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Issues of Solar Photovoltaic Inverters Connected to the Grid

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Update time : 2022-04-29 20:26:14
Governments and power companies expect photovoltaics to account for a relatively large proportion of the total energy they supply. Converting the direct current of solar cells into alternating current that is synchronized with the power grid has strict requirements in design. Photovoltaic inverters must achieve maximum efficiency over a wide power range and operating conditions, and must also meet stringent safety requirements. The performance of the photovoltaic inverter ultimately depends on the accurate measurement of the base power, so the selection of the current sensor is very important.
 
Photovoltaic systems need to realize their potential to increase power generation efficiency to reduce cost per kilowatt. As we all know, solar cell manufacturers have been working hard to improve the basic efficiency of converting solar radiation into electricity. Photovoltaic manufacturers are also designing next-generation inverters to improve power and efficiency by adding diagnostics and other features, adding intelligence and functionality. The latest trend is multi-string technology: connecting multiple strings of solar cells in series to a single inverter, where each cell has its own maximum power point tracking (MPPT) device to maximize energy production. Solar cells are power sources difficult to use. The cells are open circuited, and the output is rated at about 0.6 volts: typically there are up to 72 cells per solar panel, creating an open circuit of 44 volts. A short-circuited battery can output a certain level of current. At a point between these limits, the battery will output maximum power at a certain voltage and current. This MPPT operating conditions (such as projected solar radiation levels) vary, so the inverter must track this point to maintain peak efficiency. Designers do this through software algorithms, relying on voltage and current sensors that collect data instantaneously.
 
The output current of the photovoltaic inverter is generally 15~50Arm, and the feedback output of the pulse width modulation (PWM) sine wave controller is measured by the sensor to measure the amount of electricity entering the grid. The controllers are mainly based on microprocessors or digital signal processors, which have a +5V power supply and share a working voltage reference with other active components of the electronic control system.
 
The inverter used by the solar panel is connected to the grid through a transformer or using a direct-connected transformerless design. Depending on the layout, the former method can use a power-frequency transformer at the grid access point, or a high-frequency transformer as an isolation point inside the inverter circuit. The circuit below the low frequency transformer provides built-in protection to prevent DC from entering the AC grid, but losses in the transformer itself cause efficiency losses. Due to the inaccuracy of IGBT exchange, etc., the AC output of the inverter may have a DC component; the DC offset of the current sensor used in the inverter control loop itself shows as the DC component of the output circuit, so the offset should be minimized. There are very strict limits on the DC supply that the grid can accept; the problem for designers is not only that these limits vary from country to country, but that they are expressed either as a percentage of the rated current (eg 0.5%) or as an absolute limit as low as 20mA. In all cases it is necessary to measure very small DC currents within large AC currents with minimal offset and drift.
 
Another safety issue is earth leakage. In the transformerless configuration, there is a ground path for the leakage capacitance of the solar panel or the impedance of the human body under any circumstances. It is necessary to use a residual current device (RCD) to detect unsafe ground currents, or use a current sensor of the appropriate size again to turn the RCD function. Embedded inverter design. In this way, the system can start up and operate at different recognized safety levels specified by the standard, while withstanding the strong AC ground currents generated by the capacitance between the solar cell installation and the nearby ground.
 
In order to synchronize with the grid, special control of the output of the inverter is required. The inverter must output sinusoidal alternating current, so harmonics are minimized while reacting quickly to current changes on the grid side. The sensors used here must have fast response times and low zero drift. Reducing zero drift due to temperature changes also helps reduce the need for complex compensation algorithms.
 
HANGZHI Precision Electronics Co., Ltd. works closely with photovoltaic inverter manufacturers to lay the foundation for technology development and contribute to the competitive advantage and market share of the solar industry.
 
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