Solar Inverters

Overview
Capturing solar energy with the aid of photovoltaic (PV) panels has become very popular with the improved cost and efficiency of such panels. There are a number of different systems employed in going from the panel DC voltage to a quality AC voltage that can be fed into the grid. Some domestic systems employ an electronics module for each panel. These modules are called micro-inverters if they produce AC output power and micro-converters in they produce DC output power. For the micro-inverter systems a central DC/AC inverter is required to generate an AC output. Other popular types of solar architectures see a number of panels, typically 4-7, wired in series to produce a higher DC output voltage which in turn is fed into a central inverter.

System Challenges
There are a number of common challenges whichever architecture is selected. One of the key parameters of any solar PV inverter is its efficiency in converting input dc power into output ac power. A great deal of design effort is placed on optimizing various parts of the signal chain to get this figure as high as possible. Also critical is the reliability of operation over a typical equipment lifespan of 20 years. This must be achieved in ambient conditions which can be harsh in both summer and winter. Finally, the generated power destined for the grid must be of a quality that meets regulatory requirements in terms of frequency stability and dc current injection levels.

Measurement / Sensing
Typical DC current sensors employed on the panel side include Hall Effect (HE) sensors and shunt resistors. Devices like the AD8212 are high side current monitors producing an analog output proportional to the current flow while a device like the AD7400 is sigma-delta modulator producing an output 1's density stream proportional to current flow. This device incorporates its own isolation barrier inside the package to provide galvanic isolation between the sensed current and the digital output. On the AC output side where galvanic isolation is mandatory, current transformers (CTs) and Hall Effect sensors have been widely used to date while shunt resistors have been less popular here. However due to the growing volume of Solar PV installations connecting to the Grid, there is also a growing concern from utilities and power distribution operators as to the potential for DC current injection into the grid. Since DC current injection can lead to grid inefficiencies, lower permissible levels from each inverter are under consideration in many grid systems. These tighter limits will have an impact on an inverter's output current-measuring capabilities since now a wider dynamic range will be required than previously needed. Each sensor type has its own advantages and disadvantages but all require precision, low noise, low power op amps like ADI's OPX177 family to complete the signal conditioning demanded by the signals of different current sensors.

Control / Processing
In terms of control, ADI's latest low power fixed point dsp controller family is very suitable for the control and communications functions required in this application. The BF50x family delivers 400MHz of processing performance in a dual multiply and accumulate architecture equivalent to 800 MegaMACS of performance. The BF50x includes an integrated 12-bit Dual SAR ADC and delivering 11.7 ENOBs. With this advanced converter performance, product designs can now offer greater accuracy than ever before. Another major feature of the BF50x is an integrated 4 Megabyte synchronous, parallel flash which eliminates the need for external memory, reducing cost in price sensitive applications.

From the perspective of I/O, the BF50x offers a powerful combination of peripherals including two 3 Phase PWM units for advanced power switching applications, a removable storage interface that support multiple standards for external storage devices as well as wireless, and a CAN controller for industrial and automotive applications.

Communications
Finally a communication channel will be needed both to report on the performance of the system to a central controller and to receive instructions from that source. When communicating at high data rates or over the relatively long distances that might be found in a wind farm, differential data transmission offers superior performance to single-ended transmission. Popular protocols for this communication task are RS-485 and CAN. Analog Devices offers a wide range of iCoupler®-based isolated RS-485/RS-422 transceivers to suit this application with the ADM248xE and ADM258xE families which include integrated isolated DC/DC converters. More recently, Analog Devices' has released the first of a family of isolated CAN transceivers, the ADM3052/3053, with and without an integrated isolated dc/dc converter. For proprietary protocols there are also numerous standard iCouplers available such as the ADuM140x family, which are quad channel isolators.