*2.6. Control of the Grid-Connected Photo Voltaic Inverter*

A typical grid connected solar power conditioning system consists of a three-phase two level PV-inverter for converting DC power to AC power, a sine filter to smoothen the AC output and a transformer to couple the inverter and the grid. The transformer also provides isolation between AC side and DC side. Figure 5 shows a control block diagram for a grid connected PV-inverter. In this system, the PV array voltage and currents are to be monitored for MPP tracking and the grid voltage is to be monitored for the phase-locked loop (PLL). The controller senses the charging current or discharging current of the battery and the MPP of the PV array and then calculates the maximum possible power that can be fed to the grid. The current reference is generated based on the maximum possible power and the PLL output. Three phase grid voltage is applied to the PLL to find out the angle wt. Angle wt obtained through the PLL is used to generate Id and Iq components from three phase grid currents. After comparing the reference Idq currents and actual Idq currents, the error signals are given to the PI controllers for the active and reactive power control. The PI controller outputs are converted back to three Phase modulating signals and given to the PWM generator to generate inverter gate pulses [14,15].

**Figure 5.** Control block diagram of a grid-connected photo voltaic inverter.

The perturb and observe method is used for maximum power point tracking (MPPT). In this method, the following activities are carried out:

	- In this state, PV voltage = Voc and the PV current = 0
	- Initialize PV power reference MPP = 0
	- Minimum PV power reference (MPP\_Ref\_Min) is limited to 0.
	- Maximum PV power reference (MPP\_Ref\_Max) is limited to 113 kW.
	- Measure PV voltage and current and calculate the new PV power (Ppv\_New)
	- If Ppv\_New is more than Ppv
		- Ppv == Ppv\_New
		- MPP == MPP + 500 Watt
	- If Ppv\_New is less than Ppv
		- Ppv == Ppv\_New
		- MPP == MPP − 500 Watt

The normal operation sequence of the system is explained below:


In this work, a new power balancing control algorithm for the proposed configuration is developed to meet the operational requirements of the system. The proposed algorithm is explained in the next section.

#### **3. Power Balancing Control of the Grid Energy Storage System in Photo Voltaic Applications**

Power control of PV with ESS for off-grid applications is presented in [16]. In the system presented, the battery and PV arrays are connected to the common AC load through independent converters i.e., as an AC-centric system. During charging of the battery, the PV array supplies power to the AC load and battery. When the battery is fully charged, the battery and PV arrays supply power to the common AC load. Conditions for battery charging and discharging and control of power converters during Mode1 and Mode2 operation are explained briefly. Experimental results were presented to show the dynamic response of the system during mode changeover.

Control for high power PV + a fuel cell plant with hybrid energy storage consisting of a battery and the supercapacitor is presented in [17]. Each energy source and energy storing element are connected to a common DC link through independent converters in this configuration. Energy management among the different sources, control for charging the super-capacitor and batteries is explained briefly. Experimental results were presented for explaining the dynamic characteristics of the system and plant performance during long load and short load cycles. Since the system presented is for off-grid applications, Mode3 operation i.e., charging the battery from the grid supply is not covered in this work. Real-time simulation of the hybrid energy system with wind-PV-battery storage is presented in [18]. In the presented system, independent converters for battery, PV modules, and the wind are used. Based on the power availability of all the sources, an algorithm is developed for battery charging, discharging and load shedding.

The systems presented in [16–18] are for off-grid applications hence the control during Mode3 of operation is not covered. System configurations presented in the above works require independent converters for each source, which may increase the cost of the system and also may increase the complexity of the control algorithm. The above mentioned drawbacks can be mitigated with the proposed system configuration and with the power balancing control algorithm explained in the next subsection.

The proposed system is the combination of three phase PV inverter and a DAB-based battery charger explained in Section 2. Power flow through the inverter can be controlled over a wide range through current control. Battery current can be controlled in both directions through DAB using a phase angle control. Power balancing among the three sources in the presently proposed system is achieved by controlling the power flow through the battery charger and inverter. The power balance control algorithm shown in Figure 6 is explained below:

	- If the PV voltage (Vpv) is less than the minimum PV voltage required (Vpv\_Min) then MPP of the PV array is zero. Vpv\_Min is the minimum DC required to match the inverter output voltage with the transformer secondary voltage. In this system, 620 V is the minimum DC link voltage required, as shown in Table 2.
	- In case the MPP is higher than minimum value i.e., P\_PV\_Min then the system is in Mode1.
	- In this mode of operation, the battery will be in charging state and the PV array provides the power for battery charging.
	- The remaining power after battery charging will be transferred to the grid.
	- Based on the SOC of the battery, the reference battery charging current I\_Batt\_Ref is obtained.
	- Through the power balancing equation, the inverter reference current Id\_Inv\_Ref is calculated based on the MPP and battery current:
		- Inverter power reference = MPP − battery power reference

(the battery Power reference is positive during charging mode and negative in discharging mode)

	- ➩ Id\_Inv\_Ref = √2 \* Iabc\_rms\_ref
	- ➩ Id\_Inv\_Ref = <sup>√</sup>2 \* (MPP <sup>−</sup> [I\_Batt\_ref \* V\_Battery])/(√3 \* Vabc\_rms)
	- In this case based on the backup time adjusted by the user, the reference battery current I\_Batt\_ref is calculated. In case the backup time adjusted by the user is 6 h, then the battery current reference is calculated as follows:
		- ➩ I\_Batt\_ref = rated Amp-hour rating of the battery/backup time
		- ➩ I\_Batt\_ref = 288 Ah/6 h = 48 A
	- Through the power balancing equation, the inverter reference current Id\_Inv\_Ref is calculated based on the MPP and battery current:

Id\_Inv\_Ref = √2 \* (MPP – [I\_Batt\_ref \* V\_Battery])/(√3 \* Vabc\_rms)


(4) After determining the battery reference current I\_Batt\_Ref and inverter reference current Id\_Inv\_ref, the controller implements the closed loop current control through PI controllers and releases the gate pulses to the inverter stack and battery charger stack.

**Figure 6.** Algorithm for power balancing control of grid energy storage system in photo voltaic applications.

The proposed algorithm was tested on the real controller with the help of Hardware-In-Loop simulations. The need for HIL simulations, features of the real-time digital simulator and the setup built for HIL simulation for the proposed configuration are explained in the next sections.
