Accurate current measurement is essential to advance solar power generation

Governments and ... ​Newer considerations in the PV market are the expectations that installations of all sizes will be connected to the grid; even single-family systems can deliver and sell electricity to the utility if they generate more than the local load requires. For PV systems to realize their potential, they must become more efficient to reduce the 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; PV manufacturers are also designing next-generation inverters to increase power and efficiency by adding diagnostics and other features, adding intelligence and functionality. The latest development is multi-string technology: multiple strings of solar cells connected in series are connected to a single inverter, where each cell has its own maximum power point tracking (MPPT) device to maximize the energy produced. Solar cells are not easy to use . The battery is an open circuit and outputs a nominal voltage of about 0.6 volts: typically each solar panel has up to 72 cells, forming an open circuit of 44 volts. A short-circuited cell 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 maximum power point varies with operating conditions, such as the level of projected solar radiation, so the inverter must track this point to maintain peak efficiency. Designers do this through software algorithms that rely on voltage and current sensors that collect data instantaneously. The inverter output current is typically between 15 and 50 Arm, and the amount of power going into the grid is measured by the sensor, which measures the output of the feedback to a pulse-width modulated (PWM) sine wave controller. The controller is mainly based on a microprocessor or digital signal processor, which has a +5 volt supply and shares the operating voltage reference with other active components of the electronic control system. LEM's HMS current sensors operate from a +5 volt supply. The internal reference voltage (2.5 volts) is provided through a separate pin, making them easy to use with DSPs or microprocessors. However, they can also accept the external reference of these DSPs (between 1.5 and 2.8 volts) and generate their own reference from it. This makes the entire application more efficient and helps eliminate reference drift when calculating errors.

[page] Inverters used in solar panels are connected to the grid either through a transformer or using a direct-connect transformerless design. Depending on the layout, the former method uses a power frequency transformer at the grid connection point, or a high frequency transformer as an isolation point inside the inverter circuit . Circuits based on low-frequency transformers provide inherent protection against DC injection into the AC grid, but the transformer's own losses cause efficiency losses. The inverter's AC output may have a DC component due to , for example, inaccurate IGBT switching; the DC offset of the current sensor used in the inverter control loop itself appears as a DC component in the output circuit, so the offset should be minimized. The DC supply acceptable to the grid is subject to very strict restrictions; the problem facing designers is not only that these restrictions vary from country to country, but also that some are expressed as a percentage of the rated current (such as 0.5%), while others are expressed as absolute limits as low as 20mA (British standard). In all cases, it is necessary to measure a small DC current in the midst of a large AC current with minimal offset and drift.