A cycling and flow of knowledge and ideas.

Monday, 10 February 2014

Power factor and asymmetric loads

Two things which give power companies headaches are poor power factor and asymmetric loads.

Power Factor.

I have been looking at power factor explanations on the Internet and have been astounded that for something that appears to be straight forward, it often appears explained in a confusing and laboured way. Even Wikapedia avoids giving a full explanation. In fact, I have only found a single source on Youtube worth listening to. Sadly I have not found a single web source that explains it clearly. So I thought I would have a go with this article on power factor.

Most explanations of power factor only give half the story and although, "not" incorrect they are misleading.  Invariably, the explanations are from individuals or companies trying to correct a problem which, "as yet," does not exist for the typical domestic customer.

There are two main types of power factor: one is caused by reactive power such as fridges and the other is caused by non-linear loading such as electronic equipment, predominantly using switch mode power supplies.  The typical domestic electromechanical electricity meter, naturally ignores reactive load and only measures actual power consumed.

  • The caveat is that changes are afoot with the way metering and the mechanisms of metering. Consequently, the issue may rear its head at a later stage if domestic meters do not automatically compensate for reactive loads as they do today. These older meters are being phased out and there is a question if their replacement, "smart meters," will meter reactive loads.

    The domestic customers most likely to be affected are those with solar power arrays, who are exporting their generated power back into the low voltage mains network. Old electromechanical meters will happily run backward when power is being exported out into the low voltage domestic distribution network. Consequently, distribution companies replace these old mechanical meters with digital ones, that do not allow reverse operation. These new meters may not intrinsically compensate for a reactive loads and so may be affected by poor power factor.

As all billing is calculated with the reading from the distributor supplied meter, readings from these should always be considered the bottom line. When you see a lot of the demonstrations on products to correct power factor, they generally use readings from multimeters or various digital power meters, rarely are they the electromechanical meters used to meter the property.

Power factor is a measure of how effective an appliance is at consuming power. It is a rating, given as a ratio or percentage, and shows how close an appliance is to a perfect load, by power companies. Perfect is 100% and poor is 0%. Poor power factor is deemed to cause excessive loading to the power distribution network.

It is possible to have an appliance with a poor power factor but using power more efficiently than appliance with excellent power factor.

  • Take the example of an old tubular fluorescent light with a magnetic ballast.  This magnetic ballast will cause a poor power factor but the overall apparent power will still be less than that of the equivalent tungsten filament incandescent bulbs, with perfect power factor. A fluorescent light will use 4 to 5 times less power to produce the same lumen light output as the equivalent tungsten filament incandescent bulb. So, power factor is not a statement of efficiency.

  • Take the second example of two identical fluorescent tubes but one with an old magnetic ballast and one with a modern electronic ballast. They would produce the same light output and so the fluorescent light generating part of the appliance would have an identical efficiency.  The old magnetic ballast will have a poor power factor compared to the  modern electronic equivalent.  Consequently the apparent power would appear higher in the poor power factor version giving the impression it is consuming more power when it is not.  Thus power factor is a rating of efficacy rather than efficiency.

Power factor is a serious concern for larger companies who are penalised by the distribution companies if they have poor power factor. For domestic customers, the best point to fix poor power factor issues with appliances is at manufacture of the product in question. Having recognised this, the European Union (EU), has brought out legislation to combat this issue (IEC EN 61000-3-2). This standard has been adopted by non EU manufacturing nations such as China and the United States.

If you're interested to know more, how power factor is calculated then please read my article on power factor. Alternatively my article includes a link to a more definitive Youtube lecture on the its derivations and calculation.  The lecture takes the viewer from first principles and the topic, power factor, is well explained with plenty of diagrams.  Sadly the audio is bad and the lecturer is not a natural presenter but he does explain power factor clearly, in a way that no other source does (at this point in time anyway).

Asymmetric loads.

Asymmetric loads on the other hand, are really a problem for large buildings and companies which are supplied by 3 phase power.  With asymmetric loading, more power can be drawn from one phase than the other 2 combined.  This is a problem as the power drawn is unevenly distributed between the phases even though they are all equally supplied by the distribution network. The most likely offenders tend to be large office buildings with computer floors or small manufacturing enterprises. Energy distribution companies discourage this by charging the client at the rate of the highest phase for all phases.

When I worked with a UPS manufacturer this was one of their selling points.  The argument was as follows:

  • If you are supplied 3 phases and have all your appliances drawn from a single phase then you are charged at a rate equivalent to 3 times that of an evenly balance installation. In such situations, this problem is simply alleviated by using Uninterruptable Power Supplies (UPS) used as a load balancer.

  • The UPS takes the power evenly from all phases and supplies the internal wiring as required. This gives the owner and flexibility to add or remove appliances at will, without having to worry about asymmetric loading on any particular phase. Essentially, the UPS effectively decouples the mains distribution network from the customer's application of it.  As you can imagine a UPS of this type is neither small nor cheap.

Looking for hard evidence of this charging structure, and the associated meters, has come back without satisfactory references.  So I will have to put a question mark around this as a, "real issue," because I do not have any proof or evidence of this charging structure.

Fortunately it is not a problem for the standard domestic installation as most houses in the UK are supplied with a single phase.  Your neighbour might be on a different phase but that isn't a problem for the homeowner but more a problem for the distribution network.

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