6.2.3 Phase-Loss Detection Logic

The RMS voltage measurement accurately measures the voltages of the three-phase sinusoidal signals and ensures accurate monitoring of the three-phase sinusoidal signals. This will help in identifying a single-phase fault condition.

The ADCC peripheral is used for calculating the RMS voltage. The PIC18F56Q24 device has a 10-bit ADC with computation modes.

The Waveform Clicks generate emulated three-phase sinusoidal signal with the amplitude in the range between 0 to 3.3V. The phases L1, L2 and L3 of three-phase signal are connected to the ADCC channels, while the ADCC measures the instantaneous value of the A/C signal and gives a digital equivalent value.

Note: In case of A/C mains power, the three-phase sinusoidal signal of the power supply must be scaled down and level shifted to conform to the ADCC input range requirements.

The application firmware takes 35 instantaneous samples of the 50 Hz and 60 Hz A/C signal. The RMS voltage value is then calculated on these samples by using the averaging method.

Figure 6-3 (below) gives the firmware flow for the phase-loss calculation API. Calculated RMS voltage level is used to check for the phase-loss event.

Figure 6-3. Phase-Loss Flow Diagram