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Whether the hybrid active power filter with injection circuit (IHAPF) gets desirable effect of harmonic suppression mainly depends on the detecting method. For it is a dynamic harmonic filtering equipment, it has high request in system's instantaneous. The delay and convenience on the performance of IHAPF is focused on and an improved method with simple integration and delay and gain to substitute the low pass filter of conventional ip-iq method is proposed. This method can reduce the detecting delay to 1/6 of source period and cost. This method also can be applied to a single-phase circuit, three-phase four-wire circuit and the unbalanced three-phase load. On the other hand, an improved synchronous-reference-frame method for detecting currents with reactive power and harmonics is proposed. By synchronizing the positive sequence vector of the grid voltage, this improved synchronous reference frame method is very simple to implement without the phase locked loop and omit complex calculations for sinusoidal functions. Experimental results verify the perfect filtering performance of the shunt IHAPF with the novel detecting method.