CN103973121B - single-phase power electronic transformer - Google Patents

Abstract

The present invention provides a novel single-phase power electronic transformer, comprising: an input stage circuit, an intermediate isolation stage and a DC-AC inverter; The first bridge arm and the second bridge arm; the first bridge arm includes two connected capacitors, and the connection line between the two capacitors is connected to an input terminal of high-voltage AC; the upper bridge arm of the second bridge arm The half-bridge arm and the lower half-bridge arm include multiple modular multilevel SMs, and the connection line between the upper and lower bridge arms is connected to another input end of high-voltage AC. In this way, the input stage as a whole forms a single-phase half-bridge rectification circuit, which is suitable for a high-voltage single-phase rectification system. At the same time, the power electronic transformer provided by the present invention has an AC-DC mixed input interface, which can be adapted not only to a single AC or DC power distribution system, but also to an AC-DC hybrid power distribution system, and realizes the transformation between power grids of different forms and voltage levels isolation and power transfer functions.

Description

Single Phase Power Electronic Transformer

technical field

The invention relates to the field of electric power technology, in particular to a single-phase power electronic transformer.

Background technique

Since the transformer was invented in the 19th century, it has become one of the most basic and important components of the power system. It is widely used in power distribution systems to achieve voltage level conversion and isolation. However, the traditional transformer has many disadvantages: (1) It is huge, and with the increase of the voltage level, the volume and weight of the transformer increase sharply; (2) It is sensitive to power quality, and the distortion generated when it is connected to non-linear loads such as uncontrolled rectification The current will enter the power grid through the coupling effect of the transformer, which will pollute the bus grid and cause the quality of power supply to decline; (3) When a grounding short circuit fault occurs on the load side, the traditional transformer cannot effectively isolate the fault, resulting in the expansion of the fault range; (4 ) requires a special relay protection device for protection. When the transformer core is saturated, it is easy to generate a large excitation inrush current, which brings harm to the power system; (6) The function is single, without functions such as voltage regulation, power factor correction and power flow control, which cannot meet the needs of smart grid construction.

Power electronic transformer (PET) is a new type of power transformer developed gradually with the development of high-power power electronics technology in recent years. It can overcome the shortcomings of traditional transformers and take advantage of the advantages of power electronic converters and high-frequency transformers. On the basis of realizing the basic functions of traditional power transformers such as voltage transformation, isolation and energy transmission, power electronic transformers can also realize functions such as fault isolation, power quality control, and distributed DC power supply access.

PET is used in high-voltage power distribution applications, and generally uses a traditional multi-level topology. However, as the voltage level and the number of levels increase, the complexity of the required power electronic devices and topology will increase greatly. In the modular multilevel converter (SM) topology, the bridge arm of the converter is not composed of multiple switching devices directly connected in series, but adopts the cascading method of sub-modules, which is especially suitable for high-voltage applications. occasion. And the lower bridge arm voltage change rate (dv/dt) and current change rate (di/dt) greatly reduce the stress on the switching device, and the harmonic content rate and total harmonic distortion rate are also greatly reduced, so that Can reduce or even save the large-capacity AC filter. The modular structure also makes capacity expansion and redundant design easier.

Electronic power transformers in the prior art generally include cascaded H-bridge rectifiers, isolation stages and inverters, and a small number of modular multi-level rectifiers are used, but they are limited to three-phase three-leg circuit structures and cannot be applied to in a single-phase system.

Contents of the invention

The object of the present invention is to provide a single-phase power electronic transformer that can be applied in a single-phase system.

In order to achieve the above object, the present invention provides a single-phase power electronic transformer, comprising:

Input stage circuit, intermediate isolation stage and DC-AC inverter;

Wherein, the input stage circuit includes a first bridge arm and a second bridge arm connected in parallel between the positive and negative input terminals of the high-voltage DC bus; the first bridge arm includes two connected capacitors, and the two capacitors The connection line between them is connected to an input terminal of high-voltage AC; the second bridge arm includes a plurality of series-connected modular multilevel SMs, and the connection line between the upper half-bridge arm and the lower half-bridge arm is connected to the high-voltage AC connected to the other input terminal; the intermediate isolation stage includes multiple DC-DC isolators, each DC-DC isolator is connected to an SM, and the output terminals of multiple DC-DC isolators are connected in parallel to the DC-AC inverter Inverter; the DC-AC inverter is connected to the low-voltage AC output terminal.

Preferably, the DC-DC isolator is a bidirectional full-bridge converter DAB.

Preferably, both the upper half-bridge arm of the second bridge arm and the lower half-bridge arm further include a current-limiting reactor, one end of the current-limiting resistor is connected to the input end of the high-voltage AC, and the other end is connected to the connected to the SM.

Preferably, the DC-DC isolator includes an inverter, an intermediate frequency transformer and a rectifier, the DC side of the inverter is connected to the DC output terminal of the SM, and the AC side is connected to the original side of the intermediate frequency transformer; The AC side of the rectifier is connected to the secondary side of the intermediate frequency transformer, and the DC side of the rectifier is connected to the DC-AC inverter.

Preferably, the SM shares a capacitor with the inverter connected to it.

Preferably, the rectifier and the output stage share a capacitor.

In the single-phase power electronic transformer provided by the present invention, a first bridge arm is connected between the positive and negative input terminals of the high-voltage DC bus, and the first bridge arm includes two connected capacitors, and the connection between the two capacitors The line is connected to an input terminal of high-voltage AC, so that the input stage forms a single-phase half-bridge rectifier circuit as a whole, which is suitable for high-voltage single-phase rectifier systems. At the same time, the power electronic transformer provided by the present invention has an AC-DC mixed input interface, which can be adapted not only to a single AC or DC power distribution system, but also to an AC-DC hybrid power distribution system, and realizes the transformation between power grids of different forms and voltage levels isolation and power transfer functions.

Description of drawings

Fig. 1 is a schematic structural diagram of a single-phase power electronic transformer provided by an embodiment of the present invention;

Fig. 2 is a structural representation of a preferred SM in the single-phase power electronic transformer provided by the present invention;

Fig. 3 is a schematic structural view of a preferred DC-DC isolator in the single-phase power electronic transformer provided by the present invention;

Fig. 4 is another kind of structural representation of a kind of single-phase power electronic transformer provided by the present invention;

Fig. 5 is a control schematic diagram of SM in a control method of a single-phase power electronic transformer provided by the present invention;

Fig. 6 is a working principle diagram of the inner loop controller in the SM control process in a control method of a single-phase power electronic transformer provided by the present invention;

Fig. 7 is a schematic diagram of DC voltage control in the SM control process in a control method of a single-phase power electronic transformer provided by the present invention;

Fig. 8 is a control schematic diagram of the intermediate isolation stage in a control method of a single-phase power electronic transformer provided by the present invention;

Fig. 9 is a control schematic diagram of the output stage in a control method of a single-phase power electronic transformer provided by the present invention.

detailed description

The specific implementation manners of the present invention will be further described below in conjunction with the drawings and examples. The following examples are only used to illustrate the technical solution of the present invention more clearly, but not to limit the protection scope of the present invention.

Embodiment one

Embodiment 1 of the present invention provides a single-phase power electronic transformer, as shown in Figure 1, including:

input stage circuit, intermediate isolation stage and DC-AC inverter 300;

Wherein, the input stage circuit includes a first bridge arm and a second bridge arm connected in parallel between the positive and negative input terminals of the high-voltage DC bus; the first bridge arm includes two connected capacitors 111 and 112, and the two The connection line between the capacitors is connected to an input end of high-voltage AC; the second bridge arm includes a plurality of modular multilevel SMs (120) connected in series, and the connection line between the upper half bridge arm and the lower half bridge arm is connected to The other input terminal of the high-voltage AC is connected; the intermediate isolation stage includes multiple DC-DC isolators 200, each DC-DC isolator 200 is connected to an SM120, and the output terminals of multiple DC-DC isolators 200 are connected in parallel to DC - AC inverter 300; the DC-AC inverter 300 is connected to a low voltage AC output.

In the single-phase power electronic transformer provided by the present invention, a first bridge arm is connected between the positive and negative input terminals of the high-voltage DC bus, and the first bridge arm includes two connected capacitors, and the connection between the two capacitors The line is connected to an input terminal of high-voltage AC, so that the input stage forms a single-phase half-bridge rectifier circuit as a whole, which is suitable for high-voltage single-phase rectifier systems. At the same time, the power electronic transformer provided by the present invention has an AC-DC mixed input interface, which can be adapted not only to a single AC or DC power distribution system, but also to an AC-DC hybrid power distribution system, and realizes the transformation between power grids of different forms and voltage levels isolation and power transfer functions.

In practical application, the upper half bridge arm and the lower half bridge arm of the second bridge arm should contain an equal number of SMs, and the structures of each SM are the same, and the capacity of the two capacitors of the first bridge arm is the same.

As shown in Figure 2, it is a structural schematic diagram of a preferred SM in the single-phase power electronic transformer provided by the present invention, including two power devices 121 and 122 connected in series, and a DC energy storage capacitor 123, each power device includes An insulated gate bipolar transistor and a freewheeling diode, and the insulated gate bipolar transistor and the freewheeling diode of each power device are connected in an H-bridge structure.

As shown in Figure 3, it is a schematic structural diagram of a preferred DC-DC isolator in the single-phase power electronic transformer provided by the present invention, including an inverter 230, an intermediate frequency transformer 240 and a rectifier 250, and the DC of the inverter 230 The second capacitor 220 is connected in parallel, the DC side of the inverter 230 is connected to the DC output end of the modular multilevel SM120, and the AC side of the inverter 230 is connected to the intermediate frequency transformer 240 On the primary side, the AC side of the rectifier 250 is connected to the secondary side of the intermediate frequency transformer 240, the DC side of the rectifier 250 is connected in parallel with the first capacitor 210, and the DC side of the rectifier 250 is connected to the low-voltage DC power grid.

As shown in Figure 3, in one of the preferred embodiments, the inverter 230 includes four groups of insulated gate bipolar transistors and freewheeling diodes, and the four groups of insulated gate bipolar transistors and the freewheeling diodes form an H bridge Structural connections.

Preferably, the DC-DC isolator is a bidirectional full-bridge converter DAB. As shown in FIG. 3 , in one embodiment, the rectifier 250 includes four groups of IGBTs and freewheeling diodes, and the four groups of IGBTs and the freewheeling diodes are connected in an H-bridge structure. .

As shown in Figure 1, both the upper half bridge arm and the lower half bridge arm of the second bridge arm 110 also include a current-limiting reactor 122, one end of the current-limiting resistor 122 is connected to the input end of the high-voltage AC, and the other end Connect to SM in series.

Embodiment two

The difference between the embodiment of the present invention and the first embodiment lies in that the SM in the input stage shares a capacitor with the inverter connected to it.

Embodiment Three

The difference between the embodiment of the present invention and the first or second embodiment is that the rectifier of the middle isolation stage and the DC-AC inverter share a capacitor.

Figure 4 provides an electronic power transformer showing the case where the SM in the input stage shares a capacitor with the inverter connected to it and the intermediate isolation stage shares a capacitor with the DC-AC inverter.

Its specific implementation process is:

(1) Input stage

The structure of the input-stage SM control system is shown in Figure 5, which consists of an inner-loop current controller, an outer-loop DC voltage regulator, a phase-locked synchronization link, and trigger pulse generation. The inner loop current controller realizes the direct control of the current waveform and phase of the AC side of the converter to quickly track the reference current. The DC voltage control of the outer loop realizes constant DC voltage control according to the SM system-level control objectives. The phase signal output by the phase-locked link is used to provide the reference phase required for voltage vector directional control and trigger pulse generation.

The inner loop current controller adopts PI control which is more convenient to realize. Considering that there is coupling between the d and q axis variables in the SM mathematical model, and there are u _d and u _q disturbance signals, the feedforward decoupling strategy is considered, and the introduction The voltage coupling compensation items ωLi _d , ωLi _q and the AC grid voltage feedforward items u _d , u _q , and the inner loop controller are shown in Fig. 6 .

In the figure, the current controller output quantities v _dref and v _qref respectively correspond to the d-axis and q-axis components of the sinusoidal reference fundamental wave voltage expected to be output by the SM. The current reference values i _dref and i _qref in the figure can be obtained from the output of the outer loop controller, and the inner loop current control adopts current feedback and grid voltage feedforward, which improves the tracking response characteristics of the current controller, and at the same time eliminates Steady-state error of current tracking.

The outer loop DC voltage regulator generates the inner loop current reference value according to the control requirements to realize the DC voltage control. Figure 7 shows the DC voltage controller used, the comparison of the DC voltage and its set value and the PI link are used to make the DC voltage U _dc track its reference value U _dcref . The DC voltage control is to maintain a certain DC voltage by adjusting the electric energy input from the AC system to the DC energy storage capacitor of the sub-module. Therefore, the sub-module capacitor voltage balance control is also introduced to realize the balanced charge and discharge of N sub-modules in each bridge arm to achieve the basic balance of capacitor voltage.

(2) Intermediate isolation level

DAB uses phase-shift control to make the DC voltage on the low-voltage side reach a specified value, as shown in Figure 8. First, compare the DC voltage on the low-voltage side with the set reference value to obtain their difference, and then adjust the phase shift angle according to the voltage difference by the PI controller, so as to achieve the purpose of controlling the DC voltage on the low-voltage side. When the high-voltage side system is unbalanced , the low-pressure side system can still maintain the original equilibrium state.

(3) Output stage

The single-phase full-bridge inverter uses a double closed-loop control of the inner loop of the inductor current and the output voltage of the outer loop to achieve constant frequency and constant voltage output. The control block diagram is shown in Figure 9. Comparing the reference value vref with the output voltage, the error signal outputs the reference value of the inner loop current through the voltage PI regulator, and then compares the current reference value with the inductor current, the obtained error signal passes through the current loop PI regulator, outputs the modulation wave, and performs SPWM modulation generates pulse drive. The inner loop of the inductor current can quickly suppress the influence of load disturbance and obtain better system dynamic response performance. The voltage outer loop can improve the output voltage waveform and improve the output accuracy.

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the technical principle of the present invention, some improvements and modifications can also be made. These improvements and modifications It should also be regarded as the protection scope of the present invention.

Claims (6)

1. A single-phase power electronic transformer, characterized in that, comprising: an input stage circuit, an intermediate isolation stage and a DC-AC inverter; Wherein, the input stage circuit includes a first bridge arm and a second bridge arm connected in parallel between the positive and negative input terminals of the high-voltage DC bus; the first bridge arm includes two connected capacitors, and the two capacitors The connection line between them is connected to an input terminal of high-voltage AC; the second bridge arm includes a plurality of series-connected modular multilevel sub-modules SM, and the connection line between the upper half-bridge arm and the lower half-bridge arm is connected to the high-voltage AC The other input end of the AC is connected; the intermediate isolation stage includes multiple DC-DC isolators, each DC-DC isolator is connected to an SM, and the outputs of multiple DC-DC isolators are connected in parallel to the DC-AC Inverter; the DC-AC inverter is connected to the low-voltage AC output terminal; The upper half-bridge arm and the lower half-bridge arm of the second bridge arm contain an equal number of modular multilevel sub-modules SM, and each modular multi-level sub-module SM has the same structure. 2. The transformer according to claim 1, wherein the DC-DC isolator is a bidirectional full-bridge converter. 3. The transformer according to claim 1, wherein the upper half bridge arm and the lower half bridge arm of the second bridge arm also include a current-limiting reactor, and one end of the current-limiting resistor It is connected to the input end of the high-voltage AC, and the other end is connected to the input end of the SM connected in series. 4. The transformer according to claim 1, wherein the DC-DC isolator comprises an inverter, an intermediate frequency transformer and a rectifier, and the DC side of the inverter is connected to the DC output terminal of the SM, The AC side is connected to the primary side of the intermediate frequency transformer; the AC side of the rectifier is connected to the secondary side of the intermediate frequency transformer, and the DC side of the rectifier is connected to the DC-AC inverter. 5. The transformer according to claim 4, wherein the modular multilevel sub-module SM shares a capacitor with the inverter connected thereto. 6. The transformer according to claim 4, wherein the rectifier shares a capacitor with the output stage.