There is a lot of discussion on high-frequency electromagnetic compatibility (EMC) effects, with confusion between high frequency EMC effects, which are generally localised, and harmonics on a power system, which can be cumulative. They are quite separate, with different effects and different solutions.

High-frequency EMC effects tend to cause interference to sensitive data and measuring circuits by stray coupling paths. With few exceptions, if harmonics cause disturbance, it is through direct electrical connection and not through stray paths. Screening is therefore rarely a remedial measure for harmonic problems (with the possible exception of telephone interference).

Harmonics are caused by any process whereby the current drawn by equipment does not faithfully follow the sinusoidal supply voltage waveform - variable speed drives being a typical example of this. Most electronic circuits use DC internally, so they have a rectifier to convert the incoming AC to DC. Unless it has special additional active features, the rectifier draws its current in a series of short pulses.

The supply current waveform is generally referred to in terms of the harmonics of the supply frequency in contains. The harmonic current causes harmonic voltage fluctuations by other equipment connected to the same supply.

Because harmonic voltage can cause disturbance or stress to other electrical equipment connected to the same supply system, there are regulations designed to limit it. If installations contain a high proportion of variable speed drives and/or other power electronic equipment such as uninterruptible power supplies (UPS), then they may have to satisfy the supply authorities’ harmonic guidelines before permission to connect is granted.

In addition, it is prudent for users to ensure that harmonics levels within their own plant are not excessive.

Some of the practical problems that may arise from high harmonics levels include:

• Poor power factor, i.e. high current for a given power – larger electrical plant requirements (cables, circuit breakers & transformers) for a given power throughput. Larger capital costs, larger maintenance costs.
• Interference to equipment that is sensitive to voltage waveform
• Excessive heating of neutral conductors (single-phase loads only)
• Excessive heating of induction motors
• High acoustic noise from transformers, bus-bars and switchgear
• Abnormal heating of transformers and associated equipment
• Damage to power factor correction capacitors

There are two kinds of regulations that have to be considered:

• Regulations for installations
• Standards for equipment

The electricity supply authorities impose regulations to protect other electricity consumers from harmonics effects. These are based on an agreed level of voltage distortion that can be tolerated by correctly designed equipment. This is specified in terms of a total harmonic distortion (THD) – measured at the point of common coupling (PCC) with other power consumers.

The internationally accepted maximum ‘compatibility level’ in a low voltage system is 8%. To achieve this with a high degree of confidence it is usual to aim for the lower ‘planning level’, typically 5%. Individual harmonics are also subject to limits.

The harmonic levels within a plant may be higher because of the impedance of cables and transformers. However, since there are no statutory requirements, a more relaxed attitude may be taken – but it is not advisable to exceed the 8% level since most equipment is designed to be immune only up to this level.

Calculating the voltage distortion of a site can be expensive, since the harmonics have to be measured over all conditions at all times of day. The system parameters such as source impedances have to be derived and the effects of the planned new load have to be estimated.

For a large installation with a high proportion of the load comprising electronic equipment, it is nevertheless sensible and cost-effective to go through this exercise to avoid the costs of later remedial action. For simpler cases, a full analysis is overkill.

Regulations such as the new UK G5/4 provide simplified ‘staged’ procedures to permit connection based only on harmonic current data supplied by equipment manufacturers. This involves making simplifying assumptions with a bias towards caution.

Life is easier if a product conforms to a relevant harmonic standard, so that it can be connected without reference to the supply authority. The international standard for equipment rated at less than 16 amps is IEC61000-3-2 (CENELEC standard EN61000-3-2). These are applied to consumer products and similar equipment used in large numbers, where individual permission to connect would not be practical.

Small variable speed drives rated at less than about 650 watts shaft power fall within the scope of this standard and can be made to conform to it by the application of suitable measures. However, when they are used in large quantities in a single installation, it may be more cost-effective to assess their total current and obtain permission to connect the scheme as a whole.

In future, there will be a further standard – IEC61000-3-12 (EN61000-3-12) – covering equipment rated up to 75 amps.

Don’t panic! Harmonics problems are unusual, although with the steady increase in the use of electronic equipment, problems may become more common in future. The most likely problem areas are office buildings with a very high density of personal computers and in cases where most of the supply capacity is used by electronic equipment such as drives, converters and UPS systems.

As a rule of thumb, if the total rectifier loading on a power system comprises less than 20% of the total, then harmonics are unlikely to be a headache. In most industries, the capacity of the supply considerably exceeds the installed load and the load generating harmonics is often a small proportion of this. Uncontrolled AC induction motors and resistive heating elements generate minimal harmonics.

If rectifier loading is more than 20%, then a harmonic control plan should be in place. This requires that existing levels be assessed and a harmonic budget be allocated over and above the capital costs for new equipment.

The following is a simplified action plan to reduce the harmonic levels.

1. If there is a choice, connect the equipment to a point with a high fault level (low impedance), as harmonic voltage caused by a given harmonic current is proportional to the system source impedance e.g. main bus-bars rather than downstream of long cables shared with other equipment.
2. Use three-phase drives where possible – the harmonic current for a three-phase drive of given power is only 30% of that for a single-phase drive.
3. Use additional inductance – series inductance at the drive input gives a useful reduction in harmonic current, particularly effective for small drives.
4. Use a higher pulse number – 12-pulse or higher. Standard three-phase drives up to about 200 kW use 6-pulse rectifiers. A 12-pulse rectifier virtually eliminates the crucial fifth and seventh harmonics.
5. Use a drive with an active input stage – for example, the Control Techniques Regenerative Unidrive has an active input stage, which generates negligible harmonic current, as well as permitting the return of braking power to the supply.
6. Use a harmonic filter – harmonic filters are available to attenuate specific harmonics

The above advice is available in detail in the Control Techniques’ booklet ‘Harmonics Guide’ from any Control Techniques Drive Centre.

In addition, Control Techniques provides comprehensive EMC data sheets for all of its AC drive products, giving harmonic currents at full load, with, and without, 2% AC line reactors.

Control Techniques has also produced a spreadsheet running under Microsoft Excel, which estimates the harmonics produced by any combination of Control Techniques AC drives and tabulates the results with the G5/4 limits for ease of assessment. It includes the effect of variation in load, the use of AC line reactors and the operation of some or all of the drives in 12-pulse configuration.

For advice on harmonics and any of the items mentioned above, contact your local Control Techniques Drive Centre.

ENDS