ORIGINAL CONTRIBUTION

Switching Logic for Converting Off-grid PV Customers to On-grid by Utilizing Off-grid Inverter and Battery

A. R. Anishkumar1 • P. Sreejaya1

Received: 13 March 2015 / Accepted: 4 January 2016 / Published online: 25 May 2016

� The Institution of Engineers (India) 2016

Abstract Kerala is a state in India having a very good

potential for solar PV energy production. The domestic

customers in Kerala using PV system are approximately

15 % and almost all of them are using the off-grid PV

system. When these off grid customers move to on-grid

system, off grid system accessories such as inverter and

batteries become redundant. In this paper, a switching logic

has been developed for the effective utilization of off grid

accessories and reducing islanding power loss for on grid

customers. An algorithm is proposed for the switching

logic and it is verified using simulation results and hard-

ware implementation.

Keywords Off grid solar PV system �Grid connected PV system � Micro inverter �ON grid inverter � Switching logic for islanding problem

Introduction

Photovoltaic generation system can be divided into the off-

grid solar inverter system and the grid-tied solar inverter

system. The off-grid solar inverter system is mainly used in

composition-independent photovoltaic power generation

system, applied in the family, the countryside, island, and

remote areas of the power supply, and urban lighting,

communications, testing and application of the system of

power supply. Figure 1 is a system block diagram that

shows the main components of the off grid solar inverter

system, which consists of a solar panel, maximum power

point tracking (MPPT) charge controller, battery, dc-ac

inverter and ac load.

While the grid-tie solar inverter system is mainly used

in parallel with the traditional utility grid, the solar

inverter converts the energy from the PV panel to the

traditional utility grid. The main components are solar

panels and microinveters (Fig. 2). Microinverter converts

direct current generated by a single solar module to AC.

Output from several microinvertes are combined and fed

to the electric grid. Microinverters have several advan-

tages over conventional inverters. The main advantage is

that small amounts of shading, on any one solar module,

do not disproportionately reduce the output of the entire

array. Each microinverter harvests optimum power by

performing maximum power point tracking for its con-

nected module.

Grid-tie inverters are also designed to quickly discon-

nect from the grid if the utility grid goes down. This is an

national electrical code (NEC) requirement that ensures

that in the event of a blackout, the grid tie inverter will shut

down to prevent the energy it transfers from harming any

line workers who are sent to fix the power grid. This is

called an islanding condition.

Methodology

This paper focuses on the off grid users in Kerala. When

off grid users move to on grid system, off grid system

accessories such as inverter, MPPT charge controller and

batteries become redundant. Another issue related with on

grid system is the power loss at the time of islanding; the

load does not get any power from the panel or from the

grid.

& P. Sreejaya

sreejaya@ieee.org

1 Department of Electrical Engineering, College of

Engineering, Trivandrum, India

123

J. Inst. Eng. India Ser. B (December 2016) 97(4):581–588

DOI 10.1007/s40031-016-0240-x

For the effective utilization of off grid accessories and

reducing islanding power loss, a switching logic has been

developed. This switching logic is incorporated in a

MATLAB model of off grid system which can be con-

nected to an on grid model with micro inverter. The

working of the logic is verified using simulation results and

hardware implementation.

Switching Logic

Switching logic improves the efficiency of both on grid and

off grid system. In on grid system, at the time of grid fault,

PV panel power is wasted because of islanding. The

problem can be solved by incorporating an off grid system

with an on grid system. By using the switching logic, the

system can be operated in two modes. Mode 1 represents

the normal working condition and mode 2 represents grid

fault condition or islanding.

Mode 1

From Fig. 3, at normal working condition, switches S1 and

S2 are closed and micro inverter is tied to the grid and S6 is

closed to charge the battery. Battery charging is limited to

60 % and the balance charging is done under islanding

condition. Load is supplied by closing S3. All the other

switches are in open condition.

Mode 2

At the time of grid fault S1, S3, S5 and S6 are opened and

S2 and S4 are closed so that on grid system is changed to

off grid system. The balance charging is done in this

mode.

Algorithm

Algorithm is shown in flowchart (Fig. 4)

1. Start

2. Initial settings are done that is, the load reference

(Idref) is set to I1 and panel reference current (Ipref) is

Fig. 1 Off grid solar inverter

system

Fig. 2 Grid tie solar inverter system

Fig. 3 Switching logic block

diagram

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set to I2, grid voltage reference (Vgrid) is set to V1.

All switches in off condition.

3. Panel current is measured.

4. Check whether the measured panel current (Ip) is

greater than or less than the reference panel current.

If panel current is greater than reference panel

current, then switch on S1, S2. If panel current less

than reference panel current (PV output is less), then

measure the battery charge (Bc). Then check whether

battery charge is greater than Bdm (Battery charge

minimum) or not. If yes, then turn on S4, so that load

is supplied from the battery and repeat the loop,

otherwise go to step 8.

5. Measure the grid voltage (Vg). Calculate the differ-

ence between grid voltage and reference grid voltage

(Vd).

6. Check whether the difference is less than or greater

than the threshold value Vdm (voltage difference

minimum). If the Vd is less than the threshold value

then the grid is healthy and so turn on S5, S3 and turn

off S2. Now the system is ONGRID. The load is

supplied by the grid and also by the panel.

Fig. 4 Flowchart

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7. If Vd is greater than the threshold value then the grid

is not healthy and so turn on S4 and turn off S1 and

the system is OFFGRID. Load is supplied by the

panel. Go to step no 9.

8. Check the battery charge (Bc). If the battery charge is

greater than BCM (battery charge maximum), the loop

is repeated, if not, turn on S6 to charge the battery up

to BCM.

9. Measure the load current (IL), compare the load

current with the reference value. If the measured

current is less than the reference current, turn off S1,

S5, S3, S6, and S4. That is no load in the system. If

the measured current is greater than the reference

current then repeat the loop. Reference current is zero

(no load).

10. Stop.

Simulation

PV panel, MPPT charge controller, boost converter, off

grid inverter and on grid inverter are modeled in

MATLAB [1, 2]. The perturb and observe (P&O) method

is used for MPPT [3, 4]. Figure 3 is simulated and

depending on the grid condition, system can be made to

work both on grid and off grid with the help of six

switches [5]. Let us consider two modes of operation;

mode 1 is normal condition, where the system is working

as an on grid system. In this mode, load is powered by

both grid and PV panel.

In mode 2, for voltage sag from 0.2 s to 0.4 s, the sys-

tem is switched to off grid system and the load is served

from the solar panel.

Simulation Results

Simulation parameters are given in Table 1. Figure 5

shows the output voltage waveform of 95 V PV panel.

The panel is connected to the on grid system and off

grid system by switches S1 and S2 respectively. In

normal working condition (mode 1), S1 is closed and S2

is opened. So the panel is connected to the on grid

inverter. In mode 2, S1 is opened and S2 is closed so

that the panel is connected to off grid inverter. The

algorithm is explained earlier and is applied. The fol-

lowing results are obtained.

Figure 6 shows the grid voltage and load voltage when

there is a voltage sag in the grid from0.02 s to 0.04 s. The load

voltage remains constant because of the switching action.

Since switch S6 is closed in normal condition up to

0.02 s, battery is charged from both grid voltage and PV

panel voltage as shown in Fig. 7.

From 0.02 s to 0.04 s, there is a voltage sag in the grid

when S2 and S4 are closed and all other switches are open.

Table 1 Simulation parameters

PV Output Voltage 200 W 95 V 2 A

Battery 150 V

Voltage Sag 0.02–0.04

Load RL load R = 100 X; L = 0.01 lH

Fig. 5 PV panel output voltage

Fig. 6 Grid voltage and load

voltage

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Then the battery charging is done only from the PV panel

output.

Load voltage, load, grid and panel currents are shown in

Fig. 8. Up to 0.02 s, load current is shared by the grid and

PV panel. During 0.02 s to 0.04 s, grid is disconnected

from the supply and grid current is zero. Then the full load

current is supplied by the panel.

Hardware Implementation Circuit Diagram

Circuit Description

Circuit consists of a microcontroller (AT89C51), an analog

to digital converter (ADC0808), a timer (NE555), 6

switches (MOSFET, IRF540) and their gate drive circuits

Fig. 7 Battery SOC and

voltage

Fig. 8 Load voltage, load

current, grid current and panel

current

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(IR2110), a voltage regulator (IN7805) and an op-amp

(LM324) as shown in Fig. 9.

The controller controls all the switches after getting

appropriate commands. Since the controller reads only

digital data, the analog values of voltage and currents from

different equipments are converted to digital values by

ADC. Both ADC and controller get the supply from the

battery in the system by voltage regulator. The clock signal

for ADC is obtained from 555 timer.

The grid voltage, inverter voltage, battery voltage,

voltage corresponding to panel current and voltage cor-

responding to load current are given to different channels

of ADC (CH0,CH1,CH2,CH3,CH4 respectively). These

analog values are converted to digital values and given to

the controller. Controller checks the conditions and gives

appropriate commands to the driver circuits of MOSFET.

6 pins of controller (P1.0, P1.1, P1.2, P1.3, P1.4, and

P1.5) are used for sending the switching signals to driver

IC.

Fig. 9 Circuit diagram

Fig. 10 Experimental setup

Fig. 11 Grid share in normal condition

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Hardware Set Up and Results

The experimental setup includes 250 W, 30 V PV panel,

250 W micro inverter, 24 V, 1kVA off-grid inverter,

1 kW MPPT charge controller, 24 V 100 Ah lead acid

tubular battery, a charging circuit and 60 W lamp load as

shown in Fig. 10. The system works as per the algorithm

explained. Bdm is taken as 10 % and BCM is taken as

60 %. In normal condition, PV panel is connected to grid

through micro inverter and the load is supplied by both

grid and micro inverter. At the same time, the battery is

charged through the converter. If the battery is charged to

60 % then it is disconnected from the grid. If grid is

disconnected from the system, the load is supplied by

panel through off-grid inverter. If both the grid and panel

are out of service, load is supplied by battery through off-

grid inverter.

Case 1: Grid and PV Sharing the Load

Figure 11 shows the voltage, current and power supplied to

load from grid during normal condition. The grid share is

16.09 W and the rest of the load power is shared by the

micro inverter.

Case2: Power Flowing Towards the Grid

When the load is switched off, the solar power of 42.77 W

flows towards the grid (opposite sign) and it is shown in

Fig. 12.

Case3: Islanding (Grid under Fault)

Since the micro inverter disconnected PV from grid, load is

supplied from Off grid inverter and the output is shown in

Fig. 13. dc output of PV panel under this condition is

shown in Fig. 14.

Conclusion

This paper proposes the model of an on grid micro inverter

connected to an off grid system. By using a switching

logic, the load can be served by both the grid and the PV

panel. Under islanded condition, it is served from the

panel. Both the simulation and experimental results verifies

the working of the switching logic.

Thus an existing off grid customer can use his off grid

inverter and batteries when he goes for an on grid con-

nection while rectifying the major issue of islanding

problem.

Acknowledgments The authors would like to thank College of

Engineering, Trivandrum, Kerala, India for funding the project.

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Fig. 12 Solar power to grid under no load condition

Fig. 13 Off grid inverter output in islanding condition

Fig. 14 PV panel dc output in islanding condition

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