DC Flexible Voltage Control Strategy Considering Distributed Energy Storage Participation

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2017/06/19 15:29
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Authors He Yan, Li Yong, Cao Yijia, and Wang Weiyu, researchers of the School of Electrical and Information Engineering, Hunan University, wrote an article in the tenth issue of the “Journal of Electrotechnics” in 2017, addressing the problem of poor voltage stability and difficult control of multi-node DC distribution networks. A flexible voltage control strategy for DC distribution networks considering the participation of distributed energy storage is proposed.

Firstly, the topology structure of DC distribution network is analyzed, and the deficiency of the existing control strategy in multi-nodal voltage regulation and the feasibility of distributed energy storage participating in voltage regulation are pointed out. Secondly, the voltage-frequency coupling of the AC/DC interface in the distribution network and the cascading behavior of the bidirectional DC interface are deduced. For the AC/DC section of the distribution network port, the frequency fluctuation of the AC power grid is linked with the DC voltage fluctuation through the virtual inertia effect, and a virtual voltage difference is generated to control the frequency fluctuation of the energy storage response and reduce the access of the DC distribution network to the AC power grid. Impact.

According to the distribution network-microgrid converter cross-section, the control strategy of the distributed energy storage unit is designed according to the different types of interfaces, so that it responds to changes in the voltage of the distribution network node, and enhances the dynamic stability of the DC voltage during power fluctuation, and reduces The possibility of switching the operating mode of the distribution network. This strategy requires only local node information, no communication, and good scalability.

Finally, a typical simulation model of the DC distribution network at both ends is established. The effectiveness of the proposed flexible voltage control strategy is demonstrated through system simulation.

With the development of environmental issues and energy crisis, distributed energy sources and micro-grid with regional autonomy have become hot topics in recent years. High-permeability renewable energy access will greatly change the operating characteristics of traditional power systems and optimize users. The power supply performance of the distribution network also brings new challenges to the safe and reliable operation of the power grid.

The traditional AC distribution network is not suitable for the wide access of distributed energy sources because of the problems of power coupling, complex design of protection and operation controllers, large line loss, and sensitivity to harmonic pollution. DC distribution network has the advantages of high transmission capacity, small power consumption, good compatibility with distributed new energy, and reduction of multi-level energy conversion frequency. Therefore, DC distribution network will become the trend and hot spot for the construction and development of distribution networks in the future. .

The relationship between the DC grid voltage and power balance will determine the stable operation of the DC distribution network, so voltage stability is an important indicator of the stability of the DC distribution network. At present, there is no mature scheme for the voltage control strategy of DC distribution network, and the voltage control strategy of flexible DC transmission and DC microgrid is mainly used for reference.

The literature proposes a master-slave control strategy based on high-bandwidth communication. That is, a converter station is selected as the balance node, and the rest of the converter stations are set to power control. This strategy is simple to implement, but it is too dependent on the communication between stations, and the reliability is not high. The literature proposes an adaptive droop control strategy that uses the slope relationship between a given DC voltage and the power of the converter station to control the voltage of the multi-terminal DC system. This strategy requires no communication, but there is a stable operation deviation, and the setting of the droop coefficient is more complex. Inappropriate choice of coefficients may cause confusion or even instability in the system flow.

The literature proposes a layered DC microgrid voltage control strategy. Based on a differential droop control, secondary voltage adjustment is introduced to achieve voltage quality improvement. However, due to the large number of nodes in the DC distribution network, the flow is complicated. The method is difficult to implement under the premise of distributed control. In addition, due to the widespread use of power electronic devices, it is no longer appropriate to regard an external AC power grid connected to a DC distribution network as an infinite power supply. While considering the voltage stabilization, distribution network port converters need to properly consider Frequency effect on the AC grid.

In order to reduce the influence of power fluctuation characteristics of distributed power supplies, energy storage (ES) devices have been widely used. Most of the energy storage units in the DC distribution network are distributed layouts: an energy storage unit that is independently configured and is connected to the distribution network node through a bidirectional DC-DC converter to track DC voltage changes, and control modes and port switching. The flow station is the same; the other is an energy storage unit configured inside the microgrid.

The energy storage unit configured inside the microgrid has two operation modes: 1 The energy storage unit does not start when the microgrid is connected to the network, and is started as a standby power supply when the microgrid is operating, and the power of the distribution network is connected during the grid operation. The adjustment burden is heavy; 2 According to the load forecasting data, the energy storage unit bears the total net power variation during grid-connected operation, and the unsteady part is borne by the distribution network. The above operating modes have the disadvantage that the idle rate of the stored energy in a single mode is too high or the control accuracy of the historical data is low.

With the expansion of the DC distribution network, the number of nodes increases, and the node voltage control away from the converter station becomes more complex and difficult. Second, the random high frequency of renewable energy power fluctuations also brings about extreme stability of the system. Great challenge.

This paper first analyzes the composition of typical DC distribution networks and the control methods of different nodes. Then it studies the virtual inertia of the AC/DC interface and bidirectional DC interface of the distribution network and the cascade characteristics of power cascades. Based on this, it proposes a distributed energy storage system. Participate in the distribution network voltage regulation control strategy. This strategy requires no communication, which can improve the dynamic stability of the distribution network voltage and reduce the impact of power fluctuations of the DC distribution network on the AC grid. Finally, in order to prove the effectiveness of the proposed control strategy, a simulation model of DC distribution network was built and compared with the traditional control strategy.

Figure 1 Schematic diagram of control strategies for different types of distribution network nodes

in conclusion

This paper proposes a voltage regulation strategy based on distributed energy storage for DC distribution networks. By theoretically deducing the voltage-frequency coupling relationship of the AC-DC interface of the distribution network and the cascaded droop characteristics of the bidirectional DC interface, the corresponding node energy storage unit is controlled to compensate voltage fluctuations and grid frequency fluctuations, and its control strategy and basic parameters are designed. The effectiveness of the proposed control strategy is proved by system simulation.

The research results show that based on the proposed voltage control strategy, the fluctuation of bus voltage of DC distribution network and its influence on the large AC power grid can be effectively reduced when the power changes, and the robustness and fault traversing ability of the system are improved, and the energy storage unit is protected. At the same time, make full use of its power regulation margin. This paper provides a new method for system-level control of voltage control and energy storage unit of multi-terminal multi-node DC distribution network.