7SG15 MicroTAPP Description of Operation
©2013 Siemens Protection Devices Limited
Chapter 1 Page 8 of 38
The following standard system conditions are catered for with minimal or no adjustment to the MicroTAPP: -
1 Where a transformer is in parallel with other transformers, either within a site or across a network, when set
to TAPP mode, the relay operates in order to: -
•
maintain the system voltage at the correct level
•
operate at a tap position where minimal reactive circulating current flows from or into any system
transformer which is a part of the network
2 In the event of a failure of communications either between grouped transformers or from a remote control
centre, the relay operates in a stand-alone mode until the fault is rectified
3
If a transformer in a group is switched IN, no significant change in voltage will occur
4 If a transformer in a group is switched OUT by use of the ‘prepare to switch out’ function, no significant
change in voltage will occur
5 The Load Drop Compensation (LDC) method maintains the voltage at the correct level regardless of the
number of transformers connected to a common busbar
6 Settings applicable to different network or busbar running arrangements can be applied to each relay and
implemented by a single instruction (either from a remote source or locally) or plant status change (operation
of a bus-section CB for instance)
7 Each relay independently protects against incorrect operation which would allow abnormal voltages to be
applied to the network
Up to 16 transformers operating in parallel can be controlled as a group.
4.2 Transformers in Parallel
With traditional schemes where 2 or more transformers are connected in parallel either within the same site or
across a network, a reactive circulating current will flow between them unless the following conditions are met: -
•
The transformers are identical
•
The transformers have the same number of taps and tapping interval
•
The transformers are always on the same tap position
•
The transformers have the same impedance
•
The transformers are fed from the same primary source or, more correctly, have the same voltage applied to
the primary winding connections
These conditions put constraints on power system design, which are eliminated by the MicroTAPP voltage control
system that is designed to detect reactive current and bias the relay target voltage in such a way that the
circulating current is reduced to a minimum. Two methods of control are provided:
1
The TAPP system which uses an enhanced negative reactive circulating current principle.
2 Detection of circulating current between transformers connected to the same busbar but not through a
network.
For the purpose of explanation the circulating current method is described first, the most widely used and
preferred system, however, is the TAPP method which allows for transformers to be operated in parallel at any
point in a network, i.e. operate groups of transformers at different substations in parallel.
4.2.1 Circulating Current Control
As described previously the MicroTAPP uses a communication system for the transfer of load information to other
relays thus allowing each relay to determine the summed site load and power factor (
I load
) shown in Figure 4.
