![ole miss electrical engineering calculators ole miss electrical engineering calculators](https://engineering.wp2.olemiss.edu/wp-content/uploads/sites/301/2020/10/online_classes_IT3.jpg)
![ole miss electrical engineering calculators ole miss electrical engineering calculators](https://news.olemiss.edu/wp-content/uploads/2019/02/Lei-Cao-2048x1150.jpg)
Now that we know the per unit line and load impedances, let’s draw them on the per unit circuit diagram: The line per unit impedance and load per unit impedance are calculated as shown below: We will start by calculating the per unit line impedance and per unit load impedance first before we tackle the transformers. Now that we have calculated the base impedance for each zone, we can start calculating the per unit impedances of each system element using the following formula: (Go back to top) Step 4: Calculate the Per Unit Impedance for Each Zone We can solve for impedance using voltage and power, and if we use the base voltage and base power in each zone, then the resulting impedance will be the base impedance for each zone as shown below: Now that we have the assigned base values for power and voltage, we’ll need to calculate the base impedance for each zone so that we can use it to calculate the per unit impedances later on. (Go back to top) Step 3: Calculate Base Impedance for Each Zone 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:
![ole miss electrical engineering calculators ole miss electrical engineering calculators](https://news.olemiss.edu/wp-content/uploads/2012/12/DamonWall-231x300.jpg)
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
![ole miss electrical engineering calculators ole miss electrical engineering calculators](https://engineering.wp2.olemiss.edu/wp-content/uploads/sites/301/2020/05/AdamSmith.jpg)
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.