Visit Count: 593 The Role of Zero Sequence Current Have you wondered ever that in spite of the simple Yyn connection of transformer; why it is necessary to provide zero-sequence filtration for differential protection or for radial feeders; why receiving end line CB trips on distance protection? T he cause of such things is the formation of zero sequence currents during faults. How they are formed, the detailed calculations were done by one of our members Er. J. R. Kulkarni. Besides great insight into the power system, it is a very tedious and time-consuming job. Letus go through the excel sheet to maintain this enthusiasm He has shared the respective Excel sheet here. Here is a screenshot for the excel sheet. To download the excel sheet click on the screen shot. Here are the comments from him about the calculation. TFR LV side faults Due to the tertiary or due to the tank 'phantom tertiary' effect, the zero-sequence current on the primary side does not maintain the TFR ratio with the zero-sequence current on the secondary side. The total ATs on HV shall be equal to the Secondary ATs plus tertiary ATs. Although I1 and I2 are in the TFR ratio (Io is not as per TFR ratio), due to the above, the phase currents shall not be in the TFR ratio. This may cause mal-operation of the TFR diff relay on through faults. Hence Io removal reqd. Grid faults The zero-sequence currents do no maintain their ratio due to the presence of the tertiary or the tank phantom effect. This can cause mal-operation of the diff relay. Single-phase units: If the tertiary is not used, and if there is a source on LV side too, I1, I2, and Io are transferred between HV and LV Phase currents maintain their ratio. However, the presence of the tertiary helps reduce voltages in un-faulted phases. But then Io does not maintain the ration bet HV and MV. Hence removal of Io is required. As an additional exercise; the calculations are validated using PowerWorld software. As an additional exercise; the calculations are validated using PowerWorld software. It is good for enthusiastic to download PowerWorld free edition for the student. It is always a good habit to read computer and operating system requirements before installing any s oftware into our computer. It avoids further frustration. Download the software from here PowerWorld Simulator – Version 21 This version is licensed for educational use and and limited to 13 bus (may confirm....). The following are screenshots for the downloading procedure. After downloading the pw21EduEvalSetup.msi file you can install the software on your computer. These are screenshots for validation done through PowerWorld. You can download the respective case file of PowerWorld from here- 3WdgIct.zip.