Digital control of grid-connected converters

During the last decades, the amount of power electronic converters connected to the grid has increased rapidly.  Since regulations such as EN-61000-3-2 limit the amount of harmonics in the input current of these converters, many converters use power factor correction (PFC). This means that a low harmonic distortion of the input current and a low phase shift between the input current and the grid voltage are obtained.
In many cases, the control algorithms for PFC are still implemented as analogue circuits. However, with the advent of fast digital signal processors (DSP) embedding control peripherals such as PWM generation units, AD converters, etc. new and more complicated control algorithms become feasible. In EELAB the consequences of going towards a digital control have been investigated and new control algorithms have been developed.

Applications

Digital control of a boost PFC converter in a wide power range

As input stage of many switching power supplies, a boost converter topology is used. This converter has a double objective: to control the waveform of the converter input current so that the regulations are met, and to control the dc voltage at the output of the converter to a (in most cases) constant level. Therefore the controller contains two control loops, a current loop acting on the duty ratio of the switch S and a voltage loop acting on the input conductance ge to balance the input and output powers of the converter. The converter scheme is shown in Fig. 1 together with its control loops. Fig. 2 shows a picture of an experimental set up of a boost PFC converter, controlled by a DSP.

 

Fig. 1. A boost PFC converter with control scheme

 

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Fig.2. Test set up of a digitally controlled PFC converter

When using a digital controller, some difficulties have to be overcome to obtain the same results as those obtained with a standard analogue controller: the timing of the current sampling is critical, while the response of a digital pulse-width modulator is different from its analogue equivalent. However, once these problems are countered, the wide flexibility of a digital controller allows improving the performance of the PFC converter:

  • When the output voltage is sampled synchronously with the line voltage, the influence of the voltage control loop on the input current harmonics disappears. As a result the bandwidth of the voltage controller can be increased while a very low harmonic distortion of the input current is maintained. These results are summarized in Figs. 3 and 4.

 

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Fig. 3. Left: transient behaviour of the input current and output voltage with analogue controller; right: line current distortion caused by the analogue voltage controller.

 

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Fig. 4. Left: transient behaviour of the input current and output voltage with digital controller; right: line current distortion caused by the digital voltage controller.

  • Duty-ratio feedforward is a technique to help the controller calculate the correct duty-ratio by forwarding its ideal value to the output of the current controller. With this technique, the phase shift between the converter input current and the grid voltage are decreased, as shown in Fig. 5.

 

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Fig. 5. Input current and line voltage at nominal power (1 kW)

  • When PFC converters operate at reduced load, they start working in the discontinuous conduction mode during parts of the line cycle. Since in this mode, oscillations of parasitic components appear and the dynamic behaviour of the converter changes intrinsically, the input current waveforms will be distorted. To guarantee good operation of the converter throughout the entire power range, it is necessary to make some corrections to the control algorithm: a new, more complex sampling algorithm and an extension of the duty-ratio feedforward algorithm to the discontinuous conduction mode. With these changes, a good operation of the converter is obtained at low power, as shown in Fig. 6 for a 250 W load.

 

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Fig. 6. Experimental waveforms of the PFC converter at 128 W with standard control for CCM (left) and with improved control for DCM (right).


Converters for grid connection of distributed generation units

The research of the control of PFC converters is closely related to the research in the field of power systems, since the input impedance of grid-connected converters may influence disturbances on the distribution grid. Moreover, converters employed as front-end converters for connecting distributed power generation to the distribution grid can be controlled with a similar digital control strategy. When a digital controller is used, the sampling algorithms, current and voltage controllers operate in the same way and also the duty-ratio feedforward algorithm is extended for use with this converter.

Read more about the application of this type of converter on the Power Systems page

Relevant publications

  • K. De Gussemé, W.R. Ryckaert, D.M. Van de Sype, J.A. Ghijselen, J.A. Melkebeek and Lieven Vandevelde, "A boost PFC converter with programmable harmonic resistance,"IEEE- Trans. Ind. Applic. Vol. 43, No. 3, May 2007, pp.
  • K. De Gussemé, D.M. Van de Sype, A.P. Van den Bossche, and J.A. Melkebeek, "Input current distortion of CCM boost PFC converters operated in DCM," IEEE-Trans. Ind. Electron. Vol. 54, No. 2, Apr. 2007, pp.
  • K. De Gussemé, D.M. Van de Sype, A.P. Van den Bossche, and J.A. Melkebeek, "Digitally controlled boost power factor correction converters operating in both continuous and discontinuous conduction mode," IEEE-Trans. Ind. Electron., Vol. 52, No. 1, Feb. 2005, pp. 88-97. 
  • D.M. Van de Sype, K. De Gussemé, A.P. Van den Bossche, and J.A. Melkebeek, "Duty-ratio feedforward for digitally controlled boost PFC converters," IEEE-Trans. Ind. Electron. Vol. 52, No. 1, Feb. 2005, pp. 108-115.
  • D.M. Van de Sype, K. De Gussemé, A.P. Van den Bossche, and J.A.A. Melkebeek, "A sampling algorithm for digitally controlled boost PFC converters," IEEE-Trans. on Power Electr., Vol. 19, No. 3, May 2004, pp. 649-657.

Contact

For more information: Alex.VandenBossche@ugent.be