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 equation used for RMS voltage calculation is: $V_{r m s} = \sqrt{\frac{{V 1}^{2} + {V 2}^{2} + {V 3}^{2} + ............ + {V n}^{2}}{n}}$

If the computed RMS voltage on any of the phases is zero volt, it means that the corresponding phase is lost.

Figure 6-3. RMS Voltage Calculation Flow Diagram

The ADC peripheral is used for calculating the RMS voltage. The AVR128DA48 device has a 12-bit ADC and it is used in this application with Differential mode configuration.

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 ADC channels, while the ADC measures the instantaneous value of the AC signal and gives a digital equivalent value.

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

The application firmware takes 80 instantaneous samples of the 50 Hz AC signal and 64 instantaneous samples of the 60 Hz AC signal. The RMS voltage value is then calculated on these samples by taking the integral of their squares. Figure 6-3 shows the firmware flow diagram of the RMS voltage measurement.

Figure 6-4 gives the firmware flow for the phase loss calculation API. Calculated RMS voltage level is used to check for the phase loss event.

AN3607

6.2.3 Phase Loss Detection Logic