Using Digital Instrument Transformers to Reduce Substation Design Costs

Authors: Dylan Stewart, GE, Canada, Allen Rose and Rich Hunt, GE, USA

The value in the intelligent digital substation is the ability to design and build substations faster and more efficiently, use fewer materials for the physical infrastructure of the substation, operate and maintain the substation (and substation equipment) at lower effort and cost, and unlock the value of data to improve the performance of the substation, substation equipment, and the power system itself.

The digital substation is, right now, moving from concept to reality, by expanding beyond simply replicating traditional SCADA concepts to using process bus. Process bus is nothing more than distributed I/O for protection and control, digitizing all the inputs to the protection and SCADA systems (currents, voltages, equipment status) at their source at the primary equipment.

The digital substation is essentially the virtualization of analog data. It is obvious that digitizing all analog information at the source has great value in the design and operation of protection, control and SCADA systems.

Once data is freely available and easy to access, it can be used to unlock the value the substation through better asset performance management and system operations. This leads to interesting questions: can we use the virtualization of data to improve the physical design of the substation and substation equipment? Can we go ever farther with substations in building more efficiently, using fewer materials, and maintaining them more cost effectively?

The obvious way that process bus improves physical design is by replacing the numerous copper cables required for measurements and equipment status with far fewer fiber optic cables: less design of cabling, less wiring, less installation effort. This can also permit the replacement of cable trench with simple conduit. 

A small air-insulated distribution substation may use 3,600m of copper cables across the switchyard for measurement, status, control, and power distribution, requiring investment in space and money for cable trench.

Process bus can reduce this to 400m of fiber optic cable, and 600m of copper cable for power distribution only. Beyond the savings due to less design of cabling, less wiring, and less installation effort, there is the physical savings afforded: a 0.1m diameter conduit as opposed to a 1m wide cable trench. In transmission substations, it is possible to realize even greater savings, reducing copper cables, and the facilities to support copper cables, by close to 90%. However, simply reducing copper cabling is not enough in terms of substation and equipment design. Virtualization must allow equipment to become smaller, lighter, more flexible, and more adaptable in terms of application, and easier to maintain and support.

One way to address design is to push digitizing data as close to the source as possible. Process bus starts at the terminal blocks of primary equipment, but the goal should be to remove the terminal blocks and be natively digital if possible. The place to start, because proven solutions exist, is with conventional wire-wound current transformers (CTs).

CTs are a physical device that add cost to every part of substation design. Their size and weight (which increase with accuracy class and voltage level), directly influence equipment design, equipment standards and specifications, substation layout, and substation footprint.

CTs influence operations, due to considerations around accuracy, linearity, and precision. They force engineers to mitigate the impacts of saturation and transients in both device algorithms and device settings. CTs even touch organization structure: they are primary, physical equipment that will therefore belong to the primary equipment group.

Process bus as a concept doesn’t directly address any of the issues of CTs. The present process bus design simply retains these issues. Process bus wires a merging unit, that digitizes signals, directly to the terminal blocks of the CT. Obviously, merging units are necessary because CTs are the installed base, and part of equipment specifications.

Merging units also match well with existing utility organization structures, as they maintain the boundaries between equipment, protection and control, and SCADA groups. However, non-conventional instrument transformers (NCITs) are a solution that directly addresses the limitations of conventional instrument transformers, improving substation design, substation performance, and operating and maintenance costs.

NCITs are any measurement transducer that uses a measurement technique other than the traditional CT, voltage transformer (VT), or capacitive voltage transformers (CVT). Examples include low-power CTs, Rogowski coils, fiber optic current sensors, and capacitive and resistive voltage dividers.

A digital instrument transformer (DIT) is an NCIT integrated with electronics to natively provide digital sampled values as an output.

NCITs use mature measurement techniques, that are well understood and trusted. Rogowski coils were first applied early in the 20th century. Fiber optic CTs have been used since the 1980s.

The limiting factor to their use has been the interface, as NCITs don’t produce the high energy 1A/5A/120V output that relays and meters expect. DITs remove this limit through adoption of common data formats under the IEC 61850 Standard.

One DIT, that replaces CTs with a fully digital substation, is the fiber optic current transducer (FOCT). FOCTs replace conventional terminal blocks with digital communications, and allow for smaller, more flexible substation equipment.

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