Abstract

In the current diploma thesis, it is examined the problem of harmonics in low voltage electrical networks with high penetration of photovoltaic systems.

In the fist place it’s investigated the mechanism of harmonic generation in the networks voltage due to the distorted current that a non linear electrical device consumes or injects. Furthermore, problems that can occur in a network whose voltage is “polluted” as long as ways of harmonic mitigation are addressed.

The harmonic problem is studied for specific electrical devices which are the basic functional units of the photovoltaic grid- connected plants. These are the power inverters of which the output current is in fact remarkably distorted.

Thereafter, the power quality standards are evaluated from the perspective of allowable harmonic current emission of an electrical device but also for the requirements (focusing on voltage harmonics) that an electrical grid must meet.

In the next stage, a theoretical analysis is performed, regarding the harmonic voltage generation mechanism due to harmonic current sources in low voltage networks.

The experiments which are conducted aim to the identification of the parameters that affect the intensity of the emitted current harmonics of the laboratory inverter and for this reason harmonic current emissions are measured in various values of its output power. Also, it is studied the inverter harmonic current emission by technically polluting (with voltage harmonics), the voltage that it is being connected. Still, harmonics at frequencies higher than the standards are covering (greater than 2kHz) are measured. Finally, the phase angles of current harmonics in inverters of the same type are measured and it is being investigated the relation between angles of the same order.

In the final stage of the thesis, simulations are performed using the PSCAD-EMTDC software. In the context of a harmonic study, low voltage networks are modeled (emphasizing on rural networks) with harmonic sources in correspondence with the penetration of PV inverters in actual low voltage networks. The harmonic sources utilized for this study are, firstly Rigid harmonic sources (with fixed current harmonic amplitudes and phase angles), measured waveforms from actual inverters and finally a fully modeled single- phase inverter.

The simulations are defined by a variety of scenarios with different characteristics (network loads and sources of harmonics) with the aim of drawing conclusions as to the intensity of the problem of harmonics due to the penetration of photovoltaic in LV networks, as long as the validity of different modeling of harmonic sources.

Key words: photovoltaic systems, harmonics, inverter harmonic emission, harmonic simulation, harmonic measurement, single-phase inverter modeling, harmonic sources modeling, PSCAD

Author: Iasonas Kouveliotis-Lysikatos

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

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

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