The nominal line-to-line system voltages are normally used as the base voltages with 3-phase power being used as base power. Selection of the latter has some advantage of commonality when many studies are made while the former choice means that the impedance or reactance of at least one significant component will not have to be converted to a new base. The MVA assigned may be the MVA rating of one of the predominant pieces of system equipment or a more convenient number such as 10 MVA or 100 MVA. Base amperes and base ohms are then derived for each of the voltage levels in the system. It is common practice to assign study base values to MVA and KV. ![]() When any two of the four are assigned the other two can be derived. There are four base quantities: base MVA, base KV, base ohms, and base amperes. Any per unit impedance will have the same value on both the primary and secondary of a transformer and is independent of voltage level.Ī network of per unit impedances can then be solved using standard network analysis. These normalized impedances are known as per unit impedances. To use the per unit method, we normalize all the system impedances (and admittances) within the network under consideration to a common base. By removing the impact of varying voltages, the necessary calculations are simplified. The per unit system of calculation is a method whereby system impedances and quantities are normalized across different voltage levels to a common base. ![]() Input Motor Sub-tansiant Impedance (X" in Per Unit) :Ĭalculation of Motor Impedance on a Different MVA and Voltage Base Per-Unit Impedance Calculation for Motors For example, if you want to convert from meters to micrometers you would convert from 'base unit' to micro. Input Base Line-to-Line Voltage (KV LL ): Converters > Metric Units Conversion Calculator Metric Units Conversion Calculator Calculator Use Convert among units for any base unit of measure such as gram or meter or second or byte, etc. Input Capacitor Bank 3-Phase Rating ( KVAR): Input 3-Phase Short Circuit Current (kA):Ĭapacitor Bank Per-Unit Impedance - Given KVAR
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