Lighting represents a significant portion of global energy use, in some cases approaching 10% of all electrical consumption. Thus, replacing incandescent bulbs with light-emitting diodes (LEDs), which use 75% less energy, offers a viable energy efficiency opportunity. However, because LEDs are non-linear loads they can have significant impacts on power quality. This article reviews these concerns and assesses the impact of power quality disturbances on LED lighting.
Compared to an incandescent light bulb, LED lighting technology is significantly more complex. It comprises many different components, each of which may have an impact on power quality. The semiconductor diode employed to emit visible light consists of a substrate and primary optical unit. The device transfers heat, generated in the LED, away from the semiconductor diode into a heat dissipation system (a heatsink). There are usually a few LED modules within a single luminaire, as well as an electronic driver circuit, wiring, connectors, and optical reflector, all within a housing.
The electronic driver circuit converts the AC electric power supplied to it into a DC supply which the LEDs can convert into visible light. Some drivers allow for light dimming to be implemented too.
Harmonic emission (or harmonic generation) is a primary power quality concern. When AC power is converted to DC power via a semiconductor, high frequency current, known as harmonics, is generated. The harmonic spectrum and magnitude will vary based on the semiconductor technology used and the type of network connection (single- or three-phase). LED lamps are typically single-phase loads and are connected between phase and neutral. Single-phase DC rectifiers are notorious for generating a wide harmonic spectrum – from second to 50th order – with dominant zero sequence third harmonic order. Elevated harmonic distortion levels will usually stress the electrical distribution network when the total demand distortion (TDD) exceeds the IEEE 519-2014 harmonic standard limits.
Zero sequence harmonics
When dealing with a single phase rectifier, specific attention must be given to the zero-sequence harmonics, particularly the third harmonic order as it will have the highest magnitude in the harmonic spectrum. The singularity of zero sequence harmonics (3rd, 6th, 9th, 12th, 15th, …) is that they add up perfectly in the neutral conductor, and the neutral current magnitude can reach 1,7 times the level of current flowing in the phase conductor. This presents a risk for neutral conductor overload. To resolve this issue, the most common solutions involves using a neutral conductor with a cross-section of the phase conductor.
Figs. 1a, b and c show the phase current and the neutral current generated by single-phase rectifier loads and their generic harmonic profile. Fig. 2 shows the current waveform of a 15 W LED lamp: the waveform deviates greatly from a regular sinewave because it’s rich in harmonic current. The current harmonic magnitude extracted from the waveform is shown in Fig. 3.
Harmonic emission limit for LED lighting equipment
Like any other appliance, LED lamps must comply with several directives. The IEC 61000-3-2 standard sets harmonic limits for equipment which draws 16 A or less. According to this standard, equipment is classified within four classes, in which lighting is class C. This includes light dimmers.
For lighting equipment with an active input of more than 25 W, the harmonic current is limited according to Table 2 of the IEC standard. Lighting equipment with an active input power smaller than, or equal to, 25 W shall comply to one of the following two sets of requirements:
Harmonic emission in electrical networks by LED lighting
Based on information reviewed for this article, LED lighting equipment will generate a relatively high level of harmonic distortion in current waveforms. For example, a 25 W LED lamp which respects the IEC 61000-3-2 standard and injects 85% of third harmonic and 61% of fifth harmonic, obtains a THD(I) of 105%, which is extremely high.
Fortunately, LED lamps have very low power consumption, only drawing a few mA. Also, newer LED driver designs incorporate power factor correction capacitors. This not only improves power factor (PF), but also offers some harmonic filtering. This can reduce the THD(I) to 43%.
If diversity of load is introduced in the circuit, some natural harmonic attenuation will occur via current vector cancellation, as well as when resistive loads are added to the same circuit resulting in further harmonic attenuation.
Normally, since the harmonic current flowing through the network impedance is low due to the low power consumption of LED lamps, it has very little impact on the total harmonic distortion on voltage (THD(V)) – usually below 5%.
A harmonic issue could arise in an electrical circuit if a very high density of LED lighting is present with a low diversity factor. In this case harmonic overload may occur on the bus, on the auxiliary transformer, and in the neutral conductor. For this type of situation, external harmonic mitigation such as an active harmonic filter device should be considered.
Power factor of LED lights
There are several types of LED drivers on the market and the PF impact will vary based on the type of rectifier used, if they are uncompensated, compensated with capacitors and if they have built in passive filter or active filtering device. Socket type bulbs with power below 10 W tend to be uncompensated and they have a PF ranging from 45 to 80% lagging.
LED panels, which consist of several strips of LEDs placed in parallel in the panel and fed from an external ballast, normally have a PF greater than 90% lagging when operating above 50% loading and a PF ranging from 30% to 85% when operating below 50% loading. The dimmer, which is an integral part of the ballast, has a big impact on the PF.
Experiments conducted on street lighting, show that for LED luminaires ranging from 30 to 200 W have a PF in the range of 92 to 98%. The Energy Star programme requirements for integral LED amps gives us one more reference. Where lamp power is 5 W or less, there is no PF requirement, while for lamps with a power greater than 5 W, the PF must be greater than 70% lagging.
However, a wide range of results and references are possible. The trend is that LED lamps of 10 W or less (and older LED technology) will have low PF, and that LED luminaires with more than 10 W (and newer technology) will have PF greater than 90%. In the event of an installation with poor PF due to LED lighting, external capacitor-based compensation equipment can be used to correct the PF. In such a case, it is recommended to use de-tuned capacitor systems as they are designed to correct PF in harmonic rich networks.
Inrush current caused by LED drivers
In the initial moments after the luminaire has been energised, a significant transient current appears (up to 253 times the rated current according to studies by Schneider Electric). The duration of the transient current can be less than 1 mS for one luminaire. The transient is due to the capacitors used to perform the PF correction. The PF of such compensated luminaires is generally greater than 90%.
In any electrical network the choice of circuit-breaker depends on the nature of the load powered. The rating depends on the cross section of the cables to be protected and the over-current protection curves are chosen according to the load’s inrush current.
Since no normative tripping threshold curves exist for transients which last for less than 10 mS, Schneider Electric performed measurements at a commercial building’s lighting installation. This was deemed to be a typical installation which will help with circuit-breaker selection.
A B-curve 20 A rated circuit-breaker tripped upon energising 25 luminaires each with a rating of 56 W. The following was observed:
It is therefore necessary to select the correct circuit-breaker during the design phase. The peak value of the current at switch-on depends upon the energising time, the number of luminaires forming the lighting circuit and the short-circuit power and architecture of the network (Fig. 4).
The curves presented in Fig 5 (a, b and c) provide information relating to the number of luminaires which can be connected downstream of a circuit breaker depending on its characteristics (the rating and tripping curve of the magnetic protective device). These curves were produced based on the unfavourable conditions which generate a maximum current peak at circuit energisation.
Also impacted by the LED lamps’ high inrush current are the commuting contactors. Contactor deratings given by manufacturers must be considered in the design phase to obtain the best adapted coordination with LED luminaires and performances compliance (electrical endurance).
For three-phase circuits (power supply of luminaires between a phase conductor and the neutral conductor), switch gear and control gear of the three-pole type is preferable to a control device of the four-pole type. Not switched, the neutral conductor will help to prevent a harmful voltage surge at power frequency from being applied across the terminals of the luminaire if the neutral conductor fails to close.
Alternatively, a “smart” solid-state contactor, activated at the voltage zero crossing can be used for LED energisation. When the control voltage is applied to the relay input, an internal static component performs the switching function at zero crossing of the voltage waveform. The precision at switching (connection to the network) is excellent. The inrush current is then reduced. Thus, it is possible to use circuit breakers without derating. The number of luminaires which can be powered by a single circuit is limited only by the thermal withstand of the smart relay.
LED earth leakage current
The measurements taken show that the leakage current is at maximum for switching on at the voltage peak. The frequency of this transient current is about 100 kHz, which is high. For switching on at zero voltage, the leakage current is practically zero (Fig. 6).
The permanent earth leakage current at 50 Hz is generally less than 1 mA for a luminaire. Given that lighting circuits are protected by earth leakage protection devices of 300 mA rating, many luminaires can be installed downstream of a protective device. At 100 kHz, the current is not detected by the earth leakage protection device.
LED lamps are non-linear loads which generate harmonics in low voltage networks. Inrush current will have the biggest impact on the power factor of electrical networks, so manufacturer derating recommendations should be respected to avoid nuisance tripping of the circuit-breaker. Harmonic issues can be encountered when many LED luminaires are installed on a network, however this can be addressed by installing external reactive compensation systems to correct the PF if this becomes an issue with the utility.
Contact Prisca Mashanda, Schneider Electric, Tel 011 254-6400, firstname.lastname@example.org
Source: EE plublishers