That is the case for this example.įor base power, I’ve arbitrarily picked T1’s MVA rating for the system and the voltage ratios of the transformers for the base voltage in each zone: If all transformer ratio’s match, then the secondary voltages of all upstream transformers are equal to the primary voltage all downstream transformers and vice versa, then your voltage base in each zone will be equal to the primary and secondary voltages of each transformer. Similar for voltage, it is advantageous to pick one transformer and use either the primary or secondary voltage as the base voltage in a particular zone, and then use the remaining transformer ratios to step up or down the voltage base for each neighboring zone accordingly. Why this is advantageous will become clear when we run the math. If the problem does not assign the base values for you, then it is advantageous to pick one of the existing MVA values in the system such as the apparent power rating of one of the machines. If it does, use them accordingly as the answer choices will most likely still be in per units and using a different base will change the resulting per unit system values. The problem might tell you to use specific values for base power and voltage. The base power will be the same in for each zone, but each zone will have a different base voltage. The next step is to choose the base values for power and voltage. (Go back to top) Step 2: Assign Base Values The first step is to illustrate this by drawing a straight line through each transformer: In this example, there are two transformers that divide the system into three different voltage zones that are created by the stepping up or stepping down of voltage by each transformer. The usefulness of the per unit system is in converting all system impedances to per unit impedances and re-drawing the circuit without having to worry about the different voltage levels from each transformer. (Go back to top) Step 1: Separate by Voltage Zones Assume both transformers are either delta – delta or wye – wye connected and that there is no phase shift between primary and secondary current and voltage. Using the Per Unit system and taking into account the transformer percent impedances, solve for the current in each part of the three-phase system shown below. Step 11: Check Your Work Using the Transformer Ratios.Step 10: Calculate the Actual Current in Each Zone.Step 9: Calculate the Base Current in Each Zone.Step 8: Use Ohm’s Law to Calculate the Per Unit Current.Step 7: Calculate the Per Unit Current and Per Unit Voltage.Step 6: Calculate the Per Unit Impedance for Transformer T2.Step 5: Calculate the Per Unit Impedance for Transformer T1.Step 4: Calculate the Per Unit Impedance for Each Zone.Step 3: Calculate Base Impedance for Each Zone.What’s in this article? – Per Unit Example – How To, Tips, Tricks, and What to Watch Out for on the Electrical PE Exam: To help clear things up, here is an in-depth break down on an example question that is similar to what you can expect to see on the NCEES Electrical Power PE Exam.Įlectrical PE Review – Base Changing Percent Impedance and Per Unit ImpedanceĮlectrical PE Review – Calculating Base Impedance with Three Phase vs Single Phase Values If you’ve never used it before, it is easy to become confused with all of the individual pieces that make the system work. You can opt out of Google Analytics tracking by installing this browser extension Requests for Informationīy entering and submitting information in any Request for Information form on the University of Arizona Online website, you consent to being contacted via phone, email, text message and/or pre-recorded message by the University of Arizona Online and its affiliates.Click here to print this article for your exam references! There is a lot of confusion surrounding the per unit system, and rightfully so. More information about Google’s user data security policy for Google Analytics Google Analytics Demographics and Interest Reporting Google Display Network Impression Reporting Google Advertising features we use may include: We utilize first- and third-party cookies to measure website and campaign effectiveness, but no personal information is collected automatically. The University of Arizona Online website collects anonymous visitor data using Google Analytics. View the University of Arizona’s electronic privacy policy Google Analytics View the University of Arizona's Privacy Statement Electronic Privacy Policy
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