Research on power control of AC/DC hybrid microgrid with power electronic transformer

Introduction: Researchers at the National Electric Energy Conversion and Control Engineering Technology Research Center (Hunan University), Lan Zheng, Tu Chunming, etc., wrote in the 23rd issue of the Journal of Electrical Engineering, 2015, on power electronic transformers for AC/DC hybrid microgrids. (PowerElectronicTransformer, PET), analyzed the two modes of operation, grid-connected and off-grid, and designed the corresponding control strategy.

Lan Zheng and Tu Chunming, researchers of the National Electric Energy Conversion and Control Engineering Technology Research Center (Hunan University), wrote in the 23rd issue of the Journal of Electrical Engineering, 2015, on PowerElectronicTransformer for AC/DC hybrid microgrid. , PET), analyzed the two modes of operation, grid-connected and off-grid, and designed the corresponding control strategy.

In the grid-connected mode, the control PET input interface makes the AC-DC hybrid microgrid equivalent to a “resistive load” or “current source”, while controlling both the AC and DC output interfaces to be equivalent to a constant voltage source. For the off-grid mode, hybrid power droop control is proposed, which can be used to obtain the power to be exchanged between microgrids according to the frequency and voltage information at the interface and the drooping characteristics of the hybrid microgrid. The simulation model of AC/DC microgrid system and power electronic transformer is built. The simulation results show that PET can accurately and quickly adjust the power flow between the main network, AC microgrid and DC microgrid under the condition of distributed energy power fluctuation. The stable operation of AC/DC hybrid microgrid is realized, which proves the correctness of the proposed control strategy.

The demand for distributed energy (Distributed Energy Resources, DER) drives the continuous development of power systems. Microgrid is an effective solution for large-scale intermittent DER access. DER is connected in DC mode, which can save a lot of commutation links, and does not require phase and frequency tracking. The controllability and reliability will be greatly improved. DC is the ideal form of access for DER. In recent years, DC micro-grid has gradually gained people's attention. However, the AC microgrid is still the main form of the microgrid at this stage. AC access will still be the main form of DER grid connection. Therefore, the hybrid microgrid with AC and DC coexistence will be a long-term microgrid structure in the future.

DER operation is subject to natural conditions, power generation is intermittent, and a large number of DER accesses will result in bidirectional power flow on the network. The public connection point (Point of Common Coupling, PCC) is the transfer station for energy flow between the distribution network, the AC micro-network and the DC micro-network. The coordinated management of energy at the PCC will be crucial, and the precise coordination of power needs to be achieved. A reliable "energy router".

PowerElectronic Transformer (PET) consists of high-frequency transformer and power electronic conversion circuit. With high-voltage and low-voltage AC interface and DC interface, it has the functions of voltage transformation, isolation and energy transmission. It can become an “energy router” and realize PCC. Energy coordination management.

At present, the research on the control method of power and power transformer is only based on its basic control, and it does not involve the coordinated operation of the microgrid. The microgrid droop control method only considers the voltage signal on the AC or DC side, and does not consider the signal pairs on both sides. The influence of the working condition of AC/DC microgrid, the literature [two approximation methods of bidirectional droop control of power electronic converters are proposed, but the control links are redundant, which reduces the reliability of the system.

This paper mainly studies the application of power electronic transformers in AC/DC hybrid microgrid, and studies the operation strategies of hybrid microgrid in grid-connected and off-grid modes. In the grid-connected mode, the current and voltage at the interface of the PET main network are controlled to be in phase, and the AC and DC output interfaces are controlled to a constant voltage source.

In particular, for the off-grid mode, hybrid droop control is proposed, which can combine the frequency and voltage information at the AC-DC micro-network interface with the drooping characteristics of the AC-DC microgrid to obtain the power to be exchanged between the AC-DC microgrids. . The proposed control strategy can accurately control the bidirectional flow of power between PET interfaces, and adjust the power distribution between the main network, the AC microgrid and the DC microgrid to achieve stable operation of the AC-DC hybrid microgrid.

Figure 1 PET topology diagram

in conclusion

AC/DC hybrid microgrid will be a long-term microgrid structure in the future. This paper analyzes the operation modes of grid-connected and off-grid for the power electronic transformers applied to it, and designs the operation strategy accordingly.

In the grid-connected mode, the input interface PET is controlled so that the AC-DC hybrid microgrid appears as a "resistive load" or "current source"; at the output interface, PET appears to be a constant voltage source. In particular, for the off-grid mode, hybrid power droop control is proposed to coordinate the power between the AC and DC microgrids to accurately and quickly control the power flow.

The simulation analysis model is built. The simulation results show that PET can accurately and quickly adjust the power flow between the main network, AC micro-network and DC micro-grid under the condition of micro-network distributed energy power fluctuation, and realize the AC-DC hybrid micro-grid. Stable operation proves the correctness of the control strategy proposed in this paper.

In this paper, only the DER power variation is simulated. The AC microgrid frequency and the DC microgrid bus voltage change in the dynamic process are not analyzed in depth, and the stability of PET and microgrid is analyzed when DER is off grid. This is a problem that needs to be addressed and solved in later research work.

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