FITNESS - Paving the way forward for GB's digital substation journey

Authors: P. Mohapatra, C. Patterson and J. Mackenzie, SP Energy Networks, UK, C. Popescu, ABB, UK, and M. Wehinger, OMICRON electronics, Austria

FITNESS demonstrates digital substation technology with a combination of non-conventional instrument transformers (NCITs), associated merging units (MUs) and standalone merging units (SAMU) publishing sampled value streams to the process bus for protection and control IEDs based on the IEC 61850 edition 2 standard. The project aims to demonstrate that the designed architecture can support all the application needs of a protection, automation and control (PAC) system. The main objective of this project is to test and prove multi-vendor interoperability at the station, bay and process bus level.

Business Case:

FITNESS addresses several areas where operational costs can be reduced through reduction in the number and length of planned outages. Planned outages for maintenance and refurbishment constrain the network and power flow. Currently majority of the testing is done on site, unplanned delays in site commissioning and tests further add to the costs of the TO and DNO. Requiring shorter outage windows significantly reduces the risk of over-run, as the scheme can be fully tested prior to site work starting, and leads to increased flexibility.

 Substation costs that are related to substation functionality and operation include primary and secondary equipment, cabling, and engineering and commissioning. Engineering and commissioning costs are reduced by maximizing the amount of testing at the factory and minimizing the amount of testing required on-site. Substation costs related to civil works, project management and design in new build or replacements require large investment, can be reduced by smaller physical size, fewer supporting structures, and smaller cable ducting and reducing engineering effort. A significant contribution to the minimization of these costs is through standardization of design, equipment and procedures.

Substation environmental and carbon impact of use of copper, oil and gas in substations is reduced through implementation of digital substations. Oil and gas usage is reduced by 80% through replacement of conventional instrument transformers with NCITs, and copper usage is greatly reduced by replacement with optical fiber. Thermal losses associated with conventional CTs and secondary wiring will also be reduced through replacement with NCITs. Risk to life of working with CT secondary circuits; while the hazard is mitigated by design and working practices, it would be eliminated through replacement of conventional CTs with NCITs thus enhancing substation safety.

FITNESS Architecture

In FITNESS two bays are considered. Bay 1 is based on PRP architecture, whilst Bay 2 is based on HSR. The individual bay architectures consist of three levels namely station level, bay level and process level. There is also a dedicated check synchronization bus ring carrying the voltage required for the check synchronization. This information is shared between the two bays.

The FITNESS architecture uses Ethernet based network as the backbone. It implements the IEC61850 Edition 2 unifying communication standards to facilitate information sharing and interoperability. IEC 61850-8-1 is used for exchanging digital information over station bus and process bus. 

Analogue information is transmitted from field devices at process level to the respective IED's using IEC 61850-9-2LE sampled values. The protection IEDs use 80 samples per cycle. Time critical information is exchanged via GOOSE and there is a need to ensure that this information is transferred reliably within a specified time frame. Communication redundancy is addressed by the standards IEC62439-3 which define two protocols HSR and PRP. These two protocols provide the required network redundancy with zero recovery time.

Time synchronization is of paramount importance for maintaining reliability and availability of the process bus and sampled values, and consequently, the availability of critical protection and control functions in a digital substation.

Depending on the regulations, utilities may not accept loss of protection functionalities for more than 1-3 secs, and such loss cannot be a frequent occurrence. In the FITNESS project time synchronization is achieved predominantly by application of the IEC 61850-9-3 standard. IEC 61850-9-3 (2016) part of IEC 61850 standard specifies a precision time protocol (PTP) power utility profile which allows compliance with the highest synchronization classes of IEC 61850-5 and IEC 61869-9.

FITNESS Offsite Test Set-Up and Methodology

In FITNESS, the focus has been on testing the robustness of the architecture, stability and performance of the network and, most importantly, reliability and availability of the key protection and control functionalities for the designed system. Extensive offsite testing using state of the art software and testing tools has been performed on the FITNESS setup. This ensures minimal to no unidentified issues during commissioning on site and achieves maximum efficiency through deployment of digital substations.

The project undertook a level of offsite testing that is expected to be carried by the system integrators in order to discover potential product or interoperability related issues in advance thus avoiding fixing them later on at higher costs during commissioning, operation or maintenance phase. The key business drivers behind extended offline testing for project FITNESS are as follows:

FITNESS being a pilot demonstration for SPEN with a multitude of new technologies and concepts being deployed live on site for the first time. It is important that SPEN and its partners on the project identify issues with the configurations and architecture ahead of substation commissioning

IEC 61850 and other new standards trialed through this project are constantly evolving. In a business, as usual project generally there will be less opportunity of upgrading firmware as per evolving standards and repeating tests both onsite and offsite as time will be of essence. FITNESS aims to deliver on site as per business processes, however being an innovation project we allowed the vendors to demonstrate the best of the technologies according to the latest standard to help identify any new gaps and ensure the fixes applied in the past to the standard and the supply chain products do deliver better functionality

One main objective of FITNESS is to prove that extended offsite testing (Figure 1) saves time during commissioning as only a subset of the tests need to be performed during site acceptance testing. It will result in large benefits through reduction outage duration and less costs of engineers spending time on site

FITNESS demonstrates a unique collaboration between two major vendors, SMEs, academia and utilities. In the period of offsite testing it has already shown that this level of collaboration is required to really prove IEC 61850 interoperability

Offsite testing provides utility engineers an insight into the type of tools available with vendors for configuration and testing of an IEC61850 engineered solution and it provides necessary knowledge to maintain and manage the system in operation. As the concepts and techniques are new, utilities require this level of engagement and training to increase confidence in 61850 based solutions.

The offline test procedure for FITNESS was categorized into three major categories:

  • Overall system and network testing including positive and negative tests for:
    • Reliability and availability tests for both redundant architectures (HSR and PRP) Figure 2, Figure 3
    • SV, GOOSE and MMS interoperability between multi-vendor IEDs and SCADA systems. Figure 4, Figure 5
    • Edition 2 test and simulation mode
    • Time synchronization
    • Network performance testing Figure 5
    • System configuration file (.scd) verification Figure 6
  • Protection and control functionality testing:
    • Main protection (Line differential and Distance) Figure 7
    • Backup protection (Overcurrent, Earth Fault and Circuit Breaker (CB) Fail) Figure 5
    • Common control functions (synchronizing and delayed auto reclose (DAR))
  • Substation supervision and data acquisition (SCADA) system test:
    • Supervision and Control
    • Alarm/Indications
    • Measurements

Observations: Following are selected observations from FITNESS offsite testing. Figure 2 and Figure 3 show the results for HSR and PRP performance. Figure 2 is under steady condition and Figure 3 shows GOOSE availability on PRP at different locations with changes in network availability.

Figure 4 shows multi-vendor interoperability and availability of SV streams and GOOSE in the HSR bay during current differential operation.

Figures 1-4

The inter-bay communication from PRP to HSR bay and the associated GOOSE propagation delay was measured and is shown in Figure 5. It is to be noted that the GOOSE propagation delay is in the range of micro-seconds. This proves multi-topology interconnection is robust and interoperable between different redundancy architectures and vendor devices in digital substations.

The overall configuration of the substation is implemented using a .scd (substation configuration description) file.

It is important to validate this file and fix any discrepancies (between the system configuration and the actual data) in the network found before functionality and system testing as shown in Figure 6.

Figures 5-6

Figure 7 proves that the standard protection functions have the same reliability and performance in a digital substation based on SV measurement streams and GOOSE tripping sequence as compared to conventional substations. The automated test sequence created during offsite testing phase can be re-used in all phases of the project (offsite, site and operation and maintenance testing).

In FITNESS the time synchronization architecture comprises of two GPS based MCs, acting as grandmasters, directly connected to the two station LAN switches on PRP, configured to act as TCs. From station level, the PTP signals are distributed to the bay level switches and IEDs.

These devices participate in selecting the best master clock and could act as master clocks if no better clocks are detected.

Once all devices have been configured and connected, a series of test cases were run to demonstrate time synchronization under the following conditions:

  • Clocks change over whilst in GPS mode (Clock class 6)
  • Clocks change over whilst in holdover mode (Clock class 7 - system running on local oscillators)
  • Clocks availability during LANs failure combined with loss of one or both GM clocks

The observed behavior of the system was as follows:

  • The best-known clock on the system became the Best Grand Master Clock (BGMC)
  • The other MC clock went into passive mode
  • All devices (IEDs, switches and Merging Unit) had the same time as the BGMC
  • All (IEDs, switches and Merging Units) shown a synch locked indication
  • All IEDs, switches and Merging Units) shown the Grandmaster identity (the same BGMC)
  • When the current grandmaster clock went down, the passive clock was elected and became the new grandmaster

The behavior of the overall system was in line with the IEC61850-9-3 standard - the switching between the two Grand Master Clocks was seamless under both test scenarios (GPS and non-GPS conditions).

The Merging Units - the most demanding device when it comes to PTP synchronization - exhibited a much better holdover time than the one specified in the standard (minimum 5sec). A software tool was used to sniff the network. This allowed us to detect abnormalities in the network traffic and automatically log all events with the corresponding detailed information.

In FITNESS NCITs from two vendors will be deployed proving multi-vendor interoperability at process level for protection and control applications as per the system architecture. The same accuracy class level as for a conventional instrument transformer is applied to NCITs to meet utility specifications for protection and metering purposes. The test setup used to test the NCITs included the following equipment Figure 8, Figure 9.

  • Current/voltage source simulators: to simulate the current/voltage
  • NCIT: current/voltage sensors
  • MU: received the proprietary signals from the sensors and publishes IEC 61850-9-2LE streams
  • Accuracy Measurement Unit: calculated the accuracy of the sensor under test for protection and metering applications
  • Visualization Tool (PC): provided control and visual measurement results
  • Time Synch signal: Synchronized all the devices in the setup to avoid the inaccuracy caused by out of synchronization

Accuracy tests were performed at ambient temperature for the accuracy limits class 0.2S according to IEC 60044-8 and 11, Sub. cl. 9.4. Measurements were carried out at 1%, 5%, 20%, 100% and 120% of rated current (2.5 kA) with the extended current factor (1.2).

The errors are plotted in two separate graphs, as shown in Figure 9, Figure 10.  As it can be seen from the graphs, the sensor head along with its merging unit is compliant with the IEC accuracy class 0.2S for metering and protection applications, according to IEC 60044-8 at a rated current of 2.5 kA and rated frequency of 50 Hz.

Site Commissioning

The FITNESS solution was commissioned on site between May-July 2018. The commissioning was done in parallel to the conventional solution and wider reinforcement works. The FITNESS commissioning included the following steps:

  • Delivery of containerized portable relay room (PRR), with all IEDs mounted in panels and pre-tested offsite
  • Installation of NCITs. The optical CTs were installed on the switchgear using a bracket solution avoiding use of a foundation and the non-conventional VTs were mounted on the same foundation with conventional equipment (Figure12, Figure 13)
  • Installation of the pre-mounted bay marshalling kiosk (BMK) specially designed with a higher environmental rating to house the MUs, SAMUs and SCUs  (Figure 14)
  • Installation of remote end conventional protection panels to prove interoperability between the digital and conventional line end protection solution.
  • Installation of fibers and power cables to implement the complete digital solution
  • Primary Injection and plant position tests
  • Network, protection and control and line end to end commissioning tests

One interesting observation during FITNESS commissioning was identification of noise caused on unshielded Ethernet (copper) cables in HV compound as compared to optical connections from test equipment which typically have RJ45 ports. The conventional protection system typically need 4-5 weeks of outage to fully commission and test all the protection and control functions. In comparison, the FITNESS digital solution required 2 days of testing of the BMK interface with the primary, 2 days of network testing and 3 days of protection and control testing. The tests were repeated on site to ensure that functionality of this pilot solution. In future extensive offsite testing and delivery of pre-tested digital substation solution could potentially minimize site commissioning tests to tests related to primary interface NCITs, MUs and plant connections only.

Testing and Simulation Modes - an "Outageless" Approach to Maintenance

IEC 61850 Edition 2 defines the mechanism for isolating and testing IEDs that use both GOOSE and Sample Values services in an IEC 61850 system. Basically, it allows one or more IEDs to be tested, without affecting other IEDs on the same network. When a device is in test (or test-blocked) mode it will accept GOOSE messages from other IEDs that are also in test mode.

IEC 61850 simulation mode defines whether the IED accepts signals such as GOOSE and Sampled Value generated by simulation equipment test sets. When an IED is in simulation mode, it will accept any available simulated messages. The other IEDs that are not in simulation mode, cannot accept simulated messages.

These new testing features have been demonstrated during FITNESS bench testing. As such, tests were successfully conducted for both protection and control areas and the observed behavior was as expected - in the figure below, it can be clearly seen the status of protection whilst under various test regime, i.e. a distance protection trip instigated via simulated SV, whilst the plant position (CB) does not change. It's obvious now that these new features lend themselves a simpler approach to onsite maintenance, allowing testing whilst in service, without operating the plant, or requiring outages (Figure 15).

Way Forward

FITNESS proves that interoperability issues related to SVs, GOOSE and MMS between the chosen vendors have been minimal to none. The only interoperability issues identified are related to integration of different IEDs to the two SCADA systems. The protection performance tests must be conducted in conjunction with network and time synchronization tests to prove reliability, availability and robustness of the entire system.

These tests require specialized test equipment and software tools enabling automated test sequences which are desirable in the context of a digital bay. Once all the information needed for testing is made available in an IEC61850 format, it is a simple procedure to start building up test routines for all conceivable testing scenarios.

There are areas of SPEN's business where the use of IEC 61850 is very much business-as-usual - such as the use of station bus on our distribution network sites, with a single vendor. At a transmission level, we have a more limited number of sites where a multi-vendor solution is under way. Now with the FITNESS project, SPEN has an opportunity to add even more to our knowledge of best practices, and work towards a solution that delivers all of the benefits that a fully digital substation promises. As we plan our future works, we can start to see projects where the ability to test offsite reduce civils and wiring costs and so on begin to make a financial case for moving forward with non-conventional installations.

There's a lot of work to be done to translate the initial projects into internal policies and procedures - to think through what has worked and what hasn't, and apply a standard approach to designing and testing future systems. We need software tools that are up to the job, training for the new skills involved, as well as consistent and usable documentation of the installations. FITNESS project has paved the way forward in these areas.

By working to pull together the knowledge and expertise of departments across the company, and bringing them all on board with the technology, we can make the transition from a digital substation being an innovation project to being a standard option when we plan new projects. 

Biographies

Priyanka Mohapatra is working as Innovation Lead at SP Energy Networks, as a part of her role she is actively working on rolling-out digital substation and other innovation technologies to business-as-usual. She is the technical lead for project FITNESS. Prior to joining SPEN, Priyanka was working with Siemens AG for 8 years. She started at Siemens Ltd., India as an electrical engineer before moving to Siemens AG Global R&D, Germany working as a software developer and project manager for SCADA, EMS, DMS systems. She then worked with Siemens Protection Devices Ltd., UK as a product owner and senior engineer designing and developing protection devices.

Christopher Patterson is a senior engineer in the directorate of SP Energy Networks. Christopher is the project lead for project FITNESS and is also involved in delivery of other transmission innovation projects. He has a background in protection and control design and is currently involved in engineering of FITNESS P&C and SAS system.

Jennifer Mackenzie works as the Protection & Control Engineer for Non-Load Substation projects in the SP Energy Networks RIIO T2 team. She spent 5 years in the company as the P&C design engineer for a variety of major transmission network projects, including sites utilizing IEC 61850 solutions. Jennifer holds an MEng in Electrical and Electronic Engineering.

Constantin Popescu is a Technology Manager for ABB's Power Grid - Grid Automation Business Unit in the UK. He received his B.Sc. degree in Electrical Engineering from the University of Craiova, Faculty of Electrotechnics, Romania. Previously a principal protection system engineer with TRANSELECTRICA, Constantin moved to the UK in 2002. In his over 13 years' tenure with ABB UK, Constantin held various engineering roles. He is currently leading the process of migrating protection and control system designs from conventional to full digital within ABB Power Grid.

Matthias Wehinger received a MSc for Integrated Product Development at the University of Applied Sciences Vorarlberg. He started at OMICRON in product development of testing tools. Since 2014 he has been working as product manager with focus on power utility communication. As an engineering manager, he is involved in many customer projects.

BeijingSifang June 2016