Combined transformer based on multi-winding transformer and converter and control method thereof

Abstract

The application discloses a combined transformer based on a multi-winding transformer and a converter and a control method thereof, and relates to the field of transformers.The combined transformer comprises a single-phase circuit topology and a three-phase circuit topology, wherein the three-phase circuit topology is combined on the basis of the single-phase circuit topology; the single-phase circuit topology comprises an electromagnetic multi-winding transformer, an AC / AC power electronic converter and a bypass switch K.Each phase of the electromagnetic multi-winding transformer is composed of a set of high-voltage windings W HV and two sets of low-voltage windings W lv1 and W lv2 which are wound on the same core.In the application, the output voltage and capacity of the converter are only 1 / (n+1)(n>1) of the output voltage and capacity of the combined transformer, so that the capacity and cost of the converter are reduced.The output voltage u2 of the converter is adjusted to realize the adjustment of the supply voltage u an of the load, and the input current i x of the converter is adjusted to realize harmonic control and reactive power compensation.

Description

Technical Field

[0001] This invention relates to the field of transformers, and in particular to a low-cost, high-reliability combined transformer based on a multi-winding transformer and a converter, which has voltage regulation, harmonic mitigation, and reactive power compensation capabilities, as well as its control method. Background Technology

[0002] Power transformers are among the most important electrical devices in power systems. They function as interconnectors between power grids of different voltage levels and facilitate power exchange. Their safe and stable operation directly impacts the safety and stability of the power system. With the large-scale integration of new energy sources and the increase in high-precision and sensitive loads, higher demands are being placed on the quality of power supply. While traditional transformers offer advantages such as low cost and high reliability, they cannot meet the requirements for voltage regulation, harmonic mitigation, and reactive power compensation. Therefore, there is an urgent need to research low-cost, high-reliability power transformers with multiple functions, including rapid voltage regulation.

[0003] Power electronic transformers combine power electronic converters with high-frequency transformers. Besides possessing the basic functions of traditional power transformers—voltage transformation, electrical isolation, and energy transfer—they also offer active voltage control, reactive power compensation, and harmonic mitigation. However, current power electronic devices are limited by the level of power semiconductor devices, resulting in high cost and large size. They also struggle to operate continuously under power semiconductor device failure, restricting their widespread application in high-voltage, high-capacity applications. On-load tap-changing transformers use tap adjustment for voltage regulation, which cannot achieve rapid and smooth voltage regulation. Therefore, they cannot address voltage dips or perform harmonic mitigation and reactive power compensation.

[0004] To overcome the problems of existing technologies, patent CN201711335407.X proposes a three-stage power electronic transformer with AC input and AC output, which can reduce the number of conversion stages and power semiconductor devices in the power electronic transformer, thereby reducing costs. However, due to the complexity of the modulation technology, the power conversion efficiency is relatively low. Patent CN202010443606.8 proposes a capacitor-isolated power electronic transformer, which can reduce the number of high-frequency transformers, but it is difficult to apply to high voltage ratio applications. Patent CN201910634405.3 proposes an on-load tap-changing circuit for an interleaved parallel multi-stage transformer, which can reduce the switching frequency of the on / off switch, increase the service life of the corresponding tap-changing switch, and eliminate the need for skip-stage tap-changing. However, on-load tap-changing transformers can only adjust in fixed steps, resulting in insufficient waveform transition during voltage regulation, making precise voltage regulation impossible, and they also lack reactive power compensation and harmonic mitigation capabilities. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing power electronic transformers, such as high cost, inability to quickly and smoothly regulate voltage on load, and limited functionality. It proposes a combined transformer based on a multi-winding transformer and a converter, and its control method.

[0006] The technical solution adopted in this invention is:

[0007] A combined transformer based on a multi-winding transformer and a converter is characterized by comprising a single-phase circuit topology and a three-phase circuit topology, wherein the three-phase circuit topology is formed by combining the single-phase circuit topology; the single-phase circuit topology includes: an electromagnetic multi-winding transformer, an AC / AC power electronic converter, and a bypass switch K; each phase of the electromagnetic multi-winding transformer consists of a set of high-voltage windings W wound on the same iron core. HV and two sets of low-voltage windings W lv1 W lv2 Composition, low-voltage winding W lv2 The number of turns is for the low-voltage winding W lv1 1 / n, where n>1; the AC / AC power electronic converter input and low voltage winding W lv2 The AC / AC power electronic converter output is connected in parallel with the bypass switch K, and the AC / AC power electronic converter output is connected in parallel with the low-voltage winding W. lv1 The load is supplied with power in series, and the voltage u of the load is... an and the output voltage u2 of the AC / AC power electronic converter, the low-voltage winding W lv1 The low-voltage output voltage u1 satisfies u an＝ u1+u2.

[0008] Furthermore, the three-phase circuit topology includes four combinations based on the single-phase circuit topology: high-voltage side delta connection / low-voltage side star connection, high-voltage side delta connection / low-voltage side delta connection, high-voltage side star connection / low-voltage side star connection, and high-voltage side star connection / low-voltage side delta connection.

[0009] Furthermore, the three-phase circuit topology uses three single-phase transformers or a three-phase common-core transformer.

[0010] Furthermore, the AC / AC power electronic converter includes an AC / DC rectifier, a DC / AC inverter, and a DC capacitor C. dc The AC / DC rectifier includes power semiconductor switching devices S1, S2, S3, S4 and a filter inductor L. i The DC / AC inverter includes power semiconductor switching devices S5, S6, S7, S8 and a filter inductor L. f and AC filter capacitor C fThe specific connection method is as follows: the drains of power semiconductor switching devices S1, S3, S5, and S7 are connected to a DC capacitor C. dc The positive terminal of the power semiconductor switching devices S2, S4, S6, and S8 is connected to the DC capacitor C. dc The negative terminal; the source and drain of the power semiconductor switching device S1 are connected to the input filter inductor L. i Filter inductor L i The other end is connected to output terminal x; the source of power semiconductor switching device S3 and the drain of S4 are connected to AC output terminal y; the source of power semiconductor switching device S5 and the drain of S6 are connected to output filter inductor L. f Filter inductor L f The other end is connected to the output terminal z; the source and drain of the power semiconductor switching device S7 and S8 are connected to the AC output terminal t; the AC filter capacitor C f One end is connected to AC output terminal z, and the other end is connected to AC output terminal t.

[0011] The control method for a combined transformer based on a multi-winding transformer and a converter is characterized by the following steps:

[0012] (1) Inverter control method in AC / AC power electronic converter:

[0013] During normal operation, the reference voltage u at the output terminal of the combined transformer of multi-winding transformer and converter is... an_ref The difference between the output voltage u1 of the electromagnetic multi-winding transformer and the reference voltage u of the AC / AC power electronic converter inverter is used as the reference voltage u. 2_ref The inverter output voltage control adopts a dual closed-loop control strategy of voltage and current, with reference voltage u. 2_ref The difference between the actual output voltage u2 of the AC / AC power electronic converter and the inductor current inner loop reference value i is generated by the first proportional resonant controller PR. L_ref Filter inductor L f Current inner loop reference value i L_ref With filter inductor L f Actual current value i L The difference is used by the first proportional-integral controller (PI) to generate the inverter reference voltage u. c_ref Inverter reference voltage u c_ref The first PWM modulation generates drive signals for power semiconductor devices S5, S6, S7, and S8.

[0014] (2) Rectifier control method in AC / AC power electronic converter:

[0015] During normal operation, the AC / AC power electronic converter rectifier adopts a dual closed-loop control strategy for voltage and current, with the DC capacitor reference voltage u... Cdc_refWith actual voltage u Cdc The difference is processed by the second proportional-integral controller (PI) to generate a portion of the current inner loop reference value, i. dc At the same time, through the load current i load Extracting harmonic current components i h and reactive current component i q i dc -(i h+ i q (i) is the inner loop current reference value. x_ref ; Current inner loop reference value i x_ref With the actual value of current i x The difference is used by the second proportional resonant controller PR to generate the rectifier reference voltage u. r_ref rectifier reference voltage u r_ref The second PWM modulation generates drive signals for power semiconductor devices S1, S2, S3, and S4.

[0016] Furthermore, when the AC / AC power electronic converter fails, the bypass switch K is closed to bypass the converter, thus ensuring continuous power supply to the load; when the power grid experiences a short circuit, the AC / AC power electronic converter's rapid shutdown capability isolates the fault.

[0017] The advantages of this invention are:

[0018] The AC / AC power electronic converter of the present invention consists of a non-isolated rectifier and an inverter. The output voltage and capacity of the AC / AC power electronic converter are only 1 / (n+1) (n>1) of the output voltage and capacity of the combined transformer, respectively, thereby significantly reducing the capacity and cost of the converter.

[0019] In this invention, taking single-phase as an example, the inverter section of the AC / AC power electronic converter adopts a dual closed-loop control strategy of AC voltage outer loop and AC current inner loop to control the inverter's AC output voltage u2 to achieve control over the transformer's output voltage u. an The AC / AC power electronic converter rectifier section adopts a dual closed-loop control strategy with an outer DC voltage loop and an inner AC current loop. Load current harmonics and reactive current are extracted and added to the reference value in the inner current loop. Harmonic mitigation and reactive power compensation are achieved by controlling the rectifier current. This control method is also applicable to the proposed three-phase combined transformer.

[0020] In this invention, when the AC / AC power electronic converter fails, bypassing the converter by closing switch K allows for continued power supply to the load via an electromagnetic transformer, offering higher reliability than existing full-capacity power electronic transformers. When a grid short-circuit fault occurs, rapid fault isolation is achieved through the blocking of the power semiconductor devices in the AC / AC power electronic converter. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of a single-phase circuit of a combined transformer based on a multi-winding transformer and a converter according to the present invention.

[0022] Figure 2 This is a schematic diagram of a three-phase circuit of a combined transformer based on a multi-winding transformer and a converter according to the present invention.

[0023] Figure 3 This is a schematic diagram of the AC / AC power electronic converter circuit of the present invention;

[0024] Figure 4 This is a flowchart of the AC / AC power electronic converter inverter control method of the present invention;

[0025] Figure 5 This is a flowchart of the AC / AC power electronic converter rectifier control method of the present invention. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0027] Example 1

[0028] like Figure 1 As shown, a combined single-phase transformer based on a multi-winding transformer and a converter includes an electromagnetic multi-winding transformer, an AC / AC power electronic converter, and a bypass switch K; each phase of the electromagnetic multi-winding transformer consists of a set of high-voltage windings W wound on the same iron core. hv and two sets of low-voltage windings W lv1 W lv2 Composition. Low-voltage winding W lv2 The number of turns is only that of the low-voltage winding W lv1 1 / n (n>1). AC / AC power electronic converter input and low voltage winding W lv2 In parallel, the output of the AC / AC power electronic converter is connected in parallel with the bypass switch K, and the output of the AC / AC power electronic converter is connected in parallel with the low-voltage winding W. lv1 When the load is powered in series, the load voltage u an The output voltage u2 and low-voltage winding W of the AC / AC power electronic converter lv1 The low-voltage output voltage u1 satisfies u an＝ u1+u2.

[0029] The winding connection methods of the combined three-phase transformer based on multi-winding transformer and converter include four types: high-voltage side star connection and low-voltage side star connection, high-voltage side star connection and low-voltage side delta connection, high-voltage side delta connection and low-voltage side star connection, and high-voltage side delta connection and low-voltage side delta connection. The transformer core can adopt either a three-phase common core or an independent core. Figure 2 This is a circuit diagram showing a high-voltage side delta connection and a low-voltage side star connection, with the iron core using a three-phase independent iron core.

[0030] like Figure 3 As shown, the AC / AC power electronic converter includes an AC / DC rectifier, a DC / AC inverter, and a DC capacitor C. dc The AC / DC rectifier includes power semiconductor switching devices S1, S2, S3, S4 and a filter inductor L. i The DC / AC inverter includes power semiconductor switching devices S5, S6, S7, S8 and filter inductor L. f and AC filter capacitor C f The specific connection method is as follows: the drains of power semiconductor switching devices S1, S3, S5, and S7 are connected to a DC capacitor C. dc The positive terminal of the power semiconductor switching devices S2, S4, S6, and S8 is connected to the DC capacitor C. dc The negative terminal; the source and drain of the power semiconductor switching device S1 are connected to the input filter inductor L. i Filter inductor L i The other end is connected to output terminal x; the source of power semiconductor switching device S3 and the drain of S4 are connected to AC output terminal y; the source of power semiconductor switching device S5 and the drain of S6 are connected to output filter inductor L. f Filter inductor L f The other end is connected to the output terminal z; the source and drain of the power semiconductor switching device S7 and S8 are connected to the AC output terminal t; the AC filter capacitor C f One end is connected to AC output terminal z, and the other end is connected to AC output terminal t.

[0031] The control method for a combined transformer based on a multi-winding transformer and a converter includes the following steps:

[0032] (1) Inverter control method in AC / AC power electronic converter:

[0033] like Figure 4 As shown, during normal operation, the reference voltage u at the output terminal of the combined transformer of multi-winding transformer and converter is... an_ref The difference between the output voltage u1 of the electromagnetic multi-winding transformer and the reference voltage u of the AC / AC power electronic converter inverter is used as the reference voltage u. 2_ref The inverter output voltage control adopts a dual closed-loop control strategy of voltage and current, with reference voltage u.2_ref The difference between the actual output voltage u2 of the AC / AC power electronic converter and the inductor current inner loop reference value i is generated by the first proportional resonant controller PR. L_ref Filter inductor L f Current inner loop reference value i L_ref With filter inductor L f Actual current value i L The difference is used by the first proportional-integral controller (PI) to generate the inverter reference voltage u. c_ref Inverter reference voltage u c_ref The first PWM modulation generates drive signals for power semiconductor devices S5, S6, S7, and S8.

[0034] (2) Rectifier control method in AC / AC power electronic converter:

[0035] like Figure 5 As shown, during normal operation, the AC / AC power electronic converter rectifier adopts a dual closed-loop control strategy of voltage and current, with the DC capacitor reference voltage u... Cdc_ref With actual voltage u Cdc The difference is processed by the second proportional-integral controller (PI) to generate a portion of the current inner loop reference value, i. dc At the same time, through the load current i load Extracting harmonic current components i h and reactive current component i q i dc -(i h+ i q (i) is the inner loop current reference value. x_ref ; Current inner loop reference value i x_ref With the actual value of current i x The difference is used by the second proportional resonant controller PR to generate the rectifier reference voltage u. r_ref rectifier reference voltage u r_ref The second PWM modulation generates drive signals for power semiconductor devices S1, S2, S3, and S4.

[0036] When the AC / AC power electronic converter fails, the bypass switch K is closed to bypass the converter, thus ensuring continuous power supply to the load. When there is a short circuit in the power grid, the AC / AC power electronic converter's rapid shutdown capability isolates the fault.

[0037] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A control method for a combined transformer based on a multi-winding transformer and a converter, characterized in that, The combined transformer includes a single-phase circuit topology and a three-phase circuit topology, wherein the three-phase circuit topology is formed by combining elements of the single-phase circuit topology. The single-phase circuit topology includes: an electromagnetic multi-winding transformer, an AC / AC power electronic converter, and a bypass switch K. Each phase of the electromagnetic multi-winding transformer consists of a set of high-voltage windings W wound on the same iron core. HV and two sets of low-voltage windings W lv1 W lv2 Composition, low-voltage winding W lv2 The number of turns is for the low-voltage winding W lv1 1 / n, where n>1; the AC / AC power electronic converter input and low voltage winding W lv2 The AC / AC power electronic converter output is connected in parallel with the bypass switch K, and the AC / AC power electronic converter output is connected in parallel with the low-voltage winding W. lv1 The load is supplied with power in series, and the voltage u of the load is... an and the output voltage u2 of the AC / AC power electronic converter, the low-voltage winding W lv1 The low-voltage output voltage u1 satisfies u an= u1+u2; The method includes the following steps: (1) Inverter control method in AC / AC power electronic converter: During normal operation, the reference voltage u at the output terminal of the combined transformer of multi-winding transformer and converter is... an_ref The difference between the output voltage u1 of the electromagnetic multi-winding transformer and the reference voltage u of the AC / AC power electronic converter inverter is used as the reference voltage u. 2_ref The inverter output voltage control adopts a dual closed-loop control strategy of voltage and current, with reference voltage u. 2_ref The difference between the actual output voltage u2 of the AC / AC power electronic converter and the inductor current inner loop reference value i is generated by the first proportional resonant controller PR. L_ref Filter inductor L f Current inner loop reference value i L_ref With filter inductor L f Actual current value i L The difference is used by the first proportional-integral controller (PI) to generate the inverter reference voltage u. c_ref Inverter reference voltage u c_ref The first PWM modulation generates drive signals for power semiconductor devices S5, S6, S7, and S8. (2) Rectifier control method in AC / AC power electronic converter: During normal operation, the AC / AC power electronic converter rectifier adopts a dual closed-loop control strategy for voltage and current, with the DC capacitor reference voltage u... Cdc_ref With actual voltage u Cdc The difference is processed by the second proportional-integral controller (PI) to generate a portion of the current inner loop reference value, i. dc At the same time, through the load current i load Extracting harmonic current components i h and reactive current component i q i dc -(i h+ i q (i) is the inner loop current reference value. x_ref ; Current inner loop reference value i x_ref With the actual value of current i x The difference is used by the second proportional resonant controller PR to generate the rectifier reference voltage u. r_ref rectifier reference voltage u r_ref The second PWM modulation generates drive signals for power semiconductor devices S1, S2, S3, and S4.

2. The control method for a combined transformer based on a multi-winding transformer and a converter as described in claim 1, characterized in that, The three-phase circuit topology includes four combinations based on the single-phase circuit topology: high-voltage side delta connection / low-voltage side star connection, high-voltage side delta connection / low-voltage side delta connection, high-voltage side star connection / low-voltage side star connection, and high-voltage side star connection / low-voltage side delta connection.

3. The control method for a combined transformer based on a multi-winding transformer and a converter as described in claim 2, characterized in that, The three-phase circuit topology uses three single-phase transformers or a three-phase common core transformer.

4. The control method for a combined transformer based on a multi-winding transformer and a converter as described in claim 1, characterized in that, The AC / AC power electronic converter includes an AC / DC rectifier, a DC / AC inverter, and a DC capacitor C. dc The AC / DC rectifier includes power semiconductor switching devices S1, S2, S3, S4 and a filter inductor L. i The DC / AC inverter includes power semiconductor switching devices S5, S6, S7, S8 and a filter inductor L. f and AC filter capacitor C f The specific connection method is as follows: the drains of power semiconductor switching devices S1, S3, S5, and S7 are connected to a DC capacitor C. dc The positive terminal of the power semiconductor switching devices S2, S4, S6, and S8 is connected to the DC capacitor C. dc The negative terminal; the source and drain of the power semiconductor switching device S1 are connected to the input filter inductor L. i Filter inductor L i The other end is connected to output terminal x; the source of power semiconductor switching device S3 and the drain of S4 are connected to AC output terminal y; the source of power semiconductor switching device S5 and the drain of S6 are connected to output filter inductor L. f Filter inductor L f The other end is connected to the output terminal z; the source and drain of the power semiconductor switching device S7 and S8 are connected to the AC output terminal t; the AC filter capacitor C f One end is connected to AC output terminal z, and the other end is connected to AC output terminal t.

5. The control method for a combined transformer based on a multi-winding transformer and a converter as described in claim 1, characterized in that, When the AC / AC power electronic converter fails, the bypass switch K is closed to bypass the converter, thus ensuring continuous power supply to the load. When there is a short circuit in the power grid, the AC / AC power electronic converter's rapid shutdown capability isolates the fault.

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