A techno-commercial review on grid connected photovoltaic system

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

A techno-commercial review on grid connected photovoltaic system Shantanu Chatterjeea, Prashant Kumara, Saibal Chatterjeeb, a b

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Department of Electrical Engineering, NIT, Arunachal Pradesh, India Department of Electrical Engineering, NERIST, Arunachal Pradesh, India

A R T I C L E I N F O

A BS T RAC T

Keywords: Anti-islanding Filter Maximum power point tracking (MPPT) Neutral point clamp inverter(NPC) Phase locked loop(PLL) Photovoltaic(PV) Pulse width modulation(PWM) Synchronous reference frame(SRF)

Owing to rapid depletion of fossil fuels and environmental hazard, different non-conventional sources of energy like wind, geothermal, nuclear, photovoltaic etc, have been taken into consideration for power generation. Environmental familiarness, reduced installation cost, improved power quality, abundant availability of source makes photovoltaic based distributed generation as one of the most popular alternative source of energy production. Improvement in power electronics technology makes synchronization of PV system with grid more viable. Power output from the PV module changes continuously with time depending upon the climatic condition. In order to get maximum output from the PV system different types of MPPT algorithm present in literature are studied and improvement proposed are described in this paper. Furthermore, different types of inverter topologies along with different grid synchronization technique and PWM topologies used to connect the PV system with the 3-phase AC grid are presented. In order to minimize the harmonic content of the inverter output, different types of filters used are presented in the proceeding section. Nextly different commonly used advanced islanding detection techniques for the safety purpose of PV based distribution generation system have been addressed and based on advantages and limitation of anti-islanding techniques, a comparison table has been presented. Afterward based on parameters like input, output voltage & current, MPP range, used standards etc, a comparison table between different commercially available grid tied PV inverters are presented in this paper. Finally, drawbacks of the prevailing grid connected PV system have been discussed.

1. Introduction Increasing power demand, scarcity of fossil fuel, environmental hazard have led the use of different non-conventional sources of energy production. Moreover, the power production from different nonconventional energy is environmental friendly (As PV can reduce CO2 emission by 970 g/kWh of electrical energy). In non-conventional energy, growth of solar based power production has shown steady upraise worldwide with a growth rate of 30% in past three decade (Fig. 1). Till date many solar projects with capability more than 100 MW have been commissioned in different countries like Germany, China, Japan, USA, India etc [1]. The 850 MW Longyangxin Dam Solar Park project in China, 579 MW Solar Star project in the United States, 300 MW Cestas Solar Farm project of France, 166 MW Solar Park Meuro project in Germany, 148 MW Eurus Rokkasho Solar Park project in Japan are some of the biggest projects in different countries worldwide. The solar irradiance received by India is 4 − 7 kWh /m2 /day with 270 sunny days on average, which makes India one of the most suitable

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country for producing electrical energy using solar power. Some of the major solar power projects in India are 360 MW Kamuthi Solar project in Tamil Nadu, 345 MW Charanka Solar Park project in Gujrat, 151 MW Welspun Solar MP project in Madhya Pradesh. Out of 44783.33 MW installed grid connected renewable power capacity, 8062 MW are solar power based grid connected systems. According to National Solar Mission of India, the installed capacity will be increased up to 20 GW by 2020, 100 GW by 2030 and 200 GW by 2050 while reducing the cost/kWh less than 0.10$ . PV system can be divided into two categories (1) Standalone PV system and (2) Grid connected PV system. Power produced in the standalone system is being utilized at the place where it is produced and it is not possible to transmit over a long distance. For that reason grid connected PV system is gaining much attention nowadays. For high voltage range PV modules are connected in series or parallel combination. There are different types of architectures to interface multiple PV modules with the utility grid that are as follows [2]: (1) Central Power Conditioning System (PCs) (2) String Power Conditioning System (3) Multistring Power Conditioning System (4)

Corresponding author. E-mail addresses: [email protected] (S. Chatterjee), [email protected] (P. Kumar), [email protected] (S. Chatterjee).

http://dx.doi.org/10.1016/j.rser.2017.06.045 Received 4 October 2016; Received in revised form 8 March 2017; Accepted 16 June 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: Chatterjee, S., Renewable and Sustainable Energy Reviews (2017), http://dx.doi.org/10.1016/j.rser.2017.06.045

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the PV modules different type of MPPT algorithm like Perturb & Observe (P & O), Incremental Conductance (IC), Fractional Open Circuit Voltage (FOCV) etc have been proposed. Section 3 describes different MPPT algorithm along with their advantages over the other. PV inverter is the core part of PV based DGs. The function of PV inverter is to convert the DC into AC. Beside this the inverter is responsible for controlling power supplied to grid, DC link voltage control, grid synchronization etc [3]. Section 4 discusses about some basic as well as advanced inverter topologies used for grid connected PV system. Electrical grid are effected by many disturbance like disturbance and resonance due to flow of harmonic current through the line, fault due to lightning strikes, and gross error in the operation of electrical equipment [4]. Some of the grid requirements are: (1) Operation with certain power factor that are close to unity, (2) Limited harmonic content of injected current, (3) Continuous operation under voltage distortion, etc. So it is important to monitor the requirements of grid and synchronize the variable according to the requirement of grid [5]. Section 5 explains different relevant methods for grid synchronization techniques used in PV system. Section 6 discusses controller structure for grid connected PV system. To get the desired output from the inverter with low harmonics content, the inverter needs to switched by using PWM technique. Section 7 presents different types PWM topologies used in literature. The output current of inverter contains harmonics but according to IEEE 519 maximum allowable harmonics in current is 5%. So filters are introduced at the output of inverter to suppress that harmonics [6]. Section 8 focuses on some advanced version of filters used in grid connected PV system. Islanding is a critical safety issue which occurs when grid is

Fig. 1. Annual growth of PV installation by different countries [1].

AC module Power Conditioning System. A comparison between these PV PCs is given in Table 2. This paper mainly focuses on grid connected PV system and its architecture. The generic block diagram of grid connected PV system is shown in Fig. 2. In order to connect the PV inverter to the grid certain standards must be maintained, otherwise the utilities connected to the grid will be malfunctioned. Section 2 describes different important standard which should be maintained while installing PV system and during operation. For the purpose of extracting maximum power from Table 1 List of important Standards for grid connected PV system. Standards

Year

Title

Ref.

Grid connection IEEE 1547 DIN EN 50530 IEEE 2030

2003 2011 2011

[128] [129] [130]

IEC 62446

2009

Standard for Interconnecting Distributed Resources with Electric Power Systems Overall efficiency of grid connected photovoltaic inverters Draft guide for smart grid interpretability of energy technology and information technology operation with the electric power system, and end-user applications and loads Grid connected photovoltaic systems - Minimum requirements for system documentation, commissioning tests and inspection Photovoltaic (PV) systems - Characteristics of the utility interface Photovoltaic systems - Power conditioners - Procedure for measuring efficiency (IEC 61683:1999) Standards for inverters, converters and controller for use in independent power system

[9] [132] [133]

Photovoltaic (PV) standalone systems - Design verification Concentrator photovoltaic (CPV) modules and assemblies Design qualification and type approval (IEC 62108:2007) Standard for conformance test procedures for equipment interconnecting distributed resources with electric power system

[134] [135] [136]

2000 2010 2014 2015 1998 1999 1999

Recommended practice for utility interface of photovoltaic system Electromagnetic compatibility testing end measurement technique Photovoltaic devices - Part 8: Measurement of spectral responsively of a photovoltaic (PV) device Photovoltaic (PV) array On-site measurement of current voltage characteristics Photovoltaic system performance monitoring Guidelines for measurement, data exchange and analysis IEEE recommended practices and requirements for harmonic control in electrical power systems Public distribution voltage quality

[137] [138] [139] [140] [141] [142] [143]

2008 2012–01

Utility-interconnected photovoltaic inverters - Test procedure of islanding prevention measures. Test procedure of islanding prevention measures for utility-interconnected photovoltaic inverters (IEC 62116:2008, modified) Electrical installations of buildings - Part 7–712: Requirements for special installations or locations - Solar photovoltaic (PV) power supply systems

[144] [145]

Automatic disconnection device between a generator and the public low-voltage grid Low-voltage fuses - Part 6: Supplementary requirements for fuse-links for the protection of solar photovoltaic energy systems Safety of power converters for use in photovoltaic power systems - Part 1: General requirements Photovoltaic (PV) module safety qualification - Part 2: Requirements for testing (IEC 61730–2:2004, modified)

[14] [147]

PV-power converter IEC 61727 2002 DIN EN 61683 2000–08 IEEE 921, UL1741 2010 Design & Testing Procedure verification CEI 62124 2004 DIN EN 62108 (VDE 0126–33) 2008–07 IEEE 1547.1 2005 Measurement and Analysis IEEE 929 IEC 61000–4–15 IEC 60904–8 IEC 61829 IEC 61724 IEEE 512 EN 50160 Islanding IEC 62116 DIN EN 62116 (VDE 0126–2): IEC 60364–7–712

2002

Safety VDE 0126–1–1 IEC 60269–6

2006 2014

IEC 62109–1 DIN EN 61730–2 (VDE 0126– 30–2)

2010 2007–10

2

[131]

[146]

[148] [149]

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Table 2 Grid connected PV inverter Configuration.

disconnected from the utility grid while load are connected with power generation unit. The major concern of islanding are uncertain intermit of irregularity like automatic voltage shutdown, human

error, discrimination of maintenance services etc [7]. Section 9 discusses different types of islanding detection methods and their anti-islanding methods; Section 10 gives a technical compar-

Fig. 2. Block diagram of grid connected PV system with control structure.

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2.3. IEC 62116:2008 – Utility-interconnected photovoltaic inverters – test procedure of islanding prevention measures

ison between commercial available PV-inverter in the market. In Section 11 different drawbacks in prevailing technology have been addressed. This paper aims in giving a broad comprehensive overview of all the components present in a grid connected PV system by studying more than 100 research papers. The comparison table of different commercially available PV inverter system by some of the technology leaders in this field, will enable the researchers working in this area, to have a clear picture of the overall PV market. Section 11 of the paper where the drawbacks of the prevailing technology is discussed will help the readers to identify the areas in which they can progress their research in order to mitigate the shortcomings of the existing system.

IEC 62116-Utility interconnected photovoltaic inverters- test procedure of islanding prevention measures was introduced in 2008 by TC-82 [12]. There are two editions of IEC 62116. Initially first edition was documented, later on the first edition had been replaced by second edition with some modification. In IEC 62116, Voltage and frequency trip settings according to National standards and/or local code have been described [13]. This standard describes guidelines for testing the performance of automatic islanding prevention measures installed with single or multi-phase utility interactive PV inverters connected to the utility grid. The test procedures and criteria described in this standard have minimum requirements that allows repeatability [4]. Additional requirements or more criteria may be specified if demonstrable risk can be shown. Inverters and other devices meeting the requirements as per this standard are considered for non-islanding operation.

2. Standards for grid connected PV system As the electricity generation using photovoltaic system are increasing, safety of such system and grid connected to it has also became an important issue [4]. Lots of standards have been developed by various countries for improving safety, quality factor, islanding problem etc. These standards are presented by various commissions. Some of them as follows: (a) International Electrotechnical Commission (IEC) (b) Institute of Electrical and Electronics Engineers (IEEE), (c) German Commission for Electrical, Electronic & Information Technologies of DIN and VDE (DKE) (d) National Electrical Code (NEC), (e) National Renewable Energy Laboratory (NERL). The different standards for grid connected PV system is shown in Table 2. Some of widely used standard are described as follows:

2.4. VDE 0126-1-1:2006 – automatic disconnection device between a generator and the public low-voltage grid VDE-0126-1-1 is the most important German safety standard. This standard refers for automatic disconnection device between a generator and the public low voltage grid [14]. In order to achieve automatic disconnect an ENS device was introduced. This device was initially presented as hardware equipment, later on as a software based algorithm. The principle of the ENS device was to observe impedance change with resolution 0.5Ω . In software algorithm, the grid impedance detection limit was extended from 0.5Ω to 1Ω due to problem like unnecessary tripping, degradation in power quality, etc. This standard also includes the detection technique for overdrive voltage and frequency and it have following specification: Leakage current limit = 300 mA, Active monitoring of fault current with sensitivity = 30 mA, Isolation = > 1 kΩ /V . It is also not allowed for the installation of ≥30 kW Ac output power. VDE 0126-1-1 allows only three methods for automatic disconnection under grid fault condition; by using impedance measurement technique, three phase voltage monitoring and fault detection using a grid frequency oscillator [4]. Table 3 shows a comparison of different standards used in grid connected PV system.

2.1. IEEE 1547:2008 – Standard for interconnecting distributed resources with electric power systems For interconnection of distributed resources with electric power Systems in distributive grid connection, the institute of electrical and electronics engineering had created an standard IEEE 1547 which has significant effect on those industry that does business related to electrical energy. IEEE 1547 has helped to modify distribution generation and energy storage technology as well as it also supports to provide a beginning rule for integrating clean renewable energy technology [8]. IEEE 1547 has delivered information related to voltage regulation, grounding problem, disconnects, islanding etc. IEEE 1574:2003 was first series of standard that were developed by Standards Coordinating Committee 21 (SCC21) for photovoltaic, energy storage etc. There are some limitation of the standard IEEE 1547 like this is applicable only to all distributed resources technologies with aggregate capacity 10 MVA or less.

3. MPPT MPPT algorithms are defined in such a way that the PV panel can operate at its maximum power point(MPP) while depending on the states of load, PV power generation capacity, PV temperature, solar irradiance and vibration [15]. Ref [16,17] have proposed PC-based MPPT system in which neural network detects the optimal operating condition under different operating condition and PC has been used to execute the control algorithm and store the data. Sometime the operating regions are partially shaded exhibiting multiple local maximum power points or region with rapidly changing irradiance condition, in this under varying situation tracking algorithm are different. Ref [18] proposed a low cost method to predict the global MPP region which evaluated the I-V characteristics curve first and then used a combination of the measured current at each stair to predict the global MPP region. Ref [19] has proposed a modified method for tracking MPP under partially shaded condition in which different groups are made on the basis of voltage detected at each PV module. Each of those groups which have maximum power locates the global and local MPP. This cuts down the tracking time and also avoids blind scanning. [20] has proposed an improved perturb and observe MPPT algorithm based on variable step Newton-Raphson method. Different types of MPPT algorithms are categorised in Fig. 3 and explained as follows:

2.2. IEC 61727:2004 – Photovoltaic (PV) systems – characteristics of the utility interface In the grid connection requirement IEC 61727, Photovoltaic system- characteristics of utility interference is prepared by IEC technical committee 82 [9]. This standard applies to utility interconnected photovoltaic power system. In utility power system for conversion of AC to DC, static non-islanding inverter are used in parallel with the utility. This standard is used for the system which has power rating of 10 KVA or less. The main focus of this standard is on power quality parameter, voltage and frequency range, dc injection, flicker, harmonics and waveform distribution. The document was prepared with reference to many other previous existing standards like IEC 61000-32, electromagnetic compatibility [10]. This standard is developed for minimizing the effect of unintentional generation, propagation and reception of electromagnetic energy with reference to unwanted effect, besides this IEC 61034-7-712 is the process of installation of building [11]. IEC 61724 are prepared for photovoltaic system performance monitoring guidelines for measurement, data exchange and analysis. 4

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Table 3 Comparison between different parameters of standards used in Grid connected PV system [128–149].

Fig. 4. P-V characteristics of PV array under different condition [21].

operating voltage. As the temperature and radiation change the maximum power also changes. The graph of change in power with respect to the array terminal voltage is shown in Fig. 4. In this method, power drawn by the PV modules are calculated at every instant and accordingly the PV arrays are adjusted in an appropriate point of maximum power [21]. The algorithm for P & O method is shown in Fig. 5. Some P & O MPPT tracking method are given for sudden change in the irradiance by using current perturbation, adaptive control, and

Fig. 3. Classification of MPP tracking methods.

3.1. Perturb and observe (P & O) Perturb and observe method is the simplest and widely used method. The PV array current and power depends on the terminal

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Fig. 7. Flow chart Fractional open circuit Voltage MPPT algorithm [34].

Incremental conductance can be realized using state space control and modeling method. So rigorous assessment of PV system robustness under different condition and stability of INC algorithm can be checked by using control design tools [31]. Another method for MPP tracking is a combination of IC method and fuzzy logic in which fuzzy logic controller enables the intelligent control of tracking parameter and improvises the efficiency. Besides this for tracking of MPP there are many modification and implementation in INC algorithm and its hardware architecture has been proposed by researchers in [32,33].

Fig. 5. Flow Chart for P & O method [22].

variable control algorithm [22]. By these methods the oscillation near the MPP can also be reduced. Ref [23] has used P & O method with particle swarm optimization. By using this algorithm, the searching space for MPP gets reduced which in turn reduces the time for finding the MPP. Ref [24] proposed a MPPT algorithm for PV system to operate at MPP under partially shaded condition. It shows guaranteed convergence to MPP and improved transient response. Besides many modification have been done in P & O based MPPT algorithm in order to achieve oscillation free global MPP in very less time, some of them are explained in [25–30].

3.3. Fractional open circuit voltage (FOCV) Fractional open circuit voltage method comes into existence due to linear relationship between voltage at maximum power point and voltage at open circuit of PV array under different condition. This relationship can be illustrate as Eq. (1).

3.2. Incremental conductance (INC) Due to some limitation of P & O method like improper tracking of MPP, oscillation at MPP under rapidly changing atmospheric condition etc. INC method came into focus. In this method the terminal voltage are adjusted as per maximum power operating point (MPOP) voltage or by differentiating the power with respect to voltage [31]. The algorithm of incremental conductance is shown in Fig. 6.

VMPP = ki VOC

(1)

where ki=Proportional constant whose value is about 0.76 and 0.92 [34]. For tracking of MPP, FOCV algorithm is shown in Fig. 7. In this method the reference voltage is taken as operating voltage and open circuit voltage (Voc) are calculated by opening the circuit for millisecond time. Maximum power point voltage is calculated by using Eq. (1). This is a low cost and high efficient MPP tracking method. 3.4. Fractional short circuit current (FSCI) Under varying irradiance, the maximum power of the PV array is proportional to the operating short circuit current. FCSI is accurate and high speed MPP tracking method which is based on finding optimum current by observing short circuit current. Here the relation between optimum operating current and short circuit under different of luminance is given in Eq. (2) [36].

Iop = kIsc (E )

(2)

where k=Proportional constant 3.5. Fuzzy logic control (FLC) Due to decision making tendency of fuzzy logic controller, it shows flexibility in operation as well as adaptive in nature. Ref [35] has proposed a method of MPP tracking using fuzzy logic controller. Initially they have calculated the value peak power of PV array then, using this peak value as a reference signal the MPP has been estimated. The flow chart for Fuzzy logic based MPPT algorithm is as shown in Fig. 8.

Fig. 6. Flow chart Incremental Conductance algorithm [31].

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crossing point of grid voltage and by using the updated reference signal, it is easy to track MPP rapidly against changing illumination level. 3.9. One cycle control (OCC) One cycle control method is a non-linear control method which is based on considering the average value of measured solar array voltage & current for controller design. and it allows the constant switching frequency mode operation. In this method, the average value of voltage and current are controlled by single stage inverter according to reference signal. By tracking the reference signal, the MPP can easily found for different irradiance level [41]. The use of single stage converter make cheap and reliable to the whole system. OCC based MPPT method requires lesser number of hardware components due to absence of multiplier or digital signal processors. 3.10. Hybrid MPPT Hybrid MPPT is combination of different conventional MPPT methods. [18] uses the combination of P & O and artificial neural network. The flow chart for hybrid MPPT are as shown in Fig. 12. In this method the maximum power are set by ANN on the basis of observation of power at different point by P & O method. Also the value of maximum power point power is set according to PV system and environment condition. The proposed method does not use any individual irradiance sensor that helps to decrease the cost of system. There have been various hybrid methods prosed for tracking MPP [42–53]. These hybrid MPPT algorithm apply combination of different conventional MPP methods & advance algorithm like Bang-Big Crunch (BB-BC )algorithm [44], Estimation and Revision (ER) method [46], Artificial fish swarm algorithm [53] and their strategies contrast in terms of unpredictability, speed of reaction, measure of speculation, the number and sorts of sensors required and the hardware execution.

Fig. 8. Algorithm for Fuzzy logic controller [35].

3.6. Artificial neural network based method MPP tracking using neural networking is a smart and smart way with good accuracy and rapidly. In this method the MPP tracking is divided into two process; first is the estimation of the maximum power of PV module and second is estimation of the normal operating point of PV module at any given solar radiation and surface temperature [37]. Hence by calculating difference between these two points, Maximum power are assigned and controller sets the PV module at desired MPP position. [38] proposes a configuration of neural network for designing the controller which is shown in Fig. 9(a). The neural network contains three layers: input layer, hidden layer and output layer. The input node has five nodes on which different input condition are applied on it. Ref [38] has proposed flow chart for Artificial neural network based controlling process which is shown in Fig. 9(b).

3.11. Advanced MPPT algorithm In Ref [54] an Radial Basis Function Network (RBFN) based MPPT algorithm has been proposed, The proposed algorithm performs better than conventional P & O and INC methods in terms of generating maximum O/P power, reducing conduction loss and improved static gain. In Ref [55] a novel bee colony based MPPT algorithm gas been proposed. This algorithm has advantages like simple configuration, use of less control parameter, better convergence, robustness, and efficient tracking of MPP under varying atmospheric condition. In Ref [47] a PSO based single stage MPPT algorithm has been propose. The proposed algorithm uses voltage of the PV panel as the particle of search space and generates the suitable reference voltage for the DC-link capacitor in order to operate the system at MPP. The proposed algorithm has advantage like reduced oscillation at MPP, efficient tracking of MPP under partially shaded condition, simple structure etc. Ref [56] proposed a Levy distribution based Cuckoo search optimization algorithm which presents increased robustness, increased efficiency, high convergence rate etc.

3.7. Power analysis MPPT Power analysis MPPT method is based on operating the PV module in power region of the I-V characteristics [39]. The power region of I-V characteristics is determined by taking the effect of series and shunt resistance on output voltage & current of PV panels. The algorithm for power analysis MPPT method is shown in Fig. 10. By using this method, a robust control can be achieved without any oscillation in steady state. Also the overall cost of this hardware is less so it can easily be implemented.

4. Inverter In grid connected PV system, single phase or three phase inverters are utilized for power conversion. But in three phase inverter, due to lower switching frequency, destitute spectral performance has been recorded in output voltage and current. The designing of inverter are done in such a way by which it can able to produce less harmonics that should not exceed the harmonic limit as per the grid utility standard [57]. Sometime due to high switching frequency, high radio interference is present so, the inverter should also able to control this interference. Different of PV inverter topologies divided according to number of levels is shown in Fig Fig 13.

3.8. Sliding mode control Sliding mode observe method does not use current drawn by PV array. In this method a sliding mode observer which is designed using system dynamic equation are presented (Fig. 11) that gives information about the current [40]. The current signal is feed to controller to generate appropriate reference signal for MPP tracking. For constructing system dynamic equation, it is difficult to calculate steady state value. So the reference voltage is updated at every zero7

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Fig. 9. Artificial neural network based method [38].

Fig. 11. Block diagram for sliding mode observer [40].

nent. Besides in bipolar modulation, switching ripple present in current is equal to switching frequency which is drawback of this type of inverters. Ref [58] has proposed a modified full bridge inverter which is a combination of unipolar and bipolar modulation with dead-time freewheeling mode and zero-vector freewheeling mode. Ref [60] has proposed a five level full bridge inverter that shows high power density and higher conversion efficiency. H5 has advantages of having common mode current and increased efficiency. H5 inverter is an inverter that uses a switch to disconnect the side during the freewheeling stage in order to keep the common mode voltage constant [59]. The schematic diagram is shown in Fig. 15. It avoids high frequency voltage at input terminal and allows high efficiency due to its simple and cost optimized circuit layout. This is done by using bridge circuit comprising switching elements and freewheeling element. It has advantage of avoiding lower core losses. But in his inverter the conductive losses are very high. HERIC inverter is formed by adding bypass system in AC side as shown in Fig. 16. HERIC inverter stands to be the highly efficient and

Fig. 10. Algorithm of power analysis MPPT [39].

Full bridge is one of the widely used inverter in grid connected PV application. The basic full bridge PV inverter is shown in Fig. 14. For better efficiency unipolar, bipolar and hybrid modulation strategies are used in this inverter. On switching there is no possible state in which the output current will be zero which is advantage in full bridge inverter and also output voltage only contains grid frequency compo8

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Fig. 15. H5 Inverter (SMA) [59].

Fig. 12. Algorithm for P & O and artificial neural network based hybrid MPPT [18].

Fig. 16. HERIC Inverter (Sunways) [61].

Fig. 13. Classification of Inverter.

Fig. 17. REFU Inverter [62].

Fig. 14. Full bridge inverter [58].

reliable inverter concepts. This bypass legs enables it to prevent reactive power exchanges that helps to increase efficiency [61]. HERIC inverter has approximately same feature as of H5 inverter. It have feature as of having very low leakage current and Electromagnetic interference (EMI). A generic REFU inverter is shown in Fig. 17. This topology is the upgraded version of high bridge topology by adding the AC bypass to create zero voltage with minimum losses [62]. It also shows high efficiency while achieving low leakage current and EMI.

Fig. 18. Full Bridge Inverter with DC Bypass (FBDCBP) (Ingeteam) [63].

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Fig. 19. Full Bridge Zero Voltage Rectifier [64].

The full bridge inverter with DC-bypass is shown in Fig. 18. It contains two extra switch in DC link as well as two extra diodes for clamping the output with ground. This inverter has tendency to operate at any power factor as well as common mode voltage does not get generated in this inverter, but high conductive loss has been detected [63]. Fortunately it does not affect the systems overall efficiency. Full bridge zero voltage rectifier (FB-ZVR) is another modification of FB inverter topology. In this inverter zero voltage can be achieved also. The schematic for FB-ZVR is shown in Fig. 19. This inverter has also features like low leakage current and reduced EMI [64] while achieving 96% of efficiency. But this inverter increases the losses across the filter as it provides bipolar output voltage during dead time clamping. Neutral point clamped inverter(NPC) is the most widely used inverter topology nowadays due to its versatility and can be used as single phase and three phase inverter with different types of levels. The schematic diagram of NPC inverter is shown in Fig. 20. It has features like reduced switching loss, very low leakage current and almost zero EMI as well as high efficiency [65]. But unbalanced switches create some problem in some cases. Flying capacitor is an alternative to the diode-clamped topologies. The circuit diagram of flying capacitor is shown in Fig. 21. In this topology more capacitors are used in comparison to diode clamped topology. With respect to earth potential, the capacitor floats which is reason behind calling it flying capacitor [66,70]. This inverter can work on that voltage where switching element and diodes of power cell stops working.

Fig. 21. Flying Capacitor Inverter [66].

Fig. 22. Z-source Inverter [67].

In Ref [67], Z-source inverter (ZSI) has been proposed for PV based DG system. There is an additional impedance network in Z-source inverter which makes it different from VSI and CSI. The schematic diagram of Z-source inverter is shown in Fig. 22. The inductor L present in ZSI allows safe shoot through of inverter arms. Z-source provides more flexibility and control freedom while using it in PV system. In [68], a there phase four leg Inverter has been proposed for grid connected PV DGs. Proposed topology eliminates the leakage current component. Reduction of the leakage current make the system satisfies the standard VDE-0126-1-1. The use of this topology also eliminates the common mode voltage which is generated by the PWM technique. The limitation of four leg three phase inverter is that the modulation index must be less than 0.666 or more than 1.108. Fig. 23 depicts the circuit diagram of the four leg inverter where the 3 leg are connected with the grid and the 4th leg is connected to the neutral point of the capacitor bank. Ref [69] has proposed a soft computing based three phase PWM inverter with active resonant commutated leg link snubber (ARCS). The configuration consists of PV sources at input side and three bidirectional ARCS connected between each phase and the midpoint of the inverter leg (Point A, B, and C) as shown in Fig. 24. The soft switching based configuration enables the inverter to be used in very high power applications.

Fig. 20. Neutral point clamped Inverter [65].

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Fig. 26. Classification of Grid synchronization Technique. Fig. 23. Three phase four leg Inverter [68].

5.1. Phase locked loop (PLL) Phase locked loop are the most widely used grid synchronization method for tracking the phase of time varying input signal. PLL are designed in such a manner that it has can provide accurate synchronization of information with a motive of higher immunity and insensitivity towards disturbance, harmonics and unbalance in the input signal [72]. The basic building blocks of PLL are as shown in Fig. 27. PLL contains phase detector (PD), loop filter (LF) and voltage controlled oscillator (VCO). The voltage, v, from the point of common coupling is given as input to the phase detector which is compared with the output voltage provided by the internal oscillator. Phase detector generates an error signal, εpd, which is feed to the loop filter for attenuating its higher frequency AC components. The different topologies of PLL is shown in Fig. 28. Fig. 24. Bi-directional soft switching based Inverter [69].

5.1.1. Synchronous reference frame PLL (SRF-PLL) The operation of synchronous reference frame PLL is based on synchronization of PLL rotating reference frame to the utility voltage frame in order to detect the instantaneous phase angle θ. The basic structure of synchronous frame PLL is shown in Fig. 29. The system under consideration is single phase and unlike three phase system, single phase system doesn't produce orthogonal signal inherently. Hence in single phase SRF-PLL an orthogonal signal generation system must be employed along with the PLL in to order produce orthogonal signal in αβ reference frame [4]. The orthogonal signal Vα, Vβ for a three phase or single phase system are transformed into rotating dqreference frame. In order to archive grid synchronization the quadrature axis reference voltage (Vq) is set to zero by PI controller as a result of which the reference is locked according to the phase angle of the grid voltage vector. The expression of reference voltages Vd is given in Eq. (3).

5. Grid synchronization technique In grid connected PV system, the factors related to stable and safe operation of grid are very vital issues. In grid synchronization, the internal reference signals generated by control algorithm of grid connected power converter is brought into line with a particular grid variable, usually the fundamental component of grid voltage. The processes of monitoring the grid variables include monitoring of phase angle, amplitude and frequency of utility voltage. The principle of grid voltage monitoring is mainly based on accurate screening of the grid voltage at the point of common coupling (PCC) in order to trip the disconnection procedure, when they go beyond the limits given as per the grid codes. Zero crossing detection is one of the simplest method for monitoring of grid variables. By counting the zero crossing of grid voltage, the frequency of fundamental can be evaluated. In this method as shown in Fig. 25, the phase of grid voltage can be calculated by integrating the estimated frequency. No phase controller is involved in this method [71]. Grid synchronization can be mostly done using phase angle, besides this it can also be used for anti-islanding algorithms. The grid synchronization methods can be classified into two groups which is shown in Fig. 26.

vd = vα sin θ + vβ cos θ

(3)

vd = V cos θ. sin θ − V sin θ. cos θ

(4)

vd = −V sin (θ − θ )

(5)

Where θ and θ are the phase angle of the PLL and Grid respectively.

Fig. 25. Grid monitoring using zero crossing detection without using any phase controller [71].

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Fig. 27. A generalized block diagram of PLL.

Fig. 30. ZCD based PLL [71].

Fig. 31. PLL with an ideal in-quadrature PD [73].

5.1.4. PLL based on T/4 transport delay In single phase application PLL based on T/4 transportation delay is the simplest way to generate the correct orthogonal signal [74]. PLL based on T/4 transportation delay is shown in Fig. 32(a). The transportation delay block is programmed with a buffer, the size of which is one fourth of the sample present in the fundamental frequency. Hence if the applied voltage is purely sinusoidal in nature the transportation delay block works efficiently to generate correct orthogonal signal without creating any error in the phase synchronization. PLL with dual delay block is shown in Fig. 32(b).

Fig. 28. Different topologies of PLL grid connected system.

Fig. 29. Basic structure of SRF-PLL [4].

Beside this when the grid voltages are in balanced condition there is no fundamental negative sequence component present, but negative fundamental sequence component exists when the grid voltages are in unbalanced condition.

5.1.5. PLL based on Hilbert transform Hilbert transform based PLL are based on property of Hilbert transform as this transform shifts the phase angle of input signal, depending upon the sign of their frequency calculated in Fourier analysis [75]. The type of PLL is shown in Fig. 33. For a input signal V(t) the Hilbert transform is defined in Eq. (6).

5.1.2. ZCD based PLL In ZCD based PLL, the reference phase angle of PLL is produced by zero crossing detection technique. Zero crossing detection based PLL is shown in Fig. 30. By setting the phase angle difference to zero the PI controller generates the reference phase angle [71].

H (x ) =

1 π

∞

∫−∞ tV−(ττ) dτ = πt1 *V

(6)

For frequency domain it can be defined by Eq. (7).

⎛1⎞ f (H (V )) = f ⎜ ⎟ F (V ) = [−jsign (ω)] f (V ) ⎝ πτ ⎠

5.1.3. PLL with an ideal in-quadrature PD Fig. 31 shows the schematic diagram of PLL with an ideal inquadrature phase delay. In PLL with in-quadrature phase delay, an extra phase detector is introduced while designing the PLL in order to cancel out the oscillation present in phase angle error signal which is greater than twice the grid frequency, [73]. As there are no steady state oscillations, so the bandwidth of PLL increases which enables it to calculate the phase angle quite efficiently.

(7)

For this equation it seen that Hilbert transform are used as a multiplier operator. 5.1.6. PLL based on park transform The park transformation is expressed by Eq. (8).

⎡ Id ⎤ ⎡Cosθ Sinθ ⎤ ⎡ Iα ⎤ ⎢ ⎥=⎢ ⎥⎢ ⎥ ⎣ Iq ⎦ ⎣− Sinθ Cosθ ⎦ ⎣ Iβ ⎦ 12

(8)

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Fig. 35. PLL based on Inverse Park transform [4].

expressed by Eq. (9).

⎡ Iα ⎤ ⎡Cosθ − Sinθ ⎤ ⎡ Id ⎤ ⎢I ⎥ = ⎢ ⎥⎢ ⎥ ⎣ β ⎦ ⎣ Sinθ Cosθ ⎦ ⎣ Iq ⎦

(9)

And the input of inverse park transform are

⎡ I * (s ) ⎤ ⎡ I *⎤ * (s ) = ⎢ α ⎥ = G−1 ⎢ d ⎥ Iαβ * ⎢⎣ Iβ ⎥⎦ ⎣⎢ Iq* (s )⎥⎦

(10)

Where G is park transform matrix. The schematic diagram for PLL based on Inverse park transform is shown in Fig. 35. This PLL is very suited for single phase application due to easy to implement with conventional PLL as two additional low pass filters [4]. Fig. 32. PLL based on a T/4 Transport Delay [74].

5.1.8. Adaptive lattice synchronous frame PLL The adaptive lattice synchronous frame PLL are based on adaptive filtering method. By adaptive filtering technique, the error which may be time invariant can be found by using recursive algorithm. This method provides very low THD of injected current and high power factor whenever the grid is polluted [77]. The block diagram for adaptive filter based PLL is shown in Fig. 36. Here in order to reject higher order odd harmonics in natural reference frame (NRF), if any variation in grid frequency occurs it automatically varies the tuning frequency. It rejects the varying frequency harmonics present at PCC while tracking the reference signal accurately. Fig. 33. PLL based on Hilbert Transform [75].

5.1.9. Multi-sequence harmonics decoupling (MSHDC) based PLL MSHDC-PLL is mainly combination of αβ -PLL and MSHDC network that provides better dynamic response and attenuation of undesirable oscillation. MSHDC block is used for cancelling out the oscillation of positive sequence voltage vector. MSHDC-based PLL has fast and accurate response. It also improves the quality of injected power under abnormal condition. It has a tendency to provide support under balanced and unbalanced grid fault condition. Due to abnormal grid condition the representation of rotating voltage vectors are very useful in order to decouple and estimate all the contained vectors accurately. These vectors contain DC and Oscillation terms. MSHDC enables to estimate desirable DC terms and attenuate oscillation [78]. The MSHDC block mainly uses crossfeedback subtraction that makes first order low pass filter to become

Id,Iq are rotating reference frame, Iα , Iβ are orthogonal stationary frame and θ is rotating angle In this PLL, the input voltage is transformed into orthogonal form based on park's transformation as shown in Fig. 34. Based on this transformation the error signal that is provided by the PI-controller can be estimated [76]. 5.1.7. PLL based on inverse park transform For generating orthogonal signal for PLL implementation the inverse park transform are used. Inverse park transform can be

Fig. 34. PLL based on Park transform [76].

Fig. 36. Adaptive Lattice Synchronous frame PLL [77].

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Fig. 37. Multi-sequence Harmonics Decoupling based PLL [78].

much more complex recursive filter to give better stability. The block diagram of MSHDC based PLL is shown in Fig. 37. Here the MSHDC detects the clear positive sequence of grid voltage very rapidly and dynamically provide oscillation free positive sequence Vdq + 1 signal to αβ -PLL. MSHDC has been used to cancel out the oscillation of three most significant harmonics h1, h2 & h3 as shown in Fig. 37.

Fig. 39. Sinusoidal amplitude integrator based PLL(SAI-PLL) [80] (a) SAI-PLL (b) Schematic of Positive sequence SAI block.

another virtual flux are used for phase angle calculation. The main disadvantage of this PLL is slower response in time domain. The dual virtual flux PLL is shown in Fig. 40.

5.1.10. Enhanced PLL Another adaptive filter based PLL is enhanced PLL (EPLL). This PLL has ability to lock the phase and amplitude of output voltage. The diagram for enhancement PLL is shown in Fig. 38. If the frequency and angle of the input signal matches with the reference signal, the output of this PLL due to its adaptive nature become zero which enables the multiplier PD to cancel out the signal oscillation and thereby the phase angle of input signal are properly detected [79].

5.2. Frequency locked loop (FLL) In FLL, instead of phase the frequency of grid voltage and current are estimated which enables the system to perform better in terms of accuracy, reliability and dynamic response. The improvement is because of the fact that the performance of FLL remains unaffected even when there is a sudden change in phase or phase jump at the input signal. So in order to achieve better grid synchronization under weak grid condition FLL based technique proves to be more efficient. The transfer function of first order FLL loop can be given by Eq. (11) and the block diagram of basic FLL is shown in Fig 41.

5.1.11. Sinusoidal amplitude integrator based PLL(SAI-PLL) Sinusoidal amplitude integrator (SAI) based PLL is able to estimate the amplitude and phase angle of fundamental component of grid voltage in a very quick time with higher accuracy. This type of PLL operates on the basis of complex vector notation. For extracting the DC and AC component of grid, SAI structure and a separate block is used. The schematic diagram of the separate block which is used to detect the positive sequence component of frequency is shown in Fig. 39(a). For grid synchronization, the SAI unit is connected in the base loop with another separate block that completely separate the AC and DC component of dq voltage which leads to accurate extraction of phase by PLL [80]. The block diagram of SAI based PLL is shown in Fig. 39(b).

ω G = ωref s+G

(11)

Where G is the gain of FLL, ωref is reference signal frequency and ω is detected frequency. Ref [82] proposed a Second order generalized integrator (SOGI) based FLL. The block diagram of SOGI-FLL is shown in Fig. 42. The SOGI-FLL performs better than PLL in terms of efficiency in adapting the centre frequency. Ref [82] proposed a multiple second order generalized integrator

5.1.12. Dual virtual flux PLL (DVF-PLL) Dual virtual flux PLL utilizes the concept of virtual flux for grid voltage estimation. The dual virtual flux PLL gives faster time response, small amplitude attenuation and precise phase shift [81]. In this one

Fig. 38. Enhanced PLL [79].

Fig. 40. Dual Virtual Flux PLL (DVF-PLL) [81].

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Fig. 41. Basic Building block of FLL.

Fig. 43. Bode plot of the Ideal and Non-Ideal PR controller.

Fig. 42. SOGI-FLL [82].

(MSOGI) in combination with FLL. It can capture the centre frequency of the SOGI more efficiently as compared to SOGI-FLL. The multiresonant frequency adaptive property of MSOGI-FLL enables it to capture not only the positive and negative sequence component of the fundamental voltage but also adapts the other sequence components of the harmonics voltage. This property of the MSOGI-FLL enables it to perform better in weak grid condition as compared to SOGI-FLL. 6. Controllers Fig. 44. Bode plot of the PI and PR Controller.

PR controllers are now one of the most widely used controllers in renewable energy integration with the grid. The preference of using PR controllers over PI controller is due to reduced steady state error, perfect decoupling, ease of tuning, less settling time, elimination of selective harmonics, regulation of sinusoidal voltage and robustness. The transfer function of the PI and the Ideal PR controller are given in Eq. (12) and (13).

GCon PI (s ) = kP +

kI s

GCon PR Ideal (s ) = kP +

7. PWM The ac output voltage of inverter is usually non-sinusoidal in nature and contains harmonics. In single phase full bridge inverter it is simple to increase the amplitude but controlling of voltage is difficult. Controlling of voltage implies controlling the fundamental component of voltage. The harmonic content in single phase full bridge inverter is given by Eq. (15).

(12)

2kI s 2 s 2 + wGrid

(13)

VAB =

Where kP, kI and ωgrid are the proportional gain, integral gain and resonant frequency respectively. From the Fig. 43, it can easily be seen that at resonant frequency when the ideal PR controller is used, there is a very high gain within a very narrow frequency and that frequency is called the resonant frequency. The presence of this resonant peak reduces the steady state error i.e the reference signal will be tracked in a much smaller time. But this peak hampers the stability of the system. So to have the same performance without any resonant peak the Non-Ideal PR controller is implemented having transfer function as given in Eq. (14).

GCon PR Non − Ideal (s ) = kP +

2kI wCut s 2 s 2 + 2wCut s + wGrid

⎡ ⎛π 4 α ⎞⎤ VDC sin ⎢n ⎜ − ⎟ ⎥ nπ 2 ⎠⎦ ⎣ ⎝2 π (2

α ) 2

(15)

− where β = By varying the value of α from 0 to π, the amplitude of output 4 voltage can be increased from π VDC to 0. But here the output is α proportional to cos 2 . Hence this is not a linear control. But in UPS or other applications the output should be proportional to the function we use. Also in an inverter as the number of level increases the number of switches also increases thereby increasing the complexity of the system [83]. In full bridge inverter, when load are connected in between the inverter arm mid point and the mid point of the dc link voltage the triplane order harmonic current also come in existence, but when it is connected through neutral point of the load than there has no existence of triple order harmonic current. Still the waveform contains 5th , 7th , 11th etc order harmonics. At this stage for controlling the amplitude of the inverter output voltage the DC link voltage has to be changed, which is not feasible. So to eliminate harmonics a filter is used at the output of the inverter, but it increases the cost of system and also for wide range of frequency the design of filter becomes an overwhelming task. Due to such type of problem the concept of modulation comes into existence. In all

(14)

Where ωcut is the cut off frequency. The bode plot of PI and PR controller is shown in Fig. 44. From Fig. 44, it can be seen that the amplitude of the resonant peak reduces and gets spread over a wide range of frequency. If the value of kI increases the damping peak also increases, thereby decreases the steady state error. Decrease in the value of the kI decreases the damping peak and makes the system stable but it accounts for increase in steady state error. 15

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Fig. 45. Classification of PWM.

modulation technique pulse width are economical and beneficial to reduce harmonic and for obtaining the output just as the fundamental of desired output [83]. The different topologies of PWM is shown in Fig. 45. 7.1. Sinusoidal pulse width modulation (SPWM) In sinusoidal PWM, the sine waveform is compared with the carrier waveform. The carrier waveform is generally fixed amplitude triangular waveform whose frequency is odd multiple of the sine waveform frequency. Due to which the width of pulse varies according to the amplitude of sine waveform. Also due to large odd symmetry of carrier frequency phase of the pulse varies according to sine wave fundamental [58]. The generated modulated signal are feed to the switches of half bridge inverter and full bridge inverter. Here the switches must have constant switching because the carrier have constant period. So when amplitude of sine wave is greater than triangular wave, top switch will be ON due to which voltage drop across load will be VDC /2 . When amplitude of sine wave is less than triangular wave, bottom switch will be ON due to which voltage drop across load will be −VDC /2. In three phase inverter the frequency of carrier waveform is even multiple of the sine waveform frequency. The lower order components of harmonic like 3rd, 5th, 7th etc will be highly suppressed due to this PWM modulation technique. Beside this the current drawn by the remaining harmonic present in load voltage VAo is nearly sinusoidal and have small ripple. Thus by using sine triangular wave PWM we get the fundamental amplitude variation control now to limit the fc and sideband.

Fig. 46. Space Vector diagram for 3-phase basic Inverter [84]. 1

where a = − 2 + j

Selective Harmonics Elimination PWM has tendency to control the harmonics present in fundamental voltage or current generated by inverter as well as reduce the total harmonic distortion. The main objective of Selective Harmonics Elimination PWM are to get a fundamental sinusoidal variation by putting notches in such a manner by which the positive half cycle that means 1st 180° and 2nd 180° period are symmetric to sine wave. Harmonics can be controlled by using more number of notches also at the same time the fundamental voltage as required by load can be controlled. In this PWM voltage waveform are fallen apart using Fourier series. SHE-PWM has several features [86]: (1) elimination of low order harmonics (2) low switching loss (3) improves THD (4) high voltage gain as well as wide converter bandwidth (5) less need of filtering. 7.4. Current hysteresis controlled PWM

Space Vector Pulse Width Modulation is an advanced PWM technique in which the PWM pulses are generated forming a state space representation of voltage by taking the active and null voltage vector states. The advantages of SVPWM over SPWM are higher utilization of DC link voltage, reduced switching loss and lower THD [84]. To operate the three phase basic inverter 3 switches have to be turned on at the same time. The number of legal switching combination can be calculated by SCn - (Illegal Combination), where S = Number of switches and n = Number of switches to be turned on. The no of illegal switching combination in a 3-ϕ inverter = 3×4, as for every legs there are 4 illegal combination. Hence the number of legal combination will be 8 and for each legal combination there will be a corresponding switching vector from V0 to V7. The voltage vector diagram is shown in Fig. 46. Here V0 and V7 are null voltage vector and V1 to V6 are active voltage vector states. Also during the sampling time tS the average voltage produced by the reference voltage vector VS* should be same as the average voltage produced by the inverter. So, VS* is produced by the combination of zeros and non-zero voltage vector. This voltage space vector VS in terms of phase output voltage of inverter can be presented by Eq. (16) [85].

Vs* =

2 [Va + aVb + a2Vc] 3

1 (t1 v1 + t2 v2 + t3 v3) τs

and Ts = t1 + t2 + t3

7.3. Selective harmonics elimination PWM (SHE-PWM)

7.2. Space vector pulse width modulation (SVPWM)

Vs =

3 2

To generate PWM signal, the current ripple in hysteresis controller is confined within a hysteresis band. Current limiting capability, stability, quick response makes current hysteresis control PWM most suitable to be used in grid connected PV inverter. By hysteresis current control it is easy to eliminate voltage distortion, bias current and discontinuous current. There are mainly two methods in current hysteresis control [87]: single side band hysteresis current controlled inverter and double band hysteresis current controlled inverter. In single side band as shown in Fig. 47 at every time the current error reaches its lower hysteresis limit (i0 − h ), the positive output voltage is produced by the inverter and if the current error reaches its upper hysteresis limit (i0 + h ), the negative output voltage is produced by the inverter 8. Filters IEEE 519, IEEE 1992, IEEE 1547 indicates the permissible level of harmonics that can be injected into the grid. The different types of filters used in PV system are broadly categorized as shown in Fig. 48.

(16)

8.1. L-Filer

(17)

The L-filter is first order filter thus it has a lower attenuation power of higher frequency region and restricted dynamics response. In order 16

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Fig. 49. Filter connected between grid and inverter.

G 1 (s ) =

Fig. 47. Single-band hysteresis current controller [87].

=−

IGrid (s ) VInv (s )

VGrid =0

sCf RDam + 1 s 3LInv L Grid Cf + s 2Cf RDam (LInv + L Grid ) + s (LInv + L Grid )

G 2 (s ) = =

IGrid (s ) VGrid (s )

(18)

VInv =0

s 2LInv Cf + sCf RDam + 1 s 3LInv L Grid Cf + s 2Cf RDam (LInv + L Grid ) + sDam (LInv + L Grid ) (19)

Fig. 50 shows that the LCL filter with passive damping shows higher peak overshoot in comparison to the L-filter and LC-filter. 8.3.1. LCL filter with active damping The control strategies with grid current feedback and inverter current feedback are shown in Fig. 51 and 52 respectively. The control loop gain for the system with filter gain GFilter(s) are given by Eq. (20).

Fig. 48. Different types of filters used in grid connected PV system.

Gloop (s ) = Gc. GINV . GFilter . H

(20)

Fig. 53 shows the bode plots of open loop transfer function for grid current feedback and inverter current feedback. In WAC based control, weighted average value of inverter side current and grid side current are used feedback to control the inverter. The WAC based control with nondamped LCL filter is shown in Fig. 54 [92]. If 1 − β and β are considered as weighted value for two inductor current (I1 and I2), then current feedback and open loop transfer function is given in Eq. (21) & (22). In control loop gain, the peak amplitude present in LCL filter at resonant frequency is canceled by using this methodology.

to attenuate switching current ripple of the inverter to an acceptable amount a high value of inductances is required which on the hand make the system bulky and increases power losses [88]. This type of filter is used for high switching application. 8.2. LC-Filter LC filter is 2nd order filter so it has a higher attenuating power as compared to L filter at high frequency region but due to the presence of a shunt capacitor, it presents a resonant peak at resonant frequency which disturbs the stability of the system [89]. In order to eliminate the resonant peak two types of damping technique are used: (1) Active damping and (2) Passive damping. In passive damping, a resistance/ damping resistance is added in series with the capacitance but addition of the damping resistance increases the power loss which in turns increases heat sink requirement and cost of the overall system. On the other hand passive damping method decreases the attenuation power at the high frequency region. In active damping the grid side and converter side current are taken feedback and thereby no power loss or lower attenuation is obtained in this method.

i f = (1 − β ) βi1 i2

(21)

8.3. LCL filter Nowadays among all filters LCL filter has taken much attention. LCL Filter is third order filter with attenuation of 60 dB/decade for frequency above full resonant frequency [90]. For this purpose this filter is used. Transfer function of LCL filter is given by Eq. (18) and Fig. 49 shows complete circuit diagram of the system. Transfer function of LCL Filter with damping resistance in given by Eq. (18) and (19) [91].

Fig. 50. Bode plot of different types filter.

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Fig. 51. LCL filter with grid current feedback control [92]. Fig. 55. LCCL filter [93].

As per the value of the capacitor Cf1 and Cf2 the feedback control strategies can be altered as grid current feedback and inverter current feedback. If Cf2 are taken as zero, the controller work as grid current feedback and if Cf1 is taken zero, it will act as inverter current feedback control. 8.5. LLCL filter

Fig. 52. LCL filter with inverter current feedback control [92].

LLCL filter is a 4th order filter and is used to capture the switching frequency component. The switching bandwidth of this filter can be written by Eq. (24). Eq. (24) can also be used to find out the value of the Lf.

ωsw =

1 L f Cf

(24)

LLCL filter is shown in Fig. 56 and transfer function can be derived by Eq. (25) and (26) [94].

Gui → i1 (s ) ug =0 =

Gui → ig (s ) u

g =0

=

(L 2 + L f ) Cf s 2 + 1 (L1 L 2 Cf + (L1 + L 2 ) L f Cf ) s 3 + (L1 + L 2 ) s

L f Cf

s2

(25)

+1

(L1 L 2 Cf + (L1 + L 2 ) L f Cf ) s 3 + (L1 + L 2 ) s

(26)

Within half of the range of switching frequency the LLCL gives same response as that of LCL from where it can be concluded that introduction of LLCL filter will not introduce any control complexity. Moreover the smaller magnitude of grid side inductance (L2) will be more advantageous in terms of bandwidth control as it boosts up the resonant frequency [95].

Fig. 53. Bode plot of LCL filter with grid current feedback control and inverter current feedback.

9. Anti-islanding Islanding is the one of the important safety issue in grid connected PV system. Islanding problem gets created when the utility grid has been terminated from the local load but the PV still continuous to supply power. This results in a huge amount of charge accumulation at the point of common coupling (PCC) which form island of charge near PCC. This phenomena is termed as islanding which is shown in Fig. 57 [96]. The accumulated charge may create following problems [97]: (1) It may be dangerous for the maintenance operators. (2) If the connection gets re-established between the island and the grid, large current may flow through the connection which can damage the equipment connected to it. So, detection of islanding is very important. A better way for

Fig. 54. Weighted Average Current based Control (WAC) [92].

G (s ) =

I f (s ) Vi (s )

=

(1 − β )(1 − α ) LCs 2 + Rdamp Cs + 1 Ls (α (1 − α ) LCs 2 + Rdamp Cs + 1)

(22)

Here L is the inductance of inductor used in L-filter, and the value L of α is L1 . 8.4. LCCL filter If the feedback capacitor Cf is slitted into two parts Cf1 and Cf2 and the feedback current is taken from the middle point as shown in Fig. 55 [93]. The transfer function for this filter can be described by Eq. (23).

GLCCL (s ) =

(Cf 1 + Cf 2 ) + Cf 1 Cf 2 Ls 2 i12 (s ) = Vi (s ) Cf 1 Cf 2 L2s 3 + (Cf 1 + Cf 2 ) Ls

(23)

Fig. 56. LLCL filter [94].

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Fig. 57. Concept of Islanding [96].

Fig. 59. Algorithm for Passive method [99].

Fig. 60. OUF/OUV range [99]. Fig. 58. Classification of Islanding detection method.

detection of islanding is usually to monitor the amplitude and frequency variation of the grid voltage. Standard regarding the antiislanding capability of the Grid connected PV system are IEEE Std. 929–2000, IEEE 1547 etc [98]. The classification of islanding detection method is shown in Fig. 58 and has been explained as follows: 9.1. Passive method Passive method attempts to detect transient change in the grid variables and use that information for determination of whether or not the grid has failed, or some other condition has resulted in a temporary charge. The algorithm for passive method is shown in Fig. 59. 9.1.1. Under/over voltage(OUV) and under/over frequency(OUF) This method is based on limiting the operating range of voltage or frequency at PCC. The over/under voltage and frequency limit of this method is shown in Fig. 60. If the amplitude of voltage or frequency at PCC exceeds from its corresponding limit of voltage and frequency, the supply provided by inverter has to be stopped. According to [99], by controlling the active and reactive power of the load the islanding situation can be controlled as these power depends upon the voltage and frequency. The equation for active and reactive power are given in Eq. (27) and (28).

Pload =

2 VPcc , Rload

(27)

Qload =

2 VPCC (1 − LCω 2 ) ωL

(28)

Fig. 61. Phase jump detection [100].

continuously, then it is easy to detect islanding situation. This method is very accurate method because there are very less possibilities of occurrence of disturbance in the phase angle. Besides, this method has less impact on transient response and output power quality [100]. 9.1.3. Harmonic detection method Inverter voltage and current contains some percentage of harmonics. So for detection of islanding, harmonic can be considered an important parameter. The reasons behind this harmonic in islanding situation are as follows [100]: (1) The switching process of the inverter is one of the cause of existence of harmonic current. (2) The grid impedance goes high as the utility gets disconnected from the supply. In harmonics detection based anti-islanding method, when the total harmonics present in the inverter output voltage oversteps from its threshold value then a trip signal gets generated which disconnects the

9.1.2. Voltage-phase jump detection When the islanding condition occurs, it changes the phase angle of the voltage at the PCC. If the difference between the phase angle of inverter current and voltage at PCC as shown in Fig. 61 can be observed 19

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Fig. 62. Order of Harmonics.

inverter from distributed generation system (DGs). Ref [101] has shown that the third harmonic component of the voltage plays a vital role for islanding detection as shown in Fig. 62. 9.1.4. Wavelet based islanding detection Wavelet based islanding detection is a passive islanding detection technique as it doesnt inject by external disturbance signal to observe the change in grid parameters [102]. The time frequency localization property of the wavelet transformation enables it to detect the islanding situation in a very efficient and faster way. The wavelet transformation basically splits the signal into many samples of different frequencies which is known as wavelet co-efficient [103]. If the value of the wavelet co-efficient exceeds the predefined reference threshold signal and retain in that position for a specified duration of time then the islanding phenomena is detected and the system gets cutoff from the grid. Wavelet islanding detection technique can reduce the NDZ to almost zero level but will degrade the quality of power [104]. The algorithm of wavelet transformation based islanding detection is given in Fig. 63.

Fig. 63. Algorithm for Wavelet based islanding detection [104].

situation the grid is not present. As a result the phase of the inverter increase rapidly with respect to load impedance. This phenomena leads to increase of inverter frequency beyond the frequency trip point and thereby the inverter will be turned off [106]. 9.2.3. Sandia frequency shift In Sandia frequency shift method the positive feedbacks that are provided by frequency estimator is applied at the frequency of voltage present at PCC. This method is also called active frequency drift with positive feedback [107]. In normal operating condition when the utility grid is connected through distributed generation system(DGs) the positive feedback frequency tries to change the frequency of the PCC but due to stability of utility grid the frequency of the voltage at the PCC remains stable [108]. In islanding situation when the utility grid is disconnected from the system the frequency of voltage at PCC goes high and shows an error in chooping factor that can be given by Eq. (31). The flow chart of this method is shown in Fig. 64.

9.2. Active method Active method attempts to detect a grid failure by injecting small disturbance signal into the grid variables in order to detect any change in grid parameters. Active methods are described as follows: 9.2.1. Active frequency drift method Active drift method is based on detection of zero crossing of voltage at PCC. In this method comparison between the frequency of current and voltage are done by setting lower and higher value of grid current. The current are set with small deviation and the frequency of voltage at PCC are observed by Eq. (29) and (30) [105]:

finv (k ) = fPCC (k − 1) + Δf0

cf =

δf 2T2 = T f + Δf

cf = cf0 + k ( fPCC − fline )

(31)

The frequency of voltage at PCC increases continuously and at a point it reaches the over frequency trip point. In response of which the inverter gets tripped and decreases its output power.

(29)

(30) 9.2.4. GE frequency shift In GE frequency shift method, positive feedback derived from frequency estimator are fed to the reactive current present at PCC [110]. Due to which the reactive power goes higher. In islanding situation the reactive power goes very high enough to cross the over voltage protection limits and thereby the inverter gets tripped. The flow chat of GE frequency shift method is as shown in Fig. 65 [109].

Where cf is the chooping fraction. When utility grid is connected to the inverter,due to stability of grid no deviation will be observed in grid frequency but when grid has been disconnected and islanding situation occurs, the frequency deviation creates too much disturbance in load current. Due to this reason over frequency protection comes into action. 9.2.2. Slip mode frequency shift Slip mode frequency shift is based on applying a positive feedback to the phase of voltage present at PCC. In normal situation when the utility grid is in operating condition, this algorithm stabilises the system by adjusting the phase angle between the inverter and the RLC load or load impedance with unity power factor. But in islanding

9.2.5. Harmonic injection method Harmonic injection based islanding detection are based in injection of harmonic current into grid impedance and resulting voltage harmonics are extracted. According to value of voltage harmonics the islanding situation can easily be detected [111]. The block diagram of 20

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Fig. 67. PLL based detection method [112].

Fig. 64. Sandia frequency shift method [108].

small variation is detected in power component, then it is reported to the inverter by control signal. 9.2.7. PLL based detection method In this method PLL are used to detect the situation of islanding due to input output proportional feature of PLL. Any change in PLL input changes the output angle of the PLL which are feed to the inverter [7]. So if any changes in current due to islanding condition occurs, the inverter output also changes which can be easily detected on the PCC. Ref [112] proposed a method of islanding detection which uses perturb & observe method. For the detection of change in the output of the inverter, they have used Goertzel Algorithm. The flow chart of this method is as shown in Fig. 67. 9.2.8. Hybrid islanding detection method Ref [114] proposed a combination of wavelet & neuro fuzzy based islanding detection technique. The use of neuro fuzzy technique along with wavelet enables the system to avoid the threshold value evaluation mechanism and thereby the hybrid algorithm multiplies the NDZ without comprising the power quality. Different combination of active and passive methods have been also been proposed to increase the efficiency of detection, reducing the NDZ, false operation and reduction of configuration cost [115]. Hybrid algorithms enable the algorithm to have the advantage of all combined algorithm while eliminating the demerits of each one. The comparison between islanding detection method in terms of power quality, NDZ etc is presented in Table 4.

Fig. 65. GE frequency shift [109].

harmonics injection method are shown in Fig. 66. Here disturbance current signal idis is injected and according to the voltage harmonic fluctuation islanding is verified.

9.2.6. Grid impedance estimation method As per standard VDE 0126-1-1(ENS), if grid impedance is increased by 0.5 at perfect balance load condition, it creates complicated situation like islanding problem. In such case, the islanding can be detected by considering the active reactive power variation [113]. If

10. Techno-commercial review According to IHS report of 2015, German based company SMA recorded the highest market share of 14% in earning revenue from PV inverter supply followed by Huawei (9%), Sungrow, ABB and Solaredge. Different types of standard are used by different countries so there inverter specifications are also different. Here, Table 5 provides a comparison of grid connected PV inverters which have been manufactured by inverter supplier companies in terms of specific parameter used by different countries [116–125]. 11. Gaps in prevailing technology Though photovoltaic power is one of the adequately available, never ending and environmental hazards less resource but there are many scopes of improvement in standalone as well as grid connected

Fig. 66. Harmonic Injection method [111].

21

22

Sandia Voltage Shift

Harmonic Injection

PLL based detection Method

Hybrid Method

Communication Based method

9

10

11

12

Slip Mode Frequency Shift

5

8

Active Frequency Drift Method

4

GE Frequency shift

Harmonic detection

3

7

Voltage Phase jump Detection

2

Sandia frequency Shift

OUF-OUV Method

1

6

Islanding Method

Sr. NO

Not Specified

Not Specified

Not Specified

Not Specified

Voltage shift, Positive feedback on voltage

Not Specified

Accelerated frequency drift, Active frequency drift with positive feedback

Slide mode frequency shift, Phase lock loop slip

Frequency, Bias, Frequency shift up/down

Detection of impedance at a specific frequency

Power factor detection, Transient phase detection

Standard Protection Relays, Abnormal voltage detection

Similar Methodologies

Table 4 Comparison table of anti-islanding methods.

First harmonics present in voltage at PCC Amplitude of Frequency or Voltage at PCC Status of circuit breakers and recluse at utility

Voltage harmonics at PCC

Reactive current reference Amplitude of voltage at PCC

Chopping frequency of PCC voltage

Frequency of PCC voltage

Frequency of current and voltage at PCC

Total harmonic distortion

Difference in Phase angle of Voltage and current

Voltage or Frequency

Parameter to monitor

Load inside the island

For Very high value of Q

low Q it shifts toward capacitive loads High Q loads when resonant frequencies are very near to the line frequency.

For small chopping fraction (cf < 1%) it is same as SMS. For

High value of Q,Low distortion outputs

−5%, ≤ΔQ ≤ 5%

for OUV −17%, ≤ΔP ≤ 24% for OUF −5%, ≤ΔQ ≤ 5%

Effect of NDZ

0.1–0.2 in, function to set threshold 0.105 to 0.115

<200 ms

Not applicable

Not applicable

Trip time

Under normal functioning loads this method does not have an NDZ,No effect on output power quality and impact system transient response

Implementation is easy in micro-controller inverters.,Highly effective when used combination with Sandia frequency shift method.

Easy to implement, Highly effective in multiple inverter applications, Less effect on output power quality and impact system transient response with good effect on islanding detection. Easy to implement, Less effect on output power quality and impact system transient response with good effect on islanding detection.

Low cost islanding detection method, Used for several other reason besides islanding prevention, also used in other islanding prevention method. Implementation is easy,No effect on output power quality and impact system transient response, No change in effectiveness under multiple inverter connection. Operation under wide range of grid conditions,Under multiple-inverter case effectiveness of this should not change significantly. Implementation is easy in micro-controller inverters.

Advantages

Requirement of transmitter on utility system,Very large expense due to installation of transmitter

Requires very small reduction in output power quality,Utility system transient response and power quality may be affected.

Output power quality of PV inverter should be high for this method.

Transient problem and System level power quality problems under very high gains and penetration levels in feedback loop.

Difficult to choose threshold for reliable islanding detection,Nuisance trips in PV inverter under certain load condition if thresholds are set too low. Difficult to choose threshold for reliable islanding detection,Nuisance trips in PV inverter under certain load condition if thresholds are set too low. Radiated and conducted radio frequency interference (RFI) due to discontinuous waveform,Some specific rules to follow under multiple-inverter condition.

Relatively large NDZ, Reaction time may be variable or predictable

Limitation

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Table 5 Comparison between different parameters of commercial available inverter used in Grid Connected PV system [116–125].

will reduce the cost per watt. Switching losses are pretty much evident during the high switching frequency operation of the hard switching based conventional inverter. This increases the size of the heat sink and also reduces the efficiency of the overall system. The schemes present for grid synchronization in the literature are not reliable for high power industrial applications. The control of multi Level inverters must be designed in a way so as to ensure high power quality, system robustness, high reliability and support grid voltage and frequency stability. According to the survey none of the commercially available grid connected inverter has employed grid supportive ancillary feature. Researches are going on to earn revenues from the PV system during the night [126,127]. The solar inverter is fully utilized during the noon, partly used during rest of the day time and remains totally unutilized during the night time i.e almost 70% of the time the inverter remains idle. Utilizing the Inverter as STATCOM for reactive power compensation helps not only to increase connectivity to the neighbouring wind farm but also increase power transmission capability, improve power regulation and power factor correction. Besides there are some commercial complexity present in the prevailing system. Electricity industry infused with short- to long term risks are difficult to commercialise correctly or allocate to industry participants.

photovoltaic system. The gaps in prevailing technology can be presented as shown in Fig. 68. Present day commercially available inverters have an average life time ranging from 5 to 10 years. So the inverters become the most unreliable component in the whole chain linking the PV to the grid. So the inverter life span must be increased. The available multilevel inverters used in literature are mainly experimental prototype and are used for low power application. Mostly the PV power is able to meet only the local electrification to household. So to provide power to industry or large scale application, now the tend is on improving the design of the inverter for higher power ratting. This

Fig. 68. Grid connected PV system Prevailing Technology Gaps.

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This paper concludes with gaps in prevailing technology along with techno-economic comparison of commercially available PV inverter like maximum DC voltage, power, current MPP voltage range, frequency, power factor etc. It is expected that this paper will deliver an insightful reference for the researchers working in the field of grid connected PV system. It will also help them in selecting appropriate topology for their particular application.

12. Conclusion This paper presents a technological review of almost every parts of the grid connected PV system. At the very beginning of the paper a brief review of all the grid codes applicable for a grid tied system has been documented in Table 1. A comprehensive analysis of different widely used important standards has been presented in Table 3 based on different parameter like nominal power maximum, allowable limit of current THD, DC current injection limit etc. In the subsequent part of the paper detailed review of different MPPT algorithm has been discussed. It could be concluded that different bio-inspired soft computing based and hybrid MPPT algorithm gives better performances in terms of convergence rate, oscillation of obtained near the MPP, tracking efficiency, probability of obtaining the global MPP, degree of freedom in controller designing, robustness, reliability etc as compared to conventional techniques. Inverter plays an important part in grid connected PV DGs. Increasing power demand and advancement in power electronic front have lead towards the use of different improved version of power inverters with high efficiency, lower switching losses and better harmonics performance. In this paper some of the conventional and advanced inverter topologies like Z-source inverter, soft computing based inverter etc have been presented. Some of the inverter topologies patented by leading companies like REFU, Sunways, SMA (H5 Inverter), Ingeteam. In this paper different grid synchronization techniques have been reviewed. It can be concluded that time domain based synchronization technique performs superior than the frequency domain techniques in terms of tracking efficiency, faster dynamic response, compatibility, robustness etc. Time domain based synchronization is broadly classified into 2 categories: PLL and FLL. Different types of PLL have been presented in this paper. It is observed that three phase system can produce quadrature signal inherently in αβ - reference frame while single phase systems employ T /4 transportation delay, Hilbert transform and park transform in order to generate proper orthogonal signal. FLL based grid synchronization has high adaptability as compared to PLL. Moreover the computational burden also gets reduced in FLL as it doesnt use any trigonometric function for grid synchronization. Many PWM techniques like SPWM, SVPWM etc have been used in PV application. SVPWM performs better than SPWM not only in terms of maximizing the utilization of DC bus voltage but also reduces switching loss by optimizing the switching sequence. Section 8 of this paper presents a detailed report of different types of filters used in grid connected PV connected system. From this section, it can be concluded that as the order of the filter increased the attenuation power at high frequencies region also increases. Therefore the use of LCCL or LLCL filters proves out to be the most appropriate solution, but the use of 2nd, 3rd and 4th order filters bring with it a resonant peak at resonant frequency which hampers the stability of the overall system. In order to minimize the amplitude of the resonant peak passive damping procedure has been introduced where the resistance is connected in series or parallel with the filter capacitance. But the introduction of passive damping produced overheating and decreased attenuation. In order to overcome this drawback active damping procedure is introduced where the grid side or the inverter current is taken as feedback to the current loop. Islanding detection is integral measures for safety of grid connected PV system. Passive islanding detection method detects grid failure by directly monitoring the change in grid parameters while the active islanding detection method injects a disturbance signal in order to detect any change in grid variable. In this paper almost all types of anti-islanding methods have been addressed and a comparison table based on trip time, NDZ, reliability etc has been presented. However, proper selection of antiislanding method depends upon the user priority of parameter selection. Each anti-islanding method has its own merits and demerits so, it become very difficult to choose proper anti-islanding scheme.

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