Harmonic Current in Power Supplies and IEC 61000-3-2

The harmonic current produced by switched-mode power supplies (Figure 1) can severely compromise the voltage quality of mains, and the international standard IEC 61000-3-2/
EN 61000-3-2 exists to set limits on the amount of harmonic current that equipment may output.

 

Figure 1. An example of an AC-DC power supply, which must be in compliance with IEC 61000-3-2, meaning that harmonic current must be addressed. Image used courtesy of CUI Inc

 

However, without an understanding of the nature of harmonic current and what can be done to mitigate it, compliance with IEC 61000-3-2 can seem overwhelming.

To better understand this concept, this article will dive into problems with harmonic current, its relationship with power factor, and as well as the causes, regulation, limitation, and mitigation of harmonic current. 

 

The Potential Problems with Harmonic Current

In general, a harmonic current is a sinusoidal current with a frequency that is an integer multiple of the fundamental frequency of the mains or usually 50 or 60 Hz.

Additionally, only the fundamental frequency's current creates active power, measured in watts, which can also be referred to as true or real power. On the other hand, harmonic current does not produce active power and circulates in the distribution network rather than powering the load.

All in all, harmonic current can prove to be problematic.

For example, it increases the current in distribution cables, switches, and transformers, which leads to power dissipation that produces heat that must be managed.

It can also saturate transformers if they are not sized for the harmonic current, and harmonic current means that larger cables and transformers are necessary, which increases system cost.

Additionally, it can also lead to increased current and overheating in the neutral conductor of 3-phase systems, as well as malfunctions in other equipment connected to mains. 

 

Relationship Between Harmonic Current and Power Factor

The presence of harmonic current also causes a decrease in the power factor (the ratio of active power to apparent power). Since only the fundamental frequency produces active power, the volt-amp product of the harmonics contributes to reactive power, which increases the apparent power without changing the active power. 

To better understand this, a review of power triangles is in order.

For a linear system, the power triangle, shown in Figure 2, applies and the equation for the associated power factor is shown below, where P is real power and S is apparent power in the circuit.

 

\[ PF = \frac{P}{S} \]

 

Figure 2. A power triangle for a linear power system. Image used courtesy of CUI Inc

 

For non-linear power systems, however, distortion power provides an additional axis to the power triangle that is orthogonal to active and reactive power, as shown in Figure 3.

 

Figure 3. Notice the distortion reactive power is the third axis of the power triangle. Image used courtesy of CUI Inc

 

The total harmonic distortion provides information on how much loss (or heating) the harmonics cause as they circulate through resistive cables. The equation for total harmonic distortion (THD) takes into account the current associated with each harmonic according to this formula:

 

\[ THD = \sqrt {\sum_{n=2}^{\infty} \frac{I_n^2}{I_1^2} } \]

 

Once determined, the total harmonic distortion can be used to calculate the distortion power factor (DPF):

 

\[ DPF = \sqrt {\frac{1}{1 + THD^2} } \]

 

To obtain the total (and far more accurate) power factor (TruePF), multiply the displacement power factor (PFD) by the distortion power factor (DPF):

 

\[ TruePF = PF_D \cdot DPF \]

 

The displacement power factor (PFD) is obtained as follows, referring back to Figure 2:

 

\[ PF_D = \cos {\theta} \]

 

While power supplies often have small displacement factors, they can create a large amount of distortion,  causing a low power factor. A low power factor leads to increased current for the same active power, but that current merely circulates in the system producing losses and heat.

 

What Causes Harmonic Current

To mitigate the effects of harmonic current, it is important to understand what causes it. In short, harmonics result from non-linear loads (i.e., loads whose current vs. voltage characteristics do not form a straight line) that create distortion, of which rectifiers on the front end of power supplies are a good example.

Because the ac input voltage is a sinusoidal waveform a linear system will produce a sinusoidal current, while non-linear systems will produce non-sinusoidal current. The resulting current of a non-linear system is distorted and often extremely complex, but using Fourier Series can be expressed as a weighted sum of sinusoids where the frequency of each sinusoid is an integer multiple of the fundamental frequency (a harmonic). 

Figure 4 shows an example of the typical input waveform of a power supply with no power factor correction.  It can be clearly seen that the input current is not sinusoidal, but instead a short periodic pulse, which means that the power supply as a load is non-linear and will lead to harmonic current.

Figure 4. Notice the non-linear nature of the input current due to the presence of harmonic distortion. Image used courtesy of CUI Inc

 

Regulation of and Limits for Harmonic Current

Harmonic current limitations are regulated by IEC 61000-3-2/EN 61000-3-2, which applies to equipment with an input current of up to 16 A per phase. The goal of this standard is to maintain high power quality and reliability of the mains to protect the distribution network and connected devices and it has serious implications for power supplies.

In this standard, there are application-specific classes with different requirements, namely Classes A - D. Class D covers televisions, personal computers, and monitors that consume ≤600 W. Class C applies to lighting equipment, Class B is for portable tools, and Class A covers everything else. 

Of these four classes, Class A and B have absolute limits on harmonic current while Class C is relative to 50Hz current and Class D is relative to mains power.  The limits specified by IEC 61000-3-2/EN 61000-3-2 are summarized in the table below (note that λ for harmonic order 3 represents the circuit power factor).

 

Table 1.

 

Mitigating Harmonic Current

In order to comply with the IEC 61000-3-2/EN 61000-3-2 standard, harmonic distortion must be mitigated.

For power supplies, such mitigation is best achieved using either passive or active power factor correction, the circuits of each are shown down below in Figure 5.

 

Figure 5. Comparison between passive and active power factor correction circuits. Image used courtesy of CUI Inc

 

Passive power factor correction involves adding fixed line frequency filtering at the input of the power supply and is most effective for lower power systems, due to the large size of reactive components resulting from the low line frequency.

In addition, it does not work well with universal input ranges and does not support multiple PSUs as the values are fixed and designed for a single operating point. Otherwise, it is a simple and reliable approach to harmonic current mitigation and works extremely well with Class A equipment.

Active power factor correction electronically controls the waveform of the input current by using a power converter, typically a boost converter, running at a high frequency. The boost converter uses the ac input voltage as a reference and modulates the switches such that the input current is in phase and of the same frequency as the input voltage.

So, for a sinusoidal input voltage, you would get a matching sinusoidal current waveform.

The high-frequency operation of the power converter allows for a significant reduction in the size of the reactive components and the internal feedback loop makes it effective across a wide range of conditions.

Active power factor correction is most effective when used with higher power systems. It also works well with a universal input range and supports multiple PSUs. On the other hand, it does require more components, is more complex, and not as reliable as passive power correction.

 

Conclusion

Harmonic currents of switched-mode power supplies can seriously impact the power quality of the mains, negatively affect other devices operating on the same lines, and lead to such issues as overheating, saturated transformers, and the need for larger cables.

Compliance with IEC 61000-3-2 is necessary to maintain high power quality and protect other mains connected devices.

At CUI, our team of engineers and power supply experts will work with you to find the right product from our extensive inventory of AC-DC power supplies as well as access the data, tools, and other resources you need to make a solid decision.

Our team will also help you navigate the complex world of IEC 61000-3-2 compliance, selecting the right approach to address issues of harmonic distortion and meet any other global standards you need to meet. All in all, CUI has thought ahead and designed active power factor correction into most of its AC-DC power supplies for 100 W and above.

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