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Abstract---This work presents a photovoltaic (PV) system, connected to a three phase grid. This work focuses on fault analysis in a grid connected photo-voltaic (PV) energy system. In this work, a three phase Multi-level Inverter connected with an AC grid fed by photovoltaic systems with advanced sine PWM control scheme is presented. The proposed modulation technique uses single reference signal and number of high frequency carrier signals to generate the PWM signal. Now a day, most of the photovoltaic (PV) power sources are connected to the AC grid. When photovoltaic power sources are connected to grid, the grid connected PV system is affected by various power quality issues like voltage sag, voltage swell, voltage disturbances, waveform distortions and three phase fault. One of the main power quality problems is three phase fault and it is appeared in the grid due to short circuit condition between two phases and ground. Fault analysis is carried out by creating a LG, LL, LLL and LLLG fault in the grid connected systems. Grid side voltage, current and power waveforms at the grid side are analysed with fault conditions. A detailed simulation has been done for the Multi-Level Inverter and the validation of system is verified through MATLAB/SIMULINK and the results are presented. Keywords--Photo Voltaic (PV) Systems, Multi-Level Inverter, Grid connected system, Modulation Scheme, Three phase fault, Total Harmonic Distortion.

I. INTRODUCTION

Renewable energy sources are a very good solution in the global energy problem. The renewable energy source with grid integration is carried out by power electronics circuits. The energy generated by photovoltaic power sources contributes the large part of the electrical energy obtained from various renewable energy. Various renewable energy sources such as solar energy, wind energy, geothermal energy etc., are used to generate the electric power specifically solar energy is used to harvest the solar energy. Solar energy has the advantages of no pollution, low maintenance cost, no installation area limitation and no noise due to the absence of moving parts. The major use of PV systems

in power systems can be divided into two main fields: off-grid or stand-alone applications and on-grid or grid-connected applications. The standalone systems are used in places where there is no connection to the utility grid. They provide electricity to small rural areas and are usually used for low power loads (refrigeration, lightning). Their power ratings are around 1 kW and they offer a good alternative to meet the energy demands of off-grid communities. The first large grid-connected photo voltaic power plant was installed in Lugo, California, U.S.A with energy producing capacity of 1 MW [3]. Even though solar energy system has many advantages, major disadvantages of PV energy system are the requirement of very high system installation cost and the low photo voltaic energy conversion efficiency. To overcome these problems, efficiency of conversion of solar array can be increased and output power from the solar array can be maximized.

II. GRID CONNECTED PV SYSTEM

The configuration of grid connected photovoltaic system is represented in Fig.1.This system consists of solar PV panel as a DC source, DC/DC converters, a multilevel DC/AC inverter, a power transformer, and an AC grid. The input supply is feed through PV module and this output voltage magnitude is increased to require level by DC/DC converter. Output of DC/DC boost converters is the DC power and it is supplied to the multilevel DC/AC inverter to convert the applied DC voltage into AC voltage. This AC voltage is transformed to the AC grid through interfacing transformer.

[email protected]

Research Scholar, Department of Electrical and Electronics Engineering, Jerusalem College ofEngineering, Chennai, India.

[email protected], [email protected],

PG Scholar, Department of Electrical and Electronics Engineering, Jerusalem College ofEngineering, Chennai,India.

Professor, Department of Electrical and Electronics Engineering, Jerusalem College ofEngineering, Chennai,India.

V.Jamuna#1, N.Saritha*2, N.Nanthini#3,

Fault Analysis on Photo Voltaic Fed GridConnected Systems

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International Conference on Recent Advances in Mechanical Engineering and Interdisciplinary Developments [ICRAMID - 2014]

ISBN 978-93-80609-17-1

mailto:[email protected]



Fig.1 Grid connected PV system Grid connected PV systems can produce and

transform the power directly to the utility grid. Depending on the number of modules, the PV array converts the solar irradiation into specific DC current and voltage. DC to DC boost converter is used in the case of the voltage required by the inverter is very low. Energy storage devices can be included in to store the energy produced in grid.

Fig.2 Grid connected photovoltaic system with filter

Grid connected photo voltaic system with filter inductance is given in Fig.2. In this system, the power conversion is realized by a three-phase multilevel inverter which delivers the energy to the grid. High frequency harmonics that appear due to power semiconductor switching devices are reduced by connecting the filter. Filter components are designed to minimize the ripple current of inverter and reactive power supplied by inverter.

In this work, grid connected PV system is simulated for various kinds of faults. A fault is defined as any failure which interferes with the normal current flow. A fault can produce high value current called as short-circuit current. This short circuit current flows through the network and reaches the faulted point. When short-circuit current flows in the photo voltaic grid connected system, it can genertae heat which is proportional to the square of the current magnitude; this huge amount of heat may damage the insulation of power system devices such as bus bars, cables, circuit breakers and switches.

Fault analysis can be broadly divided into symmetrical and unsymmetrical faults. A fault involving all the three phases on the power system is known as symmetrical fault or three-phase fault while the one involving one or two phases is known as unsymmetrical fault. Unsymmetrical faults are classified into single line-to-ground(LG fault), line-to-line (LL fault) and double line-to-ground faults. The various reasons of faults are lightning, insulation aging, heavy winds, trees falling across lines, vehicles colliding with poles, birds, kites, etc. During fault period,power system components are affected in several ways. The first effect is generation of overhet and this over heating may damage electrical equipments such as bus-bars, generators and transformers. The second effect is the reduction in voltage profile of the system to unacceptable limits as a result of fault. Frequency drop may lead to instability.

Power system fault is one of the basic problem in power system engineering. The results of power system fault analysis are used to determine the type and size of the protective system to be installed on the system so that continuity of supply is ensured even when there is a fault on the power system.

To provide proper interface between grid-connected PV systems and the utility grid, some conditions must be satisfied, such as phase sequence, frequency and voltage level matching. Providing these conditions strongly depends on the power electronics technology of PV inverters. In grid connected PV system, the balanced voltage feed to the three phases of grid is given as:

𝑽𝒂 = 𝑽𝒎 𝒔𝒊𝒏𝝎𝒕 (1) 𝑽𝒃 = 𝑽𝒎 𝒔𝒊𝒏 (𝝎𝒕-120˚) (2) 𝑽𝒄 = 𝑽𝒎 𝒔𝒊𝒏 (𝝎𝒕+120˚) (3) 𝑽𝒎 = 𝑽𝒑 × √𝟐

√𝟑 (4)

Where Vp is the peak amplitude voltage.

III. MULTILEVEL INVERTER TOPOLOGY

For grid connected photo voltaic fed inverter systems, seven-level inverter is employed and it consists of traditional H-bridge inverter and auxilliary switches and a capacitor voltage divider. This capacitor volatge divider is created by connecting C1,C2 and C3 capacitors as given in Fig.3. In this Fig.3 traditional H-bridge inverter is replaced by modified H-bridge inverter. This topology can provide more advantages over other topologies, i.e., uses less no of power switches, power diodes, and capacitors for inverters with same number of levels.

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Fig. 3 Seven-level inverter with grid

In this grid connected PV system, photovoltaic (PV) arrays are connected to the inverter through a DC to DC boost converter. The output power generated by the inverter is applied to the power system network. The DC to DC boost converter is required ,to boost up the PV panel output voltage,because the voltage produced by the PV system is lower than the grid voltage. To filter the ripples in the output current a filtering inductance Lf is used. By properly switching the inverter switches, it can produce seven output-voltage levels. The output volatge levels of seven level inverter are given by, 0, +Vdc/3, +2Vdc/3, +Vdc, -Vdc/3, -2Vdc/3,-Vdc as given in the table 1.

TABLE 1. SWITCHING SEQUENCE OF SEVEN LEVEL INVERTER

V0 S1 S2 S3 S4 S5 S6

Vdc On Off Off On Off Off 2Vdc/3 Off Off Off On On Off Vdc/3 Off Off Off On Off On

0 Off Off On On Off Off 0* On On Off Off Off Off

-Vdc/3 Off On Off Off On Off -2Vdc/3 Off On Off Off Off On

-Vdc Off On On Off Off Off

IV. PWM MODULATION

In this work, all the switches of seven level inverter are triggered with the use of sine PWM technique. In order to generate the PWM switching signals, sine PWM technique is introduced. For seven level inverter, three reference signals (𝑉𝑉𝑟𝑟𝑒𝑒𝑓𝑓 1, 𝑉𝑉 𝑟𝑟𝑒𝑒𝑓𝑓 2,

r𝑒𝑒𝑓𝑓 3) are compared with a high frequency carrier signal (𝑉𝑉 carrier). The reference signals had the same frequency and amplitude and are in phase with an offset value that is equivalent to the amplitude of the carrier signal. If 𝑉𝑉 𝑟𝑟𝑒𝑒𝑓𝑓 1 has exceeded the peak amplitude of 𝑉𝑉 carrier, 𝑉𝑉

𝑟𝑟𝑒𝑒𝑓𝑓 2 is compared with 𝑉𝑉 carrier until it exceeds the peak amplitude of 𝑉𝑉.

In the proposed Multi Level Inverter (MLI) configuration, one leg of the inverter is operated at a very high switching frequency that is equivalent to the frequency of the carrier signal, while the other leg is operated at the rate of the fundamental frequency (i.e., 50 Hz). Switches 𝑆𝑆5, and 𝑆𝑆6 are operated at the rate of high frequency carrier signal. Remaining switches are operated at the rate of fundamental frequency. Initially, the seven level inverter is simulated with sine PWM modulation technique and it is presented in Fig.4.

Fig.4 Seven level inverter with sine PWM technique

Fig.5. Sinusoidal PWM signal

Fig.6 Seven level inverter.output voltage

Fig.7 Thirty one level inverter without Modulation scheme

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For the seven level inverter, the generated sine PWM signal is presented in Fig.5. To reduce total harmonic distortion, sine PWM signal is applied as switching signals for switching devices of inverter. The simulated output of seven level inverter is presented in Fig.6. In multi-level inverter topology, as a number of level increases the measured value of total harmonic distortion decreases. In this work, simulation of multi-level inverter is extended up to thirty one level inverter as represented in Fig.7 and its output voltage waveform is shown in Fig.8. Its FFT analysis is given in Fig.9. It clearly shows that thirty one levels MLI can produce very low value of THD 12.36%.

Fig.8 Thirty one level inverter output voltage

Fig.9 FFT Analysis of thirty one level inverter

With the use of Sine PWM modulation scheme, the above mentioned THD value can be further reduced without any additional control technique. This technique can eliminate the choice of large filter inductor and capacitance. In order to reduce the THD value, the thirty one level MLI is simulated with sine PWM technique as shown in Fig.10 and Fig.11.

Fig.10 Thirty one level inverter

Fig.11 Generation of sine PWM signal.

Fig.12 Sinusoidal PWM signal

The waveform of sine PWM signal which is applied to the power switching devices of thirty one level inverter is shown in Fig.12. Produced output voltage waveform is presented in Fig.13. In this method, frequency of carrier signal is chosen as 10 kHz, it is considerably very high as compared with the frequency of carrier signal of seven level inverter.

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Fig.13Thirty One Level inverter output voltage

Fig.14 FFT Analysis of Thirty One Level inverter

To validate the advantage of thirty one level inverter output with modulation over without modulation, FFT analysis is used. It indicates that THD value is very low i.e.4.76% with modulation scheme.

V. FAULT ANALYSIS

To analyze an effect of three phase fault in grid connected PV system, three phase seven level inverter is connected to grid via transformer using MATLAB/SIMULINK. The grid connected PV system is analyzed for various fault conditions. The system is simulated for various types of three phase fault conditions. The three phase fault is generated on the grid side. The fault duration is set for 0.08 second only. It starts from 0.02 and ends at 0.1 second. During the fault period, the grid side voltage and current waveforms and inverter side voltage and current waveforms are taken and observed. To validate the performance of grid connected systems, FFT analysis is also done and THD values are observed and tabulated. For different fault conditions, active and reactive power values are observed and tabulated. Initially the system is simulated with LLL fault condition.

In this work, the three phase multi-level inverter is simulated for 260 V DC input and the output is

connected with three phase grid with the use of coupling transformer as shown in Fig.15.

Fig.15 Multi-level inverter with grid connection

A three phase to ground (symmetrical) fault is created on the grid side. The grid current before initialize the fault (<.02sec) is maintained at 1.5× 104A, when a fault is initialized at 0.02 sec, the current reduces to 2A.

A. LLL Fault

For grid connected photo voltaic system, fault analysis is studied by generating only two types of faults i.e. LLL fault and LLLG fault. Because, these two types of faults may severely affect the power system components. This LLL fault is called as symmetric three phase fault, since all the three phases (A-Phase, B-Phase and C-Phase) of grid side output voltage waveforms getting collapsed and the magnitude of current waveform raised to very high value. During fault period voltage and current waveforms at inverter side as well as at grid side is shown in Fig.16 to Fig.19. The system maintains the zero values of reactive power at grid side during no fault period and during fault period it shoots up to 18×104 MVAR. This value of reactive power is very high when compared with other three types of faults like LL fault, LG fault, and LLG fault. During this LLL fault the active power decreases up to 4×104 W.

At inverter side, the system maintains zero value of reactive power during no fault period and during fault period it is changed to -3.5 MVAR. This value of reactive power is very high as compared with other three types of faults like LL fault, LG fault and LLG fault. During this LLL fault the active power decreases to 0.4×107 W. Under this fault condition THD of voltage is increased from 0% to 2.9% at inverter side and 3.15% at grid side. The THD of current is also increased from 0% to 2.7 % at inverter side and 3.9 % at grid side.

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Fig.16 Voltage at grid side under LLL fault

Fig.17 Grid current under LLL fault

Fig.18 Inverter voltage under LLL fault

Fig.19 Inverter Current under LLL fault

B. LLLG Fault

During LLLG fault period voltage and current waveforms at inverter side as well as at grid side is shown in Fig.23 to Fig.26. The system maintains the zero value of reactive power at grid side during no fault period and during fault period it shoots up to 3.5×104 MVAR. This value of reactive power is very high when compared with other three types of faults like LL fault, LG fault, and LLG fault. During this LLL fault period, the active power decreases up to 3.5×107 w

At inverter side, system maintains the zero value of reactive power during no fault period and during fault period it is changed to 16×104MVAR. This value of reactive power is very high when compared with other three types of faults like LL fault, LG fault, and LLG fault. During this LLL fault the active power decreases to 0.5×107 W. Under this fault condition THD of voltage is increased from 0% to 2.36% at inverter side and 3.34% at grid side. The THD of current is also increased from 0% to 1.64% at inverter side and 3.28% at grid side as shown in Fig.23 to Fig.26. From voltage and current waveforms under various kinds of fault conditions, it has been observed that LLLG fault is more severe than LG, LL, LLG and LLL fault. It has been also concluded that fault on the grid side will have severe effect compared with inverter side.

Fig.20 Grid voltage under LLLG fault

Fig.21 Grid current under LLLG fault

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Fig.22.Inverter voltage and current under LLLG fault

Fig.23 Voltage THD at inverter side

Fig.24 Current THD at inverter side

Fig.25 Voltage THD at grid side

Fig.26 Current THD at grid side

Thus three phase fault analysis on grid side and inverter side have been performed for different types of fault conditions like; LLL and LLLG faults. The change in value of active and reactive power and voltage and current THD without fault condition and with fault conditions are observed and analyzed. For different types of faults, inverter side and grid side parameters are compared and presented in TABLE2.

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TABLE 2. COMPARISON OF SYSTEM PARAMETERS UNDER VARIOUS FAULTS.

S.No Type of fault

Inverter Side Grid Side

1 LLL THDV:2.9%

THDI:2.7%

Active Power(P): 0.4×107 w

Reactive Power(Q): 3.5×104 MVAR

THDV:3.75%

THDI:3.9%

Active Power(P): 4×104 w

Reactive Power(Q): 18×104 MVAR

2 LLLG THDV:2.36%

THDI:1.64%


Reactive Power(Q): 16×104 MVAR

THDV:3.34

THDI:3.28


Reactive Power(Q): 3.5×104 MVAR

This table clearly shows that LLL and LLLG fault can produce the severe effects in power system when compared with other types of faults. Fault analysis is very important factor when designing the protection circuit.

VI. CONCLUSIONS

In this proposed work, advanced sine PWM switching scheme is presented for the multilevel inverter. In this method, three reference signals and a triangular high frequency carrier signal is utilized to generate PWM switching signals. The performance of the proposed multilevel inverter is analyzed in detail. By controlling the modulation index, the desired number of levels of the inverter’s output voltage can be achieved with reduced THD. And also performance of grid connected system is studied under different fault conditions like; LLL and LLLG fault. Various parameters under fault period are tabulated and compared. It has been concluded that the most severe fault on power system is LLL fault.

REFERENCES

[1] Abhijit Kulkarni, Student Member, IEEE, and Vinod John,

Senior Member, IEEE ,”Mitigation of Lower Order Harmonics in a Grid-Connected Single-Phase PV Inverter ‘,IEEE Trans. Power Electron., vol 28, No. 11, November 2013

[2] Amr Ahmed A. Radwan, Student Member, IEEE, and Yasser Abdel-Rady I. Mohamed, Senior Member, IEEE,” Analysis and Active Suppression of AC- and DC-Side Instabilities in Grid-Connected Current-Source Converter-

Based Photovoltaic System”, IEEE Trans. On sustainable energy vol, 4, No. 3, July 2013.

[3] Xianwei Wang, Student Member, IEEE, Fang Zhuo, Member, IEEE, Jing Li, Lin Wang, and Song Ni,”Modeling and Control of Dual-Stage High-Power Multifunctional PV System in d–q–o Coordinate”, IEEE Trans. industrial Electron., Vol. 60, No. 4, April 2013

[4] Jaume Miret, Member, IEEE, Miguel Castilla, Antonio Camacho, Matas,”Control Scheme for Photovoltaic Three-Phase Inverters to Minimize Peak Currents During Unbalanced Grid-Voltage Sags’, IEEE Trans. Power Electron., vol 27, No. 10, October 2012.

[5] Sibasish Pandaa, Anup Kumar Pandab and H.N Pratiharia, “ Fault Analysis on Grid Connected MPPT based Photovoltaic System”, International Journal of Current Engineering and Technology.Vol 22, No.4.April 2012.

[6] Nasrudin A. Rahim, Senior Member, IEEE, Krismadinata Chaniago, Student Member, IEEE, and Jeyraj Selvaraj “Single-Phase Seven-Level Grid-Connected Inverter for Photovoltaic System”,IEEE Trans. Power Electron., vol.. 58, No. 6, June 2011

[7] Juan Luis Agorreta,, Luis Marroyo, Member, IEEE, “Modeling and Control of N-Paralleled Grid-Connected Inverters With LCL Filter Coupled Due to Grid Impedance in PV Plants “,IEEE Trans. Power Electron, Vol. 26, No. 3, March 2011

[8] Blaabjerg.F , R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of control and grid synchronization for distributed power generation systems,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1398–1409, Oct.2009

[9] Michael E. Ropp, Member, IEEE, and Sigifredo Gonzalez, “Development of a MATLAB/Simulink Model of a Single-Phase Grid-Connected Photovoltaic System” IEEE Trans. Energy Conversion, April 2009.

[10] Alepuz.S , and J. Rodríguez, “Control strategies based on symmetrical components for grid-connected converters under voltage dips,” IEEE Trans. Ind. Electron., vol. 56, no. 6, pp. 2162–2173, Jun. 2009.

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