ABB RELION REX640, Technical Manual, Page: 1709 / 1960

Negative Reactance Principle NRP
This parallel control scheme is suitable for power transformers with different
ratings and step voltages. Since no communication between the regulators is
needed, this principle can be applied even when the parallel transformers are
located at different substations. To start the parallel operation, the active operation
mode has to be set to "NRP" for all the regulators of the connection. The active
operation mode can be changed via function block inputs or by setting it either
locally or remotely.
When applying this principle, each regulator has a phase angle setting φ
Load
(setting
parameter
Load phase angle) towards which it tries to regulate the current. The
setting value is chosen according to the expected power factor of the load (positive
setting value equals inductive load). When the actual phase angle of the load current
is the same as the setting and the transformers and their tap changer positions are
identical, the currents of the transformers are in the same phase as the total load
current. If the tap changer positions are different, the circulating current flows and
the currents of different transformers either lag or lead the load current.
Figure 918
shows that the circulating current is the reactive component which separates the
measured current vector from the expected angle value.
I
LOAD
x sin(
φ
LOAD
)
I
TR1
x sin(
φ
1
)
φ
LOAD
φ
1
U_A
Ici = circulating
current
I
LOAD
I
TR1
I
LOAD
= I
TR1
x
│
cos(
φ
1
)
│
/cos(
φ
LOAD
)
Figure 918: The expected phase angle of the load supplied by the transformers
operating in parallel is entered as a setting value φLoad
The regulators calculate the circulating current with the equation
I I
ci Load TR
= − ⋅ ⋅
(sin tan cos )
ϕ ϕ ϕ
1 1 1
(Equation 376)
I
TR1
Average of the currents I_A, I_B and I_C
φ
1
Phase angle between U_A and I_A
φ
Load
Set
Load phase angle
of the load current
In the negative reactance method, the circulating current is minimized by changing
the control voltage according to the measured circulating current. The regulator
1MRS759142 F Control functions
REX640
Technical Manual
1709