Figure 1: P153 general assembly
2.1 Overview of Hardware design
The P153 hardware design overview is explained with the help of the schematic diagram. The P153
hardware consists of three sets of internal Current Transformers (CTs). These internal CTs are
basically designed to cater to Protection & metering requirements. Dedicated CT is available for the
earth fault protection requirements. The relay also has provision for the built-in SMPS unit which
accepts power supply input of 24-50V DC or 110-230 V AC/DC from external source and outputs 12 V
and 24 V DC for internal circuitry. The current signals acquired as analogue inputs get processed
through operational amplifier, filter circuit, multiplexer, ADC (Analog to digital converter) and finally fed
to CPU.
The CPU design is a hybrid of the digital signal processor (DSP) and high speed microcontroller which
runs complex algorithm for deriving the fundamental & harmonic component from the input current
signals. The digital inputs and outputs modules are designed to interface the monitoring, control and
protection signals through optically isolated circuit as per the field requirements. The other peripherals
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