Enabling Modern, Compact and Resilient Substations through Non-Conventional Instrument Transformers and Process Bus

 By Farel Becker and Gaston Ortega

Different solutions for non-conventional instrument transformers (NCIT) have been developed and are now available for use in high voltage gas insulated switchgear. Special electronic devices, called merging units, have been developed for the pre-processing and merging of signal in accordance with the IEC 61850-9-2 standard. This ensures the compatibility as well as interoperability of any protection device with the appropriate digital interface. The article describes the benefits NCIT technologies and IEC 61850 process bus technologies for use in high voltage gas insulated switchgear.

The novel combination of non-conventional instrument transformers (NCIT) and IEC 61850 9-2 process bus technologies has the potential to substantially change the architecture of high voltage gas insulated substations in the near future. High-voltage gas insulated switchgear (HV GIS) manufacturers are developing a package offering a non-NCIT directly connected with process bus to further reduce the size and cost of GIS products.

Process bus becomes an enabler for the use of NCIT; it presents a new protection, control, and metering integration concept replacing yard copper cabling with optical fiber, impacting new distribution and transmission substations, with substantial financial, safety, and operational benefits.

The Siemens HV GIS design using NCIT is a combined eCT/eVT solution mounted in a cast resin insert within a metal partition. It is based on redundant Rogowski coil sensors for measurement of current and electric field probe sensors with the capacitive dividers for voltage measurement. Six sensors are in each partition. The partition is equipped with one passive sensor-connection box for each phase. In addition, this connection box contains overvoltage protection and the EMC measures. This “combi-sensor” design offers several advantages to the end user customer.

The advantages of HV GIS with NCIT

One of the main deciding factors for choosing GIS over Air Insulated Switchgear (AIS) technology stems from expected savings in necessary space. The more compact NCIT sensors in the HV GIS cast resin partitions account for 5-10% decreased size and weight—compared to GIS bays with conventional instrument transformers. Other operational advantages include:

• 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 design 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)
• environmental benefits due to the reduction in the volume of SF6-insulating gas

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. Now, with integrated NCIT technology, the space necessary for the construction of a substation can be reduced even more.

How does process bus work?

Conventional application devices such as protection, control, fault recorders, and meters, read current and voltage analog inputs (AI) directly from copper instrument transformer connections, and connect to copper wired binary inputs and outputs (BIO) to exchange signals with the corresponding circuit breakers.

With process bus technology, the same devices are split in two electronic components: the main application device, to be housed in a protection panel in a substation room, and the merging unit, a device located on the yard at each circuit breaker cabinet connecting to the necessary AI and BIO. In addition, the merging unit contains the analog to digital converters, an internal or external satellite synchronized clock, redundant fiber paths, and optional logic. The two components, the main application device and the merging unit, connect with each other via the Ethernet based fiber optic network, known as process bus network.

Sampled measured values (SMV) are the current and voltage signals digitalized by each merging unit and sent as multicasts, via the process bus network, so application devices can subscribe to the data as needed. Generic Object Oriented Sequence Events (GOOSE) messages are also sent via the process bus network, to replace all copper wired signals, such as circuit breaker status, tripping, and any control operations.

Both SMV and GOOSE messages are non-routable multicast protocols. To further enhance the cyber security of the application outside of the substation room and within the substation fence, access to the merging unit configuration can be restricted after commissioning.

The advantages of process bus

Process bus presents an array of quantitative and qualitative benefits in a HV GIS substation.

• Construction Cost Savings
  Unlike a convention substation where a substantial reduction in copper wiring and expensive trenching during the construction of a substation can result in savings between 50% and 80%, in a HV GIS substation process bus become an enabler for the use of NCITs. Notwithstanding the 5-10% size and weight reduction of the HV GIS itself, there is a reduction in the copper connecting both circuit breakers for interlocking between bays and NCIT, to electronic devices in the substation control room.
• Operational & Maintenance Cost Savings
  With process bus, it is possible to detect failed communication links and failed devices from remote locations with alarms, when copper connections would not have given such indications. Additionally, testing applications becomes easier and safer. The minimal copper wiring between the circuit breaker and the merging unit would typically be done by the breaker manufacturer. The utility has the option to test all applications from a remote workstation, by utilizing test bits and simulation bits to inject simulated GOOSE and SMV, saving time on travels and efficient testing routines.
• Reduced threat of copper theft
  Available cooper can reduce the threat of theft. With no underground copper in the yard, even thieves will be less likely to attempt to steal copper.
• Resilient substation design
  Process bus substations recover faster from catastrophic failures such as fires or floods. Optical fiber enables visibility of damage and faster recovery than possible with copper CTs and PTs. When damaged the time, cost and effort in replacing trenched copper is much larger than installing new fiber.
• Immunity against Electromagnetic Pulses (EMP)
  EMP, whether for natural causes or as potential targeted attack, can disrupt the power grid. EMP can result in equipment damage, or miss operations. By design, fiber in the process bus network completely isolates substation breakers from the electronics in the control room.
• Advanced functionality
  A process bus network allows integration of wider range of data sources and independent signal distribution. This means, multiple main application devices can access information on multiple merging units with data from breakers, CTs and PTs), for simpler or more efficient schemes. For example, one protective relay could be protecting and controlling multiple feeders, and a fault recorder would simply read data on multiple CTs directly from the process bus network.
• Supplier interoperability
  Since process bus is based on the IEC 61850 standard architecture it enables multi-vendor applications. The signal processing between the process bus margining unit and protective relay is completely standardized between relay manufacturers by the IEC 61850 9-2 standard. In 2015, the first Utility in North America deployed a successful substation pilot with protection devices subscribed to merging units from different suppliers.
• Enhanced flexibility and scalability
  With merging units in all breakers, it is easier than ever to add new main application devices into the network for additional purposes, such as protection, fault recording, control, or metering.

The implementation of the combination of NCIT IEC 61850 Process Bus offers substantial reduction in the size and weight of the HV GIS of approximately 5-10%. Other operational advantages are also seen. The implementation of process bus offers another set of customer benefits ranging further enhance the design and functionality of the HV GIS substation.

For a downloadable copy of  May 2016 eNewsletter which includes this article, please visit the IEEE Smart Grid Resource Center.

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