EMC and Mains harmonics

Harmonics: Their causes and problems

Non-linear loads generate harmonic currents which in turn create harmonic voltage distortion due to the finite (and complex) impedance of the mains supply network.

These harmonic currents can overload neutral conductors and distribution transformers, cause nuisance tripping of protective devices, and create EMC problems. Harmonically distorted mains waveforms can cause a variety of reliability problems with electronic apparatus.

Examples of non-linear loads include:

++ Transformers and DOL induction motors (usually fairly small contributors);

++ Rectifier systems used in electrolytic processes;

++ Fluorescent lamps;

++ The DC power supplies of any electronics (e.g. adjustable-speed motor drives, information technology and telecommunication equipment, etc.) whether linear or switch-mode;

++ Three-phase power convertors (six-pulse, twelve-pulse, etc.);

++ The DC power supplies of microwave magnetrons and klystrons, and other radio-frequency generators;

++ Arc welding, arc furnaces, electric smelting, etc.

Where the mains supply suffers from severe waveform distortion, even linear loads such as resistive heaters draw harmonic currents which then pollute their supply networks.

Of all the above it’s the electronic DC power supplies that are causing the most concern these days, due to the ubiquity of electronic devices such as TV sets in domestic premises (soon to be increased by the use of variable speed drives for many appliance motors); computers and computer technology in commercial buildings; and adjustable-speed drives in industry. In most cases the building mains and earthing networks were installed when large harmonic currents were not a consideration.

Traditionally, harmonics was only a concern for larger systems and installations, particularly for power generation and distribution and heavy industry. But the modem proliferation of small electronic devices, each drawing perhaps only a few tens or hundred of watts of mains power, and usually single-phase (e.g. personal computers), has brought the problem of mains harmonics to almost every type of system and installation.

Triplens

Emissions of "triplen" harmonics (multiples of 3: 3, 6, 9, 12, etc.), or triplen harmonic distortion of the incoming supply, add constructively in Neutral conductors and are known to reach 1.7 times the phase current in some installations. Single-phase non- linear apparatus has a much greater propensity to emit triplen harmonics than three- phase equipment, which is partly why the problem of triplen harmonics in neutral cables and transformers of systems and installations is a particularly modern one. Other non-triplen harmonics have phase rotations which are either faster or slower than the rate of the fundamental, and can even rotate backwards. These can cancel out to some degree leaving a complex ripple current (a detailed analysis is usually required to determine the degree of cancellation).

--- Addition of harmonics

Transformer and Neutral conductor overload

Harmonic currents in the Neutral conductors present serious reliability risks, and even safety risks, where Neutral conductors have not been suitably dimensioned. Many modern installations use Neutral conductors of the same cross- sectional area (csa) as their associated phase conductors, and some (usually older) buildings are known to use half-size or smaller Neutral conductors, with obvious problems. In some countries (e.g. South Africa) it’s possible to find buildings wired with quarter or even eighth-sized Neutrals.

As well as creating problems with Neutral conductors, these excessive "zero-phase" currents cause excessive zero-phase flux in delta-wound transformers, leading to possible overheating (often only at particular locations within the transformer). Most distribution transformer manufacturers will provide formulae which allow correct dimensioning on the basis of known harmonic currents.

Unexpected tripping

Overcurrent protection devices (fuses, MCBs, etc.) can trip even though the phase current appears to be much less than their rating. The real issue here is that the protection device is working properly, but the electrician's AC ammeter will read low (-30% is not unusual) unless it’s a true RMS type. The danger is that the electrician will fit a higher-rated protection device to prevent "nuisance tripping" and the cables will then not be adequately protected, resulting in increased fire safety hazards.

Conductor skin effect

Harmonic currents cause significantly greater ohmic heating in conductors than do 50 or 60Hz currents. This is due to the effects of skin depth at higher frequencies. E.g., the density of the current in the cross- section of a copper conductor at the 7th harmonic of 50Hz (or the 5th harmonic of 60Hz) is reduced to around 32% (one skin-depth) at a depth of around 5mm from the outer surface. Since higher frequency currents are forced to flow through less of the conductor's cross section, they experience greater resistance and hence produce greater heating effects.

This is a greater problem for larger diameter cables. A similar problem of increased thermal losses also occurs in the windings of distribution transformers, and possibly also in their magnetic cores.

Effects on electronics

Electronic circuits that rely upon zero-crossing detection can mis-operate, due to harmonically distorted waveforms. Consequences range from increased acoustic noise and/or energy inefficiency, through worsened reliability, to "explosive disassembly". Electronic power supplies may provide much lower unregulated voltages than expected, since they usually rely on charging to the peak voltage of the sinusoidal mains. Harmonic waveform distortion (mostly due to the use of such power supplies)

results in "flat-topping" and the power supplies don't charge up to as high an unregulated voltage as would be expected from a voltmeter reading of the supply.

Consequently the regulated outputs of these power supplies may fall below their minimum specified levels, with unpredictable results for the functionality of the equipment, when the RMS mains voltage is only slightly below nominal.

Motor failure

Direct-on-line motors operated on distorted waveforms will try to rotate at the speed of phase rotation of the various harmonics. They will try to run at 3 times the rate of the fundamental for the 3rd harmonic, 6 times for the 6th harmonic, etc. At the very least the harmonic distortion of the supply waveform will cause a decrease in efficiency and a rise in winding temperature. Higher levels of distortion can cause serious overheating of the motor with consequent shut-down (assuming full protection is in place), excessive acoustic noise and vibration, damage to bearings (which can wear-out at as little as 10% of their nominal life), and disassembly of enclosures and mountings due to high levels of vibration. At even higher levels of harmonic waveform distortion motors may even refuse to start, drawing such high levels of third harmonic currents (the slip speed for the 3rd harmonic is much greater than for the fundamental) that their protective devices open.

Voltage distortion due to resonances

System resonance effects at the harmonic frequencies can create areas of the power distribution network where the voltage is more heavily distorted than elsewhere, and/or has significant over- or under-voltage. Also, some areas of the network can suffer from much higher levels of current than elsewhere in the network, at a few harmonic frequencies. At 50 or 60Hz most systems don’t suffer from resonances, but at the higher frequencies of the harmonics, with the levels of capacitance increasingly seen due to the use of EMI filters in modem apparatus, it’s becoming increasingly possible for power networks to resonate at harmonic frequencies.

Power factor correction

Power factor correction capacitors can fail, sometimes explosively (either the large ones used by the electricity suppliers or in the distribution room, or the smaller ones fitted to electrical apparatus and fluorescent lamp ballasts). This is due to the much higher harmonic currents they experience because of their lower impedance at higher frequencies, leading to overheating. Sometimes harmonics can cause overvoltage breakdown.

Radiated interference

Harmonic currents in mains supply cables give rise to emissions of electric and magnetic fields from the supply cables themselves, and also from the earth network which suffers from increased levels of harmonic currents due to the effects of the distorted supply voltages on the earthed capacitors in RF filters. These emissions especially cause problems for audio induction loop systems, and can also cause problems for VDUs, audio systems in general, and other sensitive equipment.

Increased earth leakage

Harmonic distortion of mains voltage causes protective earth conductors to carry higher levels of harmonic leakage currents, due to the lower impedance of EMI filter capacitors at these higher frequencies. These currents can cause the earth leakage to exceed safety norms. They also create potential differences between different parts of the earth network and hence give rise to common mode LF conducted disturbances on signal and power lines.

Harmonic solutions

It’s important to make sure that all electrical measuring instruments in use by electrical engineers measure true RMS with a bandwidth of at least 2kHz, otherwise inaccurate readings of voltage and current will result, with possible unreliability and even safety consequences. Test equipment is also available from a number of suppliers that will display harmonics as individual bar graphs or readings.

To deal with mains harmonics it’s often necessary to involve the local electricity supplier, especially where the problem is due to waveform distortion on his supply.

Waveform distortion should typically be held to < 4% THD (total harmonic distortion) according to the electricity supply industry's guidelines, but might in future be relaxed to 8% throughout Europe due to the increasing difficulty experienced by power generators and distributors in maintaining a pure sinewave. The problem is compounded because European countries have different supply topologies, some of which are more susceptible to harmonic pollution than others. In some countries waveform distortion can be much greater than 4% THD: in parts of mainland China it’s reputed to be so bad that the mains waveform is a reasonable square wave.

Standards

Standards for harmonic emissions from apparatus include the older IEC 555-2/EN 60555-2, which applies to domestic electrical equipment, and IEC/EN 61000-3-2 (which has superseded it), IEC/EN 61000-3-4, and IEC/EN 61000-3-6.

IEC/EN 61000-3-2 becomes mandatory on 1st January 2001 for all apparatus offered for sale in the EU that operates from public low-voltage supplies (400/230 Vac)

and draws under 16 Amps per phase. This standard is an attempt to control the ever- increasing levels of harmonic pollution on the public mains supplies.

IEC/EN 61000-3-4 is for all equipment running on public low-voltage supplies which draws up to 75 Amps per phase. This will probably never be a mandatory EMC standard, instead it provides a number of standard forms allowing the hopeful user of a new apparatus to negotiate with his power supplier on the basis of the harmonics emitted by the new apparatus. Only equipment which meets IEC/EN 61000-3-2 (regardless of its current consumption) is to be allowed unfettered connection to the public mains supplies without permission from the supplier of the electricity.

IEC/EN 61000-3-6 provides a set of application forms, similar to IEC/EN 61000- 3-4, except that it applies to apparatus supplied from MV or HV.

Remedies

Remedial measures for systems and installations include:

++ Oversizing HV, MV, or LV/LV transformers;

++ Use star power distribution for different applications by means of different feeders or transformers;

++ Use an adequate csa for the neutral conductor, possibly even larger than the phase conductor;

++ Equalize the load sharing between phases (does nothing for harmonics but reduces the heating effects in the neutral);

++ Use only apparatus that conforms to IEC/EN 61000-3-2 or the appropriate sections of IEC/EN 61000-3-4 or -6;

++ Fit resonant filters "tuned" to the harmonic(s) which are causing the problems. These may be series or parallel resonant, depending upon the effect required;

++ Fit active harmonic correction equipment. These are now available from several manufacturers, in a variety of sizes, and are usually intended to provide local correction for a machine or process, or a floor of information technology equipment. Active harmonic correction equipment is connected in parallel with the mains supply and draws current when the voltage waveform is too high compared with a pure sine-wave, storing the energy in internal capacitors and releasing it to the supply when the voltage waveform is too low. Apart from inevitable inefficiencies (minimized by their use of switch-mode technology) these units don’t consume power.

They protect the "upstream" mains distribution network from the harmonic currents flowing in the "downstream" network, and may possibly increase the harmonic currents in their downstream network. They also help to preserve supply waveform quality, but may not be able to be used where the incoming supply is significantly distorted - check with the manufacturer.

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