Juhamatti Korhonen

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.

Active

Active du/dt is a new output-filtering method to mitigate motor overvoltages. The inverter pulse pattern edges are broken down into narrower pulses, which control the filter LC circuit. This results in an output voltage that does not have to exhibit the overshoot typically seen in common LC circuits in output-filtering applications. Furthermore, the shape of the output-voltage edge has properties well suited for output-filtering applications. An appropriate filter rise time is selected according to the motor-cable length to eliminate the motor overvoltage. The basis of the active du/dt method is discussed in brief. Considerations on the application of the active du/dt filtering in electric drives are presented together with simulations and experimental data to verify the potential of the method.

du/dt

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.

—New Output-Filtering Approach for Inverter-Fed Electric Drives

Modern high power grid inverters use usually LC-or LCL-filter at grid interface. Simpler L-filter does not prove sufficient harmonic attenuation without resulting in to very large and bulky filter components. Higher order filters realize better attenuation with smaller components. However, these filters introduce additional resonances to the system, which may cause instabilities without proper care. Connecting more than one inverter with individual LC- or LCL-filters in parallel will result in shifted resonances. The identification of the resonances in parallel connected LC- and LCL-filter system is essential for proper operation of the grid-connected inverter. For instance the efficiency of active damping is highly depended on the accurate knowledge of the filter resonances. In this paper the transfer functions for parallel-connected filter systems are derived and resonances analyzed. A method for resonance identification is presented.

Control of an inverter output active du/dt filtering method

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.

Identification of resonances in parallel connected grid inverters with LC- and LCL-filters

This paper presents a method for minimizing the circulating current between parallel connected photovoltaic inverters. The method is based on a common mode current measurement and on changing the modulator switching frequency. Prior methods for limiting circulating current in parallel connected inverters have not been efficient in long distances. The control method works wirelessly and is not limited by the distance between the inverters.

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

Inverter-fed motor drives can be subject to transient overvoltage problems caused by switching operations. Motor terminal overvoltages may occur if a motor is fed through a long motor cable. These overvoltages may result in a premature motor insulation failure. The phenomenon exists whether the motor is fed by a single inverter or parallel inverters. A modulation strategy and a parallel inverter topology that mitigate the motor terminal overvoltages are presented in this paper. The topology consists of parallel inverters with a common point of connection at the motor terminals. The proposed method does not use any passive filters. Experimental measurements were conducted with standard industry frequency converters to verify the method. Modulation sequences resulting in smallest possible overvoltage for two and three parallel inverters applying the method were found.

Wireless circulating current control for parallel connected photovoltaic inverters

Multilevel inverters have become a well proven technology in medium voltage drives, especially due to their improved power quality. This paper presents a new five-level inverter topology that is based on a neutral point connection and a flying capacitor. The operation of the topology is presented, taking into account the balancing of the flying capacitors. The performance of the topology is demonstrated with simulations, and compared with five-level active neutral point clamped inverter.

Active motor terminal overvoltage mitigation method for parallel two-level voltage source inverters

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.

Five-level inverter with a neutral point connection and a flying capacitor

Multilevel inverter topologies have been a widely researched topic, especially for high-power applications. Its more sinusoidal output voltage waveform is preferred over traditional two-level inverters. Common-mode voltages are known to cause motor bearing currents and to be a source for electromagnetic interference. Traditional multilevel modulation methods still produce common-mode voltage, and this can result in bearing failure. In this paper, a modulation method that reduces the common-mode voltage and mitigates the motor terminal voltage oscillation caused by long motor leads, is discussed. The method is compared with a traditional multilevel inverter modulation method and another common-mode voltage reducing method.