Modeling and experimental testing activity of the Voltage Optimization Unit. : Testing activity of the distribution transformer with single-phase on-load tap changer in an experimental low voltage network and investigation of different control logics.

Abstract

In the EUDP project ‘Energy saving by voltage management’ two reports are provided by Technical University of Denmark (DTU) covering the simulation studies and experimental work. The first report presented the simulation results regarding the technical evaluation of on-load tap changers in solving the voltage problems in presence of photovoltaic distributed generation, using Bistrup distribution grid (DONG Energy) as a test case. This second report presents the experimental results of the testing activity of the Voltage Optimization Unit (VOU), i.e., a distribution transformer with single-phase on-load tap changers (OLTC) in an experimental low voltage grid. Both the transformer internal behavior and its effective operations under different unbalanced bidirectional power flow conditions have been investigated. Moreover, different control logics are analyzed based on their control objectives and control inputs, which include network currents and voltages that can be measured either locally or remotely. The experimental validation has been performed in the research infrastructure SYSLAB-PowerLabDK, a laboratory facility for the development and test of control and communication technology for active and distributed power systems, located at DTU Risø campus. The experimental test validates the control performance of the OLTC transformer and the test indicates that, using remote measurement, the voltage of the system can be kept in a safe operational band. However, the remote measurement implies additional cost investments of to the system operator, thus a proactive tap algorithm is developed and tested in this project, relying on local measurements. In addition, we also compared the experimental result with the one simulated in the DigSilent PowerFactory software environment (the software used in the first report), and the results show that the system components have been properly modeled. Note that the system tested in this report is a simplified network that cannot characterize all the features of a real distribution network, even though it is able to catch the most important ones. Future study could include two aspects: 1) investigate the OLTC’s application in an active distribution network characterized by several subfeeders, where higher penetration of different distributed generations is present; and 2) combine the   OLTC control with smart-metering technology, where the measurements from the smart meters can be used as inputs for the control actions of the OLTC transformer.