers with the required electricity carries with itself the necessity to monitor the power quality. When incorporating renewable energy sources within power distribution networks it is very important to provide power quality analysis at many nodes in the network. The nature of renewable energy sources is such that they produce either a DC voltage – as is the case with solar panels and energy containers in the form of supercapacitors or an AC voltage of varying frequency as is the case with wind turbines and wave turbines (Ackerman T. ed. 2005), (Gilbert M. 2004). The renewable energy sources must be either synchronized or converted to alternating current before their energy can be injected into the grid. The power electronic devices that convert AC voltage to DC voltage and DC voltage to AC voltage are inverters and during the conversion process disturbances in the form of harmonics, voltage sags and overshoots are created which have to be kept under control. Also many loads that are now connected to the power networks exhibit nonlinear dependence of drawn current on supply voltage. These nonlinearities are the source of higher current harmonics that are injected into the grid. As the impedance of the power