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 561: The expected phase angle of the load supplied by the transformers
operating in parallel is entered as a setting value φ Load
The regulators calculate the circulating current with the equation
I
I
ci
Load
TR
=
−
⋅
⋅
(sin
tan
cos
)
ϕ
ϕ
ϕ
1
1
1
(Equation 187)
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
The 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
calculates the circulating current compensation term U
ci
using the equation
U
I
I
Stability
U
ci
ci
n
n
=
−
⋅
⋅
100
(Equation 188)
I
ci
Circulating current
Stability
Stability setting (the recommended value depends on the loop impedance)
If the transformers operating in parallel have different rated currents, the value of
the
Stability factor setting of the regulator should be proportional to the rated
currents, that is, the higher the rated current, the higher the
Stability factor setting
value.
By comparing the reactive components of the currents measured by the different
regulators it is possible to find out if the circulating current has been minimized.
The circulating current is minimized when the reactive components are equal.
The negative reactance method gives satisfactory results only if the phase angle of
the load current is known relatively accurately. If the actual phase angle deviates
from the phase angle setting, a regulating error occurs. However, for the cases
1MRS757644 H
Control functions
620 series
Technical Manual
1057
