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DMC3S - Manual - 08 - 2021
Asynchronous grids
For asynchronous grids (e.g. two separate distribution grids) the final relay is operated (synchro-
nisation ok) once the comparison between the amplitudes and frequencies has been passed. The
close command is issued in advance of the null phase shift condition, when the measured phase
shift is equal to the value calculated from the delays (relay + breaker close time) and frequency
difference between the two grids (slip frequency). In this way, the contacts close in a condition of
perfect synchronisation.
If a power transformed is present between the two voltage measurement inputs, the amplitude dif-
ferences on both sides must be compensated for. This compensation can be done externally (using
adapter transformers) or internally (software).
The software compensation is programmed using the parameter Phase correction V1-V2 in the
menu Set \ Base
[1]
, so as to compensate for the phase shift without the need for external adapter
transformers. If there is no power transformer or external adapter transformers are used, a null
phase shift must be set.
Control Functions
The following parameters are controlled cyclically:
• Instability of the frequency measurement: if the measurement is unstable, a report is issued, and if
the difference between consecutive measurements is greater than the adjustable value of parame-
ter Rof>
SYNC
, execution of the parallel is suspended. Setting Rof>
SYNC
= 0 disables stability control.
• Status of the line VT monitoring function; if faults are detected on the secondary VT connections or
due to intervention of function 74VT, execution of the parallel is suspended.
Operating logic
The synchronisation control sequence can be started by a pulse command generated by:
• automatic reclosing, internal to the protection relay,
• a local command on the front keypad with the
(closing) button
• a command over the comms interface.
When synchronisation control is requested, the congruence controls are run (see the preceding par-
agraph). If a non-permitted state is detected, the Fault Sync report is issued and synchronisation
control is not run.
If, on the other hand, the checks are positive, the synchronisation control sequence is initiated (re-
port SYNC - n IN PROGRESS; with n = 0, 1, 2, depending on the operating mode)..
Only the enable conditions for the current operating mode are checked.
If both voltages are incoming (Mode 0), the device checks whether the two grids are synchronous
or asynchronous by comparing the measured frequency difference to the threshold Df-
GRID
; if the
difference is less than the threshold, the grids are deemed to be synchronous.
The various operating situations are reported by messages:
If the conditions are satisfied, the synchronisation function issues a close enable signal.
The duration of the enable signal can be adjusted by setting the parameter Operating mode to
PULSE and the duration (Minimum pulse duration) to the desired value for the final relay
programmed in function 25; the parameters are available in the menu Set \ Control board 1 outputs.
You can set the synchronisation attempt timeout (timeout
-SYNC
) in the menu Set \ Configuration
parameters A \ Synchrocheck relay - 25 \ Common configurations; if the conditions are not satisfied
within this time, the sequence is suspended.
Close command advance
For asynchronous grids the final relay must be commanded (synchronisation ok) in advance to
ensure that the breaker closes in conditions of perfect synchronisation.
Depending on the breaker closing time and the frequency difference (slip frequency) and phase shift
of the measured voltages V1 and V2, the device determines the next synchronisation time. The close
commanded is issued in advance of the synchronisation time by the advance time calculated on the
basis of the frequency difference (∆f).
The electrical advance angle required to compensate for the closing delay of the breaker is defined
by:
A
= 360 ∙ (t
CB-CLOSE
+ t
MEAS
) ∙ ∆f
where:
A
: electric angle
t
CB-CLOSE
: breaker closing time in seconds
t
MEAS
: protection relay measurement time (acquisition + measurement + final relay time) in seconds
∆f: difference between the measured values of the two grid frequencies (slip frequency) in Hz
For example, with the following values:
t
CB-CLOSE
: breaker closing time in seconds = 80 ms
t
MEAS
: protection relay measurement time (acquisition + measurement + final relay time) = 40 ms
f
1
: frequency of V1 signal = 50 Hz
f
4
: frequency of V signal = 50.2 Hz
∆f: difference between the measured values of the two grid frequencies (slip frequency) = 0.20 Hz
Note 1 This parameter can only be modified at Level 1
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