CN105515401A - Solid state transformer in modularized multi-level structure - Google Patents

Abstract

The invention relates to a solid-state transformer with a modular multi-level structure, which is composed of an input stage, an isolation stage and an output stage. The input stage is a three-phase modular multilevel (MMC) converter, which is composed of cascaded sub-modules. The AC side is connected to the 110KV distribution network, and at the same time connected to the isolation stage through the high-voltage DC bus; the isolation stage consists of multiple subunits Each sub-unit consists of a single-phase MMC inverter, a high-frequency transformer and a single-phase MMC rectifier in turn. The single-phase MMC inverter and rectifier are cascaded by sub-modules. The isolation stage passes high, The low-voltage DC bus is connected to the input stage and the output stage respectively; the output stage is a three-phase modular multi-level converter, which is also formed by cascading sub-modules, and the AC side is connected to the 10KV distribution network. The invention can reduce the volume and weight of the transformer, adopts a modular design, has good flexibility, has good control performance, can realize bidirectional flow of power, and can compensate the power quality of the grid.

Description

A Solid State Transformer with Modular Multilevel Structure

technical field

The invention relates to a power electronic transformer, in particular to a solid-state transformer with a modular multilevel structure.

Background technique

With the continuous development of the economy and society, the installed capacity and annual power generation of the power system continue to increase, and the scale of the power grid continues to expand. The safe and stable operation of the power system has become a prominent problem. More and more nonlinear loads are connected to the power grid, which seriously affects the power quality of the power grid. At the same time, many sensitive loads in the power grid have higher and higher requirements for power quality, and power quality problems have become increasingly prominent. As the most basic power transmission and transformation device, the traditional power transformer has a single function, but also has a large volume and weight. The output voltage fluctuates with the fluctuation of the load. The insulating materials such as transformer oil have potential pollution to the environment. When the iron core is saturated Harmonic generation, poor controllability and other deficiencies have become increasingly difficult to meet the requirements of modern power systems.

Solid-state transformer, also known as power electronic transformer, is a new type of intelligent transformer developed on the basis of high-power power electronic conversion technology. In addition to the functions of traditional power transformer voltage transformation, isolation and energy conversion, it also has the functions of harmonic suppression, reactive power compensation, distributed energy access and power flow control. The prototype of the solid-state transformer was first proposed by the American GE company in 1970. So far, many topological structures have appeared, including the initial primary structure, secondary structure and the tertiary structure currently used by most solid-state transformers. Although the solid-state transformer with three-stage structure increases the complexity of the circuit and the number of components, it can realize more functions such as reactive power compensation, power flow control and distributed generation access. What the present invention adopts is exactly the solid-state transformer of three-stage structure.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of traditional transformers, and propose a solid-state transformer with small volume and light weight, which is suitable for 110KV/10KV conversion. In addition to the functions of voltage transformation, isolation and energy conversion of traditional power transformers, it also With the power quality control function, when the power grid has voltage instability, three-phase asymmetry, large harmonic content, etc., it can still ensure high power quality at the load side. The purpose of the present invention can be achieved through the following technical solutions:

A solid-state transformer with a modular multilevel structure, characterized in that, based on a modular multilevel structure, comprising:

An input stage: a three-phase modular multilevel converter, each phase of the three-phase modular multilevel converter is divided into an upper half bridge arm and a lower half bridge arm, and an upper half bridge arm and a lower half bridge arm The half bridge arm is formed by cascading n sub-modules and a bridge arm inductor, and the middle point of each phase bridge arm is connected to the 110KV distribution network through an inductor;

1. Isolation stage: The isolation stage is formed by cascading k subunits. Each subunit in turn includes a single-phase MMC inverter, a high-frequency transformer and a single-phase MMC rectifier. stage and the output stage are connected; the single-phase MMC inverter and the single-phase MMC rectifier have two bridge arms, each bridge arm is formed by cascading sub-modules, and two equal inductances are connected in series in the middle of the bridge arms, The midpoint of the two bridge arms of the single-phase MMC inverter is connected to the two lead wires corresponding to the primary side of the high-frequency transformer, and the midpoint of the two bridge arms of the single-phase MMC rectifier is connected to the two lead wires of the secondary side of the high-frequency transformer;

An output stage: a three-phase modular multilevel converter, each phase of the three-phase modular multilevel converter is divided into an upper half bridge arm and a lower half bridge arm, and an upper half bridge arm and a lower half bridge arm The half bridge arm is formed by cascading m sub-modules and a bridge arm inductor, and the middle point of each phase bridge arm is connected to the 10KV distribution network through an inductor;

In the above-mentioned solid-state transformer with a modular multi-level structure, the sub-module is composed of two fully-controlled devices with anti-parallel diodes in series and then connected in parallel with a capacitor; the fully-controlled device can be IGBT or MOSFET or IGCT, the device material can be Si or SiC or GaN material. In different parts of the present invention, the devices in the sub-modules can be selected from different types, such as the fully-controlled switching devices with anti-parallel diodes in the sub-modules in the isolation stage select high-frequency switching devices; Low, but the current is large, and its fully controlled switching device with anti-parallel diodes can choose a switching device that can withstand a large current.

In the above-mentioned solid-state transformer with a modular multi-level structure, as shown in Figure 2, in the input stage, the fully controlled device in the sub-module can be IGBT or MOSFET or IGCT; the device adopts Si or SiC or GaN material; the access or bypass of capacitors is controlled by controlling the switching devices in the sub-modules. During normal operation, n capacitors are connected in series in each phase at each moment, and the sum of the voltages of these n capacitors is is the high voltage DC bus voltage.

In the above-mentioned solid-state transformer with a modular multi-level structure, the switching devices in the isolation stage are all fully-controlled switching devices with anti-parallel diodes, and are high-frequency switching devices; the subunits , including a single-phase MMC inverter, a high-frequency transformer, and a single-phase MMC rectifier, the three are connected in sequence, and k subunits are cascaded on the DC bus; each single-phase MMC inverter contains two bridge arms, and each bridge The arm is formed by cascading 2x sub-modules, and two equal inductors are connected in series in the middle of the bridge arm. The midpoint of the two bridge arms of the single-phase MMC inverter is connected to the two leads corresponding to the primary side of the high-frequency transformer; each single The phase MMC rectifier also contains two bridge arms, and each bridge arm is formed by cascading 2y sub-modules. Two equal inductors are also connected in series in the middle of the bridge arms. The midpoint of the two bridge arms of the single-phase MMC rectifier is connected to the high frequency The two leads on the secondary side of the transformer are connected; each high-frequency transformer has the same transformation ratio, and is connected to the corresponding single-phase MMC inverter and single-phase MMC rectifier.

In the above-mentioned solid-state transformer with a modular multi-level structure, the output stage is a three-phase modular multi-level converter, which is connected to the isolation stage through a low-voltage DC bus, and the three-phase modular multi-level Each phase of the converter is divided into an upper half bridge arm and a lower half bridge arm. The upper half bridge arm and the lower half bridge arm are formed by cascading m sub-modules and a bridge arm inductor. The middle point of each phase bridge arm It is connected to the 10KV distribution network through an inductor; the switching devices in the output stage are all fully-controlled switching devices with anti-parallel diodes, and these devices are required to withstand large currents.

The present invention has the following advantages: 1. Adopting the design of modular multi-level structure, it can be directly connected to the power grid, and its modular design makes the design, assembly, maintenance and capacity expansion of the solid-state transformer very convenient and flexible. 2. The isolation stage uses multiple sub-unit cascading methods to divide the voltage, which reduces the voltage on the high-voltage side of the high-frequency transformer. 3. The high-frequency transformer is used to reduce the volume and weight of the transformer. 4. The control is flexible. By controlling the modular multi-level converter, it can ensure the stability of the high and low voltage DC bus voltage, realize the bidirectional flow of power, and compensate the power quality of the grid.

Description of drawings

FIG. 1 is a schematic diagram of a topological structure of the present invention.

Fig. 2 is a schematic diagram of the topological structure of the input stage in the present invention.

Fig. 3 is a schematic diagram of the topological structure of the single-phase MMC inverter in the isolation stage of the present invention.

Fig. 4 is a schematic diagram of the topological structure of the single-phase MMC rectifier in the isolation stage of the present invention.

Fig. 5 is a schematic diagram of the topological structure of the isolation-level neutron unit in the present invention.

Fig. 6 is a schematic diagram of the topology structure of the output stage in the present invention.

FIG. 7 is a schematic diagram of a topological structure of a neutron module in the present invention.

Specific implementation method

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figure 1, the present invention adopts a three-stage conversion structure of AC-DC-AC, which is composed of an input stage 1, an isolation stage 2 and an output stage 3;

An input stage: a three-phase modular multilevel converter, as shown in Figure 2, each phase of the three-phase modular multilevel converter is divided into an upper half bridge arm and a lower half bridge arm, Both the upper half bridge arm and the lower half bridge arm are formed by cascading n sub-modules and a bridge arm inductor L ₁ , and the middle point of each phase bridge arm is connected to the 110KV distribution network through an inductor L ₂ ; The fully controlled device can be IGBT or MOSFET or IGCT; the device is made of Si or SiC or GaN material; the access or bypass of the capacitor is controlled by controlling the switching device in the sub-module. There are n capacitors connected in series to the circuit, and the sum of the voltages of these n capacitors is the high-voltage DC bus voltage.

One isolation stage: as shown in Figure 1, the isolation stage of the present invention is formed by cascading k subunits, and each subunit includes a single-phase MMC inverter (Figure 3), a high-frequency transformer and A single-phase MMC rectifier (Figure 4), the isolation stage is connected to the input stage and output stage respectively through the high-voltage and low-voltage DC bus; each single-phase MMC inverter in the subunit contains two bridge arms, and each bridge arm is composed of 2x Two sub-modules are cascaded, and two equal inductances L ₃ are connected in series in the middle of the bridge arm. The midpoint of the two bridge arms of the single-phase MMC inverter is connected to the two leads corresponding to the primary side of the high-frequency transformer; each sub-unit A single-phase MMC rectifier also contains two bridge arms, and each bridge arm is formed by cascading 2y sub-modules. Two equal inductors L ₄ are also connected in series in the middle of the bridge arm. The midpoint of the two bridge arms of the single-phase MMC rectifier It is connected with two lead wires on the secondary side of the high-frequency transformer; each high-frequency transformer has the same transformation ratio, and is connected to the corresponding single-phase MMC inverter and single-phase MMC rectifier. The switching devices in the isolation stage are full-control switching devices with anti-parallel diodes, and are high-frequency switching devices.

An output stage; as shown in Figure 6, it is a three-phase modular multilevel converter, which is connected to the isolation stage through a low-voltage DC bus, and each phase of the three-phase modular multilevel converter is divided into the upper half bridge arm and the lower half bridge arm, the upper half bridge arm and the lower half bridge arm are formed by cascading m sub-modules and a bridge arm inductor L ₅ , and the middle point of each phase bridge arm is distributed with 10KV through an inductor L ₆ The switching devices in the output stage are all fully controlled switching devices with anti-parallel diodes, and these switching devices are required to withstand large currents.

The sub-module, as shown in Figure 7, is composed of two fully-controlled devices with anti-parallel diodes in series and then connected in parallel with capacitors; the fully-controlled devices can be IGBTs, MOSFETs, or IGCTs, and the device materials can be Si, SiC, or GaN material. In different parts of the present invention, the devices in the sub-modules can be selected from different types, such as the fully-controlled switching devices with anti-parallel diodes in the sub-modules in the isolation stage select high-frequency switching devices; Low, but the current is large, and its fully controlled switching device with anti-parallel diodes can choose a switching device that can withstand a large current.

The present invention adopts a modular structure, which has many advantages. Through the flexible control of each part, it has the function of power quality compensation. When the power grid has voltage instability, three-phase asymmetry, and large harmonic content, it can still ensure that the load side higher power quality.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which the present invention belongs can make various modifications or supplements to the described specific embodiments or adopt similar methods to replace them, but they will not deviate from the spirit of the present invention or go beyond the definition of the appended claims range.

Claims (5)

1. a solid-state transformer for modular multilevel structure, is characterized in that, based on modular multilevel structure, comprising:

One input stage: be a three-phase modular multilevel current transformer, each of described three-phase modular multilevel current transformer is divided into first brachium pontis and second brachium pontis mutually, first brachium pontis and second brachium pontis form by n submodule and the cascade of a brachium pontis inductance, and the intermediate point of each phase brachium pontis is connected with 110KV power distribution network by an inductance;

One isolation level: isolation level unit cascadedly to be formed by k son, and each subelement comprises a single-phase MMC inverter, a high frequency transformer and a single-phase MMC rectifier successively, and isolation level is connected with input stage, output stage respectively by high and low pressure DC bus; Described single-phase MMC inverter and single-phase MMC rectifier all have two brachium pontis; each brachium pontis forms by sub module cascade; the inductance that series connection two is equal in the middle of brachium pontis; the mid point of single-phase MMC inverter two brachium pontis goes between connected with two of the former limit of corresponding high frequency transformer, and single-phase MMC rectifier two brachium pontis mid point goes between with high frequency transformer secondary two and is connected;

One output stage: be a three-phase modular multilevel current transformer, each of described three-phase modular multilevel current transformer is divided into first brachium pontis and second brachium pontis mutually, first brachium pontis and second brachium pontis form by m submodule and the cascade of a brachium pontis inductance, and the intermediate point of each phase brachium pontis is connected with 10KV power distribution network by an inductance.

2. the solid-state transformer of a kind of modular multilevel structure according to claim 1, is characterized in that, described submodule, with Capacitance parallel connection forms after being connected by two wholly-controled device with anti-paralleled diode again; Wholly-controled device is IGBT or MOSFET or IGCT, and device material is Si or SiC or GaN material.

3. the solid-state transformer of a kind of modular multilevel structure according to claim 1, is characterized in that, in described input stage, the wholly-controled device in submodule can be IGBT or MOSFET or IGCT; Device adopts Si or SiC or GaN material; Come access or the bypass of control capacitance by the switching device controlled in submodule, when normally working, each phase of each moment has n capacitances in series place in circuit, and this n capacitance voltage be high voltage dc bus voltage.

4. the solid-state transformer of a kind of modular multilevel structure according to claim 1, is characterized in that, the switching device in described isolation level is the full-controlled switch device with anti-paralleled diode, and is high frequency switching device; Described subelement, comprise single-phase MMC inverter, high frequency transformer, single-phase MMC rectifier, three connects successively, and k son is unit cascaded on DC bus; Each single-phase MMC inverter contains two brachium pontis, and each brachium pontis is formed by 2x sub module cascade, the inductance that series connection two is equal in the middle of brachium pontis, and the mid point of single-phase MMC inverter two brachium pontis goes between connected with two of the former limit of corresponding high frequency transformer; Each single-phase MMC rectifier is equally containing two brachium pontis, and each brachium pontis has 2y sub module cascade to form, two equal inductance of connecting equally in the centre of brachium pontis, and the mid point of single-phase MMC rectifier two brachium pontis goes between with high frequency transformer secondary two and is connected; The no-load voltage ratio of each high frequency transformer is identical, connects corresponding single-phase MMC inverter and single-phase MMC rectifier.

5. the solid-state transformer of a kind of modular multilevel structure according to claim 1, it is characterized in that, described output stage is three-phase modular multilevel current transformer, it is connected with isolation level by low-voltage direct bus, each of this three-phase modular multilevel current transformer is divided into first brachium pontis and second brachium pontis mutually, first brachium pontis and second brachium pontis form by m submodule and the cascade of a brachium pontis inductance, and the intermediate point of each phase brachium pontis is connected with 10KV power distribution network by an inductance; Switching device in output stage is the full-controlled switch device with anti-paralleled diode.