Abstract

This diploma thesis examines and compares the two basic categories of voltage regulation methods in distribution networks, local voltage control and coordinated voltage control. Voltage control is necessary in distribution networks due to the voltage rise phenomenon that is caused by the increased penetration of dispersed generation, especially that of photovoltaic systems. In this framework, we conducted two experiments of real time simulations, one for the study of a local voltage control system and one for the study of a coordinated voltage control system.

Initially, we summarise the basic characteristics of distributed energy resources and present the effects of their large-scale integration in distribution networks. Afterwards, we examine the most popular methods of mitigating these effects, while stressing the comparison of local and coordinated voltage control systems. Also, we emphasize on the control of the reactive power of inverters as a means of voltage regulation, as well as on the transformers with On-Load Tap Changers (OLTC).

Next, we present the real-time digital simulator (RTDS) of the Power Systems Lab of NTUA. Its function and characteristics are thoroughly analyzed, as well as those of its corresponding programming environment (RSCAD), that is used for creating the model networks and acquiring the results.

Regarding the experiments that we conducted, in the first one we model and simulate a benchmark low voltage microgrid in RSCAD to study the operation of a local voltage control system. We chose the method of locally controlling the reactive power of the photovoltaic inverters as the voltage regulation method of the system, using reactive power- voltage droops. We also modeled an active power curtailment system for the inverters. The simulations that were executed concerned the daily behavior of the microgrid and we conducted one for its grid-connected mode and one for its islanded mode. Finally, we executed one final simulation to test the response of the microgrid in the transients that are caused by the real-time transition of the microgrid’s mode.

The second experiment refers to a Software in the Loop (SIL) simulation, where we model a medium voltage network in RSCAD and develop the algorithm of a central controller in the programming environment MATLAB, in order to study the operation of a coordinated voltage control system. Initially, we describe the network that was modeled and after that we explain the logic operation of the algorithm that is used by the central controller to achieve the desirable voltage control. The network elements that contribute in the voltage regulation are the inverters of the photovoltaic systems and a transformer with OLTC.

Afterwards, we describe the means of communication between RTDS and MATLAB and then the final algorithm that was developed for modeling the central controller is presented and explained. Finally, the results of the simulation are presented, analyzed and compared to the corresponding results of the simulations of the same network for scenarios without voltage regulation and with Local Voltage Control.

Key words: dispersed generation, photovoltaic systems, voltage control, local control, coordinated control, benchmark networks, real time digital simulator, RTDS, microgrid, inverter, reactive power control, active power curtailment, transformer, OLTC, algorithm, optimization, MATLAB

Author: Marios Maniatopoulos

Responsible PhD: Panos Kotsampopoulos / kotsa@power.ece.ntua.gr

Supervising Professor: Nikos Hatziargyriou / nh@power.ece.ntua.gr

PDF: Full version (Greek) and Short version (English)