Thytronic DMC3S - Negative Sequence Overcurrent - 46

42
DMC3S - Manual - 08 - 2021
— Negative sequence overcurrent - 46
Preface
The protection against unbalanced loads calculates the negative sequence current I
2
from the
measured phase currents; the function has two settable thresholds with programmable trip time.
The trip characteristic for the two thresholds can be selected with definite or inverse time per IEC
60255-3/BS142.
A constant reset time can be set for each threshold, to reduce the time for eliminating intermittent
faults (while being no less than the protection start time).
Each protection threshold can be enabled or disabled.
The first threshold can be inhibited by the start of the second threshold.
Measurement criterion
The protection calculates the negative sequence current I
2
from the values of the three phase cur-
rent phasors I
L1
, I
L2
, I
L3
using the formula:
I
2
=(I
L1
+e-
j120°
·I
L2
+e
+j120°
·I
L3
)/3
where e
-j120°
=-1/2-j√3/2, e
j120°
=-1/2+j√3/2.
Operation and settings
The negative sequence current I
2
is compared with the set thresholds (I
2
>, I
2
>>); exceeding a thresh-
old starts it (START) and starts the respective timer count (t
2
>, t
2
>>). If the threshold violation per-
sists, when its trip time expires, the threshold trips (TRIP), otherwise the threshold is reset.
The trip characteristic for the two thresholds can be selected with definite or inverse time per the
following characteristic curves (symbology for the first threshold):
• IEC 255-3/BS142 type A or SIT, inverse time: t = 0.14 · t
2
>
inv
/ [(I
2
/I
2
>
inv
)
0.02
- 1]
• IEC 255-3/BS142 type B or VIT, very inverse time: t = 13.5 · t
2
>
inv
/ [(I
2
/I
2
>
inv
) - 1]
• IEC 255-3/BS142 type LTI, long time inverse: t = 120 · t
2
>
inv
/ [(I
2
/I
2
>
inv
) - 1]
• IEC 255-3/BS142 type C or EIT, extremely inverse time: t = 80 · t
2
>
inv
/ [(I
2
/I
2
>
inv
)
2
- 1]
Where:
t: operating time
I
2
>: trip threshold
t
2
>
inv
: operating time regulation
The following applies to all inverse time characteristics:
• The minimum trip current is 1.1 times the set threshold (asymptotic reference time).
• The characteristics are defined between 1.1 and 20 times the set threshold;
[1]
if threshold regula-
tion exceeds 2.5 I
n
, the upper limit of the measurement range is 40 I
n
.
• The minimum operating time t is 0.1 s.
• For definite time trip characteristics, the upper limit of the measurement range is 30 I
n
.
Each protection threshold can be enabled or disabled by setting to ON or OFF the parameter I2>
Enable and/or I2>> Enable in the menu Set \ Protections \ Negative sequence overcurrent - 46
\ Threshold I2> (Threshold I2>>) \ Parameters.
The characteristic can be set to definite or inverse time with I2>Curve, I2>>Curve (DEFI-
NITE, NIT, VIT, EIT, LIT) in the menu Set \ Protections \ Negative sequence overcurrent - 46 \
Threshold I2> (Threshold I2>>) \ Parameters.
Note 1 For input values greater than 20 times the threshold, the operating time is limited to the value corresponding to 20 times the threshold
I2
I2>>
t2>
t2>>
I2>
t
Trip characteristic of the two thresholds of the negative sequence overcurrent protection - 46
TRIP