Welcome to this course on the fundamentals of short circuit analysis. Here you will deal with asymmetrical three phase short circuit and fault analysis using system modeling. As a prerequisite to this course you should be familiar with per unit analysis and symmetrical components, both of which are the subject of two courses, having the same name and offered on this site pspp Academy. I do however, go through a quick review during this course of the fundamentals. But the affer mentioned courses will provide a more in depth understanding, which will be a great benefit to the subject of short circuit analysis. What are short circuit currents?

Short Circuit currents are currents that arise during unplanned fault conditions. These faults bridge two or more phases and or ground introducing large amounts of destructive energy in the forms of heat and magnetic force into a power system. Their reliability and safety of electrical power distribution systems depend on accurate and thorough knowledge of short circuit fault currents that can be present and on the ability of protective devices to satisfactorily interrupt these currents. knowledge of the computational methods of power system analysis is essential to the engineers and technicians responsible for planning, design, operation and troubleshooting of power systems. The risks associated with short circuit currents would be damage and or deterioration of the equip Also improperly protected short circuit currents can injure and or kill maintenance personnel. Why do I need short circuit analysis?

Short circuit analysis is required to ensure that existing and new equipment ratings are adequate to withstand the short circuit energy available at each point in the electrical system. A short circuit analysis will help to ensure that personnel and equipment are protected by establishing proper interrupting ratings of protective devices such as circuit breakers and fuses. If an electrical fault exceeds the interrupting rating of protective devices, the consequences can be devastating. It can be serious threat to human life and is capable of causing injury Extensive equipment damage and costly downtime. on large system, short circuit analysis is required to determine both the switchgear ratings and the relay settings. No substation equipment can be installed without knowledge of a complete short circuit values for the entire power distribution system.

The short circuit calculations must be maintained and periodically updated to predict equipment in lives. It is not safe to assume that new equipment is properly rated. Performing short circuit studies provides the following benefits. It reduces the risk a facility could face and it helps to avoid catastrophic losses. It increases the safety and reliability of power systems and their related equipment. it evaluates the applications of protective devices and equipment.

It identifies problems in the system. And it identifies recommended solutions to existing problems. There are several types of faults that can occur. I'm going to show them on a power line here but they could just as easily apply to a generator to a bus to a transformer to a feeder line. But for now we're just going to show how they apply to a tower line. The first one being your face to ground short circuit.

In other words, one of the lines would be touching the ground and and fault current would flow to the ground. You could have a face to face short circuit without touching any of the ground equipment where just two phases are touching together. This usually happens during windy conditions. Possibly lines would blow together. But it's just two phases that are being shorted together. You can also have a phase two phase two ground, and that would present a different set of parameters that you'd have to analyze.

You could have a three phase short circuit where you have all three phases shorted together. Now, the likelihood of this occurring naturally is very, very rare because one of the above two conditions are going to happen first either two phases are going to touch first, and the relay will be fast enough it'll take it out before the third one shorts with it. Usually though a three phase short circuit could occur when maintenance is being done on a line or a transformer and the maintenance crews leave a set of ground clamps on that is used for safety during the maintenance operation. That will definitely give a thief Face short circuit condition, you can also have a high resistance face to ground, which is could even be classified as an open circuit. And you could have an open face where there isn't any current flowing or large amounts of current flowing.

But the phase is not is broken and it's not continuing continuing the circuit through this type of fault is very, very difficult to detect. However, it is one of the few that we want to classify here. One of the things that we have to understand in the study of short circuit analysis is the phase relationship of face to neutral face to face quantities. So I'm going to spend just a couple of slides here just to make sure that you understand that in a balanced system, that is When the voltages in this case voltages and we could be talking about currents as well, but we're going to be just talking about voltages for now. The red to neutral, white to neutral and blue to neutral voltages are equal in magnitude. And they are separated by 120 degrees.

And if the rotation is counterclockwise as indicated here, then the red to neutral voltage leads the white to neutral voltage by 120 degrees and you'll pardon me for using green instead of white. If I put white you wouldn't see it. So I'm using green to indicate the white to neutral voltage and the blue to neutral voltage lags the white to neutral voltage by 120 degrees. So they're all separated equally by 120 degrees. If we were to measure the phase, two phase values of these factors, then the red to white voltage and the blue to red fold The voltage and the light to blue voltage would form an equal lateral triangle, as you can see here, and the internal angles of an equal lateral triangle, of course, are 60 degrees. If we zoom in on the bottom part of this triangle and look at the triangle that's borne by the blue to neutral, white to neutral and the white to blue voltage, you will see that it forms a triangle that has 30 6090 degree internal angles on it.

And the ratios of the magnitudes or the sides of those triangles are in the ratio of one, two and root three. That's just straight trigonometry. What this means is that the We are going to look at the magnitude of the voltage is only. And that is the length of the phasers that the magnitude of the red to neutral voltage over the magnitude of the red, the white voltage is equal to two, all over two times the root of three. And of course, the two twos cancel out, and we're left with the equation if we get rid of the fraction sign is root three times the red to neutral voltage is equal to red to white voltage. Now we're only talking magnitudes here for the present time.

This This means that regardless of what we're talking about in a balanced three phase system, the phase two phase quantity is equal to root three times the phase two neutral quantity Or the face to neutral quantity is equal to the magnitude of the face to face quantity divided by the root of three. Let's look at the phase angles of these phasers now. And I'm going to look at only the red to neutral and the red to white phasers in this diagram. So let's move the red the white phasor parallel over it over here. So you can see that the it forms a 30 degree angle with a red to neutral voltage. Now, I can go through the proof and fairly obvious but I'll let you do that on your own suffices to say that the red the white voltage is 30 degrees ahead of the red to neutral voltage.

So you can say the face to neutral is 30 degrees behind Phase two phase voltage given the rotation of being counterclockwise, or you could say the phase two phase voltage is 30 degrees ahead of the face to neutral voltage. or more commonly, it is stated that the face to neutral lags the face to face by 30 degrees or the face to face leads the face to neutral by 30 degrees. And this ends chapter one