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.

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