Standalone hybrid generation system for the remote area of Thar, Pakistan
Fig. 2.33. Generalized power electronics and control of a battery energy storage system
During discharge mode, the power flows from the BESS to the utility. In this mode, the DC-DC
converter maintains the DC bus voltage for the inverter, and the grid-connected inverter controls the
active and reactive power flow. The control of the utility connected inverter, as shown in Figure 52,
provides constant power control (Ye et al. 2006). Many control functions to deal with practical issues
are not shown in the diagram, such as the negative sequence regulation, DQ decoupling, etc. The
inner control loop regulates current, and the outer control loop regulates power. In some cases, the
reactive power reference, Qref, could be a power factor reference. By controlling this reference, the
injected current to the utility can be maintained at unity power factor. The output of the control
system is the high-frequency sine PWM signals for the voltage-source inverter switches. More details
of d-q based controller can be found in Appendix B. When the active power reference is increased,
the inverter draws more power from the DC bus, thereby decreasing the DC bus voltage. The DC-link
voltage regulator, as shown in Figure 52, tries to maintain the constant DC voltage by changing PWM
switching pattern for the three-phase controller rectifier switches.
2.6.4. Battery Charging/discharging Control algorithm
When sizing the battery bank, much attention is paid to the minimum and maximum energy limits
beyond which it shouldn’t discharge and charge, respectively. In other words, the battery is allowed to
charge and discharge between Ebb
peak
and E bb
min
. A battery peak control model “Charging and
discharging control mechanism” is devised based on the energy limits of the battery (or SOC), the
load demand and the supply. The following algorithm depicts how the charging and discharging of the
battery happens.
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