CN103208821A - Power mixed conversion system - Google Patents

Abstract

The invention provides a power hybrid conversion system, including MMC and a DC-DC isolator, the AC side of the MMC is connected to a high-voltage AC power grid, the DC side of the MMC is connected to a high-voltage DC power grid, and the positive and negative DC input ends of the DC-DC isolator are respectively connected to the high-voltage DC power grid The DC-DC isolator includes capacitors connected in parallel to its DC input terminal and DC output terminal respectively, and the DC output terminal of the DC-DC isolator is connected to the low-voltage DC power grid. In the power hybrid conversion system of the present invention, the DC-side output capacitors of the DC-DC isolator are connected in parallel. When the DC capacitor voltage of the MMC is unbalanced, the part with high capacitor voltage in the MMC passes through the DC-DC isolator by means of the low-voltage DC power grid. Then through the DC-DC isolator, the capacitor with low voltage is charged to increase the voltage value, thus ensuring the stability of the DC side voltage, thereby realizing the automatic balance of the DC capacitor voltage of the MMC.

Description

Electric Hybrid Conversion System

technical field

The invention relates to the technical field of power systems, in particular to a power hybrid conversion system.

Background technique

With the continuous development of the world economy, the demand for various forms of electricity is growing faster and faster, and the problems of energy shortage and environmental protection are becoming more and more serious. Renewable energy power generation technologies represented by wind power and solar power are gradually becoming the future power generation technology. The development direction and research hotspots of system technology. However, these clean energy sources often have characteristics such as scattered locations and far away from power user centers. The high-voltage direct current transmission system based on voltage source converters can connect these small distributed power systems to the AC grid in an economical and environmentally friendly way. High voltage and high power The voltage converter is the core component of this type of system. Power electronic devices in high-voltage and high-power conversion applications need to have the highest possible voltage level and power handling capability, and one of the key technologies to realize high-voltage and high-power converters is the high-power converter topology. In recent decades, multi-level conversion technology (mainly referring to voltage-type multi-level converters) has been continuously promoted. However, due to the use of direct series technology, the inconsistency of device parameters will not only cause device voltage equalization problems, electromagnetic interference problems, but also cause large switching losses due to excessive switching frequency, which is difficult to use in the field of high-voltage direct current transmission.

The Modularize Multi-level Converter (MMC) topology was first proposed by the Bundeswehr University in Munich in 2002 and applied to the field of HVDC transmission systems. It is becoming a research hotspot due to its unique structure and technical advantages. . This topology does not require direct series connection of devices, and has a modular structure. Each module can use the same hardware structure without a phase-shifting transformer, and has broad application prospects. Although some progress has been made in the related theoretical and technical research of modular multilevel converters, there are still some problems that have not been fully resolved. For example, the circulation problem of MMC in special applications such as asymmetric grid/load, the existence of commutation will bring some negative effects, but there is no effective solution yet. The capacitor voltage on the DC side of the MMC power supply contains a large number of fundamental frequency and second harmonics, which will lead to non-negligible low-order harmonic components in the AC output bridge arm current, which will affect the output characteristics of the system. The DC capacitors of each power unit of the MMC are independent of each other, and the main circuit components are non-ideal components, which may easily lead to DC capacitor voltage imbalance and DC capacitor voltage pulsation.

At present, the balance control of the DC capacitor voltage in the hybrid conversion system based on MMC is mainly realized through an external balance control circuit. Although the method of the external balance control circuit can simplify the algorithm design of the control program, it requires additional hardware circuits and control systems. , which increases the cost and complexity of the system and reduces the reliability of the system.

Contents of the invention

Based on this, it is necessary to provide a power hybrid conversion system that can easily realize DC capacitor voltage balance control in order to solve the problem that the DC capacitor voltage balance control is not easy to achieve in the general MMC-based power hybrid conversion system.

A power hybrid conversion system, including a modular multilevel converter and a DC-DC isolator, the AC side of the modular multilevel converter is connected to a high-voltage AC grid, and the modular multilevel converter The DC side is connected to the high-voltage DC grid, the DC-DC isolator includes a first capacitor and a second capacitor, the positive and negative DC input ends of the DC-DC isolator are respectively connected to the positive and negative of the high-voltage DC grid, and the DC- The DC output terminal of the DC isolator is connected to the low-voltage DC grid, the DC input terminal of the DC-DC isolator is connected in parallel with the second capacitor, and the DC output terminal of the DC-DC isolator is connected in parallel with the first capacitor.

In one of the embodiments, the power hybrid conversion system further includes a DC-AC inverter, the DC side of the DC-AC inverter is connected to the low-voltage direct current grid, and the AC side of the DC-AC inverter is connected to Low voltage AC grid.

In one of the embodiments, the DC-DC isolator further includes an inverter, an intermediate frequency transformer and a rectifier, the DC side of the inverter is connected in parallel with the second capacitor, and the DC side of the inverter is connected to the The high-voltage DC grid is connected, the AC side of the inverter 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 in parallel with the first A capacitor, the DC side of the rectifier is connected to the low-voltage DC grid.

In one of the embodiments, the modular multilevel converter is a three-phase three-leg circuit structure, and each phase bridge arm in the three-leg structure includes an upper bridge arm and a lower bridge arm, and the upper bridge arm Both the bridge arm and the lower bridge arm include multiple sub-modules and a current-limiting reactor, and the multiple sub-modules are connected in series with the one current-limiting reactor.

In one of the embodiments, the sub-module includes a half-bridge circuit and a capacitor, and the half-bridge circuit is connected in parallel with the capacitor.

In one embodiment, the half bridge circuit includes two groups of insulated gate bipolar transistors and freewheeling diodes, and the two groups of insulated gate bipolar transistors and freewheeling diodes are connected to form a half H bridge structure.

In one embodiment, the inverter includes four sets of insulated gate bipolar transistors and freewheeling diodes, and the four sets of insulated gate bipolar transistors and the freewheeling diodes are connected in an H-bridge structure.

In one embodiment, the rectifier includes four sets of insulated gate bipolar transistors and freewheeling diodes, and the four sets of insulated gate bipolar transistors and the freewheeling diodes are connected in an H-bridge structure.

In one of the embodiments, the DC-AC inverter includes four sets of insulated gate bipolar transistors and freewheeling diodes, and the four sets of insulated gate bipolar transistors and the freewheeling diodes are connected in an H-bridge structure .

In the power hybrid conversion system of the present invention, the DC-side output capacitors of the DC-DC isolator are connected in parallel. When the DC capacitor voltage of the MMC is unbalanced, the part with high capacitor voltage in the MMC passes through the DC-DC isolator by means of the low-voltage DC power grid. Then through the DC-DC isolator, the capacitor with low voltage is charged to increase the voltage value, thus ensuring the stability of the DC side voltage, thereby realizing the automatic balance of the DC capacitor voltage of the MMC. Generally speaking, the power hybrid conversion system of the present invention is a system that can easily realize DC capacitor voltage balance control.

Description of drawings

Fig. 1 is a schematic structural diagram of an embodiment of the power hybrid conversion system of the present invention;

Fig. 2 is a schematic structural diagram of an embodiment of the power hybrid conversion system of the present invention;

Fig. 3 is a schematic structural diagram of a DC-DC isolator in the power hybrid conversion system of the present invention;

Fig. 4 is a schematic diagram of the sub-module structure of the MMC in the power hybrid conversion system of the present invention;

Fig. 5 is a schematic structural diagram of a DC-AC inverter in one embodiment of the power hybrid changing system of the present invention.

Detailed ways

As shown in Figure 1, Figure 3 and Figure 4, a power hybrid conversion system includes a modular multilevel converter 100 and a DC-DC isolator 200, the AC side of the modular multilevel converter 100 Connected to a high-voltage AC grid, the DC side of the modular multilevel AC 100 is connected to a high-voltage DC grid, the DC-DC isolator 200 includes a first capacitor 210 and a second capacitor 220, and the DC-DC isolator The positive and negative DC input terminals of 200 are respectively connected to the positive and negative terminals of the high-voltage DC grid, the DC output terminal of the DC-DC isolator 200 is connected to the low-voltage DC grid, and the DC input terminal of the DC-DC isolator 200 is connected in parallel with the The second capacitor 220 , the DC output terminal of the DC-DC isolator 200 is connected in parallel with the first capacitor 210 .

In this embodiment, the first capacitor and the second capacitor may be the same capacitor or different capacitors, where the same capacitor refers to the same type of capacitor. The first capacitor is connected in parallel to the DC output terminal of the DC-DC isolator as the DC output capacitor of the DC-DC isolator, and the second capacitor is connected in parallel to the DC input terminal of the DC-DC isolator as the DC input capacitor.

In the power hybrid conversion system of the present invention, the DC-side output capacitors of the DC-DC isolator are connected in parallel. When the DC capacitor voltage of the MMC is unbalanced, the part with high capacitor voltage in the MMC passes through the DC-DC isolator by means of the low-voltage DC power grid. Then through the DC-DC isolator, the capacitor with low voltage is charged to increase the voltage value, thus ensuring the stability of the DC side voltage, thereby realizing the automatic balance of the DC capacitor voltage of the MMC. In general, the power hybrid conversion system of the present invention is a power hybrid conversion system that can easily realize DC capacitor voltage balance control.

In addition, due to the existence of the DC-DC isolator, the high-voltage AC and DC sides of the power hybrid conversion system of the present invention are in a high-voltage state, and the low-voltage AC and DC sides are in a low-voltage state, realizing perfect isolation of the high-voltage system and the low-voltage system. In this way, when a fault occurs, it is convenient to disconnect the DC and AC power grids, so the power hybrid conversion system of the present invention also has the characteristics of high reliability and flexible control.

As shown in Figure 2, in one of the embodiments, the power hybrid conversion system further includes a DC-AC inverter 300, the DC side of the DC-AC inverter 300 is connected to the low-voltage direct current grid, and the DC - the AC side of the AC inverter 300 is connected to the low-voltage AC grid.

The DC side of the DC-AC inverter is connected to the low-voltage DC grid, and the AC side is connected to the low-voltage AC grid. The function of this structure is that when the low-voltage DC grid power fails, the low-voltage AC grid can output DC power through the inverter to support the voltage of the low-voltage DC grid. Normally, due to the existence of the inverter, the low-voltage AC grid and the low-voltage DC grid can also support each other.

As shown in Figure 3, in one embodiment, the DC-DC isolator 200 further includes an inverter 230, an intermediate frequency transformer 240 and a rectifier 250, and the DC side of the inverter 230 is connected in parallel with the second Capacitor 220, the DC side of the inverter 230 is connected to the high-voltage DC grid, the AC side of the inverter 230 is connected to the primary side of the intermediate frequency transformer 240, and the AC side of the rectifier 250 is connected to the intermediate frequency On the secondary side of the transformer 240, the first capacitor 210 is connected in parallel to the DC side of the rectifier 250, and the DC side of the rectifier 250 is connected to the low-voltage DC grid.

In this embodiment, the inverter is the high-frequency modulation part, the DC side of the inverter is connected to the DC output of the MMC sub-module, and the AC side is connected to the original side of the intermediate frequency transformer; the intermediate frequency transformer couples the high-voltage AC power of the original side to the The low-voltage side of the auxiliary side realizes the electrical isolation between high and low voltage; the rectifier, that is, the high-frequency reduction part, usually has the same structure as the inverter, which is an H-bridge structure. The AC side is connected to the auxiliary side of the intermediate frequency transformer, and the DC side is connected to the low-voltage DC. net. The isolation conversion part realizes the conversion of DC into AC and is coupled to the secondary side and then restored to DC, using open-loop pulse width modulation control.

In one of the embodiments, the modular multilevel converter is a three-phase three-leg circuit structure, and each phase bridge arm in the three-leg structure includes an upper bridge arm and a lower bridge arm, and the upper bridge arm Both the bridge arm and the lower bridge arm include multiple sub-modules and a current-limiting reactor, and the multiple sub-modules are connected in series with the one current-limiting reactor.

In this embodiment, the MMC adopts a three-phase MMC structure. The high-voltage AC power grid in the power grid is connected to the AC side of the MMC sub-module, and the positive and negative high-voltage DC are respectively connected to the positive and negative sides of the MMC sub-module. Like the traditional MMC structure, in this embodiment, the MMC has 6 bridge arms in total, and each bridge arm is composed of n identical half H-bridge sub-modules connected in series with a bridge arm inductor. The structure of a single half H-bridge sub-module includes two sets of IGBTs and freewheeling diodes and a DC energy storage capacitor. Each sub-module has three different switching states, which are input, cut-off and blocking states. The input state means that the IGBT of the upper bridge is turned on, and the IGBT of the lower bridge is turned off. The current always flows through the upper bridge of the half H bridge, and the output voltage of the sub-module can be considered to be equal to the voltage of the DC energy storage capacitor; The cut-off state means that the upper bridge IGBT is turned off, the lower bridge arm is turned on, the current always flows through the lower bridge of the half H bridge, and the output voltage of the sub-module can be considered to be equal to 0; the locked state means that the upper and lower bridge arms Simultaneous turn-off of IGBTs occurs mainly during system start-up, fault, and switching dead-time phases. Under normal operating conditions, the converter only switches between input and disconnection states, and its upper and lower bridge arms conduct complementary conduction. The role of MMC is to control the output voltage of the sub-modules through the switching of the switch state, while maintaining the stability of the total DC voltage and obtaining the maximum DC output voltage, so as to meet the technical requirements of the converter for the high-voltage and high-power flexible DC transmission system. .

As shown in FIG. 4 , the sub-module includes a half-bridge circuit and a capacitor, and the half-bridge circuit is connected in parallel with the capacitor.

As shown in FIG. 4 , the half bridge circuit includes two groups of insulated gate bipolar transistors and freewheeling diodes, and the two groups of insulated gate bipolar transistors and freewheeling diodes are connected to form a half H bridge structure.

In one embodiment, the inverter includes four sets of insulated gate bipolar transistors and freewheeling diodes, and the four sets of insulated gate bipolar transistors and the freewheeling diodes are connected in an H-bridge structure.

In one embodiment, the rectifier includes four sets of insulated gate bipolar transistors and freewheeling diodes, and the four sets of insulated gate bipolar transistors and the freewheeling diodes are connected in an H-bridge structure.

As shown in FIG. 5, in one embodiment, the DC-AC inverter 300 includes four sets of insulated gate bipolar transistors and freewheeling diodes, and the four sets of insulated gate bipolar transistors and the freewheeling diodes The flow diodes are connected in an H-bridge configuration.

In the above several embodiments, the half bridge circuit, the inverter, the rectifier and the DC-AC inverter all include four groups of insulated gate bipolar transistors and freewheeling diodes, and four groups of insulated gate bipolar transistors and freewheeling diodes connected in an H-bridge structure. The H-bridge structure is a modular structure. In the above-mentioned several embodiments, the half-bridge circuit, inverter, rectifier and DC-AC inverter all adopt the H-bridge structure of this module, which is conducive to optimizing the power mixing of the present invention. Transforming the structure of the system is also convenient for integration, production, management and centralized control.

The above-mentioned embodiments only express several implementation modes of the present invention, and the description thereof is relatively specific and detailed, but should not be construed as limiting the patent scope of the present invention. It should be pointed out that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent for the present invention should be based on the appended claims.

Claims (9)

1. A power hybrid conversion system, characterized in that it includes a modular multilevel converter and a DC-DC isolator, the AC side of the modular multilevel converter is connected to a high-voltage AC power grid, and the modularized multilevel converter The DC side of the multilevel AC is connected to the high-voltage DC grid, the DC-DC isolator includes a first capacitor and a second capacitor, and the positive and negative DC input ends of the DC-DC isolator are respectively connected to the positive and negative terminals of the high-voltage DC grid. Negative, the DC output terminal of the DC-DC isolator is connected to the low-voltage DC power grid, the DC input terminal of the DC-DC isolator is connected in parallel with the second capacitor, and the DC output terminal of the DC-DC isolator is connected in parallel with the first capacitor. 2. The power hybrid conversion system according to claim 1, further comprising a DC-AC inverter, the DC side of the DC-AC inverter is connected to the low-voltage direct current grid, and the DC-AC inverter The AC side of the transformer is connected to the low-voltage AC power grid. 3. The power hybrid conversion system according to claim 1 or 2, wherein the DC-DC isolator further comprises an inverter, an intermediate frequency transformer and a rectifier, and the DC side of the inverter is connected in parallel with the For the second capacitor, the DC side of the inverter is connected to the high-voltage DC grid, the AC side of the inverter is connected to the primary side of the intermediate frequency transformer, and the AC side of the rectifier is connected to the secondary side of the intermediate frequency transformer. On the other hand, the DC side of the rectifier is connected in parallel with the first capacitor, and the DC side of the rectifier is connected to the low-voltage DC grid. 4. The power hybrid conversion system according to claim 1 or 2, characterized in that, the modular multilevel converter is a three-phase three-leg circuit structure, and each phase bridge arm in the three-leg structure Each includes an upper bridge arm and a lower bridge arm, and each of the upper bridge arm and the lower bridge arm includes a plurality of sub-modules and a current-limiting reactor, and the plurality of sub-modules are connected in series with the one current-limiting reactor. 5. The power hybrid conversion system according to claim 1 or 2, wherein the sub-module comprises a half-bridge circuit and a capacitor, and the half-bridge circuit is connected in parallel with the capacitor. 6. The power hybrid conversion system according to claim 5, wherein the half-bridge circuit includes two groups of insulated gate bipolar transistors and freewheeling diodes, and the two groups of insulated gate bipolar transistors and freewheeling diodes The diodes are connected in a half H-bridge configuration. 7. The power hybrid conversion system according to claim 3, wherein the inverter comprises four sets of insulated gate bipolar transistors and freewheeling diodes, the four sets of insulated gate bipolar transistors and the The freewheeling diodes are connected in an H-bridge structure. 8. The power hybrid conversion system according to claim 3, wherein the rectifier comprises four sets of insulated gate bipolar transistors and freewheeling diodes, and the four sets of insulated gate bipolar transistors and the freewheeling diodes The diodes are connected in an H-bridge configuration. 9. The power hybrid conversion system according to claim 1 or 2, wherein the DC-AC inverter includes four sets of insulated gate bipolar transistors and freewheeling diodes, and the four sets of insulated gate bipolar transistors Type transistors and the freewheeling diodes are connected in an H-bridge structure.

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