Simple Discrete Control of a Single-Phase Voltage Source Inverter in a UPS System for Low Switching Frequency

Abstract

Previous research has shown that the inverter output voltage distortions are higher for low switching frequencies, and it is impossible to increase the controller gains because it will cause oscillations of the output voltage. The main reason is the real frequency domain characteristic of the PWM modulator and measuring traces, which can be modelled as switching periods delays. The thesis of this paper is that by using the control system that takes care of these delays, it is possible to decrease output voltage distortions for the standard loads for relatively low switching frequencies (e.g., 12,800 Hz). The Luenberger observer was implemented in the multi-input-multi-output passivity-based control of the system with the delay, in order to predict the state variables of the inverter. It is shown that state variable prediction is unnecessary for high switching frequencies (e.g., 51,200 Hz). The theory, simulations and breadboard verification, using the inverter model controlled with the real-time interface MicroLabBox, are presented.

Improving Low-Frequency Digital Control in the Voltage Source Inverter for the UPS System

“…Only output voltage was used as the output variable in this prediction. The result of the control with this prediction was promising [17]. However, in this paper the Luenberger observer with all the three measured variables treated as outputs in the prediction will be used, giving lower distortions of the output voltage.…”

“…Today the control is only digital, using microprocessors or FPGA. We need a discrete model [14][15][16][17] of the inverter where the output voltage, the inductor current and the output current are state variables and the output voltage is an output variable of the inverter model.…”

“…The control loop can be a single-input single-output (SISO) type when only the output voltage is measured and controlled or a multi-input single-output (MISO) when the measured output voltage, output current and the output filter inductor current are inputs of the controller and the output voltage is controlled [16,17]. MISO control is more effective [15] because in SISO control, the output current is the unmeasured disturbance.…”

“…We consider two switching frequencies: 12,800 Hz assumed as the low switching frequency and 51,200 Hz as the high switching frequency. For the high switching frequency, the delay is so low that it does not matter in control [17]. For the low switching frequency, the control can cause an oscillation component in the output sinusoidal voltage.…”

“…However, we want to solve the problem in a simplified way-to design an MISO PBC controller for the system without delay in the measurement traces and implement it in the system with delays using the prediction of the measured state variables. This approach was shown in [17] where the simplified Luenberger observer was used for prediction. Only output voltage was used as the output variable in this prediction.…”