Voltimum UK Managing Editor, James Hunt, introduces power harmonics.
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| James Hunt, Managing Editor |
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Harmonics are power quality problems and they are increasingly polluting the voltages used by, to take just a few examples, production plant, pumping stations, data processing and telecommunication systems, and TV broadcast studios.
Harmonics also often take up a significant proportion of the available current – electricity consumption can therefore increase. In addition, the hidden costs of accelerated equipment ageing can be very high, and excessive harmonics make compliance with EMC standards almost impossible.
Power quality in this context refers to electromagnetic disturbances related to voltage and current at any given time and location in a power system. The International Electrotechnical Commission (IEC)’s classification of low frequency electromagnetic disturbances that affect the electrical power quality is as follows:
· AC and DC networks.
· Frequency deviations.
· Harmonics and interharmonics.
· Low frequency induced voltage.
· Voltage dips and short interruptions.
· Voltage fluctuations.
· Voltage imbalance.
· Voltage supply.
Harmonic disturbances affect the voltage characteristics of electricity supplied in a number of different ways. Because of this, and the increasing use of non-linear electrical and electronic loads (such as AC to DC power conversion circuits, and even the humble fluorescent tube), the importance of harmonics has been growing over the past few years. Put simply, harmonic disturbances can cause many problems and cost money. Power conversion circuits draw only short pulses of current from the supply network. They also combine with the source impedance, and the result is supply voltage distortion.
Harmonics are currents. In most cases, they are in multiples of the supply fundamental frequency, produced by non-linear loads. Taking an example: on a 50Hz supply, the 5th harmonic is 250 Hz, 7th harmonic is 350 Hz, and so on. Being exact multiples of the supply frequency, these are called integer harmonics.
Specifically, voltage and current harmonics can cause the following difficulties:
· Damage and disruption to standby generators.
· Efficiency loss in power generation, transmission, and utilisation.
· Electronic equipment malfunction and / or failure.
· Electric motor overheating.
· Electromagnetic interference with TV, radio, communication systems.
· Error measurements in metering equipment.
· Excessive flicker on VDUs.
· Harmonic level amplification (because of series and parallel resonance).
· Interference with ripple control systems.
· Lost data in computers systems.
· Overloading, overheating and failure of power factor correction capacitors.
· Overloading / overheating of distribution transformers and neutral conductors.
· Resonance through capacitor / harmonics interaction.
· Shortening of electrical plant component life.
· Spurious operation of circuit protection devices (fuses, circuit-breakers etc).
Standards:
There are a number of standards that apply. In the UK, Engineering Recommendation G5/4 (2001) sets planning levels for harmonic voltage distortion to be used when connecting non-linear load equipment. It recommends that the total harmonic distortion should be less than 5%.
Moreover, Engineering Recommendation G5/4 (2001) recommends that continuous harmonic, sub-harmonic and inter-harmonic voltage distortions lie within the range 0 to 2500Hz. Short burst of harmonic distortion and voltage notching are other phenomena with recommendations made.
More information on this can be obtained from The Electricity Association at www.electricity.org.uk. Engineering Recommendation G5/4 (2001) can be obtained from www.ena-eng.org.
Standards that apply in Europe include:
· IEC 1000-3-2 or EN 61000-3-2 for low voltage (LV) appliances absorbing currents of under 16 A.
· IEC 1000-3-4 or EN 61000-3-4 for LV appliances or installations absorbing currents exceeding 16 A.
· EN 50160 defines the voltages supplied by utility LV networks.
· EEEI 519 (Recommended Practices for Harmonics Control in Electrical Power Systems) is a joint recommendation by utility operators and subscribers to limit the impact of non-linear loads through a joint effort to reduce harmonics.
· IEC 1000-2-2, and recommendations by CIGRE (Conférence Internationale des Grands Réseaux Electriques) for public LV networks.
· IEC 1000-2-4 for industrial installations using LV and MV.
How to overcome harmonics problems:
There are various ways of reducing or eliminating the effects of power harmonics. The main ones are phase shifting and / or staggering, and the use of passive or active filters.
Phase shifting involves the creation of two supplies 30º out of phase from that of the original supply. Phase staggering is broadly similar in that it involves shifting the phase of individual loads such that the harmonics produced by the load(s) cancels the harmonics produced by other loads.
Passive filters are series capacitor and reactor resonant circuits. They are tuned to give a low impedance path to a specific harmonic frequency. Although effective if applied properly, passive filters can suffer from resonance problems, especially if more than one is installed.
Active filters can be very effective and can reduce harmonic current distortion to below 5% - better than that for passive filters. Such a filter can continuously monitor the harmonic current and generates an adaptive waveform, which precisely matches the shape of the non-linear portion of the load current. This adaptive current is then injected into the load. *
*This information has been gained in part from AIM Technologies Europe (www.aimeurope.com).
Harmonics measurement:
Whichever method of overcoming harmonics problems is chosen, initially the exact nature of the problem will have to be found, and this requires harmonic analysis. There are a number of test instrument manufacturers providing, for example, true RMS instruments that can provide all the information required. An example is the LEM instrument that can be seen in a related article entitled ‘What’s happening?’ also available on Voltimum.
For detailed information on power harmonics, please download the various attachments in VoltiBULLETIN March 2005.
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