In this research work, design and implementation of a 154 kV, +/- 50 MVAr Transmission STATCOM (T-STATCOM) has been carried out primarily for the purposes of reactive power compensation and terminal voltage regulation, and secondarily for power system stability. The implemented T-STATCOM consists of five 10.5 kV, +/- 12 MVAr Cascaded Multilevel Converter (CMC) modules operating in parallel. The power stage of each CMC is composed of five series connected H-Bridges (HB) in each phase, thus resulting in 21-level line-to-line voltages. Due to modularity and flexibility of implemented HBs, a CMC module power density of 250kVAr/m(3) is reached, thus making the mobility of the system implementable. DC link capacitor voltages of HBs are perfectly balanced by means of the Modified Selective Swapping Algorithm proposed. The field tests carried out at full load in the 154 kV transformer substation where T-STATCOM is installed and put into service have shown that the steady-state and transient responses of the system are quite satisfactory.