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The findings of recent research reported in the paper relate to some of the properties of reactive-power compensators based on saturated reactors when forced by external faults into momentary operating modes which have a high degree of unbalance between the separate phases. Pronounced unbalance impairs the effectiveness of harmonic cancellation measures internal to reactor systems. In fault operating periods, harmonics are much greater than those in all normal operation in the steady-state. Using a very detailed representation of the multilimb structures of the main-and auxiliary-core systems of saturated reactors in a z-plane electromagnetic transient formulation, harmonic levels are evaluated in the paper for a range of fault operating conditions. The principal factors on which harmonic levels most directly depend are examined including those of fault level, fault type, fault location and the prefault loading conditions in saturated reactors. Comparisons between recordings from livesystem tests and solutions from computer simulation verify the closeness with which the analysis and evaluation facilities used can reproduce system-fault operating conditions. From the basis which harmonic evaluations can provide, the transient loading of harmonic filters in fault operating periods is investigated together with the requirements for protection to which this leads. A study is developed of the co-ordination of filter protection with other protection functions in a complete system which includes reactive-power compensators based on saturated reactors.