7SG15 MicroTAPP Application Guide
The actual level of LDC for each of the two bus-bar groups will be proportional to the load on each of the
respective bus-bars and these levels will be maintained (by virtue of the MPPC) at the correct level even if one
transformer is taken out of service in either of the groups.
If the two groups are now interconnected either by closing the bus-coupler or operating the transformers in
different system groups then the resulting levels of basic voltage and LDC will be a compromise based on the
average settings and loading of each transformer in the new group.
If a single setting is not found to be satisfactory for all busbar configurations, alternate setting groups for different
set-point levels and LDC etc. can be implemented from bus coupling CB auxiliary switch operation or from a
SCADA signal.
4.4 Parallel networks
The MicroTAPP system uses a modified negative reactance design for the detection of circulating current. When
selected for TAPP operation (modified negative reactance circulating current mode), the relay operates to
minimise circulating current between transformers at the same site and also when transformers are operated in
parallel across networks.
For optimum performance, an accurate normal network power factor must be entered as the “System Power
Factor’ setting. Switching to alternative Setting Groups can be used to match step changes system power factor
due to the reactive compensation switching in or out of service. There are eight setting groups so it is possible to
adjust the “System Power Factor” to eight distinct values.
4.5 Embedded Generation
When generation is embedded in a network, where tap changing transformers are used, consideration must be
given to the method of voltage control employed. The choice of voltage control is influenced by the type of
generator, the generator rating and the relative capacity of the local network. Figure 3 shows a typical
arrangement.
MicroTAPP MicroTAPP
Load
12
G
Figure 3
©2011 Siemens Protection Devices Limited Chapter 5 Page 7 of 21
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