Voltage Transformers - Siemens 7sg15 microtapp Operation Manual

MicroTAPP
When the compensator is switched in, reactive current will flow through the LDC CT in the 'wrong direction'
causing the apparent power factor to change. If reactive circulating current control is used for control of parallel
transformers, the change in power factor will tend to make T2 operate at a higher voltage. A solution is to arrange
for a group setting change with a new load power factor setting to be initiated automatically by closure of the
compensator bank switch as shown in Figure 9. Obviously the original settings will be automatically re-applied
when the switch is opened.

7 Voltage Transformers

7.1 Operational Considerations
The preferred location of the VT is at the transformer secondary connections.
In the case of a busbar connected VT it is important to disable MicroTAPP whenever the transformer secondary
circuit breaker is open. If this precaution is not taken the regulating relay will be allowed to control a tap-changer
without being able to measure its output voltage. Thus if the voltage is out of band (or drifts out of band) an
unstable situation will arise caused by ineffective raise or lower instructions being issued by the relay to its tap-
changer. The tap-changer will soon arrive at either its upper or lower limit producing an abnormal voltage.
Disabling is achieved by means of an auxiliary switch in the transformer's lower voltage circuit breaker which is
arranged to disconnect the MicroTAPP 'enable' input or by applying a signal to implement a 'Tap change block'
command to the relay.
For more complex substations further auxiliary switches may be considered to be necessary in the bus-section
and/or bus coupler circuit breakers in order to ensure that all possible operating conditions are catered for.
In common with most other voltage control schemes the MicroTAPP system can operate with either bus-bar or
transformer connected VTs but the operational restrictions imposed by bus-bar VTs still apply and the ability of
MicroTAPP to automatically match secondary voltages before putting transformers back on load (thus avoiding
sudden step changes) cannot be utilised.
One significant advantage of the MicroTAPP system is that if an idle transformer is inadvertently restored to
service on an unsuitable tap position then the inherent coupling features of MicroTAPP relays together with their
"fast tap down" feature will quickly restore all transformers at the site to the optimum tap-positions. This applies to
both types of VT connection but in particular to bus-bar VT schemes.
The VT used for measurement of voltage need not be related exactly to the transformer nominal output. The
actual value is entered in the 'system settings/transformer' menu together with the transformer details, from which
the control algorithms determine the actual voltage value.
©2011 Siemens Protection Devices Limited
Status Input
MicroTAPP
1
Summed
CTs
IC1
IC1
Load
Figure 9
7SG15 MicroTAPP Application Guide
2
I
IC2 C1+2
IC1
IC1
CB aux. switch
Chapter 5 Page 12 of 21