Study On Digital Controlled Variable Frequency Supply For ACRF Test System

For high voltage(HV) on-site testing, the AC frequency-tuned series resonant test system(ACRF system) have been recognized to be the most cost-effective and only practicable solution because of the advantages of the ACRF system: a very good weight-to-test power ratio and very low power demands. The ACRF system is tuned into resonance with the test object by a voltage of variable frequency supplied by a variable frequency PWM inverter. Consequently, the variable frequency supply is the heart of a ACRF system. A variable frequency SPWM inverter for ACRF system is described in this dissertation. The analysis of the transfer function of the ACRF system indicates that this system is a typical second-order oscillation section with small damping ratio and the phase-frequency characteristic is non-linear function of frequency. By linearizing the phase-frequency characteristic around the operating point, the linearized characteristic of the system can be easily obtained. Based on the system modeling and simulation, two kinds of PWM modulation methods used in single-phase bridge are analyzed. As a result, a subsection synchronization unipolar SPWM modulation strategy and a average voltage PI control scheme is applied in DSP controller for achieving a wide frequency range of output voltage. Based on DFT algorithm, the phase of voltage in the test object is gained and the digital controller is presented for tracing the system resonant frequency. A prototype of the ACRF test system is developed, which uses DSP TMS320F240 as the core of control system. Experimental results demonstrate that the output voltage of the system based on subsection synchronization unipolar SPWM modulation and DFT algorithm have high steady-state precision and symmetrical waveform, furthermore, the phase and frequency of the voltage is both precise and stable.