Power Quality Partnership

Power Quality Partnership
Harmonics in Power Systems Copper Development Association is a non-trading organization funded by copper miners and refiners to encourage the proper use of copper and copper alloys in all its applications. It is part of a world wide network of similar organization. CDA operates a free technical helpline and publishes most of its information on its Web site.

Power Quality Partnership
Copper Development Association Fluke (UK) Ltd MGE UPS Systems Ltd Rhopoint Systems Ltd

Copper Development Association
Established 1933 website - Technical helpline IEE Endorsed Provider

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 I shall talk first about the background to harmonics and then go on to discuss the problems that arise and some of the available solutions. I will speak briefly about the problems of earth leakage being caused by electronic equipment, and then give a short presentation on electrical energy efficiency. Steve Taylor of Fluke UK will deal with practical harmonic measurements and talk about PQ site surveys and Shri Karve of MGE UPS systems will talk about managing harmonics with active conditioners.

Fundamental with third and fifth harmonics
… a very quick recap. Harmonics are sinusoidal components of a complex waveform that have a period that is integrally related to the period of the waveform. In other words, a one kilohertz square waveform is made up of sinusoidal components of 1 kHz, 3 kHz, 5 kHz, etc. This slide shows a fundamental sine wave (black), the third harmonic at 60% in red and the fifth harmonic at 40% in blue.

Composite waveform Adding them together gives the resultant waveform seen here. Note that the wave shape depends on the relative magnitudes and phase relationships of the fundamental and harmonics - and the eye is not a good analyser. Often, as in this case, the resultant waveform has more than two zero-crossings per cycle. Any complex waveform can be analysed into a fundamental and a series of harmonics.

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 Now lets look at the kinds of load that produce harmonics. This is not an exhaustive list, but it probably contains the most common culprits. Switched mode power supplies now turn up in almost every electronic device, but are most commonly associated with personal computers. They are small and lightweight and can be made to fit into almost any space. A 150 watt multi output PC power supply, complete with internal fan and case weighs about the same as a 50 VA transformer alone. It is no surprise that they are now universal. Electronic lighting ballasts are often preferred, largely because of the ease with which feedback can be applied to control lighting levels. Efficiency is similar to the best magnetic ballasts, but the harmonic profile can be alarming. Variable speed drives have been in use for many years and their harmonic profiles are well known. Uninterruptible power supplies fall into two basic groups - the small ‘single user’ systems, which have a profile similar to a switched mode power supply and the large central 3 phase units which have profiles similar to motor controllers. All these loads are non linear ie the current drawn is not proportional to applied voltage.

How harmonics are generated – linear load
Lets look at how this happens. This slide shows a linear load line - the top right hand quadrant - with current on the vertical axis and voltage on the horizontal axis. The lower right hand quadrant shows the voltage waveform with time advancing up the vertical axis. The upper left hand quadrant shows the resultant current, with time advancing to the right on the horizontal axis. The load line can be considered as a mirror, so, as would be expected with a linear load, the current waveform is a sinusoid, just like the applied voltage.

How harmonics are generated – non-linear load
This slide shows what happens with a non-linear load. This is an idealised load line. Current flows only when the applied voltage is above a certain voltage threshold. Mirroring the voltage waveform in the load line gives the current waveform shown here. In practice, for a switched mode power supply, fore example, the threshold would be different for rising and falling voltage, and different for each half cycle, so the current waveform wuld be much more complex than this.

A Common non-linear load

Current waveform for a typical Personal Computer
This is a real current waveform for a personal computer, fairly typical of a switched mode power supply - one current pulse per half cycle, nothing like a sine wave. This, and the following, measurements were taken with a Fluke 41 Power Analyser on a relatively clean supply in a domestic environment. And here…

Harmonic profile of a typical Personal Computer
And here is the harmonic profile or spectrum. Notice that only odd harmonics are present, with the third at over 80% of the fundamental and the fifth at nearly 60% of the fundamental, with significant harmonics up to the eleventh. This spectrum is also typical of small ‘desktop’ (single phase) UPS systems.

Harmonic profile for electronic fluorescent ballast
This is the profile for an electronic lighting ballast, with harmonics up to the 17th. For comparison..

Harmonic profile for magnetic fluorescent ballast
For comparison this is the spectrum for a traditional magnetic ballast. The best magnetic ballasts are very nearly as efficient as electronic ballasts, but without the harmonics.

Six-pulse bridge Variable speed drive controllers and large UPS systems use multiphase rectifiers in the input stage. This is the simplest form, the three phase or six-pulse bridge. Assuming an inductive load….

Typical harmonic profile - six-pulse bridge
Assuming an inductive load the harmonic profile looks like this. Notice that the only harmonics present are multiples of the number of pulses plus and minus 1, i.e. 5th, 7th, 11th, 13th, 17th, etc. The magnitudes are predictable - in the ideal case, where the load is inductive, the magnitude of each harmonic is the fundamental divided by the harmonic number. So the fifth has a magnitude of 20% and the eleventh has a magnitude of 9 %. This is a plot of actual values, but the agreement with theory is quite close. This is true at full load - at partial load the relative harmonic magnitudes are higher.

Twelve-pulse bridge Distortion levels can be reduced by the use of a twelve pulse bridge which consists of two six pulse bridges in parallel, fed by a six phase supply produced from a transformer with star and delta secondary windings. If the windings and rectifiers are perfectly symmetrical, this arrangement results in harmonic orders which are multiples of 12 plus and minus 1. In practice...

Typical harmonic profile - twelve-pulse bridge
In practice, the spectrum typically looks like this, with the 5th, 7th, 17th and 19th significantly reduced, but not eliminated, and the others effectively the same as for the six pulse bridge. Further phase multiplication can be used to further eliminate harmonics, but, of course, at increased cost.

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 Harmonics have been present on the network for many years caused by the rectifiers used for railway electrification and for variable speed drives. Radio and television receivers also made a significant contribution. The situation has been made worse by the rapid increase in the number of these loads and the quantity of electronic equipment in use. Most of these loads are rich in triple N harmonics - that is the odd multiples of three (3rd, ninth, fifteenth etc.) - and we shall see shortly what effect these have. Finally, the design of equipment has changed. Until the early 70s, television receivers, for example, used valves. About half the load was perfectly linear - about 70 watts was required to heat the filaments with another 70 watts - half-wave rectified - for the HT supplies. A modern receiver would consume about two thirds of the power, all of it a non-linear load.

Equivalent circuit of a harmonic generating load
A non-linear load can be represented as a linear load in parallel with a number of harmonic current generators - one for each harmonic present. These harmonic currents flow through the circuit impedances in the installation and the supply, causing harmonic voltage drops to develop. The effects of these on sensitive equipment has to be considered. If the supply is common to other uses, that is there is no dedicated transformer, other parties will get a share of your harmonics, and you will get a share of theirs. Looking first at the effect of harmonic currents within the installation...

Harmonic Diversity

Harmonic Diversity - THDI

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 Having looked at typical harmonic profiles, we now turn to look at the problems they can cause. There are there areas of concern, currents in the installation, voltages in the installation and currents in the supply. All these aspects must be examined in order to arrive at a solution. To clarify this, lets look at the equivalent circuit of a non-linear load.

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 In a balanced three phase circuit the neutral current sums to zero and for this reason neutral conductor sizes were frequently reduced. This practice is still formally permitted in the wiring codes of some countries, but it is no longer encouraged in the UK. One problem is that triple-N harmonics, that is odd multiples of three (3rd, 9th, 15th) add in the neutral rather than cancel.

Harmonic neutral currents
This slide shows this graphically. At the top are the three phase currents. Below, separated for clarity are three third harmonic currents, and below again, the sum of these currents. We’ve already seen that switched mode power supplies, lighting ballasts and UPS systems produce large third harmonic components, so you would expect IT rich environments to experience high third harmonic neutral currents. Demonstration?

Neutral conductor sizing

Neutral conductor sizing
Neutral currents can easily approach twice the phase currents - sometimes in a half-sized conductor. IEEE recommends that neutral busbars feeding non-linear loads should have a cross-sectional 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. The only solution here is to use an adequately rated conductor for the neutral. IEEE recommends 173 % of the phase cross sectional area for busbars, and 200% seems a more convenient size when using cables. Five core cable is a simple solution, using three cores for the phases and two for the neutral, six would be better, allowing another core for the protective conductor. On the other hand, adding a separate conductor alongside an existing four core cable is not a good idea. The mutual inductance between the phase cores and the external neutral is lower than that between the phase and neutral cores, so the impedance of the external core appears to be much greater, whatever size you make it, and it carries little current - especially at harmonic frequencies. The solution here is to disconnect the existing neutral and replace it with an external conductor.

Sizing the neutral conductor
BS 7671: From January 2002 where neutral current is expected to exceed phase current where neutral cross-section is less than phase cross section - neutral overcurrent protection is required Where a circuit is wired in single core cables the situation is fairly straightforward. The neutral cable should be selected for the actual neutral current, measured with a true rms meter. Where the current cannot be measured, for example where the final occupancy of the building is unknown, use a double sized neutral. Care is needed though - the effect is the same as having another single phase circuit running alongside and an allowance for this, known as ‘grouping factor’ must be taken into account. The voltage drop in the neutral will be a distorted voltage containing triple N components and this will appear in series with all the phase to neutral voltages.

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 Where a circuit is wired in single core cables the situation is fairly straightforward. The neutral cable should be selected for the actual neutral current, measured with a true rms meter. Where the current cannot be measured, for example where the final occupancy of the building is unknown, use a double sized neutral. Care is needed though - the effect is the same as having another single phase circuit running alongside and an allowance for this, known as ‘grouping factor’ must be taken into account. The voltage drop in the neutral will be a distorted voltage containing triple N components and this will appear in series with all the phase to neutral voltages.

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 Multi core cables are a little more difficult. Multi core cables are e rated for only three current carrying cores - I.e. it is assumed that the load is balanced and that there are no triple N harmonics. When triple N harmonics are present, the neutral is a current carrying conductor and the heat generated in it has to be taken into account. Effectively, a cable rated for three units of current - one for each phase - is carrying more - because of the current in the neutral. To account for this the cable has to be de-rated. Again, grouping factor has to be applied for other circuits running in the same duct.

Sizing the neutral conductor - thermal
Taking a simple thermal approach gives this graph. It shows the size increase multiplier required against the percentage triple N content in the phases. Taking a simple example, suppose that the triple N content is 50% in each phase, giving 150% in the neutral. The total cable current is 450% (3 X 100% plus 150%) instead of the design value of 300%, so we must use a cable that is 1.5 times larger than expected. This is a very simple method to apply in practice.

Sizing the neutral conductor - IEC
IEC has an infomative annex that includes a table method of estimating the cable size required. Plotted as a graph it looks like this. At 50% triple N content, the result agrees with the thermal method!

Looking at the two curves together, it can be sen that there is little difference in the range of most interest in commercial installations - between 30% and 60% triple N content. The important thing is that the cable is de-rated - by whatever method - and that the practice of using of under-sized neutrals is stopped.

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.

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 There are two problems affecting transformers. Firstly, the triple N currents circulating in the delta winding increases the resistive losses, and since it is a part of the load, reduces the load capacity of the transformer. Secondly, the magnetic and eddy current losses are higher because of the higher frequency of the harmonic currents. The consequent increase in working temperature further reduces the capacity of the transformer.

Increased Eddy current losses in transformers
Increase in eddy current loss can be calculated by: 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 There are two important frequency related losses in transformers; eddy current loss and stray magnetic losses. This is the relationship between normal, fundamental frequency, eddy current losses and those at harmonic frequencies. Effectively the multiplier is the sum of the square of the harmonic current times the square of the harmonic number.

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. So there are two basic approaches; either build the transformer for the expected duty or de-rate a standard unit to allow it to cope. In the US, UL have established a figure of merit which describes how well a unit can cope with harmonic loads, known as K Factor. In Europe, a standard has been produced to specify how a transformer should be de-rated, known as factor K.

K-Rating of Transformers - US System
First, calculate the K factor of the load according to: where: h is the harmonic order Ih is the RMS current at h in per unit of rated load current K factor is defined like this. Note that it is very similar to the eddy current loss equation we saw earlier.

Applying this to the PC load that we saw earlier, the K factor is 11.6. In other words, the eddy current losses would by nearly twelve times higher than if the same load were purely fundamental. For this typical PC load, the K factor is 11.6 (See IEE for a worked example)

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. It is then a simple matter of selecting a suitably rated transformer from the standard range of K ratings. Note that - in the US case - K factor is a measure of the increase in eddy current losses only. Since the eddy current loss accounts for about 10% of full load losses, a K factor of 11.6 translates to a de-rating to about 70% capacity.

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 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 The European standard gives this equation for the de-rating factor K. If you unwind it, it is not too dissimilar from the US equation - both involve summing the losses for individual harmonics for example. This version values the harmonic order less, since the harmonic order n is raised to the power 1.5 to 1.7 instead of two, and goes a stage further by turning the excess loss into a de-rating.

K-Rating of Transformers - European System
Applying this to my favourite PC load gives a derating to 78% capacity, which is not too far away from the US figure of 70%. However, if you calculate the effect of individual harmonics by the two methods the results can be quite different. For the same PC load, the de-rating factor is 78%

K Factor The methods for rating transformers are discussed in CDA Publication 144 In addition, calculation software is available on our web site:

K-Rating - Calculation software

Harmonic Diversity - K Factor

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 So, is it better to use a K rated transformer, or to de-rate a standard one? The decision probably depends on availability and lead time, but it is worth bearing in mind that a K rated unit will have been designed to have lower losses and will have a triple rated star point.

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 On the other hand, a de-rated standard transformer will have, effectively, an oversized primary, so that sizing primary over current protection is difficult. Either it will be too high to offer protection to the secondary, or it will be too low to survive the inrush current. In future, long after the derating calculations have been lost, someone will read the rating plate and decide to add more load. It is less efficient because the extra loss is merely being dissipated rather than designed out. And, of course, the neutral point will only have a 100% capacity.

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 Now we turn to the effect of Triple N currents on transformer losses

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. So here is a delta star transformer. The triple n harmonicas circulate in the delta winding, so there are no triple n harmonic components in the primary currents and no triple n harmonic voltage components on the primary voltages. This is good news - the harmonic problems in the installation are isolated from the supply - but the price is a reduction in supply capacity. This is why the supply companies are beginning to worry about harmonic loads - they will have to invest in plant capacity.

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. Skin effect, the apparent increase in resistance due to the tendency for current to flow on the outside of the conductor, becomes important in busbars above about 300 Hz. This increases cable losses and heating and should be taken into account when determining conductor sizes.

Skin effect - penetration depth
where: 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

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. Nuisance tripping has two common causes. Residual current breakers operate by comparing the current in phase and neutral conductors. This summing may fail at the higher harmonic frequencies, leaving the breaker over-sensitive. Additionally, the current flowing is often greater than expected because of the presence of harmonics and ordinary measuring instruments do not measure true RMS. Ken West will have more to say on this subject.

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 Now we turn to problems caused by harmonic voltages.

Voltage distortion Voltage distortion is caused when a non-linear load current, as shown at the right hand side, flows through the source and cable impedances. The distorted voltage affects every other load on the circuit, so that even the current flowing in linear loads is distorted.

Reducing Voltage Distortion by Circuit Separation
The effect can be substantially reduced, but never eliminated, by separating circuits for non-linear loads and sensitive linear loads. The degree of improvement depends on the relative magnitudes of the source and cable impedances. This is particularly useful in situations where there is a large population of distorting equipment, such as IT and process control equipment, together with sensitive loads, such as induction motors. We have to keep in mind that in many commercial premises, almost all of the lighting load and a large part of the power load is non-linear. Obviously, the lower the impedance of the cabling, the better, and this is probably a good point to mention efficiency. BS 7671, the Wiring regulations, specify the minimum conductor sizes consistent with safety, and allow conductor temperatures of 70 or 90 degrees C, depending on the insulation material. This is a great waste of energy. Using larger conductors is actually much more economic; the cost of the cable and of installation is not proportional to size, so a cable twice the size doesn’t cost twice as much, but it halves the cost of wasted energy - and it gives the benefits of lower impedance. For those who are not convinced, there is a computer with calculation software on it over there!

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) Zero sequence harmonics produce a stationary field, causing over-heating and reduced efficiency Induction motors are very sensitive to harmonics for two reasons. Firstly, the magnetic and conductor losses are higher at harmonic frequency leading to higher operating temperature, reducing efficiency and shortening motor life. Secondly, each harmonic generates magnetic field in the stator and rotor. The stator field may rotate in the same direction as the fundamental, rotate in the opposite direction, or remain stationary. Stationary fields, including the TripleNs, merely add to the heating effect, but the the rotating fields are much more important.

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 Rotor harmonic Harmonic rotation F B F B F B F Counter rotating pairs such as the 5th and seventh harmonics, which we saw earlier were produced by the standard three phase bridge, can cause pulsing and oscillation in motor -load systems and reduced torque. Harmonic currents are induced into the rotor as detailed in this chart, (which includes only with the harmonics resulting from a three phase bridge). Since the fifth harmonic is contra rotating in the stator, and the rotor is rotating forward, the rotor current current is at the sixth harmonic. Similarly the seventh harmonic is forward rotating, so the induced rotor harmonic is again the sixth. The result is overheating of the rotor, pulsing and reduced torque.

Motor de-rating curve for harmonic voltages
Because of these effects, motors on a distorted supply must be de-rated. This slide shows the curve recommended by NEMA, the National Equipment Manufacturers Association (USA). Efficiency falls off rapidly as the Harmonic voltage factor increases.

Harmonic voltage factor
The Harmonic Voltage Factor (HVF) is defined as: 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 Harmonic voltage factor is calculated from this formula - it is simply the root of the sum of each harmonic voltage divided by its harmonic order. For the harmonic limits given in EN that is, the levels you can expect on your incoming supply - motor rating should be reduced to 97%.

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. Sites which draw harmonic currents cause harmonic voltage distortion on the supply, the magnitude depending on the size of the current and the impedance at the point of coupling. Although the triple Ns will be trapped at the first delta winding they see, other harmonics will propagate onto the next voltage level, 11 kV, and spread very widely. The pattern of television viewing is reflected in the total harmonic distortion level measured on the 11 kV grid. The fact that the triple Ns do no propagate does not mean that the supplier sis unconcerned about them - they cause heating in distribution transformers which effectively reduces system capacity. There are specified limits for the levels of harmonics that a site is permitted to put onto the network and there are several standards to … be aware of. I was going to say help but that might be an overstatement!

Harmonic Standards ISBN 1 - 55937 - 239 - 7
Electricity Association Engineering Recommendation G 5/4 (2001) BS EN 61000 IEEE Std Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems ISBN These are just three of the multitude of standards which have some bearing on harmonics. G.5/3, last revised in 1976, is currently under revision which will be issued as G.5/4. G5/3 defines limits for the magnitude of harmonic current that the user can ‘export’ onto the grid.

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) 2

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

Harmonic Compatibility
Disturbance Level Total supply network disturbance Probability Density Compatibility Level Susceptibility of local equipment Immunity (test) levels Planning levels Emission limits for individual sources 3

Compatibility levels v Planning levels
Compatibility levels in IEC & , 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

Stage 1 Applies only to lv connected loads
Requires reference to other IEC standards e.g. IEC emissions from lv connected equipment <16A IEC ditto >16A (To be ) 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 5

Aggregate loads G5/4 requires that aggregate non-linear loads be considered An individual non-linear equipment complying with can be connected without consideration Groups of non-linear equipment with aggregate rated current <16A and complying can be connected For >16A either or should be used to assess emissions using diversity rules from if necessary

Example of application - the problem
Connection of communication centre equipment 15 off rectifier equipment type R each equipment is rated at 12.37A each equipment meets BS EN 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

The solution Data must be available - cannot claim BS EN compliance otherwise! Data was obtained simply by ing the manufacturer in New Zealand Simplified calculations were carried out on a spreadsheet to check compliance

Product data sheet

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

Table 7: Stage 1 Max Harmonic RMS Current Emissions for aggregate loads and equipment rated >16A per phase

Sample spreadsheet Emissions in Amps (RMS)

Example flow chart for lv connection
Example flow chart for lv connection START Complies with Complies with 6.3.1 Less than 16A N N N Y Y Y <5 kVA Complies with 3 phase N N Complies with Y Y Complies with Table 6 Y N N Complies with 6.2 Complies with Table 7 N Y Y Y N Connect to network Mitigation required Go to Stage 2

Stage 2 This applies only to:
a load or aggregate load that doesn’t meet IEC and , 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

Harmonic Measurements
9

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)

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

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

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

Harmonic solutions Steps to be taken to reduce voltage distortion on the supply include, for example: Passive harmonic filters Isolation transformers Active harmonic conditioners When the current or voltage distortion caused by a site exceeds the stage 2 or 3 limits of G.5/3 or equivalent, steps must be taken to reduce it. Listed here are three common approaches.

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 Passive filters are useful where the harmonic profile is well known and stable, variable speed drives for example, and where the problem harmonics are well away from the fundamental. They are normally tailored to the particular installation. One problem is that the harmonic profile can change, either due to a change in the harmonic distortion of the incoming supply or changes in the in-plant equipment. The filter may be overloaded as a result. You can find yourself importing harmonics to filter.

Passive harmonic filter
This is a passive filter added to the equivalent circuit. The filter acts as a bypass for the harmonic current, keeping it away from the source. The filter may be positioned centrally or close to the polluting equipment.

f Power Factor I V Ip Iq p 2

Power Factor POWER Ip p 2

Power Factor POWER Iq p 2

f Power Factor L I V I 1 I 5 I 7 p 2

Power Factor G active reactive power power M

Power Factor Correction
G CAPACITOR active reactive power power M

Diversity Self Excitation Harmonics M M M M

Control M M M M

Transformer overloading Step voltage Bank Size Switch-fuse & Cable load ratings Load make/break rating of main isolator/switch-fuse M Control

Required improvement in % wattess
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

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

Harmonic Resonance M AMPLIFIED HARMONICS TO POWER SYSTEM LV CONVERTOR

Detuned or Blocking Banks
Capacitive Fo = 189 to 204 Hz Inductive 11th 5th 7th

Filter Banks - 5th Harmonic
Capacitive Fo = 235 to 245Hz 7th Inductive

Filter Banks 5th & 7th Harmonic

Third harmonic filters
10 Amps R 30 Amps N Load S 10 Amps 10 Amps T E

Third harmonic filters
10 Amps R v I3 = 0 Amps 30 Amps N Load S 10 Amps 10 Amps T E

Delta Interconnected-Star Transformer

Harmonic reduction transformers
Load I3 Interconnected Star Transformer sized for harmonic currents only

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

Isolating transformers

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

Harmonic solutions Keep circuit impedances low
Rate neutrals and multi-core cables generously 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 Whether you use passive or active filters or isolation transformers, or some combination, it has to be remembered that the cabling on the load side is still carrying the harmonic currents and must be rated to suit. There will always be situations where the correction equipment has to be removed, either for maintenance or because it has failed, so the cabling on the supply side should also be fully rated to carry the harmonics.

Copper Development Association
Harmonics in Power Systems Copper Development Association Copper Development Association is a non-trading organization funded by copper miners and refiners to encourage the proper use of copper and copper alloys in all its applications. It is part of a world wide network of similar organization. CDA operates a free technical helpline and publishes most of its information on its Web site.

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