Control Intelligence Improves Renewable Energy Efficiency

Sep 1, 2007 12:00 PM
By Arefeen Mohammed, C2000 Applications Engineer, Texas Instruments, Dallas

Inverter Control Design

Fig. 6 shows the F2833x DSC used to control the power-stage inverter in a solar-powered system. (A wind-driven system would appear very similar, though with a wind-turbine collector.) Inputs from sensors in the panel array are fed to the controller's ADCs to provide data on the instantaneous voltage and current available from the array for conversion. Inputs may also provide information such as cell and ambient temperatures, used to protect the panels, and feedback that meters power output from the cells, used to track MPP.

All sensing inputs must be scaled so that peaks and spikes do not exceed the 3-V level of the ADCs. The data is first fed into a power control loop. And there may be more than one loop, depending on the design. Other real-time tasks that are being performed also provide inputs to the power control loop. Among these tasks are metering power returned to the grid, monitoring grid power levels for protection, regulating battery charging, tracking MPP and communicating with parallel controllers handling other systems.

Fig. 2 shows that PWMs are used to control both the dc-dc and dc-ac stages in the inverter. Depending on the power level of the system, a single or interleaved multiphase dc-dc configuration can be implemented. The dc-dc input and output voltage can be monitored and controlled using the controller's ADCs. The dc-ac stage, using an H-bridge as shown in Fig. 3, can be controlled using four PWM outputs. More than 12-bit PWM resolution is maintained at a PWM switching of 20 kHz — high enough to offer transient response and control over the ac output voltage.

This voltage is synchronized with the ac line by measuring both the line voltage and zero crossing, the latter detectable by using any of the controller's I/O lines. The low-interrupt latency of the F2833x ensures fast response and synchronization between the inverter output and the ac line voltage.

As an alternative, the system could use a three-phase inverter at the output instead of an H-bridge converter. In this case, dc-ac stage control would require six PWMs.

An important aspect of the design is in real-time fault management. A fault that occurs relatively slowly, such as overheating in the inverter, can be detected and managed using a dedicated ADC input, which monitors the temperature and initiates the appropriate system response. By contrast, a critical fault such as overvoltage, undervoltage or overcurrent requires immediate response to avoid severe system damage. The F2833x provides dedicated fault lines, called trip zones, to manage these critical faults. Trip-zone pins deactivate mapped PWM outputs within a couple of DSP cycles after receiving a fault signal, ensuring proper shutdown of the system to protect it from severe damage.

Key to Renewable Energy

Renewable energy systems are continually being improved to achieve greater efficiency and lower cost per kilowatt. While much attention is deservedly paid to improving PV panels and wind turbines, intelligent inverters can also contribute to making the technology more feasible. Floating-point DSCs represent an enabling technology for designing intelligent inverters because of their performance and flexibility. These characteristics will be especially valuable in helping designers address the varying regulatory and operational requirements of different renewable energy applications.

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