# 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
#### 4.1.2 Parameter Design of the Energy Storage System  
As the most important part of the energy storage system, the capacity configuration and parameter design of the super-capacitor is very important. If the capacity setting is too small, the excess energy at the DC bus side during LVRT may not be absorbed, and the system requirement cannot be met. If the setting is too large, it may cause waste and increase economic costs. For a system with a rated power of 5 kW, the power transfer efficiency of the isolated bidirectional DC converter is about 75%. Assuming the capacitance of the super-capacitor is \(C_{sc}\), the initial voltage is \(U_{sc\_init}\), and the rated voltage is \(U_{sc\_final}\), the charge and discharge power at the maximum instantaneous moment is expressed by Formula (8).  
\[P_{sc\ max}\geq P_{r\ max}\cdot\eta=5000W\times 0.75=3750\ W \tag{8}\]  
The rated voltage on the DC side is 540 V, the super-capacitor voltage is set to 150 V, and the LVRT time is set to 0.4 s. According to Formula (9), the capacity of the super-capacitor can be deduced.  
\[C_{sc}\geq\frac{2\eta\cdot P_{r\ max}\cdot T_{lwtt}}{U_{sc_{final}}^{2}-U_{sc_{ init}}^{2}}=\frac{2\times 3750\times 0.4}{540^{2}-150^{2}}F=11.14\ mF \tag{9}\]  
Consider a certain margin, set the rated capacity of the super-capacitor 12 mF. Firstly, to make the super-capacitor not overcharge or over-discharge in the process of LVRT, the energy storage system is protected. Secondly, considering the current withstand capability of the power electronic device in the converter, the operating voltage is set higher. It can protect these power electronics from over-current and breakdown damage.  
Filter Inductor \(L_{sc}\) Design  
The design value of filter inductance needs to meet the working requirements of both the Buck and Boost circuits [24]. At the early stage of LVRT, a bidirectional DC/DC converter operates in Buck mode and is charged to the super-capacitor utilizing DC bus capacitors. In the recovery phase of LVRT, the bidirectional DC/DC converter operates in Boost mode, and the super-capacitor is used. The constant current discharge mode feeds back the stored energy to the DC bus \(U_{dc}\). Since the average current in the loop mode is lower than the maximum current in the Buck mode, the filter inductor size can be designed according to the maximum ripple current in the loop mode of the Buck mode, as Formula (10)  
\[L_{sc}=\frac{D\cdot(U_{dc}/n-U_{sc\_init})}{\Delta I_{scp}\cdot f_{s}} \tag{10}\]  
\(U_{dc}\) is the rated voltage of the DC bus. \(U_{sc\_init}\) is the initial voltage of the super-capacitor (150 V). \(n\) is the ratio of the isolation transformer. \(D\) is the duty cycle in the Buck mode. \(I_{scp}\) is the peak value of the rated current of the super-capacitor in the Buck mode. \(\Delta I_{scp}\) is the maximum ripple current allowed by the circuit, usually set to 15% of the peak current rating. \(f_{s}\) is the switching frequency of the tube in a bidirectional DC/DC converter.