This study presents a 30-bus, 330 kV Nigerian transmission network and applies five network improvement methods to assess transmission expansion planning with loss reduction and congestion relief as the objective functions. The base-case network indicated six topologically weak transmission lines, six overloaded lines and six buses having voltage magnitudes below the statutory ±5% limit with active and reactive power losses of 219.08 MW and 409.83 MVAr respectively. To evaluate the reduction in network loss and the relief from congestion, five network compensator methods (two shunt and three series) were applied to the base-case network independently. The Shunt compensation involved static VAR compensators (SVCs) and shunt capacitors, each installed separately at the weak buses. The series compensator included a fixed series capacitor, thyristor-controlled series compensation (TCSC) and duplicate line reinforcement on the weak transmission lines. The weak bus voltage magnitudes were normalised using each of the five methods considered, but the results show varying power loss and varying congestion reductions relative to the base-case network. The congestion was completely eliminated by installing duplicate lines along both the weak and overloaded transmission lines. Active and reactive power loss reductions are: 185.46 MW, 96.61 MVAr (SVC), 189.183 MW, 130.087 MVAr (Shunt capacitor), 178.183 MW, -383.087 MVAr for series capacitor, 174.06 MW, -395.932 MVAr (TCSC) and 166.144 MW, -269.134 MVAr (duplicate on weak lines) and 109.67 MW, -734.28 MVAr (duplicate on both weak and overloaded lines). Although the SVC and shunt capacitor improved the voltage profile, overloaded lines still persist. These results show the limits of the shunt compensator (SVC and shunt capacitor) in removing congestion. However, series compensators installed on weak transmission lines provided significant relief from congestion, especially on the overloaded lines. The complete elimination of congestion by duplicate circuits on the overloaded lines was observed. The findings support evidence-based selection of upgrade options for transmission expansion planning.