Digital Technology at station bus level is today widely spread and has also extended to process bus at the primary equipment level. An analogue merging unit takes analogue inputs from instrument transformer sensors (CTs and VTs) and outputs sampled and time-stamped digital data onto the IEC 61850 9-2LE process bus. This provides safer and more economical cross-site communication using fibre optics and allows the primary and secondary plant to be decoupled.
As part of the qualification of an analogue merging unit (AMU) a system wide test requirement was identified. This was to confirm that the IEC 61850 9-2LE streams from several AMUs could be sent to the correct destinations and there would be no impairment of the protection operation. This required a model of a representative power system with a typical mix of Protective devices. The chosen system had two transmission lines, a power transformer and a busbar with distance protection on one line, current differential protection on the second line, transformer differential covering the transformer and busbar differential protection. Additional protective devices were deployed at the remote line ends to complete the protection schemes. In total, ten logical nodes were active feeding data to six protective devices.
The system was modelled using the RTDS® Simulator system installed within AP Labs at Stafford, United Kingdom (UK). Analogue feeds for current and voltage were provided to a total of ten AMUs each providing one of the logical nodes in the system. All the IEC 61850 9-2LE signals were sent to a switch and the six protective devices were also attached to the same switch. There was no separation of the data streams by the use of different virtual networks. The streams intended for the remote line ends were included in this data concentration to represent other connections that were not modelled. The operating times of all the monitored protective devices were measured and recorded.
A series of system disturbances were applied, checking the operation of all of the protection in the system. This checking was amplified by the close interaction of the elements included; for example an internal line fault was also an external fault for all the other protective systems.
This style of system wide testing can, currently, only be carried out with the use of a real time simulator. The RTDS® system installed in AP Labs, Stafford, UK could also have provided some of the IEC 61850-9-2LE streams and interfaced through GSE streams in place of the AMUs and conventional connections used.
B.Smith, A.Perks, P.Horton, Alstom Grid UK, Presented at PACWorld Conference 2014, 23-26 June, Zagreb, Croatia