7SG15 MicroTAPP Description of Operation
©2013 Siemens Protection Devices Limited
Chapter 1 Page 14 of 38
When the load current is removed from the transformer to be switched out, the remaining relays return to normal
tap change control. If the load current is not removed, after a period of time the relays reset to normal operation.
The switch-out command can be initiated either by a SCADA signal, from a PC via a communications network or
from a hard wired local control switch.
4.7 Transformer Switch in
Normally when a second transformer of a pair is switched into service the busbar voltage will increase. If the
transformers have a high impedance and the load is high, the voltage increase can be significant. The MicroTAPP
relay can avoid this increase by tapping the unloaded transformer prior to it being loaded.
The transformer should be energised by closing the HV CB and waiting for the unloaded transformer to complete
any tap changes. The MicroTAPP relay will immediately operate the tap changer of the unloaded transformer to a
position whereby the open circuit terminal voltage is equal to the busbar voltage. Once the NORMAL LED is
illuminated the transformer can be energised by closing the LV CB.
The MicroTAPP controlling the loaded transformer sends load data to the unloaded transformer via the MPPC.
This allows the unloaded transformer to be tapped to the desired voltage target prior to the LV CB closing. It also
allows any LDC compensation to be included into the tap change control.
When the circuit breaker is closed, there will be no significant change in busbar voltage and the relays will then
tap to minimise circulating current in the normal way. The advantage of this control switching sequence is the
customer does not experience a large voltage fluctuation whilst switching transformers in and out of service.
4.8 Relay
Settings
Settings applicable to a particular site can be applied to the relay either locally from the relay display, a PC via the
relay fascia serial port, or remotely over a communications link via the rear mounted fibre optic connections.
8 groups of settings can be stored by the relay, at any time only one group is used by the relay for control. When
the relay is energised, it will operate with the settings group that was last applied.
The MicroTAPP is designed to function as an integral control device within the Transformer/Site configuration, in
this respect information relating to the installation is used by the relay as follows: -
Site data
- number of transformers forming a group etc.
Transformer Data
- rating, impedance, VT and CT details etc.
Tap change
- number of steps, type etc.
Network data
- Power Factor, system voltage, group capacity etc.
Voltage control -
Basic, band, LDC etc.
4.8.1 Basic Set-point
The basic setting determines the operational target voltage for the relay with the transformer at no load. If LDC is
not used the target voltage will be the basic setting.
4.8.2 Normal voltage deadband
A deadband setting with a range that will enable the voltage to be controlled within satisfactory limits for a
practical number of tap changing operations is provided.
If the voltage fluctuates about the deadband, a corrective tap change operation will take place if the average
voltage level deviates from the relay setting.
4.8.3 Load Drop Compensation (LDC)
The relay LDC corrective effect is based on system group capacity at the system power factor. Use of the LDC
setting is discussed in the applications section of this manual.
