Till Boller

Synchronous Optimal Pulsewidth Modulation for Low-Switching-Frequency Control of Medium-Voltage Multilevel Inverters

This paper presents the mechanism and details of synchronous optimal pulsewidth modulation (PWM) generation for control of medium-voltage induction motor drives using multilevel inverters at low switching frequency. Multilevel inverters allow operation at multiple of dc-link voltage and reduce the total harmonic distortion (THD). Synchronous optimal PWM control permits setting the maximum switching frequency to a low value without compromising THD. Low switching frequency reduces the switching losses of the power semiconductor devices. An optimal control procedure is explained in detail. The performances of three- and five-level inverter topologies are compared. The experimental results of a five-level inverter drive using optimal PWM control are presented.

Optimal Pulsewidth Modulation of a Dual Three-Level Inverter System Operated From a Single DC Link

Operating a dual three-level inverter system from a single dc link circuit is an economic solution for generating five-level output voltage waveforms and to double the output power compared to a single three-level inverter. Such inverter system is used for feeding a drive motor with open stator windings. A mechanism is then required to reduce the common mode voltage components of the motor voltages. This paper describes an off-line optimization method that minimizes both the harmonic distortion of the motor currents and the common mode voltage components. The optimization permits reducing the switching frequency to a very low value of 200 Hz without compromising on harmonic distortion. High performance operation of the drive system at low switching frequency is experimentally demonstrated.

Neutral-Point Potential Balancing Using Synchronous Optimal Pulsewidth Modulation of Multilevel Inverters in Medium-Voltage High-Power AC Drives

This paper presents neutral point potential (NPP) balancing while maintaining low harmonic distortion using optimal pulsewidth modulation (PWM) of multilevel inverter for medium voltage high power industrial AC drives. This method is applicable for five-level inverters or higher. High performance of the machine is observed experimentally at low switching frequency operation employing the proposed technique. In the past, low distortion and optimal common mode voltage at low switching frequency control have been reported using proposed synchronous optimal pulse modulation.

Replacement of electrical (load) drives by a hardware-in-the-loop system

This paper presents an interesting approach for hardware-in-the-loop testing of voltage source inverters for drive applications. For this purpose the inverter under test is not connected to a real machine, but to a second inverter instead, which behaves like an electrical machine. The power capability of the so-called “Virtual Machine” is increased by sequential switching of parallel connected standard inverters. The parallel connected inverters can be of the same type as the inverter under test. Hence there exists no power limit for drive inverter testing with respect to the product range of the manufacturer.

Optimal pulsewidth modulation of multilevel inverters for low switching frequency control of medium voltage high power industrial AC drives

This paper presents an optimal pulsewidth modulation (PWM) technique for low switching frequency control of multilevel inverters for medium voltage high power industrial AC drives applications. Synchronous optimal PWM permits setting the maximum switching frequency to a low value without compromising in THD. Low switching frequency reduces the switching losses of the power semiconductor devices, resulting in higher inverter power output and efficiency. Experimental results of a five-level inverter drive using optimal PWM are presented. Two types of topologies are discussed: 1) Isolated dc link topology and 2) Common dc link topology with common mode inductor.

Optimal pulsewidth modulation of a dual three-level inverter system operated from a single dc link

Operating a dual three-level inverter system from a single dc link circuit is an economic solution for generating five-level output voltage waveforms and to double the output power compared to a single three-level inverter. Such inverter system is used for feeding a drive motor with open stator windings. A mechanism is then required to reduce the common mode voltage components of the motor voltages. This paper describes an off-line optimization method that minimizes both the harmonic distortion of the motor currents and the common mode voltage components. The optimization permits reducing the switching frequency to a very low value of 200 Hz without compromising on harmonic distortion. High performance operation of the drive system at low switching frequency is experimentally demonstrated.

Neutral point potential balancing using synchronous optimal pulsewidth modulation of multilevel inverters in medium voltage high power AC drives

This paper presents neutral-point potential (NPP) balancing while maintaining low harmonic distortion using optimal pulsewidth modulation (PWM) of a multilevel inverter for medium-voltage high-power industrial ac drives. This method is applicable for five-level inverters or higher. A high performance of the machine is observed experimentally at low-switching-frequency operation employing the proposed technique. In the past, low distortion and optimal common-mode voltage at low-switching-frequency control have been reported using proposed synchronous optimal PWM.

Increased power capability of standard drive inverters by sequential switching

This paper presents a new approach for increasing the power capability of standard two level voltage source inverters while increasing the effective PWM frequency by sequential switching of parallel connected inverters. The improved power capability and increased PWM frequency is used for an electrical test bench to test drive inverters under real power conditions. Thereby the parallel connected inverters of the test bench can be of the same type as the inverter under test. Hence there exists no power limit for drive inverter testing with respect to the product range of the manufacturer.

Sensorless Control of 3-Phase PWM Rectifier in Case of Grid Phase Disconnection

Standard drive systems are usually connected to the utility grid using six pulse diode rectifiers. This kind of the rectifiers has many disadvantages, for example non sinusoidal supply currents, non unity power factor, and the DC-link voltage cannot be controlled. To overcome these disadvantages the use of controlled PWM rectifiers seems to be appropriate. There are many control schemes proposed for active rectifiers. One of the most popular schemes is the voltage oriented control (VOC) method. It is easy to implement and simple to analyze especially because of its analogy to the well-known field oriented control (FOC) for the synchronous and asynchronous drives. One of the problems connected with this method is the need for grid voltage measurement. Voltage measurement systems are costly and each failure in the measurement device can lead to the destruction of the PWM rectifier. Due to these problems voltage estimation methods often are implemented. One of the many sensorless strategies, a phase tracking concept was presented on the PESC03 conference. The method proved to be robust to system parameter changes when being used for line current control. It was also successfully tested under the distorted voltage supply like over-voltage and under-voltage. After the presentation of the respective results on the PESC04 conference some questions arose from industry concerning the behavior of the sensorless PWM rectifier when one of the grid phase voltages disrupts. This paper shows the possible solution for this particular problem. The sensitivity of the phase tracking method to the voltage phase asymmetry is also presented. The control was performed on a rapid prototype system for power electronics control according to A. Linder (2001). Experimental results of the test are presented

Hardware‐in‐the‐Loop Systems with Power Electronics: A Powerful Simulation Tool

The chapter points out that sequential switching is not only applicable to IGBT modules with integrated diodes mounted closely on a heat sink, but also to complete industrial standard two‐level voltage source inverters (VSIs). Thus, the power capability and the overall switching frequency of these products can be increased by using several of them in parallel with sequential switching. Using this approach, a novel electrical test bench for drive inverters can be set up with a minimum effort on design and development. This provides an alternative for manufacturers of drive inverters to test their complete product range under real power levels without the requirement for a multitude of real machines.The discussion provided in the chapter proves that the so called virtual machine (VM) is a Hardware‐in‐the‐Loop system allowing an inverter to be tested at real power levels without the need for installing and operating real machines. The VM has the same characteristics as a real induction motor or even a synchronous motor. Different machines and their respective load conditions can be emulated by software, which means that the drive inverter under test can operate in its normal mode (as usual). No modification has to be done to the inverter or to the control unit.The chapter points out that sequential switching is not only applicable to IGBT modules with integrated diodes mounted closely on a heat sink, but also to complete industrial standard two‐level voltage source inverters (VSIs). Thus, the power capability and the overall switching frequency of these products can be increased by using several of them in parallel with sequential switching. Using this approach, a novel electrical test bench for drive inverters can be set up with a minimum effort on design and development. This provides an alternative for manufacturers of drive inverters to test their complete product range under real power levels without the requirement for a multitude of real machines.The discussion provided in the chapter proves that the so called virtual machine (VM) is a Hardware‐in‐the‐Loop system allowing an inverter to be tested at real power levels without the need for installing and operating real machines. The VM has the same characteristics as a real induction motor or even a synchronous motor. Different machines and their respective load conditions can be emulated by software, which means that the drive inverter under test can operate in its normal mode (as usual). No modification has to be done to the inverter or to the control unit.