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In most installations, process control requires the use of additional equipment such as computers, sensors, etc. These devices are usually installed close to each other, which makes it possible to interfere with each other. The sources of electromagnetic interferences are, among others, converter devices in which high frequency of current changes and large steep current pulses caused by commutation of very fast power semiconductor devices cause electromagnetic interference emitted to the environment and via the power grid. It is assumed that disturbances below 10 MHz are spread mainly through conductivity, also called hard wire emission, above 30 MHz by radiation. Intermediate frequencies have their share in both types of emissions.
There are two main mechanisms of interference:
- low frequency - harmonics,
- high frequency - electromagnetic
Each waveform (current or voltage) can be expressed as the sum of the fundamental frequency (50 or 60 Hz) and its higher multiples. In balanced three-phase systems only odd harmonics occur. The even harmonics, due to symmetry, are suppressed. Harmonics are generated by non-linear loads, i.e. those that draw non-sinusoidal current. Typical sources of harmonic currents include bridge rectifiers, power supplies in office devices and fluorescent lamps. The three-phase rectifier generates higher harmonics of the 6xn ą 1 rows, i.e. 5,7,11,13,17,19, etc. The increase in frequency causes the reduction of the harmonic currents. Higher harmonic currents do not increase power, but they constitute an additional current flowing in the cables. A typical result of this phenomenon is overloading of wires and reduction of the power factor, which can lead to malfunctioning of measurement systems. The voltages caused by the flow of these currents through transformer reactances can also cause damage to other devices or interfere with IT supply lines.
