Instantaneous Power mismatch measurement in via pu and in actual measurement using 3 phase instantaneous power measurement block

I am getting exact instantaneous active and reactive power with marginal deviation i measured using pu voltage and pu current; (Pout=0.9994pu or 10000W; Qout=0.2433pu or 2433vars) . but when i calculated it by actual voltage and actual current, it is giving instantaneous active and reactive power exactly 1/3 the expected power (Pout=3331watts, Qout=810vars). I am using 3phase instantaneous power measurement block in MATLAB SIMULINK. if i take base value S=VLL*ILL, it is giving active and reactive power close to expected value but significant variation (i.e. Pout=9700W and Qout=2317 var; Expected output, P=10000W and Q=2500 var) .

Answer 1

[Ayush]

Hi Jaynendra,

I understand that you’re seeing correct active and reactive power results when using per-unit (pu) voltage and current, but when using actual measured voltages and currents, the calculated power is about one-third of the expected value.

This happens because, in a three-phase system, the total power is three times the power calculated for a single phase (when using phase-to-neutral voltage and phase current). If your calculation or measurement block is using only phase values without multiplying by three, you'll get only one-third of the total three-phase power.

The per-unit (pu) system automatically accounts for this scaling, so your pu calculations match the expected total power. However, with actual values, you need to ensure you are either summing the power from all three phases or multiplying the single-phase result by three to obtain the total power.

Additionally, small variations between your calculated and expected power values can be caused by system unbalance, where the three phase voltages or currents are not equal or not exactly 120 degrees apart, which is common in practical systems.

Hope this helps!