Asymmetry issues in power system restoration

This paper is one of series presented on behalf of the Transmission Operati on Subcommittee with the intent of focusing attention on the extra high vol tage (EHV) asymmetry issues during restoration. Many existing EHV lines hav e unsymmetrical (horizontal) conductor spacing without being transposed. Th ese 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 c ranking of thermal units. The paper describes a simple and approximate method for assessing the exten t of NSC in EHV lines and then determining corrective measures. It recommen ds that when attempting to provide an off-site source to a thermal power pl ant the extent of NSC be determined and if unacceptable, either the operati on be deferred to after the initial restoration phase, or the appropriate " underlying load" be determined for connection to the receiving end of the E HV lines.