Name / Hans Jorgen Jorgensen / Session / 2
Company / DEFU / Block / 1
Address / Rosenorns Allé 9 / Question n° / Update
DK-1970 Frederiksberg C, Denmark / Language used on the floor / English
Phone / +45 35 30 07 72 / Accompanying visuals on file ? / Yes
e-mail /
Risks incident to harmonics in compensated Mv networks
In my paper, I have discussed the possible risks caused by harmonic components of the earth fault current in resonant earthed MV systems. If the rms values of the harmonic voltages are close to limits given in EN 50160, high earth potential rises and consequently also high touch voltages can occur unless the resistance to earth is sufficiently low. This is illustrated in Figure 1 for a 20 kV network. The network has a total capacitive earth fault current of 160A and the arc-suppression coil is set at 170A at 50 Hz and nominal operating voltage.
Figure 1. Contributions to the earth potential rise from 50Hz and the 5th harmonic in a 20 kV network in case of a single-phase earth fault.
This risk is not clearly described in CENELEC HD 637 S1:1999 (Power installations exceeding 1 kV a.c.), which gives the guidelines for the design of earthing systems. Here, harmonic current is mentioned but not treated in detail. The only reference is given in Figure 2.2.b of HD 637, shown below. The equation in the figure is not very precise, as the harmonic current, IH, is actually a part of all the three other currents on the right hand side, i.e. IC, IL and IR, and not a separate current.
Figure 2. The only reference to harmonic current in HD 637 is given in this figure (Figure 2.2.b)
A part of the Danish Heavy Current Regulation is presently being reformulated and the structure of HD 637 will be adopted. At the end of May 2003 it will be discussed how the aspects of harmonic current could be clarified in the Danish version, and I would recommend that a similar clarification be introduced in HD 637 at the next revision of this document.