Harmonics and Flicker - the low frequency end of the EMC spectrum

At audio frequencies, I am of course thinking of harmonic distortion of the mains and at very low frequencies, flicker. Low frequency mains harmonics and flicker are quite different phenomena the former being the consequence of a non linear load and the latter a varying low impedance load, both imposed on the mains power. Historically they have been linked together since they were often caused by large electrical machines in which the magnetizing current of magnetic components produced the required non linearity and the power requirements of the operation the varying load. This is reflected in their inclusion in EN 60555-2,3 "Disturbances in supply systems caused by household appliances and similar electrical equipment" where the Standard gives as typical examples "appliances for cooking or heating, motor operated or magnetically driven appliances, portable tools, light dimmers and radio and television receivers." The growth of consumer electronics has meant that the average home has a plethora of mains driven electronic devices not just television sets. Invariably these electronic devices have mains rectification which is the dominant cause of mains harmonic distortion. This being the case it is not surprising that when the time came to revise the Standards dealing with harmonics and flicker the scope was widened and attention given to dealing with the contributions from low power devices. The new Standards arrived in the form of EN 61000-3-2 and 3 in 1995 and after some false starts will become mandatory from 1 January 2001 onwards.

Figure 1: Implementation of EN 61000-3-2 and 3

Low Frequency Mains Harmonics

Figure 2: Current and Voltage Waveforms with single phase rectification.

Current is drawn from the supply when the input voltage exceeds that on the smoothing capacitor. When this occurs the current is only limited by the source impedance of the mains and the resistance of the diode and capacitor. As a consequence a current waveform is created rich in harmonics. Harmonics are created by any mains power supply using capacitive smoothing irrespective of whether isolation is provided by a transformer or not. The leakage inductance and resistance of the windings will reduce the effect but transformer designs tend to minimize their magnitude and so an isolation transformer has little effect.

Analysis of the current waveform produced will show it to consist of the fundamental 50 Hz component, a third harmonic at 150 Hz, a fifth at 250 Hz and so on. The number of harmonics present is determined by the rise and fall time of the current and their relative magnitudes by the particular wave shape formed. The Standards impose limits on frequencies up to 2 kHz. The method of measurement is essentially very simple; a low value shunt resistor or current transformer is placed in circuit and the voltage generated monitored with an audio frequency analyzer.

En 61000-3-2 Limits for harmonic current emissions

Figure 3: Determining the Equipment Class Type

All equipment is classified as Class A with absolute harmonic current limits unless it is not. The limits are unchanged from EN 60555-2. Portable tools become Class B, again with absolute harmonic current limits unchanged from EN 60555-2. Class C covers lighting equipment including dimmers and here relative limits are introduced for the first time. Dimmers for incandescent lamps whether integral or separate are required to meet Class A limits. Lighting equipment with power not greater than 25W is exempt. The Standard does not specify particular electronic products such as personal computers but rather specifies an input current wave shape, or rather, limits to the current wave shape. If equipment has an input power between 75W and 600W and its input current wave shape falls within the "top hat" ( 95% of the time) it is categorized as Class D and new proportional limits apply. The lower power limit will fall to 50W in the future. Professional equipment of whatever category is now included but only up to a power limit of 1 kW.

Figure 4: Class D Special Wave Shape

The above refers to steady state conditions and while the initial switch on/off with its inevitable transients may be ignored (10s interval) EN61000-3-2 does not exclude the fluctuations which may occur during normal operation of the equipment. For even harmonics from 2 to 10 and odd harmonics from 3 to 19 values up to 1.5 times the class limit are allowed for 10% of the time of any 2.5 minute observation window. The standard limits apply to all other transitory or fluctuating harmonics.

Table 1: Harmonic Limits specified by EN 61000-3-2 (odd hamonics only).

Figure 5: Limits for Fluctuating Harmonics.

The Problem of Flicker

Flicker is concerned with the visual disturbance of filament lamps due to voltage fluctuations. It is caused by the finite source impedance of the mains supply and a changing .load impedance. EN 61000 defines it rather verbosely as: "Impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time".

Voltage fluctuations and flicker are concerned with very low frequency phenomena where we are concerned with fluctuations in the RMS mains voltage. Flicker is a special case of the more general problem of voltage fluctuations induced by a varying load across finite mains impedance. Fluctuations are specified in relative voltage change with respect to the nominal operating voltage.

Figure 6: Voltage Fluctuation Limits

The relative voltage change, d, is the change in relative voltage between periods of steady state of duration one second or more. Not all voltage fluctuations will produce a noticeable change in the illumination from a 60W filament lamp driven off the same supply. When such flicker occurs it is a serious nuisance to personnel. The level of severity of the flicker as seen by an individual will depend up on the magnitude and shape of the changes as well as their repetition rate.

The simplest case of flicker involves rectangular equidistant voltage changes. There are two flicker indicators defined in the Standard:

• Short term Flicker Indicator PST The flicker severity evaluated over a short period (minutes)
• Long term Flicker Indicator PLT The flicker severity evaluated over a long period (hours) using successive PST values.

Figure 7: Determining Flicker short-term Indicator.

The long-term flicker indicator is derived from a number of short-term flicker measurements derived over a 2 hour period.

The long-term flicker indicator can be derived from the formula:

While it is possible to determine the flicker indicators by observation and hand calculation typical test apparatus available to measure low frequency harmonics will in general determine all the relevant parameters for voltage fluctuations and flicker automatically. If this all looks rather daunting then it is worth noting that EN 61000-3-3 states "Tests shall not be made on equipment which is unlikely to produce significant voltage fluctuations or flicker". Most manufacturers of electronic equipment will not be troubled by this requirement.

Which Standard to apply and when

Products not within the scope of EN 60555-2,3 are covered by the generic standard until 1st January 2001. This of course refers to EN 60555 and hence no action need be taken until 1st January 2001. The apparent contradiction created by the generic standard and EN 61000-3-2,3 is removed if one notes that the latter is a product family standard and takes precedence over the generic standard. Again, manufacturers may choose to apply EN 61000-3-2 and 3 if they so wish.

EN 61000-3-2 and EN 61000-3-3 become mandatory from 1st January 2001.

* Dr. Philip Slade is EMC Manager at the School of Engineering and Computer Science, Exeter University. The Electronics Centre at the School provides a complete range of EMC services from fully compliant testing to low cost pre-compliance as well as the practical design expertise to solve problems. For more information contact 01392 263629.