## 4 DC-DC Bidirectional Converter Control  
Battery storage system is essential for a stand-alone wind energy supply system to maintain the balance between the generated power from wind turbine and required load power through charge/discharge energy to/from this storage system. In this paper, the batteries bank is connected to the DC-link voltage through a DC-DC bidirectional buck-boost converter; the control of this converter can maintain the DC-link voltage at constant value as a reference value in addition to charge/discharge current to/from the batteries bank according to the required load power. The batteries bank voltage can be kept lower than the reference DC-link voltage (\(V_{dc}^{\ast}\)) and hence less number of batteries need to be connected in series. In the proposed system the batteries bank voltage is kept at about 300 V while \(V_{dc}^{\ast}=690\) V. In this paper the rating of the batteries bank, considering 60% depth of discharge (DOD), is decided based on the assumption that even when the wind power is zero it should be able to supply power to the load of a 15 kW for approximately an hour. The calculation of the battery rating is  
\[\mathit{Battery}\ \ \mathit{rating}=\frac{15\,kW\times 1 hr}{300\times 0.6}=83.333 \ \mathit{Ahr}\]  
Figure 6: Generator power reference _versus_ generator speed.  
Hence, 85 \(Ahr\), 12 V battery rating is considered and therefore, twenty five numbers of batteries are required to connect in series.  
The DC-DC bidirectional buck-boost converter shown in Figure 1 is operated in continuous conduction mode. The IGBTs Q1 and Q2 are switched in such a way that the converter operates in steady state with four sub intervals. The circuit operations in steady state for these different intervals are elaborated below:  
* Interval 1 (Q2 ON); in this interval the lower switch Q2 is turned ON and the upper switch Q1 is turned OFF with diode D1, D2 reverse biased as shown in Figure 7(a). During this interval the converter operates in boost mode and the inductor is charged and current through the inductor increases.
* Interval 2 (D1 ON); during this interval both switches Q1 and Q2 are turned OFF. The diode D1 of the upper switch Q1 starts conducting as shown in Figure 7(b). The converter in this case supplies power to the DC-link voltage.
* Interval 3 (Q1 ON); the upper switch Q1 is turned ON and the lower switch Q2 is turned OFF with diode D1, D2 reverse biased as shown in Figure 7(c). During this interval the converter operates in buck mode and transfer power to the batteries bank.
* Interval 4 (D2 ON); during this interval both switches Q1 and Q2 are turned OFF, the diode D2 of the lower switch Q2 starts conducting as shown in Figure 6(d).  
The control strategy of the DC-DC bidirectional buck-boost converter uses a PI controller to regulate the output DC voltage at a reference value is shown in Figure 8. In this control technique, the DC voltage \(V_{dc}\) is sensed and compared with the reference DC voltage \(V_{dc\_ref}\). The error signal is processed through the PI controller. The output signal is the duty cycle of the switches Q1 or Q2  
Figure 7: Converter operation modes. (**a**) Interval 1; (**b**) Interval 2; (**c**) Interval 3; (**d**) Interval 4.  
according to the case of charging or discharging. Where, the parameters of the PI controller are obtained by using Zeigler Nichols tuning method.  
The presence of inductor in batteries bank side results low ripple current which achieves higher efficiency and longer lifetime.