University of South Wales Master of Sciences Thesis

 
Standalone hybrid generation system for the remote area of Thar, Pakistan
 
 
 
There are different modes of operation for the BESS connected to the utility. The BESS can either 
send power to the utility by discharging or it can receive power to charge itself (Chiang et al. 1998). 
The operation mode control block, as shown in Figure 52, decides the charging/discharging operation 
for the BESS. The control design for power electronic systems is different for charging and 
discharging modes. Based on the mode in which the battery is operating, the control signals from 
charging or discharging blocks are connected to the power electronics system by the selector switches 
Sw1 and Sw2. 
The operation mode control block is designed based on a simple charge-discharge schedule of the 
BESS. The BESS can send the power to the utility during the peak-load period (i.e., from 6 p.m. to 10 
p.m.) only if the voltage of BESS is greater than the nominal value (Vb,nom). During the discharge 
mode—based on the present voltage (Vb) and the state-of-charge (SOC) of the BESS—the Pref signal 
determines the amount and rate of discharge to be generated. The BESS can be charged any other 
time, provided the SOC of the battery is smaller than the maximum storage capacity (SOC max). The 
switch control signal is generated based on a lookup table. According to the design, the signal “0” 
means no charge/discharge, “1” means discharging, and “2” indicates charging. This signal 
simultaneously controls switches Sw1 and Sw2. Based on signal status, the power electronics 
converters are either connected to the charging block or the discharging block. Other charge-discharge 
schedules can also be programmed in the operation mode control depending on the application.
When the mode of operation for the BESS is charging, the power flows to the battery system through 
power electronic converters. The DC-DC converter determines the voltage at the battery terminals 
(Vb) based on which the battery is charged. The battery voltage regulator generates the PWM pattern 
based on the reference battery voltage (Vb*) (coming from operation mode control) such that the (Vb) 
follows this reference voltage. For the proper control, the DC-DC converter requires a constant DC 
input. The DC-AC converter works as the controlled rectifier and the controller maintains the DC bus 
voltage (Vdc) at a preset value. This control design is a variation of the constant power control (Ye et 
al. 2006). Instead of using the active power reference, a DC bus voltage is regulated while the input to 
the inverter acts as a constant power source to represent the prime mover. In this case, the output of 
the DC bus regulator is proportional to the active power. 

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