Joint IEC/IEEE Standard to improve substation operation

Precise time synchronization for grid automation needs a broadly accepted Standard

By Morand Fachot

Communication between equipment and systems for the electric grid is an essential element of power utility automation, and central to the introduction of Smart Grids. The IEC and the Institute of Electrical and Electronics Engineers (IEEE) have joined forces to develop IEC/IEEE 61850-9-3:2016, a dual logo International Standard specifying a precision time protocol (PTP) profile for IEC and IEEE Standards applicable to power utility automation. 

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Switchgear and controlgear installation in substation (Photo: ABB)

No reliable electric power system without IEC International Standards

IEC TC 57: Power system management and associated information exchange, and its Working Groups (WGs), “prepare International Standards for power systems control equipment and systems including Energy Management Systems (EMS), Supervisory Control And Data Acquisition (SCADA), Intelligent Electronic Devices (IEDs), distribution automation, teleprotection, and associated information exchange for real-time and non-real-time information, used in the planning, operation and maintenance of power systems”.

For optimum operation of the electric power process, the equipment and systems must be interoperable, meaning that interfaces, protocols and data models must be common. International Standards in the IEC 61850 series, Communication networks and systems for power utility automation, prepared by IEC TC 57, play an important part in ensuring this is the case.

IEC TC 57 is a customer of Standards developed by IEC TC 8: Systems aspects for electrical energy supply, and by IEC TC 65: Industrial-process measurement, control and automation. It also maintains an internal liaison with IEC SC 65C: Industrial networks.

IEEE developed a standard for a precision clock synchronization protocol for networked measurement and control systems, IEEE Std 1588-2008, which was adopted by IEC as the dual logo IEC 61588:2009. On this basis IEC and IEEE developed IEC/IEEE 61850-9-3:2016, a dual logo International Standard specifying a PTP profile for power utility automation. This profile is a subset of IEC 61588 extended by performance specifications that allow compliance with the highest synchronization classes of IEC 61850-5:2013, Communication requirements for functions and device models, a dual logo International Standard specifying a PTP profile for power utility automation The need to specify a PTP profile for these two Standards, which would allow compliance with the highest synchronization classes of IEC 61869-9:2016, Instrument transformers – Part 9: Digital interface for instrument transformers.

IEC/IEEE 61850-9-3 was prepared under the IEC/IEEE Dual Logo Agreement by IEC TC 57, in cooperation with IEC SC 65C/WG 15: High Availability Networks, and with IEEE Power Systems Relaying Committee WG H24/Substation Committee Working Group C7 of the Power & Energy Society of the IEEE.

In addition to normative references, terms, definitions, abbreviations, acronyms, and conventions, this Standard gives details of supported clock types, protocol specifications, seamless redundancy, default settings, Protocol Implementation Conformance Statement (PICS) and management objects.

Technical details, requirements and other elements underlying the development of IEC/IEEE 61850-9-3 are outlined below by Christoph Brunner, Roman Graf and Hubert Kirrmann, three key experts from IEC TC 57/WG 10: Power system IED communication and associated data models.

Grid automation requires extreme precision

Electrical protection, monitoring and control are highly demanding applications that call for precise time synchronization.

While typical industrial control systems rely on millisecond synchronization, e.g. through the Network Time Protocol (NTP) over the internet protocol, grid automation relies on precision that is a thousand times better.

Ground faults are detected by measuring current values on all conductors to an object such as a bus bar (differential protection); the compared values must be sampled within 3 microseconds relative to each other. Instabilities that precede a blackout in large electrical grids are detected by a Phasor Measurement Unit (PMU), a device which measures the electrical waves on the electricity grid. PMUs request an absolute time synchronism better than 4 microseconds between PMUs lying hundreds of kilometres apart. Protection relays must be operated in synchronism with the zero-crossings of the current below 1 ms of precision.

The state of the art used to be to deploy dedicated, individual wirings to the protection and control devices (IEDs) carrying Inter-Range Instrumentation Group time code B (IRIG-B) signals. The large grids’ PMUs receive synchronization through satellites, e.g. GPS or Galileo, but many utilities do not trust satellites because this signal could be interrupted in case of war or be spoofed by a cyber-attack. Instead they prefer time distribution over the data network, relying on atomic clocks.

Ethernet transmission for time synchronization with sub-microsecond precision

Since the introduction of the IEC 61850 series of International Standards, IEDs have been interconnected by Ethernet (ISO/IEC/IEEE 8802-3:2014, Standard for Ethernet).  IEC 61850-8-1:2011, Communication networks and systems for power utility automation – Part 8-1: Specific communication service mapping (SCSM) – Mappings to MMS (ISO 9506-1 and ISO 9506-2), recommends synchronization by the Simple Network Time Protocol (SNTP), providing a precision of about 1 ms, which is sufficient for time-stamping events but too imprecise for differential protection or PMUs in wide area networks.

Since 2008, the IEEE 1588 Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems specified time synchronization over Ethernet with sub-microsecond precision. IEEE 1588 allows a large number of variants and therefore IEEE standardized a profile for grid automation as IEEE C37.238 in 2011.

For its part, IEC TC 57/WG 10 opted for a profile already developed for industrial automation by IEC SC 65C/WG 15: High availability networks. This profile offers the same basic services as IEEE C37.238, but provides more flexibility of the network engineering and seamless fault-tolerance. It was published in IEC 62439-3:2016, Industrial communication networks – High availability automation networks – Part 3: Parallel Redundancy Protocol (PRP) and High-availability Seamless Redundancy (HSR).

IEC/IEEE 61850-9-3 to allow much improved substation operation

To avoid the emergence of two competing standards and a de facto split between the US market and the rest of the world, IEC and IEEE decided on an IEC/IEEE Joint Development in which the requirements of both communities are considered, while allowing for extensions to further profiles. The result was the IEC/IEEE 61850-9-3 Standard, which has now been approved both by IEC and IEEE.

IEEE plans to base additional developments on IEC/IEEE 61850-9-3. Within IEC, amendments to the power utility substation automation standards, IEC 61850-8-1:2011 and IEC 61850-9-2:2011, the substation network engineering guidelines IEC TR 61850-90-4:2013 and the wide area network engineering guidelines, IEC TR 61850-90-12:2015, will reference IEC/IEEE 61850-9-3.

The IEC/IEEE 61850-9-3 standard will support the deployment of the digital substation, especially of the process bus that ties the primary measuring equipment to the substation automation system, with considerable improvement to the engineering, commissioning and operation of substations. 

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ABB switchgear Switchgear and controlgear installation in substation (Photo: ABB)