Negative Reactance Circulating Current Control - Tapp; Commissioning - Siemens 7sg15 microtapp Operation Manual

Upper
Band
A
Lower
Band
It is not possible for this method of circulating current control to be used when networks are operated in parallel as
the summed load at that site can also contain reactive current flowing between remote transformers.
4.2.2 Negative Reactance circulating current control – TAPP
A modification of the circulating current principle is used to overcome the limitations of the circulating current
system described in the previous section and allow operation of transformers in any configuration, in parallel at a
site, or across a network.
A network Power Factor setting is used to calculate the magnitude of circulating current as the vector difference
between IT1 (and IT2) and the transformer target load line at the target power factor. Figure 6 shows the situation
where T1 is exporting reactive current, either to an adjacent transformer or into the network. The relay will operate
to bring the voltage to the correct level (as described previously) in such a way as to reduce the magnitude of the
reactive current. If, as for the previous example, two transformers are in parallel at the same site the circulating
current will flow into T2 which will also act to correct the voltage while at the same time reduce the circulating
current to a minimum.
Unlike the previously described circulating current method, the TAPP system does not require load information
from other transformers in order to minimise reactive current. As will be seen in the commissioning section of this
manual, the instrument display gives a reading of the network power factor which can be used to set the optimum
operating point of the relay.
Voltage control systems that use a conventional negative reactive circulating current method based on a 90°
VT/CT connection, are not accurate at typical system power factors. As the load on a transformer increases the
apparent measured voltage rises, causing the transformer to tap down, see Figure 7.
©2013 Siemens Protection Devices Limited
B
C
D
Figure 5
IT2
IT1
IT1circ
Figure 6
7SG15 MicroTAPP Description of Operation
Time
T1 measured voltage
System voltage
T2 measured voltage
Voltage
IT2circ
Load line at
Target PF
Chapter 1 Page 10 of 38