Testing Requirements and Precautions for Transformers Using FIW Winding as per IEC 62368-1

With the continuous development of electronic technology, insulated transformers are gradually moving towards miniaturization, lightweight, and high power density. Compared with traditional Triple Insulated Wire (TIW) and film-covered insulated wire, Fully Insulated Winding Wire (FIW) has the advantage of smaller size. Meanwhile, FIW has better winding performance and soldering performance than TIW, thus showing a trend to replace triple insulated wire and become the first choice for the new generation of insulated transformers. However, the adoption of FIW also means facing stricter international safety standards. In this regard, the third edition of IEC 62368-1 (2018) newly adds the usage conditions and testing requirements for transformers with FIW, as follows:  

1. The FIW used in transformers shall comply      with the certifications of IEC 60851-5:2008, IEC 60317-0-7, and IEC      60317-56.

2. In the transformer structure, the basic      insulation, double insulation, or reinforced insulation between windings      shall meet the dielectric strength test voltage required by Table G.4.      Meanwhile, if mechanical separation is provided between windings, the electrical      clearance and creepage distance between windings may not need to be      considered.

3. Transformers shall be evaluated through      thermal cycling tests:
        a) Three transformer samples shall undergo 10 cycles of the following      temperature cycles:
        ⅰ. 68 hours at a temperature 10K higher than the maximum winding      temperature measured under normal operation, with a deviation of ±2℃, but      not lower than 85℃;
        ⅱ. 1 hour at 25℃ ±2℃;
        ⅲ. 2 hours at 0℃ ±2℃;
        ⅳ. 1 hour at 25℃ ±2℃.
        During each temperature cycle test, twice the working voltage shall be      applied between the transformer windings.
        b) After the thermal cycling treatment for the three samples:
        Two of them shall be further evaluated through a damp heat test (48      hours), and then the dielectric strength test shall be conducted;
        The other one shall be subjected to the dielectric strength test (with the      voltage value referred to Table G.4) immediately at the end of the highest      temperature in the last cycle of the thermal cycling test.
        c) No insulation breakdown shall occur during the above tests.

4. If the peak voltage Ut of the FIW across      insulation is greater than 750V, a partial discharge test shall be      conducted:
        ⅰ. At room temperature, carry out the partial discharge test on the two      samples that have undergone the thermal cycling test and damp heat      treatment. Apply the measured repetitive peak voltage Ut to the      transformer for the partial discharge test, with t1 being 5s and t2 being      15s;
        ⅱ. The partial discharge at t2 shall be less than or equal to 10pC.

Precautions:

1. FIW is only applicable to Overvoltage      Category (OVC) I or OVC II power systems (equipment), and cannot be used      for OVC III or OVC IV.

2. FIW shall be incorporated into the U.S.      insulation system, such as UL OBJY2, and the FIW insulation system shall      comply with relevant requirements.

3. The FIW grade (Grade 3 to Grade 9) and wire      diameter shall be matched with the withstand voltage requirements and      selected according to Table G.5.

BTL currently has the testing capability of Appendix G.5.3.4 of IEC 62368-1, can provide complete testing services for manufacturers, issue test reports with relevant qualification recognition, and provide customers with more efficient one-stop services.