P.3.274
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Communication and Timing
an Ethernet network that carries mixed traffic, 87L traffic should be assigned
the highest priority (7). In addition, all other traffic should be assigned a lower
priority.
SEL Recommended
Network Topologies
You can use either serial or Ethernet communications for current differential
protection. However, the non-deterministic nature of Ethernet
communications can affect network dependability (on-time delivery of
traffic), causing current differential protection to be unavailable. Because
network dependability is crucial for protection applications in general, and for
current differential protection in particular, the network topology must ensure
dependable Ethernet traffic. Therefore, if you choose Ethernet
communications, SEL recommends the following two practices:
1. Dedicated Ethernet packets transported over SONET channels
by SEL integrated communications optical networks (ICON™)
2. A dedicated Ethernet network engineered to provide
appropriate bandwidth, traffic congestion control, and
acceptable latency
Appropriate Bandwidth
Each Ethernet 87L packet is 544 bits, plus 152 bits for the preamble and frame
gap overhead for a total of 696 bits. Relays send Ethernet packets every 4 ms.
Ignoring jitter (see Acceptable Latency and Jitter), the relay allows a deviation
of 0.1 ms from the expected time interval between packets (4 ms). This means
that the network must be able to send the complete frame, including the
preamble and frame gap overhead, in 0.1 ms. With this in mind, calculate the
Ethernet packet bandwidth as follows:
Bandwidth = 696 bits/0.0001 seconds = 6.96 Mbps. If the application requires
four relays, the total bandwidth is 6.96 • 4 = 28.84 Mbps. Although networks
with lower bandwidth are possible, SEL strongly recommends that you select
the 100 Mbps bandwidth for the following reasons:
➤ A network with a lower bandwidth may perform satisfactorily
for a period of time. However, as networks change over time,
more traffic may inadvertently be routed through the switches
in the network. This may cause buffering that can result in the
87L traffic jitter exceeding limits established by the relay
design and configuration. Excessive buffering may cause lost
packets. Because the 87L protection is lost for a minimum of
two cycles (see Table 3.145) when the jitter is high or packets
are lost, higher bandwidth maximizes 87L protection
availability.
➤ For long power lines using fiber optic communication, each
hop introduces a time delay which increases the operating time
of the protection. While choosing the 100 Mbps bandwidth
does not reduce the number of hops, it provides more
bandwidth to reduce the effect of delays caused by hops in the
network.
➤ The 100 Mbps bandwidth minimizes packet delay resulting
from priority issues in switches (also see Traffic Congestion
Control). Although VLAN tagging and Priority Tagging Class
of Service can be used to segment and prioritize 87L traffic,
buffering can still be a problem. Since 87L traffic from all
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