The concept of a digital substation has long
been an insubstantial
thing – an ideal vision of
networked into an
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 fiberoptic networks for
the reliable and efficient transport of operation and supervision
data from automation systems in substations – and even
power line 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.
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. ABB has long championed
industry adoption of IEC 61850, a standard with which the
company has been intimately involved since its inception.
“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 and
ABB has been selling fiber-optic “backbones” for more than
Between the station level and the bays, fibers can carry digital
data – conforming to IEC 61850 – but to become a true digital
substation the standard has to extend even further.
The world beyond the bays is still predominately 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 (see image 1). The IEDs receiving that data
perform fi rst-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 (see image 2).
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 fi t, and every bit
as robust and reliable as the analog equipment it is replacing
or interfacing to (see image 3).
Robustness and reliability requirements apply to new technologies
such as ABB’s fiber-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
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 fiber 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 (as ABB did in 2013) 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 nonconventional 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.
Fiber optics can then replace the copper cables connecting
the primary equipment to the protection and control IEDs.
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 fi ber optics or a wireless
network, such as ABB’s Tropos technology. 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 percent – a substantial cost saving and, more
importantly, a signifi cant safety enhancement.
The process bus also adds flexibility: Digital devices can
speak directly to each other (see image 4 on page 6). 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. ABB’s ASF range of Ethernet switches fully
supports this critical aspect of substation messaging.
ABB has been heavily involved in IEC 61850 since its inception.
The standard is essential to ensure that utilities can mix
and match equipment from different suppliers, but, through
compliance testing, it also provides a benchmark against
which manufacturers can be measured.
ABB deployed the first commercial IEC 61850-9-2 installation
in 2011 at the Loganlea substation, for Powerlink Queensland.
The use of ABB’s IEC 61850-9-2- compliant merging units
and IEDs, not to mention NCITs, makes the deployment a
landmark in the evolution of substation design.
That project was part of an upgrade of an existing station, an
upgrade that saw it move into an IEC 61850 future, adopting
digital standards for effective future-proofing. ABB created a
retrofit solution based on specifications from Powerlink that
can be applied to another five Powerlink substations when
they are ready for refitting.
Two of those stations, Millmerran and Bulli Creek, were already
upgraded in 2013 and 2014, respectively. The refurbished
substations have a MicroSCADA Pro SYS600 system and
RTU560 gateway that manage Relion 670 protection and
control IEDs, with REB500 busbar protection. These all
communicate over IEC 61850-9-2 to the merging units and
over IEC 61850 to the station-level devices.
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.
ABB’s extensive IEC 61850 experience and portfolio of NCITs,
merging units, protection and control IEDs as well as station
automation solutions eases utilities into the digital world.
Flexible solutions allow utilities to set their own pace on
their way toward the digital substation.