Axino-Tech Consulting & Services
14th April, 2011
The recent demise of yet another Compact Fluorescent Lamp (these are also inappropriately known as 'long-life bulbs') after less than one year in service, reminded me I had some appliance power quality measurements that may be of interest. The appliances measured were a Draco DVB-T receiver, a CFL, a Samsung TV and a Zalman ATX computer power supply. Results for these are shown further down the page. Go to measurements
Power quality is essentially the 'goodness' of the AC power mains. Ideally the AC power mains should be rock steady at 230 volts and 50Hz (or per your local standard) for 24 hours a day. Many factors degrade power quality and result in variation of voltage and frequency over time. These variations may happen over long terms such as months, medium term over hours, short term in minutes and seconds. Then there are factors like transient conditions, surges and flicker. Distortion of the mains is caused by poor load power factor and introduced harmonics. The entire subject is comprehensive and is of considerable interest to power engineers, power utility companies and operators of large industrial plants as well as some larger commercial operations.
This paper demonstrates poor power factor due to harmonics of current created by domestic appliances. There is considerable reading on the theory of power factor available on the internet. I do not intend to cover the theory in detail and add (much) to the 'noise' out there.
I will add that some of the information that I found via the internet is wrong. I know this solely because some sources that I read will state one thing and others are contradictory. They cannot both be right. There are some internet links to the theory of power factor at the end of this article.
A long time ago, when I did my electrical theory training, the definition of power factor (PF) was defined as simply the cosine of the angle between voltage and current. When the current is perfectly in phase with the voltage, the angle is zero and the cosine of the angle is 1. This is the best possible power factor. PF=1 (or sometimes stated as 100%) is good. If, for example, the current was exactly 90 degrees from the voltage, the cosine is 0 and this would be the worst possible power factor. So, power factor goes from 0 (very poor) to 1 (best possible). You can have it negative or positive but that only tells you whether the current is ahead of, or behind, the voltage. For the purposes of this argument, the sign doesn't matter. The current vector can be resolved into the in-phase component (resistive or real) plus the quadrature component (reactive or imaginary). I dislike the term 'imaginary' because it implies something which exists only in someone's mind. However, you will see this term used frequently in this context. The quantity does exist but just try getting the term 'imaginary' past company accountants when you need funds for an even larger transformer than the load might imply. Since power=voltage x current, the voltage and current vectors can be made into a power triangle. The product of voltage and current is then the hypotenuse S (the apparent power) -another hideous term. The VA product multiplied by cos theta is the real or true power, which is the part that does work. The VA product multiplied by the sine of theta is the reactive power. This power does no actual work but has to exist; otherwise things like motors, coils and capacitors would not operate.
Appliances such as hot-water cylinders, incandescent light bulbs and radiant heaters have perfect displacement power factor by their nature. They are pure resistances and theta=0. Appliances with motors have less than perfect DPF and it varies depending on the type of motor and the load on the motor at any one time. So, your fridge, air-conditioner, fan heater, fan oven, motorised garage door, pool pumps, clothes dryer et al all draw current somewhat out of phase with the applied voltage. An oven or fan heater for example still has a very good displacement power factor (>0.9), because most of the load is still resistive (the heating elements) and only the small fan motor is creating a slight reduction of power factor for the overall appliance.
The definition of power factor was later expanded to include the degradations caused by introduced harmonics. Such problems are introduced by rectifiers, switching type power supplies and solid-state switching devices such as SCR's and Triacs. My feeling is that a separate definition should have been created so that the causes (and therefore costs or cures) for each issue could remain independent. However, we are now stuck with a two-part definition of power factor. So, the original definition discussed above is now called the 'displacement power factor' (DPF) because it relates to the displacement of current from voltage. The added part is called the harmonic power factor, and you guessed it- the combination of the two is called the 'Total Power factor'. A recipe for confusion, because unless writers are concise, you don't know whether they mean just DPF or harmonic PF or the total power factor. In addition, it is not always clear if a particular measuring instrument is measuring one or both parts.
Total Power Factor = DPF x harmonic PF
The result is still a number between 0 and 1. However, unless there are no harmonics of current present, the total power factor (TPF) is always lower (worse) than the DPF. Harmonic power factor is calculated using the total harmonic distortion (THD). There is more than one THD calculation method, but the common one in the power industry is the IEEE method.
THD(i)= 100 x √(sum of the squares of each harmonic current)/fundamental current (i1).
Usually harmonics up to the 50th are enough. In most cases, only odd order harmonics exist because despite appearances, the distorted current usually is symmetrical about the time axis. The IEEE method results in a percent figure which may go over 100%. This will seem odd to those of us used to making audio distortion measurements, but then, this is the power industry.
Harmonic PF= 1/ √(1+(THDi)2)
And as mentioned above, Total Power factor = DPF x Harmonic PF.
Small modern appliances which include rectifiers and switching power supplies can create a forest of harmonics. Ideally, an appliance would draw current at only the fundamental frequency of the AC mains which is 50Hz here, or 60Hz in some places. For these distorting appliances, some of their current is drawn off the mains at 150Hz, 250Hz, 350Hz.etc up to more than 2500Hz. The harmonic power factor is an attempt to quantify the relationship between current at the fundamental to total current of harmonic components. This kind of product usually has relatively good DPF. The better appliances include circuits to reduce harmonics and thus improve their power factor. These ones are said to be power factor corrected. They cost more. Most small appliances including compact fluorescent lamps do not have power factor correction, although you can buy types that do nowadays.
Below are the results of four recent appliance measurements: Note that the WT210 instrument auto-scales the graph so the ranges are not the same for each appliance. The full scale range is shown in the left corners of the graphs.
1. Draco DVB-T receiver model HDTV5500
This receiver (set-top box) has typical power quality for the class of product. This is the worst example of a recent check of eight such receivers. The DPF is good because the current spikes (green trace) are more or less coincident with the voltage peaks (yellow trace). However a current spike of that shape is always going to be rich in harmonics. The chart at left shows current drawn by the receiver at each harmonic frequency. Note how even harmonic currents are very low and only odd harmonics are significant. The third harmonic is only 1.6dB below the fundamental and the 5th harmonic is only 4.3dB down. Even the 49th harmonic (2450Hz) is just 35dB down. The data table shows that current THD is 118.82% (poor) and it is this result that reduces DPF of 0.971 to a total PF of 0.625. Total current draw for the receiver is 90.8mA, of which only 58.4mA is at 50Hz.
2. Samsung 32" TV model LA32C450E
This TV has active power factor correction. Although the current waveform (in green) looks a little ragged, it is recognisably sinusoidal and that is also illustrated by the low level of harmonics on the left hand chart. Compare this with the Draco receiver above. The Samsung 3rd harmonic is 14dB down and the 5th is 17dB down. THD is a relatively low 27.86% which makes the total power factor 0.865. These numbers represent a good result. Total current taken here is 266mA, of which 256mA is at 50Hz.
3. Compact Fluorescent Lamp type DEC3A rated 26W
This is a shocker. The lamp draws a sharp spike of current; rich in harmonics. Although the DPF is 0.957, the total harmonic distortion is a staggering 157.27%, resulting in a low total power factor of 0.513. The 3rd harmonic of current is only 3.1dB down on fundamental, the 5th is 6dB down, the 7th is 4.7dB down; you can follow the nasty trend apparent here. The lamp is drawing 173mA from the mains but only 90.9mA is at 50Hz.
4. ATX PSU Zalman ZM600-HP
The Zalman ATX supply has active power factor correction. Although the current waveform only vaguely resembles a sine-wave, only the low order harmonics are still significant. 3rd harmonic of current is 10.2dB below the fundamental and the 5th is 18.7dB down. This 600 watt supply is lightly loaded here; only 73 watts is drawn from the mains. The pc case fans are running and this might account for the noise bursts riding on the current waveform. With THD(i) of 35.37%, the total PF is 0.873, which is a reasonably good figure. At this loading, the PSU draws a total current of 353mA, of which 333mA is at 50Hz.
The measurements above were recorded at my workshop while the appliance was connected to local AC mains, which already has distortion of the voltage. You can see this by the flattening of the voltage peaks. This is a typical situation for mains supplies around the world, although the extent varies from place to place and even between localities. In this case the voltage distortion was recorded by the Yokogawa WT210 as being variously from 3.3% to 4.45% (measurements taken at differing times). This voltage distortion will introduce a small error in the current distortion measurements. In this case, the THD of voltage results in an insignificant error of the current THD shown, of less than 0.3%.
Your power grid must supply the current taken by an appliance regardless of whether that current is in phase with the voltage. This is why power generators, distribution systems, switchgear and every component in the chain supplying power to that appliance must be rated according to total VA and not just true wattage. When you are talking of thousands of appliances of all types, that situation is important to lines and generating companies. To imagine the scale of this, if a load has a DPF of 0.7, where the current is 45 degrees out of phase with the voltage, that current drawn is 41% higher than what it would be if the DPF was equal to 1. That 41% increase in current doubles the power losses of the lines and transformers in the AC distribution system. You can see why the supply authorities have an interest in keeping the DPF of large AC power consumers as near to 1 as possible. This is why industrial consumers and large commercial operations have to pay a premium for poor DPF. And to avoid those price premiums, industrial and commercial operators will install DPF correction devices at their premises.
None of this is significant for domestic households. There are two main reasons. First, the DPF of a typical domestic household is not that bad overall. Much of the electrical load of a house is caused by plain heating such as ovens, water heating, ordinary electric space heaters and incandescent lights. This will swamp any poor DPF appliances such as air-conditioning pumps, washing machines, fridge pumps and so on. Such devices may result in worse household DPF momentarily as they start and stop. Net household DPF will be better than 0.85 to 0.9 for most of the time. Secondly, it is impractical to correct for the power factor of a domestic house. The only effective methods involve installing appropriate capacitors for each and every appliance which has a poor displacement power factor. Devices have been marketed which claim to improve household power factor and reduce your power bill. They are poor at the former and cannot alter your power bill. Don't' bother.
Supply authorities do not (at least presently) record displacement power factor of domestic premises. The homeowner pays the amounts recorded on their energy meters each month and these respond only to the real component of power. So you would achieve no reduction of your power bill even if you did go to the trouble of correcting your DPF. But you don't get away with it altogether. The rate that utilities charge a domestic user for electricity does account for an assumed DPF, because the losses caused by less than perfect DPF forms part of the costs of supplying electricity to you. In any case, they can still charge domestic users what they think they can get away with, regardless of how good your household power factor.
The harmonics of current caused by appliances that are non-linear result in voltage distortion of the mains and too much voltage distortion has many unwanted consequences. As mentioned earlier, appliances that are non-linear are things like compact fluorescent lamps, variable speed drives, dimmers, switch-mode supplies in computers, TV's, VCR's, set-top boxes and the myriad of other small appliances about the home. High mains harmonic distortion can result in:
Supply authorities are concerned about harmonics on the mains. They refer to a standard; IEEE-519, which sets limits of voltage distortion caused by too much harmonic current. The limit in this case is 5% voltage THD, measured at the customers' point of interface, which in a domestic house would be at the main switch. Critical commercial premises such as hospitals and airports apply more strict criteria, such as 3% THD, meaning all appliances used in those premises must have good power factor.
In practice, it is not easy to measure THD of domestic dwellings, because it varies depending not only on the proportion of distorting loads operating at any one time, but it varies with the 'stiffness' of the local mains supply as well. That stiffness is a measure of the loading on individual street transformers, which in turn is governed by how generously rated it is, as well as how many houses in the street are connected to it and the power factor of each of those houses at any one time. So it is virtually an impossible mission to determine an individual house quality unless that house were to be disconnected from the mains supply and fed with a known source of voltage from a portable generator. This would be too much trouble for any authority in the domestic case.
In my local area, for instance, I know the mains stiffness, or regulation, is poor because the voltage varies considerably over the course of minutes as well as over any one day. Here, I observe the voltage go from 224 volts to 245 volts over the course of an hour and that is a regular pattern. Even over the course of a few minutes, variations of up to 10 volts are common at times of peak loadings. In N.Z, supply authorities know that there is not a lot of margin in some areas, meaning harmonic distortion is becoming a dire issue. Looking at the test results above, my workshop AC voltage distortion reached 4.45% at one stage. The distortion of mains voltage resulting from an appliance depends on the level of harmonics it generates (the THD), as well as the load of that appliance. A large load with 125% THD, for example, will result in more voltage distortion than a small load of the same distortion. Poor power factor caused by harmonics generated in non-linear loads cannot be corrected by using shunt capacitors, as are used on large industrial plants to correct displacement power factor. Instead a filter must be used. In a domestic household, a single low-pass filter at the entry point would, in theory, be possible. This prevents your harmonic currents getting out to the grid and also prevents other's harmonics from getting in. It will not stop the damaging effects of the harmonics within your house. However, the science of designing harmonic filters is not simple. The design of the filter will depend on a whole lot of factors including total load, which harmonics are predominant and how it fits with any correction capacitors that might already be present for displacement power factor correction. Ad-hoc design could lead to resonances, which are very bad; leading to possible destruction of plant, so such design is best left to professionals. In the domestic situation, it would be a whole lot better if people could buy high power factor appliances.
New Zealand now applies the standard AS/NZS 61000.3.2 for domestic appliances, but only applies it for those appliances that draw 25 watts or more. Under 25W, which includes all CFL's currently, appliances must conform to a table for harmonic limits, or a limit for 3rd harmonic of 86% of fundamental current plus a limit on 5th harmonic of 61% of fundamental current. This latter table is quite loose and I believe, quite inadequate. IEC now requires lighting to have a PF greater than 0.96, although, they too mitigate the standard under 25 watts. ANSI now applies a maximum THDi per appliance of 32%. This is much better and AS/NZS61000.3.2 also specifies the limit of voltage THD at a customer premises of 5%.
An NZ study (Watson) estimated that 5% THD could be reached at the connection point of a domestic house when operating only 14 standard(low pf) CFL's rated at 20 watts each. My concern is that household non-linear loads do not comprise only CFL's, but rather a large number of other small appliances, numbers of which are destined to increase over time. The equivalent effect of 14 low pf CFL's is easily exceeded in most households. Better regulations are needed.
The MEPS program based on AS/NZS 4847.2 is a good start but their goal is not specifically about harmonic reduction. The MEPS requirement for CFL power factor is to exceed only 0.55. (or 0.9 for those claiming to be high power factor). This needs an upgrade. The additional cost of buying high power factor CFL's for example has been estimated as being only 60c per lamp. (10% additional). Of course, legislating for low THD (high power factor) for all small appliances has a cost to consumers, but that incremental cost is not great and it is easy to argue that spreading the cost to those who buy larger numbers of appliances is better than forcing huge increases in power generating capacity and over-sized infrastructure; the cost of which has to be borne by all.
Most of the base-load of a domestic household is by low power, poor factor, appliances. Go and count up the number of cordless phones, digital clocks, TV's, satellite receivers, set-top boxes, personal video recorders, computers, printers, scanners, fax machines, routers, webcams, sensor lights and phone chargers in your house that stay powered (or on standby) most of the time. From many measurements made by myself, most small appliances have even worse power factor when on standby than when operating.
The situation will get worse. For example, although many N.Z households (71% at April 2011) now have some form of digital television, the remainder will be forced to do so by the end of 2013. That means 478,000 households have to buy a digital TV, set top box or personal video recorder. In fact that is the minimum, because many householders will elect to buy a second or even third such device in order to service other rooms or to permit use of older TV's. The number of households too is increasing at a faster rate than the population generally, because of a trend to apartment living, smaller family units and retirees. That means an increase of number of households of 1.2% per year. So, the problem of ever increasing domestic power consumption has to be countered by use of more efficient appliances. However, that has to be done properly. The CFL debacle is a case in point. Allowing penetration of poor power factor appliances is another mistake. The general public will not easily be able to distinguish a high quality and high power factor device from any other, unless labelling is mandated, so the best way is to legislate against low P.F devices.
I had better say that I am definitely for reducing household appliance energy consumption. The energy star and MEPS programmes are helpful in identifying the better appliances. I think they do not go far enough because they barely mention power factor. Perhaps they imagined it was going to be too hard a sell to force people to buy more expensive appliances if owners were not going to benefit directly through their power bill. High power factor appliances mean that a country could defer building more power generators, bigger lines and bigger sub-stations when compared to the use of the same appliances having poor power factor. So yes, replacing incandescent lamps with CFL's will save you money on your bill and save the country some money as well, but the country will save even more if those CFL's had a good power factor. This translates to improving of greenhouse gas emissions since less generating capacity is required. I would hope that consumers could look for the term 'high power factor' or 'low power distortion' on the packaging. As a last note, I still regard CFL's as having a lot of issues, which means they are not suitable to replace each and every incandescent lamp. I don't intend to reiterate all those here; if interested please read my earlier articles.
The Economics of Improving Power Factor
Ed Kwiatkowski, BSEE, MS President, Staco Energy Products Company
Harmonics And How They Relate To Power Factor
W. Mack Grady; The University of Texas
University of Wollongong Power Quality Centre Tech Note 3: March 2000
Installation of compact fluorescent lamps; Assessment of benefits:
Parsons Brinckerhoff Associates
Lighting Answers Power quality NLPIP Vol2 No.2
Standby and Baseload in New Zealand Houses - A Nationwide Statistically Representative Study
BRANZ Conference paper No.124
Axino-Tech Consulting & Services April 2011