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Merging Units need much better than 1 millisecond .. in fact they need better than 1 microsecond accuracy.
Consider a substation with a main bus VT with its MU, and then a series of 20 feeder located CTs and MUs
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But with separate independent MUs, the relay algorithms can easily get confused if it uses voltage sample from MU-V count #2345 within the 1-second window and current sample from MU-I1 count #2345 but they were taken say 1 millisecond different in real time
The need to use IEEE 1588 v2 Precision Time Protocol (2008)
Distance relays only need samples (as per IEC 61869-9) at 4800 samples per second (sps) [96 samples per cycle (spc) at 50 Hz or 80 samples per cycle (spc) at 60 Hz] i.e. one sample taken every 208 microseconds.
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The use of these clocks is clarified in the IEEE C37.238 PTP Profile.
Looking ahead, IEC 61869-9 has Sampled Values as fast as 96 kHz - i.e. one sample every 10 microsec so coherency will probably need to be better than say 0.1 or even 0.01 microsec