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Small-Signal Terminal Characteristics Modeling of Three-Phase Boost Rectifier with Variable Fundamental Frequency

Fig. 1. Interleaved transformerless series voltage injection converter
The stability of integrated power systems has become a concern due to the interaction between controls, leading to potentially unstable operation and requiring the verification of system operation before a design can be implemented. The conservative stability criteria for dc distributed power systems is based on the ratio of the source and load impedances. In order to design a robust power system for which stable operation can be guaranteed, th precise identification of small-signal dq impedances is necessary. The characterization of small-signal dq impedances is usually implemented either with voltage or current injections. The injection of single-phase wide-bandwidth signals into three-phase ac power systems significantly reduces the measurement time required, and uses the a minimal number of hardware components.

The interleaved transformerless H-bridge converter (Fig. 1) is optimized to operate as a series voltage injector, increasing the low-frequency injection range by avoiding transformer saturation problems. Decoupling control is implemented, providing an effective way to balance dc capacitor voltages and regulate the series injection voltage. The modular and scalable single-phase impedance measurement unit (IMU) is designed and constructed to inject chirp, multitone and sinusoidal signals in series with the three-phase ac power systems. The measurement system is designed to inject all three types of signals in the full frequency range, offering a trade-off between measurement time, precision, and the number of identification points. The effectiveness of the proposed identification approach is verified with an online estimation of source and load impedances of an actively controlled, programmable voltage source and a three-phase resistive load, as shown in Fig. 2.

Fig. 2. Source small-signal dq impedance calculated with FFT algorithm (straight red line) and cross-correlation method (straight blue line): (a) Zdd(s) (b) Zdq(s) (c) Zqd(s) (d) Zqq(s).
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