Time in IEC 61850 based substation protection and control systems

by Alexander Apostolov. USA

Time Related Requirements
Time synchronization has been a requirement for electric power systems protection, automation and control systems (PACS) since the introduction of microprocessor based multifunctional PAC and recording devices.
The need was to be able to analyze the operation of multiple devices at different physical locations in response to an electric power system event. This was further extended based on the need to measure electric power system parameters in different physical locations.
The IEC 61850 standard uses the PICOM (Piece of Information for Communication) concept developed by CIGRE Working group 34.03 and published in Technical Brochure B5 180 “Communication requirements in terms of data flow within substations.” It is focused by definition on the exchanged data between two application functions or subfunctions within a substation. One of the components of the PICOM is the Time tag defined as an absolute time to identify the age of the data if applicable.

IEC 61850 defines different requirements for the development of the standard that include also time related requirement as follows:

  • Accuracy - depending on the application, different time accuracy is required
  • The time stamp shall be based on an existing time standard (UTC is generally accepted as the base time standard)
  • The time model shall be able to track leap seconds and provide enough information to allow the user to perform delta time calculation for events crossing the leap second boundary
  • The time stamp model shall contain sufficient information that would allow the client to compute a date and time without additional information such as the number of leap seconds from the beginning of time
  • The timestamp shall be easily derived from commercially available time sources
  • The overall time model shall include information to allow computation of local time
  • The time model shall allow for ½ hour offsets for Local Time
  • The time model shall indicate whether Daylight Savings is in effect or not
  • The format shall last at least 100 years
  • The timestamp format shall be compact and easily machine manipulated

Event Time Definition: The industry has been looking at the topic of time stamping for many years in an effort to standardize the definitions based on a common understanding of the types of time-stamps required. IEC 61850-5 defines three different kinds of events, which need a dedicated time allocation procedure:

  • If an event is defined as the result of computation (internal or calculated event) allocation of time (time tagging) shall be done immediately within the time resolution of the clock. No special measures are needed
  • If an event is defined as change of a binary input the delay of the debouncing procedure of the input contact must be considered. The event time shall be locally corrected
  • If an event is defined as change of an analogue input the delay of the filtering procedure of the input circuit has to be considered. The event time shall be locally corrected

This strong event time definition ensures that the processing of the time stamp becomes independent from the communications system latency and does not require correction by the receiving function.

Table 1 shows the time performance classes for almost all events related applications and depend on the supported functionality.
With the introduction of synchrophasor measurements the need for much more precise time synchronization arose. This is due to the fact that a 1 ms accuracy (T1 class in Table1) results in a phase angle measurement error of 18 deg in a system with a nominal frequency of 50 Hz. To reduce this error, the time-synchronization has to be in microseconds range with different performance classes being defined in IEC 61850. T3 accuracy will be OK for most applications, but it is commonly accepted that PMUs need to be synchronized to meet T5, i.e. with an accuracy of 1 µs.
With the introduction of optical current and voltage sensors or stand-alone merging units used for process interface in digital substations, a similar requirement was established for their time synchronization.

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