Abstract: A novel variable reactor based on transformer is proposed.In the variable reactor,a transformer is chosen,the primary winding voltage is detected and functions as a reference signal.A voltage source inverter is adopted to generate an adjustable voltage,which is applied across the secondary winding of the transformer.When the secondary controlled voltage is proportional to the primary winding voltage,the equivalent impedance of primary winding of the transformer will change continuously.The novel variable reactor can be applied to an active power filter.When the variable reactor is connected in series between the power utility and harmonic source,check the harmonic voltage of the primary winding and a voltage source inverter is used to follow so as to generate a voltage,which is applied across the secondary winding of the transformer.When the applied harmonic voltage and the fundamental current component of the power utility satisfy a definite relation,the series transformer will appear as a very low impedance to the fundamental harmonic and high impedance to harmonics.The novel series active power filter features low capacity,excellent filtering characteristics and easier strategy.The experimental results from two types of harmonic sources have verified the validity of the novel series active power filter.