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On 24th September, 2006, a severe disturbance occurred on the National Grid System of Pakistan which caused cascading outages of transmission lines and generating stations that ultimately led to system wide collapse/blackout. This major blackout was one of the worst power system failures of WAPDA, the main electric utility of Pakistan. This paper provides an insight of the investigations to identify system constraints that caused blackout The bulk power transfer on long 500 kV transmission lines in northern part of WAPDA, prior to blackout, had already loaded them close to their stability limits. As a result, the system, after a disturbance, was stressed further beyond its capability and experienced small signal and voltage instabilities, which led to its collapse. In this paper, detailed analytical work including steady state and dynamic simulations/analyses have been carried out to study the behaviour of VVAPDA power system during collapse. Later, the effectiveness of the power system controllers, i.e., power system stabilizers (PSS) and flexible AC transmission system (FACTS) has been demonstrated as remedial measures to prevent such major system blackout.