Time synchronization for transmission substations using GPS and IEEE 1588

Time synchronization systems that utilize the global navigation satellite systems (GNSS) are widely used in the monitoring, control, and protection of transmission networks. They ensure that phasor measurement units (PMUs) can accurately monitor voltage phase angles, increase the accuracy of fault locators, enhance the capabilities of disturbance recorders, and allow differential feeder protection to use re-routable communication networks. However, concern about the reliability of GNSS receivers used in intelligent electronic devices (IEDs) have been reported; problems include mal-operations of differential protection, erroneous satellite timing/location messages, inappropriate installations, and blocking of satellite signals due to illegal use of GNSS jammers in vehicles. Utilities now require a timing system less dependent on the use of low cost GNSS receivers integrated into IEDs, but one that uses Grandmaster clocks, slave and transparent clocks, and an Ethernet communication network. The IEEE 1588-2008 synchronization protocol uses the Ethernet to disseminate a global time reference around a substation. A future substation will probably include duplicate 1588 grandmasters, each incorporating stable oscillators with GNSS and terrestrial receivers, in conjunction with a 1588 compliant Ethernet data network with slave and transparent clocks, and redundancy boxes for interfacing with IEDs. Although IEEE 1588 protocol is promising for future substation automation systems, its performance and impact has to be fully evaluated before it can be used in real substations. This paper describes how an IEEE 1588 time synchronization testbed is designed, constructed, and tested. Testing involves measuring the time offset when the Ethernet is heavily loaded with other traffic and the holdover capability of 1588 clocks. Additional delay introduced by IEEE 1588 traffic is also measured. As there is limited testing on GPS receivers within the power industry, this paper also uses the testbed to evaluate the steady state and transient behavior of GPS receivers. The results show a 1588 time synchronization system is accurate, secure, and ideally suited for protection and control applications, compared to a timing system merely based on GPS receivers. The information described in this paper should increase a utility's confidence in applying IEEE 1588 timing in a real substation.