and
X0Transformer
are used to input the zero-sequence impedance of the
earthing / main transformer. If the settings are applied, the controller can
consider the transformer when calculating network parameters.
Using this network model, the controller determines network parameters G
symm
,
B
symm
, G
asymm
, B
asymm
, G
coil
and B
coil
.
G
symm
Real part of symmetrical part of the total network admittance. The sign is pos-
itive and it represents the shunt losses of the network (excluding the losses of
the central coil and the parallel resistor). If there are distributed compensation
coils in the network, their shunt losses are included in this value.
B
symm
Imaginary part of symmetrical part of the total network admittance. The sign
is positive (capacitive) and it represents the capacitive uncompensated earth-
fault current of the network. If there are distributed compensation coils in the
network, their effect is included in this value.
G
asymm
Real part of asymmetry admittance. The sign may be positive or negative
depending on the actual level of asymmetry.
B
asymm
Imaginary part of asymmetry admittance. The sign may be positive or negative
depending on the actual level of asymmetry.
G
coil
Real part of the coil admittance. The sign is positive and it represents the
resistive losses of the central coil including the parallel resistor.
B
coil
Imaginary part of the coil admittance. The sign is negative and it represents
the inductive current of the central coil, which defines the compensation de-
gree of the network.
All previous admittance terms can be easily converted into equivalent current values
by multiplying them by the system nominal phase-to-earth voltage U
n
.
I
ef_network
= (B
symm
+ B
asymm
) · U
n
Uncompensated earth-fault current
I
loss_network
= (G
symm
+ G
asymm
) · U
n
Network shunt losses
I
coil
= B
coil
· U
n
Coil inductive current
I
loss_coil
= G
coil
· U
n
Coil loss current
Network parameter calculation is based on the forced change in network zero-
sequence voltage. This is achieved by changing the coil position or by changing
the connection state of the parallel resistor of the coil. All the Monitored data values
expressed in Amperes are scaled with reference to U
0
primary voltage.
9.11.6.4
Coil automatic tuning
Coil automatic tuning mode is enabled with setting
Controller mode = "Automatic".
In this mode, the
TUNING_ACT
binary output is activated. The coil automatic tuning
procedure consists of two parts. First, sufficient zero-sequence voltage variation is
obtained using coil movement or resistor switching. Secondly, network parameters
are calculated.
The automatic tuning procedure requires a sufficient change to be detected in
the measured zero-sequence voltage. This can be achieved by coil movement or
switching the parallel resistor of the coil. The mode is selected with
Tuning mode
setting. The available options are "Coil movement" and "Resistor switching".
In the coil movement mode, the controller first moves the coil a little towards the
coil's default position and determines the direction of raising Uo voltage. Then
the controller moves the coil preferably to the direction of raising zero-sequence
voltage until the measured zero-sequence voltage is changed by the amount
defined with
V Res variation setting. V Res maximum setting defines the maximum
Control functions
1MRS759142 F
1738
REX640
Technical Manual