A method implemented by a diagnostic system for performing diagnostics on a target transformer that includes a plurality of phases, the method comprising: controlling, by a processor (120,505) of the diagnostic system, switch circuitry to couple a voltage generator to each of the plurality of phases (205, 215) of the target transformer; for each phase, the processor (120,505) performs acts including: controlling the voltage generator to output an AC voltage at a first frequency; measuring a magnitude and a phase angle of a first excitation current flowing into the target transformer associated with the first frequency, the first excitation current having inductive and capacitive components; the method is characterised by controlling the voltage generator to output a voltage at a second frequency; measuring a magnitude and a phase angle of a second excitation current flowing into the target transformer associated with the second frequency, the second excitation current having inductive and capacitive components; determining a magnitude of an inductive component and a magnitude of a capacitive component of the target transformer at a normal operating frequency of the target transformer as function of excitation currents at the first and the second frequencies to thereby determine a pattern of magnitudes of the inductive and capacitive components of the excitation current for the target transformer; comparing, by the processor, the determined pattern for the target transformer to patterns associated with one or more benchmark transformers having known electrical characteristics; and when the pattern associated with the target transformer matches a pattern of one of the benchmark transformers, determining the electrical characteristics of the target transformer to match electrical characteristics of the one benchmark transformer, wherein comparison of the pattern for the target transformer to the patterns of the one or more benchmark transformers having known electrical characteristics facilitates determining whether the target transformer is defective. controlling, by a processor (120,505) of the diagnostic system, switch circuitry to couple a voltage generator to each of the plurality of phases (205, 215) of the target transformer; for each phase, the processor (120,505) performs acts including: controlling the voltage generator to output an AC voltage at a first frequency; measuring a magnitude and a phase angle of a first excitation current flowing into the target transformer associated with the first frequency, the first excitation current having inductive and capacitive components; the method is characterised by controlling the voltage generator to output a voltage at a second frequency; measuring a magnitude and a phase angle of a second excitation current flowing into the target transformer associated with the second frequency, the second excitation current having inductive and capacitive components; determining a magnitude of an inductive component and a magnitude of a capacitive component of the target transformer at a normal operating frequency of the target transformer as function of excitation currents at the first and the second frequencies to thereby determine a pattern of magnitudes of the inductive and capacitive components of the excitation current for the target transformer; controlling the voltage generator to output an AC voltage at a first frequency; measuring a magnitude and a phase angle of a first excitation current flowing into the target transformer associated with the first frequency, the first excitation current having inductive and capacitive components; the method is characterised by controlling the voltage generator to output a voltage at a second frequency; measuring a magnitude and a phase angle of a second excitation current flowing into the target transformer associated with the second frequency, the second excitation current having inductive and capacitive components; determining a magnitude of an inductive component and a magnitude of a capacitive component of the target transformer at a normal operating frequency of the target transformer as function of excitation currents at the first and the second frequencies to thereby determine a pattern of magnitudes of the inductive and capacitive components of the excitation current for the target transformer; comparing, by the processor, the determined pattern for the target transformer to patterns associated with one or more benchmark transformers having known electrical characteristics; and when the pattern associated with the target transformer matches a pattern of one of the benchmark transformers, determining the electrical characteristics of the target transformer to match electrical characteristics of the one benchmark transformer, wherein comparison of the pattern for the target transformer to the patterns of the one or more benchmark transformers having known electrical characteristics facilitates determining whether the target transformer is defective.