Improved AC immunity of ground fault protection in span power systems

The past decade has seen a resurgence in span powering with the proliferation of broadband services. Span powering offers an economical method to power a variety of DSL equipment over large distances from the central office (CO). The reach of span power has increased in recent years with the acceptance of new 200 Vdc limits. However, in order to take advantage of this higher voltage level, reliable DC ground fault protection must now be provided. The twisted copper pair wires used to deliver the span voltage are often in close proximity to power transmission lines. Power line interference can be induced onto the telephone lines resulting in an unwanted common mode AC noise. Small DC ground fault currents become difficult to measure in the presence of these induced AC currents. This paper will discuss an adaptive filtering approach, based on the LMS algorithm, to eliminate the unwanted interference. Guidelines from Telcordia GR-1089-Core, chapter 5 will be used to evaluate the effectiveness of the adaptive filter.