The term "Direction of Current" is one of the most complex terms in power engineering because that term has 4 divergent definitions.

The term "Direction of Current" is one of the most complex terms in power engineering because that term has 4 divergent definitions.

This article is part 2 of the first article on the direction of Current. If you need context, please read the first article.

Below is the first article I previously wrote on Direction of Current        

Above is the first article I previously wrote on Direction of Current
        

In this article, current stands for Alternative Current (AC).

Electrical power engineers, unfortunately, use the term direction of current in 4 different meanings. On some occasions, those meanings can contradict themselves.

Meaning 1- Definition of Direction of Current:?Current has no direction. See Figure 1 below, which shows sinewaves of current in Kansas, USA Power Grid.

Figure 1: Sinewaves of AC balanced currents in Kansas showing current has no direction

Meaning 2- Definition of Direction of Current: Current has a direction defined by this arrow on software or paper. See Figure 2 below, which shows the arrow next to current.

For this meaning, the direction of current is up.

Figure 2: Arrow next to the current showing direction of current is up

Meaning 3- Definition of Direction of Current: Current has a direction determined by 2 parameters:

Parameter 1: Current behavior

Parameter 2: CT polarity

See Figure 3 below, which shows direction of current: reverse and forward. Although the figure states fault current, the concept is the same for the nominal current.

In this definition, Current is reversed or forward depending on the 2 parameters mentioned above.

Figure 3: The current direction is either forward or reverse based on 2 parameters. The picture was taken from the SEL university manual.

Meaning 4: Current has a direction defined by 3 parameters

Parameter 1: current behavior

Parameter 2: CT polarity

Parameter 3: Line angle impedance (Z1). Other alternative options exist, such as negative or zero sequences, but we will simplify the article and use Z1. See Figure 4 below.

In this definition, Current is reverse or forward depending on the 3 parameters mentioned above.

Figure 4: Direction of current is either forward or reverse based on 3 parameters. The picture was taken from SEL 351S user manual.

4 people can talk about the current direction. Each talks about something different; sometimes, the meaning is entirely different.

Experts understand each other and have no issues; However, why is that field built for experts? Why do average electrical power engineers have to go through that hustle?

Explanation of Meaning 1 and Meaning 2.

Meaning 1 Current has no direction:

Everyone knows that AC changes direction 100 or 120 times per second, depending on your country. Electrical engineers agree that current has no (Absolute) direction.

Meaning 2: Current has a direction, as shown in the arrow in Figure 2

Electrical engineers needed ways to visualize circuits and mathematically solve power flows and circuit analysis problems, so arrow direction conventions were created. Unfortunately, we decide to use the same term direction of current and assign an arrow to it.

The arrow will be pointing toward the load during nominal conditions. So, if you have a motor, for example, during nominal conditions, the direction of current on the motor will be pointing toward the motor since it is a load.

Similarly, if you have a generator, the arrow will point away from it. You will also often see in some books where the author writes that the direction is assumed to be the flow during the positive half-cycle of the sine wave. It is similar to saying that current flows toward the load during nominal conditions.

See Figure 5 below from the famous book " Protective Relaying Principles and Applications" by J. Lewis Blackburn and Thomas J. Domin.

The direction of current is different when there is a fault because the fault becomes the load, so current flows toward the fault that becomes the load.

So now we use the direction of current to both say current has an arrow direction and at the same time current has no direction.

In meaning 2, the definition of current does not have the same connotation as in meaning 1. An electrical power engineer needs to understand which definition we use and the context; otherwise, you may be lost.

The most challenging of the definitions occur between meaning 3 and meaning 4. They even contradict themselves on some occasions.

In a later article, we will discuss those nuances and explain how an engineer made a mistake that led to a relay trip.

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