What will happen if I install a 180Hz zero-sequence current high impedance blocking filter at the load-side of a distribution transformer?
In most applications, this type of current blocking filter will substantially increase voltage distortion (THDv) at the loads. Several independent NETA companies reported THDv values in excess of 20%. One UPS/PDU manufacturer, which was obliged to install this type of filter at the output of its PDU, reported a 15% THDv increase after its installation. In this instance, a system that was marginal with a 4% THDv, before the installation of the filter, rose to 19% after the installation. A designer or facility manager, who specifies this type of filter, assumes that the load-generated zero-sequence harmonic currents have no alternative parallel zero-sequence path than the secondary windings of the distribution transformer. This assumption has caused serious apparatus and system failures. In addition, some sensitive electronic loads and distribution system devices will reject the highly distorted voltage as being unsuitable. For example, small single-phase UPSs may transfer their loads to
