Power Factor Correction, Harmonic Currents & EN61000-3-221st February 2023
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Power Factor Correction, Harmonic Currents & EN61000-3-2, what’s the problem?
You’re designing a piece of electronic equipment and looking at all the safety and EMC legislation you need to comply with. Some areas are relatively straightforward, but then within the EMC directive, you come across EN61000-3-2 which relates to levels of harmonic current your equipment is allowed to draw.
I’m all for harmony, but what do you mean?
Let’s first look at where our electricity comes from. Most of the electricity we use today derives from some sort of generator, be it steam, wind or water powered. These generators produce a sinusoidal voltage, commonly known as “mains power”, which changes from positive to negative 50 times per second. For these generators to operate at peak efficiency, it is vital that the current drawn has the same waveform as the voltage – see below:
Without delving into the maths, this waveform has what is known as a unity power factor (PF=1). Power factor is defined as the ratio between the real power and apparent power (This is what we get charged for). If the current waveform deviates from this model, either in terms of the current waveform getting out of phase (capacitive or inductive loads will force the red line to shift left or right), or, and this is relevant to switch mode power supplies, the current is taken in “gulps”, the power factor will reduce and the generating station becomes less efficient.
Why are power supplies such an issue?
Let’s initially consider the sheer volume of industrial and consumer electronic equipment that is out there, and continues to grow. AC to DC power supplies in their simplest form take the incoming mains voltage, rectify it (swap the negative half of the cycle to make it positive), and then smooth it in an energy store called a capacitor. The graph below shows that current waveform (Icap) differs considerably from our ideal model:
Again, without getting into the detailed maths, this kind of waveform will have a power factor somewhere between 0.6 and 0.7.
What does this mean for the power station?
An electronic load with a low power factor will draw more current than a load with a high power factor for the same amount of useful power transferred. In the case of simple power supplies around 30% – 35% will be wasted. As a result electrical utilities will usually charge a higher cost to industrial or commercial customers who operate at a low power factor.
What is being done about this?
Global legislation has been brought in to encourage equipment manufacturers to move away from the simple models, and design power supplies which much higher power factors. IE61000-3-2 is an internationally recognised standard that limits mains voltage disruption by prescribing the maximum values for harmonic current distortion. IEC 61000-3-2 (EN61000-3-2 in Europe) applies to all electrical and electronic equipment with a rated current up to 16A, and defines limits for different classes of product.
Which part of EN61000-3-2 applies to me?
EN61000-3-2 has broken electrical and electronic equipment down into 4 basic classes…
Class A – balanced three-phase equipment and all other household appliances, excluding those specified as belonging to Classes B, C or D. Let’s park this for a moment and explore B, C & D in more detail.
Class B – Portable tools (This has probably the least stringent limits on equipment, but is also a relatively small subset of where power supplies are used). Class B harmonic currents are defined in terms of absolute currents for each harmonic.
Class C – Lighting equipment including dimmers. Here the harmonic current limit is expressed as a percentage of the 50 Hz input current (%).
Class D – PCs, monitors, radios, TVs and other IT equipment with an input power of <600W. Limits for class D equipment are expressed in terms of power related current with maximum values given for each harmonic with absolute maximum values also stated.
Equipment covered under classes C&D will generally require a power supply which has active power factor correction and a PF of >0.95
Class A then covers everything else and is defined in terms of absolute levels of harmonic current and is not related to the power of the equipment. From practical experience simple switch mode power supplies with input circuits like that featured above, will generally meet the limits of Class A up to about 75W. Thereafter we would recommend using power supplies with active power factor correction.
There can be a grey area with power supplies in the range 75 – 120W. On first inspection they will meet the requirements of the class A limits, but as the equipment warms up, the capacitors within the power supply will start to take larger, and shorter gulps of current (capacitance is increasing with temperature). The result is a decrease in power factor. It could be that the equipment will operate quite happily for 10 minutes and then fail the EN61000-3-2 type test.
Sounds like a problem for the power supply manufactures
Yes and no. There are techniques available where the power factor can be improved. The simplest is to add some passive filter elements before the main storage capacitor, but these can be bulky and also surprisingly expensive.
Active power factor correction is a technique which typically adds an extra power conversion stage, but allows the current waveform to be very precisely controlled and harmonics considerably reduced.
This is going to cost me, isn’t it?
Yes, it will add some cost, but often less than you think. Modern PFC controller circuits are now so common that prices have been driven down. Also, the secondary power conversion stage (400V to <48Vdc) can be a very simple high efficiency circuit.
I’m still not sure what I need to do to be compliant
Firstly, pick up the phone and talk to Relec about your project. We have over 40 years of experience, and will find the right product for your application. We can guide you through the different industry safety standards, EMC issues and make sure you have the most reliable and cost-effective power supply for your application.
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