Reverse Feed of Step Down Transformer 10MVA

In addition to explaining the alternate to ANSI vector diagrams

DYN11 being a Delta Wye transformer with secondary leading the primary by 30 degrees. (most common distribution configuration in US)

https://electricalnotes.wordpress.com/2012/05/23/vector-group-of-transformer/

This explains how winding configuration can improve transmission side voltage regulation, by coordinating the 30 degree phase shifts across the step-up transformer, since voltage regulation is controlled by secondary side generator excitation.

The winding configuration is dictated by the need for power system grounding and line protection, where ground faults are the most common event on a power transmission system.

No help on the construction difference though. The only practical difference I know of is anecdotal reports that the magnetizing current for a step-up design can be 25 times rated current if energized from the high voltage side, versus 12 times rated current for a step-down transformer design, energized from the primary side.

Voltage regulation across a transformer is a function of the impedance of the transformer, what it feeds/is feeding it, as well as the state of the overall system; and has little to do with the generator excitation, though they do interact with each other.

The winding configuration deals with the internal construction of the transformer, while vector group, phase shift, and grounding refer to the external connections at the terminals.

The construction differences are very critical as outlined in #12 in the previous thread.

"...the magnetizing current for a step-up design can be 25 times rated current if energized from the high voltage side, versus 12 times rated current for a step-down transformer design, energized from the primary side..." Sorry but it's just the opposite, energization at the low voltage side may result in up to 20 times rated current vs. up to 10 times when energized from the high voltage side. The energizing current also depends upon the where in the cycle it was previously de-energized, subsequently re-energized, and whether the transformer has any load attached, among other things. These are not "anecdotal reports" and can be achieved using well known calculations that have been verified via actual tests.

Unfortunately Power System Engineering does not lend itself to engineering via blogs, search engines, and/or handbooks, anymore than Googling on Cardiac Surgery will prepare you to do bypass surgery on yourself; you'll still have a fool as your patient.

Voltage regulation of a transformer is as you describe, voltage regulation of a power system is controlled by the supply and demand of reactive power, matching the demand of the loads and the components, such as power transformers, within the power system. As such, the utility generators control the voltage of your power system, along with transformer tap changers, banks of shunt capacitors, those are the typically manipulated variables to keep the power system voltage in control.

I believe that some unit generator step-up transformers are wound with the high voltage windings next to the core, with the low voltage windings as the outer windings. This can help with power system voltage regulation, as mentioned in the earlier link I referenced. That is my understanding why the magnetizing current characteristic is opposite of normal step-down transformer behavior, the low voltage winding has the higher air core reactance. I have not experienced this, but I have heard of this, anecdotally.

My statements are not anecdotal, but are based upon sound engineering practice, coupled with actual field experience. You can verify them by looking at your copy of the Westinghouse (now ABB) Transmission and Distribution Engineering Handbook, The J&P Transformer Book, and Stability of Electric Power Systems by P. Kundur, as well as others in the field.

btw- there are only two main variables that provide real time fine control of a modern grid; the amount of steam passing through the turbine, and the level of current in the generator field. Fuel/steam flow controls the frequency and real power flow, while field current controls the generator terminal voltage and reactive current flow. All other means of load flow control, such as phase shifters, tap changers, capacitors, etc., provide non-realtime, incremental control over slowly changing load, line, and voltage conditions. As time progresses, the FACTS (Flexible AC Transmission System) will enhance real-time electronic control of system wide operational variables.

Thank you so much for your reply.. I found the link really very informative.