P.3.48
SEL-411L Relay Protection Manual Date Code 20151029
Protection Functions
87L Differential Elements
User-Programmable
87L Communications
Bits
The relay provides user-programmable communications bits embedded in the
87L communications packets over the 87L media (serial or Ethernet). These
communications bits are fully programmable and are meant for custom
applications. SEL
OGIC control equations in the transmitting relay drive these
transmitted bits, and the associated received bits (in the receiving relay) are
available as Relay Word bits. By default, no relay functions use the user-
programmable communications bits. These bits, however, can be applied
freely to facilitate custom applications that include, but are not limited to,
direct transfer tripping from breaker failure protection, disabling
autoreclosing, and changing setting groups.
Note that the 87L communications packet already includes a number of 87L
function bits for distribution among the relay terminals as necessary. Among
these bits are an external fault detection bit, test bit, blocking bit, 87 direct
transfer tripping bit, charging current compensation active bit, external time
locked bit, and in master mode bit. There is, therefore, no need to apply the
user-programmable communications bits to transmit those 87L functions bits
already included in the communications packet. The user-programmable bits
are active if you use the E87CH setting to configure the 87L application, even
if the E87L setting is set to N.
The relay transmits and receives user-programmable 87L communications bits
on all active channels (see Configuration, Monitoring, Alarming, and Logic
for 87L Channels). The relay sends user-programmable communications bits
every 4 ms. Neither sending nor receiving relays add any intentional delay.
When evaluating the end-to-end response time of the 87L user-programmable
communications bits, you should factor in the 4 ms transmission rate, the relay
processing times for Relay Word bits and SEL
OGIC control equations, channel
latency, and your settings for debounce time.
The user-programmable 87L communications bits are channel-dependent, so
they must be set at fail-safe values when the relay receives no valid data. The
default fail-safe values are logic 0. The 87CHpOK Relay Word bit flags
whether a received value maps to its associated Relay Word bit or whether the
relay substituted a fail-safe value (p in the channel number, p = 1, 2 or 3,
accordingly). If 87CHpOK is asserted, received values map into the Relay
Word bits; if 87CHpOK is deasserted, the relay uses fail-safe values instead.
The 87CHpOK deasserts if the communications channel fails, BCH detects
corrupted data, the packet sequence numbers are nonsequential, or the
transmitting relay address differs from the address the receiving relay expects.
Depending on the application, the fail-safe value you want for a given
communications bit may be logic 1. In such cases, you can accommodate the
proper fail-safe response in your SEL
OGIC control equations that use the
received bit, or you can invert the bit at the receiving end so that logic 1
becomes the natural fail-safe value for the received bit.
In 87L applications over serial channels, four user-programmable bits are
available per communications channel. Therefore, a relay transmitting over
two ports can send four individual bits on each of the two ports, for a total of
eight bits.
In 87L applications over Ethernet, each relay transmits 32 user-programmable
bits, regardless of the number of relays in the 87L scheme. Each relay
broadcasts these 32 bits to all other relays in the 87L scheme. Therefore, each
relay transmits 32 bits, but it can receive a total of 32, 64, or 96 bits from its
one, two, or three remote peers.
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