New technologies in lighting have today’s designers focusing a keen eye toward power quality. Power quality is the ability of an electrical system to deliver power in the safest and most efficient manner. Power quality is an important consideration in all industries, but with the advent of LEDs, Compact Fluorescent Lighting (CFL), High Intensity Discharge (HID) lighting and others, the commitment to an efficient power distribution system is more essential than ever.
Many new lighting technologies, while certainly more efficient, can adversely affect power systems through harmonics which decrease power quality. Inefficiencies in power quality may limit the amount of devices that can be placed on the distribution network. It may also cause equipment to under-perform or to behave erratically. In the most severe cases, it can even harm the system and the devices along the network. Poor power quality management inevitably leads to an increase in operational costs and places an unnecessary strain on already dwindling resources. Therefore, lighting systems should be driven by efficient and sustainable power sources that will not unnecessarily burden the power grid while still providing the perfect electrical environment for advanced lighting systems. The ultimate reason that we are interested in power quality is economic value. There are economic impacts on utilities, their customers and suppliers of load equipment.
Harmonics in power systems
With the connection of increased electronic devices, it is necessary that we observe the quality of power. Stable voltages and undistorted waveforms are the two most desired qualities in power systems. Harmonics are a mathematical model to analyse distorted waveform.
Any periodic waveform can be expressed as a series of sine waves with varying frequencies and amplitudes. That is, we can create a series of sine waves of varying frequencies and amplitudes to mathematically model this series of pulses. The frequencies we use are multiples of the fundamental frequency, 50 Hertz. We call these multiple frequencies harmonics. The second harmonic is two times 50 Hertz, or 100 Hertz. The third harmonic is 150 Hertz and so on.
In our three phase power systems, the ‘even’ harmonics (second, fourth, sixth, etc.) cancel, so we only need to deal with the ‘odd’ harmonics. This is because when both the positive and negative half cycles of a waveform have identical shapes, the Fourier series contains only odd harmonics. This offers a further simplification for most power system studies because most common harmonic-producing devices look the same to both polarities. In fact, the presence of even harmonics is often a clue that there is something wrong – either with the load equipment or with the transducer used to make the measurement.
In a three phase power system the third harmonic is the harmonic of primary interest. The higher order harmonics do not affect system performance to a very large extent. The third harmonic of each of the three phase conductors is exactly in phase. When these harmonic currents come together on the neutral, rather than cancel, they actually add and we can have more current on the neutral conductor than on phase conductors. Our neutral conductors are no longer protected as we would like.
These harmonic currents create heat. This heat over a period of time, will raise the temperature of the neutral conductor. This rise in temperature can overheat the surrounding conductors and cause insulation failure. These currents also will overheat the transformer sources which supply the power system. This is the most obvious symptom of harmonics problems; overheating neutral conductors and transformers. Other symptoms include:
- Nuisance tripping of circuit breakers
- Malfunction of UPS systems and generator systems
- Metering problems
- Computer malfunctions
- Overvoltage problems.
Remedies for neutral heating due to third harmonics
- Oversizing Neutral Conductors: In three phase circuits with shared neutrals, it is common to oversize the neutral conductor up to 200% when the load served consists of non-linear loads. For example, most manufacturers of system furniture provide a #10 AWG conductor with 35 amp terminations for a neutral shared with the three #12 AWG phase conductors. In feeders that have a large amount of non-linear load, the feeder neutral conductor and panel board bus bar should also be oversized.
- Using Separate Neutral Conductors: On three phase branch circuits, another philosophy is to not combine neutrals, but to run separate neutral conductors for each phase conductor. This increases the copper use by 33%. While this successfully eliminates the addition of the harmonic currents on the branch circuit neutrals, the panel board neutral bus and feeder neutral conductor still must be oversized.
- Oversizing Transformers and Generators: The oversizing of equipment for increased thermal capacity should also be used for transformers and generators which serve harmonics-producing loads. The larger equipment contains more copper.
Lighting systems and their effects
Hence it can be seen that it is of very high priority to eliminate or reduce the effect of harmonics. The analysis of the effect of harmonics on different lighting systems has been carried out in order to address this issue. Lighting comprises approximately 17.5% of global electricity consumption.
As the world transitions from incandescent to Solid State Lighting (SSL) technology, utilities and government regulatory agencies worldwide are concerned that, as this large segment of the consumption base switches to SSL, it will increase infrastructure costs. This is due to the reactive nature of LED-based solid state lighting, which results in higher distribution currents that adversely affect Power Factor (PF) and, in turn create a larger demand on the power grid.
The move to LED-based solid state lighting promises a significant reduction in the carbon footprint of the electrical power grid simply due to the dramatic reduction in real power consumption. However, if power factor is not managed, the grid will still need to be able to provide a much higher power level than is actually needed at the load, eliminating a significant portion of the benefits of moving to solid state lighting.
Historically, incandescent bulbs have had near-perfect power factor. Therefore, solid state lighting is being held to a much higher PF standard compared to legacy AC/DC power supplies. In most cases, power supplies are free from any form of power factor regulation for supplies rated up to 75W. However, for solid state lighting, PF regulations typically kick in as low as 5W or below.
CFLs are an energy efficient alternative to traditional incandescent lamps because they offer similar light but use one-fourth the electricity and last up to eight times longer. CFLs are also cost effective because the initial cost can be recouped within two to three years in the form of reduced electricity bills.
Moreover, CFLs can be used by electric utilities in Demand Side Management (DSM) programs to reduce peak demand levels and defer the cost of expensive infrastructure upgrades. CFL is a nonlinear load, therefore it injects harmonic to the network. In past, due to lower application of CFL, these harmonics were ignored, however today by the widespread application of CFLs; these small sources are combined and have high effect on power distribution networks.
With regulations dictating power factor requirements for solid state lighting, designers need to incorporate power factor correction circuits into the driver design. A clear understanding of the end requirements based on the intended application of the luminaries determines the type of power factor correction that needs to be implemented to enable a brighter, greener future.
Harmonic analysis of a few commercial lighting systems
A Fluke 434 II Energy Analyzer was used for measuring the voltage, current, power factor and THD content for different lighting systems. The experimental readings are shown in Table 1.
Utilising the new Energy Loss Calculator function, the 434 II measures the fiscal cost of energy wasted due to poor power quality. This energy monetisation capability allows you to identify the most energy-wasteful areas of your facility so you can determine potential energy saving solutions. Add basic power quality measurements to the package and you’ve got yourself one powerful troubleshooting tool.
From the experimental results we can observe that the 3rd harmonic current in CFL is in the range of 9% to 33.6%, for LEDs in the range of 9% to 41.6% and for fluorescent tubelights around 8.5%. Lights which are not branded gave
even worse results and have not been included here.
The above experiment can be done by the consumer which the following intent:
- Energy monetization: calculate the fiscal cost of energy waste due to poor power quality
- Energy assessment: quantify the before and after installation improvements in energy consumption to justify energy saving devices
- Frontline troubleshooting: quickly diagnose problems on-screen to get your operation back online
- Predictive maintenance: detect and prevent power quality issues before they cause downtime
- Long-term analysis: uncover hard-to-find or intermittent issues
- Load studies – verify electrical system capacity before adding loads
Hence consumers must be careful in the choice of the lighting. Based on these studies there is need to formulate a policy on monitoring the harmonic content injected into the grid due to these lighting systems. Standards have to be strictly enforced to ensure that manufacturers maintain the harmonics at permissible levels.