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Modern power electronics are capable of regulating loads with bandwidths so high that they essentially enforce constant power on millisecond timescales and contribute to system-wide voltage instability problems. Active front-end control of such loads that implement a power buffer function has been shown to mitigate instability, but has relied on complicated hybrid control techniques. This paper proposes a geometric control surface that implements the power buffer function by coupling the input impedance to the stored energy and by altering the source and load dynamics. The surface is derived from optimal control theory where importance is placed on maintaining continuous input impedance and retaining as much local energy as possible. The optimal control is a tradeoff between the needs of the system and the needs of the load. This paper introduces a geometric control surface based on a change of variables that simply and effectively implements a power buffer function. The formulation and implementation of the optimal surface are presented, in addition to experimental validation of the new power buffer control law.