Method and apparatus for selective cancellation of subsynchronous resonance

An apparatus for the selective cancellation of harmonic frequencies of current of a multiphase dynamic stabilizer for a multiphase AC electrical network, comprising:

• plurality of multiphase dynamic stabilizer means for providing electrical stabilization to the electrical network;
• control means producing a control signal for controlling the operation of the stabilizer means on a periodic basis to produce the stabilization;
• and
• transformer means having a multiturn primary side and a multiturn secondary side electrically interconnected intermediate the phases of the electrical network and the phases of each of the stabilizer means via the primary side and secondary side, respectively, the turns ratios and the leakage reactances of the primary and secondary sides means rotating the phases of the selected harmonic frequencies of currents produced in each of the stabilizer means with respect to those of the other stabilizer means by about 30 degrees such that the selected harmonic frequencies are substantially cancelled in each of the stabilizer means.

An apparatus for the selective cancellation of harmonic frequencies of current of a three-phase dynamic stabilizer for a three-phase AC electrical network, comprising:

• first stabilizer means for dynamically compensating an electrical power system and having a plurality of variable reactance means connected in delta;
• second stabilizer means for dynamically compensating the electrical power system and having a plurality of variable reactance means connected in delta;
• each variable reactance means comprising a series combination of a pair of thyristors connected in reverse parallel and reactance, the phase angle of conduction of the thyristors determining the value of the reactance in the stabilizer means and producing harmonic currents of the orders 1, 5, 7, 11, 13 . . . n, where n is an odd integer not divisible by 3;
• transformer means for electrically interconnecting the first and second stabilizer means to the electrical network comprising:
• a plurality of multiturn primary windings electrically connected in delta forming a primary side, the primary side being electrically connected to the electrical network with each primary winding cooperating with a single phase thereof;
• a plurality of multiturn principal secondary windings electrically connected in wye forming a secondary side, the principal secondary windings being in corresponding phase relationship with the primary winding;
• two multiturn bifilar auxiliary secondary windings per principal secondary winding and having the same phase thereof;
• the free end of each principal secondary winding electrically connected to an end of one of the other two auxiliary secondary windings of the other two principal secondary windings with the free ends of each of the auxiliary secondary windings so connected being electrically connected to the phase of the first and second stabilizers corresponding to the phase of the principal secondary winding electrically connected to the other ends thereof;
• the electrically connected combination of principal secondary winding and auxiliary windings being such that each phase is present therein;
• and
• the ratio of the number of turns of the bifilar auxiliary secondary windings to the peripheral secondary winding being about ##EQU11## with the leakage reactance of the principal secondary winding being equal to about twice the common leakage reactance of the bifilar auxiliary secondary windings .+-.10% whereby the leakage reactance of the secondary side rotates the phase of the harmonic currents produced in each stabilizer means by an amount of about 30 fundamental electrical degrees causing the substantial cancellation of the harmonic currents of each stabilizer characterized by the series 5, 7, 17, 19 . . . n, p where n=5+z;
• p=7+z;
• and z=multiples of 12.

An apparatus for the selective cancellation of harmonic frequencies of current of a multiphase dynamic stabilizer for a multiphase AC electrical network, comprising:

• plurality of multiphase dynamic stabilizer means for providing electrical stabilization to the electrical network;
• control means producing a control signal for controlling the operation of the stabilizer means on a periodic basis to produce the stabilization;
• and
• transformer means electrically interconnected intermediate the phases of the electrical network and the phases of each of the stabilizer means for rotating the phases of the selected harmonic frequencies of currents produced in each of the stabilizer means with respect to those of the other stabilizer means such that the selected harmonic frequencies are substantially cancelled in each of the stabilizer means, comprising:
• a plurality of multiturn primary windings comprising a primary side, the primary side being electrically connected to the electrical network with each primary winding cooperating with a single phase thereof;
• a plurality of multiturn principal secondary windings forming a secondary side, the principal secondary windings being in corresponding phase relationship with the primary winding, one end of each principal secondary winding electrically connected in common;
• a plurality of multiturn auxiliary secondary windings each having an electrical phase relationship, with the auxiliary secondary windings and principal secondary windings having corresponding phases being positioned adjacent each other in the secondary side with the auxiliary windings so positioned being substantially bifilar, an auxiliary winding electrically interconnected intermediate the free end of the principal secondary windings and each phase of each stabilizer means utilized such that all the electrical phases of the multiphase system are present in each electrically interconnected combination of principal secondary winding and auxiliary secondary windings with the phases of the stabilizer means so connected corresponding to the phase of the principal secondary windings, the auxiliary secondary windings rotating the electrical phases of the currents therein with respect to each other and the phases of the electric network whereby substantial cancellation of selected harmonic frequencies of the dynamic stabilizer means occurs with the selection of cancelled frequencies being dependent upon the number of dynamic stabilizer means connected.