index may be expressed in terms of active reactive and apparent power and may be used as an adaptive triggering criterion for further control actions the mbvsmc provides a means such as a computer volt and current meters and any other suitable equipment or method for measuring current and voltage phasors at boundary buses of a load center area by using an equivalent method to aggregate a total load of the load center area to a fictitious bus and estimating a thevenin equivalent to represent the external power system the mbvsmc then calculates the vsm index and compares the vsm with a pre set threshold to decide whether to take the control actions it should be appreciated that a control system computer relay and any other suitable devices may be used to monitor the system perform calculations and perform control actions referring to fig1 an entire power system 10 may be decomposed into two parts a load center 11 and an external power system 12 power is transferred from the external power system 12 to the load center 11 through boundary buses 13 the power transfer from the external power system 12 to the load center 11 may be calculated using a measured current and or voltage phasor at the boundary buses 13 for purposes of the calculations below the general boundary bus number is denoted by u201c i u201d the power transfer.