P.3.24
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Theory of Operation
It is important to realize that the relay uses time for 87L protection only if it
receives instruction to do so via the channel synchronization method setting. If
you use this setting to select the channel-based mode, the relays will not use
time for 87L protection, even if a valid time is available to the relays. If valid
time is available at both relays of a given channel, the relays perform extra
channel monitoring and statistics measurements that are possible only when
both relays of a given channels are synchronized to the absolute time. The
relays will, for example, measure channel asymmetry when absolute time is
available at both relays working over a given channel; without absolute time,
the relays can measure only the channel’s round-trip delay.
The relay supports several time fallback strategies for the external-time-based
synchronization mode. These strategies range from immediate disabling of the
87L function should a required time source become unavailable, to continuing
operation in the channel-based synchronization mode if the channel needing
time proves to be symmetrical prior to loss of the time reference. Operation in
this latter case continues for as long as round-trip time does not change,
suggesting the channel was not switched and that it therefore remains
symmetrical. Table 3.141 describes the time fallback modes and associated
settings in detail.
Security With Respect
to Communications
Events
Line current differential relays are exposed to several types of
communications events, particularly regarding security, for which there must
be consideration in relay design.
Noise in the communications channel can corrupt data. The term noise refers
to such issues in the communications channel as interference coupled to the
channel media or electronics, failing components in the electronic devices
comprising the network, poor quality of fiber terminations and associated
losses, and marginal power budget for fiber transceivers.
The relay uses a 32-bit Bose-Chaudhuri-Hocquenghem (BCH) code to protect
87L data integrity. With a packet size of 255 bits, i.e., BCH (256, 223), the
minimum distance for error detection is 10 bits, meaning that the relay detects
all 9-bit errors. Assuming a binary symmetric channel (a uniform distribution
of probability for corruption in any single bit in the packet), the probability of
an undetected error is below 1.2 • 10
–10
. The relay uses no error-correction
algorithms, because these would degrade the strength of data protection.
The relay rejects corrupted packets, and the relay algorithm falls back
gracefully, optimizing its response according to whether a single packet is lost
or if more packets are unavailable.
Any data integrity protection has a finite non-zero probability of defeat. We
realize that 87L relays will send, receive, and use significant numbers of
packets during their service lives, so a second layer of protection against
corrupted data is necessary. A relay sends packets every 4 ms, about 7.884
billion packets a year. A 32-bit data integrity check is sufficient if the channel
is relatively noise-free. However, protective relaying applications need to
assume a worst-case scenario of standing noise in the communications
channel such as results from a failing component in the communications
equipment. As the failing component slowly degrades, the number of burst
errors received by the relay increases. In this case the relay could receive more
than one billion corrupted packets per year. Even though the probability that
the receiving relay would fail to detect that one of the packets is corrupted is
less than 1.2 • 10
–10
per corrupted packet, eventually it will happen, and that
could result in an unwanted 87L operation. A state-of-the-art protective relay
cannot tolerate such an event.
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