# Grid-Friendly Active Demand Strategy on Air Conditioning Class Load
## 4 Active Prevention Strategy on LVRT and Reactive Compensation
### Energy Storage System Based on Isolated DC/DC Converter  
#### 4.1.1 The Topology of the Energy Storage System  
To enable the system LVRT ability, the internal structure of the IRC can be skillfully utilized, and the energy storage system is added to the DC bus of the converter. The DC side and the energy storage system are joined together by a bidirectional DC converter, which is an isolated structure to achieve LVRT control, as shown in Figure 5.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Harmonic/Number** & **5** & **7** & **11** & **13** & **15** & **17** & **19** \\ \hline
**Before filtering \(i_{\text{a}}\)/\(A\)** & 37.94 & 19.81 & 15.12 & 11.48 & 9.50 & 8.07 & 37.94 \\
**APF filtering \(i_{\text{a}}\)/\(A\)** & 3.20 & 2.97 & 3.03 & 3.15 & 3.04 & 2.91 & 3.20 \\
**Active and passive filtering \(i_{\text{a}}\)/\(A\)** & 2.58 & 2.21 & 0.68 & 0.41 & 0.38 & 0.35 & 2.58 \\ \hline \hline \end{tabular}
\end{table}
Table 2: THD of odd harmonic before and after filter.  
Figure 5: Fuzzy adaptive PI control of energy storage.  
The DC side of the full-bridge can withstand the change of high power, and the energy storage system of the super-capacitor is connected to the push-pull side. The equivalent model of the super-capacitor uses a series connection of capacitor SC and internal resistance R to function as a bidirectional flow of energy. During the process of frequent charge and discharge of the energy storage system, transient fluctuations are easily induced by the sudden change of the inductor current. Therefore, a freewheeling inductor L2 and a diode are connected in parallel on the push-pull side to make a direction change of the current easy of the conduction. When the bidirectional DC converter operates in the buck mode, the current flows from the loop of L1. In the Boost mode, the current flows from the loop of L2, which can effectively solve the shortcoming of transient fluctuation of the current during the charge and discharge switching of the single inductor. In this paper, the isolated bidirectional DC/DC converter is used to realize the control of LVRT. When the DC bus voltage is greater than the rated voltage during the grid malfunction, the bidirectional DC converter works in the buck mode. At this time, the switch tubes S5 and S6 are equivalent. The diode plays the role of synchronous rectification. By controlling the conduction and deactivation of S1-S4, the excess energy on the DC side is stored in the right energy storage element. During the complementary conduction of the switching tubes S1 and S4, as well as S2 and S3, the DC-side voltage is stored in the inductor L1 through the single-phase full-bridge and the transformer, and the inductor L1 passes through S5 and S6 during the simultaneous shutdown of S1-S4. Perform freewheeling to release energy. When the DC bus voltage is less than the rated voltage, the converter operates in the boost mode. At this time, the switch tubes S1-S4 correspond to the diodes, which play the role of synchronous rectification. The purpose of the energy storage system to transfer energy from the push-pull circuit to the DC side is achieved by controlling the switching tubes S5 and S6. During the alternate conduction of the switch tubes S5 and S6, the energy storage element transfers energy to the left DC bus through the push-pull converter and the transformer, where the inductor L2 stores energy and the current rises. During the simultaneous conduction of S5 and S6, the inductor L2 continues to flow through S5 and S6, releasing energy and decreasing the current. Therefore, the push-pull full-bridge bidirectional converter can operate in a step-up or step-down mode according to changes in the DC-side voltage during a grid malfunction, maintaining the DC bus voltage in the allowable range, and achieving LVRT of the entire system.