Electrical Per-unit Calculation

The per-unit method of calculation in its basic concept is to develop a counterpart impedance or reactance network diagram of the power system involved and resolve these impedances down to a single impedance value through delta, wye, series, parallel conversion equations. While it is possible to do the calculation with the actual system ohmic values, it becomes very complex when several voltages levels are involve since ohmic values at different voltages are not directly compatible. With the aid of the per-unit mathematical technique, it has facilitated this difficulty. While the equivalent diagram and the per-unit mathematics are two separate fields, they usually are used together and referred to as the per-unit method of determining short circuit values.

For Medium voltage power system usually benefit the application of the per-unit
method because the analysis normally undertaken involves a relatively few specific points in an existing system. Also in these system, reactance usually far exceeds resistance high X/R ratio, permitting resistance to be ignored, which greatly simplifies the mathematics.


The primary advantages of the per-unit system are as follow:

1. The per-unit values for transformer impedance, voltage and current are identical when referred to the primary and secondary line ( no need to reflect impedances from one side of the transformer to the other, the transformer is a single impedance).

2. The per-unit values for various components lie within a narrow range ragardless of the equivalent rating.

3. The per-unit values clearly represent the relative values of the circuit quantities. Many of the ubiquitous scaling constants are eliminated.

4. Ideal for computer simulations.

The need for fault calculation in design

After my seminar last March 2007 it was a three days seminar on fault calculation
at Cebu Philippines. I become aware with my work when making Design and Estimate, it
is also an eye opener for all Electrical Engineer especially to the newly engineers,
this what happen.

Contrary to the practice of many local engineers, Fault Calculation must precede any effort to procure system protection devices. This activity is supposed to be one major part of the design process, but is oftentimes skipped or omitted.

Several provisions of the National Electrical Code, the Philippines Electrical Code
& IEEE publications ralate to proper system protection. Safe and reliable operation
of the industrial plant based on these provision mandate that electrical system must
be protected adequately & effectively.

While over-current protection devices are provided for overloaded protection for system components such as switchgears, busses, wires & cables, motor controllers, etc. it is also necessary to place protection for more damaging events such as faults. To obtain a reliable operation and to assure that system components are protected from damage during abnormal events, it is necessary to firts calculate the fault duties at various points in the electrical system while still on the drawing boards and adequate protective devices must subsequently be in place to anticipate these faults.

For all possile conditions, it is the responsibility of the system designer to design
electric power systems with adequate control of short circuits as one major consideration. It is also the plant engineer's responsibility to see to it that the protective devices are armed to predetermined settings either by himself or by consultants. As can be recalled, uncontrolled short circuits can cause service outages with accompanying production downtime and associated inconvenience, interruption of essential facilities, extensive equipment damage, personnel injury or fatality and a possible full-blown fire.

Again as the system designer is responsible for the selection of the right equipment, and would generally have the task of calculating system short circuits, procedures and techniques for these calculations are not generally available in on reference dissertation but are scattered among many publication and technical papers.


Fault calculations result to at least three very significant outputs which will
become the bases of the following:

1. First : proper selection of protective equipment ratings as circuit breakers or fuses that suit to system requirements :

2. Second : realistic arming up of protective relays to trigger operation of circuit breakers once faults do occur :

3. Thirdly : Proper coordination of operation of these protective devices to effect selective interruptions of the only required breakers to trip faulted circuits without the hassle of rendering the other portions of the system powerless.

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