IEEE PES T&D Conference & Exposition 2022 - 130

these data exchanges: point-to-point and Ethernet networkbased
communications architectures. Following are the
main advantages and disadvantages of these architectures.
Point-to-Point Communications Architecture
Point-to-point communications are as simple as wired connections
between two devices except the information exchange is
done digitally and normally through a fiber-optic cable pair.
Point-to-point devices are simple to connect as there is no network
between the devices.
Point-to-point communications systems have direct
fiber-optic connections between merging units and protective
relays, as Figure 9 illustrates. In some applications,
one merging unit can have point-to-point communications
with multiple protective relays. The main advantage of this
system is its simplicity because it does not require network
switches between merging units and protective relays and
external clocks are not necessary. For this reason, these systems
have low latency and low jitter. The main disadvantage
is that information sharing with multiple relays is limited,
limiting the complexity of the protection schemes employed.
For example, sharing busbar voltages may require multiple
merging units.
Network-Based Communications Architecture
A network-based communications architecture uses a data
server that publishes measurements to a network with no
knowledge of how many devices are listening or subscribing
to the published data. The listening devices subscribe
to the data published by one or several merging units. This
approach implies that the internal clocks of publishers and
subscribers require a synchronization method. The traffic-engineering
backup path for network reconfiguration is
much more involved than when using the physical point-topoint
architecture.
The network-based communications architecture, shown in
Figure 10, includes merging units, ethernet switches, precision
time protocol (PTP) clocks, and protective relays. The
network clock distributes precise time to the merging units and
protective relays using PTP according to the IEEE 1588 standard
for time synchronization. An ethernet switch manages
large amounts of network data traffic and acts as a PTP transparent
clock to ensure time synchronization of the network
devices within 100 nanoseconds. The switch defines paths
only for specific ethernet packets and discards unexpected ones
based on pre-determined rules. The main advantage of this
architecture is that voltage and current measurements and Boolean
information can be shared across the whole network, which
requires fewer merging units. The main disadvantage is that the
sample value (SV) architecture requires switches and clocks
that reduce the reliability and performance of the network.
There are many factors to consider in relay designs that
use SVs. IEC 61850-9-2 merging units publish the sampled
data at 4,800 Hz (for 60 Hz systems) or 4,000 Hz (for 50 Hz
systems). Binary information, like breaker status, is published
similarly to SVs using IEC 61850 GOOSE messaging.
Merging units monitor and publish these binary
statuses for relays to subscribe to these data. Protective
relays should identify bad data in an IEC 61850-9-2 data
stream. Once the bad measurements are identified,
the
protection functions that depend on these measurements
should be disabled, and the devices should send an alarm
to the operator. Furthermore, the bad data received by the
device should not cause protection misoperations.
Line Current Differential Protection
Application With a Point-to-Point DSS
The 87L functionality can be implemented using point-topoint
or network architecture DSSs and/or conventional
technologies. The sampling process in an 87L terminal is
effectively the same in either implementation. In both architectures,
the remote terminal current measurements arrive at
the local terminal with a known latency, and it is the job of the
local terminal to accommodate that latency. Figure 11 shows an
Protective
Relay
Protective
Relay
Process Bus LAN
Merging
Unit
Primary
Equipment
Merging
Unit
Primary
Equipment
Switchyard
figure 10. DSS with ethernet-based communications.
130
ieee power & energy magazine
April 2022 Show Issue
Merging
Unit
Primary
Equipment
PTP
Clock
Protective
Relay
Protective
Relay
Protective
Relay
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