P.3.275
Date Code 20151029 Protection Manual SEL-411L Relay
Protection Functions
87L Communication and Timing
relays is set to the highest priority, all line current differential
packets are treated with equal priority, which means that these
packets may still be buffered if the bandwidth is not adequate.
Acceptable Latency and Jitter
Latency, or channel delay, is defined as the one-way measurement of the time
from the remote relay sending a packet to the local relay receiving the packet.
The differential element can operate with a constant latency of up to 50 ms,
but a channel delay results in an equivalent delay in tripping time. For every
millisecond of latency, the relay receives the packets one millisecond later and
relay operating time is one millisecond slower.
Because channel delay is a function of a particular network, the relay
calculates the channel delay time and adjusts the expected arrival time of
Ethernet packets from each remote relay. Once the relay establishes the
channel delay time, it must receive packets from all remote relays within this
time frame.
The channel delay time is an arbitrary reference time and, if less than 50 ms,
does not affect relay algorithms. Jitter (slight variations in the channel delay
time), however, can cause the differential elements to be disabled. As
mentioned in Appropriate Bandwidth on page P.3.274, the differential element
is designed for Ethernet packets to arrive every 4 ms (plus 0.1 ms tolerance),
but jitter can cause the packets to arrive either earlier or later than expected.
Use the 87ETHJT Port 87 setting to extend the maximum jitter which the
relay will tolerate. At the maximum 87ETHJT setting of 3.5 ms, the maximum
expected packet arrival is 7.6 ms (4 ms + 0.1 ms + 3.5 ms = 7.6 ms). If
network conditions such as buffering causes a packet delay longer than 4.1 ms
plus the 87ETHJT setting, the relay declares the packet lost. When a packet is
lost, the 87L elements are disabled for a minimum of two cycles.
Note that 87L trip time is increased by the time selected for the 87ETHJT
setting. Select non-zero values of 87ETHJT only when necessary to force the
relays to tolerate high network jitter.
Traffic Congestion Control
When Ethernet packets are transmitted, they leave the egress port in sequence,
and only upon the completed transmission of the previous packet. Therefore,
when two or more Ethernet packets compete for a network path, such as to
egress the port of a switch, the packet(s) not being transmitted is buffered in
the switch. If the buffered packets exceed the port’s capacity, the buffering
depth increases, and eventually packets may be discarded. When the switch
discards an 87L packet, relay logic detects this as a lost packet, causing the
87L function to be unavailable for a minimum of two cycles (see Table 3.145).
SEL recommends that strict traffic congestion control engineering be applied
to networks transporting 87L protection traffic. The focus for this engineering
is to make sure the available link bandwidth and network appliance backplane
bandwidth exceeds the required summation of all 87L traffic bandwidth and
the overall latency of the network is less than 50 ms.
The relay complies with IEEE 802.1Q. Use the 87VLAN setting to create a
protection broadcast domain and limit the 87L traffic to this domain. For this
protection broadcast domain, set the 87LPRI to 7 to assign 87L traffic the
highest priority.
NOTE: Select non-zero value of
87ETHJT only when necessary to force
the relays to tolerate high network
jitter.
NOTE: Use the COM 87L report
Maximum Receive Delay and Receive
Delay Histogram to determine the
variation in your network latency to
assist in selection of the 87ETHJT
setting value. SEL suggests you hook
the SEL- 411L relays up to the chosen
network and let them sit for a 24-hour
period in order to get a good sample
size of data for the COM 87L report.
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