Low Voltage_ Low Frequency Harmonic Emission Limits

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					Low Voltage, Low Frequency Harmonic Emission Limits

By Merv McInnis for CSA Technical Committee 311.4 June 30, 1997 (Second Draft corrected according CIRED publication) Scope This paper will describe recommended harmonic emission levels for electronic equipment for use in Canada, based on IEC 1000-3-2, expressed as harmonic currents. These emission levels are based on the same rationale that was used to derive the harmonic limits listed in IEC 1000-3-2 for 50-Hz systems prevalent in Europe. The limits have been adjusted to reflect a standardized Canadian source impedance at 60 Hz and at different operating voltages (120, 208, 240, 347 and 600 VAC, 60 Hz). 1.0 Emission Classes IEC 1000-3-2 applies to equipment rated up to 16 amps (16 amps per phase). Class distinctions apply as follows: Class A - less than 1 kW rating, with current base-width that exceeds 60 electrical degrees conduction. Class B - less than 1.5 KW rating, portable tool Class C - lighting equipment Class D - 600 Watts or less, with current basewidth of 60 electrical degrees or less Professional or Class E (proposed class currently under study by IEC committees) covering equipment rated greater than 1-KW but less than 16 amps. For Canadian purposes, in practical applications, IEC 1000-3-2 would apply to equipment rated to 12 amps (since the 15R-type plug generally covers the vast majority of equipment that is cord connected and pluggable). The calculations are carried out, however, all the way to 16 amps, to cover any equipment that might wind up being connected via a 20Rtype plug and receptacle. 2.0 Canadian Source Impedances The standardized Canadian source impedances were extracted form the paper Power Supply Impedances of Residential and Industrial Distribution Systems authored by Roger Bergeron, Amodou Oury and Andre LaPerriere of Hydro-Quebec’s Research Institute.
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Table 1 shows these source impedances at the point of common coupling (PCC) to the public low-voltage distribution system, or the “residential panelboard”. Voltage (VAC) 120 208 240 347 600 Phase Impedance (Ohms) 0.20+0.060i 0.10+0.036i 0.20+0.081i 0.29+0.068i 0.29+0.067i Neutral Impedance (Ohms) N/A 0.10+0.036i N/A 0.30+0.040i N/A Phase-Neutral Phase-Phase Impedance Impedance (Ohms) (Ohms) N/A N/A 0.20+0.070i 0.20+0.070i N/A N/A 0.58+0.11i N/A N/A 0.57+0.14i

Table 1. Canadian Source Impedances at 60 Hz

3.0 Calculation Methodology In order to maintain consistency with the methodology used in developing IEC 1000-3-2, calculations for proposed Canadian limits were based on a review titled Rationale for Harmonics Emission Limits, prepared by TF01 of IEC SC 77A/WG01, identified as Weiler 05/22/96 -1- vintage (Appendix A). The same assumptions were made and current levels were calculated using the various Canadian source impedances and the same admissible contribution levels of voltage distortion as outlined in the review. 4.0 Class A and B Limits Table 2 shows proposed Canadian current harmonic limits (through the 13th harmonic) for Class A (ClA) and Class B (ClB) based on achieving IEC 1000-3-2 voltage compatibility levels using Canadian source impedances at the 120 VAC level. Voltage Z Real Z Imag Harmonic Max Amps Z - Ohms V Drop - % V Drop - Volts Cl.A in Amps Cl.B in Amps 120.00 0.20 0.06 1.00 12.00 0.21 3.00 3.60 17.23 25.84 120.00 0.19 0.19 3.00 12.00 0.27 0.85 1.02 3.85 5.77 120.00 0.19 0.31 5.00 12.00 0.36 0.65 0.78 2.14 3.21 120.00 0.19 0.44 7.00 12.00 0.48 0.60 0.72 1.51 2.27 120.00 0.19 0.56 9.00 12.00 0.59 0.40 0.48 0.81 1.21 120.00 0.19 0.69 11.00 12.00 0.71 0.40 0.48 0.67 1.01 120.00 0.19 0.81 13.00 12.00 0.84 0.30 0.36 0.43 0.65 120.00 0.19 0.94 15.00 12.00 0.96 0.25 0.30 0.31 0.47

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Table 2. Class A and B characteristic values at for low order odd harmonics using Canadian source impedance values - 120 VAC, 60 Hz fundamental Table 3 shows calculated values of even harmonic currents based on Canadian source impedance values at 120 VAC.
Voltage Z Real Z Imag Harmonic Max Amps Z - Ohms V Drop - % V Drop - Volts Cl.A in Amps Cl.B in Amps 120.00 0.20 0.06 1.00 12.00 0.21 3.00 3.60 17.41 26.11 120.00 0.20 0.13 2.00 12.00 0.23 0.30 0.36 1.54 2.31 120.00 0.20 0.25 4.00 12.00 0.32 0.20 0.24 0.75 1.13 120.00 0.20 0.38 6.00 12.00 0.42 0.20 0.24 0.57 0.85 120.00 0.20 0.50 8.00 12.00 0.54 0.20 0.24 0.45 0.67

Table 3. Even harmonic currents based on Canadian source impedance at 120 VAC, 60 Hz. For higher order harmonics, current values are calculated based upon: Odd Harmonics: Even Harmonics:
In  I 15  15  n , Where 15  n  39 In  I 8  8  n , Where 8  n  40

Table 4 shows a summary of Class A and Class B limits proposed for 120 VAC 60 Hz.
Harmonic Order n 1 3 5 7 9 11 13 15 2 4 6 8 Maximum Permissible Harmonic Current Class A (Amps) 17.41 3.75 2.11 1.50 0.80 0.67 0.43 0.31 0.31(15/n) 1.54 0.75 0.57 0.45 .45(8/n) Maximum Permissible Harmonic Current Class B (Amps) 26.11 5.62 3.16 2.25 1.21 1.01 0.64 0.47 0.47(15/n) 2.31 1.13 0.85 0.67 .67(8/n)

15  n  39

8  n  40

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Table 4. Proposed Class A and Class B harmonic current limits based on Canadian source impedance at 120 VAC, 60 Hz. 5.0 Class D limits Class D equipment harmonic limits of IEC 1000-3-2 were derived for a conduction angle of minimum 60 degrees during each half cycle, and are proportionate to input power up to a level of approximately 600 watts. The harmonic current levels are once again limited by the values of Class A. Table 5 shows a derivation of proposed Class D limits based on the IEC 1000-3-2 scaled to match the proposed Canadian Class A limits.
Harmonic order n 1 3 5 7 9 11 13 Max Class A IEC Class A Canada/IEC ratio for Class D IEC (Amps) (Amps) Class A (Amps) (ma/watt) 17.41 14.35 1.21 3.75 2.3 1.63 3.4 2.11 1.14 1.85 1.9 1.50 0.77 1.95 1 0.80 0.4 2.01 0.5 0.67 0.33 2.03 0.35 0.43 0.21 2.05 0.30 Class D Canada (ma/watt) 5.54 3.51 1.95 1.01 0.71 0.61

Table 5. Class D Canadian limit derivation. Table 6 shows combined proposed Class D limits for use with Canadian source impedance at 120 VAC, 60 Hz.
Harmonic order n 3 5 7 9 11 Maximum Permissible Harmonic Current Per Watt Class D (mA / W) 5.54 3.51 1.95 1.01 0.71 7.88/n Maximum Permissible Harmonic Current Class D (Amps) 3.75 2.11 1.50 0.80 0.67 See table 4

13  n  39 Table 6. Proposed Class D limits on harmonic current emissions based on Canadian source impedance at 120 VAC, 60 Hz.

6.0 Class E (Professional Equipment) Limits IEC 1000-3-2 classifies Class E equipment as greater than 1 kW and less than 16 Amps. In Canada, at 120 VAC, the practical upper limit for current is 12 amps, based on a 15-amp receptacle operating at 80% of its current rating. The high end of the Class B range, which is 1.5 kw, represents 12.5 amps at 120 VAC, so for up to 12 amps, the professional and Class B limits could be considered identical.

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For the range of current between 12 and 16 amps, there may be occasions when professional type equipment is fitted with a 20-amp plug and used with a 20 amp receptacle. Therefore, it probably makes sense to extrapolate the Class B limits based on percentage of fundamental that exists up to 12 amps. Table 7 shows these percentages referenced to 12 amps.

Harmonic Number 1 3 5 7 9 11 13 15 2 4 6 8

Class B Limit Amps 26.11 5.62 3.16 2.25 1.21 1.01 0.64 0.47 2.31 1.13 0.85 0.67

Class B Limit % of 12A 217.60 46.83 26.36 18.73 10.05 8.38 5.37 3.91 19.27 9.41 7.07 5.57

Table 7. Proposed Class B limits as % of 12 Amps Table 8 shows proposed Class E limits up to 16 amps at 120 vac, 60 Hz. Class E limits for less than 1 kW will be the same as Class A, and limits from 1 kW up to 12 amps will be the same as Class B.
Class E Limit @ 16 A (Amps) 34.82 7.49 4.22 3.00 1.61 1.34 0.86 0.63 3.08 1.51 1.13 0.89

Harmonic Class B Number Limit % of 12A 1 3 5 7 9 11 13 15 2 4 6 8 217.60 46.83 26.36 18.73 10.05 8.38 5.37 3.91 19.27 9.41 7.07 5.57

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Table 8. Proposed Class E limits for harmonic currents based on Canadian source impedances at 120 VAC, 60 Hz, for Fundamental current 12 Amps  If  16 Amps

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Appendix A (Copy of IEC SC77A-WG1/TF1 WEILER MAY 22, 96 THIS INTERNAL DOCUMENT WAS ISSUED FOR DISCUSSION WITHIN THE IEC TF1)

RATIONALE FOR HARMONICS EMISSION LIMITS

Scope
This short review has been prepared by TF01 of IEC SC 77A/WG01. It is aimed at giving a brief review of the effects of low frequency conducted emissions, i.e. the results and sources of distortions of the supply voltage and some background ideas on the numerical values proposed. This paper contains three parts: A. Effects of distortions B. Explanation of the emission limits C. Limits for Professional Equipment The limits which are finally proposed are the result of intense discussions within the TF and outside of it and the conviction, that a compromise had to be found between such conflicting and justified wishes as cheap manufacturing and limitation of the power supply distortion.

A.

Effects of distortions

Actually, in most countries the line voltage departs from the ideal sinusoidal waveshape and presents a flattened top similar to the following Figure 0-1
400 300 200 100 0 -100 -200 -300 -400 0

0.2

0.4

0.6

0.8

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Figure 0-1 Typical actual voltage waveshape; components are, in ascending harmonic order, 100%, 1%, 3.5%, 1.7%, 0.2%

u=sqrt(2)*230*(sin(wt) - 0.01*sin(3*wt) - 0.035*sin(5*wt).+0.017*sin(7*wt) + 0.002*sin(9*wt))

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The flattening is due to harmonics of the voltage, mainly of order 5 and 7, but also all other low order harmonics are present. These voltage harmonics can have effects on other equipment present in the supply system1. Several main groups are affected in a noticeable way:  Capacitor banks (reactive power compensators)  Motors  Electronic equipment  Protective equipment For capacitor banks, additional currents will flow proportionate to the voltage of the harmonic and its order. E.g. a 5% harmonic of order 5 will cause an additional current of 25% (compared to the fundamental). These additional currents cause overheating and premature ageing of the capacitor banks, leading eventually to their destruction. For motors (especially three phase induction motors), the voltage harmonics will cause additional (rotating or stationary) magnetic fields in the machine and thus additional currents in the rotor windings and iron core. These currents can be calculated using well known models for these machines; however they have, as compared to the fundamental, very high slip values. Considering again a fifth harmonic of 5%, the result will be an additional current of the order of magnitude of the fundamental magnetising current with the corresponding additional losses, overheating (damaging of the insulation due to hot spots localised in the machine windings) and thus reduction of lifetime and reliability. For other electronic equipment (like control parts of drives, audio equipment a.s.o.), disturbances can also occur, especially due to a strongly decreased immunity to voltage dips. Furthermore, the control itself which often is based on zero crossings of the supply voltage, can produce uncontrolled misfunctions. Mains signalling is affected in a special way. Given the selectivity of modern filters, voltage harmonics normally are not a real problem. However during the conduction time of the diodes (and thus insertion of a very high capacitance) of the simple supply of most electronic equipment (using rectifiers with capacitive smoothing), a noticeable decrease of the amplitude of the signalling voltage can be detected leading to misfunctions. Also protective equipment can operate unpredictably under certain distortions and trip seemingly without reason. A rule of thumb says that problems will occur if the total harmonic distortion exceeds 8%. This has been (unvolontarily) verified experimentally by several utilities. In order to reduce the probability of damages due to the voltage distortions, expected maximum values for the individual harmonics have been worked out (compatibility levels, see IEC 1000-2-2).

B. Foundations for Emission Limits: General Considerations
In order to assess the limits, a simplified model of the supply system is considered. The supply side, whose general structure is given in Figure 0-1, can be described directly by its impedances Zi collected into a unique mains impedance ZL.

1 For more details see e.g. Wagner, Balda et al. "Effects of Harmonics on Equipment", IEEE Trans. on

Power Delivery, Vol. 8 Nr. 2, April 1993 and IEEE Std. 1159-1995

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MV Bus Z tm p Transformer s Z't Z To Loads Load considered
d

LVDistribution ZL

ZL

c

Load considered

Figure 0-1 Schematic of distribution system, left complete, right simplified (capacitors not considered). If the load considered is non-linear, i.e. causing emission of distorted currents, a non-sinusoidal voltage drop will occur across ZL, resulting in a distorted voltage at this point and also in other points nearer to the transformer. According to IEC 1000-2-2, the following values (Table 1) of percentage harmonic voltages are considered as upper expected values, which shoulf not to exceeded in most of the cases and most of the time (Compatibility levels): Odd harmonics, not multiples of 3 Ordinal number n 5 7 11 13 17 19 23 25 >25 Level as % of rated voltage 6 5 3.5 3 2 1.5 1.5 1.5 0.2+0.5x25/n Odd harmonics, multiples of 3 Ordinal number n 3 9 15 21 >21 Level as % of rated voltage 5 1.5 0.3 0.2 0.2 Even harmonics

Ordinal number n 2 4 6 8 10 >12

Level as % of rated voltage 2 1 0.5 0.5 0.5 0.2

Table 1 Compatibility levels according to IEC 1000-2-2 Note: It is proposed the change the values for the harmonics >17 to generally (0.5 + 1.5 17/n) %, those for the triplens to 0.2%

Compatibility levels have no direct relation to the individual voltage distortion which may be allowed for an individual piece of equipment, just the (weighted) sum of the individual distortions shall not exceed the compatibility level. For Medium and High Voltage distribution systems, ICU has introduced corresponding planning levels. For practical purposes (especially seen from a manufacturer’s view point) it is preferable to have an indication on the maximum allowable current emission from a piece of equipment, as this value can be measured or calculated in the premises of the manufacturer. Therefore, standards were developed considering this requirement.

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The values for current emission limits that are given in IEC 1000-3-2 for class A equipment where in fact derived for a given assumption of the superposition of different distorting loads with phase angle control and assuming, besides the mains' impedance, an individual allowable voltage distortion as given in Table 2 Ordinal number n 3 5 7 9 11 Admissible contribution to voltage distortion (% of nominal voltage) 0.85 0.65 0.60 0.4 0.40 Ordinal number n 13 15 to 39 2 4 to 40 Admissible contribution to voltage distortion (% of nominal voltage) 0.30 0.25 0.30 0.20

Table 2 Formerly assumed admissible contributions (per equipment) to voltage distortion For the calculation of the admissible harmonic current emissions, both this admissible voltage distortion and the impedance of the supply system must be known. In low voltage supply systems, investigations have shown that in 95% of the cases the following impedances will not be exceeded:

Connection single phase connection: ZL = three phase connection: ZL = ZL =

Impedance (0.4 + j 0.25)  (0.24 + j 0.15)  (Line) (0.16 + j 0.10)  (Neutral)

Table 3 Standardised impedances for 50 Hz Public Low Voltage systems Note: This are 95% maximum values, in many actual supplies the values will be lower. For 60Hz systems, corresponding values have not yet been published From these calculations result the following maximum limits for current emissions: n Z/ U/V Cl.A in Amps Cl.B in Amps 1 0.47 6.90 14.63 21.94 3 0.85 1.95 2.30 3.45 5 1.31 1.49 1.14 1.71 7 1.79 1.38 0.77 1.15 9 2.28 0.92 0.40 0.60 11 2.78 0.92 0.33 0.50 13 3.27 0.69 0.21 0.32

Table 4 Characteristic numerical values: n=harmonic order, Z=impedance at that n, U=absolute voltage drop at that n Cl.A=maximum allowable current at that n, Cl.B=1.5 times Cl.A. These values can be used for low power equipment whose harmonics' phase angles are widely variable. If the phase angles of the harmonics emitted by different pieces of equipment are more or less coherent (as is the case for most electronic equipment with an input rectifier followed by capacitive smoothing), lower values must be assumed, as is the case for Class D equipment2 in IEC 1000-3-2.

For equipment with higher rated power, a more elaborate consideration is appropriate:

2 For this class, harmonic current limits were derived for a conduction angle of minimum 60 o during each

half period; the limit values are proportionate to the input power up to a level of approx. 600W from where on the harmonic currents are again limited by the values of Class A.

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For the system's impedance, knowledge of the actual supply system’s impedance is required. A classical measure for this impedance is the short circuit power Scc at 50Hz for the PCC considered.

For the superposition of the emissions from different pieces of equipment, different aspects have to be considered:  transference downstream the supply levels of harmonic voltages originating from other sources of harmonics ("background noise");  low order harmonics have similar phase angles, so they tend to add linearly;  high order harmonics normally have largely different phase angles, so their summation can be approximated by a square law;  special harmonics may have opposite signs, depending whether they are caused by single phase or genuine three phase equipment (the 5th is a typical example);  other special harmonics of single phase equipment tend to be in phase in all three line conductors, giving a zero sequence system and leading to nearly tripling of the voltage drop in the neutral (the 3rd harmonic is a typical example). This may result in overheating of the neutral and/or tripping of the corresponding protective means;  not all equipment generating harmonics is in service at the same time;  the actual voltage distortion influences the current harmonics emissions (this point was not considered rigourously).

In the draft for IEC 1000-3-4, the following served as a basis: 1. Only one quarter of the total compatibility level is actually at disposal for equipment connected at a certain point; the remainder of the distortion is transmitted from the higher voltage level to the public low voltage supply system actually considered. 2. As a measure of the supply system’s impedance, the short circuit power Scc at the PCC was introduced. This is of course highly variable, so it was assumed that IEC 1000-3-3 and IEC 1000-3-5 apply, stipulating that the voltage drop due to a single piece of equipment should, at 50Hz, not exceed 3%. 3. Admissible emission limits may increase with [Equipment Power]0.4 to 0.5 as shown by several independent investigations. Taking these starting points, the following values can be calculated for the maximum allowable current harmonics as percentage of the fundamental:

n calculated Cl.B at 16 Amps

3 13.89 21.569

5 10.00 10.69

7 5.96 7.21

9 1.39 3.77

11 2.65 3.10

13 1.92 1.98

Table 5 Calculated limit values in % of fundamentalfor current emissions, assuming ¼ of the compatibility level at disposal and 3% voltage drop for the fundamental, the lower row gives the values according to Cl. B from Table 4. This shows that for the most crucial 5th harmonic a current of only 10% (compared to the fundamental) can be permitted if it is considered that the total load is of the distorting type. However, this value was adjusted to 10.7% considering the 16A limit for Class B equipment of 1000-3-2 (avoidance of a discontinuity). All other harmonic limits where then adjusted accordingly, using the ratio of the values in Table 1 of IEC 10003-2 (here Cl. A in Table 4) between the 5th harmonic and the other harmonic orders. Note: For the detailed calculations, see the Annex. As mentioned, higher emission limits can be permitted when the total load is made up of many lower power items considering the superposition laws for the harmonics and the superposition statistics of the different pieces of equipment. Of course the system impedance plays a major role in determining the level of supply

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system voltage distortion: this is reflected in the quantity Rsce3 which relates the short circuit power to the actual power of the equipment (and thus indirectly the total number of pieces of equipment admissible at the PCC under consideration). Several independent investigations have shown that for this case the emission limits can be increased proportionate to
0. 4 Rsce .

This led to the values in Table 6 for single phase equipment; for genuine three-phase equipment not generating triplen harmonics (adding in the neutral conductor), somewhat higher values can be allowed.

n Rsce 66 120 175 250 350 450 >600

I3 23 25 29 34 40 40 40

I5 11 12 14 18 24 30 30

I7 8 10 11 12 15 20 20

I9 6 7 8 10 12 14 14

I11 5 6 7 8 9 12 12

I13 4 5 6 7 8 10 10

Table 6 Limit values for different Rsce , single phase equipment In order to limit the measurement burden, only the harmonics mentioned in Table 6 were limited individually, the higher order harmonics are limited in a sweeping manner using the Partial Wheighted Harmonic Distortion Factor PWHD defined by

I  PWHD   n n  . 14  I 1 
40

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C. Emission limits for Professional Equipment > 1kW and <16A Input Current
Dealing with the requirement to set emission limits for professional equipment in the input current range below 16A, the TF discussed several approaches. Finally, on the proposal of a TF-member representing equipment manufacturers, it was resolved to assimilate them to high power equipment and to assume an R sce of 120 (irrespective of the otherwise used system's impedance according to Table 3). This then gave the limit values in Table 7 for the emission at 16A, the maximum input current for which IEC 1000-3-2 is still relevant.

Single phase Three phase

I3 25 n.a.

I5 12 16

I7 10 12

I9 7 n.a.

I11 6 11

I13 5 8

Table 7 Emission limits in % of the fundamental current for professional equipment at 16A

3 Note that the minimal R

sce

that can be considered here is 1 / 3% = 33.

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For lower input currents, it was assumed that at 1kW, the values according to Class A of IEC 1000-3-2 hold; they increase linearly with power up to 1.5 kW where Class B is reached. From there on, they increase linearly to the values given in Table 7 (see Figure 0-1 ).

see Table 7 Class B Class A

1kW 1.5 kW

16A

Figure 0-1 Slope of emission limits Formulas as well as values for PWHD are given in the Draft for the revision of IEC 1000-3-2 related to this equipment. These higher emission limits as compared to all other (electrically similar) equipment should however be used carefully; they are in fact against the priciple of equal rights that must govern all limitations and can be justified only for very specific equipment not in a widespread use.

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