3 phase transformer tap changes savings 5

[gwbissett]

Somebody mentioned that by changing the taps on a three phase transformer, it is possible to have a consumption saving.I do not agree cause, if the voltage goes up, the current will go down equally and supply the same amount of power OR because the load stays the same the voltage goes up and the current goes up- thus power goes up - more consumption? should lower the voltage-lower current - lower power - lower consumption?
Is there an impedance savings or something I am missing?

 

[Marmite]

There are indeed savings to be had. Several of the large supermarket chains in the UK have done this and saved a lot of money. There is also a trial running in some distribution Companies of a product called VPhase which can be applied to domestic properties. The savings come from the standardisation of voltages across Europe at 230/400V. This means that all new equipment/motors/lighting etc are designed to work satisfactorily on that voltage. The realistic voltage a customer receives in the UK is likely to be somewhere between 240-250V. The excess voltage results in wasted heat, particularly in induction motors, so if you reduce the voltage you reduce the waste heat.
Regards
Marmite

 

[dpc]

In general, if voltage is reduced, power demand will go down. How much it goes down depends on the types and mix of loads. Motors look somewhat like constant kVA loads, so there will not be much improvement there. But heating, lighting and other loads behave more like constant impedance loads. So lowering the voltage to these loads does reduce the power they draw.

However, energy usage may not change in the same proportion as power demand. An electric water heater operating at a lower voltage will draw less power, but it still takes the same energy to raise a volume of water to a particular temperature, so the heating element just stays on longer.

We work with utilities that routinely reduce voltage to their customers in order to reduce their peak demand charges for wholesale power.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[waross]

The small utility that I used to work with for a time tracked the billing on a transformer by transformer basis. A distribution transformer was replaced with one with the taps inadvertently set high. The billing on that transformer increased about 5% in relation to the other transformers on the system.

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

 

[ppedUK]

The short answer is, it depends on what type of load you are looking at. dpc got it spot on!

If you are talking about resistive loads and lighting, then there are possible savings. If you are talking about induction motors, there are none, in fact if you reduce the voltage for a given absorbed shaft power, then current has to increase to develop that same power and the winding IR losses increase accordingly, so the motor actually generates more heat than at a higher voltage.

Be careful in motor installations designed at say 415V and then later reduced to 400V. The losses in the cabling will also increase because the volt drop will increase and then you might be operating the plant outside of the Regulations it was designed to.

 

[redfurry]

Look up the terms "constant power", "constant current" and "constant impedance" used to describe idealized loads.

A constant power load will draw the same power regardless of voltage.
A constant current load will draw power proportional to voltage.
A constant impedance load will draw power proportional to the square of the voltage.

While real loads don't fall neatly into these categories, they are typically responsive to voltage.

On some systems when they are close to brownout conditions (heavy system loading), a standard procedure is to drop the distribution system voltages by a few percent in order to reduce loads.

 

[stevenal]

http://www.beckwithelectric.com/infoctr/appnotes/App16.pdf