IEEE Power Electronics Society Student Branch chapter at Amrita School of Engineering, Bengaluru organized a Distinguished Lecture Program (DLP) on “Hardware-in-the-Loop Systems with Power Electronics- a Powerful Simulation Tool” by Dr. Ralph M Kennel, TU Munich on January 20, 2016. Dr. Ralph M Kennel is Chair and Professor for Electrical Drive Systems and Power Electronics at The Technische Universität München (Germany). His main areas of interest are sensorless control of AC drives, predictive control of power electronics and Hardware-in-the-Loop systems. Dr. Kennel is a Senior Member of IEEE, a Fellow of IET and a Chartered Engineer in the UK. Within IEEE he is Treasurer of the Germany Section, as well as Distinguished Lecturer and ECCE Global Partnership Chair of Power Electronics Society (IEEE-PELS). Dr. Kennel has received the Harry Owen Distinguished Service Award from IEEE-PELS in 2013.
Dr. Kennel started his talk by highlighting the need of Power Electronics for human society. He noted that its financial research funding, however, did not cover the essential needs till political leaders included that issue in their public announcements. Illustrating this point, he showed the inspirational public talk by Barack Obama, President of the United States of America and Horst Seehofer, Bavarian Prime Minister to illustrate that point.
Dr. Kennel explained that Hardware in loop systems consists of a simulation computer and a hardware which is part of the real world. The hardware could be simulated in the computer, but it requires exact modelling of the hardware, especially with respect to physical behavior of energy. He presented an interesting approach for Power-Hardware-in-the-Loop (PHiL) testing of voltage source inverters for drive applications. For this purpose the inverter under test is not connected to a real machine, but instead connected to a second inverter, which behaves like an electrical machine (virtual machine). This virtual machine must provide better performance than the inverter/device under test to enforce any current reference provided by the model.
The IGBT has higher switching frequency (> 50 kHz) and the basic idea of sequential switching is used to reduce switching losses, where several IGBTs are connected in parallel and each device is switched sequentially at reduced frequency. However, the free wheeling diodes cannot be switched sequentially, hence, all the free wheeling diodes are loaded with full switching frequency. The diodes with the lowest voltage drop are heated up more than other devices, leading to unsymmetrical load and unstable operation. To overcome this, the concept of magnetic free wheeling can be used. Magnetic free wheeling is simpler, but the inductance required for it makes the size bigger and needs a separate core design for this purpose.
The controller in the hardware in a loop system cannot be dealt with using a classical PI controller, as the current control of the Virtual Machine is significantly faster than the control of the inverter under test. Hence, the control of the inverter under test cannot react on the enforced current. The control of the inverter under test is fighting against the control of the Virtual Machine. One possible solution is a T- Filter between inverters instead of inductance. Here the current of Virtual Machine is allowed to be different to the current of the inverter under test and the control of the inverter under test does not fight against the control of Virtual Machine.However, T- Filter is more complex than an inductance with parallel windings. In addition, the current of Virtual Machine is not identical to the current of the inverter under test.
The successful solution is an Inverted machine model where instead of calculating machine currents as a reaction on terminal voltages, the model is applied to calculate the induced machine voltages as a reaction on enforced currents. The advantages of this model is that it avoids conflicts between current controllers and provides a scheme without voltage sensors at the output of the inverter under test. The inverter under test can be operated in the same way as a real AC 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. As the “Virtual Machine” can be scaled, there is no practical power limitation for drive inverter testing with respect to the product range of the manufacturer.
The lecture was very interesting and beneficial to industrial experts, academicians, researchers, PG and UG students who attended the Distinguished Lecture Program (DLP). The session was concluded with vote of thanks and felicitation of the speaker. The seminar was organized very well by the IEEE PELS Student Branch chapter members, with the wholehearted support of the department of Electrical and Electronic Engineering staff of Amrita School of Engineering, Bengaluru.