Design of an Industrial Power Converter for Adjustable Speed Drives

Electrolytic capacitors are nowadays widely used as DClink for industrial Adjustable Speed Drives (ADS), however they present a lower lifetime with respect to other capacitor technologies. For this reason, there is the trend to replace electrolytic capacitors with film ones to increase the system reliability. In addition, film capacitors are characterized by a much smaller capacitance per volume with respect to electrolytic ones. Thus, the DC link capacitance can be drastically reduced, leading to the concept of slim DC link power converters. However, by decreasing of orders of magnitude the dc-link capacitance, resonances between the dc-link capacitors and the grid inductance can occur. These resonances cause very high dc-link voltage ripple and the increase of grid current Total Harmonic Distortion (THD). This problem has to be properly solved at control level to avoid the use of other hardware components. The goal of my master thesis is the design of a slim dc-link drive system for ADSs featuring low THD of input grid current, being suitable for data centers’ cooling system. This is done through the following steps: electrical stress calculation, components choice, losses computation and cooling system design. In addition, the thesis proposes a method to actively damp the DC link resonances, using motor control. Moreover, this work is an integral part of a research contract. As mentioned before, the main issue in the slim dc-link converter design are the resonances related to the interaction between the dc-bus capacitance and the grid inductance. To stabilize the system I decided to use a Voltage Modulation based on Virtual Positive Impedance Concept . Dc-link voltage consists of the three main components: DC (mean) component, one component related to the bridge rectifier at six time the grid frequency (300Hz) and one component related to the resonance between DC link and the grid. The idea behind the adopted method is to actively damp the dc-link resonances using the electrical machine. This is done by a proper calculation of the dc voltage signal used to compute the duty cycles of the inverter, the calculation of the dc voltage is a combination of a low pass and a resonant filter tuned at 300 Hz. These filters are necessary to isolate the high frequency (resonant) component that must be damped, without interfering with the natural behavior of the rectifier bridge, causing 300 Hz oscillations on the dc-link voltage. I have first simulated the system behavior using the software PLECS. The simulation trsults shows that, when the compensation starts, the voltage resonance component is damped reducing drastically the voltage ripple. Moreover, in the grid currents there are no more resonant components, therefore, the THD drops down reaching approximately 32%, very close to the theoretical limit (31%). Based on the current and voltage stresses, I selected first the power electronic module employed for the rectifier and inverter. Then, based on the loss calculation, I selected the heatsink. A thermo-fluid dynamic analysis is performed to validate the thermal resistance and the heatsink temperature distribution. Finally, I developed the schematics to create the PCB of the converter. Moreover, the simulation results are tested and confirmed experimentally in a test bench.