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The continued use of high-voltage thyristor devices in industry and their increased use in high-voltage dc transmission systems call for more attention to the properties of these devices. One of the important thyristor parameters is their turn-off time tq, which can be a limiting factor when applying thyristors at elevated switching frequencies. Hence, the accurate measurement of tq and its variation versus the operating conditions remains a crucial task for thyristor converters operating at elevated switching frequencies. In this paper, a proper test circuit for measuring this parameter with a high level of accuracy has been designed and built. Owing to the test circuit specificity, the variation effects of several electrical and physical constraints, such as the forward current IF, the reverse applied voltage VR, the operating temperature To, and the ramp rate of the forward reapplied voltage dVD/dt, on the tq parameter of thyristors are also studied and analyzed based on the physics of semiconductor devices and associated simulations. [ABSTRACT FROM PUBLISHER]