IEEE PES T&D Conference & Exposition 2022 - 107

control and data acquisition
(SCADA) and other devices performing
protection, automation,
and control are based on digital
communications, predominantly
using IEC 61850 communication
protocols. Typically, merging units
(MU) are used to convert analog
signals from the secondary CTs
and VTs to a digital interface. The
interface to breakers and disconnect
switches is digitized by switchgear
control units (SCU). SCUs
have a hardwired interface to the
breaker trip/close coil to operate
the breaker by receiving GOOSE
messages from the protection and
control IEDs. They also collect and
send status information from the
breaker through the communication
network. Within this paper,
we will refer to a process interface
unit (PIU) for an IED that combines
both MU and SCU functionality.
As can be seen in the aforeSCADA
Control
Status
PIU
Network
Protection
IED
PIU
Control
D
Control
IED
y11
Status
Control
PIU
Control
IED
Protection
IED
Status
Protection
IED
Control
IED
mentioned
reference, the architecture
of digital substations is
typically distributed as illustrated
by Figure 1 with PIUs providing
the interface between the primary substation equipment and
all devices in the bay level over the substation's local area
network. The interfaces are based on GOOSE and sampled
values (SV) messages. The interfaces between the multifunctional
IEDs themselves are typically performed via
GOOSE messages in the process or station bus. Finally, client/server
(MMS protocol) communication over the digital
substation's station bus is used for the interface between bay
and SCADA levels.
figure 1. Digital Substation Typical Architecture and Components.
For simplicity, logical nodes are represented in Figure 2
by the four-character logical node class name. Taking Feeder
1 as an example, the following logical nodes are specified
for its protection function: TVTR (voltage sensor VT1),
XCBR
Logical Nodes and Function Allocation
The data model defined in the IEC 61850 standard allows
users to start designing the system with an abstract definition
not tied to a specific set of relay models. The first
step in the system specification is to define which functions
and data are needed to fulfill station requirements. In IEC
61850, the different functions in the system are decomposed
into logical nodes (LN) that may reside in one or more
physical devices. Logical nodes provide the foundation for
users to start building blocks to form a function or overall
scheme. Figure 2 shows an example of a simplified singleline
diagram with the representation of the primary equipment
(transformer, feeder lines, breakers, CT, VT, bus). Primary
equipment and protection and control functions were
decomposed and logically specified by the logical nodes
represented in the single line.
April 2022 Show Issue
CB1-T1
TCTR CT1-T1
T1
TCTR
TVTR
VT1
TCTR
PTOC
RBRF
PTRC
XCBR
XCBR
CT1
PTOC
PDIF
CT2-T1
CB2-T1
PDIF
TCTR
CB1 XCBR
Feeder 1
CT2
CB2
Feeder 2
figure 2. System Functional Specification.
ieee power & energy magazine
107
PTOC
RBRF
PTRC
PTRC
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