17.18
SEL-400 Series Relays Instruction Manual Date Code 20171006
IEC 61850 Communication
Sampled Values
Primary/Secondary Scale Factor
SV messages provide current and voltage measurements in terms of the primary
side of the instrumental transformers. SEL SV relay protection calculations are
based on traditional secondary quantities. Thus, the received digital samples must
be scaled to the secondary properly. For example, if the SV stream comes from a
merging unit that is connected to a 1200/5 CT, the SEL SV relay CT ratio settings
should be 240.
SV Network Delays
The SV merging unit and process bus network act as the remote data acquisition
system for an SV relay. There are time delays introduced by this remote data
acquisition system. The delays of an SV stream include the merging unit process-
ing delay and the process bus network delay. The sum of these is called the net-
work delay. SEL SV relays measure and report this network delay. The measured
network delay for each SV subscription is stored as an analog quantity and
reported via the COM SV ASCII command. See Section 9: ASCII Command
Reference in the product-specific instruction manual for more detailed informa-
tion.
SEL SV relays buffer samples, and the buffer length is controlled by the
CH_DLY settings. Set the CH_DLY settings to at least one sample interval
greater than the total network delay. SEL SV relays wait to start resampling until
samples arrive for the configured CH_DLY. This design also provides a consis-
tent delay (CH_DLY) to protection and control operations, which overcomes the
non-deterministic delays caused by the Ethernet process bus network.
If SV messages of the first SV subscription, which is listed first in the COM SV
command response, are delayed by more than CH_DLY, they are considered lost.
If less than three consecutive messages are delayed or missing, the SEL SV relay
interpolates for these delayed or lost messages. If more than three samples are
delayed or missing, the SEL relay ASCII command COM SV reports
SV STREAM
LOST
for this scenario.
The protection and control operation times are delayed by the configured
CH_DLY. Use caution when setting the relay coordination times to account for
this added delay.
Coupled Clocks Mode
SEL recommends configuring a high-quality time source for the SV relays.
Depending on the time-synchronization status, an SEL SV relay operates in one
of two modes: the freewheeling mode and the coupled clock mode. SEL SV sub-
scribers use the same logic as SEL SV publishers to determine a local smpSynch
based on its time-synchronization status. When the incoming smpSynch of the
first SV subscription is non-zero and matches the local smpSynch, the relay oper-
ates in coupled clocks mode and SVCC asserts. When operating in coupled
clocks mode, the relay can calculate the network delay for incoming SV streams.
These delays are stored in analog quantities SVNDmm where mm is the subscrip-
tion number. The delays are also reported in the COM SV command response.
When operating in the freewheeling mode, the SV relay will not provide the net-
work delay statistics.
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