Asymmetry issues in power system restoration

This paper is one of a series presented on behalf of the Transmission Operation Subcommittee with the intent of focusing attention on EHV asymmetry issues during restoration. Many existing EHV lines have unsymmetrical (horizontal) conductor spacing without being transposed. These characteristics generate unacceptable negative sequence currents (NSC). Under light-load condition and during restoration, NSC has caused cascade tripping of a number of generators resulting in an area wide blackouts, has prevented synchronization of incoming generators, and has blocked remote cranking of thermal units. The paper describes a simple and approximate method for assessing the extent of NSC in EHV lines and then determining corrective measures. It recommends that when attempting to provide an offsite source to a thermal power plant the extent of NSC be determined and if unacceptable, either the operation be deferred to after the initial restoration phase, or the appropriate "underlying load" be determined for connection to the receiving end of the EHV lines.