Abstract

This thesis presents various types of droop control methods in single-phase, isolated, low-voltage microgrids, for inverters connected in parallel that support a common load. The main feature of the project was the controlled sharing of power (active and reactive) between the inverters, but also maintaining the desired limits for the microgrid frequency and the inverter’s output voltage, without having any kind of additional communication between them, beyond their basic electrical interconnection.

The study was conducted aiming to find an ideal control method between two types of inverters (battery and PV), so that ultimately the result of their parallel use in a microgrid is completely dependent on the user’s selections, both in terms of the frequency and voltage limits, and the sharing of power between them. The main concern when programming the control method was for it to be fully predictable for any changes that may occur in both the load and the PV power generation.

In the first part of the thesis the key elements of droop control are described, while also the control for the first kind of inverter is developed, which involves controlling the frequency and voltage through the measured active and reactive output power, respectively, (P-f, Q-V droop control) and is used for controlling the battery inverters. Then the reasoning behind the control mode of the second kind inverters (PV) is analyzed, to which active and reactive output power control is applied, based on the measured microgrid frequency and its output voltage, respectively (f-P, V-Q droop control).

In the second part of the thesis, simulations are conducted on a battery inverter to test the response of the control to different parameters, and then the sharing of active power between such two inverters connected in parallel is studied. Next, simulations for parallel connection of a battery and a PV inverter take place, while the combination of their control parameters was such in order to get the optimal output power from the PV panels. At this point, laboratory experiments were held in order to determine whether the simulation results match those of an actual microgrid.

In the third part of the thesis, simulations were conducted for a microgrid which operates autonomously on a daily basis, while a control method via the the battery state of charge (SoC) was incorporated.

All simulations were conducted in Simulink, the data flow graphical programming language tool of the Matlab environment, while the laboratory experiments took place at the SmartRue Lab of the Electrical and Computer Engineering department at NTUA.

Key words: droop control, battery inverter, PV inverter, active power control, reactive power control, frequency control, voltage control, microgrid, islanded operation

Author: Dimitris Papalexis

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)