P1495/D2

01-26-01

Draft Guide for

Harmonic Limits for Single-Phase Equipment

Sponsored by theTransmission and Distribution Committeeof theIEEE Power Engineering Society

Copyright © 2001 by the Institute of Electrical and Electronics Engineers, Inc.345 East 47th StreetNew York, NY 10017, USAAll rights reserved.

This is an unapproved draft of a proposed IEEE Standard, subject to change. Permission is hereby granted for IEEE Standards Committee participants to reproduce this document for purposes of IEEE standardization activities. If this document is to be submitted to ISO or IEC, notification shall be given to the IEEE Copyright Administrator. Permission is also granted for member bodies and technical committees of ISO and IEC to reproduce this document for purposes of developing a national position. Other entities seeking permission to reproduce this document for standardization or other activities, or to reproduce portions of this document for these or other uses, must contact the IEEE Standards Department for the appropriate license. Use of information contained in this unapproved draft is at your own risk.

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07-10-00 P1495/D2

Introduction

(This introduction is not part of IEEE P1495, Guide on Harmonic Limits for Single-Phase Equipment.)

With increasing quantities of non-linear loads being added to electrical systems, it has become necessary

to establish criteria for limiting problems from system voltage degradation. Presently, IEEE Standard

519-1992 addresses harmonic limits at the consumer and service provider interface. The intent of IEEE

519 is to limit harmonic current injection into power systems and ensure voltage integrity. This standard

is manageable and practical when properly applied to industrial and commercial three-phase consumers.

However, when IEEE 519 is applied to single-phase system connections (particularly residential

consumers), it can become highly impractical. Furthermore, engineering studies indicate that the

cumulative effect of single-phase non-linear loads may potentially cause voltage degradation on power

distribution systems even with individual single-phase consumer IEEE 519 compliance.

The following guide recommended practice/standard for equipment is developed to help preserve voltage

integrity by limiting harmonic current injection of single-phase loads which are likely to appear in

increasing numbers in power distribution systems. By addressing harmonic current distortion at the

individual sources, system problems may be avoided, particularly in areas where IEEE 519 limitations

are not likely to be followed. The harmonic current limits established in this standard are proposed with

the intent of minimizing the impact on existing equipment design. Coordination with existing industry

practices, and international harmonic standards is also considered.

______________________________________________________________________________________

At the time this standard was completed, the working group had the following membership:

The following persons were on the balloting committee: (To be provided by IEEE editor at time of publication.)______________________________________________________________________________________

Note: new text (i.e., changes from previous version) is added in italics. Deletions are shown with a strikethrough.

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07-10-00 P1495/D2

Contents

Introduction................................................................................................................................................

1. Overview............................................................................................................................................. 4

1.1 Scope................................................................................................................................................

1.2 Purpose.............................................................................................................................................

2. References.............................................................................................................................................

3. Definitions........................................................................................................................................... 5

4. Loads covered in guidestandard..........................................................................................................6

5. Background......................................................................................................................................... 9

6. Existing related industry standards and practices...............................................................................11

6.1 IEEE 519...................................................................................................................................... 11

6.2 IEC 61000-3-2 and 61000-3-4.......................................................................................................11

6.3 Social Security Administration requirements for IT equipment.....................................................13

6.4 California Energy Commission electric vehicle charger requirements...........................................13

7. Recommended distortion limits.........................................................................................................14

7.1 Differences in distribution design.................................................................................................15

7.2 Consideration of multiple nonlinear loads....................................................................................17

7.3 Diversity effects............................................................................................................................ 17

7.4 Cancellation effects...................................................................................................................... 17

7.5 Load levels and harmonics............................................................................................................ 18

7.6 Limits........................................................................................................................................... 18

8. Testing requirements......................................................................................................................... 18

9. Bibliography...................................................................................................................................... 23

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07-10-00 P1495/D2

Guide for Harmonic Limits for Single-Phase Equipment

1. Overview

The growth in power electronics has impacted many loads that traditionally were considered linear in nature. As a result, the number of nonlinear loads has increased and is expected to increase dramatically in the years ahead.

High harmonic content should be regarded as an undesirable load characteristic much like low power factor or low efficiency. With each of these, it’s a matter of degree. While some can be tolerated, an excessive amount should be avoided.

Although the number of problems caused by harmonics is not yet a significant widespread problem, the potential for such problems is much greater today than it was 10 years ago, and the effects of nonlinear loads are becoming an increasing concern in many parts of the world today. In addition, single-phase converters, found in many devices, have particularly high amounts of all odd harmonics, particularly the third harmonic.

A variety of single-phase equipment can be categorized as nonlinear today . Switch-mode power supplies are found in a variety of devices like computers, televisions, VCRs, etc. Compact fluorescent lamps are now available with electronic ballasts. Variable speed control is evolving to include washers, dryers, space heating and air-conditioning. Battery chargers are being developed for electric and hybrid-electric vehicles, and the list continues.

There have been a number of different studies which investigated one of these new technologies to see if and when a potential problem might surface on the distribution system due to single-phase harmonic sources. For the most part, these studies did not consider these loads in combination.

What we find is found is a wide range of nonlinear devices that have been identified which collectively need to be considered for future power distribution systems. In addition, there are some newer technologies being explored which could add to the previous mix.

The objective is to arrive at a reasonable set of limits for single-phase equipment such that the likelihood of a problem caused by harmonics is small.

1.1 Scope

Current distortion limits for single-phase utilization equipment rated under 600 V and up to 40 A.

1.2 Purpose

To establish equipment harmonic limits, much like IEC 1000-3-2, which deals with both three-phase and single-phase limits for devices under 16 A and with 230 V nominal rating. Existing Std 519 contains site limits on harmonics, and is primarily three-phase.

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07-10-00 P1495/D22. References

In this document, two types of "references" are used: those that are directly related to the subject being discussed - references - and those that provide supporting information to the subject being discussed - bibliographic citations. For the convenience of the reader in not breaking the pace of reading, "references" and "citations" are briefly identified in the text as described below.

References contain information that is implicitly adopted in the present document; complete implementation of the recommendations made in the present document would require the reader to consult that reference for the details of the subject. This first type is introduced in the text as [xx], and the listing is provided below, in this section.

Citations are not essential to implementation of the recommendations, but are provided for the use of readers seeking more detailed information or justification. This second type is introduced in the text as [Ayy] and the listing is provided in the Bibliography.

This document shall be used in conjunction with the following publications:

[1] IEEE Std 100-1996, Dictionary of Electrical and Electronics Terms

3. Definitions

3.1 Abbreviations

ECM electronically-commutated motorEV electric vehicleTHD total harmonic distortionTHDI total harmonic distortion of currentTHDV total harmonic distortion of voltage______________________________________________________________________________________

3.2 Definitions

Whenever possible, the definitions of the terms used in this document are those found in IEEE Std 100-1996 [1] or Std 519-199x [Azz]. In some instances, the IEEE definition in the current dictionary may be either too broad or too restrictive; in such a case, an additional definition is included in this section.

3.1 distortion factor: The ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity, often expressed as a percent of the fundamental. Also referred to as total harmonic distortion.

3.2 harmonic: A sinusoidal component of a periodic wave having a frequency that is an integral multiple of the fundamental frequency.

3.3 harmonic distortion: The mathematical representation of the distortion of a pure sine waveform. See: distortion factor.

3.4 linear load: An electrical load device which, in steady state operation, presents an essentially constant load impedance to the power source throughout the cycle of applied voltage.

3.5 noise: Electrical noise consists of unwanted electrical signals, which produce undesirable effects in the circuits of the control systems in which they occur. For this Guide, "control systems" is intended to include sensitive electronic equipment in total or in part.

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07-10-00 P1495/D23.6 nonlinear load: Electrical load that draws current discontinuously or whose impedance varies during the cycle of the input ac voltage waveform.

3.7 total demand distortion: The total rms current distortion in percent of maximum demand current.

3.8 voltage distortion: Any deviation from the nominal sinewave of the ac line voltage.

4. Loads covered in this guidestandard

This section will characterize the more common types of single-phase nonlinear loads, including switch-mode power supplies, electronic lighting ballasts, variable speed heat pumps and air conditioners, and battery chargers for electric vehicles.

4.1 Switch-Mode Power Supplies

A variety of ac electronic equipment is powered internally by dc. In the early days of power supply design, the common single-phase power supply was linear, more or less. In Fig. 4-1, a schematic of a linear power supply, the output voltage is controlled by means of a transistor (not shown) which operates in its active region. Comparatively speaking, the linear power supply has high power losses (overall efficiency in the 30-60% range). In addition, the 60 Hz transformer represents a significant weight to the power supply and results in a more costly design.

Fig. 4-2 is a schematic of a switch-mode dc power supply. The dc voltage change is attained through dc-to-dc converter circuits which employ transistors or MOSFETS to perform the switching (on or off). Unlike the linear power supply, the solid-state devices do not have to operate in their active region, resulting in reduced power losses. In this design, the 60 Hz transformer has been eliminated, resulting in reduced weight and improved overall efficiency (in the 70-90% range). There is still a transformer (not shown), but it is smaller, lighter and less costly because it operates at high frequencies.

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Fig. 4-2 Switch-Mode dc Power Supply

Fig. 4-1 Linear Single-Phase dc Power Supply

Fig. 4-2. Switch-Mode Power Supply

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With a switch-mode power supply, the ac voltage is converted to dc through diode rectifiers. The regulated dc output is attained by means of high frequency (e.g., 20-100 kHz) switching. Although not shown, a high frequency isolation transformer is used. This transformer is much lighter and smaller than the one used in the linear power supply. The current draw is controlled by the voltage appearing across the low voltage capacitor. The resulting current waveform appears like that shown in Fig. 4-3, in which the current has a zero value until some minimum capacitor voltage is reached, and then rises to a peak value and back to zero again. As contrasted with the linear power supply, the current appearing on the ac side lasts for only a portion of each half-cycle. As a result, the characteristic is nonlinear and the device is a source of harmonics. The term crest factor, the ratio of peak to rms current, is often associated with harmonics. For a pure sine wave, the crest factor is 1.414. Common switch-mode power supplies can have crest factor values exceeding 5.

Measurements [from Grady] indicate the current waveform for a color television has a THD I of 121% and a third harmonic content of 84%. Similar measurements for a desktop computer with a laser printer indicate a THDI as high as 140% and a third harmonic content of 91%.

4.2 Electronic Ballasts for Fluorescent Lighting

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Fig. 4-3 Voltage and Current for a Switch-Mode Power Supply

07-10-00 P1495/D2Fluorescent lamps with magnetic ballasts are three to four times more efficient than incandescent lamps. Electronic ballasts for fluorescent lamps result in a further 20-30% in efficiency over the magnetic ballasts. Most of the older high-loss magnetic ballasts can no longer be sold in the United States. Like the switch-mode power supply, electronic ballasts employ high frequency switching, usually in the 20-45 kHz range. When electronic ballasts were first introduced, the THD I was around 100%, with a large third harmonic component.

Many commercial services employ three-phase 4-wire distribution systems, with the neutral conductor often a smaller size than the phase conductor. With this type of electrical design, the third harmonic current (and odd multiples of 3rd) from each phase adds in the neutral. It was not surprising to find neutral currents greater than phase currents where many of the early electronic ballasts were installed. In fact, the neutral current could easily be twice the magnitude of the phase currents due to the high third harmonic content.

A relative newcomer is the compact fluorescent lamp (CFL) with electronic ballast. Generally found in the range of 15 to 27 W, CFLs are used to replace 40 to 100 W incandescent lamps. Many utilities’ energy conservation programs encouraged their customers to install these more efficient lamps and later discovered they introduced a significant source of harmonics. Pileggi et al concluded that a feeder with just 2 or 3 compact fluorescents per household would lead to excessive voltage distortion [1]. Obviously, the amount of current distortion from such a device can vary.

Following complaints resulting from high neutral currents in three-phase commercial and industrial environments, ANSI established limits on current harmonics. The maximum THD I is 32% and the triplens (third harmonic and odd multiples of third, i.e., 3 rd, 9th, 15th, etc.) are limited to 30% per ANSI C82.11-1993.

4.3 Variable Speed Heat Pumps and Central Air Conditioning

Some newer heat pump and central air-conditioning designs incorporate variable speed control of the compressor, blower and fan motors. If the temperature is very hot or cold, the heat pumps run at maximum speed; however, if the temperature is milder, they run at low speeds. The result is more energy efficient, more comfortable and quieter than conventional systems.

Employing electronically-commutated motors (ECMs), the input of a variable speed heat pump resembles a large switch-mode power supply (e.g., 3 kW or more). The ECM is a dc brushless motor with stator construction similar to a three-phase ac induction motor. The stator winding current is sequenced and switched creating a rotating magnetic field. This switching is performed electronically by an inverter. The rotor uses permanent magnet construction.

Conventional heat pumps have a current THD of 13% with around 9% third harmonic content; some of the newer ECM designs have current THD values of 123% with the third harmonic content of 85%.

Because of the large load, there is concern that it would only take relatively few installations of this type of heat pump to cause voltage distortion problems. In [2], the authors found the voltage distortion on a distribution feeder reached 10% when the penetration rate for these variable-speed drives reached 10%, i.e., when 10% of the homes installed the new ECM designs. Though a 10% THDV would be deemed unacceptable by most of us, it does give us a sense of how much of a particular nonlinear load would cause problems on a residential feeder.

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07-10-00 P1495/D2

4.4 Electric Vehicle Battery Chargers

Many utilities are presently marketing electric vehicles (EVs) or are providing the battery chargers for EVs. At issue is the fact that the technology exists to produce low distortion chargers. However, without any harmonic limit to meet, manufacturers will likely opt for a lower cost, higher distortion design. Considering that the single-phase chargers represent a 240 V load of around 6 kW, the potential for harmonic problems is genuine, particularly at the residential level. Many distribution engineers have focused more on the load impacts of chargers on transformers and feeders. The load impacts can be significant; however, the harmonic effects are potentially large too.

Some of the least expensive battery chargers have THD I values of 91%. Based on [2] and the assumption that the EV charger load is twice that of the variable-speed drive, the penetration rate would only have to be 5% before the THDV reaches 10%. Obviously, to meet the recommended THDV limit of 5%, the penetration would have to be less than 5% or the equipment harmonic limit would have to be lower.

5. Background

An approximate expression for determining the amount of each current harmonic present for single-phase converters is given by the following equation:

where: n = harmonic order (3, 5, 7, 9, 11, 13, etc.)

A similar approximate expression for determining the amount of each current harmonic present for three-phase (6-pulse) converters is given by the following equation:

where: n = harmonic order (5, 7, 11, 13, 17, etc.)

In both of these expressions, the harmonic is expressed in terms of the fundamental frequency term. The results of these two expressions are shown in the table below.

Harmonic 3 5 7 9 11 13 15

1-phase converter 67 40 29 22 18 15 13

3-phase converter x 20 14 x 9 8 x

As can be seen, most single-phase converters produce all the odd harmonics at about twice the amount of their three-phase converter counterparts. In addition, single-phase converters have significant amounts of third harmonic content, which the three-phase converters do not have.

It should be noted that the single-phase equation underestimates the % of each harmonic and the three-phase equation overestimates the harmonic content in most cases. The current total harmonic distortion

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Table 4-1. Approximate Current Harmonic Content (%) for 1-Phase and 3-Phase Converters

07-10-00 P1495/D2(THD), which is the weighted or rms assessment of all harmonics, approaches 100% for many of these devices. The expression for THD is as follows:

where: THDI = current total harmonic distortion

In = harmonic rms current (in amps or %)

I1 = fundamental frequency rms current (in amps or 100%)

A similar equation for voltage THD results from V being substituted for I in the above. Using the values from Table 1-1, the THDI for a single-phase converter is around 95% versus 30% for a three-phase converter, based on odd harmonics up to the 50 th. Up until now, because the typical system impedance up to a harmonic source is reasonably low and the typical harmonic source is a relatively small load, the resulting THDV (in %) will usually be in the single digits.

The harmonic content and THD can be obtained for different levels of load for some devices. For example, a battery charger has a variable load characteristic and the harmonic content and THD varies as a function of load. It us usually sufficient to note the THD and % of individual harmonics at rated load. At lower levels of load, the resulting percentages (of individual harmonics and THD) are usually offset by the lower base current at that load. For example, with two loads having the same base current, one that produces 20% THDI at 50% load is no worse than one which produces 10% THD I at 100% load. They both produce the same distortion in amperes.

INCLUDE RESULTS OF VARIOUS SYSTEM STUDIES

6. Existing related industry standards and practices

6.1 IEEE 519

IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems, established limits on harmonic currents and voltages at the point of common coupling (PCC), or point of metering [6].

The limits of IEEE 519 are intended to:

1) Assure that the electric utility can deliver relatively clean power to all of its customers;

2) Assure that the electric utility can protect its electrical equipment from overheating, loss of life from excessive harmonic currents, and excessive voltage stress due to excessive harmonic voltage.

Each table from IEEE 519 lists the limits for harmonic distortion at the point of common coupling (PCC) or metering point with the utility. The voltage distortion limits are 3% for individual harmonics and 5% THD.

Table 6-1 contains current distortion limits which apply for user voltage levels ranging from 120 V to 69 kV. The harmonic current limits are based on the size of the user with respect to the size of the power system to which the user is connected. The ratio of ISC/IL is the short circuit current available at the PCC

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07-10-00 P1495/D2to the maximum fundamental load current or that current associated with the measured demand of the load.

It is recommended that the load current, IL, be calculated as the average current of the maximum demand for the preceding 12 months. The values listed in Table II should be used as system design values for the "worst case" of normal operations. As noted earlier, the TDD is normally based on the maximum demand current.

The heaviest electrical users have the most stringent harmonic limits. However, as the size of the user load decreases with respect to the size of the system, the user can inject a larger percentage of harmonic current into the utility system. This protects other users on the same feeder as well as the utility, which is required to furnish a certain quality of power to its customers.

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Note that all of the harmonic limits in IEEE 519 are based on a customer load mix and location on the power system. The limits are not applied to particular equipment, although, with a high amount of nonlinear loads, it is likely that some harmonic suppression may be necessary. As the load mix changes, it is often necessary to reevaluate the limits of IEEE 519. This is one of the difficulties with the Standard.

6.2 IEC 61000-3-2 and IEC 61000-3-4 (formerly 1000-3-2 and 1000-3-4)

6.2.1 IEC 61000-3-2 (1995-03)

Specifies limits for harmonic current emissions applicable to electrical and electronic equipment having an input current up to and including 16 A per phase, and intended to be connected to public low-voltage distribution systems. The tests according to this standard are type tests. Test conditions for particular equipment are given in annex C. For systems with nominal voltages less than 220 V (line to neutral), the limits have not yet been considered.

6.2.2 IEC/TS 61000-3-4 (1998-10)

The recommendations of this technical report are applicable to electrical and electronic equipment with a rated input current exceeding 16 A per phase and intended to be connected to public low-voltage ac distribution systems of the following types:

- nominal voltage up to 240 V, single-phase, two or three wires;

- nominal voltage up to 600 V, three-phase, three or four wires;

- nominal frequency 50 Hz or 60 Hz

These recommendations specify the information required to enable a supply authority to assess equipment regarding harmonic disturbance and to decide whether or not the equipment is acceptable for connection with regard to the harmonic distortion aspect. This document is not to be regarded as an International Standard.

The European standards, IEC 61000-3-2 & 61000-3-4, placing current harmonic limits on equipment, are designed to protect the small consumer's equipment. The former is restricted to 16 A; the latter extends the range above 16 A. Two particular classes of equipment covered are:

Class C Lighting equipment

Class D Equipment having current with a "special waveshape"

Table III gives the limits for lighting equipment. Class D is defined by a special waveshape (Fig. 1). If the current waveshape of a nonlinear load is within the limits of Fig. 1 for 95% of a half-cycle, then it is deemed to be Class D equipment and must satisfy the limits of Table IV.

Class D equipment is more representative of many of the single-phase nonlinear loads that were previously discussed. Referring to Table IV, the current limit for a given harmonic is based on the input power (mA/W) as well as a maximum current value. Table V shows the latest proposal for limits.

It is not clear from the tables in IEC 61000-3-2 and 61000-3-4 how restrictive the limits are as a function of equipment rating. The table below shows what the resulting current THD limit is for a 220 V device designed to meet the latest IEC proposals.

Wattage THD Limit (%)

< 600 90

1000 – 1500 60

> 3500 26

Fig. 2 show the same information contained in this table, but also shows the linear decline between intervals (i.e., between 600 W & 1000 W, and between 1500 W & 3500 W).

By comparison with the limits suggested by IEEE 519, the IEC limits permit higher levels of harmonic distortion.

6.3 California Energy Commission electric vehicle charger requirements

7. Recommended distortion limits

At this point, we can either develop our own limits or we can adopt limits of others, like the IEC. We are fortunate in that a number of studies in the U.S. that have already been undertaken. The reports on these efforts cover the spectrum of low power devices like compact fluorescent lamps [Pileggi & Emanuel], larger loads like variable speed heat pumps and central air conditioning [Grady] and large electrical loads like battery chargers for electric vehicles [Grady]. These studies determine how much of a given nonlinear load a distribution system can tolerate before one exceeds 5% voltage THD. This is probably a more meaningful starting point since these studies involve system models conforming to the distribution systems found in the U.S. and the harmonic spectrum of modern single-phase converters.

Table 6-1. THD Limits from IEC 61000-3-2 and 61000-3-4

Figure 6-3. IEC Harmonic Limits as a Function of Wattage

The other option is to adopt the proposed IEC limits (i.e., 61000-3-2 and 61000-3-4). If the distribution systems were similar enough around the world, and if the use of such a standard was validated by field experience, this would make sense. However, there are two basic distribution systems found around the world and the two are different enough – in terms of harmonic implications.

7.1 Differences in distribution design

A typical distribution system found in the U.S. (and in North America in general) operates at 12.5 kV (phase-to-phase). It is radial in that the loads are connected to only one source of supply. Main feeders are 4-wire (3 phases and a neutral) and branches are usually single-phase with a limited amount of three-phase used for branches. Transformers are connected wye wye. The large majority of transformers are single-phase connected from phase-to-neutral at 7.2 kV, and stepping the voltage down to 120/240 V (3-wire) for residential and most single-phase commercial loads. A typical transformer rating for residential application is 25 kVA or 50 kVA with services run to between 6 and 10 homes.

The European style of distribution system operates at 11 or 12 kV (phase-to-phase), but does not have a neutral. Thus, the main feeders are 3-wire circuits. Transformers are three-phase connected delta wye; they are more like a miniature substations with 500, 750 or 1000 kVA ratings being popular. Secondaries operating at 400/230 V are usually tapered (i.e., 2 or 3 conductor sizes are used, with the conductor size close to the transformer is considerably larger than that found near the end of the secondary) and cover extensive distances to provide service to 60 or 100 customers. Individual 230 V (2-wire) services are tapped off the secondary system.

A much less common system in the U.S. is secondary network distribution, in the form of either a secondary grid network or a spot network. Grid networks are found in the downtown areas of many older cities to supply office buildings and other commercial loads. With secondary grid networks, 6 or more feeders and network transformers in parallel supply a low voltage grid (208/120 V or so). Transformers are large (e.g., 500 or 1000 kVA) compared to radial distribution and are connected delta wye, similar to the European distribution systems, making the third harmonic of little concern. Due to high rise construction, spot networks have replaced grid networks in popularity. With spot networks, each major load is served from a combination of 3 or more circuits and transformers in parallel (at 480/277 V typically). With the grid or spot networks, the equivalent source impedance to the point of common coupling is an order of magnitude smaller than that found in a radial system. Thus, the ability of a secondary network system to handle harmonics is much greater than a radial system. However, it should be remembered that the secondary network is not the typical distribution system; most utilities utilize radial distribution as their standard.

Table 6-2 summarizes the important differences between the two systems. The U.S. distribution system is more susceptible to problems brought about by the third harmonic. Third harmonics readily pass through the wye wye connected transformers in the U.S., while the European delta wye transformers provide a closed path for third harmonics in the delta winding, thus providing third harmonic isolation between the secondary and the primary. In addition, stray voltage can also result due to the presence of third harmonics on the neutral of U.S. systems [Conrad et al].

The equivalent impedance to the customer is greater with the European style of distribution largely due to the extensive amount of secondary used. However, the service transformer is usually the point of common coupling for U.S. distribution systems. Because of the smaller transformers ratings used in the U.S., the equivalent system impedance to the point of common coupling is higher with the U.S. type of distribution, suggesting that the U.S. harmonic limits ought to be lower than those for the European distribution. One could argue that the point of common coupling for the European type of distribution would be beyond the transformer, i.e., including some amount of secondary. Nonetheless, the equivalent system impedance at

SAVE for Table 6-2.

the point of common coupling would still be greater for the U.S. type of distribution, suggesting that lower harmonic limits are needed for equipment used on U.S. systems.

Since the service wires in the U.S. system connects to both 120 V and 240 V loads, suitable limits must be adopted for each. From the standpoint of the primary distribution system, a 1 amp 240 volt load will have the same impact as a 2 amp 120 V load. Thus, the ampere limits for 120 V will be twice the 240 V limits.

7.2 Consideration of multiple nonlinear loads

In each nonlinear load category, some assessment of market potential and coincident usage should be made initially to guide the limits ultimately chosen. For example, if one looks at an arbitrary ranking of low, medium and high, one can see where potential problems are likely to occur. A high market potential combined with high coincident usage indicates the worst a bad situation from a harmonics standpoint. On the other hand, a low coincident usage combined with low market potential might be judged a good is the best situation from a harmonics standpoint. These are the two extremes; all other combinations are lie somewhere between these two limits.

Coincident

Usage

High Worst Bad situation

Medium

Low Best Good situation

Low Medium High

Market Potential

In addition, the criticality of a particular type of nonlinear load can be established by the additional ranking of its wattage rating times its THDI.

While this matrix classification sorts out the extreme cases, case studies and or experience is needed to establish whether these scenarios yield acceptable performance (i.e., do not violate voltage harmonic limits).

It is important to consider nonlinear loads in combination, rather than think of principally one source of harmonics.

7.3 Diversity effects

When two or more similar loads are connected to a portion of a distribution system, their usage patterns may be different. As a result, the maximum coincident demand is usually less than the sum of the individual demands. This result is referred to as diversity.

Diversity has an important role in distribution design practice. Whenever possible, it is important to incorporate realistic use patterns for nonlinear loads to avoid overstating their impact.

7.4 Cancellation effects

Cancellation among groups of particular harmonics result principally from phase displacement for the harmonic or harmonics involved.

Cancellation due to phase-angle diversity between dispersed single-phase non-linear loads is insignificant for the 3rd and 5th harmonics, but becomes increasingly significant for higher harmonics [Mansoor].

However, in three-phase commercial service, there is phase shift introduced by having some loads connected phase-to-phase and others, phase-to-neutral. In addition, isolation transformers result in a 30 degree phase shift for the loads that they serve, making cancellation more likely.

However, for residential applications and single-phase commercial uses, the dominant use of the wye-wye transformer connection tends to minimize cancellation effects.

Like diversity, cancellation effects should be included.

7.5 Load levels and harmonics

Most often, the harmonic spectrum for a particular device is given at nameplate load. For PCs and compact fluorescent lamps, the characteristic is fixed and doesn’t vary through time. With variable speed heat pumps and EV battery chargers, there can be quite a bit of variation in the harmonic spectrum as a function of load. Care should be exercised in assigning the appropriate harmonic spectrum to a particular nonlinear load. This is particularly true for monitoring studies since most monitoring equipment computes the harmonic distortion as a function of the fundamental frequency current.

7.6 Limits

Based on the system studies done to date, and allowing for combinations of nonlinear loads, the maximum recommended THD levels are shown in the table below.

Maximum THDI 15 %

Maximum 3rd harmonic current 10 %

8. Testing requirements

This section describes the fundamental requirements for harmonic voltage and current measurements for low voltage equipment. It is explained that such measurements must be performed using a nearly perfect sinusoidal voltage with a standard internal impedance. This condition provides the basis for comparison among different non-linear loads. Another topic discussed in this paper is the importance of accurate and calibrated transducers and analyzers.

Table 7-1. Recommended Current Distortion Limits

8.1 Background

Today’s engineer takes for granted that electrical equipment manufacturers will list the major values that characterize equipment performance: rated voltage, frequency, kVA, power factor, output power, rms line current and more. Such information is needed for several reasons:

Equipment selection, installation and protection.

Selection and design of auxiliary equipment.

Comparison among similar products.

Simulations that will help prevent or explain undesirable situations.

The last two items are probably the main motivation for the correct information on the current harmonics spectrum. We should remember that, for useful simulations and meaningful predictions, it is necessary to know reasonably well (i.e., within 5%) the values of the dominant harmonic current phasors (amplitude as well as phase angle).

Very few manufacturers, however, list information describing harmonic current spectrum produced by their equipment. This is especially true for single-phase units. Even when such information is provided, the accuracy of the listed values is questionable since the industry does not yet have standards that call for certain procedures and types of instrumentation.

This section mentions the problems associated with the measurements of harmonic currents produced by single-phase loads and suggests a measurement protocol.

8.2 The voltage supply

Among many factors that control the frequency spectrum (harmonics) of the line current supplying a nonlinear load, five items are essential:

The design of the nonlinear load (circuit topology, components, geometry and physical properties of the active materials)

The rms value and the frequency spectrum of the applied voltage.

The value and frequency dependence of the Thevenin impedance measured at the bus supplying the observed nonlinear load.

The amount of power converted and delivered by the nonlinear load and the characteristics of the output power. For example, a mechanical load will require a certain torque and speed, and an arc welder will be characterized by the arc current and gap.

Nonelectrical environmental parameters such as temperature and pressure. Some electrical discharge lamps are very sensitive to such parameters.

We learn from here that, when measuring the current harmonic emission produced by a nonlinear load, it is imperative to energize the observed nonlinear load from a voltage supply with a standardized internal impedance and a nearly perfect sinusoidal voltage. This approach will help provide a common reference level for comparing different nonlinear loads.

The basic measurement circuits are presented in Fig. 8-1. The voltage source must have a frequency of 60 0.01 Hz and very low distortion (THDV < 0.5%). It is suggested that the Thevenin impedance, measured at 60 Hz and including the current transducer CT and the analyzer voltage input impedance, is:

Zs = 10 + j 50 m for a 240 V load

and

Zs = 3 + j 15 m for a 120 V load.

A tolerance of 5% to the above values is permitted. The frequency dependence of Z s must be linear (see Fig. 8-2) with a margin of error of 5%. This requirement is intended to help avoid resonance’s that will lead to erroneous measurements.

8.3 Harmonics Analyzer

The analyzer (harmonic, wave or spectrum analyzer) must operate with the maximum allowable errors shown in the following errors shown in the following table.

Measurement Conditions Max. Allowable Error

Voltage (V) ≥ 1 % VN

< 1 % VN

5 % V

0.05 % VN

Current (I) ≥ 1 % IN

< 1 % IN

5 % I

0.05 % IN

The analyzer must be calibrated, and its calibration should be traceable. Periodic, ad-hoc verifications can be implemented with the help of a perfect square or triangle input waveform of known amplitude. The spectrum of such waveforms is known and enables an immediate evaluation of the analyzer.

8.4 The Transducers

It is well known that the most significant source of errors in harmonic measurements is due to current and voltage transducers. The current transducer must have a flat frequency response in the range of 50 to 2500 Hz. If subharmonics (h < 1) are measured, then the current transducer must also be adequate for the lowest frequency measurement, and in certain applications, this can be a dc component. The same requirements apply to voltage transducers, usually included in the analyzer.

Table 8-1. Maximum Allowable Errors

8.4.1 Current Transformers

The major source of errors caused by current transformers is traced to its magnetizing current. The magnetizing current is nonsinusoidal and contains harmonics that are included in the measured current. When the measured harmonic current is of the same order of magnitude as the magnetizing current harmonic, then large phase angle errors occur and even the measurement of harmonic magnitudes may be compromised. It is imperative to use metering class current transformers, with magnetizing current harmonics (Imh ≤ 5 % Ih, i.e., the magnetizing current harmonic is less than 5% of the measured harmonic current).

The burden of the current transformer, consisting of the input impedance at the current terminals of the analyzer plus the impedance of the conductors that connect the analyzer with the secondary terminals of the current transducer, must not exceed the rated burden of the current transformer. The lower the actual burden, he lower are the phase and amplitude errors caused by the current transformer.

Clamp-on type current transformers should not be used for such measurements. Their magnetizing current is large. Oxidation, impurities and nicks on the butt-joint surfaces of the core cause the increase of the magnetizing current. Moreover, many of the commercially available clamp-on units are overly sensitive to stray magnetic fields produced by adjacent current-carrying conductors.

8.4.2 Rogowski coils

A type of current transducer that is rising in popularity is an air-core current transformer called a Rogowski coil. This is a toroidal coil wound on a rigid or flexible nonmetallic core. A sinusoidal current

will induce in the coil a voltage

where M is the mutual inductance between the transducer and the circuit that carries the distorted current to be measured. A high input impedance analyzer ( 10 M) can measure the magnitudes MhIh. The system is calibrated at 60 Hz with a known current IK that yields a reading VK. The magnitude of a certain harmonic current is determined from the expression

This method is convenient for large harmonic currents. For smaller currents, a larger value of M is required. This translates in a larger number of turns and the inter-turn capacitances may deteriorate the linearity of the frequency response.

Commercial units use precision integrators that reconstruct the signal [3]. The geometry of the coil, shielding method, length and characteristics of the coaxial cables connecting the coil with the integrator and the integrator with the analyzer, and especially the design of the integrator will affect the accuracy of the measurement. A simulated frequency response is shown in Fig. 8-4. Accurate amplitude measurements can be obtained for a wide frequency range. The phase angle, however, may be measured with a significant error. A most annoying problems is the effect caused by the location of Rogowski coil with respect to the conductor whose current is to be measured. Based on the data published in [3] and graphed in Fig. 8-5, results that it is imperative to run the instrument calibration and the actual measurements with Rogowski coil fixed in one rigid location with respect to the conductor. This should not present a problem for laboratory measurements, but may disqualify this method for precision field measurements.

8.4.3 Hall-Effect Transducers

New generations of current transducers using Hall-effect sensors and closed-loop systems[4], Fig. 8-6, are advertised to operate within 1% amplitude error. Such transducers can measure dc and subharmonic currents as well as components in the tens of kHz range. Based on the published data[4], it is still difficult to ascertain how the accuracy of such units compares with current transformers. For dc measurements however, it is the transducer of choice.

8.4.4 Noninductive Shunt Resistors

This is a most accurate method if the shunt impedance is properly built and calibrated, Fig. 8-7. The impedance of the shunt must be included in the value of ZS. This method is conveniently applicable in the laboratory in situations where the supply source and the analyzer can share one common point of connection.

8.5 Suggested Protocol

With the nonlinear load disconnected, measure the voltage source distortion to ascertain that THD V

0.5 %.

Adjust the Thevenin impedance to the prescribed value.

Check the linear frequency response of the Thevenin impedance. This procedure requires injection of harmonic currents into the voltage source. In many situations, the injection can be provided by the nonlinear load under test, otherwise a 2 kVA or larger nonlinear load, with THD I > 1.0, can produce the right harmonic source for this task. The analyzer is used to measure the harmonic voltages (Vh) and harmonic currents (Ih). The Thevenin harmonic impedance is computed from

The normalized values

and must be found within the lines

pu Zs = (1 0.05) h; h > 1

If a background harmonic voltage, VhO, (i.e., a source harmonic voltage) has the same order of magnitude as the measured voltage, Vh, then the measurement of the Thevenin impedance at this particular frequency may be compromised since, in this case, the expression of ZS is

The requirement that the supply voltage is sinusoidal addresses this need; nevertheless, it is necessary to verify that the condition

Vh = ZS (h) Ih 20 VhO

is always satisfied, especially when the measured harmonic voltage Vh < 0.5 % V1.

Connect and energize the nonlinear load to be evaluated. Allow for the unit to stabilize thermally. Measurements are taken for different input rms voltages (e.g., 0.95, 1.00 and 1.05 rated voltage) and, if applicable, a set of output powers

(e.g., no-load, 0.25, 0.50, 0.75, 1.00 and 1.10 rated load). It is recommended to start with the conditions that cause the largest temperature rise, i.e., maximum power.

9. Bibliography______________________________________________________________________________________

[B1] Power Electronics, Mohan, Undeland and Robbins, John Wiley and Sons, 1995

[B2] Grady, W.M., TU Electric Whole-House Measurements (field tests of Jan./Feb. 1991), Panel Presentation from the 1996 T&D Conference

Pileggi, D.J. et al, “The Effect of Modern Compact Fluorescent Lights on Voltage Distortion”, IEEE Transactions on Power Delivery, v8, n3, July 1993, pp 1451-59

Thallam, R.S. et al, “Estimating Future Harmonic Distortion Levels Due to Single Phase Adjustable-Speed Drive Air Conditioners, IEEE ICHPQ 1992

ANSI C82.11-1993, American National Standard for Lamp Ballasts: High Frequency Fluorescent Lamp Ballasts

L. Sulfstede, “Residential Variable Speed Technology”, The Trane Company, 1989

EPRI Journal, “Field Testing of the HydroTech 2000 Heat Pump”, December 1991

Power Quality – Harmonics, Ontario Hydro, 1990

IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems

Steciuk, P.B., K.A. Puskarich, W.E. Reid, “Harmonic Considerations on Low Voltage Systems”, presented at the 1991 IEEE Textile, Fiber, and Film Industry Technical Conference

[1] A. Mansoor, W.M. Grady, A.H. Chowdhury and M. Samotij, “An Investigation of Harmonic Attenuation and Diversity Among Distributed Single-Phase Electric Loads”, IEEE Transactions on Power Delivery, Vol.10, No.11, Jan. 1995, pp. 467-73.

[2] International Standard IEC 1000-4-7, “Electromagnetic Compatibility (EMC), Part 4: Testing and Measurement Techniques”.

[3] W.F. Ray and R.M. Davis, Wide Bandwidth Rogowski Current Transducers. Part I: The Rogowski Coil. Part II: The Integratory”, EPE Journal, Vol. 3, No.1, March 1993, pp.51-9 and No.2, June 1993, pp. 116-22.

[4] T. Twitchell, “ASIC-Based, Closed-Loop Transducer Meets Motion System Current Measurement Requirements”, PCIM, March 1998, pp. 58-62.

Table 6-2. Fundamental Differences in Distribution Design between European and U.S. Distribution Systems

Standard Design Practice Comments

United States Europe

Transformers

Connection wye wye delta wye US is limited by 3rd, Europe by 5th

number of phases mostly 1, some 3 3 US single-phase is connected phase-to-neutral

kVA rating 50 500 or 1000

number of customers 6-10 60-100

equivalent impedance to transf. sec. higher lower point of common coupling issue

main feeder or circuit

kV phase to phase 12.5 11 or 12 not much difference

number of conductors 4 3 neutral (US) is ampacity limited by primarily 3rd, also stray voltage possibility

customer services

service voltage 120/240 V 230 V need to address both 120 V limits as well as 240 V limits w US

number of wires 3 2 Europe just has 230 V limit

equivalent impedance to customer lower higher point of use