Ratmir Gelagaev

State estimation in distribution grids

Problems and techniques for estimating the state of an observable distribution grid are investigated. A distribution grid is observable if the state of the grid can be fully determined. For the simulations, the modified 34-bus IEEE test feeder is used. The measurements needed for the state estimation are generated by the ladder iterative technique. Two methods for the state estimation are analyzed: weighted least squares and extended Kalman filter. Both estimators try to find the most probable state based on the available measurements. The result is that the Kalman filter mostly needs less iterations and calculation time. The disadvantage of the Kalman filter is that it needs some foreknowlegde about the state.

Analytical Switching Loss Model for Superjunction MOSFET With Capacitive Nonlinearities and Displacement Currents for DC–DC Power Converters

A new analytical model is presented in this study to predict power losses and waveforms of high-voltage silicon superjunction MOSFET during hard-switching operation. This model depends on datasheet parameters of the semiconductors, as well as the parasitics obtained from the printed circuit board characterization. It is important to note that it also includes original features accounting for strong capacitive nonlinearities and displacement currents. Moreover, these features demand unusual extraction of electrical characteristics from regular datasheets. A detailed analysis on how to obtain this electrical characteristic is included in this study. Finally, the high accuracy of the model is validated with experimental measurements in a double-pulse buck converter setup by using commercial SJ MOSFET, as well as advanced device prototypes under development.

Fast robust gate-drivers with easily adjustable voltage ranges for driving normally-on wide-bandgap power transistors

Wide-bandgap (WBG) semiconductors, such as gallium nitride (GaN), are more and more being used in switching power devices. An AlGaN/GaN/AlGaN Double Heterojunction Field Effect transistor (DHFET) was developed in previous work and needed to be tested. The used test circuit was a buck converter. This type of converter, in addition with the normally-on switching behaviour of the GaN-based transistors, requires dedicated gate drive circuitry, resulting in the development of three types of gate-drivers. This paper presents the topology and performance of these drivers. Because of the type of converter, the drivers need to be galvanically isolated. Furthermore, because the experimental GaN transistors are normally-on, the drivers need to be robust so that they apply a negative gate-to-source voltage to switch off the transistor in case an error occurs in the driver. A third requirement for the drivers is that it has to be easy to adjust the voltage levels, in order to test the devices at different gate-to-source voltage conditions. A final requirement is that it has to be possible to construct the drivers with readily available electronic components. Because the drivers are galvanically isolated, there is a parasitic isolation capacitance in the DC-DC-converter of the drivers. This gives rise to a common-mode current which possibly can disturb the operation of the driver. The article also discusses this common-mode problem.

Numerical observability analysis of distribution systems

Observability analysis is a very important tool in state estimation. For a given set of measurements and their geographical location in the system, it determines whether the state of the whole system can be estimated. In this paper a numerical observability analysis, applicable to distribution systems is discussed and applied to the IEEE 34 bus test feeder. The method is based on the inverse function theory and the Jacobian matrix of the system. By calculating the dependent columns of the Jacobian matrix it is possible to determine the unobservable branches in the system. This also gives an indication of how many linearly independent pseudo-measurements are required to make the distribution system observable. Because the R=X ratio is not negligible in the distribution lines, a transformation has been used to simplify the observability analysis.

A novel voltage clamp circuit for the measurement of transistor dynamic on-resistance

For determining the dynamic on-resistance Rdyn,on of a power transistor, the voltage and current waveforms have to be measured during the switching operation. In measurements of voltage waveforms, using an oscilloscope, the characteristics of an amplifier inside the oscilloscope are distorted when the range of the measurement channel is not set wide enough to measure both on-state and off-state voltage, resulting in failure to accurately measure the voltage waveforms. A novel voltage clamp circuit improving the accuracy of the transistor on-state voltage measurement is presented. The measurement accuracy is improved by clamping the off-state voltage across the transistor to a lower voltage that is still greater than the on-state voltage. Unlike traditional clamping circuit, the presented voltage clamp circuit does not introduce delay caused by RC time constants keeping the voltage waveform clear even during state transitions of the evaluated semiconductor device for frequencies up to 1MHz.

Multi-objective design of a close-coupled inductor for a three-phase interleaved 140kW DC-DC converter

A multi-objective design procedure is applied to the design of a close-coupled inductor for a three-phase interleaved 140kW DC-DC converter. For the multi-objective optimization, a genetic algorithm is used in combination with a detailed physical model of the inductive component. From the solution of the optimization, important conclusions about the advantages and disadvantages of using close-coupled inductors compared to separate inductors can be drawn.

An Electromagnetic Circuit Simulator for Power Electronics

A method for solving the full equations of Maxwell for circuits with discrete nonlinear, nonideal elements is presented. To this end, the method of moments is used in combination with a classical circuit simulator. Both operate in the frequency domain. A few methods to greatly accelerate the calculations are also presented. The methods are implemented and tested in Matlab.

Design of an intra-module DC-DC converter for PV application: Design considerations and prototype

The effective efficiency of a photovoltaic system is strongly affected by varying factors such as partial shading. The centralized power conversion has limitations to mitigate that problem. As a possible solution the benefits of the distributed power conversion are explored and as an integral part of distributed power conversion an intra-module DC-DC converter is presented here to implement at substring level in photovoltaic application. In this work, the design requirements of the intra-module converter are identified and the converter is designed accordingly. The effectiveness of the designed converter is also discussed in this paper. Finally, a fully-working prototype of this converter is built and tested.

Multi-objective optimization of a flash lamp drive

To adjust the light spectrum of a flash lamp, the current flowing through the flash lamp has to be controlled. An optimal design of a switched resistor bank circuit following a predefined current profile is presented. The optimal values of the switched resistors are calculated using multi-objective optimization with genetic algorithms.

Accurately modelling of parasitics in power electronics circuits using an easy RLC-extraction method

A method for accurately modelling parasitics in power electronic circuits, is presented in this paper. The freeware software programs FastCap and FastHenry are used to create a model of the printed circuit board tracks, consisting of resistances, self and mutual inductances, and self and mutual capacitances. This model can be easily loaded into a standard circuit simulator such as Spice, together with models for other components, such as the diodes, transistors, coils and capacitances. Thus, the power electronic circuit can easily be simulated in the time domain, returning electrical currents and voltages typically being subject to ringing effects and overshoot.