Lightning Discharges

     As a result of natural environmental phenomena, there may exist lots of  positive and negative charge centers in the air. The distance between those charge centers may vary up to 500 km long and their charge distribution changes. If the electric field density in the vicinity of a negative charge center reaches a critical value ,typically 10 kV/cm, the small length of air is started to be ionized and an ionized channel is formed. This ionized channel is also called "streamer". The critical electric field value for ionization is directly proportional to the breakdown value of air. This breakdown value can be changed by humidity and temperature but ionization is most likely start in large water droplets in air. 

     As I said, the distance between the positive and negative charges centers in air may vary up to 500 km ,thus, the excessive negative charges may prefer to discharge through ground (another positive charge center) which is much more closer to negative charge center. After first streamer ,about a couple of ten microseconds later, the second streamer arises and it follows the same way with the first streamer but it goes a little bit further than the first streamer. This process continues a number of times ,each time the ionized channel propagates 10 to 200 metres, up to the ionized channel is reached to a critical distance to ground. This process is called "stepped leader". Each time the stepped leader propagates in air, an excessive amount of negative charges move towards ground. When the stepped leader reaches to a distance of 10 to 50 metres from earth, the field intensity of the earth (positive charge center) reaches a sufficient value to form an upwards streamer in order to make connection between stepped leader and earth. Then the excessive amount of current flows from earth to clouds along ionized path so that the negative charges neutralized. This neutralizing current is called "return stroke". The figure below explains the whole characteristics of lightning discharges.

Figure 1: Lightning Discharge Phenomenon

Reference: EE575 - Advanced High Voltage Techniques (METU)

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