Abstract:
In Sri Lankan National Electricity Grid, severe voltage fluctuations have been noticed
in the Grid Substation auxiliary supply. The auxiliary supply is quantitatively 20CkVA,
but very important to keep in good health because it provides power to all substation
equipment installed in GSSs, such as battery charger units and power supply modules
for various types of protective monitoring and measuring instruments. Due to these
fluctuations, not only the protective equipment, but also the small power equipment
such as computers, Air conditioners and substation lights are also had frequent
failures. It is observed all these failures to the equipment and substation lights have
been experiencedjust after the tr pping of feeders due to an earth fault.//
The objective of the research is to analyze the problem of voltage fluctuations in the
auxiliary supply in Grid Substations and find the possible remedies. The studies
presented in this report are mainly focused on the Earthing Transformer, which provides
the power to the Auxiliary equipments. The earthing transformers in bulk of the GSSs are
a Zig-Zag wound transformer with an low voltage auxiliary winding.//
Unfortunately, there are no models for Zig-Zag transformer readily available for
simulation of the problem using computer software. Therefore the purpose of
simulation of the problem in this report is mainly based on a Prototype transformer,
which was wound specifically for the purpose.//
The possible cause to the voltage surges experienced in the auxiliary supply at an earth
fault is due to the extra flux in the grounding transformer limbs and the leakage flux of
primary windings directly influencing the auxiliary winding, resulting an extra voltage
induced in the auxiliary winding. The arrangement of the winding in each limb has made
a huge impact on the problem.//
The integration of two functions has created a negative impact on the performance of the
transformer. As a solution, firstly, this report suggest to split the two functions integrated
in the grounding transformer i e use separate transformer for grounding the 33kV side of
the GSS and an other separate transformer for auxiliary and control supply for the GSS.
Secondly, re-arrange the windings in such a way that the no resultant flux induces due
to the fault current injected into the neutral of the grounding transformer
