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.

NCIT Technology in Gas Insulated Switchgear

NCIT Technology in Gas Insulated Switchgear

Utilities which are planning to construct new or upgrade existing substations want to invest into a future-proof technology, which will open the door to greater benefits in a long run. They require both standardized and interoperable products which may be formed into solutions to fulfill the vision of a fully digital substation.

Digital substations which are implemented with integrated compact NCIT-measurement devices facilitate the   optimization of   the traditional bay layout and benefit from new features such as lower cabling costs, process bus communication and management of digital data.

A.  Advantages in GIS with NCIT:  The more compact NCIT sensors which are integrated in GIS cast resin partitions account for 5-10% decreased size and weight-compared to GIS bays with conventional instrument transformers.  One of the main deciding factors for choosing GIS over AIS technology stems from expected savings in necessary space. Now, with integrated NCIT technology, the space necessary for the construction of a substation can be reduced even more.

Connections of NCIT sensors to the merging unit and further digital connections to protection devices are simple local area networks.

This is in contrast to the great quantities of cabling required for conventional CTs and VTs. Reduced wiring in NCIT technology allows for the reduction of copper, lower costs for cabling and higher reliability due to lower potential points of failure.

Our concept is to have NCIT sensors integrated in the cast resin partition and offers several benefits compared to conventional CT and VTs. This design results in a reduction of GIS dimensions, as well as environmental benefits due to the reduction in the volume of SF6-insulating gas in switchgear.

Another important point is the reduced engineering and logistics of non-conventional instrument transformers due to NCIT mounting partition; this is integral to the GIS itself. One technical aspect of a NCIT sensors worth noting is that they are suitable for a wide range of applications. For example, one Rogowski coil brings sufficient accuracy for both revenue metering and measurements of high short circuit currents. In a similar way, a capacitive voltage divider provides wide ranges of voltage measurements.

The use of NCIT results in a reduction in engineering when compared to conventional CT and VT-both by customer and the GIS manufacturer.  This multi-application approach also simplifies logistics with shorter delivery and installation times. Finally, the NCIT-sensors provide improved measurements due to high accuracy, non-saturation and elimination of Ferro resonance effects during switching operations.

NCIT technology improves GIS systems in the following ways:

  • More compact GIS design-reduced weight, lower dimensions and less cabling
  • Replacement  of  copper  material  by  optical  fiber connections
  • Higher performance in measurement of harmonics
  • Improved measurement behavior with wide dynamic range due to no saturation effect in multi-purpose current sensor
  • Simplified  engineering and logistic-only one hardware variant for current and voltage measurements
  • High performance-no  magnetic  losses,  and  no ferro-resonance effects
  • Improved  safety  due  to  minimizing  the  risk  of internal arc
  • No requirement for early definition of transformer technical data (i.e. class and burden)

B. Voltage Measurement using Field Probe:  The electrode ring for voltage indication builds a capacitance C to the primary conductor on the high voltage potential. A 50/60Hz alternating electric field causes the flow of displacement current Ic between the connection point of electrode ring and the ground potential on flange of partition. The measurement value of displacement current Ic is a measure (proportional) for the derivative of the applied AC voltage Upr. The integration of this value will be calculated in the merging unit and produce the final calculated GIS bus voltage.

C.  Current Measurement using Rogowski coils:  A Rogowski coil is an electrical device for measurement of an alternating current (AC). It consists of a helical coil of wire with the lead from one end returning through the center of the coil to the other end-so that both terminals are at the same end of the coil.  The voltage induced in the coil is proportional to the rate of change (derivative) of current in the conductor. The analog output signal of the Rogowski coil is connected to the merging unit. The value of the current will be determined by the numerical calculation in the merging unit (Figure 2).

The whole assembly is then located around the current conductor that passes through the GIS cast resin partition.

D. Combined Current and Voltage sensors in GIS (Figure 3)

The Siemens concept with NCIT for high voltage GIS is a combined eCT/eVT solution. It is based on redundant Rogowski coil sensors for measurement of current and electric field probe sensors with the capacitive divider principle for voltage measurement.

Measurement of short-circuit currents as well as rated currents also in low value range (< 400 A) is possible with the same Rogowski current sensor.

Both sensors are integrated in the cast resin partition of the GIS. The partition is equipped with one passive sensor- connection box for each phase (Figure 3).

In addition, this connection box contains overvoltage protection and the EMC measures. There is no preprocessing of signals in the connection box. Network Cables transfer analog signals from NCIT sensors to the merging unit which samples, digitizes and evaluates the measured values.

The digitalized data will then be transmitted via IEC 61850-9-2 LE protocol to the process bus module on the protective relay.

Launching Process Bus Technology

Reliability and long term stability are key elements when discussing new applications and technologies in power automation. To reduce risks, evaluation and piloting phases are implemented prior to widespread deployment of a new technology. Process bus and NCIT technologies are going to change the architecture of substations dramatically in the near future.

Therefore, a thorough evaluation phase is needed. A systematic approach is important in order to allow customers to gain the comfort level before adopting this new technology.

A.  Process bus with conventional sensors:  To limit complexity, the first step in the evaluation phase should be restricted to the secondary technology. In this first step, a protection scheme shall test the process bus using conventional sensors for a high voltage feeder circuit.

Modern protection devices have a modular expandable design   in   both   hardware   and   firmware.   This   permits protection devices to be expandable for several process bus inputs. High-performance state-of-the-art CPUs allow protection devices  to  run  even  complex  high-speed algorithms  such  as  distance  protection  twice  in  a  single device CPU.

This not only provides a cost-efficient pilot operation but also makes it easy to evaluate the results. A comparison between conventional wiring and process bus signals would easily shows any substantial differences. In order to accomplish this comparison, the protection device is fitted with a process bus input module.  

Steady state behavior and accuracy can be evaluated by this approach. Having the parallel installation in a real life substation also allows investigation under actual situations without any risk. The system is protected using conventional field proven wiring. A parallel system using process bus only runs on the same signals but does not trip the switchgear.

B.  Introducing NCIT in substations:  After evaluating process bus in combination with conventional sensors, the next step can be taken. In this evaluation phase process bus is proven technology and the evaluation can concentrate on new sensor technologies. Here NCIT technologies can be evaluated. However, the approach to evaluate the sensor behavior is similar to the evaluation of process bus (Figure 5).

Summary

The combination of process bus and NCIT technologies has the potential to substantially change the architecture of high voltage   gas insulated substations in the near future.  

IEC 61850 9-2 process bus protocol is an enabler to realize substantial reduction in copper cables and expensive trenching during the construction phase of the substation.  On the   other   hand, high   voltage   GIS   manufacturers   are developing a package offering of NCIT to be matched with their merging unit designs to further reduce the size and cost of their products.

In order for conservative utility customers to gain a comfort level with these new technologies, pilot testing will be necessary.    A two-step approach is recommended by first thoroughly comparing the use of SMVs with merging units connected to conventional instrument transformers.

Once satisfied, the next step of adding NCIT to the pilot is recommended.  This approach is easily accomplished by  adding  an  additional  process  bus input  module  to  the  protective  device  and  a  personal computer running special software to compare the conventional and NCIT systems via their independent SMV signals. 

All of this can be done with relatively little expense when compared to the long-term savings that will be realized by the reduction in GIS size, weight and complexity.  

Biographies

Holger Heine is the Product Lifecycle Manager within the Energy Automation Business Unit of Siemens Energy Management Division in Germany. Within Protection and Substation Automation group he is responsible for Communication and Process bus in Siemens SIPROTEC devices. After receiving his diploma summa cum laude in computer engineering from the Technical University in Berlin, Holger started his career with Siemens as part of the development team designing Siemens latest SIPROTEC 5 relay platform. He also developed Siemens SIPROTEC PRP and HSR solutions and participated actively in the standardization of these protocols.

Patrice Guenther has an Electrical Power Engineering graduate degree from the Leipzig Technical University. He started his career at Siemens and worked in various countries around the world with different responsibilities. The experiences in process’ controlling and troubleshooting consultations led to operation tasks as chief engineer for installation, commissioning and maintenance of switchgear substations. His current status is senior key expert and Emergency Manager for medium and high voltage switchgears (AIS and GIS).

Farel Becker was born in Pottsville, PA. Farel received his Bachler degree from Kings College in 1980 and MBA from Rochester Institute of Technology in 1991.  Since joining the Siemens Power Engineering graduate training program, he has held various Sales, Marketing and Product Management positions. Currently he is responsible for the substation automation product management at Siemens Digital Grid.

Relion advanced protection & control.
BeijingSifang June 2016