**1. Introduction**

260 Power Quality Harmonics Analysis and Real Measurements Data

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Voltage Power Supply Systems for Equipment with Rated Current Greater than

Connection of Distorting Installations to MV, HV and EHV Power Systems

Renewable energy systems such as wind farms and solar photovoltaic (PV) installations are being considered as a promising generation sources to cover the continuous augment demand of energy.

With the incoming high penetration of distributed generation (DG), both electric utilities and end users of electric power are becoming increasingly concerned about the quality of electric network (Dugan et al., 2002). This latter issue is an umbrella concept for a multitude of individual types of power system disturbances. A particular issue that falls under this umbrella is the capacitive coupling with grounding systems, which become significant because of the high-frequency current imposed by power converters.

The major reasons for being concerned about capacitive couplings are:


For these reasons, it has been noticed the importance of modelling renewable energy installations considering capacitive coupling with the grounding system and thereby accurately simulate the DC and AC components of the current waveform measured in the electric network.

Introducing DG systems in modern distribution networks may magnify the problem of ground capacitive couplings. This is because DG is interfaced with the electric network via power electronic devices such as inverters.

These capacitive couplings are part of the electric circuit consisting of the wind generator, PV arrays, AC filter elements and the grid impedance, and its effect is being appreciated in most large scale DG plants along the electric network (García-Gracia et al., 2010).

Harmonic Distortion in Renewable Energy Systems: Capacitive Couplings 263

Moreover, every PV array is considered as an independent current source with a DC current ripple independent of the converter ripple. These ripple currents are not in synchronism with the converter and produce subharmonics in the DC circuit which increase the Total

The typical maximum harmonic order h = 40, defined in the power quality standards, corresponds to a maximum frequency of 2 kHz (with 50 Hz as fundamental frequency) (IEC Std. 61000-4-7, 2002). However, the typical switching frequency of DC/DC and DC/AC converters, usually operated with the Pulse Width Modulation (PWM) technique, is higher than 3 kHz. Hence, higher order harmonics up to the 100th order, can be an important concern in large scale PV installations where converters with voltage notching, high pulse numbers, or PWM controls result in induced noise interference, current distortion, and local

A suitable model of capacitive couplings allows reproducing these harmonic currents injected not only into the grid, but also into the DC circuit of the PV installation that would lead to internal resonant, current distortion and unsafe work conditions where capacitive discharge currents could exceed the threshold of safety values of work (IEEE Std. 80-2000, 2000). The capacitive coupling is part of the electric circuit consisting of the PV cells, cables capacitive couplings, AC filter elements and the grid impedance, as shown in Fig. 1, and its

Fig. 1. Model of PV module, PV array and capacitive coupling with PV structure.

Harmonic Distortion in the current waveform (THDI) (Zhow et al., 2010).

GPR at PV arrays (Chicco et al., 2009).

effect is being appreciated in most large scale PV plants.

Power electronic devices, as used for DG, might be able to cause harmonics. The magnitude and the order of harmonic currents injected by DC/AC converters depend on the technology of the converter and mode of its operation (IEC Std. 61000-4-7, 2010, IEEE Std. 519-1992, 1992).

Due to capacitive coupling between the installation and earth, potential differences imposed by switching actions of the converter inject a capacitive ground current which can cause significant electromagnetic interferences, grid current distortion, losses in the system, highnoise level in the installation and unsafe work conditions (Chicco et al., 2009).

Several renewable system installations analyses have been reported (Bellini, 2009, Conroy, 2009, Luna, 2011, Sukamonkol, 2002, Villalva, 2009), where most theoretical analysis and experimental verifications have been performed for small-scale installations without considering capacitive coupling. Power electronics models and topologies also have been studied, but without considering the amount of losses produced by the capacitive current that appears due to the switching actions (Zhow, 2010, Chayawatto, 2009, Kim, 2009). In (Iliceto & Vigotti, 1998), the total conversion losses of a real 3 MW PV installation have been studied considering reflection losses, low radiation and shadow losses, temperature losses, auxiliary losses, array losses and converters losses. The latter two factors sum a total of 10% of the rated power where part of these losses is due to the capacitive coupling that was neglected.

Therefore, for an accurate study of power quality, it is important to model DG installations detailing the capacitive coupling of the electric circuit with the grounding system, which are detailed for PV installations and wind farms in Sections 2 and 3, respectively. These models allow analyzing the current distortion, ground losses and Ground Potential Rise (GPR) due to the capacitive coupling. The combined effect of several distributed generation sources connected to the same electric network has been simulated, and results have been presented together with solutions based on the proposed model to minimize the capacitive ground current for meeting typical power quality regulations concerning to the harmonic distortion and safety conditions.
