LED Lighting Current Harmonics
April 16, 2021
According to the most recent Commercial Buildings Energy Consumption Survey (CBECS), 17% of all electricity consumed in U.S. commercial buildings is for lighting, making it the single largest end use category of electricity. This large consumption for utility power not only brings forward the push to improve power conversion efficiency at the end load but also to reduce losses in the power distribution network.
Lighting equipment comes in a wide range of sizes and power levels. The price range varies accordingly, and economical solutions require a trade-off between performance and cost. With the drive to reduce the amount of energy use for lighting, electronic power conversion has allowed the expansion of highly efficient lights such as florescent and more recently light emitting diode (LED) products. The electronic power conversion has introduced some negative side effects which have been mitigated over the years by industry standards driven by government regulations.
One of the most significant negative impacts on the utility network is the introduction of the non-linear nature of the electronic power conversion methods. The non-linear load creates high harmonic current drawn from the utility resulting in low power factor. The consequence of the lower power factor ( PF ), distortion PF not phase shift PF in this case, is increased losses in the power distribution system and increased stress on the infrastructure.
Power supplies with Power Factor Correction (PFC) have additional circuitry to “waveshape” the input current to be drawn through all or most of the entire AC cycle. The degree of the power factor correction/harmonic distortion reduction varies with the sophistication of the “waveshaping” topology and controller selected. It is feasible to obtain near sinusoidal input current and PF ~ unity over a load and fixed line voltage. However, it becomes more challenging and costly to maintain high PF at wider load and AC line voltage range.
A “HARMONIC” is a sinusoidal component of a periodic waveform or quantity having a frequency that is an integer multiple of the fundamental frequency. An AC periodic voltage or current can be represented by a Fourier series of pure sinusoidal waves which contain the basic or fundamental frequency and its multiples called harmonics. Harmonic Distortion refers to the distortion factor of a voltage or current waveform with respect to a pure sine wave. Distortion factor (harmonic factor) is the ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity, expressed as a percent of the fundamental.
The effects of harmonic currents from nonlinear loads are better understood these days by the industry and regulation have been enacted to minimize the level or harmonic currents. Due to the low impedance of most power systems, the power system can generally source significant amounts of harmonic current demanded by the load without converting these to unacceptable voltage distortion levels. These harmonic currents are measured and must be lower than the emission limits defined in the standards.
EN61000-3-2 standard defines limits for harmonic current emissions for various types of equipment. For lighting equipment, Class C limits apply. The latest edition of EN61000-3-2:2019 was enforce and includes clarification to the lighting equipment current harmonic requirements and test method.
Lighting equipment test conditions are outlined and limits are defined depending on the power level of the lighting equipment along with consideration of dimming controls that may be included as part of the lighting equipment or luminaire. The total harmonic current measured under any dimmer/color setting within the defined min/max power range, shall not exceed the limits defined by the standard when operated at the maximum input power setting. THD is limited to the 40th harmonic with respect to the IEC61000-3-2 standard requirements.
When selecting a power supply for a given application, it is important to understand the actual maximum rated load and power demand of the end product. The power supply will likely be rated for a higher power than the end application. This is commonly the case due to thermal derating consideration in the end application, other derating practices or using a common power supply for several different lighting products. The implication here is that the power supply may meet the EN/IEC61000-3-2 class C limits for the power supply maximum rating, but in the application the actual maximum power may be significantly less, even down to 40% or less of the power supply rating. Therefore, it is important to verify the power supply will meet the harmonics at this lower “Rated Maximum” power rating of the end product. Also, depending on the power conversion method used, it is possible the harmonics could increase at lower loads which may not be an issue if the full rating of the power supply is used, but at lower “max power” rating of the lighting equipment, that may not be the case. Even with dimming applications, the harmonic currents are measured against the luminaire harmonic currents at its maximum load limits of the standard. As long as the harmonics decrease with dimming, the product should meet the standard.
SL Power Electronics develops power conversion solutions for the medical, industrial and the LED lighting industry. Lighting specific power products are designed to maximize the useful range of the power supply to meet the IEC61000-3-2 class C harmonic limits over a wide operating range.
Please contact Rojan Karmacharya, for complete white paper on LED Lighting Input Current Harmonics; Understanding Current Harmonics and requirements for EN61000-3-2 compliance.
Senior Field Application Engineer