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In this paper, a modified signal energy method called the envelope method will first be proposed to estimate the transient stability limit, and then a parallel envelope method is developed to speed up the limit search for online applications. A new weighting limit estimation formula is used for improving the estimation accuracy and minimizing of the effects of observing nodes of the original signal energy method. The effectiveness and the computation speed of the proposed method have been fully evaluated on a 20 machine 100 bus UK system using a 4 CPU SMP workstation. Simulation results show that the envelope method could achieve better accuracy than the original signal energy method and its very fast timing measurements show that even the serial envelope method could have faster computation speed than the parallel time-domain-simulation method, and the parallel envelope method is the fastest and is suitable for online security analysis and control applications.