Abstract

This diploma thesis examines the structure of a simulation experiment in real time with the laboratory microgrid, and specific the photovoltaic inverter. The simulation is called Power Hardware In Loop (PHIL). Firstly, the microgrid devices are described and the real-time simulator (RTDS), which will simulate the electrical network that will connect the Hardware Under Test (HUT) and the “intermediate» (Interface) which will make the connection of the two previous parts.

Both HIL techniques are mentioned with particular emphasis on PHIL and problems concerning stability and accuracy, are studied. The Nyquist stability criterion is presented, on which the study was based concerning the stability issue and the two types of errors that occur in a PHIL simulation are presented, namely the TFP error (Transfer Function Perturbation) and the NP error (Noise Perturbation). After that the various connection topologies of the interface (interface algorithms) are indicated by which the components of the loop can be connected, in order to overcome the issues of stability and accuracy. The operation of the AC/DC/AC amplifier in analysed, who will play the role of the interface in the experiment.

Regarding the experimental part of the work, we initially apply the PHIL technique on the voltage divider circuit. We present the topology and we model our system in the best possible way. Through measurements we find the total time delay of the loop and we model the interface as an amplifier with unit gain and influence from the output filter. Using Matlab we apply the Nyquist stability criterion as well as the necessary accuracy study, and we present the results. Following this point, the results from the simulations made in Simulink as well as the experimental results from the PHIL application, are shown. Finally, we study in Simulink the case where an unstable voltage divider circuit, in a PHIL application, becomes stable through the use of a filter in the feedback current.

In the last part of the work we apply the PHIL technique to a LV network in a rural area, in which there is photovoltaic production and we present the results of the voltage levels at the connection nodes. The network of the rural area is described and we implement some production-consumption scenarios using the PHIL technique. Then by using Simulink we saw the voltage rise on the connection nodes of the network for different scenarios of production – consumption (offline simulation). Finally we applied two ways to reduce the voltage rise using the capabilities of the photovoltaic inverters, namely the method of power curtailment and the method of power factor control.

Keywords: photovoltaic systems, microgrid, MPPT photovoltaic, RTDS, real-time simulation, PHIL, stability in PHIL, accuracy in PHIL, Interface Algorithms, AC/DC/AC inverter with 3 branches, voltage rise due to photovoltaic connection, power curtailment, power factor control

Author: Vasilis Kleftakis / vkleft@mail.ntua.gr

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

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

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