Topology of Wind Power DC Collection Converter Based on Double Bus Structure and Its Power Balancing Control Strategy

Wind power collection technology is the key to grid-connected wind power generation. Compared with AC wind integration, DC wind power collection is more advantageous. Using a triple active bridge (TAB) converter as a submodule, this paper proposes a wind power DC converter topology and its control strategy for medium-voltage dc (MVDC) grid connection. The converter adopts a double bus structure, with the MVDC bus connected to the grid and the low-voltage dc (LVDC) bus handling the mismatched power. It has independent maximum power point tracking (MPPT) control, power balancing, and autonomous voltage balancing on the output side. The converter system is simulated and verified on the MATLAB/SIMULINK software platform. The simulation results show the effectiveness of the control strategy and the ability of the converter to operate stably under multiple conditions.


Introduction
As the installed grid-connected capacity of wind power generation rises rapidly and the demand for large-scale convergence and long-distance transmission, the AC wind power collection is no longer well suited to the development.DC wind power collection has the advantages of reduced energy losses, high conversion efficiency; no harmonics, no frequency or reactive power problems; and stronger transmission capability, which is more consistent with the future direction of grid development [1] [2].
Due to the high-power level and high voltage gain of wind power collection systems, modular DC-DC converters are generally cascaded on the output side of submodules to meet the requirements.There are three main types of multi-module cascaded DC-DC converters, namely input-independent-outputseries (IIOS), two-stage conversion (TSC), and low-voltage bus-based (LVBB) [3][4] [5].The IIOS architecture is simple, and has few conversion stages, low losses and achieves independent MPPT; the TSC architecture achieves independent MPPT, but requires a two-stage power conversion with higher hardware costs and losses; the LVBB architecture, without independent MPPT that contributes to an increase in the production of energy, can achieve modular design, but also requires two-stage power conversion that harms system efficiency.All three structures suffer from unbalanced submodule output voltages due to mismatched input power, which can lead to increased losses, power commutation, MPPT failure, and even switch damage, severely testing the system [6][7][8].
This paper proposes a dual bus structure-based wind power DC converter, which uses triple active bridge (TAB) converters as submodules, for medium-voltage dc (MVDC) grid integration.Furthermore, the corresponding power balancing strategy is investigated to enable the converter with voltage balancing capability.The proposed converter is capable of independent MPPT and stable operation under various conditions.

Triple Active Bridge Converter
The topology of the TAB is shown in Figure 1.The three ports of the TAB are all full bridge units, coupled by a triple-winding high-frequency isolation transformer.In this paper, a phase shift modulation strategy is adopted for the TAB.The switching frequency of all switches is a fixed value of fs and the duty cycle is 50%.The drive signals of the upper and lower switches of the same bridge arm are inverted, and the drive signals of the upper and lower switches of the same path are the same.
The expressions for the output power of each port of the TAB converter are [9]: (1 (1 where V 1 , V 2, and V 3 are the dc side voltages of port 1, port 2, and port 3 respectively; D 12, D 23, and D 13 are the phase-shifted duty cycle between the switches of port 1 and ports 2, port 2, and ports 3 and port 1 and ports 3 respectively; L 12 , L 23 , L 31 are the equivalent inductors between the ports of the equivalent circuit of the TAB, the expressions for which are not repeated here [10].

Proposed Converter Topology and Its Control Strategy
The proposed topology of a dual bus structure DC-DC converter is shown in Figure 2. Port 1 of each TAB submodule is used as the energy input port to connect to the wind power unit; port 2 is used as the energy bi-directional port to connect to the LVDC bus in parallel; and port 3 is used as the energy output port to connect to the MVDC bus in series.Each wind power unit can be independently MPPT controlled as each unit is independently connected to a different TAB.Port 2 of the TAB submodule can absorb energy from or supply power to the low-voltage dc (LVDC) bus.
Taking a converter consisting of three TAB submodules as an example, Figure 3 illustrates the power balancing principle.As shown in Figure 3, when the input wind power varies, the submodule TAB2 with the higher input power transmits the mismatched power to port 2 of submodule TAB3 via the LVDC bus, which ultimately results in equal output power at port 3 of submodules TAB1, TAB2, and TAB3.This part of the mismatched power flowing through the LVDC bus is converted in two stages, while the rest of the input power is converted in only one stage, providing a high conversion efficiency.
Figure 4 is the block diagram of the proposed control strategy for each TAB submodule.Figure 4 (a) shows the independent MPPT control loop.Figure 4 (b) shows the control loop for output voltage balancing.Therefore, the power balancing mechanism of the control strategy is: when the input wind power of TAB i port 1 increases, the phase shift duty cycle D 13 increases, and the output voltage V 3_i of Figure 1 Topology of the triple active bridge converter its port 3 rises; as can be seen from Figure 4, when V 3_i rises, the reference voltage V 2ref_i of port 2 also rises and the phase shift duty cycle D 12 increases, making the power transmitted from port 1 of TAB i to port 2 P 12 increases and the corresponding P 13 decreases, and vice versa.Therefore, when an internal power mismatch occurs in the converter, for example when the input wind power of TAB i is greater than the input wind power of TAB j , V 2ref_i will increase and V 2ref_j will decrease, so that the input power is eventually distributed equally between TAB i and TAB j .

Simulation Verification
To further verify the effectiveness of the converter and its control strategy, a 10 kV system consisting of 13 TAB was built in MATLAB/SIMULINK software.The system parameters are listed in Table 1.As it is shown in Figure 5, an input power mismatch occurs at 0.08s.P 1 , P 2 decreases and P 3 , P 4 increases, and the port 3 voltages rise or fall accordingly.The simulation results show that V 3_1 , V 3_2 rise and V 3_3 , V 3_4 fall, but their instantaneous changes are within 0.5 V and return to the steady state value within 0.04 s.Therefore, the system has a good output voltage balancing effect when the input power is mismatched.
As it is shown in Figure 6, when the first wind power unit fails, the input power of TAB1 P 1 is 0. At this time, the TAB submodules of the wind power units have not failed to deliver power to TAB1 through the LVDC bus, making the converter input power balanced and the system output voltage balanced as well.As it is shown in Figure 7, when submodule TAB1 fails, the port 3 voltage of TAB1 is 0. The TAB submodules that have not failed still maintain normal operation, but their voltages V 3_2 ~V3_1 rise from 800 V to 866.6 V, and the voltage of the LVDC bus rises from 200 V to 331 V.It indicates that when the TAB submodule of the converter fails, the control strategy can achieve the balance of the converter system output, but cannot guarantee that the voltage value at the LVDC bus is kept around its rated value.

Conclusion
In this paper, a double bus structure DC-DC converter for wind power MVDC grid integration with TAB as submodules is proposed.The converter achieves independent MPPT, modular design, and power collection.Furthermore, the converter can realize autonomous power and output voltage balancing through the implementation of the control strategy.

Figure 2 Figure 4
Figure 2 Converter topology diagram Figure 3 Schematic of power balancing principle

Figure 5 Figure 6
Figure 5 Simulated waveform of the converter system in the case of power mismatch