This paper describes an assessment tool for potential problems associated with high harmonic voltages and harmonic resonance in power transmission systems. The assessment is made in the design phase for placement and sizing of shunt capacitors in the transmission system. The tool employs the IEEE Recommended Practice 519 which specifies the maximum harmonic current at a given transmission bus. If this limiting harmonic current is injected into the transmission system, harmonic voltages will occur. The assessment tool is the evaluation of the resulting harmonic voltage magnitude at the capacitor placement bus, and the evaluation of whether this harmonic voltage magnitude exceeds IEEE 519 limits. If the harmonic voltage exceeds the limit, the given size of the shunt capacitor and / or the proposed location is deemed to be inadvisable or worthy of additional scrutiny. A scan of possible reactive power support values is proposed to identify problematic cases. This is termed a capacitance scan because the scan corresponds to varying reactive power support at a bus. An application in a large interconnected transmission system is shown in the Southwest United States. Suggestions for software implementation are given.