*5.1. Inverter, Grid and Load Currents in Different Modes of Operation*

Figure 9 shows the current waveforms in Mode1 of operation. Since the PV array can produce more than the minimum power required for charging the battery and feeding the internal loads connected to the plant, the additional power produced by the PV array is supplied to the grid. An irradiance of 1000 W/m<sup>2</sup> is adjusted on the user interface panel, hence the PV array is producing the maximum possible power. From the presented result, it can be observed that the grid current is phase displaced by 180 degree with respect to the inverter current.

**Figure 9.** Currents of inverter, grid, load, and voltage at PCC in Mode1 of operation.

In Mode2 of operation, an irradiance of 0 W/m<sup>2</sup> is adjusted on the user interface panel; hence the PV array cannot produce any power. The battery is in charged condition and supplies the power to the load based on the Amp-hour rating of the battery and the discharging time or backup time adjusted by the user.

Since the local loads consume more than the inverter supplied current, the remaining current is drawn from the grid as shown in Figure 10. Since the load is drawing current from both the sources, the grid current and the inverter current are in phase with each other.

**Figure 10.** Currents of inverter, grid, load , andvoltage at PCC in Mode2 of operation.

In Mode3 Operation, the PV array cannot produce any power and the battery is in discharged condition. Since the battery is to be charged, the grid supplies the necessary charging current to the battery and the current required for the local loads as shown in Figure 11.

**Figure 11.** Currents of inverter, grid, load, and voltage at PCC in Mode3 of operation.

The dynamic response of the system to a step change in inverter reference power is shown in Figure 12.

**Figure 12.** Currents of inverter, grid, load for a step change in reference power.

When the inverter power reference is more than the local load requirement then the inverter is supplying current to grid and load. Since the grid is receiving the current, the phase displacement between grid and inverter currents is 180 degrees. After a step change in the reference power, since the reference power is less than the load requirement, the load current is supplied from both the inverter and the grid, hence the both the currents are in phase with each other. With the present controls, the system reaches the steady state within one cycle time.

#### *5.2. Battery Charger Input and Output Currents in Different Modes of Operation*

Battery current is considered as positive during charging and negative during discharging of the battery. The battery will be in charged condition in Mode1 and Mode3 of operations as explained earlier. During charging, depending on the SOC of the battery, the charging current reference is obtained and the controller carries the closed loop current control of the battery charger. The battery charger input and output currents for different modes of operation are discussed below. In Mode1 operation, since the battery is in charging condition, battery current and the average value of battery charger input current are positive, as shown in Figure 13.

In Mode2 of operation, when the PV array voltage is less than the minimum DC link voltage then the battery charger operates with closed loop voltage control and maintains a constant DC link voltage. The current through the battery depends on the Id Reference of the inverter which is obtained through the Amp-hour rating of the battery and the backup time required for the user. Since the battery is in discharging condition, battery current and the average value of the battery charger input current are negative, as shown in Figure 14.

**Figure 13.** Battery charger input and output currents in mode1 operation.

**Figure 14.** Battery charger input and output currents in mode-2 operation.

Similar to Mode1, in Mode3 operation battery current and the average value of the battery charger input current are also positive as shown in Figure 15. The charging current required for the battery is provided from the grid supply in this case.

**Figure 15.** Battery charger input and output currents in mode3 operation.

The dynamic response of the battery charger system is observed by applying a step change in the battery current reference. The system takes approximately 250 milliseconds to come to the steady state as shown in Figure 16.

**Figure 16.** Battery charger reference and actual currents for a step change in reference current.

### **6. Future Scope**

The results presented in this paper are obtained through real-time digital simulations. A scaled down model of the plant i.e., power circuit can also be made to test the controller and control algorithm. A predictive diagnostic in high-power transformers used in traction grade uninterruptible power supplies is presented in [20]. In similar lines, predictive diagnosis of the system components in a grid energy storage system can also be addressed as future work.

The presented system can also be extended for smart grid applications by incorporating additional energy sources along with the PV and battery, then as future work, the security and privacy problems in this smart grid can be addressed, as discussed in [21].

In this paper, design and control of grid energy storage system with a conventional PV inverter is explained in detail. Additional feature reactive power compensation can be incorporated to make the system work as PV-STATCOM. This system can also be extended for high power applications by using multilevel configurations for the PV inverter [22,23].
