Volume 4 (2013): Issue 1 (December 2013)

In this paper, sensors for the application of high voltage networks insulation diagnostics are focused on. For detection and localization of the components with insulation deterioration, observing the partial discharge transients is one of the main methods. However, as the partial discharge traces are transients of extremely short duration, monitoring of such transients is also a challenging task. This paper presents the inductive sensors that would be suitable for the on-line partial discharge monitoring and the selection of operating mode and characteristics are discussed. The issues of resonance and signal processing are analyzed for the targets of reaching highest accuracy of the measurements. Operation criteria and performance limits such as sensitivity and upper bandwidth are particularly discussed. It is presented that in order to reach highest accuracy and sensitivity the sensor should be used in damped mode. Applying the accurate air-core inductive sensors would allow the condition monitoring systems to be cheaper and thus mounted for permanent and continuous condition monitoring in various locations in the power distribution networks.

This paper presents an isolated quasi-Z-source inverter-based (qZSI) resonant DC/DC converter. The explanation of selection of the proposed topology is justified. Both the normal and the boost modes are discussed. Theoretical operation waveforms as well as basic expressions for the calculation of currents and voltages are proposed. A 1500 W laboratory prototype was built and experimentally verified at two operation points: that of light-load (300 W) and full-load (1500 W). All the experiments were also carried with resonant circuit and without it. The experimental results as well as performance of proposed qZSI based resonant DC/DC converter laboratory setup are presented and analyzed. Experimental and calculated characteristics showing the dependence of the load voltage and supply current on the load resistance in both modes were presented. The dynamic losses in the transistors were evaluated for the cases with the resonant circuit and without it. The main conclusions based on this study are summarized and the future tasks for development of proposed converter were defined.

Electrical grid modernization concept promotes the use of DC subgrids in order to improve efficiency, minimizing energy conversion count in the source-to-load chain. The present paper discusses an average current sensorless control algorithm for proposed bidirectional AC/DC converter, which is based on a dual half-bridge topology with common neutral wire that is not commutating during converter operation. The proposed current sensorless control algorithm has been obtained analytically for rectification, grid-tied and stand-alone inverter modes. The average value of inductor current tracks the reference current signal with constant switching frequency. Two control functions for inductor’s discontinuous and continuous current modes have been defined for each of the operation modes, and a sensorless transition between DCM and CCM modes has been stated. The proposed sensorless control algorithm has been also adapted for use with LCL input filter. The results of simulation in the PSIM software approved the analytical model, keeping the average inductor current to follow the reference value in inductor discontinuous and continuous conduction modes. Experimental investigation of the proposed current control algorithm provided similar results confirming the discussed theory.

This paper is focused on the control issues of the quasi-Z-source derived push-pull converter with integrated magnetic elements. The proposed converter is intended for applications that require a high gain of the input voltage and galvanic isolation, i.e. power conditioning systems for renewable energy sources, such as variable speed wind turbines with direct driven permanent magnet synchronous generators. Magnitude and frequency of the output voltage of such turbines are variable due to intermittent nature of the wind power. Despite number of advantages converter has complicated dynamic behavior. Simulations showed change of stability margin depending on current operation point of the wind turbine and output load. Closed loop control system should provide fast response and stable operation in the wide range of wind speeds. Simulations showed that the conventional PI-controller with saturation cannot satisfy those requirements. Nonlinear PI-controller was derived by adding adjustment block to the conventional PI-controller. Adjustment block is drastically changing proportional and integral gains of the controller according to sign of the output voltage error. Proposed controller is compared with conventional one by means of simulation in PSIM. Simulation results prove that proposed nonlinear control system has improved regulator performance.

Self-excitation for synchronous generator is described in the paper. The system is based on a buck converter input filter capacitor discharge through excitation winding of the generator. The buck converter is connected to the stator outputs through an uncontrollable diode rectifier, but excitation winding is used as a load. Input filter capacitor of the converter provides initial current pulse which magnetizes excitation system and produces the generator voltage increase, for this reason the capacitor is charged before self-excitation process starts.

Results of the computer simulation and physical experiment are obtained and presented. These results show that the proposed self-excitation converter in conjunction with an input capacitor pre-charged from a low-power electronic generator actually magnetizes the generator excitation system therefore generator voltage and accordingly excitation current increases. Stabilization of generator output voltage occurs with a voltage surge, though its peak value slightly exceeds the reference one.

The future investigation of the proposed self-excitation system may include definition of mathematical equations which describe transients in the generator’s self-excitation mode and development of control methods for purpose of self-excitation process control without voltage peaks. The computer model also should be improved.

Wide use of induction motor drives makes the problems related to induction motors very topical. One of such problems is the maximal utilization of torque and velocity of induction motors. In this regard the use and accurate adjustment of rotor magnetic flux observers may be helpful. The technique of observer adjustment is subject of special interest. This technique can be regarded as optimal if it ensures constant acceleration that, in turn, corresponds to constant magnitude of active and magnetizing components of stator current. In contrast, nonoptimal tuning of the magnetic flux observer creates a transient response caused by variation of magnetic and active components of the stator current resulting in changing acceleration of the motor. However, the parameters of non-optimal process can be used for fine tuning of the observer which considers the variation of the time constants obtained analyzing the drive's magnetic circuit saturation. It is possible to conclude that implementation of fine adjustment of rotor magnetic flux observer is of critical importance for induction motor torque and velocity maximum utilization.

Speed inaccuracy decreases the pump efficiency, reliability, and energy saving. This research is devoted to the determination of the ways of accurate speed control of the pump drives operated under changeable loads. The impact of speed inaccuracy on the pump performance is studied. Based on the analysis of methods for the static accuracy improvement, the drawbacks of the traditional approaches have been shown with reference to the pumping applications. A new methodology of the slip compensation has been proposed for implementation to improve the scalar drive performance. It notably decreases the speed inaccuracy of the open-ended pumping applications. The enhanced quality of the drive control at different loading conditions has been shown on a laboratory test bench. Also, for the multi-pump systems this approach results in an additional benefit from the viewpoint of the operation around the best operation point providing a safe pump control both to exclude the pump damage and to improve the process quality.