ELECTROMAGNETIC COMPATIBILITY (EMC) (JBS READY)

15.6.1. Definitions (JBS Ready)

*Copyright © IEC, Geneva, Switzerland. www.iec.ch

The International Electrotechnical Vocabulary (IEV) gives the following definition of EMC:

”The ability of an equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.” [IEV 161-01-07]

Other relevant definitions are:

Electromagnetic disturbance

Any electromagnetic phenomenon which, by being present in the electromagnetic environment, can cause electrical equipment to depart from its intended performance [IEV 161-01-05, modified]

Electromagnetic interference (EMI)

Degradation of the performance of equipment, transmission channel or system caused by an electromagnetic disturbance.

NOTE The terms “electromagnetic disturbance” and “electromagnetic interference” denote respectively cause and effect, but they are often used indiscriminately. [IEV 161-01-06]

Disturbance level

The amount or magnitude of an electromagnetic disturbance, measured and evaluated in a specified way [IEV 161-03-01, modified]

Electromagnetic compatibility level

The specified electromagnetic disturbance level used as a reference level in a specified environment for co-ordination in the setting of emission and immunity limits

NOTE By convention, the compatibility level is chosen so that there is only a small probability that it will be exceeded by the actual disturbance level. [IEV 161-03-10, modified]

Immunity level

The maximum level of a given electromagnetic disturbance incident on a particular device, equipment or system for which it remains capable of operating at a required degree of performance [IEV 161-03-14]

15.6.2. Electromagnetic disturbances on transformers (JBS Ready)

Transformers may be exposed to various types of electromagnetic disturbances when operating in public and industrial power supply systems.

Disturbance phenomena to consider are: • harmonics,

• inter-harmonics, • overvoltages, • voltage unbalance, • overcurrents,

• power frequency variation. Harmonics

Harmonics are sinusoidal voltages or currents having frequencies that are whole multiples of the power frequency at which the supply system is designed to operate.

Equipment with a non-linear voltage/current characteristic cause current harmonics. Harmonic currents produce harmonic voltage drops across the impedance of the network.

IEC 60076-1 Power transformers Part 1: General assumes that the wave shape of the supply voltage is approximately sinusoidal. This requirement is normally not critical in public supply systems but may have to be considered in installations with considerable convertor loading. If the total harmonic content exceeds 5% or the even harmonic content exceeds 1%, this should be specified in the enquiry and in the order.

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Current harmonics influence the load losses and the temperature rise, which may need consideration in the transformer design.

In systems with considerable capacitive elements, like long cables or power factor correction capacitors, there is a risk that harmonics cause shunt and series resonance in the network, which in turn causes voltage magnification even at a point remote from the distorting load.

The magnetizing current of transformers contains harmonics due to the non-linear magnetizing characteristic. The magnetizing current is in normal operation small, in the order of 1% of the rated current of the transformers, and the influence of its harmonics is negligible. However, if the transformer core is saturated due to an applied voltage higher than the rated voltage, the magnetizing current and its harmonics will dramatically increase.

Interharmonics

Interharmonic currents and voltages have frequencies which are not an integer multiple of the fundamental frequency. They can appear as discrete frequencies or as a wide-band spectrum. The sources of interharmonics can be different types of convertors, induction motors, arc welding machines and arc furnaces.

Interharmonics may need consideration in the transformer design. Overvoltages

Quasi-stationary or steady state overvoltage causing an increase of more than 5% in the flux density in the transformer core in relation to the core flux at rated voltage should be specified in the enquiry and in the order.

The saturation flux density of modern core steel is 2,03 Tesla. Further increase in the magnetic flux may cause unacceptable heating in structural details keeping the core together and in the tank. A dramatic increase in the magnetizing current and its harmonics will also take place.

The topic of temporary and transient overvoltages is treated in chapter 14. Voltage unbalance

IEC 60076-1 Power transformers Part 1: General assumes that the three-phase supply voltage is approximately symmetrical in normal service conditions.

The predominant cause of voltage unbalance is unbalanced load. Due to the distorted voltage triangle the voltage across one or two phase windings will be higher than rated. Consequently the flux density in one or two limbs of the core will increase correspondingly. If saturation in these limbs is reached, unacceptable heating in structural details keeping the core together and in the tank may occur. Increase in the magnetizing current and its harmonics in the affected phases will be another consequence. The sound level of the transformer may also increase.

It is not possible to make a general rule regarding permitted unbalance for transformers. Each case must be treated individually. If the degree of unbalance is known before ordering the transformer, it will be possible to take it into account in the design. Before connecting a dominant single-phase load to an already existing transformer, it is recommended to analyse the situation first.

Overcurrents

IEC 60076-7 – Power transformers – Part 7: Loading guide for oil-immersed transformers (in process) and IEC 60905 Loading guide for dry-type transformers provide advise regarding overloading of transformers, continuously or intermittently.

Transformers may also be exposed to overcurrents due to short-circuits in the network. Such currents may amount to 10 – 20 times the rated current of the transformer or even more. Large pulsating mechanical forces will act on the windings and their supports during these currents. In addition the high current density in the windings will rapidly increase the winding temperature. Transformers are designed to withstand certain short-circuit currents, but the duration of the currents must be limited to maximum 2 seconds by means of relays that disconnect the transformer from the energy sources, unless a longer duration is agreed in the contract.

Service experience indicates that transformers from experienced manufacturers seldom fails because of short-circuit currents.

Power frequency variation

In public supply systems the temporary frequency deviation from the nominal frequency is normally not more than ±1 Hz. The steady-state deviation of frequency from the nominal frequency is much less.

A frequency of 49 Hz instead of 50 Hz will increase the flux density in the transformer core by 2% at the same applied voltage.

In supply systems isolated from public network (for example an island system) frequency variations up to ±4% are expected. In such cases the flux density in the transformer core may increase up to 4%, and this should be taken into account when designing the transformer.

The maximum disturbance level in a network may be derived from theoretical studies or measurements. The disturbance level is not a single value but varies statistically with location and time. Because of this variability it is often very difficult or even impossible to determine the real highest level of disturbance, which may appear very infrequently. It may, in general, not be economical to define the compatibility level in terms of this highest value to which most devices would not be exposed to most of the time.

It seems more appropriate to define the compatibility level as a value that only will be exceeded by the disturbance level in very few cases, for example 1 or 2%.

The compatibility of transformers regarding overvoltages and overcurrents is verified by means of various tests described in the IEC transformer standards 60076-3, 60076-5 and 60726. When performing thermal testing on convertor transformers, the effect of harmonic currents is taken into according to IEC 61378-1.

15.6.3. Electromagnetic field in the vicinity of transformers (JBS Ready)

Distribution transformers with voltage ratio from medium to low voltage for supply to commercial and industrial consumers are often integrated into these consumers’ buildings. The electromagnetic field from such transformers may disturb the performance of electronic equipment situated near by. Or in other words, the magnetic field may exceed the immunity level of the electronic equipment. IEC 61000-2-7 “Electromagnetic compatibility (EMC) – Part 2: Environment – Section 7: Low frequency magnetic fields in various environments” refers results from measurements taken at an installation of a 315 kVA distribution transformer. The measurements were taken with field coils and are considerably influenced by the presence of harmonics. For this reason two values are stated for each location, one at 50 Hz and one at 0 kHz to 2 kHz. Typical measured maximum values at various locations are given in the following table:

Magnetic field

[A/m] Magnetic flux density in air [µT] Location

50 Hz 0 kHz – 2 kHz 50 Hz 0 kHz – 2 kHz

Adjacent to transformer connections 200 300 250 380

Above transformer 15 60 20 75

Adjacent to low-voltage cables 20 70 25 90

Adjacent to but outside the roof 5 30 6 40

*Copyright © IEC, Geneva, Switzerland. www.iec.ch

The magnetic field was detectable up to approximately 10 m from the physical enclosure of the substation.

Practical experience from measurements of electromagnetic field around a transformer indicates that the measured values are considerably depending on how the measurements are made. There is so far no international standard describing how measurements of the electromagnetic field around transformers shall be performed to achieve consistent results. Still the above example indicates the magnitude of the field that can be expected.

CENELEC report R014-001 “Guide for the evaluation of electromagnetic fields around power transformers” indicates formulas for calculation of field values for some simple geometric configurations.

In cases where the field exceeds the immunity levels of various kinds of electronic equipment in the vicinity of the transformer, appropriate shielding of the equipment or moving the equipment further away from the transformer can solve the problem. ABB has developed solutions for shielding cable boxes which reduce the electromagnetic field related to the LV cables and connections.

Contaminants on bushings of oil-immersed transformers and on windings of dry-type transformers may, especially in the presence of moisture, cause electrical discharges, which in turn may cause

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disturbance on electronic equipment. Appropriate routines for cleaning are recommended in section 4.3.6.

15.6.4. Effect of electromagnetic fields on humans (JBS Ready)

With regard to health hazards a magnetic flux density of 100 µT at 50 Hz seem to be considered as acceptable with high safety factor (250-500). For short time exposure the acceptable limit can be doubled.

Research on the effect of electric and magnetic fields on health is still not complete. Attention should be paid to the possible influence of such fields on heart pacemakers.