The concept of a digital substation has long been an insubstantial thing – an ideal vision of all-knowing substations networked into an intelligent grid. But the concept is now a lot more practical so the specifics of what makes a substation “digital,” and why that is such a desirable thing, can be discussed.
Digital signaling offers excellent reliability and capacity, and has been in use in power infrastructure for decades. Most existing electricity grids employ digital fibre-optic networks for the reliable and efficient transport of operation and supervision data from automation systems in substations – and even powerline networks carry tele-protection signals these days. But only now are the advantages of standardized digital messaging starting to extend into the deeper substation environment.
IEC 61850 for power utility automation
Without standards, the adoption of digital messaging for intrasubstation communication was piecemeal and fragmented, with mutually incompatible signaling creating an assortment of messaging within vertical silos.
IEC 61850, or “Communication networks and systems for power utility automation,” as the IEC document is properly known, is a comprehensive standard broken down into components that, for example, specify how the functionality of substation devices should be described – how they should communicate with each other, what they should communicate and how fast that communication should be. All of this is critical to realizing the benefits of a truly digital substation.
At the station level, things are generally digital, even in relatively old installations. SCADA (supervisory control and data acquisition) systems usually demand digital information.
Between the station level and the bays, fibres can carry digital data – conforming to IEC 61850 – but to become a true digital substation the standard has to extend even further.
Transition to digital substations
The world beyond the bays is still predominantly analog. The conventional primary equipment, like current and voltage transformers, is connected back to intelligent electronic devices (IEDs) using parallel copper wires carrying analog voltage signals. The IEDs receiving that data perform first-level analysis and often provide the gateway into a digital world.
But there is little advantage in keeping the data in analog form for so long and to properly earn the title of “digital substation” the transition to digital must take place as soon as the data is gathered.
Through permanent system supervision, digital equipment reduces the need for manual intervention and the adoption of the all-digital process bus allows sensitive equipment to be relocated into the bays. The digital equipment that has to be located out in the yard must be easy to fit, and every bit as robust and reliable as the analog equipment it is replacing or interfacing to.
FOCS provides robustness and reliability
Robustness and reliability requirements apply to new technologies such as ABB’s fibre-optic current sensor (FOCS) too. A FOCS can directly monitor current running through a high-voltage line without having to involve a current transformer (CT) to step down the current to a measurable value. Eliminating the CT also eliminates the risk of open CT circuits, in which life-threatening voltages can occur, and so increases safety.
A FOCS exploits the phase shift in polarized light introduced by an electromagnetic field (the Faraday effect). The shift is in direct proportion to the current flowing in the high-voltage line, around which the fibre carrying the light is wrapped. The measurement is digitized right at the source and transmitted as a digital signal, via the process bus, to the protection and control IEDs, as well as the revenue meters.
Such an optical CT takes up a lot less space than its analog equivalent. It can even be integrated into a disconnecting circuit breaker to combine the functions of circuit breaker, current transformer and disconnector in one device – halving the size of a new substation.
The FOCS is one of a range of non-conventional instrument transformers (NCITs) that can make things entirely digital. NCITs have to be every bit as reliable as the equipment being replaced – and they are: Over the past decade ABB has supplied more than 300 NCITs (combined current and voltage sensors fitted into gas-insulated switchgear) for use in Queensland, Australia, and the utility has yet to see a single failure in the primary sensor. Extensive use of NCITs makes a substation simpler, cheaper, smaller and more efficient.
Not everything can be digital – analog data will continue to arrive from conventional current and voltage transformers, for example. But there is no reason for wholesale replacement when a standalone merging unit can perform the transition to digital right beside the existing instrument transformer. Fibre optics can then replace the copper cables connecting the primary equipment to the protection and control IEDs.
Digital process bus replaces copper
As a conductor, every bit of copper in a substation is a potential risk. For example, where current is incorrectly disconnected, such as with an open secondary current transformer, arcing may occur as dangerously high voltages build and a copper line can suddenly carry high voltage, putting workers and equipment at risk. Less copper brings greater safety.
The digital substation dispenses with copper by using the digital process bus, which might use fibre optics or a wireless network. Just the removal of copper can, in some circumstances, justify the switch to digital. Going digital can cut the quantity of copper in a substation by 80% – a substantial cost saving and, more importantly, a significant safety enhancement.
The process bus also adds flexibility: Digital devices can speak directly to each other. For this, IEC 61850 defines the GOOSE (generic object-orientated substation events) protocol for fast transmission of binary data. Part 9-2 of the standard describes the transmission of sampled values over Ethernet. These principles ensure the timely delivery of high-priority data via otherwise unpredictable Ethernet links.
Digital substation improves reliability
A fully digital substation is smaller, more reliable, has a reduced life-cycle cost and is simpler to maintain and extend than an analog one. It offers increased safety and is more efficient than its analog equivalent.
Not every substation needs to be catapulted into a wholesale digital world – it depends on the substation size and type, and whether it is a new station or a retrofit of the secondary system. Different approaches and solutions are required. Flexible solutions allow utilities to set their own pace on their way toward the digital substation.