The MCU acquires analog values at the rate of 16 samples per cycle. Digital signal processing (DSP)
performs powerful numerical algorithms which converts this signal in to equivalent vectors. Once the
signal is converted into vectors, number of parameters are derived from it such as phase currents (Ia,
Ib, Ic), positive sequence current (I1) and negative sequence current (I2). The MCU also calculates
harmonic contents of the current signals (actual harmonics depend upon type of relay). All
measurements are tuned to fundamental frequency i.e. 50 Hz or 60 Hz, so that the relay will remain
stable during distorted waveform generated by modern electronics load.
The typical frequency response of tuned filter for 50 Hz is shown in figure 4.
Figure 4: Frequency response of tuned filter for 50 Hz
The measurement is totally immune to all higher harmonics which makes relay operation reliable. All
these measured values are then used for different protection functions such as over current/Earth
Fault and negative phase sequences. The actual protection function depends on type of relay and
described in the rest of the document.
2.4 Digital Input/Output Module
This module supports 6 numbers of Digital input DI and 6 numbers of Digital output DO channels for
acquiring filed signals or controlling field devices respectively. The DIs are isolated from other circuits
by using Opto isolators. The threshold voltage at which Opto coupler turns ON is controlled by
comparator based on the voltage range selected during device ordering.
2.5 Power Supply Module
The power supply module is a designed using modern PWM based switching mode technique. It
converts input supply to the 12 V and 24 Vdc low voltage supply for relay electronics and control
circuit. It also provides necessary isolation from input power supply. The normal operating voltage
range is 24 V – 230 VAC/DC.
2.6 Communication Module
The P253 supports 2 numbers of isolated ports for communication.
• (a) USB port provided on front of the relay
• (b) RS 485 (2 wired) port provided on the rear side of relay.
The relay can be accessed using the P50 Agile Configurator. Rear port supports open protocols like
IEC60870-5-103/MODBUS or DNP3.0 (ordering option) and is used for external communication.
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