Telephone Interference Criteria for HVDC Transmission Lines

The telephone interference caused by a transmission line is primarily characterized by the equivalent line disturbance current defined as: le=??n (Cn ?? In)2 where Cn = C-message weighting factor for the n:th harmonic ln = residual line current of n:th harmonic For many HVdc lines, particularly in North America, this current has been required to be < 270 mA. During the last few years, however, values as low as 100 mA have even been specified by some utilities. Since the cost for installation of dc filters in the converter stations becomes high and very progressive if the equivalent line disturbance current has to be limited to less than about 1 A, it is very important that the specified value reflects the actual need from a telephone interference standpoint. The object of the report is to identify the maximum tolerable equivalent disturbance current for transmission lines. This is done by means of a combination of calculations and measurements in which different key parameters are scanned and evaluated. Based on a general calculation of coupling impedance versus soil resistivity and frequency, specific data are defined for a typical "worst case" coupling between a HV line and a telephone system in a suburban area. Disturbance current criteria for a dc line are given in the report based on a maximum noise metallic voltage at a telephone set of 17 dBrnC (OdBrnC = 24.5 ??V across 600 ohm) and a circuit balance of 60 dBrnC.