Compatibility summary
You will need a travel adapter in São Tomé and Príncipe; plug types used there differ from Singapore.
Plug comparison
Singapore plug types: G
São Tomé and Príncipe plug types: C F
There is no common plug type between Singapore (G) and São Tomé and Príncipe (C F). You need a travel adapter.
Voltage comparison
Singapore: 230V, 50Hz
São Tomé and Príncipe: 230V, 50Hz
Voltage difference is within a range many devices tolerate; check your appliance label.