TechArchive

Original article date: December 1996

The subject of harmonics and the various EC directives relating to drives are closesly linked. Steve Barker, Siemens MasterDrives product manager, takes some of the mystery out of, EMC and power system harmonics.

Drives manufacturers are increasingly realising the benefits offered by products with improved EMC performance and the potential problems which could arise from the misinterpretation of EMC standards.

Of the many EC standards and regulations now facing electrical engineers, the EMC directive must be the least understood, most misinterpreted and seem the most costly of all. To make matters worse, in the rush to gain that all important competitive edge, some suppliers have offered misleading sales and marketing information which has perpetuated the general state of confusion.

EC Directives

Drives are categorised as power electronic components and as such are regulated by at least three specific EC directives:

• Machinery Directive:- Since variable speed drives are not machines, the Machinery Directive does not apply directly to the drives. However, the actual machine itself must comply with the Directive ? a declaration of incorporation may be given by the inverter manufacturer.
• Low Voltage Directive:- Applies to all variable speed drives and therefore CE marking is mandatory by the end of December 1996 latest.
• EMC Directive:- Applied in different ways according to the type of product and its route to market (Fig 1).

CE Marking

If an item is CE marked, this signifies that the equipment complies with all relevant directives. The most important consideration is to establish which directives are in fact relevant in each case. For most variable speed drives, the directives are:

It is essential to note that it is a legal requirement that equipment is correctly CE marked. CE marking does not signify a better quality product, nor does it absolve the users of any responsibility. Even if a drive is not CE marked, it must still comply with the essential protection requirements of the Directive.

Harmonics

The EMC directive places most emphasis on high frequency phenomena, however the effects of low frequency disturbances are currently under separate consideration by the IEC/CENELEC committees with a view to the introduction of harmonised standards.

Harmonics are measured as additional frequencies calculated as a multiple of the fundamental base frequency. The distortion of this fundamental sine wave is responsible for various conditions on and around an electrical installation.

Harmonics are produced by any electrical device which draws power in a non-linear way such as fluorescent lights, variable speed drives, PCs, TVs and UPS systems. Variable speed drives cause harmonics under load conditions due to the six pulse rectification technique.

Harmful effects resulting from this include overheating problems and erratic operation of control and protection equipment. In addition, power factor correction capacitors have even been known to explode under the excessive energy absorbed. Capacitors present a lower impedance to higher harmonic frequencies resulting in excess voltage and current stressing.

KWh meters will not record true power levels and may read higher kWh levels than actual power at the fundamental frequency. In extreme circumstances, recording of total power consumption may mean that a user connected to the mains supply inadvertently draws harmonic power due to neighbouring distortion and as a consequence, will be charged for the use of that energy.

So, how can these problems be avoided? Increasing the pulse number of the converter, from say 6 to 12 pulse, requires 30deg phase-shifted supply, which is found to give maximum harmonic cancellation on the primary side of the transformer. The resulting pattern of 12 pulses per cycle will produce significant reductions in harmonic distortion.

The introduction of harmonic filters is another, albeit potentially expensive, method of eliminating harmonics ? this must be carefully designed to ensure effective operation. Each leg of the harmonic filter may be tuned to one specific frequency, with great care taken in the design and application of these filters to avoid site resonance problems developing. Filter circuits always constitute capacitive current at the fundamental wave of the three phase system. They absorb not only the harmonic currents but also draw a capacitive current at the fundamental wave frequency to help compensate the reactive power of the system.

The introduction of sinusoidal rectifiers can also eliminate harmonics. These fully controlled, three phase bridge connected configurations use IGBT (insulated gate bipolar transistor) devices in place of the standard diode rectifier. They feature a switching arrangement which virtually eliminates low order mains harmonics. Although this is a more expensive technique than conventional methods and results in a slight reduction in overall efficiency of operation, it does offer significant benefits in terms of low order harmonics and power factor control.

The impact of the EU directives and their focus on power supply pollution should not be viewed in any negative way. Irrespective of the possible implementation of future standards on harmonics, manufacturers are addressing potential problems with a range of innovative solutions such as 12/24 pulse systems and sinusoidal rectifiers. The result of these innovations will be more reliable equipment, causing fewer power system problems which can only benefit the end user.

In order to avoid potential problems at the Grafham Dam Pumping Station of Anglian Water, Siemens supplied pseudo-24-pulse systems ? each inverter was a standalone 12 pulse drive supplied via individual cast resin phase shifting transformers. However, the Siemens transformers were supplied as matched pairs with a further phase shift between each pair. Therefore the supply system sees a 24 pulse system with the consequent further reduction in harmonics.

Following a successful installation and start-up, a detailed harmonic analysis was undertaken by Eastern Electricity to assess the impact of the drives installation. This report not only revealed that the new level of harmonic distortion remained significantly below the limits specified, but also highlighted that the total voltage distortion did not appreciably increase, compared to previous measurements taken prior to installation of the drives.

EMC Immunity

Many EMC problems can be avoided by improving immunity in a number of ways:

• Attachment of metallic cabinet panels using flat connectors with a large surface area
• Segregation of signal cables and power lines by a minimum of 20cm
• Ground cable shields at both ends using large surface area connections
• Suppress all contactor and relay coils
• Place all cables close to ‘ground plane’
• Twist unscreened conductors together
• Minimise cable lengths and ground unused conductors at both ends
• Mount any optional filter as close as possible to the source of the disturbance
• Ensure good potential equalisation between equipment

• Free guides to Power System Harmonics and EMC Directives are available from Siemens

• Steve Barker sits on GAMBICA’s power electronics technical committee which is involved in the preparation of IEC standards.

• Siemens
• Tel: 0161 446 5971
• Fax: 0161 446 5972

December 1996