En bachelorprojekt om overharmoniske strømme og spændinger, som behandler konsekvenserne for beskyttelsesudstyr og elmotorer, en analyse af, om der er eller kan være kompatibilitetsproblemer i danske industriinstallationer og en analyse af, hvem der har ansvaret for kompatibilitet.
This report concerns harmonic distortion in low voltage electrical installations.
Nonlinear loads are the reason of harmonic distortion. Nonlinear loads draw a current that do not follow the sine form of the voltage. It has shown that harmonic distortion is not active nor reactive power; it has its own component. It is in a third dimension and therefore neither active nor reactive.
When the nonlinear current passes through the line impedance, there will be a voltage drop according to Ohms law. As a result, the harmonic currents is the source of harmonic voltage.
The report is also dealing with the influence of harmonic distortion on RCCB, MCB, MCCB, fuses and asynchronous motors. Fuses and the bi-metallic strip in MCB and MCCB detect the current directly or indirectly. Whether it is a sinusoidal- or complex waveform, the over current release will function properly for these types. There is however a drawback; there is a risk of premature tripping, if the contribution of the harmonic current of the effective current value is not taken into consideration.
If the over current release is handled by an electronic MCCB it is crucial, that the harmonic distortion is taken into account. Some electronic MCCBs are measuring the peak value of the current wave and then computed the effective value, with the factor “square root of 2”. This is not a good combination with harmonic distortion because the fixed ratio between the peak value and the effective value is only applicable for sinusoidal conditions - not complex waveforms. Some types of MCCBs are computing the current with many measuring points throughout the period. These electronic MCCBs are therefore measuring the harmonic distortion contribution to the true effective value of the current.
If a “type A” RCCB is influenced by harmonic distortion, the trigger current might be rising, and the protection is thereby not satisfactory. Consequently, other types of RCCBs are in sales to cover installations where harmonic distortion is present.
Harmonic distortion is also affecting asynchronous motors. Themes like increased heating, reduced efficiency and reduced service life are some of the problems harmonic distortion is causing. What causes the most impact on asynchronous motors is the lower order of the harmonic frequencies, e.g. the 5th, 7th and 11th order.
Due to the problems of over current releases, RCCB and asynchronous motors, it is favorable to reduces the harmonic currents. Either, installing equipment with high linearity and immunity or passive/active filters can reduce the harmonic distortion.
Finally, the report also covers which recommended upper limit of harmonic distortion the manufacturers and the standardization organizations have. From these recommended limits, electrical installations in Danish industry are analyzed through measurements from Dansk Energi Analyse. This analyze has shown that there is a risk of incompatibility in many of the installations.