Siemens 7sg15 microtapp Operation Manual page 69

7SG15 MicroTAPP Application Guide
4.1 TAPP Method – System Power Factor Setting
The TAPP method uses the "System Power Factor" setting applied to the relay with which to calculate the
circulating current compensation. This setting is also used to normalise the line drop compensation applied, so
that it is not affected by any circulating current and only relates to the load power factor.
Each MicroTAPP relay can monitor the state of the KANBUS link. The "Circulating current" and "line drop
compensation" features both rely upon its operation to function correctly. An alarm can be given if a station in the
group is lost when the MPPC Failure Detection set to "Enabled".
In addition the MicroTAPP has a logic feature that will automatically switch all relays running as a group in
circulating current mode to the TAPP method, if the loss of one of the relay stations on the MPPC KANBUS link
between the relays is detected.
Therefore it is imperative that the setting applied to the relay accurately reflects the load power factor, irrespective
of whether TAPP or circulating current is being used.
Ideally the power factor should be set identically on all transformers that normally supply a particular busbar load.
The relay default power factor is 0.97 lagging and can be used initially, before the setting is refined to its'
operational value.
To find the setting to use, the relays should switched into AUTO voltage control mode and left to tap to the
NORMAL steady state. Once the relay(s) have reached the deadband the power factors can be read from the
relay Instruments display. The deadband is reached when the only the NORMAL green coloured LED is
illuminated.
When using the TAPP method is important that the System Power Factor is set accurately to reflect the true load
power factor. If there is a large error between these two parameters then the system voltage will also have an
error.
Setting Example
Two transformers running in parallel at a single substation supply a common group load. The relays, tap changer
and scheme are now fully commissioned and all relay Instruments are at expected values. The relays are
switched to AUTO and the left to settle into their voltage deadbands. The Power Factor Instrument is now
selected using the fascia keypad. The relay Power Factor and Group Load in MVA are recorded from both relays
over a period of time. The results are examined to find where the peak load occurred. The "System Power Factor"
setting applied should reflect the power factor when the load is near or at its steady peak level. The substation
load will vary during the normal daily load cycle so these values may need to be recorded for some time before
the point is reached where the load is at its maximum value.
In this example, one relay displays a power factor of 0.92 lagging and the other displays 0.94 lagging, at the point
at which the load is near its recorded peak. The average of these two values is 0.93 lagging. Both relays should
now be set with a System Power Factor of 0.93 lagging.
For loads normally supplied from two parallel substations the power factors from all transformers paralleled
should be averaged when the peak load occurs. All relays should than be set to this average value. The system
circulating current will then be minimised. Minimising the circulating current allows MVar demand to be minimised
and hence power system losses reduced.
NOTE: It is important to set the System Power Factor setting when the substation load is high, as this is when the
compensation for circulating current and Line Drop Compensation (LDC) have maximum effect. In TAPP mode, if
a significant difference exists between the true load power factor and the MicroTAPP "System Power Factor"
setting, a system voltage error will occur. Therefore it is important to find the true System Power Factor and set
the relay accordingly when the system is heavily loaded.
4.2 Independent Single Transformers
At a single transformer site a MicroTAPP relay is arranged as shown in Figure 1. Connections are made to the VT
for voltage measurement and to the CT for LDC and control of circulating current when the transformer is
operated in parallel with other transformers at remote sites. The TAPP method should be selected if it is possible
to parallel substations together, albeit as a temporary measure. If a single transformer substation is purely radial
with no possibility of parallel operation with another substation then circulating current can be selected.
©2011 Siemens Protection Devices Limited Chapter 5 Page 5 of 21