High voltage on utility transformer secondary

[jmbelectrical]

A house is supplied with a 240/120-volt, single-phase, three-wire service. No other houses are connected to the utility transformer's secondary side. The property owner is alleging that their phase-to-phase voltage rose to between 270 and 280 volts, resulting in the failure of numerous items. The utility company was allegedly notified and replaced the transformer in response.

Without knowing much of anything about the original utility transformer, and without having the opportunity to physically examine it, any thoughts as to what could have happened?

 

[RRaghunath]

It happens with transformers that are lightly loaded (in comparison to its rated capacity). The transformers are rated to have slightly higher voltage at their terminals when on no-load and this is intended to compensate internal voltage drop (in the transformer) that lowers the transformer terminal voltage as the transformer is loaded. If the transformer is loaded lightly, this becomes a problem as the voltage remains close to no-load voltage which is higher. Especially, during off-peak hours on the distribution network when the voltage on primary side of transformer tends to rise beyond the rated, the consumer voltage becomes unacceptably high.
One solution is to choose a tap (on the transformer tap switch) that is different from nominal and gives rated voltage even under no-load or lightly loaded conditions.

R Raghunath

 

[crshears]

The property owner is alleging that their phase-to-phase voltage rose to between 270 and 280 volts, resulting in the failure of numerous items.

Did the customer say this was a spike or surge, or instead claim that the condition was sustained for some time? It makes a difference . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[wcaseyharman]

I think we’d need to know more about the specifics to make a determination. It here’s a couple things I’ve seen before.
- was this continuous are confined to a certain event? We’d had low level ground faults that are too low or intermittent to trip anything, but create enough of a neutral shift to causes customer overvoltages.
- if the customer has solar panels during lightly loaded conditions the customer had export power, causing the voltage at the customer to be higher than the system. If the customer is close to a substation where the voltage is kept high, possible overvoltages can occur.
- A bad neutral, particularly between the transformer and the main panel board, can cause low voltage on one leg and high voltages on the other if the load is unbalanced.

Just some possibilities to consider.

 

[crshears]

If the customer is close to a substation where the voltage is kept high, possible overvoltages can occur.

All true, dub casey; one more factor to aid in following up on the above would be what time of day it was when the O/V occurred.

High mid-day or peak time voltage, if customer is close to the substation, could be due to heavy loading causing voltage to automatically regulate higher in order to ensure U/V does not develop at the hinterlands of the feeder.

So, as has been said often here: it depends . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[jghrist]

The utility company was allegedly notified and replaced the transformer in response.

Seems unlikely that a utility would replace a transformer in response to a high voltage complaint. They would first either change the taps on the existing transformer or adjust voltage regulators.

 

[bacon4life]

Hopefully utilities that use line drop compensation also have first customer protection enabled to prevent the substation voltage from exceeding +5%. Occasionally voltage regulators misoperate, but that affects all customers on the circuit.

When adjusting transformer taps, it can be challenging to track which transformers have had tap change adjustments. The ideal tap under the normal configuration could make voltage much worst during abnormal configurations.

There are lots of possible causes of overvoltage, but I have not thought of anything short term other than excessive solar export that would be resolved by changing out the transformer. A very unlikely possibility would be that the utility originally installed the wrong transformer, such as 13.812.47 kV/240 V instead of a 12.4713.8 kV/240 V transformer. [Edited to show an example of high voltage rather than low voltage.]

 

[waross]

13.8 kV/240 V instead of a 12.47 kV/240 V

Had something similar.
13.8kV transformer on a 13.2kV circuit.
The voltage was about 5% high.
The manager tracked the sum of the meters on each transformers to identify power theft.
He noticed that billing on that transformer was trending about 5% higher and asked me if I knew why.
I pointed out that the line crew should have set the tap don on that transformer.
The manager couldn't bring himself to forgo the extra revenue and the voltage was never lowered.
On a break even diesel system, almost all of the extra revenue went for fuel and there was little real gain.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jmbelectrical]

Did the customer say this was a spike or surge, or instead claim that the condition was sustained for some time? It makes a difference . . .

This was a sustained condition. The property owner does not reside at the house full-time, so its exact duration is unknown. The overvoltage condition was initially discovered by a technician who was in the process of replacing a pool pump.

 

[wayne440]

"...A very unlikely possibility would be that the utility originally installed the wrong transformer, such as 13.8 kV/240 V instead of a 12.47 kV/240 V transformer..."

"...13.8kV transformer on a 13.2kV circuit. The voltage was about 5% high..."

It seems to me that both of those instances would result in the "240V" side being low, not high. Am I losing my situational awareness?

 

[waross]

Memory fades, Wayne440
Good catch.
Our system was 13.2 kV
The country's National grid was 13.8 Kv.
Most of our transformers were 13.8 kV on 13.2 kV circuits with the taps set up.
13.8 kV transformers were what were available locally.
WE bought a transformer from the US instead of from the mainland, and got the correct 13.2 Kv rated transformer.
The crews set the taps as if it were a 13.8 and so the voltage was high.
Our generator step-up transformers were 480/7620 Volts so we were stuck at 13.2 kV.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[bacon4life]

You are correct Wayne. I edited to post to make the correction.

 

[Palletjack]

Couple of things..
A 13.2 xf on a 13.8 system (or vice versa) would produce voltages at 230V or 250V. Not 270 or 280
Maybe a 13.2 Xf on a 14.4 system? That would make the voltages that high, but it wouldnt have went high all the sudden. It would have been high at installation and should have been caught when the crew checked the voltage.

Not all distribution transformers have taps. Some are dual rated for something like 7.6 kV/13.2kV, but its expensive to install taps in every distribution pole transformer.

I have had transformers before that experienced some type of damage where the voltage got that high. Transformer bushing was blown out the top and several winding had shorted producing the high voltage.

Now, if the latter happened its extremely doubtful the utility will tell you that, even though they aren't liable for the damage.