Thytronic DMC3S - Second Harmonic Restraint - 2 Ndh-REST

77
DMC3S - Manual - 08 - 2021
— Second harmonic restraint - 2ndh-REST
Preface
When a power transformer is powered up, the phenomenon of inrush current occurs, teh amount
and duration of which depends on a variety of factors, including:
• Instantaneous value of the power voltage.
• Structural characteristics of the transformer, magnetisation characteristic and dimensions.
• Residual flux.
The maximum value of the inrush current is obtained when the transformer is powered with voltage
applied when the sine wave passes through zero, in the positive or negative direction, with positive
or negative residual flux respectively.
Furthermore, the ratio between the maximum inrush current and the nominal transformer current
diminishes with the size of the transformer itself.
In any case, the inrush current contains a significant second harmonic component.
To prevent the slow intervention of certain protections due to the inrush current, the second har-
monic restraint function can be enabled for each threshold of protections 46, 50/51, 50N/51N, 51N(E),
51N(Eme), 51(SQL), 67 and 67N.
Operating logic
The second harmonic component of each phase current (I
L1-2nd
, I
L2-2nd
, I
L3-2nd
) is compared with an
adjustable threshold I
2ndh>
.
When one or two of the three second harmonic components violate the threshold, the threshold
itself starts.
To keep the protection functions blocked until they are reset, the second harmonic restraint function
features an adjustable reset time t
2ndh>RES
.
Protections for which the second harmonic restraint is enabled are blocked for an adjustable maxi-
mum time; the parameter t2ndH-REST can be set in the menu Set \ Protections \ Second harmonic
restraint - 2ndh-REST.
2NDH-REST-diagram.ai
I
L1-nd
t
2ndh-REST
T0
≥ 1
≥ 1
t
2ndh-REST
t
2ndh>RES
T0
t
2ndh>RES
Logic
2ndREST
Start I2ndh>
I
2ndh>
I
L1-nd
≥ I
2ndh>
I
L2-nd
I
L2-nd
≥ I
2ndh>
I
L3-nd
I
L3-nd
≥ I
2ndh>
1/3
2/3
&
&
&
Functional diagram of the second harmonic restraint - 2NDH/REST