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Evaluation and control of active capacitor banks for a floating power modules based converter

Converter schematic
Fig. 1. (a) PIU with active capacitor banks, (b) assembly of one active capacitor, and (c) assembly of an equivalent capacitor bank
  With multiple converters connected to a power grid, the problem of instability may arise. An impedance measurement unit (IMU) is utilized as a tool to measure the system impedance to analyze its stability. A perturbation injection unit (PIU) inside the IMU injects perturbations to the grid. The voltage and current responses are then measured, and impedance is calcu- lated accordingly. Unlike a traditional rectifier, the PIU application is more complex, with multiple current harmonics. The injection frequency, which is nearest to the grid frequency, causes the largest ripple on the dc-link voltage. In order to reduce the voltage ripple, a large capacitor can be connected in parallel with the dc-bus of the PIU. In this paper, the active capacitor is utilized as a high-power-density solution to replace the passive capacitor in the PIU. The active capacitor consists of one full bridge, one inductor, and three film capacitors, and is connected in parallel with a floating H-bridge, as illustrated in Fig. 1(a). A control scheme is proposed as in Fig. 2(a) to compensate multiple power harmonics and balance the dc-link voltage in the active capacitor. The controlled voltage of the active capacitor¬ís full bridge is expressed as Fig. 3, where &Delta&lparvCa1&lpar t&rpar&rpar is the ripple extracted from the voltage vCa1&lpar t&rpar, &Delta&lparvCa1&lpart&rpar&rpar&VerticalSeperator&omegas&plus&omegap is the ripple component of &Delta&lparvCa1&lpart&rpar&rpar at &omegas&plus&omegap, and &beta is the
loss compensation factor, which is multiplied with the derivative of &Delta&lparvCa1&lpart&rpar&rpar&VerticalSeperator&omegas&plus&omegap to balance vCa2.
  Simulation results of a three H-bridges based PIU are provided to verify the effectiveness of the active capacitor solution. The voltage waveforms across Ca1, Ca2, Ca3 and dc-bus voltage are included in Fig. 2(b). It is seen that the voltage vCa3, which is generated by the full bridge of the active capacitor, follows the voltage -&Delta&lparvCa1&rpar as well. Since each dc-link voltage is the sum of vCa1 and vCa3, its value is maintained constant around 1,000 V with a small ripple. All the dc-bus ripples of the three floating H-bridges are limited within 4&percnt the dc-bus voltage.
  Fig. 1(b) and 1(c) show the assembly of the active capacitor and an equivalent capacitor bank, respectively. The comparison indicates that with a 5&percnt DC-bus ripple requirement the active capacitor solu- tion for the PIU helps reduce both the volume and weight by five times over the traditional passive capacitor solution.
Control scheme
Fig. 2. (a) Proposed voltage-mode control scheme, and (b) simulation results

Equation
Fig. 3 Controlled voltage of the active capacitor’s full bridge
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