Harmonic filtering circuit with special transformer

A filter to remove harmonics from distorted, three-phase alternating current applied to the filter, the filter comprising:

• (a) an autotransformer comprising a core of ferromagnetic material, the core including:
• (i) a first set of laminations with three parallel legs comprising a central leg and first and second side legs on opposite sides of the central leg, and a first bar portion magnetically joined to one end of each of the legs, and
• (ii) a second set of laminations comprising a bar portion magnetically linked to the other end of each of the legs, whereby each of the legs forms a winding leg for one phase of the current and, together with the bar portions, generally low-reluctance flux circuit means;
• (b) a plurality of first coils, each at a corresponding first location on a respective one of the legs, each of the first coils having respective first and second terminals;
• (c) a plurality of second coils, each at a corresponding second location on a respective one of the legs, the second location on each of the legs being spaced from the first location on that leg, each of the second coils comprising respective first and second terminals, the second terminal of the first coil on each leg being connected to the first terminal of the second coil on the same leg, whereby each pair of first and second coils on a respective leg forms part of a respective series input circuit to receive one phase of the three-phase alternating current, the second terminal of each of the second coils being connected together, whereby the second coils are wye-connected;
• (d) a region of increased reluctance in a corresponding part of each of the legs linked to the respective second coil on that leg to create a respective built-in inductor in each of the second coils;
• (e) first and second magnetic shunt assemblies, one end of the first shunt assembly extending from a location between the first and second coils on the first side leg to a location between the first and second coils on the center leg, and the second shunt assembly extending from a location between the first and second coils on the center leg to a location between the first and second coils on the second side leg to a location between the first and second coils on the center leg whereby some of the flux generated in each of the first coils by the respective phase current flowing in that first coil is by-passed through the shunt assemblies to the side legs without passing through the respective second coil on the same leg;
• (f) three capacitors, each connected in parallel with a respective one of the second coils to resonate with the built-in inductor of that coil at the fundamental frequency;
• and
• (g) three output terminals, the respective first terminal of each of the second coils being connected to a respective one of the output terminals.

A filter to remove harmonics comprising:

• (a) an autotransformer with a ferromagnetic core having a plurality of legs forming a low-reluctance flux path, a first bar forming the low-reluctance flux path between the legs at one end of the core, and a second bar forming a low-reluctance flux path between the legs at the other end of the core;
• (b) first and second coils, having first and second terminals, the first and second coils being spaced apart at a first location on one of the legs;
• (c) an electrical connection between the second terminal of the first coil and the first terminal of the second coil, whereby the first and second coils are in a series input circuit to receive the alternating current to be filtered, said current generating flux in the first leg;
• (d) a filter output terminal;
• (e) means to connect the electrical connection to the output terminal;
• (f) a region of increased reluctance in a part of the flux path linking the second coil, thereby forming a built-in inductor in the second coil;
• (g) magnetic shunt means which magnetically couples one of the legs at a third location between the first and second locations to an adjacent one of the legs, where some of the flux generated by alternating current flowing in the first coil is by-passed through the shunt means to the adjacent leg without linking with the second coil;
• and
• (h) capacitor means connected across the second coil to resonate with the built-in inductor at the fundamental frequency.