Abstract: A thermal protection system is provided for a solid state valve that includes at least one representative thyristor having a junction and a heat sink system. The protection system comprises a thermal analog model whose electrical network dynamic electrical operating characteristics are electrically analogous to the thermal characteristics of the solid state valve for any given value of instantaneous load current supplied through the valve, which thermal characteristics include the heat transfer to cooling fluid capability characteristics of said representative thyristor including its heat sink system. A first electrical signal is derived which is representative of the value of the D.C. load current through said valve. A D.C.