Investigating the phenomenon of voltage and common mode currents in three-phase inverters and ways to reduce and eliminate them

Abstract

Common-mode voltages and currents are created due to the creation of parasitic capacitance between the 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 the arrangement without a transformer, a path 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.

Key words: common mode voltage, sinusoidal pulse width modulation, vector space modulation, topology of inverters

Introduction

Renewable energy sources, especially those that have photovoltaic origin, have developed a lot in In recent years, it is mainly due to the increase in temperature and the concessions given to governments for this type of technology [1].

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 With the type of converters, there is no galvanic isolation between the photovoltaic panels and the grid, so that problems can arise.

For the mentioned reasons, 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 is due to the fact that There is a non-negligible parasitic capacitance between the photovoltaic cells and the insulation ground, 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 transmission of the leakage current caused by the common mode voltage to the grid. This capacity is usually modeled as capacitors between negative lead of solar cell and ground. In solar cells that are connected by transformer to the grid, electrical isolation of windings of the transformer and high frequency common mode voltages and currents virtually no room for the flow of common mode current and in practice resulting in no common mode current. This type of inverter arrangement and the way of switching have no effect on the issue. But in no trans arrangement must find a path to prevent the transmission of common mode voltage, leakage current from the network found.

If the level is high enough to bridge the base frequency square wave modulation was also keying. Thus electromagnetic effects between power and the electronic parts of the system are minimized. At the same time the output voltage sine wave inverter to be close and there is a great need for filtering and common mode voltage will be created. However, for low frequency square wave modulation voltage and current will cause distortion. So using sinusoidal pulse width modulation for different levels suggested by different modulation factor. Sinusoidal pulse width modulation method is simpler and more intuitive method of vector space method and its application to the inverter, two, three and multilevel unipolar and bipolar need for complicated calculations no. That is why it is aimed at optimization of the common-mode voltage is possible.