Abstract:
This dissertation investigates the feasibility of fixed tap or reduced tap transformers on the
Eskom Transmission system, thereby reducing transformer failures and reducing the cost of
new or replacement transformers.
The main analysis uses Optimal Power Flow (OPF) techniques based on a Siemens PSS/E
software platform and a minimum system loss operating point or objective function while
reducing tap movements and maintaining prudent system limits but allowing shunt VAr
devices to freely compensate for the reduced tapping. Various years and system conditions
are analysed.
The research shows that:
135 Transmission-Distribution transformers are suitable for changing to fixed tap
transformers
160 Transmission-Distribution transformers are suitable for changing to reduced tap
transformers
80 Transmission-Distribution transformers should be left as-is
A 5.7% reduction in system losses from 1118.4 MW to 1054.9 MW (63.5 MW) is
possible as a result of an optimised system
Finally it is shown that replacing some Transmission transformers with fixed-tap transformers
is a more cost effective solution.
The contribution of this research is two-fold:
1. to show that there is scope for Eskom System Operations to operate its system more
efficiently in terms of system losses
2. to show that an OPF methodology is a practical technique to limit transformer tap
movements and allow optimal VAr placement while minimising system losses
