Power electronic transformers in wind power generation systems

Power electronic transformers (PET) have received extensive attention due to their advantages of compensating reactive power, improving power quality, limiting fault current, and reducing the number of traditional transformer materials. At present, it is also an important research topic to replace conventional power frequency transformers in wind power generation systems. This paper first analyzes the topology of the primary circuit of the power electronic transformer. Several common PET topologies are introduced, and their advantages and disadvantages are analyzed. The module cascade topology and modular multilevel topology are mainly introduced, and the topological circuit diagram is given. The paper also comprehensively introduced the research status quo of wind power generation systems based on the interface of PET, permanent magnetic wind turbines and dual-feed fan wind turbines. The traditional structure of the permanent magnet synchronous generator (PMSG) system and the doubly fed induction generation (DFIG) system is shown. Finally, the problems in PET and the future development direction are summarized.


Introduction
Power electronic transformer, also known as a solid-state transformer or intelligent transformer.In general, it refers to new power electronics devices not limited to conventional industrial frequency ac transformers or high week wave transformers realized by power electronics technology (relative to the power frequency, transformer working frequency is higher).PET generally includes at least the traditional AC transformer voltage level transformation and electrical isolation functions.Based on this, studies are conducted on the DC access of new energy storage devices, fault isolation between ports, and interaction with other intelligent devices [1].
The conventional power frequency transformer can be replaced with PET, typically used in medium, high-voltage, and high-power situations.Compared with traditional transformers, PET is more suitable for improving system performance and enriching system functions [2].PET is primarily used in the onboard conversion system to traction electric locomotives, the Smart Grid Internet and the distributed renewable energy network.
Some progress has been made in applying PET technology in wind power at home and abroad.First, use PET technology to provide a low-voltage DC bus for wind power, build a low-voltage DC microgrid, and realize a compelling collection of nearby loads.Secondly, the PET isolation stage uses a highfrequency transformer, which reduces the capital cost of the power frequency transformer in transportation, installation and maintenance.In addition to realizing voltage conversion, active power transmission, and electrical isolation, PET also compensates reactive power, improves power quality, and limits fault current.Therefore, PET has been widely used in wind power generation systems.A matrix converter-based power electronic transformer for wind power system architecture was proposed by Dujic D. et al. [3].The initial research work on wind power systems based on the PET interface focused on squirrel-cage induction motor wind power systems, and the effectiveness of the proposed topology was demonstrated through simulations.The system can realize reactive power compensation, active power transmission, voltage conversion and other functions.Besselmann T. et al. proposed a PET-based permanent synchronous magnet wind turbine grid-connected system [4].This high-voltage DC side of the wind turbine is equipped with a supercapacitor energy storage device, giving it a powerful low-voltage ride-through capacity.
This paper analyzes the topology of the primary circuit of the power electronic transformer.The module cascade and multilayer phases are mainly introduced.Several new types of wind turbines currently used worldwide, such as power electronic transformers, permanent magnet wind turbines, and double fans, are briefly introduced.This paper briefly introduces some problems facing the development of wind power technology in China and puts forward the future direction of wind power technology.

Topology of power electronic transformer
PET combines electronic and high-frequency power conversion technology to convert amplitude, frequency, and voltage or current phase.The division of PET topology is mainly based on the general criterion of energy transformation number.Heinemann L. et al. do not consider high-frequency transformer AC/AC conversion and divide PET into single-phase, two-phase and three-phase types [5].However, the high-frequency transformer will also cause a loss in the AC/AC power converter process.The proposed AC/AC/AC PET topology is relatively early and has certain practicability [6].This method has the advantages of simple structure, variable voltage and electrical insulation.However, the second-order waveform only restores the first-order waveform, and its control ability is poor.The topology only contains a high-frequency (or intermediate frequency, from now on referred to as highfrequency) transformer.The overall capacity is difficult to expand, and the reliability could be higher.PET with medium and high voltage AC input and low voltage AC output intuitively distinguish the number and complexity of power transformation links of different types of PET topologies [7].
In addition, the structure of PET can be divided into two types [8].AC/AC type PET because there is no DC link directly to the AC voltage level and frequency conversion, the structure is simple, this transformer in the weight and volume control.But its realization of fewer functions and poor controllability makes it lose the core advantages of PET.This structure of PET has the advantages of solid controllability, good power quality and convenient grid connection to a DC power supply.Gao, R. et al. studied a cascaded PET topology based on the traditional AC/DC/AC PET [9].The cascaded Hbridge circuit modules of the input stage are small and compact, improving the input voltage level.LLC resonant converter with isolation stage enables flexible bidirectional power transmission, reduces switching loss and improves system efficiency.The inverter circuit in the output stage has a stable output and high-power quality.This paper adopts the classification method of literature [7] to study the topological structure of PET.In practice, the main five-level topology is used for high-voltage, highpower applications in power systems due to the limitations of power devices and magnetic materials.
The topology structure of the modular cascade PET is as follows in Figure 1.Using a new type of low-power device with an H bridge, the effective control of the PET device is realized.The isolation stage uses the double active bridge, which realizes the voltage level change, electrical isolation and twoway energy transmission through the transfer control of the whole bridge at both ends of the highfrequency transformer.The inverter stage employs a full-bridge inverter, offering clients high-quality reactive and AC power and, occasionally, voltage amplitude or frequency that deviates from the grid.Compared with the traditional cascaded PET circuit, the modular multilayer PET circuit has better DC voltage quality, lower frequency loss and fewer power electronic devices.Based on the modular multilevel PET topology, Gupta, Rick and others.In order to achieve the advantages of energy exchange and mutual support between various PET ports, they also design a PET autonomous operation control scheme that is suitable for the distribution network [10].The topology structure of the modular multilevel PET is shown in Figure 2.

PMSG wind power generation system
PMSG has no gear transmission, no excitation, no transmission, no excitation, no operation, and no operation cost.It is a research hotspot in the field of wind power at present.A power frequency transformer connects the permanent synchronous magnet generator wind turbine to the power grid.For low-voltage ride-through, add an unloaded resistor to its DC side.Its traditional construction is as follows in figure 3. To eliminate the impact of large power transformers on the grid and increase the power density of the grid, researchers at home and abroad have proposed a new topology of a permanent synchronous magnet generator wind power system.Huang, A. Q. et al. introduce a three-phase switched multi-coil PET for wind power systems [11].The system realizes power conversion using a high-frequency AC link provided by PET composed of two groups of three-phase primary side windings and one group of three-phase secondary side windings.The primary and secondary coils of PET are composed of two pairs of inverts and are connected to a 60 Hz three-phase power system, resulting in a one-week threephase high voltage with a constant load rate.A new type of rectangular conversion circuit is used to couple the PET secondary coil with the fan, thus achieving the purpose of adjusting the power factor of the PET secondary coil and suppressing the common mode.The new scheme proposed in this project can increase the system's power density, save storage capacity, eliminate the common-mode voltage of switching elements, reduce the load current, and enhance the system's reliability.Huang, A. Q. et al. established a complete grid-connected system model of solid transformer for permanent magnet direct drive wind power generation, including generator and solid transformer models [12].Combined with the dual space vector control strategy, the system has good variable speed constant frequency dynamic operation performance.The unloading unit and additional current feedforward compensation factor were used to optimize the inverter stage control of the solid transformer and improve the low voltage traverse ability of the system.As follows in Figure 4, this paper introduces a kind of power electronic transformer with a rectifier, high-frequency isolation and reverse.The system uses a three-phase fullbridge inverter as an inverter.This device uses a single-phase full-bridge inverter, single-phase fullbridge rectifier and high-frequency inverter.Rectification stage and reverse flow stage.Replace the power frequency transformer with a new power supply and connect it to the grid.On this basis, a new method is adopted to reduce the system's heat loss, and a new type of reactive power compensation load is introduced into the system so as to enhance the low voltage penetration performance.

DFIG wind power generation system
DFIG has recently been widely used in wind power generation and is one of the current mainstream models.The traditional structure of the DFIG system is as follows in Figure 5.The rotor is excited by back-to-back frequency conversion speed regulation, and the stator end is excited by frequency conversion speed regulation, which is connected to the power system.The doubly-fed generator adopts two stator and rotor methods to realize the power system's synchronous control.In the double-fed fan, the rotor-end converter is used to transmit the excitation current without interfering with the normal operation of the whole fan.This control method can well solve the problem of coupling between active power and reactive power in a power system.The power of the heat exchanger is only about 25% of the fan's power.However, the stator end of the doubly-fed motor is connected to the power system and is very sensitive to the voltage change of the power system.In practice, it still needs further improvement to improve its low-voltage characteristics.Similarly, many scholars are also studying its application in the power system based on studying the new doubly-fed power system.Syed I. et al. propose an override control model for low voltage ridethrough when a grid fault occurs [13].On this basis, a new power system structure is proposed and simulated.Comparison of operating conditions with conventional transformer-based wind power systems.The experimental results show that the proposed method can effectively suppress the disturbance caused by voltage, current and electromagnetic torque.The transient process of the DFIG wind power generation system was analyzed when the grid voltage sagged, and the grid voltage swelled [14].Based on this, an improved control strategy for setting chopper protection for PET high-voltage stage cascaded H-bridge sub-modules is proposed when the grid voltage sags.The DGIG-PET system is simulated under the same sag amplitude and duration.

Conclusion
At present, topology, control strategy, and fault diagnosis are the three main areas of study for PET system research.Using PET technology for wind turbines, the topological structure of wind turbines, the stable operation of wind turbines and the reactive power compensation of wind turbines have been deeply studied.Relevant research has achieved rich results, but further research is needed in other aspects.
In wind turbines, how to overcome the influence of low voltage and high voltage on wind turbines has always been the focus of attention.Some studies have shown that wind turbines' low voltage ridethrough capability can be designed by designing PET control strategies, but there are few related studies.
For PET, the optimization and improvement of its performance depend on the research and development of core materials such as power switches, capacitors and high-frequency transformers.Therefore, introducing new materials and technologies is an urgent problem in the PET field.
Presently, domestic and foreign studies focus on the double-sided full-bridge DAB type gridconnected wind turbine as the research object.However, MMC type PET has many advantages over Hbridge type.In addition, due to the different structures of DAB, its characteristics are also different, so in wind turbines, the topology selection of PET still needs to be further studied.

Figure 1 .
Figure 1.Module cascading PET topology[5].Compared with the traditional cascaded PET circuit, the modular multilayer PET circuit has better DC voltage quality, lower frequency loss and fewer power electronic devices.Based on the modular multilevel PET topology, Gupta, Rick and others.In order to achieve the advantages of energy exchange and mutual support between various PET ports, they also design a PET autonomous operation control scheme that is suitable for the distribution network[10].The topology structure of the modular multilevel PET is shown in Figure2.

Figure 3 .
Figure 3. Traditional structure of PMSG[9].To eliminate the impact of large power transformers on the grid and increase the power density of the grid, researchers at home and abroad have proposed a new topology of a permanent synchronous magnet generator wind power system.Huang, A. Q. et al. introduce a three-phase switched multi-coil PET for wind power systems[11].The system realizes power conversion using a high-frequency AC link provided by PET composed of two groups of three-phase primary side windings and one group of three-phase secondary side windings.The primary and secondary coils of PET are composed of two pairs of inverts and are connected to a 60 Hz three-phase power system, resulting in a one-week threephase high voltage with a constant load rate.A new type of rectangular conversion circuit is used to couple the PET secondary coil with the fan, thus achieving the purpose of adjusting the power factor of the PET secondary coil and suppressing the common mode.The new scheme proposed in this project can increase the system's power density, save storage capacity, eliminate the common-mode voltage of switching elements, reduce the load current, and enhance the system's reliability.Huang, A. Q. et al. established a complete grid-connected system model of solid transformer for permanent magnet direct drive wind power generation, including generator and solid transformer models[12].Combined with the dual space vector control strategy, the system has good variable speed constant frequency dynamic operation performance.The unloading unit and additional current feedforward compensation factor were used to optimize the inverter stage control of the solid transformer and improve the low voltage traverse ability of the system.As follows in Figure4, this paper introduces a kind of power electronic transformer with a rectifier, high-frequency isolation and reverse.The system uses a three-phase fullbridge inverter as an inverter.This device uses a single-phase full-bridge inverter, single-phase fullbridge rectifier and high-frequency inverter.Rectification stage and reverse flow stage.Replace the power frequency transformer with a new power supply and connect it to the grid.On this basis, a new method is adopted to reduce the system's heat loss, and a new type of reactive power compensation load is introduced into the system so as to enhance the low voltage penetration performance.

Figure 4 .
Figure 4. Based on PET interface PMSG structure of wind power generation system [10].

Figure 5 .
Figure 5. Traditional structure of DFIG.Similarly, many scholars are also studying its application in the power system based on studying the new doubly-fed power system.Syed I. et al. propose an override control model for low voltage ridethrough when a grid fault occurs[13].On this basis, a new power system structure is proposed and simulated.Comparison of operating conditions with conventional transformer-based wind power systems.The experimental results show that the proposed method can effectively suppress the disturbance caused by voltage, current and electromagnetic torque.The transient process of the DFIG wind power generation system was analyzed when the grid voltage sagged, and the grid voltage swelled[14].Based on this, an improved control strategy for setting chopper protection for PET high-voltage stage cascaded H-bridge sub-modules is proposed when the grid voltage sags.The DGIG-PET system is simulated under the same sag amplitude and duration.