At distribution voltage levels, accuracy in the range of 10 to 20 milliseconds may be sufficient and provided by a local clock. When time synchronization requires a resolution of plus or minus one millisecond, an external time source is needed, such as a GPS or WWV receiver. Accuracy in the microsecond range requires synchrophasors.
Sam Sciacca’s article Designing a Time Synchronization Source discusses pertinent degrees of accuracy, system design and applicable protocols and standards such as IEEE Standard 1815™, Simple Network Time Protocol (SNMP), IRIG-B, IEEE 1588™ and IEEE C37.238™.
Read this and other articles by Sam on his blog, Insight on Power, published by CSE Magazine.
Sam Sciacca is an IEEE senior member and current chair of two IEEE working groups dealing with cyber-security for electric utilities: the Substations Working Group C1, which is working on P1686; and the Power System Relay Committee Working Group H13, which is working on PC37.240. Sciacca currently is CEO of SCS Consulting, LLC.