Grid connection layout and safety issues of solar photovoltaic inverters

Governments and power companies expect photovoltaic (PV) power to account for a large proportion of the total energy they supply. Converting the direct current from solar cells into alternating current that is synchronized with the grid is a demanding design requirement, and will become even more so in the future. PV inverters must achieve maximum efficiency over a wide power range and operating conditions, and must also meet strict safety requirements. The performance of the inverter ultimately depends on accurately measuring the underlying electrical quantities. PV inverter manufacturers need to work closely with sensor manufacturers to ensure that the latest trends in PV technology are supported.

Our world requires replacing fossil fuels with "green" renewable energy sources to benefit the environment. Realistic scenarios predicting power systems in the near future include multiple energy sources, with solar energy in multiple configuration sizes, ranging from large-scale power plants covering several hectares to single-family home installations. This will drive strong growth in the photovoltaic (PV) solar inverter market. Even in the current economic downturn, the PV market is expected to reach $34 billion in 2013. A relatively new consideration in the PV market is the expectation that installations of all sizes will be connected to the grid; even single-family systems in homes can transmit and sell electricity to the power company if the power generated exceeds the needs of the local load.

For photovoltaic systems to realize their potential, they must increase efficiency 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; photovoltaic manufacturers are also designing the next generation of inverters to increase power and efficiency by adding diagnostics and other features, adding intelligence and functions. The latest trend 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 power sources. The cell 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 cell 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 maximum efficiency. Designers rely on voltage and current sensors that collect data instantly and use software algorithms to do this.

The output current of the inverter is typically between 15 and 50 Arm, and the amount of electricity entering the grid is measured by the sensor, which measures the feedback output 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 withstand 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 to eliminate reference drift when calculating errors.

The 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 can use 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 losses in the transformer itself will cause efficiency losses. The AC output of the inverter may have a DC component due to reasons such as 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%), and some are expressed as absolute limits as low as 20mA (British standard). In all cases, it is necessary to measure a small DC current in a large AC current with minimal offset and drift.

Another safety issue is leakage to ground. In a transformerless configuration, there is always a ground path from the solar panel leakage capacitance or human body impedance. A residual current device (RCD) is needed to detect unsafe ground currents, or again use a current sensor of appropriate specifications to embed the RCD function into the inverter design. In this way, the system can start operating at different recognized safety levels (several mA) (AC and DC) specified by the standards, while withstanding the strong AC ground current generated by the capacitance between the solar cell device and the nearby ground.

Today's solar inverter layouts require compact, low-cost and reliable ground current sensing solutions based on current sensors. LEM has designed the CT series sensors specifically for this purpose. They are different current devices with rated ranges of 100mA, 200mA and 400mA, providing a linear output of 5V at rated current. The response time is no more than 20ms and 60ms at 80% and 90% of rated current. The use of high-tech design ("fluxgate") is key, especially for accurately measuring very small DC or AC currents with low offset or drift; DC and AC currents up to 18kHz can be measured. The CT products can be mounted on the PCB, are small in size and light in weight, and have through holes for the phase conductors to pass through. The CAS/CASR/CKSR current sensors mounted on the PCB use the same closed-loop fluxgate technology; isolated measurements of AC and DC currents are possible, they cover a rated range of 6 to 50Arms, with a maximum measurement value of three times the rated value, and a frequency of 300kHz (+/-3dB). They have been specifically designed in accordance with the requirements of the latest inverter design trends, with improved performance in the following areas: common-mode interference, temperature drift (offset and gain; maximum zero-point temperature drift of 7 to 30 ppm/K depending on the model), response time (less than 0.3 microseconds), insulation level, +5 V supply and compact size.

In order to synchronize with the grid, the output of the inverter needs to be specially controlled. The inverter must output sinusoidal AC, so harmonics must be minimized while reacting quickly to current changes on the grid side. The sensors used here must have a fast response time and low zero drift. Reducing zero drift caused by temperature changes also helps to reduce the need for complex compensation algorithms. In contrast, at the DC input of the inverter that monitors the MPPT through sensors, the current changes should be less, so low-cost open-loop sensors can be used.

Inverters not connected to the grid, such as those charging batteries in backup systems, are not subject to the National Grid, but must meet many of the same safety and efficiency requirements.

The specifications that PV inverter designers must adhere to are likely to become more stringent. For example, as with limits on DC input to the grid, there may be some consensus on the permissible level of total harmonics in the inverter output current; currently there are a variety of local limits depending on the layout. This requires accurate current measurement at grid frequencies well above 50 or 60 Hz.

Close collaboration between sensor manufacturers such as LEM and PV inverter manufacturers will lay the foundation for the development of technologies that, when combined, will lead to real competitive advantage and market share in the growing solar industry.