To get the M.Sc) degree in Electrical - Power

Dissertation for M.Sc))

Power-Power Orientation

Abstract

Common-mode voltages and currents are generated due to parasitic capacitance between solar cells and their frame, which is usually grounded. These capacities are usually modeled as capacitors between the negative terminal of the solar cell and ground. In solar cells that are connected to the grid through a transformer, the electrical insulation of the transformer coils and the high frequency voltage and common mode current practically have no place to flow, and as a result, no specific common mode current is produced. In this way, the type of arrangement of the inverter and the way it is keyed do not have much effect on this issue. But in a transformerless arrangement, a way must be found to prevent the leakage current caused by the common mode voltage from being transferred to the network.

If the number of levels is large enough, the bridges can also be switched at the base frequency with square wave modulation. In this way, the mutual electromagnetic effects between the power parts and the electronic parts of the system are minimized. At the same time, the output voltage of the inverter will be close to the sinusoidal waveform, and there will be no need for large filtering, and the common mode voltage will not be created. Of course, for low frequencies, using square wave modulation will cause voltage and current distortion. Therefore, the use of sinusoidal pulse width modulation with different modulation coefficient is suggested for different levels. The sinusoidal pulse width modulation method is a simpler and more understandable method than the vector space method, and its application in two-, three-, and multi-level single-pole and two-pole inverters does not require complex calculations. For this reason, it is possible to optimize it for the purpose of minimizing the common mode voltage. Keywords: common mode voltage, sinusoidal pulse width modulation, vector space modulation, topology of inverters. 1 Introduction Renewable energy sources, especially photovoltaic sources, have developed a lot in recent years, mainly due to the increase in temperature and concessions given to governments for these types of technologies. [1].

The power processing of renewable energy sources is done by power converters, which have these issues such as efficiency and cost as key factors. In the special case of photovoltaic inverters connected to the grid, most power converter topologies use a transformer that works at low or high frequency, and this creates galvanic isolation between the photovoltaic panels and the power grid. Low frequency transformers are large, heavy and expensive and add additional losses to the system. The size of the isolation transformer can be greatly reduced by using a two-level topology where the transformer operates at high frequency. This method reduces efficiency, as at least two cascaded power converters are required. For this reason, a large number of inverters with transformerless topology [2] have been proposed in the last few years, which has led to the production of cheaper, more compact and more efficient power processing systems. In addition, when using inverters without transformers, some techniques for measuring insulation reactance and residual current should be used, which makes inverters without transformers even safer than inverters with transformers.

Regarding the size of power inverters connected to the grid, a pattern change has been observed in the last few years. Large central inverters with power above 100 kW have been replaced by small size inverters that provide large amounts of energy with a single string or a small group of strings. By following this method, the maximum power tracking point of large photovoltaic panel groups can be improved, because they can be subjected to very different solar radiation levels. In this context, the use of single-phase inverters up to 5 kW is of great importance.

For the reasons mentioned, a significant number of single-power topologies have been proposed to implement grid-connected single-phase transformerless inverters, in this type of converters, there is no galvanic isolation between the photovoltaic panels and the grid, so that problems can arise that require special attention, such as common-mode voltages and leakage currents at both ends of the photovoltaic panels, which are caused by the fact that an invisible parasitic capacitor There is an overlap between the photovoltaic cells and the ground insulation, and under certain operating conditions (for example, humidity, soil and installation mode), it can reach very large values. The usual values ??of this capacitance are between 50 and 150 for crystalline silicon cells and up to the values ??for thin film cells [3]. Leakage increases the harmonic common mode current in the system, reduces the quality of network current connection, causes disruption of conduction and interference of electromagnetic radiation and causes personal safety problems. In solar cells that are connected to the grid through a transformer, the electrical insulation of the transformer coils and the high frequency voltage and common mode current practically have no place to flow, and as a result, no specific common mode current is produced. In this way, the type of arrangement of the inverter and the way it is keyed do not have much effect on this issue. But in the arrangement without a transformer, a way must be found to prevent the transfer of leakage current caused by the common mode voltage to the network. (a) Common mode current and parasitic capacitor and (b) Photovoltaic system model

If the number of levels is large enough, the bridges can also be switched at the base frequency with square wave modulation. In this way, the mutual electromagnetic effects between the power parts and the electronic parts of the system are minimized. At the same time, the output voltage of the inverter will be close to the sinusoidal waveform, and there will be no need for large filtering, and the common mode voltage will not be created. Of course, for low frequencies, using square wave modulation will cause voltage and current distortion. Therefore, the use of sine pulse width modulation with different modulation coefficients is suggested for different levels. 1-2 H- full bridge or full bridge The most widely used topology in photovoltaic inverters connected to the grid is full bridge. This topology is made of four transistors connected together as shown in Figure 2-1. Because many commercial converters use this topology in combination with a low-frequency transformer, it would be interesting to study this application for transformerless inverters.

The most common modulation used in this topology is unipolar, because it has a number of advantages over bipolar modulation (for example, less current ripple at high frequency, better efficiency or less electromagnetic interference emission) [4]. However, when unipolar modulation is used in the full-bridge inverter, a high-frequency common-mode voltage is applied to the photovoltaic panels, such that a non-negligible leakage current appears due to the capacitance of the photovoltaic panels. For this reason, it is recommended against the modulation of transformerless inverters [5].

Figure 1-2. Full-Bridge Topology

To solve the no-flow problem in the full-bridge photovoltaic converter, bipolar modulation can be used. This modulation prevents the high frequency components of the common mode voltage from being applied to the panels [6], so the common mode voltage has only the low frequency component of the first harmonic, thus reducing the leakage current [7, 8, 9]. However, to limit the amount of peak leakage current, good coordination and synchronization between the gate signals of the bridge transistors is very important. Otherwise, the leakage current can increase greatly [10]. As a result, this topology is not considered as a good alternative for implementing transformerless photovoltaic converters, even if bipolar modulation is used [11]. The connection of the neutral wire of the network to the midpoint of the capacitive divider ensures an almost constant common-mode voltage, thus preventing the leakage current from passing through the parasitic capacitor of the photovoltaic module [12].