The aggressive penetration of Hybrid Renewable Energy Sources (HRES) into weak transmission network creates challenges in voltage regulation and transient stability. This paper examines the impact of high penetration of photovoltaic (PV) and wind energy on the voltage stability and transient response of the Nigerian 330-kV, 34-bus transmission network. Newton Raphson power flow and Continuation Power Flow (CPF) methods are used to find the voltage collapse proximity and loadability limits for determining the static stability. For the dynamic analyses, time domain simulations are used to assess rotor angle and speed deviations following critical three phase faults. Further, the performance of Flexible AC Transmission Systems (FACTS) devices, namely the Thyristor-Controlled Series Capacitor (TCSC) and Static Synchronous Compensator (STATCOM) in reducing the instability is comparatively analysed. The results show that the base network experiences severe voltage deficiencies at the remote busses (Gombe: 0.907 p.u., Jos: 0.949 p.u.) and is vulnerable to cascading failures under contingencies. The inclusion of HRES makes the instability worse and the critical clearing times are violated. However, the best position of STATCOM shows a better performance than TCSC in restoring the Gombe bus voltage from 0.907 p.u. to 1.049 p.u. and maintaining the transient stability margins. These findings provide a framework for grid operators to reinforce weak networks undergoing renewable energy transition.

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