Power Management
The two monitors report cell status over separate daisy chain interfaces. The daisy chain of the primary monitor is fed into a serial port. This daisy chain has bidirectional capability with device address and control signals from the microconverter being passed up the chain, and resultant cell data such as individual cell voltages and, if required, individual cell temperatures, coming down the chain.
Figure 2:More detail of the cell monitoring electronics within a single string
charge to avoid any injuries. It is sometimes forgotten that although the inverter may have been physically removed from the system for maintenance, the capacitors can still hold a lethal charge. Typically, common current and voltage measurements are taken at three locations – at least – to help with the system management. These three voltages are high voltages and can be measured via resistive dividers into the voltage channels of the devices.
Single string Cell Monitoring Referring to Figure 2, each Li-ion cell in a string has two monitors watching it: a primary monitor which is an Analog
to Digital Converter (ADC) channel directly converting the cell voltage into a 12-bit word, and a secondary monitor which is a window comparator looking for over- and under-voltage excursions of the cell voltage. The primary monitors are contained in a 6-channel device and the secondary monitors are contained in a companion 6-channel device. This approach provides a completely redundant measurement system whereby if the primary monitoring devices go ‘off-the-air’ for instance with a communications problem, the secondary or back-up monitors can still operate and maintain safety in the system.
The daisy chain offers read-only capability with cell status only (overvoltage, undervoltage, overtemperature) coming down its daisy chain. To maintain redundancy this data stream is fed directly to a GPIO (General Purpose Input/Output) port on the BF50x Blackfin processor via an FPGA. The FPGA is simply to concentrate the data from all the battery strings into a more manageable single port interface into the BF50x. Each microconverter also communicates with the Blackfin through the same or a similar FPGA arrangement.
The current flowing in each string develops a small voltage across Rshunt which is captured by the current measurement channel of the microconverter. Simultaneously the overall stack voltage is captured by the voltage measurement channel via the resistive divided pair. It might also be wise to duplicate the stack voltage measurement using another microconverter and another resistive divider pair in order to compare readings, which should be equal within allowable tolerances.
Energy storage is an important and necessary aspect of providing output power from solar photovoltaic installations and wind turbine systems over a sustained period, but it’s one that needs careful monitoring to ensure that safety procedures are adhered to. Highly specialised products are available to monitor each Li-ion cell as well as individual battery strings, ensuring that they do not overcharge or overheat.
Analog Devices |
www.analog.com John Wynne is an Energy Systems Applications Specialist at Analog Devices
www.cieonline.co.uk
Components in Electronics July/August 2010 25
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