In substation, phase advance capacitors are used to improve the power factor. Series reactors are generally connected with the capacitors to reduce the waveform distortion and suppress the inrush current. The fifth harmonic current usually flows into the series reactors for a high voltage power capacitor. Japanese Industrial Standards provide three types of temperature rise test methods. Each test method has its own problems. To alleviate them, we have devised a new test method called “Middle Frequency Test” which uses a meddle frequency between the fundamental and fifth harmonics for excitation. This paper presents an implementation method and demonstrates the usefulness of the proposed test method.