Photovoltaic Inverters
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Photovoltaic system sizes range from small off-grid systems providing hundreds of watts to large central utility power plants generating over one megawatt. There is a wide range of PV system configurations and a wide range of inverters on the market, with some inverter manufacturers offering over 20 models. Because the PV industry is evolving so rapidly, inverter manufacturers must continually design new products. This requires scalable and flexible control systems, that allow design and software reuse.
Freescale 56F8000 DSC series solutions are ideal for photovoltaic inverter control, combining the processing power of a digital signal processor with the functions and ease of use of a microcontroller — in a true single chip solution. These solutions provide high integration at the right price to reduce overall PV inverter costs while helping to improve efficiency and reliability. Integrated peripherals include:
- Dual 6-output pulse wave modulation modules
- Four 12-bit analog-to-digital converters with more than 16 channel inputs
- Up to 16 multi-function 16-bit timers
- A variety of serial communication peripherals and on-chip voltage regulator
- Power supervisor
- Temperature qualified up to 125°C
Off-Grid Systems
Off-grid systems are used to drive local loads. They normally require a battery pack and battery charging/management system. They can provide both DC and AC output power but are not connected to the utility grid. Typical output power ranges from hundreds of watts to a few kilowatts and is single phase. Control solutions for these systems include the MC56F8013 and MC56F8036.
The 56F8013 is used on primary side to control Maximum Power Point and battery charging. The 8036 controls a boost stage and the DC/AC inverter stage.
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Residential Systems
Residential scale grid-tie systems are the most common types of system today. They typically provide three to 10 kilowatts of output power and are not normally accompanied by a battery backup. Output AC power must be synchronized in phase, frequency, and voltage to the grid and have a low total harmonic distortion. String inverters, which control one or more strings of individual PV modules, are the most common types of inverters used for residential applications. Key requirements for these inverters are high efficiency and long lifetime as well as low cost. The 56F83xx series of DSCs provide single chip control solutions for both MPPT and DC/AC conversion.
56F83XX
Memory:
- Up to 576 KB Flash memory
- Up to 36 KB RAM
Integrated Peripherals:
- Up to 12-bit, Analog-to-Digital Converters with total 16 inputs
- Up to two FlexCAN (CAN Version 2.0 B-compliant)
- Two Serial Communication Interfaces (SCIs)
- Two Serial Peripheral Interfaces (SPIs)
- Up to 12-Channel PWM
- Up to 16 16-bit multifunction timers
Up to 76 General Purpose I/O pins
Commercial Systems
Large commercial and central utility power generation systems are the fastest growing sector of the PV industry today Growth is spurred by continued reduction in the price of PV electricity and by favorable government policies, including feed-in tariffs, tax incentives and the requirement for utilities to provide a certain minimum percentage of their power from renewable sources. These systems can range from tens of kilowatts for commercial rooftop systems to over 100 megawatts for utility-scale central power generation plants occupying many acres of land.
The inverters for these systems are central inverters, although a parallel combination of several string inverters may also be used. In addition to supplying AC power to the grid, central inverters may also be used for power factor correction. One control solution for commercial inverters is to use a 56F8037 for DC/DC control and a member of the 56F83xx family for DC/AC inverter control. In addition, a Coldfire MCU can be used for supervisory control and communications.
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