Fig. 11. The interface of the analysis software for TLMS calibration. u u I u UI PU cosu U cos 22 2 I sin (3) The relative measurement uncertainty model of active power P to apparent power S is: uuu PIU cos SU I 22 u sin cos (4) 2 The uncertainty budget is listed in Table 2. The expanded uncertainty of uP /S is evaluated to be better than 30 μW/VA, k=2 with power factor ranging from 0.1 to 0.001. Summary To meet the requirements for calibrating transformer loss measurement systems, National Institute of Metrology China developed a reference system that was able to provide the measurement accuracy of 30 μW/VA, k=2, i.e., the accuracy of the system is 0.3% when power factor is 0.01. Future research will utilize the system on-site and overcome the challenge of field noise and interference. In addition, international comparisons with other National Measurement Institutes will be planned for global equivalence of transformer loss measurement, which can benefit transformer manufacturers in exporting the products to global markets. Table 1 - A group of testing results at 50 kV, 500 A conditions Power factor 0.1 0.05 0.01 0.005 0.001 8 Reference System Voltage (V) Current (A) Power (W) Voltage (V) Unit Under Test Current (A) PP S 50.0804k 500.238 2.5003M 50.0853k 500.272 2.50125M 50.0825k 500.130 1.24855M 50.0868k 500.166 1.24915M 50.0803k 500.032 245.329k 50.0857k 500.040 245.736k 50.0805k 499.915 122.003k 50.0868k 499.952 123.194k 50.0820k 499.824 22.458k 50.0869k 499.857 22.985k IEEE Instrumentation & Measurement Magazine Power (W) Avg. 62 50 42 34 35 BA A (μW/VA) uA 3 3 3 3 3 Avg. 0.06 0.10 0.42 0.68 3.50 PP P BA A (%) uA 0.0 0.0 0.0 0.1 0.3 August 2022