Time synchronization

Introduction: After any disturbance in power system, analysis for cause of disturbance requires sequential events from relevant protection and control devices. Therefore, all numerical IEDs (relays and controllers) needs to be time synchronized for proper sequence of events in chronological order. 

Generally IEDs are time synchronized by one of the following method:

1. Pulse Per Minute (PPM)
2. Inter-range instrumentation group timecode-B (IRIG-B)
3. Simple Network Time Protocol (SNTP)
4. Precision Time Protocol (PTP)

PPM: Time of the relay has to be set once manually, only seconds and milliseconds will be reset to 00.000 after receiving pulse from time synchronization equipment. It is simplest way of time synchronization. Large number of relays can be synchronized by connecting in parallel. Its accuracy depends on accuracy of internal clock as after synchronization it has to run on internal clock for one minute untill next pulse is received. Being obsolete system it is rarely used these days.

IRIG-B: It is standard format for transferring time information. Information can contains data for day of year, Hours, Minutes, Seconds and milliseconds.Data is transmitted by moduling on 1KHz sine wave through co-axial cable. Accuracy of microsecond level can be achieved. 

SNTP: It is time synchronization using Local Area Network (LAN) system. GPS equipment works as server and sends time information, which contains complete data Year, Month, day, Hours, Minutes, Seconds and milliseconds. All the IEDs get time information from server and time is synchronized. Its accuracy is low as delay in network components is not compensated. Accuracy of millisecond level can be achieved.

PTP: It is used where more accuracy is required than provided by SNTP. In PTP delay in network components is calculated by system and compensated by modifying time stamps. In this way, accuracy of nanosecond level can be achieved.