Harmonic Currents

Power line harmonics is just one of the many power quality issues that arise with public utilities. Effectively, current harmonics represent a distortion of the normal sine wave provided by the utility. When a product such as an SCR switched load or a switching power supply distorts the current, harmonics at multiples of the power line frequency are generated. Two significant consequences arise as a result of harmonic generation. First, because of finite impedances of power lines, voltage variations are generated that other equipment on the line must tolerate. Second, when generated in a three-phase system, harmonics may cause overheating of neutral lines.

Power line harmonics are generated when a load draws a non-linear current from a sinusoidal voltage. The harmonic component is an element of a Fourier series which can be used to define any periodic waveshape. The harmonic order or number is the integral number defined by the ratio of the frequency of the harmonic to the fundamental frequency (e.g., 150 Hz is the third harmonic of 50 Hz; n = 150/50). A second harmonic is therefore two times the fundamental frequency of the supply line volt current. If the supply voltage had been generated by an ideal source (zero impedance), the current distortion would have little effect on the supply voltage sine wave. However, because a power system has a finite impedance, the current distortion caused by a nonlinear load creates a corresponding voltage distortion in the supply lines. This voltage distortion can subsequently disrupt operation of other sensitive equipment connected to the same line. Voltage distortion can also cause motors operating on the line to overheat.

Because neutral lines are not fused or protected by circuit breakers, overheating of neutral conductors in a three-phase line can be a significant safety hazard. Such damaging occurrences are usually attributable to the use of single-phase loads attached to three-phase/single-phase wiring systems. Excessive neutral current is caused by the existence of "triplen" harmonics, which add in series in the neutral line. Triplen harmonics are those harmonics that are an integral multiple of three times the fundamental. In three-phase, four-wire systems, each of the three phases is separated in phase by multiples of 120°. Triplen harmonics have relative amplitudes that are also 120° out of phase with the fundamental. Therefore, these harmonics, when drawn from the single phase lines, can add together in phase to cause neutral currents that exceed the phase current. While the individual phase currents are less than the circuit breaker protection current, the neutral current caused by the triplen harmonics may exceed the current rating of the wire used.

The National Electrical Code requirements prohibit installing fuses or circuit breakers in the neutral line, so the neutral cannot be protected against the excessive currents generated and may overheat. Therefore in the absence of controls on harmonic currents, neutral conductors must be sized larger than phase conductors to prevent overheating.

The original IEC document covering power line harmonic distortion, IEC 555-2, limited the applicability of the requirements to nonprofessional devices. This document, sourced by IEC subcommittee 77, covers single-phase circuits up to 240 V and three-phase circuits up to 415 V at both 50 Hz and 60 Hz operations. Limits are set only for 220/380 volts, 230/400 volts, and 240/415 V at 50 Hz.

IEC 555-2 was originally published in 1982. Amendment one was published in October 1985. CENELEC adopted IEC 555 in its document EN 60555-2 in April of 1987. Part 2 of the CENELEC document was identical to the original IEC 555-2. A withdrawal date of June 1, 1987, was specified for this document meaning that members of the European Union were required to implement procedures for requiring performance in accordance with EN 60555-2 by that date. In adopting IEC 555-2, CENELEC also adopted the scope of the document without change. In other words, only equipment designed for nonprofessional uses were covered by the requirements of the standard. The standard specifically excludes any professional use equipment.

In March of 1995, the IEC published document IEC 1000-3-2 (also sourced by Technical Committee 77). The primary difference between IEC 1000-3-2 and its predecessors is its scope of applicability. IEC 1000-3-2 is "...applicable to electrical and electronic equipment having an input current up to and including 16 A per phase...." IEC 1000-3-2 was published by CENELEC as document EN 61000-3-2 in 1995 and harmonized to the EMC Directive.

The general requirement in IEC 1000-3-2 prohibits power control systems that generate low-frequency harmonics. Specifically, systems that generate switching operations less than or equal to 40 times in a half cycle are prohibited in the control of power supplied to heating elements and thermal devices. This relates to equipment that has power current control in the form of SCRs. The SCR provides phase control of the voltage applied to the device and therefore limits the power dissipated. Resistive motor controllers or controllers designed to operate on 200 W or less are exempted from this requirement as long as they meet maximum harmonic specifications listed in Table I. The specifications set special limits for portable tools and lighting equipment. For portable tools, the limit is equivalent to 1.5 times the standard limits. The limits for lighting equipment are shown in Table II. For all other general types of equipment, the limits listed in Table I apply to all types of power connection.

Harmonic Order (n)		Maximum Permissible Harmonic Current Expressed as a Percent of the Input Current at the Fundamental Frequency (%)
2		2
3		30 • *
7		10
9		7
13		5
11 < n < 39 (odd harmonics only)		3
* is the circuit power factor
Table II. Low-frequency harmonic emissions limits for Class C equipment.

The limits listed in Table I are specified for power voltages of 230 V line to neutral and 440 V line to line. Operation at different voltages must use a correction factor determined by dividing the test voltage by 230 V or 440 V. The limits are applicable whether the harmonics are transitory or steady-state in nature, except for nonrepetitive currents lasting for no more than a few seconds.

A special class of equipment (Class D) is defined for harmonic requirements. This class includes equipment that has a defined current waveform characteristic as shown in Figure 1. The figure represents a half cycle of the current waveform. (The normal sinusoidal waveform would be 0 at the start, maximum at /2 radian (90°) and 0 at radian (180°). In order for equipment to be considered Class D, its current waveform must be within the envelope for at least 95% of the duration of each half cycle. As shown in Table III, limits for Class D equipment are specified in absolute numbers and in relative limits of milliamperes per watt. Both relative and absolute levels must be met for the system to comply. Only equipment operating at power levels less than or equal to 600 W may apply the Class D limits. (Equipment that exceeds 600 W is exempted from the Class D requirements, but must still satisfy the Class A requirements shown in Table I.)

Figure 1. Class D waveshape envelope.

Figure 2. Low-frequency harmonic emission limits flowchart.

These requirements apply to transitory continuous harmonics. The transitory harmonics may be the result of the start-up of motors, heaters, and so forth. The limits are relaxed by 1.5x if the harmonic is present for only 10% of any 2.5-minute observation time. This relaxation applies to even harmonics from n = 2 to n = 20 and for odd harmonics from n = 3 to n = 19. All other transitory harmonics must meet the limits defined in Tables I–III. However, those harmonics that appear only during the first 10 seconds after turn-on or turn-off are excluded from the requirements.

When published in the Official Journal, EN 61000-4-3 included no extended dates of implementation, which implies that the standard should become applicable immediately upon publication. Most manufacturers have not addressed harmonic requirements in any of their designs, so publication of this document created significant hardships for many of them. As a result, an amendment to the document was proposed, but never published, which extended the applicability date (date of withdrawal) to June 1, 1998. Amendments A1 and A2 were published in the Official Journal on February 27, 1999, extending the date of withdrawal to January 1, 2001. On the date of withdrawal, all manufacturers of electronic equipment will be required to meet the harmonic limits. The standard allows that all products that did not fall under the scope of EN 60555-2 but are now covered by EN 61000-3-2 are exempt from the requirement until that time. For products that fall under the scope of EN 60555-2, transitional rules apply. For the most part, because EN 60555-2 applies, it is appropriate to do harmonic measurements in accordance with that standard. Some competent bodies have interpreted the transition period to mean that any product that falls under the original scope of EN 60555-2 must prepare a TCF to demonstrate compliance with the new requirement of EN 61000-3-2 until the transition period expires.

Testing to EN 61000-3-2

Test Source

EN 61000-3-2 sets requirements for the purity of the test source. The requirements for a source apply to the voltage as follows:

Measurement Equipment

Several types of test equipment can be used to measure harmonics. These include frequency domain instrumentation such as spectrum analyzers or time domain instrumentation using discrete Fourier transform algorithms. Some requirements common to all instrumentation are