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Large harmonic filter installations must perform over a broad set of impedance conditions that change according to operational contingencies at the point of common coupling, (PCC). A complex harmonic load, such as a railway feeder station, may be compactly characterized by a Thevenin or Norton network for each harmonic. Spread-sheet based tools can be developed to rapidly assess the harmonic impacts for contingency driven variations in the PCC conditions. Maximum power transfer calculations show that voltage-based harmonic distortion standards force limits for high order harmonics that can correspond to extremely low harmonic power levels in practical systems. The potential to review the use of voltage based standards is raised.