NCIT - the Technology for Gas Insulated Switchgear

Authors: Holger Heine and Patrice Guenther, Siemens, Germany and Farel Becker, Siemens Industry, USA

In the last years’ process bus technology has moved away from laboratory grade equipment to real products. This was driven by the technological progress making high performance components like high speed CPUs available to integrated devices for harsh environments like merging units and protection devices. The introduction of process bus in relay protection scheme   enables reasonable introduction of the use of process bus technology in the substations.  From a desire to save copper cables by changing to fiber optic communication connections from hard wires, utilities all over the world are implementing the first applications of process bus. This is especially true in countries where 5A interfaces are used today; the savings are huge. 

Additionally, with the introduction of process bus, utilities are beginning to consider the use of merging units with a non- conventional   instrument   transformer   interface.   In   high voltage gas insulated switchgear, these new sensor technologies represent a significant improvement. Smaller dimensions and better performance are two of the key factors that will soon push this new technology into the power automation market, where reliability and field proven systems are of primary importance.

Interoperability in Process Bus Applications

The basic idea of process bus is to measure voltages and currents in a decentralized approach. The IEC 61850 9-2 sampled measured values are transmitted via fiber optical connections to a protection relay which is most often located in the substation control house. Decentralized measurements are not a new technology. This solution has been used for a long time in applications like decentralized busbar protection. In this application, measuring units and protection devices are provided by a single vendor. The former case made use of proprietary communication connections making it easier to implement a robust network system.

However, connecting other manufacturers’ devices in such a system is not possible. With the introduction of process bus, the restriction to a single supplier’s solutions is eliminated and interoperability is now possible among several suppliers. This is essential when introducing NCIT technology into high voltage GIS applications.

In NCIT systems, conventional 1A/5A interfaces are no longer needed. When non-conventional sensor technology is applied, low voltage signals are generated that are not directly proportional to the primary measured values. It is an important point to understand that the exact performance characteristics of these NCIT devices are only known to the sensor specific manufacturer. This prevents interoperability directly at the sensor interface. Even sensors using the same basic technology from different vendors have variation in performance. Nevertheless, it is essential to connect different vendors’ protection devices also to NCIT system to build up reliable primary and backup protection schemes; this is to assure the NCIT technology is both interoperable and cost effective in a high voltage GIS application. Merging units for NCIT applications are provided by the sensor manufacturer and connected directly to that manufacturer’s sensors. They measure the signals provided by the sensor and calculate the primary values for currents, voltages and frequency.

The output is a digitized representation of a sinusoidal wave form. This data is transmitted in the form of digital communication to a protection device via fiber optic cable. Interoperability between manufacturers comes with the standardization of the interface of the merging unit and protection device.  In process bus, at the physical layer, Ethernet is used to transport the data by way of IEC 61850 9-2 LE protocol (Figure 1).

A.  IEC 61850 based process bus: In 2004, when the first version of IEC 61850 was published, interoperability on station bus level was the main focus. This application has been well vetted with a large array of installations using IEC 61850 all over the world.

IEC 61850-8-1 MMS communication as well as IEC 61850-8-1 GOOSE are both used for communication on station bus level. The latter is a peer-to-peer communication required for high speed protection schemes. On the process bus  level,  IEC  61850-9-2  describes  the  exchange  of digital  representation  of  analog  values  in  a  standardized digital format referred to as Sampled Measure Values, SMV. IEC 61850-9-2 is the communication standard that defines the basis for process bus installations.

Although IEC 61850-9-2 describes specific protocol mappings, it does not include specific data models, datasets, sampling rates or transmission rates.

These restrictions are necessary in order to implement an interoperable process bus. To define a  common understanding for these parameters, an interoperable implementation specification, namely the “Implementation Guideline  for  the  Digital  Interface  to  Instrument Transformers Using IEC 61850 9-2” was published by UCA International Users Group. This profile of IEC 61850-9- 2 is commonly known as IEC 61850-9-2LE (Light Edition). Two of the major restrictions integrated by IEC 61850-9-2LE are the telegram format and the sampling rate.

IEC 61850-9-2LE compliant SMV telegrams include voltages and currents from all phases of a three-phase system and its zero components. The combination of voltages and currents in a single SMV data stream is advantageous especially for protection functions.

The sampling rate is specified as 80 samples per nominal line cycle for IEC 61850-9-2LE merging units. In 50 / 60 Hz systems, 4000 / 4800 telegrams per second are generated by each merging unit. For this reason, bandwidth consideration must be taken into account when designing a process bus network.

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