Mounting and Commissioning
3.3 Commissioning
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
550
• With closed circuit breakers, the power values are viewed as primary and secondary values on the front
display panel or via the operator or service interface with a personal computer.
Here, again, the „WEB-Monitor“ is a convenient help since the vector diagrams also show the allocation
between the currents and voltages (Figure 3-30). Cyclically and acyclically swapped phases can easily be
detected.
• The measured power values on the actual device or in DIGSI enable you to verify that they correspond to
the load direction (Figure 3-31):
P
positive if active power flows into the protected object,
P
negative if active power flows toward the busbar,
Q
positive if reactive power flows into the protected object,
Q
negative if reactive power flows toward the busbar.
Consequently, the powers and their components must have opposite signs at both ends.
Consider also that high load currents, which can occur on long overhead lines or cables, are capacitive,
which means that they correspond to a negative reactive power. In spite of a resistive-inductive load, this
may lead to a slightly negative reactive power at the feeding end, whereas the other end features an in-
creased negative reactive power. The lower the load current for the test, the higher the significance of this
influence. In order to get unambiguous results, you should increase the load current if necessary.
Figure 3-31
Apparent load power
• The power measurement provides an initial indication as to whether the measured values of one end have
the correct polarity.
– If the direction of the reactive power is correct but the sign of the active power is incorrect, cyclic phase
swapping of the currents (right) or of the voltages (left) might be the cause;
– If the direction of the active power is correct but the reactive power has an incorrect sign, cyclic phase
swapping of the currents (left) or of the voltages (right) might be the cause;
– if the signs of both active and reactive power are incorrect, the polarity in address
201
CT Starpoint
has to be checked and corrected.
The phase angles between currents and voltages must also be conclusive. All three phase angles
ϕ
(U
Lx
–
I
Lx
) must be approximately the same and represent the operating status. In the event of power in the direc-
tion of the protected object, they correspond to the current phase displacement (cos
ϕ
positive); in the event
of power in the direction of the busbar they are higher by 180° (cos
ϕ
negative). However, charging currents
might have to be considered (see above).
• The measurements may have to be repeated after correcting the connections.
