CHARACTERISTICS
39
NV10B - Manual - 04 - 2022
• Relay residual nominal voltage U
En
Three instances are expected:
A) residual voltage acquired with open delta VT.
B) residual voltage measure by means VT connected from star-point and earth.
C) residual voltage measure by vector calculus.
Case A
The residual nominal voltage U
En
may be estimated by means of the following general formula (ef-
fective for insulated neutral or impedance-ground connected grids):
If open delta VT with primary nominal voltage balances to the grid voltage divided by √3, the follow-
ing streamlined calculus may be used:
U
En
= 3 · VT open delta secondary nominal voltage [V] for insulated or impedance-ground neutral
grids.
Example A1
The relay residual nominal voltage U
En
must be estimated by means of the following general for-
mula:
U
En
= √3 · U
ng
/K
VT
= √3 · 6900 / (√3 · 100) = 69 V
Example A2
The relay residual nominal voltage U
En
must be indifferently estimated by means of the following
general formula:
U
En
= √3 · U
ng
/K
VT
= √3 · 6000 / (√3 · 60) = 100 V
or by means the following:
U
En
= 3 · U
ns
= 3 · 100 / 3 = 100 V
Case B
The relay residual nominal voltage U
En
must be estimated by means of the following general for-
mula:
If a VT with primary nominal voltage equal to the grid voltage divided by √3 is connected from star
neutral to ground, the following streamlined calculus may be used:
U
En
= VT secondary nominal voltage [V]
VT ratio K
VT
Grid nominal voltage U
ng
where the VTs [V] x √3 are connected
U
En
=
Es1-UEn.ai
K
VT
=
/
/
V
V
10000
3
100 3
3
= 100 ·
3
= 100 ·
/
/
V
V
10000
3
100 3
Grid nominal voltage U
ng
= 6.9 kV
VT ratio: K
VT
=
Insulated or impedance-ground neutral
/
U
ng
= 6.9 kV
U
En
52
NV10B
Es2-UEn.ai
K
VT
=
/ V
V
6000
3
100 / 3
3
= 60 ·
Insulated or impedance-ground neutral
U
np
= 6 kV
U
En
52
NV10B
3
= 60 ·
/
/
V
V
6000
3
100 3
Grid nominal voltage U
ng
= 6 kV
VT ratio: K
VT
=
/
Voltage transformer ratio K
TV
ng
where the VTs are included [V] / √3
U
En
=
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