Abstract
The huge influence of the sampling rate on the performance of the digital PID control of a voltage source inverter (VSI) is revealed. It is shown that an appropriately chosen continuous-time model of a digital controller with the PWM power converter behaves like the actual discrete-time system, which allows for a simple controller analysis and design. The variable structure nature of the inverter with both the RC rectifier and an abruptly changing resistive load with two modes of operation within the sampling period is directly taken into account. Two simulation models, a discrete-time PWM and a continuous-time, of an inverter are presented, which are used to tune the PID controller and to evaluate the control performance. The behavior of the system in both modes is explained on the basis of the root loci and frequency characteristics. The results obtained for three sampling rates: 12.8, 25.6, and 51.2 kHz, are presented and compared with an actual VSI experiment. A comparison with other results obtained for this VSI shows that properly tuned PID control outperforms the more sophisticated solutions based on the coefficient diagram method (CDM) and the passivity based control (PBC).
Highlights
DC/AC inverters, called voltage source inverters (VSI), are commonly used as a basic component of uninterruptible power supply units (UPS) which provide emergency power to a load when the mains power fails
The performance of the inverter is usually measured as the value of the total harmonic distortion (THD) of the output voltage under the the standard non-linear repetitive controller (RC) rectifier load
Contrary to the popular approach to load current as an independent disturbance, our approach is based on the variable structure nature of the inverter where there are two operating modes during the period of the output voltage: load mode and no-load mode
Summary
DC/AC inverters, called voltage source inverters (VSI), are commonly used as a basic component of uninterruptible power supply units (UPS) which provide emergency power to a load when the mains power fails. The performance of the inverter is usually measured as the value of the total harmonic distortion (THD) of the output voltage under the the standard non-linear RC rectifier load. Since the performance of simple inverters without feedback control is usually not satisfactory, a plenitude of control schemes were proposed in the literature They can be classified into two main categories: single-loop control and multiple input controllers, including multi-loop structures. Even more complicated multiloop structures with the DC-bus voltage decoupling and load current compensation are presented and discussed in [4] Another sophisticated solution based on multiple inputs is the passivity based control (PBC) [5,6,7] which uses three input variables: the output voltage, and the inductor and output currents [8]. Contrary to the popular approach to load current as an independent disturbance, our approach is based on the variable structure nature of the inverter where there are two operating modes during the period of the output voltage: load mode and no-load mode
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