Non-conventional reliable AC transmission systems for power delivery at long and very long distance

All over the world there are large and very economical hydroelectric resources, which are located at long (500-1500 km) and very long (2000 - 4000km) distance from load areas. To secure required level of reliability, transmission systems for power delivery from hydroelectric stations must meet the criterion (n-1), i.e. transmitted power doesn't have to decrease in case of emergency opening of any circuit sections. Application of double or triple-circuit interlinked transmission systems is traditional decision for realizability of this criterion. But frequently reliability condition does not allow to implement the most economical variant, for example to make use of 1150 kV single-circuit transmission system instead of 750 kV triple-circuit one. It is considered in the capacity of feasible variants for solution of reliability problem as follows: triple-circuit interlinked transmission systems; single-circuit transmission systems with spare phase; single-circuit four-phase transmission systems; single-circuit transmission systems with realization of two-phase long-duration conditions. The following aspects are discussed and analyzed: general review of variants for long and very long distance transmission; their scheme and conditions; comparison of their reliability and techno-economical characteristics.