The behavior of single-shaft combined cycle gas turbine units at frequency drop in the connected grid

In the last 15 years many companies have developed single-shaft combined cycle gas turbine (CCGT) units having one generator, which is coupled to common single shaft of gas turbine (GT) and steam turbine (ST) via a special elaborated synchro-self-shifting (SSS) clutch. This clutch connects automati- cally the drive of the ST at one end of the generator to complement the drive of the GT at the other end of the generator. Such a technical solution, in comparison to previous ones, allows economizing on one generator circuit breaker (GCB) as well as the use of a two-winding transformer instead of a three-winding transformer. This scheme of single-shaft CCGT units is taking place in many coun- tries, but cases of tripping of such units by surge protection of GT have been observed in any power plants (PP). All such trippings were accompanied by frequency drop in the high voltage (HV) grid, to which these units were connected, although these drops were within the permissible limits for the CCGT units. This paper reports the results of investigating one of these cases of tripping, discusses the possible cause of these incidents and elaborates on some measures to prevent such trippings in the future.