An interesting set of Voltage Measurements - Old forum

[charz]

http://www.eng-tips.com/viewthread.cfm?qid=173479#post.

In this old forum I have read the following statement written by "waross"

"With a floating wye primary/wye secondary, the voltages on the secondaries are inversly dependant on the loads on the secondaries.
With no load on the transformers, the secondary voltages will be dependant on the exciting currents of the VT primaries."

If anybody could elaborate this further.

 

[HamburgerHelper]

I think that sounds more elaborate than what it is. The secondaries voltages always depend on the excitation voltage. If you want to look at it as current, the voltage on the secondary will be the excitation current times the excitation impedance.

The secondary voltages being inversely proportional to the load I think must be with how the neutral floats and helps balance out the loads. If he is talking about how the secondary voltage changes with load current, the voltage drop across the windings is linear with load current.

 

[davidbeach]

The VTs were apparently saturating near peak voltage on every half cycle. The saturation reduced the primary impedance of the one transformer at a time, moving the VT neutral point around in a circle in such a manner to cause all phases to read high. Replacing the 277V primary VTs with 480V primary VTs solved the problem.

 

[prc]

Davidbeach, If you use 480 V windings instead of 277 V, will not the voltage ratios change ? or you meant overvoltage factor ie reduced working flux density for the VT with rated voltage of 277V so that during neutral shifts VT will not go to oversaturation zone?

 

[waross]

Thank you for the closure, David.
Now it makes perfect sense. I have wondered at odd times over the years what the cause of those readings was.
charz
My comments about voltage being inversely proportional to impedance have more application to power transformers.
Let's start with DC:
Two resistors in series across a 240 Volts DC. The ratio of the voltages across the resistors will be inversely proportional to the ratio of the resistances.
Now consider two 120 Volt transformers in series across 240 Volts AC. If the loads are uneven, the first effect will be similar to the case of unequal resistors across a DC voltage.
But, a 120 Volt transformer will saturate at around 130 Volts to 140 Volts. Saturation will limit the voltage swings to about 140 Volts to 100 Volts.
On a three phase, three wire wye primary connection with a wye secondary connection, the voltage across the lightest loaded transformer will tend to level off at the saturation voltage.
You probably don't want to use this connection for distribution transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[charz]

Hi David,
In case if the system is solidly grounded, normally one would apply a 277/120 rated VT, Now If there is a over voltage on Primary then
do the VTs get saturated near peak voltage on every half cycle?

Hi Waross,
When the power transformer is loaded, the phase to ground voltages becomes normal but when it is at no load, the phase to ground voltages becomes abnormal. At the same time it is advised to put resistors at the secondary of the VT?
My question is how the secondary of the VT with loading resistor (or) loading the power transformer makes the VT to give normal phase to ground voltages?

 

[davidbeach]

It was an ungrounded 480V delta system with little bitty VTs. The bus in question had three generator breakers and two breakers to the delta side of the GSUs and was being energized through the GSUs. If a generator was connected there would have been a ground reference, but the instability in the bus voltage meant that a generator breaker could never close.

When changed out, the ratio may have change, but that's just a setting change so no big deal.

 

[prc]

1) In the market that I am familiar, PTs for such applications are specified as 277/120 V with a 1.8 over voltage factor.Client/consultant will also insist to energise units at 1.8 E for some hours during testing at factory. Scottf may correct me if my understanding is not correct. Such PTs are designed with a B of 0.8 T against 1.6 T, normally used. But even with such specifications also, neutral inversion can occur in PTs in ungrounded power system.
2) I faced such a problem nearly 30 years back. We were handing over a newly installed 315 MVA 400/220/33 kV transformer bank with tertiary formation by aluminum bus. There were 3 numbers 33 kV PTs with 1.8 overvoltage factor connected for ground fault protection on tertiary side. A broken delta(we here, call them open delta) secondary of 110/3 V was provided for detecting grounding. When bank was commissioned, 20-40V appeared occasionally across the broken delta and customer refused to take over the bank claiming some defect, some where. It took me couple of months to resolve it. I had no idea of this phenomena and with much difficulty I could collect the relevant papers (remember there was no internet or google)as attached. After studying all the literature, I came to conclusion that to solve this neutral inversion, there were three solutions- add a resistor across the broken delta, load the star connected secondary of PT with star connected resistors or add shunt capacitors across the primary 33 kV bus. Client was any way planning 33 kV Surge absorbers at tertiary terminals to mitigate the transferred surges. When these were put in to circuit, problem disappeared.
3) The reason for this is due to mismatching of magnetizing reactance of PT with zero sequence capacitance of the primary bus. In an ungrounded circuit of iron cored reactance with capacitance, oscillations can set in for a range of C and X, fed by a sinusoidal voltage.So this problem is more where the bus is short ie capacitance is less. With long lines this will not be there due to higher capacitance offered. The resistor will create equivalent voltage drop in primary so as to shift the inductive and capacitive branches going in to oscillations. Please see the 1931 paper by Boyajian (From GE, Pittsfield) for more details.