Hannu Saren

Verification of frequency converter with small DC-link capacitor

Traditionally, large DC-link capacitors have been used in voltage source inverters. By using MPPF-type capacitor the line current harmonic content is improved and the unreliable electrolytic capacitor can be omitted. In this paper, the dynamic behavior of an inverter with small DC-link capacitor has been studied by simulations and laboratory tests. A new modulating principle, named differential space vector pulse width modulation (DSVPWM), can deal with heavily fluctuating DC-link voltage and produce correct voltage vector with high accuracy. The performance of the DSVPWM has been verified with simulations

Modular double-cascade converter

A new multilevel converter topology with cascaded H-bridges and a multi-winding medium frequency isolation transformer is introduced. The proposed converter topology has a modular structure, which is achieved using cascaded H-bridges. The isolation for the H-bridges is provided by medium frequency isolation transformers which are small compared with line frequency transformers with the same power rating. The building blocks of the proposed converter topology can be connected in series or in parallel. Series connection adds voltage steps to output voltage, and parallel connection increases the power rating of the converter.

Overmodulation in Voltage Source Inverter with Small DC-link Capacitor

Overmodulation (OM) is used to increase the voltage output of the PWM controlled frequency converter. Full inverter voltage utilization is important because of the cost and output power improvement perspectives. The voltage source inverter (VSI) with high overmodulation performance is less sensitive to inverter DC-link voltage disturbances. DC-link voltage drop may result in unintentional entrance to the Overmodulation region. DC-link drop is often due to line voltage sag or fault conditions. When small DC-link capacitor is used, voltage drops due to full-wave rectifier bridge are present all the time. A high performance overmodulation method improves the drive performance under such conditions

DTC driven single phase fed voltage source inverter with small dc-link capacitor

In voltage source inverters (VSI) relatively large de-link capacitors are used to provide stable dc-link voltage and energy storage. However electrolytic capacitors are large, heavy and expensive. In many cases the lifetime of the electrolytic capacitor is the main factor that limits the lifetime of the frequency converter. Large de-link capacitor causes high line current peaks during dc-link capacitor load. Power factor correction circuits are traditionally used to correct line current quality in single phase supplied VSI. To overcome the problem of the line current quality and big electrolytic capacitor, a relatively small film capacitor (MPPF, metallized polypropylene film capacitor) is selected. While overcoming the problem of the line current waveform and the life time of the electrolytic capacitor the new problem is arisen in the form of the heavily fluctuating dc-link voltage. This leads to heavily fluctuating torque. Many industrial dynamically low performance applications, such as pumps and fans, are high inertia loads. Thus even heavily fluctuating torque can be tolerated. DTC motor control is used to maintain as smooth torque behavior as possible.

Electric drive emulator using dSPACE real time platform for VHDL verification

The use of programmable logic devices e.g. field programmable gate arrays (FPGA) in the motor control devices increase the complexity of the testing and verification. Simulation and verification can be linked together using Simulink, HDL-simulation library and dSPACE real time platform. VHDL co-simulations with Simulink model allows control algorithms to be simulated together with the rest of the system model. dSPACE real time platform allows Simulink models to be executed in real time. dSPACE can be used to emulate inverter and motor in the verification phase where control algorithms run in FPGA circuit and control the whole emulated electric drive system.

Control of an inverter output active du/dt filtering method

Frequency converters are used to increase the controllability and efficiency of electric motor drives. Induction motors may experience an insulation failure caused by an overvoltage at the motor terminals. The overvoltage is caused by an impedance mismatch between the motor terminals and the motor cable. Fast pulse rise times compared with the propagation delay in the motor cable will make the terminal voltage exceed the DC link voltage level. Passive filtering techniques for two-level inverters have been developed to cancel out the overvoltage. In this paper, a control method for active du/dt filtering to reduce motor terminal overvoltage is introduced. A filter topology and the basis for setting the filter component values are presented. The size of the passive filter used in this filtering method is smaller than in traditional passive du/dt filters. Functional filtering can be achieved with a smaller filter by controlling the filter output voltage rise time with an accurately timed inverter output voltage. The control of the system is implemented on a field programmable gate array (FPGA) to provide precise gate drive signals with high time resolution.

Modular Double-Cascade converter with soft switching DC/DC isolation converter

In power electronic converters soft switching is an efficient way to reduce losses. A resonant tank in series with the windings of the isolation transformer in a Modular Double-Cascade converter is presented. Positive effects such as switching loss reduction, prevention of flux walking and saturation of transformer core are analyzed and discussed.

FPGA implementation of DSVPWM modulator

In this paper a novel field programmable gate array (FPGA) implementation of differential space vector pulse width modulator (DSVPWM) is presented. FPGA circuit was chosen because DSVPWM requires high computational power and its algorithms can be easily parallelized. The use of traditional sequential processor would need very fast and expensive hardware compared to the FPGA implementation. DSVPWM enables more accurate flux linkage formation than traditional modulators. DSVPWM is completely new modulation method and this paper presents its first implementation

Power direction control of medium frequency isolation DC/DC converter for modular double cascade converter

Multilevel converters have strongly arrived to the medium voltage drive market. A new voltage source multilevel converter topology, called modular double cascade converter, is discussed. The converter uses cascaded H-bridges as input and output converters and a medium frequency DC/DC converter for galvanic isolation between the H-bridge modules. The key feature for the DC/DC converter is the six-winding transformer, that is able to balance the DC link voltages of the connected converters. It also enables power delivery in both directions, making four quadrant operation feasible. In this paper, different DC/DC converter control schemes are researched. The objective is to find the best method for balancing the DC link voltages between the isolated modules, and to minimize the transformer winding currents.

Implementation and testing of a distributed modulator for a medium-voltage multilevel inverter

A cascaded medium-voltage multilevel frequency inverter can be built using power bridges from unmodified factory-made low-level inverters. Thus, high-voltage power electronic components can be avoided. The multilevel nature also decreases the output voltage transients thereby reducing the insulation stress of the connected electric motor. The control of such series-connected power bridges requires a special distributed control system. This paper illustrates the basic concepts of distributed phase vector modulation for a series-connected multilevel inverter. The paper discusses in detail the implementation of the control of such a system using fast programmable logic and presents measurements showing the feasibility of the approach.