System Voltage Phasing Diagrams

[redtrumpet]

Is anyone aware of any standards for, or have references for or experience in, constructing system voltage phasing diagrams?

Our client has several systems that were originally designed as radials. They now wish to tie together the secondary side of these systems to allow parallel operation if desired. All systems step down from 63 kV to 13.8 kV through delta-wye transformers.

Unfortunately, there are a hodgepodge of primary connections read H1-H2-H3: A-B-C, C-B-A, C-A-B, and B-C-A. This results in 60/180 degree phase shifts on the secondary X winding between the ccw and cw primary-connected systems, and 120/240 degree phase shifts between ccw primary-connected systems with rolled leads. We are constructing a phasing diagram to indicate the phase relationships between systems in order to document the present arrangements and to plan corrective action.

There are four engineers, including myself, with input to the diagram. However, we all have differing opinions as to how to represent this information on the phasing diagram.

I have reviewed ANSI C57.12.70 and Stevenson's treatment in "Elements of Power System Analysis". However, they seem far from definitive in actual standard practice for representing phasing on the drawing. We wish to maintain the phase sequence between the H and X windings ie. if H winding is C-B-A, then X winding is c-b-a. C57.12.70 and Stevenson seem to support arbitrary relabeling of phases on the X winding. If we maintain sequence between H and X, does this mean we only show a maximum 30 degree shift in our X phasor relative to our reference, rather than the 30, 90, and 150 degree shifts depicted in ANSI and Stevenson? We plan to use the 63 kV system phasor as a reference with A at 12 o'clock and ccw rotation. Any ideas?

 

[peterb]

Sounds like a real mess. I would suggest the following approach, based on experience with somewhat similar situations -
- Start with the system single line, showing all relevant voltage levels (it's useful to do a 3-line for this purpose, but it does get complicated)
- Show the appropriate vectors at each voltage level; I would suggest that you keep the phase sequence info constant (e.g. A-B-C) and show the actual terminal ID (e.g. X1-X2-X3) alongside the vector
- Note that this assumes that the terminal ID remains constant through a voltage level; if not, then you would need to show vectors with the ID at each piece of equipment
- You probably know this already, but its worth repeating - when you energize a 30 deg lag transformer with reversed phase sequence, it becomes a 30 deg lead transformer - just to complicate things a bit more

Hope this helps to get the discussion started.

 

[gordonl]

I've never had to deal with this problem on this large a scale, but why not just add a voltage phasor diagram to each level of the system on the single line with the 69kV as the 0 deg reference. This would provide phase rotation and phase shift info in a small package. I would also think a simple three line beside each transformer with bushing and phase identification would be handy.

Sorry no standard, only opinion.

 

[redtrumpet]

gord - this is basically what we have done. It sounds simple until you actually try it. Then the issues come up, like:
- 63 kV reference phasor, A phase at 12 o'clock, ccw rotation

Now you come to a delta-Y transformer. H1-H2-H3 are connected C-B-A. Is the secondary side phasor:
- still rotating a-b-c ccw to match the reference, or a-b-c cw to reflect the fact the transformer phasing was reversed on the primary?
- if you choose the former, how do you show that the terminals X1-X2-X3 are rotating the opposite direction of your reference phasor? Do you put a cw arrow on the "Y" of your delta-Y symbols? Do you put the terminals on the phasor diagram, as peterb suggested, in which case the reversed rotation is not explicitly shown?
- do you assume an arbitrary labeling of the X winding terminals as a-b-c, regardless of primary connection, so that your secondary phasor now has +/- 30, +/- 90, or +/- 150 degree shifts from the reference phasor instead of +/- 30 shifts only?

The answers to these questions and many more on the next episode of Soap.

 

[BJC]

It's one of my hang ups - Ilke to see the Y-D Y Dela symbols drawn correctly on drawings. Most of the time you see a "Y" and a "Delta" drawn happ -hazzardly on the drawing.
The legs of the "Y" and the sides of the deltal should be drawn parallel to each other.
That and drawing the phasors at each transformer with phase shift show can make things easier.

 

[stevenal]

Redtrumpet,

We connect two ways, either H1 H2 and H3 connects ABC or CBA. Secondary follows. The result is a secondary that lags or leads the primary by 30 degrees, and 60 degrees out between the two secondary systems. Forget Stevenson's connection. I simply use a transformer nameplate phasor diagram, and draw a CCW arrow on the first type of connection and a CW arrow on the second. Please note that ABC, BCA, and CAB are all the same as long as the secondary labeling follows the same pattern. Likewise, CBA, BAC, and ACB are the same.

If that's not confusing enough, I know a utility that buys reconfigurable transformers. What's on the nameplate may not be how it's connected inside.

 

[gordonl]

To my own thinking, ABC would always be the ref no matter what the bushing connection, so the ref is always set with A at 0. Then I would personally label abc as identified in the system, because in your plan I imagine a requirement will be that phase rotation must not change, so that the change is seamless to the user. Your phasors will always be ccw, simply the order of rotation changes, ie abc or cba. This will put all systems on the same base for easy comparison. The transformer three line with actual phase identifications as in the phasor diagram will provide the terminal connection data.

 

[peterb]

I'm with you, bjc, on the correct orientation of the wye-delta symbols being shown on drawings.
Gordonl, I think that we are saying the same thing when it comes to keeping the phasor labelling and rotation constant - it then becomes a matter of correctly labelling the equipment terminals as to which phase is actually connected to which terminal.
It gets a little more complicated when you try to show the phasing of overhead lines. What I have done there is to show a sketch layout of the conductors for each line section where there is a configuration change, identifying the phases of each conductor.

 

[stevenal]

Gordon,

Please note that transformer nameplate phasor diagrams show no phasor in the 0 degree direction (pointing to the right, please, 0 degrees is only up on a map). To adopt this standard, all diagrams must be redrawn. Also, there is no possibility of cba rotation unless secondary connections are arbitrarily designated. Phase rotation is a separate question from how (correctly rotating) phases physically hit the transformer bushings and the resulting phase shifts. The CW arrow I spoke about above shows the rotation direction of a phasor diagram consisting of Hs and Xs only. If shown, ABC and abc would always rotate in order.

For comparison, I just pulled out my copy of a BPA transformer nameplate. Since they are big and government owned, they set the standard, right? Anyway, transformer is an auto with a delta tertiary, with an ANSI standard diagram. BPA added a diagram called "BPA vectors", with bushing designations labeled with phase connections. H1 and X1 are pointed right, labeled A. X2, H2, and B are at -120 Y1, located at -30 degrees is labeled A.

Another problem is with equipment manufacturers who helpfully label bushings, CTs, etc. as ABC, when we cannot connect them that way. Numbers are better, please.