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Power Quality Partnership Harmonics in Power Systems Copper Development Association Power Quality Partnership Copper Development Association Fluke (UK) Ltd MGE UPS Systems Ltd Rhopoint Systems Ltd Copper Development Association Copper Development Association • Established 1933 • website - www.cda.org.uk • Technical helpline 01727 731200 • IEE Endorsed Provider Copper Development Association Harmonics in Power Systems Background to Harmonics, Problems, Solutions and Standards David Chapman, Copper Development Association Harmonic Measurement and Power Quality Surveys Ken West, Fluke (UK) Ltd Total Harmonic Management Shri Karve, MGE UPS Systems Ltd Applying Predictive Techniques to Power Quality David Bradley, Rhopoint Systems Ltd Copper Development Association Fundamental with third and fifth harmonics 1.100 Fundamental Third harmonic Fifth harmonic 0.000 0 90 -1.100 Copper Development Association 180 270 360 Composite waveform 1.600 0.000 0 90 -1.600 Copper Development Association 180 270 360 Loads that generate harmonics Switched mode power supplies (SMPS) Electronic fluorescent lighting ballasts Variable speed drives Un-interruptible power supplies (UPS) These are all non-linear loads Copper Development Association How harmonics are generated – linear load 1.1 Current Waveform Current Load Line Voltage 0.0 -1.2 Angle 0.0 1.2 Angle Voltage Waveform -1.1 Copper Development Association How harmonics are generated – non-linear load 1 Current Waveform Current Load Line Voltage 0 -1 0 Angle 1 Angle -1 Voltage Waveform Copper Development Association A Common non-linear load Copper Development Association Current waveform for a typical Personal Computer Desktop System 2.0000 1.5000 1.0000 Current (A) 0.5000 0.0000 0 90 180 -0.5000 -1.0000 -1.5000 -2.0000 Degrees Copper Development Association 270 360 Harmonic profile of a typical Personal Computer Desktop System 0.6000 0.5000 Current (A) 0.4000 0.3000 0.2000 0.1000 0.0000 1 2 3 4 5 6 7 8 9 Harmonic Copper Development Association 10 11 12 13 14 15 16 17 Harmonic profile for electronic fluorescent ballast Compact Fluorescent Lamp with HF Ballast 0.0600 0.0500 Current (A) 0.0400 0.0300 0.0200 0.0100 0.0000 1 2 3 4 5 6 7 8 9 Harmonic Copper Development Association 10 11 12 13 14 15 16 17 Harmonic profile for magnetic fluorescent ballast Compact Fluorescent Lamp with Magnetic Ballast 0.1400 0.1200 Current (A) 0.1000 0.0800 0.0600 0.0400 0.0200 0.0000 1 2 3 4 5 6 7 8 9 Harmonic Copper Development Association 10 11 12 13 14 15 16 17 Six-pulse bridge Copper Development Association Typical harmonic profile - six-pulse bridge Six pulse bridge - harmonic current 25 20 15 % 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Harmonic number Copper Development Association Twelve-pulse bridge Copper Development Association Typical harmonic profile - twelve-pulse bridge Twelve pulse bridge - harmonic current 25 20 15 % 10 5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Harmonic number Copper Development Association Why have harmonics become so important? Harmonic generating equipment has been in use for decades • Increase in the number of loads • Change in the nature of loads • Increase in those producing triple-Ns Copper Development Association Equivalent circuit of a harmonic generating load Copper Development Association Harmonic Diversity 80 70 60 50 % wrt RMS 40 30 20 10 0 Fund 41 3rd 20 5th Harmonic Copper Development Association 10 7th 5 9th 2 11th 1 No of Units (pairs) Harmonic Diversity - THDI THD 120 100 % wrt Fundamental 80 60 40 20 0 1 2 5 10 No of Units (pairs) Copper Development Association 20 41 Problems caused by harmonics currents within the installation overloading of neutrals overheating of transformers nuisance tripping of circuit breakers over-stressing of power factor correction capacitors skin effect voltages within the installation voltage distortion & zero-crossing noise overheating of induction motors currents in the supply Copper Development Association Overheating of neutrals In balanced three phase systems the fundamental current cancels out But triple-N harmonics add arithmetically! Non triple-N harmonics cancel in the neutral Copper Development Association Harmonic neutral currents Phase 1 Phase 2 Phase 3 Phase 1 3rd harmonic Phase 2 3rd harmonic Phase 3 3rd harmonic 0 120 240 3rd harmonic neut ral current Copper Development Association 360 480 600 720 Neutral conductor sizing Copper Development Association Neutral conductor sizing Copper Development Association Neutral conductor sizing Copper Development Association Neutral conductor sizing Neutral currents can easily approach twice the phase currents - sometimes in a half-sized conductor. IEEE 1100-1992 recommends that neutral busbars feeding non-linear loads should have a crosssectional area not less than 173% that of the phase bars. Neutral cables should have a cross-sectional area that is 200% that of the phases, e.g. by using twin single core cables. Copper Development Association Sizing the neutral conductor BS 7671:2001 - From January 2002 473 - 03 - 04 where neutral current is expected to exceed phase current 473 - 03 - 05 where neutral cross-section is less than phase cross section - neutral overcurrent protection is required Copper Development Association Sizing the neutral conductor For three phase circuits using single core cables: • Use a neutral conductor sized for the actual neutral current • If the neutral current is not known, use a double sized neutral cable • Provide overcurrent protection • But take account of the grouping factors! • Take into account voltage drop Copper Development Association Sizing the neutral conductor For multi-core cables : • Multi-core cables are rated for only three loaded cores - applies to both 4 and 5 core cables • When harmonics are present the neutral is also a current carrying conductor • Cable rated for three units of current is carrying more - three phases plus the neutral current • It must be de-rated to avoid overheating • Neutral must have overcurrent protection • Grouping factor must be taken into account Copper Development Association Sizing the neutral conductor - thermal 2.5 Cable size multiplier 2.0 1.5 1.0 0.5 0 10 20 30 40 % third harmonic current in phase Copper Development Association 50 60 70 Sizing the neutral conductor - IEC 2.5 Cable size multiplier 2.0 1.5 1.0 0.5 0 10 20 30 40 % third harmonic current in phase Copper Development Association 50 60 70 Sizing the neutral conductor 2.5 Thermal IEC Cable size multiplier 2.0 1.5 1.0 0.5 0 10 20 30 40 % third harmonic current in phase Copper Development Association 50 60 70 Neutral conductor protection Neutral conductors should be appropriately sized and provided with over-current protection. The protective device must break all the phases, but does not necessarily need to break the neutral itself. This implies a future need for 4 pole breakers with double rated neutral poles. Copper Development Association Effect of harmonics on transformers Transformers supplying harmonic loads must be appropriately de-rated Harmonic currents, being of higher frequency, cause increased magnetic losses in the core and increased eddy current and skin effect losses in the windings Triple-n harmonic currents circulate in delta windings, increasing resistive losses, operating temperature and reducing effective load capacity Copper Development Association Increased Eddy current losses in transformers Increase in eddy current loss can be calculated by: Peh Pef h h max h 1 I 2h h 2 where: Peh is the total eddy current loss Pef is the eddy current loss at fundamental frequency h is the harmonic order Ih is the RMS current at harmonic h as a percentage of rated fundamental current Copper Development Association K-Rating of Transformers Two rating or de-rating systems for transformers:• American system, established by UL and manufacturers, specifies harmonic capability of transformer - known as K factor. • European system, developed by IEC, defines de-rating factor for standard transformers known as factor K. Copper Development Association K-Rating of Transformers - US System First, calculate the K factor of the load according to: K h h max h 1 I 2h h 2 where: h is the harmonic order Ih is the RMS current at h in per unit of rated load current Copper Development Association K-Rating of Transformers - US System Desktop System 0.6000 0.5000 Current (A) 0.4000 0.3000 0.2000 0.1000 0.0000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Harmonic For this typical PC load, the K factor is 11.6 (See IEE 1100-1992 for a worked example) Copper Development Association K-Rating of Transformers - US System Next, select a transformer with a higher K rating: standard ratings are 4, 9, 13, 20, 30, 40 and 50. NB - for non K-rated transformers: The K factor describes the increase in eddy current losses, not total losses. Copper Development Association K-Rating of Transformers - European System In Europe, the transformer de-rating factor is calculated according to the formulae in BS 7821 Part 4. The factor K is given by: e I1 K 1 1 e I I nq n I1 n2 2 n N 2 0.5 e is ratio of eddy current loss (50 Hz) to resistive loss n is the harmonic order q is dependent on winding type and frequency, typically 1.5 to 1.7 Copper Development Association K-Rating of Transformers - European System Desktop System 0.6000 0.5000 Current (A) 0.4000 0.3000 0.2000 0.1000 0.0000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Harmonic For the same PC load, the de-rating factor is 78% Copper Development Association K Factor The methods for rating transformers are discussed in CDA Publication 144 In addition, calculation software is available on our web site: www.cda.org.uk Copper Development Association K-Rating - Calculation software Copper Development Association K-Rating - Calculation software Copper Development Association Harmonic Diversity - K Factor K Factor 16 14 12 K Factor 10 8 6 4 2 0 1 2 5 10 No of Units (pairs) Copper Development Association 20 41 K-Rating or De-rating? ‘K-rated’ transformers are designed to supply harmonic loads by : • using stranded conductors to reduce eddy current losses • bringing secondary winding star point connections out separately to provide a 300% neutral rating Copper Development Association K-Rating or De-rating? ‘De-rating’ a standard transformer has some disadvantages: primary over-current protection may be too high to protect the secondary and too low to survive the in-rush current the neutral star point is likely to be rated at only 100% of the phase current it is less efficient future increases in loading must take the de-rating fully into account Copper Development Association Effect of harmonics on transformers Transformers supplying harmonic loads must be appropriately de-rated Harmonic currents, being of higher frequency, cause increased magnetic losses in the core and increased eddy current and skin effect losses in the windings Triple-n harmonic currents circulate in delta windings, increasing resistive losses, operating temperature and reducing effective load capacity Copper Development Association Effect of triple-n harmonics in transformers Triple-n harmonic currents circulate in delta windings they do not propagate back onto the supply network. - but the transformer must be specified and rated to cope with the additional losses. Copper Development Association Skin effect Alternating current tends to flow on the outer surface of a conductor - skin effect - and is more pronounced at high frequencies. At the seventh harmonic and above, skin effect will become significant, causing additional loss and heating. Where harmonic currents are present, cables should be de-rated accordingly. Multiple cable cores or laminated busbars can be used. Copper Development Association Skin effect - penetration depth where: 1 d 2 105 f d is the depth of penetration, mm f is the frequency, Hz, and is the resistivity of the conductor At the fundamental, 50 Hz d = 9.32 mm At the 11th harmonic, 550Hz d = 2.81 mm Copper Development Association Circuit breakers Nuisance tripping can occur in the presence of harmonics for two reasons: Residual current circuit breakers are electromechanical devices. They may not sum higher frequency components correctly and therefore trip erroneously. The current flowing in the circuit will be higher than expected due to the presence of harmonic currents. Most portable measuring instruments do not read true RMS values. Copper Development Association Problems caused by harmonics currents within the installation overloading of neutrals over-heating of transformers over-stressing of power factor correction capacitors skin effect nuisance tripping of circuit breakers voltages within the installation voltage distortion & zero-crossing noise over-heating of induction motors currents in the supply Copper Development Association Voltage distortion Copper Development Association Reducing Voltage Distortion by Circuit Separation Copper Development Association Effect of harmonics on induction motors Increased magnetic and copper losses Each harmonic generates a field which may rotate forward (+), backward (-), or remain stationary (0) 1 2 3 4 5 6 7 8 9 10 11 12 + - 0 + - 0 + - 0 + - 0 • Zero sequence harmonics produce a stationary field, causing over-heating and reduced efficiency Copper Development Association Effect of harmonics on induction motors • The negative and positive sequence harmonics together cause torque pulsing, vibration and reduced service life • Harmonics are induced in the rotor leading to overheating and torque pulsing Stator harmonic 1 Rotor harmonic Harmonic rotation F Copper Development Association 5 6 B 7 6 F 11 12 B 13 12 F 17 18 B 19 18 F Motor de-rating curve for harmonic voltages 1 De-rating Factor 0.95 0.9 0.85 0.8 0.75 0.7 0 2 4 6 Harmonic Voltage Factor (HVF) Copper Development Association 8 10 12 Harmonic voltage factor The Harmonic Voltage Factor (HVF) is defined as: HVF n V 2 n n 5 n where: Vn is the RMS voltage at the nth harmonic as a percentage of the fundamental, and n is the order of each odd harmonic, excluding triple-Ns Copper Development Association Harmonic problems affecting the supply Harmonic currents cause harmonic voltage distortion on the supply that can affect other customers. This distortion can propagate onto the 11 kV grid and spread widely. There are limits for harmonic voltage distortion - a supplier may refuse to supply power to a site that exceeds them. Copper Development Association Harmonic Standards Electricity Association Engineering Recommendation G 5/4 (2001) BS EN 61000 IEEE Std 519-1992 Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems ISBN Copper Development Association 1 - 55937 - 239 - 7 Why revise G5/3? Levels at 132kV higher than Grid Code allows Introduction of concept of Electromagnetic Compatibility G5/3 didn’t include notching and burst harmonics Introduction of the EU Compatibility Directive and developments in IEC and European Standards Better information on network harmonic impedance (see ETR 112) Copper Development Association The Electromagnetic Compatibility concept Satisfactory operation of supply systems and users’ equipment only when electromagnetic compatibility exists between them Emission limits help fulfil this objective G5/4 seeks to limit harmonic distortion levels on the network at the time of connection to below the immunity levels of equipment Enforced via the Electricity Supply Regs, Grid & Distribution Codes, and connection agreements Copper Development Association Harmonic Compatibility Planning levels Probability Density Total supply network disturbance Compatibility Level Immunity (test) levels Emission limits for individual sources Disturbance Level Copper Development Association Susceptibility of local equipment Compatibility levels v Planning levels Compatibility levels in IEC 61000-2-2 & 61000-212, for 400V and 6.6kV to 33kV systems are based on the immunity of capacitors The margins between planning levels and the compatibility levels depend on voltage level and range from 3% at lv and 5% at mv to 0.5% at ehv The margins are necessary to make allowance for system resonance and for loads connected where there is no consent required from the DNO Copper Development Association Stage 1 Applies only to lv connected loads Requires reference to other IEC standards e.g. IEC 61000-3-2 emissions from lv connected equipment <16A IEC 61000-3-4 ditto >16A (To be 61000-3-12) Clarifies that levels may be modified by reference to relevant fault levels rather than the notional ones used to derive the table of emissions Table 7 Copper Development Association Aggregate loads G5/4 requires that aggregate non-linear loads be considered • An individual non-linear equipment complying with 61000-3-2 can be connected without consideration • Groups of non-linear equipment with aggregate rated current <16A and complying can be connected • For >16A either 61000-3-4 or 61000-3-6 should be used to assess emissions using diversity rules from 61000-3-6 if necessary Copper Development Association Example of application - the problem Connection of communication centre equipment – 15 off rectifier equipment type R2948-15 – each equipment is rated at 12.37A – each equipment meets BS EN 61000-3-2 – the connection will be at lv and single phase – future expansion expected to 30 units Can they be connected? The customer says that no data on emissions is available Copper Development Association The solution Data must be available - cannot claim BS EN 61000-3-2 compliance otherwise! Data was obtained simply by e-mailing the manufacturer in New Zealand Simplified calculations were carried out on a spreadsheet to check compliance Copper Development Association Product data sheet Product Test Harmonic Emissions Product: R2948-15 Serial #: 1040171 Date tested: 07 October 1999 Product Compliance Group 0.5 Test Parameters Input Voltage: 230v 50Hz Output Voltage: 54v at no load Output Current: 52A Ambient temperature: +20°C 12.37A EUT EN61000-3-2 IP Currenty(A) 0.4 0.3 0.2 0.1 0.0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Harmonic (f1 = 50Hz) Copper Development Association The calculations As a first estimate the current emissions are multiplied by the number of units, and the result compared with the values in Table 7 of G5/4. This shows that there is no problem The spreadsheet calculations would show that the future increase to 30 units would give values of emissions greater than the limits for triple-Ns above 21st Copper Development Association Table 7: Stage 1 Max Harmonic RMS Current Emissions for aggregate loads and equipment rated >16A per phase Harmonic Emission order ‘h’ current Ih 2 28.9 3 48.1 4 9.0 5 28.9 6 3.0 7 41.2 8 7.2 9 9.6 10 5.8 11 39.4 12 1.2 13 27.8 14 2.1 Harmonic Emission order ‘h’ current Ih 15 1.4 16 1.8 17 13.6 18 0.8 19 9.1 20 1.4 21 0.7 22 1.3 23 7.5 24 0.6 25 4.0 26 1.1 27 0.5 Copper Development Association Harmonic Emission order ‘h’ current Ih 28 1.0 29 3.1 30 0.5 31 2.8 32 0.9 33 0.4 34 0.8 35 2.3 36 0.4 37 2.1 38 0.8 39 0.4 40 0.7 Harmonic Emission order ‘h’ current Ih 41 1.8 42 0.3 43 1.6 44 0.7 45 0.3 46 0.6 47 1.4 48 0.3 49 1.3 50 0.6 Sample spreadsheet Harmonic number 3 Emission from EUT 0.42 Emissions 15 units 6.3 Table 7 emissions 48.1 Emissions 30 units 12.6 5 0.21 3.1 28.9 6.2 7 0.16 2.3 41.2 4.7 9 0.11 1.65 9.6 3.3 15 0.03 0.43 1.4 0.8 21 0.035 0.525 0.7 1.05 Emissions in Amps (RMS) Copper Development Association Example flow chart for lv connection START N N Complies with 61000-3-4 Less than 16A Y Complies with 6.3.1 N Y Y N 3 phase Y Complies Complieswith with 61000-3-2 61000-3-2 0 Y Y N Complies with Table 7 Y Y N Mitigation required Connect to network Copper Development Association N Y Complies with Table 6 N Complies with 6.2 <5 kVA Go to Stage 2 N Stage 2 This applies only to: a load or aggregate load that doesn’t meet IEC 61000-3-2 and 61000-3-6, or Table 7 current emissions, i.e. Stage 1 PCC less than 33kV i.e. at 6.6, 11 or 22kV Current emissions can be less than Table 12, or a simplified voltage assessment can be used based on the harmonic impedance just described Copper Development Association Harmonic Measurements Copper Development Association Assessment of the connection of new non-linear equipment under Stage 2 a) measure voltage distortion present at PCC b) assess the voltage distortion which will be caused by the new equipment, and c) predict the possible effect on harmonic voltage levels by an addition of the results of (a) and (b) Copper Development Association Assessment of the connection of new non-linear equipment under Stage 2 If the results of (c) are less than • the harmonic voltage planning levels for the 5th harmonic and • the THD planning level connection of the equipment is acceptable Copper Development Association Combination rules for harmonics up to and including the 5th and for all triple-Ns, the measured and calculated values of voltage distortion are assumed to peak at the same time and to be in phase - linear addition for the other harmonics, an average phase difference of 90 is assumed at the time of maximum THD - rms addition the THD is then given by the rms addition of all combined harmonics up to the 50th Copper Development Association The Challenge to keep the harmonic voltage distortion at the point of common coupling below levels permitted by G5/4 to keep harmonic currents below levels that cause equipment overload and damage within the installation that are permitted by G5/4 Copper Development Association Harmonic solutions Steps to be taken to reduce voltage distortion on the supply include, for example: Passive harmonic filters Isolation transformers Active harmonic conditioners Copper Development Association Passive harmonic filters Filters are useful when the harmonic profile is well defined – such as motor controllers the lowest harmonic is well above the fundamental frequency - but filter design can be difficult and, especially for lower harmonics, the filters can be bulky and expensive Copper Development Association Passive harmonic filter Copper Development Association f Power Factor I V Ip Iq Copper Development Association 0 2 Power Factor POWER Ip Copper Development Association 0 2 Power Factor POWER Iq 0 Copper Development Association 2 f Power Factor V IL I1 I5 Copper Development Association 0 I7 2 Power Factor G active power reactive power M Copper Development Association Power Factor Correction G CAPACITOR active power reactive power M Copper Development Association Power Factor Correction G CAPACITOR active power reactive power M Copper Development Association Power Factor Correction • Diversity • Self Excitation • Harmonics M M Copper Development Association M M Power Factor Correction Control M M M Copper Development Association M Power Factor Correction • Transformer overloading • Step voltage Control • Bank Size • Switch-fuse & Cable load ratings M M M M Copper Development Association • Load make/break rating of main isolator/switchfuse Power Factor Correction Bank Sizing Required improvement in % wattess X kW Maximum Demand equivalent to {tan(cos-1PFA) - tan(cos-1PFR)} X MD (kW) or kVArh (actual) - kVArh (required) running hours X load factor Copper Development Association Power Factor Correction • Capacitor Discharge time required for standard Power Factor banks (1 minute) • Rapidly switching loads require Zero crossing Thyristor or IGBT switched steps e.g. Spot Welders Lift motors Cranes Copper Development Association Harmonic Resonance TO POWER SYSTEM LV CONVERTOR M HARMONICS Copper Development Association AMPLIFIED HARMONICS Detuned or Blocking Banks SOURCE IMPEDANCE WITH FILTER IN CIRCUIT Capacitive 0.3000 Fo = 189 to 204 Hz 0.2500 Y = Ln (Z+1) 0.2000 Inductive 0.1500 5th 11th 7th 0.1000 0.0500 Frequency Copper Development Association 592 580 568 556 544 532 520 508 496 484 472 460 448 436 424 412 400 388 376 364 352 340 328 316 304 292 280 268 256 244 232 220 208 196 184 172 160 148 136 124 112 100 0.0000 Filter Banks - 5th Harmonic SOURCE IMPEDANCE WITH FILTER IN CIRCUIT Capacitive 0.3000 Fo = 235 to 245Hz 0.2500 7th Y = Ln (Z+1) 0.2000 0.1500 Inductive 0.1000 0.0500 10 0 10 7 11 4 12 1 12 8 13 5 14 2 14 9 15 6 16 3 17 0 17 7 18 4 19 1 19 8 20 5 21 2 21 9 22 6 23 3 24 0 24 7 25 4 26 1 26 8 27 5 28 2 28 9 29 6 30 3 31 0 31 7 32 4 33 1 33 8 34 5 35 2 35 9 36 6 37 3 38 0 0.0000 Frequency Copper Development Association Filter Banks 5th & 7th Harmonic SOURCE IMPEDANCE WITH FILTER IN CIRCUIT 0.5000 0.4500 0.4000 Y = Ln (Z+1) 7th 5th 0.3500 0.3000 0.2500 0.2000 0.1500 0.1000 0.0500 Frequency Copper Development Association 592 580 568 556 544 532 520 508 496 484 472 460 448 436 424 412 400 388 376 364 352 340 328 316 304 292 280 268 256 244 232 220 208 196 184 172 160 148 136 124 112 100 0.0000 Third harmonic filters 10 Amps R N S T E Copper Development Association 30 Amps Load 10 Amps 10 Amps Third harmonic filters 10 Amps R I3 =300Amps Amps N Load S T 10 Amps v E Copper Development Association 10 Amps Delta Interconnected-Star Transformer R S T R S T N Copper Development Association N Harmonic reduction transformers Load I3 Interconnected Star Transformer sized for harmonic currents only Copper Development Association Isolating transformers Delta-star isolating transformers reduce propagation of harmonic current into the supply. Transformers should be adequately rated to cope with the harmonics Although the transformer effectively establishes a new neutral, don’t use half-sized neutrals Provide a well rated four wire feed so that the transformer can be isolated for service Copper Development Association Isolating transformers Copper Development Association Isolating transformers Copper Development Association Isolating transformers Copper Development Association Isolating transformers Copper Development Association Active filters Where the harmonic profile is unpredictable or contains a high level of lower harmonics, active filters are useful Active harmonic conditioners operate by injecting a compensating current to cancel the harmonic current Copper Development Association Harmonic solutions Keep circuit impedances low Rate neutrals and multi-core cables generously 1.73 to 2 times normal size Always use true RMS meters Provide a large number of separate circuits to isolate problem and sensitive loads Take harmonics into account when rating transformers Provide appropriate filtration where required Copper Development Association Harmonics in Power Systems Copper Development Association www.cda.org.uk Copper Development Association