Voltage subscripts:

[rattus]

There seems to be two schools of thought on the use of double subscripts with voltages. Most of this forum seems to use the convention that Vab means the voltage on node A relative to node B. Makes sense to me because Va by itself means the voltage on node A with the reference being implied or understood. To be absolutely clear though we add a second subscript to indicate the reference node. In other words, the voltage drop from A to B.

Some however use the convention that Vab means the voltage rise from A to B which has the opposite meaning.

What say you?

 

[jim dungar]

Some however use the convention that Vab means the voltage rise from A to B which has the opposite meaning.

Subscripts indicate from and to, which means there is an inherent directionality.
I don't know how many times I have posted Vbn=-Vnb.

A single subscript is almost useless. Vb tells me nothing about where the voltage was measured; line-line, line-ground, line-neutral.

Assuming an 'always' reference point is poor engineering.

 

[charlie b]

I learned the basics of power systems engineering from William D. Stevenson Jr.'s textbook, "Elements of Power System Analysis." That was the first course I took in my master's program, and twenty five years later I still have that textbook. Stevenson's description of the dual notation convention agrees with yours. If during one half cycle, positive current is flowing from node "a" towards node "b," the voltage at node "a" will be higher than that at node "b" (due to voltage drop along the path), and therefore Vab will be positive during that half cycle.

 

[jim dungar]

After doing some more digging, it appears my texts never defined 'from-to' or 'to-from' as being associated with 'drop', nor 'rise' because the double subscript notation was used with currents and impedances as well as voltages.

When doing mesh analysis, if the direction of my answer resulted in a negative number then I simply had chosen the wrong 'from-to' (i.e. 'to-from') not that I was evaluating a source (i.e. voltage rise) instead of load (i.e. voltage drop).

In practice however, I do tend to list voltages as from high to low, so you do not know if I am discussing the voltage across a source or a load: Vab = IZab.

 

[mivey]

My reference texts also agree with rattus. I might could dig through and find some that say otherwise but, to the best of my recollection, most of them use the same notation.

I do remember some old texts that had something different on the notation but I did not commit it to memory because I did not consider it a modern standard. Was it Edith Clark's book or some earlier text from around 1900's? I would have to look when I get back to my library but it is probably not important.

 

[rattus]

In a nutshell:

In a nutshell:

My reference texts also agree with rattus. I might could dig through and find some that say otherwise but, to the best of my recollection, most of them use the same notation.

I do remember some old texts that had something different on the notation but I did not commit it to memory because I did not consider it a modern standard. Was it Edith Clark's book or some earlier text from around 1900's? I would have to look when I get back to my library but it is probably not important.

Then Vab mean the voltage rise from b to a, or the voltage at a relative to b, or with respect to b.

 

[mivey]

Then Vab mean the voltage rise from b to a, or the voltage at a relative to b, or with respect to b.

yep.

 

[mivey]

From Blackburn's "Protective Relaying" and his description of notation used in his text:

In the circuit diagrams, current or flux is shown by either (1) a letter designation...with an arrow indicator for the assumed direction of flow; or (2) a letter designation with double subscripts, the order of the subscripts indicating the assumed direction. The direction is thus assumed to be the flow during the positive half-cycle of the sine wave ... The single subscript, such as Is, is a convenience to designate currents in various parts of a circuit and has no directional indication, so an arrow for the direction must be associated with these. Arrows are not required with Iab, Ibc, or Icd, but are often used for added clarity and convenience. It is very important to appreciate that, in these circuit designations, the arrows do not indicate phasors. They are only assumed as directional and locational indicators.
...

Voltages can be either drops or rises. Much confusion can result by not clearly indicating which is intended or by mixing the two practices in circuit diagrams. This can be avoided by standardizing to one practice. As voltage drops are far more common throughout the power system, all voltages are shown and are always considered to be drops from a higher voltage to a lower voltage during the positive half-cycle
...

The consistent adoption of only drops throughout does not need to cause difficulties. A generator or source voltage becomes a minus drop because current flows from a lower voltage to a higher voltage . This practice does not conflict with the polarity of equipment, such as transformers, and it is consistent with fault calculations using symmetrical components.
...

Voltages (always drops) are indicated by either (1) a letter designation with double subscripts; or (2) a small plus ( + ) indicator shown at the point assumed to be at a relatively high potential. Thus, during the positive half-cycle of the sine wave, the voltage drop is indicated by the order of the two subscripts used, or from the ??+?? indicator to the opposite end of the potential difference... It is preferable to show arrows at both ends of the voltage-drop designations, to avoid possible confusion. Again, it is most important to recognize that both these designations in the circuit diagrams, especially if arrows are used, are only location and direction indicators, not phasors.

 

[jim dungar]

From Blackburn's "Protective Relaying" and his description of notation used in his text:

Wow, just like I use as my standard practice.

 

[mivey]

Wow, just like I use as my standard practice.

One of the better resource texts, IMO. At least on protection topics.

 

[jim dungar]

One of the better resource texts, IMO. At least on protection topics.

Another is "The Art and Science of Protective Relaying" by C. Russell Mason.

Effectively this is Westinghouse's practices versus GE's.