Solar‐powered switched reluctance motor‐driven water pumping system with battery support

Funding information Department of Science and Technology (DST), Govt. of India, Grant/Award Number: RP02979; Improvement of Science and Technology, Grant/Award Number: RP03195G Abstract This work deals with the development of an efficient and reliable solar photovoltaic-fed water pump with a battery energy storage (BES). This system ensures a continuous and rated supply of water in all working conditions. A new control logic for BES is developed, which significantly improves the overall response of the system. The support of the battery as a backup source is obtained through a bidirectional DC-DC converter. Both the charging/discharging control and maximum power tracking of the solar panel are achieved through this bidirectional converter, whereas the voltage asymmetry compensation across split capacitors at DC link and motor drive controls are supervised by switched reluctance motor converter. The performance of the present scheme is examined through both simulated and experimental responses and is found adequate under all operational scenarios.

intermittency in its power [11]. On the other side, the uncertainty in grid power supply, especially in remote and distant areas, is still the major constraint with the grid-integrated solar water pump [12]. Besides, the voltage fluctuations in the grid voltage in such areas can also damage the system and reduce the overall system reliability. These shortcomings with standalone and grid-integrated systems are needed to be considered in order to ensure a reliable and uninterruptable water supply with the efficient operation of the WPS [13,14]. Therefore, the standalone system with battery support is one of the unique solutions and is developed in this work. Again, no such system with any of the motor drives including SRM is reported as of now. In addition, the battery connected to the system also provides the constant voltage at the DC link irrespective of any voltage fluctuation in the system. Therefore, this work is aimed to develop an SPWPS, which is reliable and perfect for critical applications like water supply in hospitals, schools, industries, and so forth. The batteryintegrated system has served many purposes like storing the solar power inadequacy, suppressing or smoothing out transients that occured in the system because of PV array power supply, and provide the high magnitude of starting currents to the drive under low insolation levels.
Various attempts are made to develop an SRM-driven solar water pump [8-10, 15, 16] in which very few have the storage arrangement [10,15,16]. One such configuration is reported in [10], which has incorporated the three-phase configuration of SRM. Although the system has the capacity to offer uninterrupted pumping regardless of the environmental situations, the high-frequency switching signals utilised to operate the motor converter and its overvoltage limitation under damper failure result in the configuration to be less efficient. Besides, the constant-voltage tracing technique used for solar power optimisation is less effective and faces major limitations under continuously changing environmental conditions. Simulated results showing the battery current has a high value of ripples, which could shrink the performance of the battery. Besides, the execution of an unregulated charging/discharging control approach severely affects the working life span of the battery pack due to the frequent maintenance required for the system. Another arrangement using an 8/6 pole configuration of SRM drive for solar water pumping is given in [15]. This arrangement is a twostage system, and therefore the losses are high due to two-stage conversion of solar power before supplied to the drive. Besides, the high ripples in the battery current due to the boost converter diminish the system performance. An isolated control system without incorporating the knowledge variation in solar power badly affects battery performance and its service life. One similar configuration to the present system is discussed in [16] where the maximum power point tracking (MPPT) converter is successfully eliminated. However, the developed controller for the scheme has not been tested for all possible mode's operation. Since the PV power optimisation is achieved through the motor control, there is no such MPPT control present when solar PV array is used to charge the battery, and pumping is not required. Besides, the uncontrolled charging/discharging of the battery is also one of the major limitations of suggested system. Thus, the present work has taken care of all the deficiencies present in the aforesaid literature and successfully has overcome them. The bidirectional converter offers both solar power optimisation and the charging/discharging control of battery energy storage (BES). The PV power is optimised in all possible modes of operation, and the elimination of solar output capacitor and DC-link voltage sensor has significantly reduced the overall system cost. The initial winding current suppression at the drive side is also offered in the present system based on the inertia of the motor. The implemented battery management system has significantly improved the system's dynamic performance, especially under insolation change without using any extra sensors. The major contributions of this work and salient features are as follows.
1. An efficient arrangement of a solar power-energised water pump with a battery storage scheme is presented in this work. 2. The charging/discharging control of the battery is integrated with a bidirectional DC-DC converter, while the optimisation of PV array power is achieved using a perturb and observe (P&O) MPPT control executed through the same bidirectional DC-DC converter.
3. Due to the highly inductive nature of motor windings, SRM is the most suitable drive for such configurations. 4. A modified version of the P&O MPPT approach is utilised to optimise solar power under all weather conditions.

CONFIGURATION OF SYSTEM
The configuration of the developed system is shown in

SYSTEM OPERATION
The developed system has basically four operating modes. The symbolic power flow mechanism for the present system is illuminated in Figure 2. The battery can directly be connected to the system without any converter. However, the required size of the battery bank is larger in this case, which directly affects the cost and size of the developed system. Moreover, the DC-link voltage ripple is to be absorbed by the battery bank, decreasing its lifetime. One potential way is to rule out these limitations by utilising a bidirectional DC-DC converter with a battery bank to interface a standalone solar system as shown in Figure 2. Hence, there is a wide scope in the selection of the battery terminal voltage magnitude, offering minimisation in battery price and its dimension. The bidirectional converter operates as a conventional boost converter to supply the power from the battery to the motor if the PV array power is not able to meet the requirement.
Mode I: Throughout this mode, the solar power generation is adequate to run the pump at its full capacity. So the battery does not supply or store the energy and just helps to track the maximum power and maintain the reference DC-link voltage, which is v pvref in the present configuration.
Mode II: In this working scenario, the power coming from the solar panel is inadequate to operate the system alone at rated speed, and therefore the rest of the power is coming from the battery bank through a bidirectional converter. The bidirectional converter in this mode acts as a boost converter. The Insulated-gate bipolar transistor (IGBT) switch S boost of the bidirectional converter is operated while the reverse diode of switch S buck provides the path for the remaining part of the current flow.
Mode III: This is the situation when there is no solar power generation. In this case, the total required power is coming  Mode IV: It is the condition when the solar power is coming into the system and water is not required. Thus, available PV array power is utilised to charge the battery bank. This mode necessitates the bidirectional converter to act like a buck converter with a duty cycle of D b2 = 0.22.

DESIGN OF SYSTEM
Various operating stages of the developed system are properly designed to achieve the desired performance. The rating of SRM is 750 W with a rated speed of 1500 rpm. The peak output power of the solar panel is 860 W, which is adequate to run the motor at its peak value. The design of different components of the developed system is given incoming sub-sections. The current at MPP,

Solar panel selection
where p pv = P mpp = 860 W is the output peak power. The modules linked in series are: The number of modules linked in parallel are: Connecting 20 modules in series, with one string in parallel, a PV array of required size is designed for this system.

Design of bidirectional DC-DC converter
Since the bidirectional converter works in a boost or buck mode as per the necessity, the switches, S boost , S buck , are operated alternately. The magnitude of duty cycle D b1 under charge storage (bidirectional converter works in buck mode) is estimated as [15] v Similarly, the value of the duty cycle when the battery is discharging the power to the SRM pump (bidirectional converter works in boost mode) is estimated as Therefore, the switching of the bidirectional converter keeps on varying between these values of duty cycles, D b1 , D b2 , according to the working conditions.
According to these two values of duty cycles, there are two different sizes of an inductor, L b , corresponding to buck and boost mode operation. In buck mode of operation, S buck is on and the energy is stored into the inductor, L b . Similarly, when the bidirectional converter is in boost mode, S boost is active and the inductor, L b , discharges its stored energy. However, for both the modes, the value of the inductor is nearly the same. Therefore, the size of inductor L b is estimated as

CONTROL SCHEME
The control logic is designed for a developed system to operate efficiently under all working situations. The MPPT operation with battery management controls is achieved through a bidirectional converter. The BES control is dedicated to optimising the charging/discharging performance of the battery bank and enables the flow of power in either direction. The other control is dedicated to the efficient motor drive operation. This motor control itself is divided into two sub-parts. The first part involves providing the soft-starting to the motor when back electromotive force (emf) is zero, whereas the second part of this control is responsible for the efficient steady-state operation of the drive and voltage balancing between two split DC-link capacitors in steady-state and dynamic conditions. All these control techniques are elaborated in the following sub sections.

Modified solar power optimisation
The conventional P&O MPPT approach is not much effective and has some limitations like deviation problems under varying insolation levels. The flow-chart of the conventional P&O MPPT technique is displayed in Figure 3. Thus, to overcome the limitations of the conventional approach, a modified P&O MPPT approach is implemented in the present system to strengthen the solar panel operation and improve its working under all situations [17]. The flowchart of the executed MPPT control is illustrated in Figure 4. This flowchart is self-explanatory of the logics behind the implemented MPPT technique.

Voltage balancing approach with drive speed control
The efficient operation of developed SRM drive at initial and in the steady-state situation relies on mainly two control approaches. The first control technique ensures the softstarting of a drive without using any current sensor at the motor side, whereas the second implemented control at the motor side is responsible for the optimal selection of dwell angle for  Logic for conventional bidirectional DC-DC converter controller electronic commutation of SRM drive and to nullify the unevenness across split capacitors. Including various advantages, the utilised voltage balancing approach also offers a small switch and diode ratings for the SRM converter. The logic developed for the present drive is to work the system efficiently as well as to make voltage asymmetric compensation across two split capacitors under initial and in the steady-state situation, which is demonstrated in Figure 4,5 After measuring the voltages across both DC-link capacitors, the compensation signal is provided to the commutation block. The comparison of the resultant duty cycle and a sawtooth signal of 7 kHz frequency results in pulse-width modulation (PWM) signals, 'S 1 -S 4 '. At final, both the signals, that is, PWM pulses, 'S 1 -S 4 ' and H1'-H4' signals including self-start control are merged with each other utilising 'AND' logic. The final PWM switching scheme for a midpoint converter, 'T1-T4' is used to switch the mid-point converter.

Control of bidirectional converter
The bidirectional battery controller used in conventional schemes [15,16] has limitations of uncontrolled charging/discharging of the battery and ineffective to the change in solar parameters dynamics. The structure of the basic conventional controller is shown in Figure 6. Thus, the present developed bidirectional controller has successfully acknowledged both these features and offers better performance of the overall system. The designed controller of the bidirectional converter is responsible for seamless power flow within the system by maintaining the desired voltage at the DC link. The main logic of the developed controller for the bidirectional converter is illustrated in Figure 7. This algorithm integrates the data of instantaneous PV array as well as a preset value of lower and upper battery current limits. In this correction, the overcharging or complete discharging of the battery is controlled by online estimation of the state of charge (SoC) of the battery by using its   [18,19]. It is present in this method that if SoC is less than 20%, then the discharge rate controller provides the zero signal to the switch, S 1 , and the bidirectional converter stops working. On the other side, when SoC is more than 98%, then the controller provides the zero signal to the switch, S 2 , and there is no further charging, and the bidirectional converter becomes inactive. The maximum and minimum values of battery current are taken as 1.5 and 3.4 A. The reference value of battery current is expressed as where i b_upp. and i b_low. signify the preset limits. The term 'p pvpeak ' is the rated power of the solar panel. Thus, the designed controller regulates the battery parameters within a set band regardless of the operating conditions. The BES terminal voltage variation at rapid insolation changes has been shown in Figure 8. It is observed that the scheme with a developed battery controller has a better response of terminal voltage at insolation changes, which limits any high transient across BES, and it helps to improve the service life of the battery.

SIMULATED PERFORMANCE
The simulated performance of the developed scheme for solar water pumping is discussed and examined in detail in this part. The initial and steady-state performances are shown using simulated results in Figures 9-12. The solar irradiances have changed from 1000 to 500 W/m 2 and then zero at 0.8 and 0.95 s in order to determine the response of the system under continuously varying working conditions.

Figures 9(a) and (b) illustrate the starting and steady-state per-
formances of PV array (v pv , i pv , p pv ), battery (v b , i b ) and SRM drive parameters (i ph1 , i ph2 , i ph3 , i ph4 , N, T e and T p ) at standard test conditions (STC) (1000 W/m 2 , 25 • C). In comparison to the double stage system, less ripple is observed in PV array parameters as illustrated in Figure 9(a) and is due to Behaviour of scheme while the water pumping is suddenly started under when PV array generates its peak power (a) response of PV array and battery parameters and (b) performance of SRM parameters the removal of the DC-DC converter stage, which produces high switching ripples in the system. The smooth tracking of maximum power point (MPP) voltage and generation of maximum output power, that is, 860 W from the PV array are displayed in Figure 9(a). The MPPT tracking duration is higher in this configuration due to the relatively large value of the PV array output capacitors. The positive value of battery current at starting shows its discharging operation to provide the initial current to drive. After this, the battery current goes to zero similar to double-stage configuration, and the PV array provides the complete power to pump the water at installed capacity.
Moreover, the starting and steady-state performances of motor drive parameters (i ph1 , i ph2 , i ph3 , i ph4 , N, T e, and T p ) for this configuration are illustrated in Figure 9(b). Figure 9(b) depicts the starting and steady-state SRM drive parameters. The successful suppression of all four winding currents at starting and soft-starting of SRM are manifested in Figure 9(b). All the motor parameters achieve their peak values and show the complete discharge of water at maximum insolation level with the battery in floating condition. The motor speed steadily rises to achieve the rated operating conditions. Thus, effective and proficient working is concluded by observing Figures 9(a) and (b).

Dynamic behaviour of system
The dynamic response of PV array (v pv , i pv , p pv ), battery (v b , i b ) and SRM drive parameters (i ph1 , i ph2 , i ph3 , i ph4 , N, T e and T p ) under varying insolation levels or absence of PV array power are demonstrated in Figure 10. The performance of the system is tested under the same situation that has been used for the double-stage configuration. It is observed from Figure 10(a) that the BES offers the extra power required to the SRM pump under low irradiance conditions. However, the DC-link voltage, v dc , is always maintained at the v pvref . After 0.95 s, when the solar power becomes zero, the battery discharges at its full value to alone provide the power to the SRM pump. Although the magnitude of v pvref is zero , the DC-link reference voltage is smoothly shifted to the fixed constant voltage, that is, 310 V as shown in Figure 10(a). The behaviour of SRM parameters under this dynamic situation is also observed and exhibited in Figure 10(b). The uninterruptable operation of the SRM drive is governed by the constant speed and rated values of winding currents as illustrated in Figure 10(b). The speed of SRM is maintained at its rated value, that is, 1500 rpm and manifests the full volume of water also under dynamic environmental conditions. Exactly similar to the double stage configuration, this configuration also has a satisfactory performance under dynamic conditions or even the absence of solar power.

Voltage balancing performance across DC-link split capacitors under dynamic condition
Since a certain DC-link voltage is always present across the split DC-link capacitors, it is also very critical and important to balance out the voltage across split capacitors. As the MPPT converter is not present, the voltage balancing together with the motor drive control is carried out using a mid-point converter. The starting and dynamic behaviours of v dc1 , v dc2 with and without the voltage balancing control are illustrated in Figures 11(a) and (b). The nature of v dc1 , v dc2 in starting, steadystate, and under dynamic conditions without implementing   Figure 11(a). In comparison to a two-stage system, the response is more severe. It is observed from Figure 11(a) that there is a huge difference between the two voltages and the motor would fail to start. There is also a difference in v dc1 , v dc2 under steady state and under varying insolation changes, that is, from 1000 to 500 W/m 2 and then 1000 W/m 2 , which leads to a ripple voltage of PV array and motor parameters. On the other side, there is a significant improvement in the performance of two voltages with the effect of voltage balancing control. The response of these two voltages with voltage balancing control is shown in Figure 11(b). Including the starting situation, the difference between the two voltages is also minimised in settled and in varying solar irradiance conditions as manifested by Figure 11(b).

Performance at sudden start of water pumping
The performances of this system when the SRM drive is suddenly switched on at the moment when the PV array power is generating its peak power and charges the battery at its full capacity are shown in Figures 12(a) and (b). At the instant, when the motor operation is not needed, the battery-side converter is operated as a conventional buck converter. However, once the pumping is started, the same converter performs as a boost converter to achieve the MPP. The charging phenomenon of the battery is governed by the negative magnitude of i b as illustrated in Figure 12(a).
It is observed that the fluctuations in PV array and battery parameters are a little higher than the two-stage systems. However, all parameters are settled down within a few seconds and work normally. The behaviour of SRM drive parameters under the situation when it has suddenly started is shown in Figure 12(b). All winding currents i ph1 , i ph2 , i ph3 , i ph4 rise in a controlled fashion and manifest the successful soft-starting even under projected adverse conditions.

EXPERIMENTAL RESULTS
The authentication of the developed WPS is carried out experimentally on a developed prototype in the laboratory. The setup includes a solar simulator, a mid-point converter, a battery pack, a bidirectional converter, and an SRM coupled to a DC generator. An AMTEK manufactured ETS600 × 17DPVF Terra SAS solar PV simulator is utilised to realise the solar PV array of 860 W peak power capacity. A SEMIKRON make stack type number, MD B6U 415/560-50F+ MD B6C1 750/415-35F, converter is utilised to realise a bidirectional converter, whereas a SEMIKRON made stack type number, MD B6U 415/560-35F+ MD B2C1 600/415-30F, IGBT-based converter is used as a mid-point converter in the developed experimental setup. The selection of battery specifications is done under some assumptions. It is considered that the PV power generation is for a maximum of 8 h/day. Thus, the battery rating is selected such that the motor can operate a minimum of 7-8 h short of utilising PV power. For conceiving the needed capability of the battery stack, the terminal voltage, v b , and current, i b , are of magnitude 240 V and 28 Ah, respectively. Hence, the developed system is able to operate the system at its full capacity even in the early morning, evening, or at night. Thus, a 240/28 Ah SMF Exide battery bank with rack and enclosure is selected for the present system. The architecture of the developed signal conditioning board is demonstrated in Figure 13. To sense the voltage 'v pv ' and currents 'i pv ', 'i b ', Hall-effect-based voltage-current sensors (LV25P, LA55P) are employed, respectively. Additionally, the picture of the developed experimental setup to validate the real-time effectiveness of the system is shown in Figure 14.

Behaviour of system indices when solar PV panel alone runs the pump
The nature of v b , i b , and motor parameters in mode I, is demonstrated using Figures 15(a) to (d). The responses of v b , i b , winding current of SERM, i ph1 are illustrated in Figure 15(a). The captured results demonstrate the skilled soft-starting and perfect MPPT operation in steady-state conditions. It also shows FIGURE 16 The system behaviour when only battery drives the motor (a) starting response of i ph1 , N, v b , i b , and (b) steady-state response of i b , i ph1 , v dc1 , v dc2 that the motor speed has achieved its peak magnitude, and so the system is able to supply the full volume of water under this working condition. The battery connected to the system is in an idle situation and assists to maintain the MPPT operation without any power exchange. Figure 15(b) presents the response of motor parameters i ph1 , v ph1, i n at 1000 W/m 2 under settled condition. It is worth mentioning that the phase voltage is nearly half of the rated DC-Link voltage, and therefore the required rating of the IGBT switch for the SRM converter is also small as compared to other similar motor-driven converters. The dynamic performance of system parameters especially solar panel and motor side is shown in Figure 15(c). It is seen that in this mode I, the MPPT controller acted very effectively and optimised the solar performance through the bidirectional converter control. Since the reference DC-link voltage is the same as the MPPT voltage, the same magnitude equal to 'v pv ' appeared across the DC-link voltage. The response of the implemented MPPT controller is displayed in Figure 15(d). It is remarked that the tracking efficiency is more than 99% and governs the successful optimisation of solar power. Moreover, the generated power is 860 W, which signifies a considerable reduction in losses due to a reduction in intermediate DC-DC MPPT converter.

Battery alone runs pump
The nature of v b , i b, and motor parameters mode II, is demonstrated in Figures 16(a) and (b). The responses of indices i ph1 , N, v b , i b in both starting as well as in settled conditions are given in Figure 16(a). The controlled rising of motor phase current and battery discharging current manifest the efficacy of  Figure 16(b). Both the magnitudes of v dc1 , v dc2 are perfectly equal and balanced throughout the operation. The PWM switching operation of the motor converter is observed by the nature of winding current, i ph1 .

Dynamic performance at varying insolation levels
The behaviours of v pv , v b , i pv and i b under irradiance variation, that is, from 800 to 400 W/m 2 are illustrated in Figure 17(a).
It is observed that there is a momentary dip in solar output voltage, v pv , under the change in insolation levels from 800 to 400 W/m 2 . However, within a fraction of a second, it again attains its MPP value and operates efficiently at lower insolation level. The increase in the magnitude of battery current governs the smooth supply of required power by the storage battery to the drive for the uninterrupted full volume of water pumping. Figure 17(b) shows the condition when there is an increase in insolation from 500 to 1000W/m 2 in mode II when PV panel power is not adequate to run the pump at its full rating. As it is demonstrated by Figure 17(b), the speed of the drive is unaffected by this insolation change and runs at its full speed.
There is also a successfully maximum power tracking in this mode. Moreover, the effectiveness of the implemented MPPT approach and the achievement of desired performance of the system in mode IV are illustrated in Figure 17(c). In this case, when water output is not obligatory, the battery stack is getting charged through available PV array power.

When solar PV array power is suddenly out and the pump is running with battery
One more probable condition is usually encountered in the rainy seasons when the system is operating at its full capacity and suddenly output of solar PV array becomes zero. The behaviour of system parameters v pv , i ph1 , v dc, and i b under such conditions is demonstrated in Figure 18. There is a small drop in PV array voltage, that is, 'v dc ' is observed at the moment when solar power has gone. However, v dc again maintained its reference value, that is, 310 V. The positive magnitude of current i b in Figure 18 demonstrates the full discharge of the battery bank to run the system at its rated value.

7.5
The behaviour of the developed system when the pump is abruptly closed This section represents the behaviour of system parameters when suddenly the pump is switched off and the available solar power is diverted from the drive to the battery storage. The disappearance of the motor winding current in the middle of the operation signifies the removal of the load.
The performance of various parameters of the system is shown in Figure 19. In this case, there is fluctuation in DClink voltage, but it is settled down within a short span of time. The battery is now receiving solar power and charges to its maximum capacity. Moreover, a comparison between conventional systems [10,15,16] and the proposed topology considering various aspects has also been done and given in Table 1. It is seen that the present scheme successfully overcomes all the limitations of the existing system and has various benefits over them.

CONCLUSION
A battery-supported solar PV array WPS with an SRM drive has been used, and their applicability has been exhibited using a MATLAB/Simulink platform. Additionally, the responses achieved via simulation have been verified on an experimental setup. The battery support acts as a secondary energy source linked at the common DC link in present configurations. An efficient power streaming control scheme has been developed and implemented in the present scheme. The developed voltage balancing technique to maintain the equal voltage across DC-link split capacitors has been observed to perform perfectly. Besides, the design logic has enabled uninterrupted water pumping with the full volume of water delivery irrespective of the climatic conditions. Overall, this topology has emerged as a reliable and efficient WPS.