Zbigniew Rymarski

The discrete model of the power stage of the voltage source inverter for UPS

This article presents a new generalised discrete model of the power stage of single- and three-phase inverters including the calculations of the output filter parameters that provide an acceptable level of output voltage harmonics. The presented model can be used with the pole placement control methods such as the coefficient diagram method or the deadbeat one sampling ahead preview control. Discrete modelling of the power stage including properties of the PWM modulator makes an easy description of the chosen control law that can be directly implemented in the control software possible. The final simulation and the experimental verification of the poles placement methods in the voltage source inventer control confirm the usefulness of the presented model but some differences between the control theory and the inverter measurements are expected.

Design method of single-phase inverters for UPS systems

This article presents the complete design of a low power voltage source inverter (VSI) dedicated for a UPS system. The analysis of the rectangular PWM-AC voltage spectrum allows for a choice of the basic architecture of the inverter. Output filter parameters were calculated to reduce the maximum amplitude of the output VSI voltage harmonics for the steady-state inverter mode. The choice of the feedback loop type was based on a discussion of the inverter output impedance using a continuous model of the inverter. The parameters of the inner loop digital control for the discrete inverter model were calculated using the Coefficient Diagram Method. The influence of the step load was modelled. The time constant of the inverter closed loop system was selected to ensure sufficient system robustness. An outer feedback loop with a plug-in repetitive controller, simplified owing to the properties of the PID/CDM inner loop control, was introduced to eliminate the periodic disturbances generated by the non-linear rectifier load and the deadtime influence. The experimental verification of the design method is presented.

The influence of the properties of magnetic materials on a voltage source inverter control

Magnetic materials made of iron-powder that are used for the cores of the chokes of voltage source inverters output filters significantly change their power losses and magnetic properties with the frequency and the amplitude of the choke current in the range that is important for designers. This causes the magnitude Bode plot of the VSI control transfer function to have a higher damping coefficient and the resonant frequency that differs from the theoretically calculated one. The paper shows the influence of the properties of magnetic materials on the control of a voltage source inverter. The design of the control loop that takes into account the changes in these properties is presented and tested in the experimental model.

Control Systems of Single-Phase Voltage Source Inverters for a UPS

This article presents considerations of the effectiveness of suppressing output voltage distortions of low power single-phase voltage source inverters (VSI) dedicated for UPS systems working with the nonlinear rectifier RC load defined in the EN 62040-3 standard. Various types of control systems were tested – PID/CDM and deadbeat instantaneous controllers designed using the authors discrete model (including the design of the output filter) of VSI and the fuzzy controller. It was shown that the additional control loop with the repetitive controller or additional input variables (currents) of the controller should be used for lowering the THD of the output voltage for the nonlinear load. The results of the experimental verification are shown.

Design method of a 3-phase VSI for UPS systems

This paper presents a complex design of a 3-phase VSI inverter with methods initially used for the single-phase inverters. The design rules of the output LC filter are implemented into a generalized discrete model of the inverter. The dual discrete feedback loop ensures a good transient response after the step load change and dumps the periodical disturbances improving the inverter operation with a nonlinear rectifier load.

Measuring the real parameters of single-phase voltage source inverters for UPS systems

ABSTRACT The design of a voltage source inverter (VSI) control system requires knowledge about its continuous or discrete model. The output filter inductance and the equivalent serial resistance, which is partly the result of power losses in the core, have values at the operating point that are significantly different from the assigned nominal values, particularly for iron-powder cores. Measurements using standard laboratory equipment are not sufficient because it is impossible to get the voltage and current of the operating point. So the best solution is to measure the Bode plots of the inverter control function at the operating point, to estimate the results of the measurements using the proper analytic function and to calculate the real values of the inverter parameters. Knowledge about the equivalent serial resistance enables the estimation of the inverter power losses. Two approaches to measurements are shown – one that does not require any additional equipment and the other that uses a specialised system to automatically measure the Bode plots. The advantages and disadvantages of both approaches are presented. These measurements enable the validation of the coil core magnetic material by taking into account the power losses. The applications of the measurements in the control design are shown.

The dynamic properties of a single-phase transformer-based inverter for ups systems

This paper presents two algorithms for the creation of an analytical model of a single-phase transformer-based inverter, using either the measurements of the control frequency-dependent characteristics of the inverter or the initial in-circuit frequency-dependent measurements of the transformer. The paper discusses how the parameters of the line transformer influence the dynamic properties of the inverter and, as a consequence, the design of the instantaneous control feedback loop. Line transformers, widely used in industry, have not yet been analysed for use at higher than normal operating frequencies. The basic parameters of the transformer are the nonlinear functions of the primary voltage amplitude and frequency. The paper includes verification of both methods using the breadboard experimental model. In the final step, the inverter control transfer functions are calculated. To show the utility of both models, an exemplary discrete controller for the experimental inverter model has been designed and tested. Copyright

The Low Cost Measuring of the Magnitude of the Sinusoidal Complex Waveform

Abstract The paper presents an evaluation of a numerical computing of a sinusoidal waveform magnitude of the complex signal consisted of sinusoidal and DC components in measuring systems based on controllers embedded with AD converters. It is shown that this method is the best for systems where the sinusoidal signal is both generated and measured, like sinusoidal inverters because this method requires strict synchronization with sinusoidal waveform. The paper presents that this method is useful (however introducing significant error) for overmodulated waveforms that are typical in power systems.

Drawbacks of impedance networks

Summary This paper presents the influence of voltage-fed impedance networks, known as Z-source and quasi–Z-source, as well as some more sophisticated networks on the static and dynamic properties of voltage source inverters. The impedance networks increase output voltage distortions with the second harmonic of the fundamental harmonic and decrease the power efficiency. The distortions of the output voltage increase for the discontinuous current mode of the impedance network. The DC voltage boost factor depends on the impedance network power losses. The impedance networks add 2 resonant frequencies that are very close each other to the control transfer function of the inverter in the frequency range that is close to the fundamental frequency. The influence of the impedance network on the control transfer function depends on the effective damping resistance in the impedance network. The impedance network operation during the “shoot-through” time increases the damping of the inverter output filter. The effective damping resistances and the inductance of the coils depend on the power losses in the magnetic materials. Theoretical models along with experimental verification can help to estimate the real influence of impedance networks on the inverter properties.

Evaluation of MMA and MAG arc welding process data

The microprocessor-controlled system for real-time collection and display of welding parameters was designed, implemented and tested. The system records up to 4 digital or analog signals collected from welding process and display their run on the LCD display. The aim of the research was to examine the possibility of detecting welding imperfections by recording the instant values of welding parameters. Tests of the device performed during manual metal arc welding and metal active gas welding revealed that the occurrence of some welding imperfections is followed by changes of the welding parameters. In this case, they can be detected by the analysis of the instant values of the welding parameters. Experiments show that monitoring of gas metal arc welding and a simple statistical approach to welding parameters can help in evaluation of weld quality. (Received 15 April 2006; Accepted 8 February 2007)