Frequency ranges
Class 1
Class 2
Class 3
Hz
Test levels % U1
Test levels % U1
Test levels % U1
16 – 100
-
2.5
4
100 – 500
-
5
9
500 – 750
-
3.5
5
750 – 1 000
-
2
3
1 000 – 2 000
-
1.5
2
Class definition is the same as that described in section 3.7. The compatibility and immunity levels must obviously be higher. The reference standards for this are IEC 61000-2-2 for low voltage public power supply systems and IEC 61000-2-4 for nonpublic industrial plants. One of the parameters defined in the two standards mentioned above is Total Harmonic Distorsion (THD), equal to the sum of the r.m.s. values of harmonic components Qh up to a certain order H, generally 40, divided by the r.m.s. value of fundamental Q1.
For example, the THD value of the three classes whose waveform is illustrated in figure 25 is 12% for class 1, 17% for class 2 and 27% for class 3. By way of reference, standard EN 50160, which defines the characteristics of the voltage supplied by the public networks, establishes that the THD value in medium voltage must not be more than 8%.
