Transformer Power Factor

[foleyspark]

How is "primary power factor" affected by secondary (load) power factor?

Example: Power transformer with purely resistive load vs. all inductive load – say 0.8pf (What is pf seen by upstream distribution?)

Does pf vary/depend on the % loading of transformer?

Does pf vary/depend on the size of transformer?

 

[davidbeach]

The transformer impedance is in series with the load impedance. On the secondary the pf is driven by the load impedance; on the primary the pf is driven by the sum of the load and transformer impedances. The transformer impedance is likely to be more inductive than the load (look for an X/R value for the transformer) but is also likely to be much smaller than the load impedance.

 

[foleyspark]

So if a utility feeds a building with one main transformer serving all resistive loads, the utility will “see” an overall load at near unity pf? Even though it feeds this inductive transformer?

The reason I ask is that two of the bigger claimed contributors to low power factor is motors and transformers. So transformers are not a big contributor to low pf unless they serve motors?

 

[Skogsgurra]

It is not neccessarily true that transformers are creating bad PF. Lots of idling transformers on a grid certainly do - especially as voltage goes up in light load situations - which increases magnetising current. But well designed transformers are seldom a problem when it comes to power factor. And your power distribution would be miserable without them...

There are special transformers for distribution that minimise the idling current, the Hexaformer is one that I have seen in operation.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

The transformer can be modelled at a very simplistic level by parallel and series elements, as shown in Figure 1 of the following link:

http://claymore.engineer.gvsu.edu/~johnsodw/egr325mine/paper2/paper2.html

An unloaded transformer appears as shown in Figure 2a, i.e. it is a lagging load formed by the parallel combination of the magnetising reactance and core loss 'resistance'. The magnetising reactance is dominant and the transformer appears as a small but highly inductive load when it is unloaded. The current drawn by the magnetising reactance is small relative to the rating of the transformer. If the transformer is grossly oversized compared to the load then the current in the parallel branch may form a sizeable fraction of the actual load on the transformer, and the overall power factor will suffer as a result.

The series elements of the Figure 1 model - the winding resistance and leakage reactance - also degrade the PF, but their impedances are usually much smaller than those of the load, and the load is therefore dominant in determining the power factor rather than the transformer.


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Experience is what allows us make a completely new mistake every day!

 

[jtkirb]

for all practical purposes, ignore transformers effect on power factor. loaded transformers do not effect PF and unloaded ones draw little vars due to the low wattage loss.

Focus on reducing load pf and slightly overcorrect and you should be fine.

JTK