6.2.2 Phase Reversal Detection Logic

The ZCD and Timer peripherals are used for calculating the phase-shift between the three-phase signals. The AVR128DA48 device comes with two instances of the ZCD peripheral and both instances are used in this application. The phases L2 and L3 of three-phase signal are connected to the ZCD peripheral to detect zero-crossing. The ZCD peripheral is configured to give an interrupt on the rising edge detection for the signal. Timer peripheral of the microcontroller is used for measuring the period between the L2 phase zero-crossing interrupt and the L3 phase zero-crossing interrupt.

ZCDs are continuously monitoring the phase L2 and L3 of three-phase supply, the phase reversal fault is identified by comparing the time taken from the L2 phase zero-crossing interrupt to the L3 phase zero-crossing interrupt against to time computed on positive sequence three-phase signal.

The calculation provides a method to identify phase shift among the three-phase signals in terms of time for both the positive phase sequence and negative phase sequence conditions.

  • The frequency of the three-phase signal is 50 Hz and the phase shift between any two signals of the three-phase signal is 120°
  • Period of the signal = 1 / frequency of the signal, i.e. 1/50 = 20 ms
  • The time taken to complete a 360° cycle is 20 ms
  • The time period between the 120° phase shifted signals of the three-phase signal is 6.66 ms
    Note: When the frequency is 60 Hz, the time period between the signals is 5.55 ms
  • The time duration between the signals of the positive phase sequence is computed using a general methodology, i.e. L1 to L2 or L2 to L3 or L3 to L1 = 6.66 ms
  • When the time duration between the 120° phase-shifted signals of the three-phase signal is 6.66 ms, this is called a positive phase sequence
  • The phase reversal fault is detected by comparing the actual time duration between the signals of a three-phase signal with the time duration between the signals of the positive phase sequence

Figure 6-2 shows the phase reversal detection application firmware flow.

Figure 6-2. Phase Reversal Detection