IEC 61850 Process Bus - It is Real!

Authors: Damien Tholomier and Denis Chatrefou, AREVA T&D

Non-Conventional Instrument Transformers

The successful implementation of NCIT in various applications (AIS and GIS) requires the availability of a full range of products. Laboratory type tests and field experiments have been running for more than 15 years and successfully show the technical feasibility of sensors and their implementation in high voltage networks within the ruling specifications.

All configurations require one unique secondary electronic rack, the so-called Merging-Unit (MU). This is a device that includes sensor electronics and different kinds of interface, compatible with protection and metering devices.

Technical solutions based on Optical and Hybrid sensors integrate the best advantage of the technology in AIS substations. The CTOE "Current Transformer based on Optic-Electronics sensors" and the VTCE "Voltage Transformer based on Capacitor-Electronics sensor" are the optimum solutions proposed. However, mainly due to interface modifications there have been a limited number of industrial applications in substations. Recent works on international standards by working groups under IEC resulted in the definition of digital communications that allow some interoperability experiments between NCIT and other equipment used by automation, opening the door to complete applications in HV and EHV substation. (Figure 3)

The solution consists in the use of following devices:

  • Current Transformer based on Optical sensors and primary Electronics (CTOE)
  • Voltage Transformer based on Capacitor divider and primary Electronics (VTCE)
  • Merging Unit (MU)- an electronics device containing the necessary electronics for sensors and the digital interface according to IEC 61850-9-2 the Standard
  • Intelligent Electronic Device (IED) with protection functions, compatible with digital interface according to the IEC standard for Sampled Values communications

The CTOE and VTCE are connected to the merging unit by optical fibers transporting digital signal according to a proprietary protocol. The MU elaborates the standardized digital frame according to an IEC 61850 implementation guideline published by the UCA International Users Group: IEC 61850-9-2-LE. An Ethernet switch allows all devices that subscribe to the sampled values to connect to the merging unit.

In order to understand better the advantages of non-conventional instrument transformers, let us consider the operating principles of this sensor technology and how they are implemented in real devices.

Optical Current Sensors and Primary Electronics
Current Transformers with Optical sensors and Primary Electronics (CTOE) are devices able to measure the current of High Voltage lines for revenue metering application, as well as for protection and redundancy features.

One phase unit includes:

  • Head with a primary optical sensor (number can be up to 3 for redundancy)
  • Composite insulator, comprising optical fibers
  • Base, including a junction box containing optical connectors and 2 redundant electronics boards for digitalization and transmission to the merging unit
  • Optical cable to the MU

The Faraday sensor:
The Faraday Effect or the magneto-optic effect describes the influence of a magnetic field on a transparent optical medium. The magnetic field alters the electron path in the medium, which acquires a circular birefringence (the phenomenon of double refraction of light wave fronts  in a transparent, molecularly ordered material produced by the existence or orientation-dependent differences in refractive index) and affects the polarization state of a monochromatic light beam propagating in the same direction as the magnetic field. As a result, the light acquires a rotation of polarization state. (Figure 2)

The design of an optical sensor is a very important factor in its performance. We need to keep in mind that such devices, depending on where they are installed, may be exposed to some extreme weather conditions. The choice of ring glass solution that gives good temperature response and also important benefits, such as easiness of manufacture, industrialization and possible use of multimode components such as larger optical fiber core, easier connectors, LED (Light Emitting Diodes) instead of LD (Laser Diode). (Figure 1) The optical detection is used to transform the Faraday polarization modulation in a light intensity modulation by addition of a "polarimetric system", including two polarizers oriented at 45° from each other, with Faraday medium between them. Furthermore, the light intensity is a measurable value and can be converted into electric signals by special opto-electronics components called photodiodes .

Primary Converter in the base of the CTOE
A primary electronics board allows converting the light power traveling in the sensor in electronics signals transmitted digitally to the merging unit. These primary electronics includes: LEDs that emit a quasi-monochromatic light. This light is coupled to a fiber included in the composite insulator, transmitted to a Faraday sensor, and coupled in a return fiber.

The beam light, modulated by the magnetic field, is detected by a photodiode (PD) and then converted in an electronic analogue signal.

An analogue to digital converter associated with a micro-controller of communications used to send the sampled values of the signal to the merging unit through a classical communication optical fiber.

Secondary Converter in the Merging Unit - MU:
A secondary electronics board in the merging unit performs the signal processing necessary to make available through the process bus communications the sampled values of the currents.

Optical Cable:

 

The optical cable, between the CTOE and the merging unit is not standardized in IEC 61850. In one implementation it packages standard communication 62.5/125 multimode optical fibers that, as well as the connectors may be chosen by the user.

CTOE Unit:
Head, including primary conductor, high voltage terminals, and a housing box containing the optical sensors:

  • 2 redundant protection channels
  • 1 metering channel
  • Composite insulator, including optical fibres for the optical sensors
  • Insulator junction
  • Base with optical connection, or primary electronics, and fibre transmission.

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