This paper uses an energy based security measure to define vulnerability to voltage instability, and proposes a method of security enhancement by examining the sensitivity of the security margin to various control actions. The use of the energy function in systems with significant line losses is reviewed, and the analytic expressions for first order total derivatives with respect to relevant control parameters derived. Computational properties, such as sparsity of various vector and matrix quantities, are examined. Numerical examples of security enhancement based on these sensitivities are computed for a thirty bus example system. Potential application of this approach within a security constrained optimal power flow is also discussed. I(EY WORDS: Voltage stability, voltage collapse, security enhancement, energy functions, optimal power flow.