Single-phase single-stage multifunctional grid interfaced solar photo-voltaic system under abnormal grid conditions

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Single-phase single-stage multifunctional grid

interfaced solar photo-voltaic system under

abnormal grid conditions

ISSN 1751-8687 Received on 30th May 2014 Revised on 3rd September 2014 Accepted on 6th October 2014 doi: 10.1049/iet-gtd.2014.0533 www.ietdl.org

Chinmay Jain ✉, Bhim Singh

Department of Electrical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India

✉ E-mail: chinmay31jain@gmail.com

Abstract: This study deals with a single-phase single-stage multifunctional grid interfaced solar photo-voltaic (SPV) system. The proposed SPV system is multifunctional as it has MPPT (maximum power point tracking) and it provides harmonics elimination, reactive power compensation and feeding SPV energy into the grid at unity power factor. The PV array is connected at the DC link of voltage source converter. The MPPT controller estimates reference PV voltage and a proportional integral controller is used to maintain the PV string voltage to the reference value. Feed-forward terms are used for SPV array and load components for fast dynamic response. The performance of the system is analysed under abnormal grid (sudden sag and swell) conditions. Simulation studies are performed on MATLAB-based platform. Simulation results are verified experimentally on a developed SPV system. Under steady-state operating conditions, the total harmonic distortion of grid voltage and currents are found well under IEEE-519 standard. A wide range of simulation and experimental results are presented to demonstrate all the features of the proposed SPV system.

1

Introduction

The recent fall in the cost of solar photo-voltaic (SPV) energy, and ever increasing prices of fossil fuels, has moved the world’s attention towards SPV energy systems. The green energy sources are booming day by day. The grid interfaced SPV inverters have been proposed by many researchers. Interfacing SPV systems with the grid incorporate many challenges such as the cost, efficiency, circuit topology and control algorithm as shown in [1–5]. Conventionally two-stage grid interfaced SPV energy systems are used, in which the first stage performs MPPT (maximum power point tracking) and the second stage is used to feed extracted energy into the grid. These two stage systems suffer from drawback of two power converters of full rating. Several configurations of grid interfaced PV farms are proposed in [6]. However, increasing cost of land has moved attention towards small distributed generating systems. The concept of distributed roof top small power SPV inverter solves the problem of cost of land up to some extent.

Recently the single-stage SPV systems are introduced by some researchers. The single-phase low voltage SPV inverters are installed in the distribution system. These inverters locally inject active power into the grid and help in the reduction of losses in the distribution line and the transformer. The structure of the distribution system is changing day by day. In case of linear loads, the voltage regulation and distribution losses are the main concerns. However, the increasing power converters in the distribution system are causing serious power quality problems. The loads such as computer SMPS (switched mode power supply) and motor drives draw non-linear currents from the grid. The harmonics content of these currents causes harmonic voltage drop in equivalent grid impedance, which in turn distorts the voltage at PCC (point of common coupling). Shunt active power filters and DSTATCOM (distribution static synchronous compensator) are proposed to deal with such power quality problems [7–11]. A DSP (digital signal processor)-based implementation for the shunt active power filter under harmonic distorted grid condition is shown in [7]. An OCC (one cycle control)-based controller for only harmonic compensation is shown in [8]. A back propagation algorithm-based control algorithm for power quality improvement

is shown in [9]. A voltage regulation technique for stiff grid is shown in [10]. A control algorithm which uses neural network-based conductance estimation is shown in [11]. Hence, the researchers are working exhaustively on the devices for power quality in the distribution system.

Some researchers have proposed two stage SPV grid interfaced systems with an active power filtering feature. A Goertzel algorithm-based two stage system is proposed in [12]. A single-phase two-stage system with a half-bridge zero-voltage switching-based control is proposed in [13]. Single-stage CSI (current source inverter)-based grid connected SPV systems are proposed in [14, 15]. A three-phase CSI-based multifunctional SPV system is proposed in [14], although only simulation studies are presented. A single-phase CSI-based SPV grid interfaced system is proposed in [15].

Now, when these systems are in practice, the debate on next generation SPV inverters has started for single-stage systems. A three-phase dual function single-stage system is proposed by Singh et al. [16]. The same PV inverter pursuing the purpose of power quality improvement and feeding SPV energy into the grid brings in several advantages. The PV inverter then works as a voltage source converter (VSC) and performs additional functions along with feeding real power into the grid. Such multifunctional VSC reduces payback time for power converters. No strict guidelines are available for such multifunctional SPV inverters.

The initial cost of SPV systems is high because of the high cost of SPV panels. Once an SPV plant is installed the main focus is on extracting maximum energy output from the plant. Several MPPT algorithms are proposed in the literature [1719]. In general, all grid converters are required to have under-voltage and over-voltage protection. The minimum and maximum voltages of operation are set normally to 0.88 and 1.1 pu [20]. This range is narrow because the converter may lose control due to sudden voltage variation and an increase in converter rating. In a grid interfaced PV system, the trip on grid interfaced converter causes the generation loss. Hence, minimising the number of trips indirectly reduces the cost of per unit energy from SPV system. The distribution system voltage frequently varies in a wide range in the case of the weak grids; however, most of the researchers have proposed multifunctional SPV inverter for fixed grid voltage. IET Generation, Transmission & Distribution

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Therefore, in the proposed system, a robust controller is demonstrated and a small incremental increase in the cost of the converter can be justified to avoid generation loss.

The proposed work deals with a multifunctional single-phase single-stage VSC-based SPV system working under a wide range of grid voltage variations. The proposed SECS (solar energy conversion system) not only feeds solar energy into the grid but also performs all features of the shunt active power filter simultaneously. The proposed single-stage single VSC-based SECS serves the purpose of harmonics mitigation, reactive power compensation and MPPT along with feeding extracted energy at unity power factor. The proposed system acts as a distributed generation system feeding active power for the local loads which helps in the reduction of losses in distribution system. Moreover, the feature of harmonics mitigation and reactive power compensation also helps in reduction of distribution losses and improved utilisation of available resources by reducing rms current in the distribution line. The combined feature helps in reduction of installation area and running cost (operational, maintenance etc.) as compared to the installation of two separate systems.

A simple PLL (phase locked loop) less control is used to implement this system. The proposed control algorithm uses the load and PV feed-forward terms for the fast dynamic response. An adjustable DC-link voltage is used for MPPT control. An intuitive and simple control algorithm is presented for control of VSC of the SPV system. Moreover, the proposed control algorithm is robust to take care for abnormalities in the grid voltages such as sudden sag and swell in the grid voltage. An in depth physical interpretation of all internal signals involved in the control algorithm is also given to make control algorithm an intuitive. The robustness of the proposed control algorithm is demonstrated through simulation and experimental results. The combination of VSC and load (linear and non-linear) emulates as an equivalent resistance in all operating conditions. The operation of the proposed system is shown for a wide range of input voltage variations (170–270 V), which avoids the generation loss. The presented work is first simulated in MATLAB-based environment and then implemented experimentally. Both simulation and experimental results are shown for a wide range of grid voltage variations. The dynamic response of the system with proposed control algorithm is found satisfactory. The total harmonic distortion (THD) of grid current is found well under IEEE-519 standard ( <5%) [21].

2

System configuration

The system configuration is shown in Fig1. The proposed system consists of SPV string, a single-phase H bridge VSC, an interfacing inductors, a ripple filter, a single-phase grid and loads. The PV string which consists of a series parallel combination of small rating panels is connected across the DC link of the VSC. A single-phase H bridge inverter consists of two insulated gate bipolar transistor legs is used as a power converter module to interface PV power to the grid. The interfacing inductor is connected in series of VSC and the grid. The interfacing inductor absorbs the instantaneous voltage difference between the PWM voltage of VSC and the grid voltage. A ripple filter is connected in parallel to the grid to absorb switching harmonic ripples in the PCC voltage. The loads are combinations of linear or non-linear elements. A lagging linear load is considered for emulating a linear load and a diode bridge rectifier with RL load is considered for emulating a non-linear load. Overall, the loads are connected at the single-phase grid and VSC is connected at the PCC via interfacing inductor. The VSC is operated such that the grid experiences the combination of VSC and load as an equivalent resistance.

3

System control

The control algorithm of the proposed VSC-based system is the heart of the SPV system. The VSC is controlled such that the grid experiences the combination of VSC and load as an equivalent resistance ( +ve or −ve). When the load power is more than SPV power the combination is seen as +ve resistance else the combination is effectively shunt connected negative resistance. A PLL less control is proposed for VSC. A synchronising signal is extracted from the grid voltage. The H-bridge VSC serves the multifunction such as MPPT, feeding SPV energy to the grid, harmonics mitigation and reactive power compensation. A robust control is proposed so that the VSC is able to sustain sudden voltage sag and swell in the grid. The control algorithm consists of two main parts, which are maximum power point tracking and the control for the grid interfacing part of VSC. The control algorithm is described as follows.

3.1 Maximum power point tracking

A hill climbing MPPT technique is used in the proposed work for simplicity. In this MPPT algorithm, the operating point is perturbed in one direction and a change in the power is observed. In case, when the power output of string is increased then the operating point is perturbed in the same direction else the direction of perturbation is reversed. The MPP is achieved by keeping the SPV string voltage to Vmpp (voltage corresponding to peak power on power against voltage curve of PV string). The output of MPPT controller is the reference PV string voltage. The PV string voltage is set to reference value with the help of a PI (proportional integral) controller, which is explained in the next subsection. The PV string is directly connected across the DC link of VSC as shown in Fig. 1, hence the reference PV string voltage is also the reference DC-link voltage. The reference DC-link voltage is continuously adjusted according to ambient conditions. The reference DC-link voltage in steady-state conditions is approximately equal to Vmpp, where Vmpp is voltage corresponding to peak power on power against voltage curve of PV string. The governing equations for hill climbing MPPT algorithm are as (1), where ‘step’ is the size of perturbation for reference DC-link voltage. The dPpv and dVpv are defined as changes in PV power and voltages, respectively, in consecutive steps. The MPPT controller estimates the reference PV string voltage (or DC-link voltage), which is then given to control algorithm for grid interfacing part of VSC.

3.2 Control algorithm for grid interfacing of VSC The interfacing of VSC ensures several functions such as maintaining DC-link voltage to set reference (estimated by MPPT controller), reactive power compensation, harmonics mitigation and feeding extracted SPV energy into the grid such that effective power factor at the grid is unity. Fig. 2 shows the control algorithm for H-bridge VSC. A total of five quantities are sensed; which are grid voltage, grid current, load current, PV string voltage and its current. The sensed quantities are given to ADC (analog-to-digital convertor) of DSP controller, where a set of orthogonal unit vectors are derived from the grid voltage. The unit vector in phase with the grid voltage is the synchronising signal whereas unit vector orthogonal to synchronising signal is used for load feed-forward term calculation. The peak grid voltage is estimated as Vp=  (v2 sp+v2sq)  (2)

Vdcref(k) = Vdcref(k − 1) + step, if dPpv. 0 and dVpv. 0 or dPpv, 0 and dVpv, 0) Vdcref(k − 1) − step, if dPpv. 0 and dVpv, 0 or dPpv, 0 and dVpv. 0) 

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where vspis the grid voltage and vsqis the grid voltage with 90° phase lead.

The SPV system feeds the power into the grid at unity power factor. For feeding the power at unity power factor synchronising signal is estimated. The synchronising signal is estimated as

up=vsp

Vp, uq= vsq

Vp (3)

The output of PI controller is considered as the loss component of VSC which is estimated as

Iloss(k) = Iloss(k − 1) + Kpvdce(k) − vdce(k − 1)

+Kivdce(k) (4)

The load feed-forward term is the active power component of the load current. The sensed load current is multiplied with the synchronising unit vector and a 90° shifted load current is multiplied with the quadrature unit vector, and their sum is then

passed through a low-pass filter to estimate active power component of the load current. The mathematical equation for estimation of active power component of load current is as

iL(v0t) = ILpsin v0t + ILqcos v0t + h=1

h=2 i

h (5)

where ILpis magnitude of fundamental load current in phase with PCC voltage. ILq is the magnitude of fundamental load current quadrature to PCC voltage and ihdenotes harmonic component of load current iL(v0t + p/2) = ILpcos v0t − ILqsin v0t + h=1 h=2 i′ h (6)

where ILpand ILqare the same as in (5) and ih′ denotes the phase

Fig. 1 System configuration

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shifted harmonic components of load currents I′

Lp=iL(v0t) · sin v0t + iL(v0t + p/2) · cos v0t (7)

On substituting values of iL(ωot) and iL(ωot + π/2) in (7)

I′ Lp= ILpsin v0t + ILqcos v0t +  h=1 h=2 i h 

·sin v0t + (ILpcos v0t − ILqsin v0t + h=1 h=2 i′ h) · cos v0t (8) I′ Lp=ILp+highfrequencycomponents (9)

Passing ILp′ through a low-pass filter gives amplitude of active power component of load current.

The PV feed-forward term is estimated as

Ipvp=2Ppv

Vp (10)

The physical significance of Ipvpis peak grid current for a given solar power and the grid voltage, without load compensation for a lossless system. Only PI controller-based system can also feed power into the grid, however, the dynamic response of that system is poor. It can be observed from that the PV feed forward term consists of two terms, Ppv(power from PV string) and Vp(amplitude of grid voltage). Both these terms help with the fast dynamic response. It can easily be observed that PVFF term is directly proportional to PV power and inversely proportional to the grid voltage magnitude. The Ppvterm helps during changing atmospheric conditions, whereas the Vp helps in fast dynamic response during sudden voltage sag or swell. The amplitude of grid current is estimated from these components. The load and system losses demand positive active power from the grid. Whereas, the PV string supplies active power to the grid. Hence, considering this the peak value of grid current, for overall UPF (unity power factor) operation is estimated as

Isp=ILp+Iloss−Ipvp (11) The estimated peak current is then multiplied with synchronising signal to estimate instantaneous reference grid current. The reference and sensed grid currents are then given to the current controller and an indirect current control approach is used. The outputs of current controller are the switching pulses for the VSC. The VSC is switched with these switching pulses such that the grid current follows the estimated reference grid current, and hence an overall UPF operation is achieved.

4

Simulation results

The proposed SPV system is simulated in MATLAB Simulink and sim-power system tool boxes. The performance of SPV system is shown under different operating conditions. Fig. 3 shows the simulated performance of the proposed system. The proposed system is simulated under different loading conditions along with abnormal grid conditions. In case of linear loads, the VSC supplies the reactive power required for the load and handles all active power from PV string. In case of non-linear loads, the VSC supplies all harmonics required by the load and at the same time VSC feeds the extracted energy from PV string. The performance of the system under different loads and SPV insolation level changes are shown along with grid voltage abnormality.

4.1 Performance of system under linear loads

Fig. 3a shows the steady-state performance under linear inductive load. A linear load of 1.5 kW, 0.8 lagging (at nominal grid voltage) is considered to demonstrate the performance. It can be observed that the load current is lagging with respect to the grid voltage, whereas the grid current is out of phase of grid voltage. The out of phase grid current confirms that the power is being fed into the grid. The difference of load and PV powers is fed into the grid. The VSC in this case supplies all reactive power required by the load and real power from the PV array.

4.2 Performance of system under non-linear loads and sudden voltage sag

Fig.3b shows the performance of the system under non-linear load and sudden voltage sag. The system is under steady-state conditions before t = .3 s. The grid voltage before t = .3 s, is nominal (230 V). The grid current is sinusoidal whereas the load current is non-sinusoidal. The grid current is out of phase of supply voltage which shows that the power is being fed into the grid. The VSC supplies all harmonics required by the load along with feeding real power into the distribution system. The difference of PV power and load power is fed into the grid. At time t = .3 s, a sudden sag (from 230 to 170 V) is observed in the grid. No appreciable effect is observed in PV voltage (vpv), current (ipv) and power (Ppv) as a result of this voltage sag. An increment in the grid current is observed because of a decrease in grid voltage along with a decrease in the load current. Even under deep voltage sag, the dynamic performance is satisfactory and the VSC continues to feed the extracted SPV power and load current harmonics required to maintain sinusoidal grid currents.

4.3 Performance of system under non-linear loads and sudden voltage swell

Fig. 3c shows performance of the system under non-linear load and sudden voltage swell. Before time t = .3 s, the system is under steady-state conditions along with nominal grid voltage. At time t = .3 s, a sudden voltage swell (230–270 V) is observed. No appreciable effect is observed in PV voltage (vpv), current (ipv) and power (Ppv) as a result of swell. The load power is increased whereas PV power remains the same, and hence a decrement in the grid current is observed. Owing to an increment in voltage at PCC the VSC has to feed reduced current to feed the same real power, hence a decrease in VSC current is also observed. The dynamic performance of the system is found satisfactory even under sudden voltage swell.

4.4 Performance of system under change in SPV insolation level

Fig. 3d shows the performance of the system for the change in SPV insolation level. The system has been working under steady-state conditions before time t = .3 s. The VSC performs the function of harmonics mitigation and feeding real power into the grid. At time t = .3 s, the SPV insolation level is changed from 1000 to 700 W/m2. The PV power decreases as a result of changes in the SPV insolation level. The effect can be seen in terms of decrease in VSC current and the grid current. The load current is the same during this time and the active power component of VSC current decreases. The harmonics demanded by the load remain the same, hence that part of VSC current remains same before and after time t = .3 s.

5

Experimental results

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voltage. A power analyser (Fluke 43B) is used to capture steady-state results. A digital storage oscilloscope is used to capture transient performance. Fig. 4 shows readings recorded on the PV array simulator at different SPV insolation levels (700 and 1000 W/m2, respectively). The MPP efficiency in both the cases is observed of the order of 99%. Test results under steady-state and transient conditions are shown for different grid and loading conditions. 5.1 Steady-state performance under linear load

The steady-state performance under a linear load on the grid is shown in Fig. 5. Figs. 5a–c show the PCC voltage to the grid current, load current and VSC current, respectively. The grid

current is out of phase with PCC voltage. The load current is sinusoidal in nature and lagging with respect to PCC voltage which confirms lagging power factor linear load. The VSC current consists of the reactive power component, demanded by the load and real power component, required to feed all incoming power at the DC link from the PV array. The harmonics spectra of grid, load and VSC currents are shown in Figs. 5d–f. The grid current THD is below 5% (under IEEE-519 standard). Figs. 5g–i show power fed into the grid, consumed by load and PV array power. The load DPF is 0.8 lagging whereas the grid DPF is −1. The –ve sign of power in grid power shows that the power is being fed into the grid and DPF of order of −1 confirms the UPF operation with respect to the grid.

Fig. 3 Performance of the proposed system under a Linear load at grid

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Fig. 4 Experimental data recorded by PV array simulator a at 1000 W/m2

b at 700 W

Fig. 5 Steady-state performance under linear load at grid a–c vswith is, iL, iVSC

d–f Harmonics spectra of is, iL, iVSC

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5.2 Steady-state performance under non-linear load Fig. 6 shows the steady-state performance under non-linear load condition. Figs. 6a–c show PCC voltage with grid current, load current and VSC current, respectively. The grid current is out of phase with PCC voltage. The load current waveform is almost square in nature, which confirms non-linear load. The VSC current consists of harmonics current, demanded by load and real power component, required to feed all incoming power at the DC link from the PV array. The harmonics spectra of grid, load and VSC currents are shown in Figs.6d–f. The load current THD (Fig.6e) is of the order of 29% whereas, grid current THD (Fig. 6d ) is below 5% (under IEEE-519 standard). The VSC current THD (Fig. 6f ) has found about 13% as VSC current consists of harmonics supplied to the load as well as fundamental active power component of the current. The –ve sign of power in grid power (Fig. 6g) shows that the power is being fed into the grid and DPF of order of −1 confirms the UPF operation with respect to the grid. The grid power, load power and PV array power are shown in Figs.6h − i, respectively.

5.3 Dynamic performance of proposed system

The dynamic performance of the proposed system is shown in Fig. 7. Figs. 7a and b show system performance under voltage sag and swell, respectively. A sag of 60 V (230–170 V) is

observed in the grid voltage. Owing to decrease in the grid voltage, the VSC current increases to inject the same real power hence an increase in VSC current is observed. The load current and hence the load real power also decreases. As a result of voltage sag, the magnitude of grid current also increases. Similarly, the performance for voltage swell is shown in Fig. 7b. The VSC current decreases to inject same real power. An increment in the load current and hence the load real power is observed because of the increase in the grid voltage. The load current is non-linear in nature, whereas the grid current is sinusoidal and out of phase of PCC voltage. A reduction in grid current is observed because of the increase in PCC voltage.

The performance for an SPV insolation level change is shown in Fig.7c. The SPV insolation level is decreased from 1000 W/m2to zero linearly. The decrease in PV array current demonstrates the decrease of SPV insolation. Initially, at high SPV insolation level, the PV power is more than the load power, and hence the remaining power is injected into the grid. However, as SPV insolation decreases the PV power to be decreases and so does the VSC current. When PV power is equal to load power, the grid current is approximately zero. A further decrease in the SPV insolation level causes the PV power to be less than the load power, then the remaining part of load flows from the grid to the load. At lower SPV insolation level, the grid current is in phase with PCC voltage, that is, power is taken from the grid. Whereas, at higher SPV insolation the grid current is out of phase with PCC voltage, that is, active power is fed into the

Fig. 6 Steady-state performance under nonlinear load at grid a–c vswith is, iL, iVSC

d–f Harmonics spectra of is, iL, iVSC

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grid. At zero SPV insolation the VSC current is only harmonics current required by the load.

Fig.7d shows performance for sudden removal of the load while feeding a linear load. When the load is removed suddenly, the load power decreases to zero and all PV power is injected into the grid. An increase in grid current is observed as a result of an increase in the injected power. A little decrement in VSC current in next steady-state conditions can be observed as VSC has not to supply the reactive power component of the load current after the load removal. The grid current is out of phase of PCC voltage in both the cases.

6

Conclusions

A single-stage single-phase multifunctional grid interfaced system has been proposed for feeding solar PV energy into the distribution network and power quality improvement simultaneously. The proposed system locally feeds the power to loads which helps in reducing the distribution line losses. Moreover, it also compensates for the reactive power and harmonics, which further reduces losses in distribution line and transformer. A robust PLL-less control has been proposed for control of multifunctional VSC which has been tested under abnormal grid conditions (voltage sag/swell). The performance of the proposed control algorithm has been found to be good even under abnormal grid conditions. Moreover, the performance of the proposed control algorithm has been observed to be quite satisfactory under load transients and steady-state conditions. A wide variety of simulation and experimental results have

demonstrated all features of the proposed system. Not only simulations but the experimental studies are also carried out at full voltage and considerable power levels. The grid current THD has been observed below 5% (satisfactory according to IEEE-519 standard) even with nonlinear loads at PCC. The simulations and experimental results have shown the feasibility of the proposed system.

7

Acknowledgments

Authors are very thankful to Department of Science and Technology (DST), Govt. of India, for funding this project under Grant number: RP02583.

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Appendix

Parameters for simulation: single-phase grid voltage 230 V, frequency = 50 Hz, supply inductance = 2.42 mH and supply resistance = 0.76 Ω, interfacing inductor = 3.5 mH, ripple filter R = 5 Ω, C = 5 µF, Kp= 0.5, Ki= 0.1, PV array open-circuit voltage: 450 V, PV array short-circuit current: 10 A and PV array peak power: 4 kW.

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