Experience with IEEE 1588 Time Synchronization for Substation Process Bus

Authors: Roger Moore, RuggedCom, Canada and Maciej Goraj, RuggedCom, Spain

The project was in a 110/10kV Air Insulated (AIS) substation in China.

The world's first installation of High Voltage substation with IEC 61850 Process Bus where IEEE 1588 Time Synchronization and dynamic multicast filtering have been used provides a good opportunity to understand the challenges in implementing these new technologies in the electric power industry. Part of this 110/10kV substation had been equipped with electronic CTs and PTs and Merging Units devices that convert analog signals to IEC 61850-9-2 digital format. Time synchronization had been realized with IEEE 1588 version II, implemented features regarding this standard included Transparent Clocks (TCs), peer-to-peer path delay measurement and BMC (Best Master Clock) algorithm for dynamic selection of the clock.

The project described in this article reflects the installation in 110/10kV Air Insulated (AIS) substation in China. The substation topology comprises three 110kV lines, three power transformers and thirty six 10kV lines. However the first stage of the project included installation and commissioning of a complete automation system for two 110kV lines, two transformers and twenty eight 10kV lines. The third 110 kV line is planned to be added in the future.

The substation has breaker-and-a-half arrangement in HV side and the 10kV busbar is divided in 3 sections. Figure 2 shows single line diagram of 110kV side where IEC non-conventional instrument transformers and intelligent breaker controllers were used in the switchyard as part of the IEC 61850 Process Bus implementation. Each 110kV line was protected with one distance relay and each transformer had redundant transformer differential relays. Two busbar protection relays had been used at 110kV as well as three bus coupler IEDs with integrated breaker failure protection. Four electronic CTs and PTs had been used and a total of 14 Merging Units had been installed. IEDs from 5 different vendors had been used in this substation, most of them with integrated protection and control functionality in one box.

There were 8 Ethernet switches in the Process Bus connected in non-redundant bus topology. All the Ethernet switches were industrial grade communication devices provided by one vendor. The switches implemented IEEE 1588 v2 functionality and were configured to act as IEEE 1588 Transparent Clock (TC).  The selection of bus topology was driven by simplicity and the intentional will for not using RSTP protocol which is necessary to avoid loops in ring or mesh topologies. Despite the fact that bus topology is not redundant and not resilient to single point of failure certain degree of redundancy has been provided by duplicating the clocks, MUs and some of the IEDs. Two redundant master clocks had been used and each of the master clocks were connected via redundant links to two different Ethernet switches. Similarly redundant Merging Units and redundant IEDs were connected to separate switches (See Figure 3.)

The communications in the substation had been realized with IEC 61850 standard. This comprised two separate Ethernet networks, one for Process Bus and one for Station Bus. Time synchronization signal was distributed to all devices through the network using IEEE 1588 version 2 in the process bus and SNTP protocol in the station bus. The IEDs as well as time clocks were equipped with dual Ethernet cards that permitted simultaneous connection to both networks, station bus and process bus. The devices connected to process bus were Electronic CTs and PTs; IEEE 1588 master clocks with GPS; Merging Units; HV protection and control IEDs.

Electronic instrument transformers as well as merging units were publishing Sampled Measured Values (SMV) data streams according to IEC 61850-9-2LE frame format at sampling rate of 80 samples per cycle. The SMV traffic were received by protection and control IEDs which in turns were sending GOOSE messages containing switching commands to intelligent breaker controllers in the process bus. No MMS traffic was used in the process bus. The IEDs subscribe in dynamic way to the Sampled Values multicast traffic using GMRP protocol. Configuration of GOOSE transmission and reception were in turn done statically using an SCL based configuration software that generated and SCD file. The devices connected to station bus were:

  • HV and MV protection and control IEDs                                         
  • IEEE 1588 master clocks with GPS
  • RTU with multiprotocol support that served also as a gateway for remote connection from the level 3 control center
  • Operator workstation with local substation SCADA
  • Engineering workstation

IEDs communicate with the RTU and local SCADA system using IEC 61850 client-server services via MMS protocol. There are also GOOSE messages implemented to accomplishing bay interlocking. As mentioned earlier time synchronization is realized via SNTP at station bus. The main reasons for selecting SNTP and not IEEE 1588 for station bus were:

  • SNTP accuracy was accepted by the end user
  • Ability to use non-IEEE 1588 v2 capable IEDs
  • Higher port count of non-IEEE 1588 v2 switches

The accuracy obtained with SNTP protocol is close to 1ms and typically is in the range of 2-3 ms. There was a need to install a number of IEDs at MV level that didn’t have IEEE 1588 v2 interface and the only available option was SNTP or IRIG-B. The last criteria was the cost as at station bus 100Mbit Ethernet was used and the port count of non-IEEE 1588 v2 switch is higher than the port count of IEEE 1588 v capable switch with Gigabit Ethernet interfaces.

Relion advanced protection & control.
BeijingSifang June 2016