Based on the analysis of the influencing factors on the charge throughput of modular multilevel converter-battery energy storage system, this section will first introduce the overall control strategy of modular multilevel converter-battery energy storage system; Then, a charge throughput suppression strategy based on second harmonic circulation injection is proposed; Finally, analyze the method for selecting the optimal second harmonic circulation component.
1. Modular Multilevel Converter-Battery Energy Storage System Overall Control Strategy
The overall control strategy of modular multilevel converter-battery energy storage system is shown in Figure 1, which includes four stages: AC power control, DC power and circulation control, charge throughput suppression, and modulation strategy.
① AC power control: achieve AC power control by controlling the output voltage reference value uj *;
② DC power and circulation control: Control of DC power and circulation by controlling ucirj *;
③ Charge throughput suppression: By injecting a second harmonic circulating current to suppress battery charge throughput, the specific method for selecting the second harmonic circulating current component will be provided later;
④ Modulation strategy: Calculate the voltage reference values of the upper and lower bridge arms, use PD-PWM modulation to obtain the trigger pulse of the power device.
2. Charge Throughput Suppression Strategy Based on Double Frequency Circulation Injection
According to analysis, in medium voltage DC distribution networks, the AC and DC side voltages of Modular multilevel converter-battery energy storage system are usually fixed, and the DC bus voltage Vdc and modulation ratio m are fixed values. Therefore, a charge throughput suppression strategy based on second harmonic circulation injection is proposed.
Firstly, analyze the scope of application of the proposed strategy. If there is no zero crossing point in the bridge arm current, Modular multilevel converter-battery energy storage system will not generate additional charge throughput and there is no need to design additional charge throughput suppression strategies. Therefore, the applicable conditions for the charge throughput suppression strategy proposed in this article are as follows:
Secondly, analyze the basic principles of the proposed charge throughput suppression strategy. The double frequency circulating current component does not change the energy distribution between the bridge arms, does not affect the voltage or SOC balance of the upper and lower bridge arms, and is one of the effective means of additional control for MMC. In addition, due to the completely independent control of the Modular multilevel converter-battery energy storage system AC and DC circuits, the injection of second harmonic circulating current does not affect the harmonic characteristics of the AC output current. Therefore, this article suppresses charge throughput by injecting a second harmonic circulating current, and the expression of the injected second harmonic circulating current is as follows:
In the formula, λ The amplitude coefficient of the second harmonic current is the ratio of the amplitude of the second harmonic current to the amplitude of the alternating current; θ The phase angle of the second harmonic circulation.
After injecting a second harmonic circulating current, the bridge arm current mainly includes DC, fundamental, and second harmonic components, which can be expressed as:
By substituting the formula, the total charge throughput of each submodule battery pack in phase a after injection of circulating current can be expressed as the formula.
By adjusting the circulation component and reducing Qa in the formula, the charge throughput suppression of Modular multilevel converter-battery energy storage system can be achieved. It should also be pointed out that the second harmonic circulating current injected by the proposed method will slightly increase the effective value and loss of the bridge arm current.
3. Selection of optimal second harmonic circulation component
To achieve the best Modular multilevel converter-battery energy storage system charge throughput suppression effect, it is necessary to calculate the second harmonic circulating current injection component that minimizes Qa in the formula. Considering the difficulty in obtaining an analytical solution for the optimal circulation through formulas, an offline calculation method is adopted to solve for the optimal circulation parameters λ and θ Numerical solution for.
Based on the actual measurement values of system parameters, the injection circulation is dynamically adjusted using the lookup table method to achieve optimal suppression of Modular multilevel converter-battery energy storage system charge throughput under various operating conditions. In addition, linear interpolation can be used to further improve the suppression effect of the proposed method under variable parameter operating conditions. Firstly, initialize the circulation parameters; Secondly, input system operating parameters such as USD, Vdc, P * ac, P * dc, etc; Furthermore, calculate k and m according to the formula; Finally, determine whether k satisfies the applicable conditions of the proposed charge throughput suppression strategy, and if so, select the optimal second harmonic circulating current parameters corresponding to k and m λ and θ， If not satisfied, exit the program directly.
Under different m and k conditions, to achieve the minimum charge throughput of Modular multilevel converter-battery energy storage system, the amplitude parameter of the optimal second harmonic circulating current that needs to be injected is determined λ And phase angle parameters θ As shown in Figures 2 and 3, respectively. As shown in Figure 2, when the power of the battery is equal, the required injected second harmonic current amplitude under the condition of k>0 is smaller than that under the condition of k<0; At the same time, as the modulation ratio decreases, the amplitude of the required injected second harmonic circulation also gradually decreases. As shown in Figure 3, in most operating conditions, the optimal initial phase angle of the circulating current is 0, and it is only not 0 when k>0 and close to 1. At this time, the additional charge throughput is almost 0, and the effect of phase angle on the suppression of charge throughput can be ignored. Therefore, we will θ Set to 0.
The comparison results of Modular multilevel converter-battery energy storage system charge throughput before and after charge throughput suppression under different modulation ratios m and energy storage ratios k are shown in Figure 4. Figure 4 (a) shows a three-dimensional view of the charge throughput of two strategies, while Figures 4 (b) and 4 (c) show the top view of the Modular multilevel converter-battery energy storage system charge throughput under the proposed charge throughput suppression strategy and without suppressing the charge throughput. When the battery power is the same, compared to when k<0, when k>0, the additional charge throughput generated by Modular multilevel converter-battery energy storage system is relatively small, and the suppression effect of the charge throughput achieved by injecting a second harmonic circulating current is also better. When the proportion of energy storage power k exceeds a certain value, the additional charge throughput can be completely suppressed. It should be pointed out that this article only focuses on φ= Take the typical working condition of 0 as an example for analysis. However, the proposed method is effective for φ The same applies to working conditions that are not equal to 0. Taking m=0.8 as an example, provide φ When not equal to 0, the suppression effect and optimal circulation parameters of the proposed charge throughput suppression strategy are shown in Figures 5 and 6, respectively.
