CPD PROGRAMME


Figure 6: The variation of power factor with induction motor speed2


capacitance can add to the harmonic problems discussed below, or simply add unwanted reactance. So induction motors, whether


driving a fan, pump or lift, should be sized appropriately to meet the design requirements, otherwise they will always run at a lower power factor (and efficiency). Variable voltage AC drives and DC static converter drives have such a wide range of power factor over the speed and load range that it is very difficult to provide power factor correction to maintain values of 0.9. For large motor loads – such as those


used in lifts – variable voltage, variable frequency drives provide better all-round drive performance than variable voltage control alone, and give near unity power factor operation. (Static power factor correction must not be applied at the output of a variable speed drive, solid state soft starter or an inverter, as it will damage the electronics.) Some systems use multiple motors so that they can collectively operate with individual higher power factors (and efficiencies).3 Discharge lamps require a ballast to start


and control the lamp and, if provided by traditional wire-wound coil ballasts, need some power factor correction. LED and some compact fluorescent lamps can have power factors significantly less than 14


will normally be corrected locally within the luminaire. High-frequency electronic ballasts are available for a wide range of fluorescent and compact lamps, providing a power factor of 0.95.5 Power factor correction can be implemented using fixed capacitors or automatic variable power factor units. By installing the equipment close to the load, the kVAr


load on the supply cable


is reduced, thereby possibly allowing a smaller distribution cable to be used.


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Harmonics Modern power systems are increasingly affected by harmonic distortion. ‘Non-linear’ loads – such as computers, inverters (for speed controllers), electronic power supplies and discharge lighting– can affect the supply waveform. They ‘switch’ on and off very quickly, making step changes to the current and so creating harmonics in the electrical supply system. As illustrated in Figure 7, the harmonic components cause the (normally sinusoidal) waveform to be distorted. This will detract from the quality of the power supply and increase the ‘wattless’ current in the system – wattless, in this case, since the harmonic current has no matching voltage harmonic to produce useful power. This can cause excessive heating in motors, abnormally high current levels through system capacitors, voltage peaking and unexpected tripping, and may also affect the measurement accuracy of monitoring devices.6 The harmonic components cause greatly increased eddy current losses in transformers. These losses are proportional to the square of the frequency, so with the higher frequency harmonics, the operating temperature of the transformer will increase and equipment lifetime is shortened. Even moderately loaded transformers supplying IT loads will have much lower lifetimes than expected, unless proper precautions are taken.8 ‘Passive’ harmonic filters may be used to


reduce the harmonic current so that the non- linear device appears to the system to react in a way that is similar to a simpler linear load. The power factor can then be adjusted, using capacitors or inductors as required. So called 'detuned' capacitors may be used – these are a matched reactance/capacitance applied to 'dampen' the system harmonics at particular


frequencies as well as providing power factor correction. More flexible (and expensive) ‘active power


factor correctors’ are also used to alter the waveform of the current drawn by a load to improve its overall power factor. Under EU regulations, power supplies


(such as those used in modern desktop computers) are required to have appropriate filters to minimise the adverse effects on the power supply systems.


Benefits of power quality correction This article has provided a very brief introduction to the area of power quality in buildings. However an awareness of the issues may provide the basis for further discussions to improve systems that:


•Reduced loads on power distribution systems;


•Provide greater stability through the reduction/elimination of harmonics.


longevity; and © Tim Dwyer 2012


Further reading: CIBSE Guide K – Electricity in buildings provides an excellent resource covering the main areas of electrical application in building services. Power Quality Self-assessment Guide (and associated Leonardo Project Power Quality Application Guide documents) available as a free download from www.leonardo-energy.org provides detailed discussion of this area.


• Thanks to Les Norman at London South Bank University for his input to this CPD


module.


Capacitor voltage


and Current Line voltage 0 90 180 Degrees


Figure 7: The current drawn by an uncorrected electronic power supply (note that EU regulations require computer power supplies to incorporate measures to reduce this effect on the mains supply)7


270 360


References 1


•Reduce/eliminate network utility charges related to reactive power;


•Improve the overall performance of the power system that can increase equipment


Misakian, M. et al, Regarding Electric Energy Savings, Power Factors, and Carbon Footprints: A Primer, NIST Technical Note 1654, October 2009.


2 CIBSE Guide K, Electricity in buildings, CIBSE 2005.


3 CIBSE Guide D, Transportation systems in buildings, CIBSE 2010.


4 BRE 15/10 Specifying LED lighting, BRE 2010.


5 CIBSE Guide F, Energy efficiency in buildings, CIBSE 2004.


6 Chapman, D., Power Quality Application Guide, European Copper Institute and Copper Development Association, 2001 (EU Leonardo Project).


7 Fortenbery and Koomey, Assessment of the Impacts of Power Factor Correction in Computer Power Supplies on Commercial Building Line Losses, California Energy Commission, 2006.


8 Chapman, D., The Cost of Poor Power Quality, European Copper Institute and Copper Development Association, 2001 (EU Leonardo Project).


June 2012 CIBSE Journal


53


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