Mike Dolt Enterprises
Mike Dolt Enterprises
User: ee101
Asked on: Thursday, April 21, 2011 at 10:13 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:31 am EDT
Question: Howdy, I have a unique problem to propose to everyone. I would like some input, as I am looking for an explanation.First a back round:I work a power plant, that recently updated the entire electrical system. We have two generators that are configured in a very similar manner. The generator feeds 3 transformers from iso-phase (1 GSU and 2 AUX transformers). The axillary transformers are are all 3 winding transformers that are generator voltage on the primary, and 7200 and 4160 and the secondary and tertiary. For clarity each transformer has a letter identification A or B and feeds either 7200A, 7200B, 4160A, and 4160B. Large loads are split between the buses. All our major fans (4000HP+ are 7200V).Second the problem (not so much a problem as an unexplained observation):Lately we have been utilizing our new digital relays to observe voltage fluctuation during large fan starts. A clear pattern has started to develop. When we start a large fan (which would be on the 7200V bus) we actually see a voltage increase in the 4160 of the corresponding transformer. It happens on both units (despite one being 2 times the size of the other) irregardless to how heavily loaded the buses are (loading does slightly change the magnitude).Little more information:We are pretty sure the spikes are actually happening. We have testing using other PTs and it happens on both units (which are mostly independed of each other).The spikes are approximately equal to the voltage dip in the 7200 times the turns ratio with the same time duration (typically 100V for 20 seconds).The opposite aux transformer sees a very short voltage dip on both buses.I have my thoughts on why it is happening, but I want to see what your independent thought is.Thanks
Possible Explanation
The observed voltage increase in the 4160V bus during large fan starts on the 7200V bus can be attributed to the following phenomenon:
Magnetic Coupling between Windings
In a 3-winding transformer, the primary, secondary, and tertiary windings are magnetically coupled. When a load is applied to one winding, it induces a magnetic flux that affects the other windings.
Flux Shift Due to Fan Start
When the large fan starts on the 7200V bus, it draws a significant amount of reactive power. This causes a sudden shift in the magnetic flux in the transformer.
Flux Interaction
The shifted flux in the 7200V winding magnetically interacts with the 4160V winding, inducing an electromotive force (EMF) in the 4160V bus. This EMF causes the voltage to increase.
Turns Ratio and Voltage Amplification
The 4160V bus is connected to the tertiary winding of the transformer, which typically has a lower number of turns than the secondary winding. Therefore, the EMF induced in the 4160V bus by the flux shift is amplified by the ratio of the secondary to tertiary turns.
Time Duration
The voltage spike on the 4160V bus lasts approximately as long as the voltage dip on the 7200V bus because the magnetic flux shift is a transient effect that gradually decays.
Short Dip on the Opposite Aux Transformer
The opposite aux transformer sees a short voltage dip because the magnetic flux shift in the transformer's primary winding also influences the tertiary winding, which is connected to the 4160V bus of the opposite transformer. As the magnetic flux shifts, it creates a brief EMF in the opposite direction, causing a temporary voltage dip.
Conclusion:
The observed voltage increase on the 4160V bus during large fan starts on the 7200V bus is a consequence of magnetic coupling between the transformer windings. The sudden flux shift induced by the fan load causes an EMF in the 4160V winding, which is amplified by the turns ratio and results in a transient voltage spike. Flag for review