A power converter configured to inject or absorb reactive power for a three-phase AC system, the power converter comprising: a power-source circuit block in each phase, comprising a plurality of unitary converter blocks connected in series, wherein each unitary converter block comprises two legs and a capacitor, each leg comprising two self-extinguishing switching elements connected in series, and the two legs and the capacitor connected in parallel, the unitary converter block converting DC into AC; a delta connection block including the power-source circuit blocks in the respective phases in delta connection; and an isolation transformer between the three-phase AC system and the delta connection block, the isolation transformer comprising a winding inserted on a circuit of the delta connection, wherein: the power-source circuit block in each phase comprises a positive arm (4Up, 4Vp, 4Wp) and a negative arm (4Un, 4Vn, 4Wn) each comprising the plurality of unitary converter blocks connected in series; the isolation transformer comprises a primary winding at a three-phase AC system side, a secondary winding, and a tertiary winding; the secondary winding and the tertiary winding both in a same phase are connected in series and are inserted in the delta connection block between the positive arm and the negative arm; characterized in that the positive arm has a first end connected with a positive side of the secondary winding of the three-winding isolation transformer; the negative arm has a first end connected with a positive side of the tertiary winding of the three-winding isolation transformer; the positive arm has a second end connected with a second end provided in the negative arm of the other phase; a negative side of the secondary winding and a negative side of the tertiary winding are connected with each other in each phase and among the three phases; and the primary winding of the three-winding isolation transformer has a positive side connected with the AC system, and has a negative side connected with each other among the three phases. a power-source circuit block in each phase, comprising a plurality of unitary converter blocks connected in series, wherein each unitary converter block comprises two legs and a capacitor, each leg comprising two self-extinguishing switching elements connected in series, and the two legs and the capacitor connected in parallel, the unitary converter block converting DC into AC; a delta connection block including the power-source circuit blocks in the respective phases in delta connection; and an isolation transformer between the three-phase AC system and the delta connection block, the isolation transformer comprising a winding inserted on a circuit of the delta connection, wherein: the power-source circuit block in each phase comprises a positive arm (4Up, 4Vp, 4Wp) and a negative arm (4Un, 4Vn, 4Wn) each comprising the plurality of unitary converter blocks connected in series; the isolation transformer comprises a primary winding at a three-phase AC system side, a secondary winding, and a tertiary winding; the secondary winding and the tertiary winding both in a same phase are connected in series and are inserted in the delta connection block between the positive arm and the negative arm; characterized in that the positive arm has a first end connected with a positive side of the secondary winding of the three-winding isolation transformer; the negative arm has a first end connected with a positive side of the tertiary winding of the three-winding isolation transformer; the positive arm has a second end connected with a second end provided in the negative arm of the other phase; a negative side of the secondary winding and a negative side of the tertiary winding are connected with each other in each phase and among the three phases; and the primary winding of the three-winding isolation transformer has a positive side connected with the AC system, and has a negative side connected with each other among the three phases. the power-source circuit block in each phase comprises a positive arm (4Up, 4Vp, 4Wp) and a negative arm (4Un, 4Vn, 4Wn) each comprising the plurality of unitary converter blocks connected in series; the isolation transformer comprises a primary winding at a three-phase AC system side, a secondary winding, and a tertiary winding; the secondary winding and the tertiary winding both in a same phase are connected in series and are inserted in the delta connection block between the positive arm and the negative arm; characterized in that the positive arm has a first end connected with a positive side of the secondary winding of the three-winding isolation transformer; the negative arm has a first end connected with a positive side of the tertiary winding of the three-winding isolation transformer; the positive arm has a second end connected with a second end provided in the negative arm of the other phase; a negative side of the secondary winding and a negative side of the tertiary winding are connected with each other in each phase and among the three phases; and the primary winding of the three-winding isolation transformer has a positive side connected with the AC system, and has a negative side connected with each other among the three phases.