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83
DMC3S - Manual - 08 - 2021
— VT monitoring - 74VT
Preface
The line VT monitoring function is used to detect faults on the voltmetric inputs from the VT line relay.
Such faults may typically be due to interruptions in the secondary connections of the TVs or faults on
the relay's internal measurement circuitry.
The function is used both to provide an alarm signal and activate emergency function 51N (51N Eme)
and to block functions 25, 32P, 67 and 67(N) which may intervene incorrectly in the absence of the
measured voltages. The function detects VT faults under the following conditions:
• loss of one or two phase voltages;
• loss of the three phase voltages under load;
• absence of the three phase voltages after power up.
• Loss of one or two phase voltages;
This fault in the voltage measurement circuits causes a negative sequence voltage component to
appear, but without the negative sequence current component. The loss of one or two phase volt-
ages is detected when the negative sequence voltage component violates an adjustable threshold
( U
2VT
>) and the negative sequence current component is below an adjustable threshold ( I
2VT
>).
The negative sequence current component is calculated as follows:
I
2
= ( I
∙
L1
+e
-j120°
I
∙
L2
+e
+j120°
I
∙
L3
)/3,
The negative sequence voltage component is calculated as follows:
U
2
= ( U
∙
L1
+e
-j120°
U
∙
L2
+e
+j120°
U
∙
L3
)/3,
where e
-j120°
= -1/2 - j√3/2 e e
+j120°
= -1/2 + j√3/2
• Loss of the three phase voltages under load
Loss of the three phase voltages under load is detected when the fundamental component of all
three phase voltages falls below a configurable threshold ( U
VT
<) and all three phase currents dur-
ing the current grid parameter period ( I
L1(k)
, I
L2(k)
, I
L3(k)
) do not differ from an adjustable threshold
( D I
VT
<) from the value of the same currents measured over the preceding period ( I
L1(k-1)
, I
L2(k-1)
,
I
L3(k-1)
). By checking whether the currents vary between the current and previous grid periods we
avoid erroneous activation of function 74VT in case of a three-phase dead short close to the TVs,
when the voltages suffer a drastic drop and the currents increase rapidly. Under load, on the other
hand, the currents do not vary, or the variation due to a change in the load, is lower than that expe-
rienced during a short circuit.
• Absence of the three phase voltages after power up
The absence of the three phase voltages after power up is detected when the breaker is closed
and the three phase voltages ( U
L1
, U
L2
, U
L3
) are all below the adjustable threshold ( U
VT
<) and the
three phase currents ( I
L1
, I
L2
, I
L3
) are all below the adjustable threshold ( I
VT
>) - the latter must be
set to a value greater than the current present when the load is powered up, but lower than the
short circuit current.
If the parameter
74VT-BLK1
is set to
ON
and a logical input is set to acquire the logical block
(Block1), the function is blocked for the activation time of the logical input in question. The operating
timer is held in reset mode so that the operating time count starts when the block signal expires;
[1]
the
logical block function can be assigned to the input in the menu Set \ Input board inputs \ Input IN1-1...
(Input IN1-6) .
All parameters are available in the menu Set \ VT -74VT monitoring
Functional block (Block3)
When a fault is detected under one of the three preceding conditions, the VT monitoring function
activates the 51N emergency function (51N eme), blocks functions 25, 32P, 67 and 67N) and, after a
configurable delay t
VT-AL
, issues an alarm.
Notes 1 For a detailed description of the operation of the logical block (Block 1) refer to the paragraph “Logical block” in the chapter MONITORING AND
CONTROL. .