The coherency aggregation method has proved very successful in the determination of dynamic equivalents of sections of a power system, its main drawback being the extensive computation times required to recognise the coherent groups with full-time simulation. The paper demonstrates the advantages of using the new 'rate of change of kinetic energy' method for coherency recognition in relation to other existing methods. A fault is applied on the power system, an approximate critical clearance time is obtained and machine conditions at that stage are used for recognition. Studies in a large power system are reported. Equivalents are obtained and compared with those produced through full simulation and with the method of the singular points. The effect of fault location on the equivalents obtained is reported. The transient responses of the equivalents produced by the different methods are compared with those of the original system. List of symbols RKE = rate of change of kinetic energy P m . = mechanical power of the ith machine P e . = electrical power of the / th machine p = d/dt derivative CJ,-= machine speed 6,-= internal machine angle co 0 = synchronous speed n = number of machines [/] = vector of nodal currents [Y] = admittance matrix [V] = vector of nodal voltages li = complex value of injected current V t = complex value of nodal voltage M t = inertia a t = post fault acceleration of the ith machine