Cascaded Multi Level Voltage Sourced Inverter Test

     Inverters are power electronic devices which converts DC to AC in power systems. In variety of areas inverters are used such as UPS (Uninterruptable Power Supply), Motor Drives, STATCOM, Active Power Filter (APF), Dynamic Voltage Restorers (DVR), Renewable based systems etc. There are lots of different inverters which are design for satisfying the requirements such as harmonic spectrum, voltage level, current level, DC link voltage or current, air or water cooled, thyristor or IGBT semiconductors etc. In my application, I will intruduce an inverter which is cascaded 5 level voltage sourced inverter for 13 MVAR STATCOM application. This inverter is used to supply required reactive power to the bus for reactive power compensation. Actually, there are 3 phases and 5 H-Bridge Inverters in each phase so that each phase have 5 level operation. DC link voltage for each H-Bridge is 1200 V in normal operation; however, I will test the whole systems' fiber optical connections and control algorithm by simply connecting 9 V batteries to their DC link capacitors. Figure 1 shows the cascaded 5 level voltage sourced inverter (6.5 MVAR systems in left and right side which are connected in parallel). Figure 2 shows the connection of 9 V batteries to the DC link capacitors.

Figure 1: Cascaded 5 level voltage sourced inverter ( 13 MVAR)

Figure 2: 9V Batteries connection to the DC link capacitors of Inverter

Figure 3 : 9V Battery Array

     This test is very important for High Power applications, because before energizing the inverter we should make sure that all of the fiber optical connections and control algorithm are correct. Also one of the most important thing is that the inverter's phase sequence should be fit the grids phase sequence. If the phase sequence and phase angles of each phase in mains are not completely fit with our control system, there may be excessive currents flowing through inverter. So we should check all the necessary connections and algorithms before we connect the system to the grid. 

     Each DC link capacitor is charged up to 1200 V DC and nominal current passed through each phase will be 600 Ampers in normal operations, however, we can do the test in very small DC link voltages (9 V batteries) with no load connected to the system so that we can check whether the IGBT's are turned on at the time it is fired or not. Also, we can check if there is a wrong connection or short circuit. 

     Test Results

     The 3 phase balanced 5 level voltage scheme is observed at the Oscilloscope screen as can be seen in figure 4. Note that the peak value of the waveform is 50 V, which is 5 batteries in series. The most important benefit of this test is that we realized that some of the fiber optical cables was broken while building the system in field. 

Figure 4 : Test Result in Oscilloscope

The batteries can be supplied from the links below.

http://www.pcrush.com/product/General-Purpose-Batteries/178657/Duracell-Coppertop-Alkaline-General-Purpose-Battery

http://www.bestbuy.com/site/9V+Alkaline+Batteries+(4-Pack)/9206575.p?id=1218059945520&skuId=9206575&st=duracell%209V&cp=1&lp=1

Creating 100V DC Source Using Batteries

     Mostly for testing purposes, systems may require DC sources. These sources may have different levels from 1.5 V to some hundreds of volts. The easiest way for creating a DC source is using commercial batteries. There are variety of batteries with different voltage levels such as 1.5 V or 9 V. In my application, I needed a nearly 100V DC source for charging a capacitor. For this purpose, I cascaded ten 9 V batteries in a form as figure below so that I could measure 96.55 V using a multimeter. 

Figure 1: 96.55 V DC supply with battery stack

     Due to the shape and terminal structure of 9V alkaline batteries, we can form a rigid stack of batteries for getting a different levels of DC source. One can easily obtain DC sources in a discrete levels of 9.6 volts since a fully charged battery has 9.6 V. One of the most important thing with this stack is that it has dangerous voltage levels while the count of batteries getting increasing. However, also note that each battery has internal resistance so the current that can be supplied through batteries is limited. In other words, the DC source is very useful but it has limited power due to internal resistance.

The batteries can be supplied from the links below.

http://www.pcrush.com/product/General-Purpose-Batteries/178657/Duracell-Coppertop-Alkaline-General-Purpose-Battery

http://www.bestbuy.com/site/9V+Alkaline+Batteries+(4-Pack)/9206575.p?id=1218059945520&skuId=9206575&st=duracell%209V&cp=1&lp=1