# Research on Composite Control Strategy of Quasi-Z-Source DC-DC Converter for Fuel Cell Vehicles
## 4 Simulation and Experimental Results
### Simulation Results  
The load resistance is 400 \(\Omega\) and the reference output voltage is 400 V. When the input voltage jumps between 50 V and 60 V, the output voltage \(u_{\text{O}}\) and input voltage \(u_{\text{in}}\) are as shown in Figure 6.  
Figure 6a is the output voltage under input voltage disturbance with only feedback controller. When input voltage disturbance occurs, the output voltage maximum excursion is around 7%. Figure 6b is the output voltage under input voltage disturbance with feedforward compensation and feedback controller. The output voltage maximum excursion is less than 1.5%. The result indicates that the feedforward compensation network improve the system's ability to resist input voltage disturbance. In the process of fuel cell vehicle driving, the vehicle demand power changes frequently, which requires fast dynamic response performance and an excellent ability to resist load disturbances for the converter. The output voltage is as shown in Figure 7 during load 50% step change. When the input voltage is 40 V and the reference output voltage is 400 V, the load resistance jumps between 400 \(\Omega\) and 200 \(\Omega\). In Figure 7, it can be observed that the output voltage maximum excursion is less than 3% and the adjustment time is less than 30 ms during load 50% step change. Figure 5: Block diagram of feedforward compensation and the feedback control system. The worldwide harmonised light vehicle test procedure (WLTP) is adopted to test the converter. The WLTP consists of low speed, medium speed, high speed and extra-high speed, as shown in Figure 8[26,27]. The parameters of the fuel cell used in the simulation are shown in Table 3. The relationship between the demand power and the speed of the vehicle is as shown in Equation (30). \[P_{\text{load}}=(M\frac{dv}{dt}+MgC_{\text{r}}+\frac{\rho_{\text{air}}Sv^{2}}{ 2})v \tag{30}\]  
where \(v\) is the vehicle speed. \(P_{\text{load}}\) is the demand power of the vehicle. The vehicle weight is \(M=300\) kg. The rolling resistance coefficient is \(\text{Cr}=0.001\). The front windowal area of the vehicle is \(S=1\) m\({}^{2}\). The gravity acceleration is \(g=9.81\) m/s\({}^{2}\). The wheel radius is \(Rad=0.14\) m. The volume density of air is \(\rho_{\text{air}}=1.2\)kg/m\({}^{3}\)[28]. The fuel cell vehicles typically use the compound energy system, which consists of a fuel cell and an ultracapacitor. The bi-directional DC-DC converter is used between the ultracapacitor and the DC bus to provide instantaneous power or to recover braking energy. When the required power \(P_{\text{load}}\) in Equation (30) is less than zero, the vehicle is recovering energy. The quasi-Z-source network DC-DC converter in this paper is used between the fuel cell and the DC bus only to provide power for the vehicle. Therefore, the vehicle load demand power \(P_{\text{load}}\) is limited to zero when the \(P_{\text{load}}\) is less than zero. Then, the vehicle load demand power \(P_{\text{load}}\) is scaled down in order to be less than the converter's rated power. The simulation result of the converter load current is shown in Figure 9. In order to track the load current variation of the converter, the output current of the fuel cell is shown in Figure 10. The output voltage of the fuel cell is shown in Figure 11.  
Figure 6: The output voltage variations under input voltage disturbance. (**a**) With only feedback controller; (**b**) with feedback controller and feedforward compensation. Figure 7: The output voltage during load 50% step change. Figure 8: Worldwide harmonised light vehicle test procedure (WLTP). Figure 5:0 The fuel cell output current under the WLTP.