A comprehensive review on CHB MLI based PV inverter and feasibility study of CHB MLI based PV-STATCOM

Renewable and Sustainable Energy Reviews 78 (2017) 138–156

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

A comprehensive review on CHB MLI based PV inverter and feasibility study of CHB MLI based PV-STATCOM V. Sridhar, S. Umashankar

MARK

⁎

Department of Energy and Power Electronics, School of Electrical Engineering, VIT University, Vellore, India

A R T I C L E I N F O

A BS T RAC T

Keywords: Photovoltaic Multi-level inverter Cascaded H-Bridge STATCOM Maximum power point tracking Active power control Reactive power control

A conventional grid-connected solar Photovoltaic (PV) inverter consists of Two-Level or Three-Level configuration is not suitable for very high power ratings and the size of AC side filter required is high to maintain the power quality as per the grid codes. It is inefficient in extracting maximum power as the tracking of Maximum power point is carried out for entire PV arrays connected together instead of independent MPPT of each PV array. With a conventional PV inverter, the utilization factor is also very less, since the system will be in idle state during night times or when the irradiation is weak. Hence a conventional solar inverter consists of TwoLevel or Three-Level inverter suffers from the following drawbacks (a) Not suitable for very high Power Ratings (b) High filter size (c) Inefficient in harvesting maximum power (d) Less utilization factor. In this study, the need for the multilevel inverter (MLI) to minimize the drawbacks of the conventional inverter is discussed. Cascaded H-Bridge (CHB) configuration which is more preferred for solar power applications where isolated input DC sources are available and for STATCOM applications where there is no requirement of DC Sources is discussed in detail. The basic operation of CHB inverter, PWM techniques, and fault tolerant operations are explained through simulation results. The Independent MPPT control of each PV array using CHB inverter is reviewed. CHB inverter controls for PV applications and STATCOM applications are also reviewed. The concept of a PV-STATCOM which is required for improving the utilization factor of PV inverter is reviewed. The operation of PV-STATCOM is explained through simulation studies. Real and reactive power flow through a 11Level, CHB MLI is verified through experimental results. Feasibility study for multilevel PV-STATCOM for High power applications using CHB configuration is carried out in this paper.

1. Introduction to PV inverters and PV-STATCOM A typical grid-connected solar power conditioning system comprises of an inverter, sine filter, isolation Transformer, AC and DC breakers connected across PV array and the grid is as shown in Fig. 1. An output transformer is required to match voltages of the grid and the inverter output and for providing galvanic isolation. Conventionally a Two or Three level inverter is used for PV applications and a digital controller controls the power flow through PV inverter. For tracking of MPP, controller monitors the PV array voltage and currents. Controller monitors grid voltage for synchronizing the inverter output with grid through Phase-locked Loop (PLL). Direct Current reference (Id_Ref) is obtained based on MPP and the quadrature current reference (Iq_Ref) is obtained form the reactive power reference. In a conventional PV inverter, Iq_Ref is Zero. Power control is carried out through PI controllers by comparing the dq components of reference grid current with dq components of actual grid current. The output of PI controllers generated the reference output voltage required for the inverter gate pulse generator.

⁎

Corresponding author.

http://dx.doi.org/10.1016/j.rser.2017.04.111 Received 13 May 2016; Received in revised form 23 March 2017; Accepted 28 April 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

A review on PV inverter topologies for grid applications is presented in Ref. [1]. Single phase Inverter configurations based on commutation of devices i.e. self-commutated and line-commutated inverters are explained briefly. Centralized inverters, string, and multi-string inverters are reviewed in this paper. PV systems based on the power conversion stages i.e. single stage or multiple stage PV systems are explained and a brief review on multi-level PV inverters are also presented. In Ref. [2], a review on Grid connected micro inverters is presented. Authors presented standards and requirements of grid connected PV inverters in this work. A brief review on central inverters, string inverters, and micro inverters is also presented. Utilization factor of a conventional PV inverter is a major concern as the conventional PV inverter feeds only active power to the grid during daytime and the system will be in the idle state when solar energy is not available. This problem can be mitigated through the concept known as PV-STATCOM which can regulate the real and reactive power through the converter. In such system, Id_Ref is obtained from MPP and Iq_Ref is generated as per Reactive power requirement.

Renewable and Sustainable Energy Reviews 78 (2017) 138–156

V. Sridhar, S. Umashankar

Fig. 1. Block diagram and control structure for a PV-inverter.

discussed and presented a brief review of anti-islanding techniques. A PV-STATCOM is simulated to understand the power flow through PV-STATCOM. A load equal to the capacity of PV-STATCOM is connected at the at the AC terminals of the inverter. Power flow through PV-STATCOM is controlled by adjusting the real power reference (Pref) and reactive power reference (Qref). When Power Reference for PV-STATCOM is Zero, then the total current supplied from the PV-STATCOM is Zero and the complete Load Current is fed from Grid as shown in Fig. 2(a). When the Pref is adjusted to 1 P.U. and Qref is adjusted to 0 P.U., then complete real power required for Load is fed from PV-STATCOM and reactive Power for Load is fed from the Grid. Since the PV-STATCOM is feeding only the Active Power, PVSTATCOM phase voltage and currents are in phase with each other as shown in Fig. 2(b). Grid current is lagging the phase voltage by 90 degree, since the grid is supplying only reactive power. When Qref is also increased to 1 P.U. for PV-STATCOM, then the Grid current becomes Zero and the total power requirement is drawn from inverter as shown in Fig. 2(c). From above discussion, it is evident that a PV Inverter can also be used for Reactive Power Compensation, by varying Reactive Power reference in the range of 0–1 P.U. With this concept called PV-STATCOM, the Utilization factor can be improved and the issues related to power quality can be reduced without any additional cost. In Ref. [16], a Sliding mode control based on an extended state observer for the grid-connected converter is presented. DC-link capacitor voltage is regulated by the external control loop. It gives the current references to inner control loop based on the desired power factor. Inner current control loop maintains the actual currents equal to their reference currents. Concepts of extended state observer (ESO), Super-twisting algorithm (STA) are explained briefly. Instead of using a general PI Controller for outer voltage control loop, STA + ESO based control is adopted in this work to reject disturbances and uncertainties. Inner current control loop is based on Super twisting algorithm. From presented results, it is observed that this control is an alternative solution for grid connected inverter controls. Some of the PV-Inverter products based on Two-Level or threeLevel inverter configuration available in the market are listed in Table 1. Some of these products are available with reactive power compensation feature. In large scale systems, number of such systems

The concept of using PV Inverter as STATCOM has been proposed for optimal utilization of the system in Ref. [3]. PV inverter acts as active power provider when the solar energy is available and provides reactive power when the irradiation is low. The concept is validated through simulation studies. In Ref. [4], a PV-STATCOM which can operate as a STATCOM throughout the day is presented. Reactive power flowing through the converter depends on the inverter rated capacity and the MPP at that instant of time. Operation of a PVInverter as STATCOM for 24-h for reactive power compensation and to control the DC link voltage in a distribution utility network is discussed in Refs. [5] and [6]. A PV-Active Filter -STATCOM for Harmonic compensation and for the regulation of reactive power for grid applications is proposed in Ref. [7]. Voltage flickering, transient voltages and harmonics are the main issues with the power quality of PV generation system. A comparison on power quality issues in a conventional PV inverter and in a PV-STATCOM is presented in Ref. [8]. Operation of PV inverter as PV-STATCOM for short duration to improve the stability of voltage sensitive loads during grid faults is presented in Ref. [9], In Ref. [10], Authors presented the PV-STATCOM testing in three stages. In the first stage of testing, RSCAD software is used to test the controller. In stage two, to test the controller and control algorithm, an hardware-in-loop simulation is carried out by interfacing the DSP based controller with Real time digital simulator. System operation is tested on the 10 kW laboratory model in stage three. Presented results explain that the system can be used for voltage control and for correcting the power factor. Steady state response and dynamic response of a PV-STATCOM controller are explained through the hardware-in-loop (HIL) simulation results in Ref. [11]. In Ref. [12], a PV-STATCOM without a DC-DC converter is proposed. DC link voltage is controlled through STATCOM operation in this system. Eliminating the DC-DC converter resulted in the reduction of cost and size of this system. The impact of Distributed generation systems on the grid and the technical challenges on power quality, protection and stability are discussed [13]. Negative impacts of PV systems on the grid network and a comprehensive review of the power quality improvement i.e. voltage regulation controls and static compensation techniques are presented in Ref. [14]. In Ref. [15], Authors presented the effects of power quality issues and the issues related to islanding are

139

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GRID VOLTAGE & CURRENT

GRID VOLTAGE & CURRENT

GRID VOLTAGE & CURRENT

400

400

400

200

200

200

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-200

-200

-200

-400 0.04

0.06

0.08

0.1

-400 0.04

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0.08

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-400 0.04

0.06

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0.1

INVERTER VOLTAGE & CURRENT 400

INVERTER VOLTAGE & CURRENT 400

INVERTER VOLTAGE & CURRENT 400

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-400 0.04

0.06

(a)

0.08

(b)

0.1

-400 0.04

0.06

0.08

0.1

(c)

Fig. 2. Grid and PV Inverter Voltage and Currents waveforms. (a) When Pref =0 and Qref =0; (b) When Pref =1 P.U. and Qref =0; (c) When Pref =1 P.U. and Qref =1 P.U.

performance requirements (such as efficiency, power density, installation cost, minimization of leakage current) and Legal requirements (such as Isolation, anti-islanding, codes and standards). In this paper, various standards for PV applications in grid connected, off-grid applications, PV in rural systems are explained briefly. Various PV system configurations such as String, multi-string, central inverters, AC module inverters and different power topologies in each configuration are presented briefly. An overview of Small-scale, medium scale, and large scale PV systems is also discussed. Presented an overview of commercially available PV inverter topologies such as Two level VSI, Three level NPC, asymmetrical CHB, etc.. Various DC-DC converters used in solar power conditioning systems are also discussed in this work. PV power systems, its components, its operation for Grid applications are presented in Ref. [23]. Various PV system configurations namely Centralized configurations, String, Multi-string Configurations, modular configurations are discussed briefly in this article. Classifications of grid-connected PV power conditioning system topologies with and without output transformers, with and without DC-DC converters are also discussed in this paper. Comparison of different MPP techniques (Perturb & Observation, Incremental conductance, current sweep, Fractional Voc, Fractional Isc etc…) based on convergence speed, complexity, sensed parameters, PV array dependence, etc are also presented in this paper. Distributed MPPT controls are also discussed. Various Grid Synchronization schemes for single and three phase systems are compared based on distortion immunity, adaptation frequency, unbalance robustness, dynamic response, cost and complexity. Various anti-islanding methods are compared based on reliability, power quality, multiple inverter integrations and standardization possibilities. The design of CHB inverter for high power solar power systems and comparison of the cascaded inverter with a conventional PV inverter on basis of cost, efficiency is presented in Ref. [24]. Component level estimation, device loss calculations, and system level evaluation are carried out and compared with a conventional PV inverter. In this work, Power Control techniques for CHB are not covered. In Ref. [25], Different types of the multi-level voltage source and current source inverter configurations, modulation techniques, and control techniques

need to be connected in parallel. In Ref. [17], Authors presented the design of large scale PV system with feature of active, reactive power control and power factor correction. An algorithm is developed to meet the requirements of grid codes and validated on a 9.4 MW solar power system. Authors also explained droop control which allows PVInverters to operate in parallel with load sharing in proportion to their power rating. A review of Power quality in microgrids and need of multifunction inverters to resolve the issues related to power quality is presented in Ref. [18]. Authors reviewed various configurations of Multifunction inverters and their controls in this paper. Different active and reactive controls are reviewed for 2-Level and 3-level inverters. A comparison is made for selection of suitable topology based on the user application. In Ref. [19], different configurations of PV inverters for grid applications are presented. Reviewed various controls for the PV inverters in this paper. Three different controls namely abc, d-q and alpha-beta controls are presented in this paper. In Refs. [18] and [19], controls for higher level inverters (More than 3-Level) are not discussed. In view of the large demand for the solar power requirement, it is necessary to have a multilevel Inverter because for high Power Ratings a Two-Level Inverter is not preferable as the THD of the system is poor and the filter size requirement is more and it is inefficient in harvesting maximum power form PV arrays. The design of a Multi-level inverter shall be useful for high power ratings and the AC Line filter size also will be reduced by using higher level inverters. A comparison study of 2-Level and multilevel inverters and a brief review on multi-level inverters based on common DC or separate DC sources is discussed in Ref. [20]. An overview of grid interfaced solar photovoltaic systems is presented in Ref. [21]. Basics of the solar cells and concept of MPPT are explained briefly. Various MPPT techniques are compared. The operation of different DC-DC converters used in solar inverters is explained briefly. Structural topologies such as Centralized configurations, String, Multi-string Configurations, modular configurations are discussed. Single phase inverter topologies such as half H-bridge, Full H-Bridge, HERIC, H5, H6, NPC, Active NPC, co-energy NPC, Flying capacitor are discussed and compared. Control techniques for grid connected three phase inverters are also presented. Emerging PV converter technologies for grid-connected systems is presented in Ref. [22]. Requirements for a PV inverter namely 140

Renewable and Sustainable Energy Reviews 78 (2017) 138–156

V. Sridhar, S. Umashankar

Table 1 Solar PV inverter products for high power applications available in the market. SL.NO

Manufacturer

Model

Rating of the Equipment

Key features

1

Sungrow Power Supply Co., Ltd.

SG 2500

Power: 2800 kVA DC Voltage: 1000 V AC Voltage: 315 V

Outdoor Applications

2

Sungrow Power Supply Co.

SG 1000 HV

Power: 1100 kVA DC Voltage: 1500 V AC Voltage: 540 V

Feature of Reactive Power control available

3

Sungrow Power Supply Co.

SG 1000 MX

Power: 1100 kVA

Feature of Reactive Power control available.

DC Voltage: 1000 V AC Voltage: 385 V

Suitable for PV Array Negative grounding

Power: 1100 kVA

Step-up Transformer on AC Side for achieving High voltage

4

Sungrow Power Supply Co.

SG 1000TS-MV

DC Voltage: 1000 V AC Voltage: 10–24 kV 5

HUAWEI

SUN8000–500KTL

Power: 500 /600 kVA DC Voltage: 1000 V AC Voltage: 320 V

Feature of Reactive Power control available

6

HUAWEI

SUN8000–1000IS

Power: 1000 kVA

Feature of Reactive Power control available.

DC Voltage: 1000 V AC Voltage: 320 V

For Outdoor Applications.

7

Hitachi -Hirel

HIVERTER - NP201i Series

Power: 1000 kW DC Voltage: 1000 V AC Voltage: 300 V

Feature of Reactive Power control available

8

Hitachi -Hirel

HIVERTER - NP201i Series

Power: 1250 kW DC Voltage: 1000 V AC Voltage: 350 V

Feature of Reactive Power control available

9

GE

PSC − 1000 MV - L - QC

Power: 1000 kW DC Voltage: 1500 V AC Voltage: 550 V

Feature of Reactive Power control available

10

Schneider

Conext Core XC 733 –NA

Power: 733 kVA DC Voltage: 1000 V AC Voltage: 407 V

Feature of Reactive Power control available

11

Ingeteam

PowerMaxter 840×360 Indoor

Power: 917 kW DC Voltage: 920 V AC Voltage: 360 V

Feature of Reactive Power control available

12

TMEIC

PVL-L0500E

Power: 600 kW DC Voltage: 100 V AC Voltage: 300 V

Feature of Reactive Power control available

losses. Three types of multilevel converter topologies, i.e. CHB; diode clamped and capacitor clamped configurations their controls, applications are discussed in Refs. [27,28]. Fig. 3 shows the classification of Multilevel inverters based on Number of DC Sources required. Diode Clamped and CHB Inverter configurations are most suitable and most used topologies for High Power requirements. Various multilevel inverter configurations, controls, modulation methods and applications are briefly discussed in Ref. [29]. Various multilevel multi-function grid connected inverter configurations are reviewed in Ref. [25]. Diode-clamped, capacitor clamped, CHB configurations and other recent advances in inverter configurations are discussed. Different modulation techniques also presented. Configurations for single-phase solar power conditioning systems without isolation transformer for grid applications are discussed in Ref. [30]. The author presented a brief description of configurations like CHB; flying capacitor, NPC half-bridge with a capacitive divider and Active NPC (ANPC) in this work. In Ref. [31], performance of NPC inverter and the CHB Inverter for PV systems for grid applications is compared. Presented results, explain that cascaded H-Bridge inverters

are reviewed. Energy balance control for a CHB inverter is discussed briefly in this paper. Reactive power controls for grid connected CHB inverters are not discussed in this work. In Ref. [26], Comparison of CHB and Modular multilevel converters (MMLC) for STATCOM applications is discussed. Reactive power controls for CHB and MMLC is explained briefly. Since the work is focused on STATCOM, only the reactive power controls are discussed in this work. Since the above reviews not covered the independent active and reactive controls for a CHB inverter, this work is focused mainly on a CHB inverter based PV-STATCOM for large scale PV applications. In the next section advantages of CHB inverter, suitability of CHB MLI for PV Applications is presented. 2. Multi-level inverter configurations suitable for grid connected systems For High power applications, multi-level inverter topologies are preferred over conventional two-level inverters. Multilevel inverters are required to obtain better THD, reduced dv/dt and reduced device 141

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MULTI LEVEL INVERTERS

SINGLE DC SOURCE

NEUTRAL POINT CLAMPED

MULTIPLE DC SOURCES

FLYING CAPACITOR TYPE

CASCADED H-BRIDGE

EQUAL DC SOURCES

UNEQUAL DC SOURCES

Fig. 3. Classification of MLI based on number of DC sources. Table 2 Comparison between NPC and CHB multilevel inverters for an N-level converter. Neutral point clamped inverter

Cascaded H-Bridge Multilevel Inverter

No. of IGBTs Required is 6 X (N-1) No of Levels N Can be either Odd or Even number No. of Capacitors Required is N-1 Single DC Source is required Voltage balancing of Capacitor is required Special care to be taken while designing a power module when no of levels increase

No. of IGBTs Required is 6 X (N-1) No. of Levels N is always an Odd Number No of Capacitors required is 1.5 X (N-1) Individual Source is required for each H-Bridge Voltage balancing of Capacitor is not required Due to modular structure power, module design is easy. Due to the modular structure of CHB inverter, spares and maintenance cost reduces. Other than Output Side; the voltage at all the points is low hence the rating of components is less which reduces the price of complete system In the case of Failure in one power module; the system can be run for a low power just by bypassing that particular cell through software.

For High Voltage Applications DC Link Voltage required; Voltage rating of each Component is more which makes the system costlier In case of Failure in one power module; the system has to be Switched off Completely

four different levels of CHB inverters i.e. Five, Seven, Nine and Seventeen level CHB inverters is carried out in Ref. [34]. THD improves with higher output levels, but the complexity of the system increases. In Ref. [35], Authors discussed on CHB MLI with equal and unequal DC Sources, PWM techniques and then discussed on a hybrid CHB inverter configuration which requires only one DC source. The present work is focused on a standard CHB inverter fed from identical DC sources.

are more efficient in extractive solar power than an NPC inverter. Table 2 shows the comparison between NPC and Cascaded H-Bridge based multilevel Inverters. CHB based Inverter is ideally suitable for PV and STATCOM applications as these systems consist of independent DC links. The following are the advantages of CHB MLI (a) Low Output THD hence filter size is small (b) By increasing the number of Modules; Output can be extended to any voltage level without using output side transformer (c) Even though the Output Voltage is High; the module level voltage is very low and safe to operate. (d) Since the voltage levels are low for each module; the cost and size of the power components and auxiliary equipment are less. (e) Modular in Construction; hence replacement of one faulty module is very easy which reduces the maintenance time and cost. In the case of Failure in one module; bypassing the faulty module and running the system without shutting down is also feasible. A review of CHB configuration, its controls and harmonic elimination methods are discussed in Ref. [32]. In the next section, the basic operation of CHB inverter, PWM techniques, Fault tolerant operation and the applications of CHB MLI for Grid applications is presented.

3.2. PWM techniques The two PWM schemes which are widely used for CHB Inverters are explained briefly in following subsections. 3.2.1. Phase Shifted Pulse width modulation (PS-PWM) A phase shifted multicarrier modulation needs (N–1) carrier signals for an N-Level CHB MLI. Amplitude and Frequency of the carrier signals are equal. As shown Fig. 5(a), Phase displacement angle between adjacent carrier signals is 360 degree/(N–1). Phase voltage and AC voltages of H-bridge modules using Phase-Shifted modulation technique are shown in Fig. 6(a). A phase-shifted PWM strategy based on a conventional unipolar Modulation method for a 5-Level CHB Inverter for Standalone PV applications with a non-UPF load is presented in Ref. [36]. In Ref. [37], Authors presented the harmonic performance for practical implementations of phase shifted carrierPWM. Effects of non-uniform DC link voltages are discussed and the Mathematical expressions for describing the Harmonic number and magnitude are presented.

3. Cascaded H-bridge multilevel inverters and its applications in grid-connected systems 3.1. Basic operation Basically, a CHB inverter is built by cascading multiple H-bridges, which are fed from independent DC sources. A Series connection of multiple H-bridges results in multiple levels in the AC voltage. As shown in Fig. 4(a), with a series of connection of Two H-Bridges, 5level CHB MLI is built. This inverter generates AC output with 5-levels i.e. +2 V; +1 V; 0; −1V and −2V. Similarly, Series connection of three H-bridge modules is required to build a 7-level system as shown in Fig. 4(b). This inverter generates AC output with 7-levels i.e. +3 V; +2 V; +1 V; 0; −1V; −2V;−3V. Maximum number of levels possible with a cascaded inverter (with H number of bridges per phase) is equal to 2 H +1 which is always an odd number [33]. A comparison study on

3.2.2. Level Shifted Pulse width modulation (LS-PWM) A Level shifted multicarrier modulation needs (N–1) carrier signals for an N-Level CHB MLI.. Amplitude and Frequency of the carrier signals are equal but displaced on vertical scale as shown in Fig. 5(b). Output voltage and AC voltages of H-bridge modules obtained through a LS-PWM technique are shown in Fig. 6(b). 142

Renewable and Sustainable Energy Reviews 78 (2017) 138–156

V. Sridhar, S. Umashankar

H-Bridge-1

DC Source-1

P

H-Bridge-1

DC Source-1

P

H-Bridge-2

DC Source-2 H-Bridge-2

DC Source-2

N

H-Bridge-3

DC Source-3

N

(a)

(b)

Fig. 4. (a) Single phase 5-level cascaded inverter; (b) Single phase 7-level cascaded inverter [33].

1

0.5

0

-0.5

-1 0.04

0.042

0.044

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0.054

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0.054

0.056

0.058

0.06

(a) 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1 0.04

0.042

0.044

0.046

0.048

0.05

(b) Fig. 5. (a) Phase shifted PWM (b) Level shifted PWM for seven-level CHB inverter.

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SPWM for cascaded inverters fed from time-variant dc sources is presented in Ref. [39]. Authors also demonstrated algorithms based on number of sensors through simulation and experimental results. A modified SHE-PWM technique for transformerless STATCOM using a CHB configuration is presented in Ref. [40]. An optimal SHE through Selective Harmonic Mitigation technique (SHM-PWM) in a CHB MLI is presented in Ref. [41]. CHB MLI with three H-bridges in cascade is simulated to determine the change in number of levels (N) with the change in modulation index (MI). MI increased linearly from 0 to 1 in 1 s. Number of levels is Three when MI is less than 0.33. Number of levels is five when Modulation Index is between 0.33 and 0.67 and Number of levels is seven when modulation Index is more than 0.67. So it is understood that Number of levels is more when the system is operated near to Modulation Index 1, hence the DC source is to be selected accordingly to maintain optimum modulation index.

Output Voltage of H-Bridge-1 1 0 -1

Output Voltage of H-Bridge-2 1 0 -1 0

0.1

0.2

0.3

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0.5

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0.8

0.9

1

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1

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0.9

1

Output Voltage of H-Bridge-3 1 0 -1 0

0.1

0.2

0.3

0.4

0.5

0.6

Output Voltage of CHB 5 0

-5 0

3.3. Redundancy and fault tolerant operation 0.1

0.2

0.3

0.4

0.5

0.6

Modulation Index MI

As explained earlier, In the case of failure in one H-Bridge of a CHB inverter, the system can continue its operation by bypassing that particular module through internal switches or by using a bypass switch. To enhance the availability and to improve the reliability of a CHB based PV-inverter, a Fault tolerant operation has been explained in Ref. [42]. CHB MLI based STATCOM with fault-tolerant feature is presented in Ref. [43]. Switching Strategy to Bypass failed H-Bridge module through internal switches or through external bypass switch is explained in next subsections.

(a)

Output Voltage of H-Bridge-1 1 0 -1

Output Voltage of H-Bridge-2 1

3.3.1. One Switch Opened on Fault Fig. 7(a) shows, the bypassing method during one switch open on fault Condition. Consider switch S1 is opened on Fault, then by Switching ON devices S2, S4 and by Keeping Switch S3 in OFF State; the H-Bridge can be bypassed. Similarly, if the device S2 is opened on fault, then by switching ON devices S1, S3 and by keeping switch S4 in OFF State, The H-Bridge can be bypassed. Switching strategy to bypass the H-Bridge during only one Switch open on the fault is presented in Table 3.

0 -1

Output Voltage of H-Bridge-3 1 0 -1

Output Voltage of CHB 5

0

-5 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

3.3.2. Two switches opened on fault Switching strategy to bypass the H-Bridge during Two Switches opened on the fault is shown in Table 3. If two top switches or two bottom switches open due to a fault, then the H-Bridge can be bypassed by keeping other healthy switches in ON state as shown in Fig. 7(b). In remaining cases bypass switch to be operated for bypassing the HBridge.

1

Modulation Index MI (b) Fig. 6. (a) Voltage waveforms of First; second; third H-Bridges and Total Voltage with Increase in Modulation Index using Phase Shifted PWM; (b) Voltage waveforms of First; second; third H-Bridges and Total Voltage with Increase in Modulation Index using Level Shifted PWM.

3.3.3. One Switch shorted on fault Fig. 7(c) shows, the bypass method during one Switch Shorted on fault Condition. Consider Switch S1 is shorted on Fault, then by switching ON device S3 and by Keeping Switches S2, S4 in OFF State; the H-Bridge can be bypassed. Switching strategy to bypass the H-Bridge in the case of remaining cases also shown in Table 3.

3.2.3. Comparison of PS-PWM and LS-PWM Techniques As shown in Fig. 6(a), With Phase Shifted Modulation Scheme, the H-bridge output voltages are equal and displaced with a phase angle of 360/(N-1). Switching and conduction losses of all the devices are equal whereas, in a Level Shifted PWM Scheme, outputs of H-Bridges are not equal as shown in Fig. 6(b). Lower Level H-Bridge Module i.e. H3 will be having more conduction time than the top level H-Bridge Module i.e. H1. To make duty cycle and conduction losses equal for all the devices, a rotation in switching needs to be adopted. However, the phase voltage of CHB inverter is same with the above two PWM techniques. Selective harmonic elimination (SHE) method is an alternative technique for the pulse generation for a CHB inverter. Other switching technique known as fundamental frequency switching can also be adapted. But these techniques are rarely used. A fifteen level CHB with fundamental frequency switching is presented and explained the effects of different types of gate pulse variations in Ref. [38]. A modified

3.3.4. Two Switches shorted on fault Switching strategy to bypass the H-Bridge during Two Switches Shorted on the fault is shown in Table 3. If two Top switches or two bottom switches Shorted due to a fault, then the H-Bridge can be bypassed through the failed Switches as shown in Fig. 7(d). In remaining cases, Bypass switch needs to be switched ON for bypassing the faulty H-Bridge. Bypass switching technique for a 7-Level CHB MLI is simulated and different types of faults are simulated to verify the operation. As shown in Fig. 8(a), Number of levels is seven, when all three H-bridges are healthy. Simulated a fault on H-Bridge-1 of CHB and the reference AC output voltage is maintained constant. Output voltage waveforms of 144

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OFF

Open on S1 Fault

S3

Open on S1 Fault

ON

ON S2

S3

ON

ON S2

S4

S4

(b)

(a) Short on S1 Fault

ON

Short on S1 Fault

S3

S3

OFF

OFF

OFF

OFF

S2

S4

S2

(c)

S4

(d)

Fig. 7. (a) Bypassing H-bridge when one Switch Opened on Fault; (b) Bypassing H-bridge when Two Switches Opened on Fault; (c) Bypassing H-bridge when one Switch shorted on Fault; (d) Bypassing H-bridge when Two Switches shorted on Fault.

3.4. CHB inverter for grid connected applications

Table 3 Switch states to bypass faulty H-Bridge when one or two switches open or short on the fault. Condition

S1

S2

S3

S4

Bypass Switch

One Switch Open on Fault S1 open on Fault Open S2 open on Fault ON S3 open on Fault OFF S4 open on Fault ON

ON Open ON OFF

OFF` ON Open ON

ON OFF ON Open

OFF OFF OFF OFF

Two Switches Open on Fault S1, S2 open on Fault Open S3,S4 open on Fault OFF S1, S4 open on Fault Open S2, S3 open on Fault OFF S2, S4 open on Fault ON S1, S3 open on Fault Open

Open OFF OFF Open Open ON

OFF Open OFF Open` ON Open

OFF Open Open OFF Open ON

ON ON ON ON OFF OFF

One Switch short on Fault S1 shorted on Fault Shorted S2 shorted on Fault OFF S3 shorted on Fault ON S4 shorted on Fault OFF

OFF Shorted OFF ON

ON` OFF Shorted OFF

OFF ON OFF Shorted

OFF OFF OFF OFF

Shorted

OFF

OFF

ON

OFF OFF

Shorted OFF

Shorted Shorted

ON ON

Shorted

Shorted`

OFF

ON

Shorted

OFF

Shorted

OFF

OFF

Shorted

OFF

OFF

Two Switches Short on Fault S1, S2 Shorted on Shorted Fault S3,S4 Shorted on Fault OFF S1, S4 Shorted on Shorted Fault S2, S3 Shorted on OFF Fault S2, S4 Shorted on OFF Fault S1, S3 Shorted on Shorted Fault

The CHB configuration is ideally suited for PV Inverter and STATCOM applications. CHB requires independent DC links and it can be achieved in these two applications without any additional cost. The control of power flow through an inverter is based on below principle. When two AC sources with same frequency are connected together, then. a) Source which is having a leading phase angle provides active power to the other source. b) Source which is having higher voltage amplitude provides Reactive Power to the other source. In Grid connected applications, one AC source is Grid and the other AC source is the output of the inverter. Phase angle difference between the sources is controlled to regulate the real power through inverter,, similarly to control reactive power through inverter, the magnitude of inverter output is controlled depending on the reference powers. In Ref. [44], Controller for Grid connected PV system applications is realized through the Real-time Digital simulator and interfaced with a single-phase 11-level, Cascade multilevel inverter. In Ref. [45], Authors demonstrated the THD analysis up to 23rd harmonics for 7Level to 15-Level operations of the PV fed CHB-MLI. A comparative study of Symmetrical and asymmetrical CHB multi-level Inverters is presented through simulation results. A fifteen level CHB based PV power conditioning system is presented in Ref. [46]. The presented system eliminates output filter and transformer. Advantages of this system are more efficiency, more reliability and the lesser cost but the system has the disadvantage of non-uniform duty cycles among Hbridges due to the use of step modulation technique. In Ref. [47], Authors presented a hybrid twenty-seven level MLI based on the asymmetric CHB configuration, its suitability for photovoltaic systems. Due to the availability of independent DC sources in PV plants, CHB MLI is more suitable for large-scale systems. Output side filter size and cost are less in the case of CHB MLI. Independent MPPT of each PV array can be achieved through this inverter configuration. Fig. 9 shows a General Structure of a CHB MLI based solar power

CHB MLI after bypassing H-Bridge-1 Module is shown in Fig. 8(b). When one H-Bridge is faulty, then the output voltage levels are reduced to five but the system continues to operate hence the availability of the system is improved. 145

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0

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0.05

0.06

0.07

-2000 0

0.08

0.01

0.02

(a)

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0.04

0.05

(b)

Fig. 8. (a) Voltage Waveforms of 7-Level Cascaded inverter when all three H-Bridges are Healthy; (b) Voltage Waveforms of 7-Level cascaded inverter after bypassing Faulty H-Bridge.

Fig. 9. Grid Connected CHB based Solar Inverter.

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(d) Modular in Construction.

conditioning system. This system comprises of multiple H-Bridges connected to independent PV Arrays through independent DC Breakers based on the Power requirement. Whenever the solar power is available the DC Breaker is closed and the active power available at the solar array is pumped to the Grid. A thorough review of active power control techniques for CHB based PV Inverters is presented in Section-4. A CHB MLI based STATCOM with phase-shifted modulation techniques is presented in Ref. [48]. In this work, CHB MLI based STATCOM is compared with other MLI configurations. Sizing of the DC capacitor is also explained. In Ref. [49], authors presented the development of 50-Mvar, 21-level, 154 kV CHB inverter based STATCOM. To reduce the dc ripple voltage, authors proposed a selective swapping technique in this paper. In Ref. [50], authors explained the design procedure for a 12-Mvar, 11-level, 12 kV CHB inverter based STATCOM. SHE-PWM is adapted to obtain sinusoidal voltages. CHB MLI based synchronous series compensator is presented in Ref. [51]. In this work, a simulation study is carried out for a 154 kV system using Six H-bridge modules per phase and a scaled down prototype was built to validate system performance. General Structure of CHB based STATCOM is identical to the CHB based solar power conditioning system shown in Fig. 9. PV arrays, DC Switches are not required for STATCOM operation. As mentioned earlier, When two AC sources with same frequency are connected together, then the source which is having higher voltage amplitude provides Reactive Power to the other source. From the above, to regulate the reactive power through inverter, the magnitude of inverter output is controlled depending on the reference reactive power. Compensation of lagging and leading VAR can be achieved and harmonics can be eliminated by using STATCOM. A review of reactive power control techniques for CHB based STATCOM is presented in Section-5.

4.2. MPPT algorithms The relation between PV voltage and the current for particular values of irradiance and temperature can be obtained from I-V Characteristic curves shown in Fig. 11(a). Open circuit voltage Voc is the maximum cell voltage and the short circuit current (Isc) is the maximum cell current. Isc of a PV cell depends on the irradiance, cell temperature. The power-voltage curve shown in Fig. 11(b) is obtained by multiplying voltage and currents at all points on the I-V curves. PV cell gives maximum power when it operates at MPP. MPP varies with the irradiance. It is always desirable to operate the system near to MPP for achieving maximum output. Since the MPP of PV module always changes with irradiance and temperature, MPP tracking needs to be carried out to make a system to operate at MPP. In Ref. [52], Authors presented a review on various MPPT techniques based on (a) Tracking techniques with constant parameters (b) Tracking techniques with measurement and comparison (c) Tracking techniques with trial and error (d) Tracking techniques with the mathematical calculation (e) Tracking techniques with intelligent prediction. A review of MPPT techniques is presented in Ref. [53]. The comparison study of different MPP techniques is carried out based on tracking speed, the complexity of the algorithm, dynamic tracking under partial shading. From this review, it is observed that P & O and IC methods are appropriate for large systems. 4.2.1. MPPT based on perturbation and observation method Perturbation and observation (P & O) is the widely implemented technique as it is very simple in implementing. PV voltage and currents are the only parameters to be measured for adapting this technique. In this technique, in every cycle, instantaneous DC power is calculated and the difference in DC power is obtained. Based on the difference in DC power, controller increases or decreases the PV voltage as shown in Fig. 12(a). P & O method suffers from following drawbacks: (a) System does not reach MPP, it oscillates around MPP (b) It is not efficient when the climatic conditions change dynamically [54].

4. Control of CHB MLI in solar PV applications 4.1. Need of CHB inverters in PV applications In PV power plants, a series-parallel combination of multiple solar modules provides the desired DC voltage to the inverter. Different configurations of solar inverters are shown in Fig. 10. A simple and widely used configuration is a Single stage Inverter which is shown in Fig. 10(a). Two-stage inverter configuration is shown in Fig. 10(b). With above-mentioned configurations, MPPT can be done on the series-parallel combination of all the arrays together, which is not so efficient. An independent MPPT strategy is required for maximizing energy harvesting from PV arrays and this can be achieved by using Cascaded Inverters. Cascaded inverters are categorized as DC Side Cascaded Inverters and AC Side Cascaded Inverters. Figs. 10(c) and (d) shows the block diagram of DC side and AC side cascaded inverters respectively. AC Side cascaded Inverter has the following advantages over DC Side Cascaded Inverters (a) Low Output THD (b) Single Stage Conversion from DC to AC (c) Even though the system is rated for higher voltage, module level voltage is very low which is safe to operate PV ARRAY

PV ARRAY

4.2.2. MPPT based on Incremental Conductance (IC) Method MPPT based on IC method eliminates the drawbacks of P & O method. PV voltage and currents are to be measured for implementing this technique. In this technique, in every cycle, instantaneous conductance (I/V) and incremental conductance (ΔI/ΔV) is calculated and compared as shown in Fig. 12(b). MPP can be reached through this method which is not possible through P & O method [55]. The performance of both the MPPT techniques is studied in Ref. [56]. From the results presented in the paper, it is observed that performance of incremental conductance method is slightly superior than perturb and observe method. Both the techniques are not efficient at low Insolation levels. To attain better performance adaptive MPPT methods should be used.

PV ARRAY

PV ARRAY

PV ARRAY

DC TO DC

DC TO DC

DC TO AC

PV ARRAY

DC TO AC

DC TO DC

DC TO AC

DC TO AC

(a)

(b)

DC TO AC

(c)

(d)

Fig. 10. Block diagram of (a) Single Stage PV-Inverter; (b) Two Stage PV-Inverter (c) DC side cascaded inverter (d) AC side cascaded inverter.

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16 1000 W/Sq.m

14

1000 W/Sq.m

1

MPP Line 12

0.8

10

POWER

CURRENT

800 W/Sq.m

800 W/Sq.m

600 W/Sq.m 0.6 400 W/Sq.m

600 W/Sq.m 8 6

400 W/Sq.m

0.4 4 200 W/Sq.m

200 W/Sq.m

0.2 2 0 0

Zero W/Sq.m 5

Zero W/Sq.m 10

15

20

25

0 0

5

10

VOLTAGE

15

20

VOLTAGE

(a)

(b)

Fig. 11. (a) Current-voltage characteristics of a PV Array; (b) Power-voltage characteristics of PV Array.

power or reference current is generated to generate gate pulses to Inverter. An independent MPPT control using an incremental conductance method is presented in Ref. [58]. A generalized non-active power theory in which non-active reference is generated for the controlling the power through the inverter is also presented in this paper. MPPT using a modified Perturb and Observation technique for CHB converter based photovoltaic inverter is presented in Ref. [59]. The performance of the system during startup and during dynamic environmental conditions is demonstrated through simulation results. In Ref. [60], independent MPPT using incremental conductance method for PV systems is presented. Unbalanced currents due to unbalanced supplied power caused by PV mismatches is studied and proposed a modulation compensation technique to resolve the issues. Explained the performance of the system through experimental results. In Solar Inverter generally, a Current control will be implemented for feeding power to the Grid. Based on MPPT Point the current reference will be calculated and power transfer takes place accordingly. Authors presented a Model Predictive Control (MPC) for CHB inverters

4.3. Control techniques for CHB inverter in PV applications The main challenges involved in the solar Inverter controls are independent MPPT and Active Power Control Techniques for achieving better efficiency. In this subsection review on Independent MPPT techniques and Active power control techniques for CHB MLI based Solar Inverters is presented. As explained earlier, solar array is the series-parallel combination of multiple modules. The Irradiance on all the PV modules will not be equal as the PV modules are erected in a large area and due to partial clouding. To attain the better efficiency, individual MPPT of each PV module/array is required. An independent MPPT control in a CHB MLI is explained in Ref. [57]. An improvement is observed in system efficiency when several PV arrays of small rating are used instead of one PV array with a full power rating. Block diagram for an independent MPPT control of Single phase 5- Level CHB is shown in Fig. 13. Voltages and currents of each PV array are to be monitored for tracking the individual MPP. Since the DC links are separate in CHB inverter, independent voltage control is possible. Based on MPP, reference START

START

MEASURE PV ARRAY VOLTAGE V(K) & CURRENT I(K)

MEASURE PV ARRAY VOLTAGE V(K) & CURRENT I(K)

CALCULATE POWER P = V(K) X I(K)

NO

NO

INCREASE PWM

V(K) > V(K-1) ?

P(K) > P(K-1) ?

YES

YES

YES

NO

DECREASE PWM

NO

DECREASE PWM

V(K) > V(K-1) ?

ΔV == 0 ?

YES

ΔI/ΔV == -I/V ?

ΔI == 0 ?

NO

YES

NO

YES YES

ΔI/ΔV > -I/V ?

INCREASE OPERATING VOLTAGE

INCREASE PWM

V(K-1) == V(K) P(K-1) == P(K)

NO

YES

DECREASE OPERATING VOLTAGE

INCREASE OPERATING VOLTAGE

V(K-1) == V(K) && I(K-1) == I(K)

(a)

(b)

Fig. 12. (a) MPPT Using Perturbation and Observation Method; (b) MPPT Using Incremental Conductance Method.

148

ΔI > 0 ?

NO DECREASE OPERATING VOLTAGE

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Vpv & Ipv

V1_Ref MPPT-1

DC Link Voltage Controller-1

P1_Ref

DC Link Voltage Controller-2

P2 Ref

PWM CONTROLLER-1

SPWM-1

GATE PULSES TO H- BRIDGE1

SPWM-2

GATE PULSES TO H- BRIDGE2

V1_PV

Vg & Ig

Vpv & Ipv

V2_Ref MPPT-2

PWM CONTROLLER-2

V2_PV

Fig. 13. Independent MPPT control in a CHB based Solar Inverter.

AI1 AI1

Es1_Ref

ENERGY BALANCE CONTROLLER

AI2

Vc1

Ig_ref

+

X

Gate Pulses to Inverter

AIn Sinwt

H-Bridge-1

Ig

L Vc1

SOLAR CELL-1

AI2

Es2_Ref

ENERGY BALANCE CONTROLLER Vc2

H-Bridge-2

Vg

Vc2

SOLAR CELL-2

AIn

Esn_Ref

ENERGY BALANCE CONTROLLER Vcn

H-Bridge-n

Vcn SOLAR CELL-N

Fig. 14. Energy-Balance control for a Single phase CHB MLI [62].

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Vdc_ref

+

Kp Ki

+

+

Sin()

++

Comparator Gate signals

π

1/S Carrier Signals

Vdc Frequency

Q_ref

H-Bridge Inverters

Calculation of Modulation Index

Vsys

GRID

Fig. 15. STATCOM control system block for reactive power control.

AC BREAKER

TRANSFORMER

DC BREAKER

PV CELL

DC BREAKER

PV CELL

DC BREAKER

PV CELL

CHB INVERTER

Gate

PWM

Voltage Current of PV Array-1

Voltage Current of PV Array-N

Independent Voltage reference generator for Each H-Bridge

MPPT-1

MPPT-N

CONTROLLER Vabc_Grid

Iabc_Grid

MPP

Reactive power Ref PLL Vabc

wt

Iq Reference Calculator

ABC To DQ

Id Reference Calculator Id_Ref

Iq_Ref Iq_Grid

-

+

PI

Vq*

DQ To ABC

PI Vd*

+ -

Id_Grid Fig. 16. Block Diagram for Real and Reactive Power control through a CHB MLI based PV-STATCOM.

generated, received and stored energies of PV modules, grid and the capacitors respectively. Samples of stored energy of each capacitor are collected and controlled and through this, the DC link voltage is regulated. Overall MPP of the system is the algebraic sum of independent MPP s of each PV array. Based on overall MPP, the current reference will be

for renewable energy applications in Ref. [61]. In Model Predictive Control, through the state equations of the system, the controlled variables behavior is predicted. Single-phase CHB inverter with cascaded three-level NPC bridges is explained in this work. An energy balance control for a CHB MLI shown in Fig. 14 is presented in Refs. [62] and [63]. This control is based on balancing the 150

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Time

Time

Time

Time

(a)

(b)

(c)

(d)

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Fig. 17. Phase Voltage and Phase Current wave forms of Load, Grid and Inverter (a) When Pref =0 & Qref =0 (b) When Pref = Pload & Q ref =0 (c) When Pref = Pload & Qref= Qload (d) When Pref=0 & Qref=Qload.

A control with the help of zero sequence voltage and negative sequence current for a CHB based STATCOM proposed in Ref. [65]. Digital simulations are carried out and faults on one line and faults on two lines are simulated to validate the control scheme proposed. This control can be adapted for CHB Inverter for PV applications also. To regulate the reactive power, Authors proposed an active power Balance control for a CHB inverter In Ref. [66]. In this control, for balanced dc link voltages irrespective to the balanced or unbalanced grid voltages, the positive- and negative-sequence components are monitored. MPC technique for a Cascaded inverter based STATCOM which optimizes the tradeoff between voltage balancing and switching losses is presented in Refs. [67] and [68]. A Power balancing control for CHB Inverter based STATCOM is presented in Ref. [69]. Authors presented the basics of CHB MLI based. The performance of power balancing control is explained through experimental results.

calculated. MPP of each PV array is proportional to the voltage output of corresponding H-Bridge since the current flowing through all the HBridge modules is same. When the Irradiance on all the PV Arrays is equal, then MPP of Each PV Array is equal hence the voltage output RMS from all H-Bridges is also equal. But generally, the Irradiance on all the PV Arrays will not be equal due to mounting arrangement, partial clouding etc… In that case, the H-Bridges produce different AC voltages depend on their irradiances. However, the total CHB voltage is always maintained constant to match with the grid voltage. 5. Feasibility of CHB inverter for PV-STATCOM applications As stated in section-1, there is a need of multilevel PV STATCOM suitable for high power ratings and for better utilization factor. It is understood that by adding on to the technology of the Two-Level PV STATCOM, it is possible to design a Multi-level PV-STATCOM in large scale systems. Along with the independent MPPT and active power controls, PV-STATCOM should have the feature of regulating the power factor. Fig. 15 shows a typical Control block Diagram for Reactive power control through an inverter. The Feedbacks required for the controller is DC Link Voltage Vdc, Grid voltage, and Grid currents. voltage control loop regulates the DC Link voltage and Inner loop regulates the Reactive power. Since the DC links are isolated in a CHB inverter, the number of voltage sensors required is large hence it is expensive. Sometimes it is impractical to mount voltage sensor because of high operating voltages. A nonlinear adaptive observer which can work even with system parameter uncertainties for a CHB MLI is presented in Ref. [64]. This control can be adapted for eliminating the sensors at DC links.

5.1. Simulation of grid connected CHB MLI From the previous discussions it is observed that in addition to the active power control, a CHB inverter can also be used for reactive power control, for PF correction, for active filter applications and for eliminating harmonics. Hence a CHB MLI based PV-Inverter can be operated as PVSTATCOM by incorporating Reactive power control feature in addition to active power control features studied in section-4. By doing minor modifications to the existing algorithms, it is feasible to design and develop a CHB MLI based PV-STATCOM which can mitigate all the drawbacks of conventional solar inverters. Fig. 16 shows a generalized Control block diagram for a grid connected CHB MLI based PV-STATCOM. PV Array 151

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CHB INVERTER H-BRIDGE-1 DC SOURCE-1

Inverter Start/Stop Command (Digital Input) Reference Active Power Reference Re-active Power INV Voltage

H-BRIDGE-2

INV Current Grid Voltage

Analog Input

USER COMMANDS

DIGITAL CONTROLLER

DC SOURCE-2

Grid Switch

FILTER

H-BRIDGE-3 DC SOURCE-3

SINGLE-PHASE TRANSFORMER PCC

GRID H-BRIDGE-4 DC SOURCE-4

SINGLE PHASE R-L LOAD

H-BRIDGE-5

Grid Switch ON Command GATE PULSES

Digital Output

DC SOURCE-5

Fig. 18. Block Diagram of a Grid Connected Single Phase 11-Level Cascaded H-Bridge Inverter.

electrical and control scheme for the CHB MLI. Power circuit comprises of five H-Bridges fed from isolated DC Sources, an L-C Filter and a Grid Switch, a fixed R-L load connected to a Single phase AC Grid. Texas Instruments make TMS320F2812 processor based digital controller is used in this system. Controller receives the analog signals such as grid voltage signal for PLL, inverter voltage signal for synchronization purpose and inverter current signal for closed loop current control of CHB inverter. User commands such as Start/Stop signal is given to the controller card using push button, reference active and reactive power signals are given from variable DC source supplies. AC terminals of CHB MLI are tied to single-phase grid. Each DC source is rated for 50 V and the single phase grid is 230 V RMS stepped down to 150 V RMS using an isolated transformer. A fixed R-L local load is connected at the AC terminals of inverter. Fig. 19(a) shows standalone CHB inverter output when the modulation index is adjusted at 0.9. A phase shifted sinusoidal PWM is used and an L-C-L filter is used to get sinusoidal output. >Power references are adjusted through variable DC power supplies. Reference inverter current is generated by the controller based on the power reference adjusted. PI controllers are used as current controllers in this system. To check the dynamic response, a sudden change in active power reference is given to the controller. Fig. 19(b) shows the response of the actual current with a sudden change in the reference current. From the results presented, it is observed that the system response to the sudden changes in the reference powers is satisfactory. Fig. 19(c) shows the system behavior when there is a step change in reference power from unity power factor load to Zero power factor load. Reference active power equal to the local load requirement is given to the system and reactive power reference is adjusted to Zero,

voltages and currents are monitored for independent MPP Tracking. Based on MPP of each array the MPP of Three Phases is obtained. Grid voltage is monitored for determining the angle wt through PLL. From the MPP, Id_ref is calculated and Iq_ref is generated to regulate the Reactive power. Active and reactive Power controls are carried out through PI Controllers by comparing the actual and reference currents. To check the power controls, a grid connected CHB inverter is simulated. A fixed R-L load is connected at the AC terminal of inverter. Pref and Qref for inverters are varied and system performance under the following cases are studied. Case 1. : Pref = Zero and Qref = Zero. In this case, Inverter current is Zero and load is drawn from Grid as shown in Fig. 17(a). Case 2. -: Pref =1 P.U. and Qref = Zero. Hence Inverter supplies only real power and Grid supplies only reactive power. So Inverter power factor is unity where as the grid power factor is ZPF as shown in Fig. 17(b). Case 3. -: Pref =1 P.U. and Qref =1 P.U. In this case, Grid current is Zero and load is drawn from inverter as shown in Fig. 17(c). Case 4. -: Pref = Zero and Qref =1 P.U. Hence Inverter supplies only reactive power and Grid supplies only active power. So Grid power factor is unity where as the inverter power factor is ZPF as shown in Fig. 17(d). 5.2. Hardware Implementation of a Single Phase, Grid connected, 11Level CHB MLI An experimental setup is built to implement the decoupled power control through a single phase 11-Level, CHB MLI. Fig. 18 shows the 152

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V. Sridhar, S. Umashankar

OUTPUT VOLTAGE OF SINGLE-PHASE 11-LEVEL CHB INVERTER

REFERENCE CURRENT

INVERTER CURRENT

INVERTER OUTPUT VOLTAGE WITH FILTER

(b)

(a) LOAD CURRENT

VOL

TAG

E

C UR

R EN

T GRID CURRENT

INVERTER CURRENT

(c)

(d)

INVERTER CURRENT

INVERTER CURRENT

GRID CURRENT

GRID CURRENT

(e)

(f)

Fig. 19. (a) Inverter output voltage before and after the filter; (b) Inverter reference current and the actual current; (c) Inverter voltage and current during change over from UPF load to Zero PF lagging Load; (d) Load, Grid and Inverter currents during changeover of inverter reference current from Zero to local Load current requirement; (e) Grid and Inverter currents during changeover of inverter reference current from Zero to local Load current requirement (f) Grid and Inverter Currents during changeover of inverter reference current from Zero to double the local Load current requirement.

Load current, Grid current and inverter currents are monitored for different reference power settings. When the reference inverter power is adjusted to Zero, then it is observed that the Inverter current is zero and the grid supplies the entire current to meet the local load requirement. When the power reference is made equal to the local load requirement, then the inverter supplies all the current to meet the load requirement and the current form the Grid becomes is Zero and

then the inverter power factor is unity before the change over as shown in Fig. 19(c). By changing the reference active and reactive powers i.e. reactive power reference equal to the local load requirement and the Zero active power reference, the power factor becomes Zero i.e. current is lagging the voltage by 90 degree. From this result, it is observed that the power factor can be controlled from Zero to unity through this system. 153

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Input side isolation transformer can be eliminated. Control complexity may increase due to DC-DC converters. Independent DC-DC converter along with high-frequency isolation transformers.

Number of Voltage and Current Sensors

System with PV-Array Negative terminal Earthing

4

5

This configuration is not suitable for systems which require negative grounding for PV arrays.

Adaption of DC voltage detection methods

Redundancy and Fault tolerant operation MPP Techniques 2

Performance of P & O method and Incremental conductance method are deteriorating at low insolation levels [52]. Since DC links are isolated to each other, independent voltage and current sensors are required. Number of sensors increases with rating of the system, hence cost of the system increases.

PWM Techniques for CHB Inverter 1

Conventional solar PV inverters suffer from the following drawbacks (a) Not suitable for high power applications (b) High filter size (c) inefficient in harvesting maximum power (d) Less utilization factor.

Design Considerations

6. Conclusions

SL.NO

Table 4 Design considerations of a CHB based PV-STATCOM.

A shunt hybrid active power filter based on a CHB MLI for high power photovoltaic applications is presented in Ref. [70]. By adapting the controls, PV-STATCOM system can be extended further for the active filter applications. A review of converter configurations for Fuel Cell (FC) applications is carried out in Ref. [71]. Since a fuel cell is also a DC source, the grid connected CHB system can be extended to microgrid applications and for Hybrid PV–FC systems. An Energy Storage System with CHB inverter is discussed in Ref. [72]. By reviewing bidirectional power flow controls and battery charging techniques, the system can also be extended for grid energy storage in PV systems.

Challenge

5.5. Future scope

3

(a) Number of voltage and current sensors required is more in the case of CHB inverters which may result in additional cost. Some accurate DC voltage detection methods need to be adapted to reduce the cost of the system. (b) Communication between H-Bridge modules, which is necessary for synchronizing the reference signal and carrier signals, is complex in larger systems. (c) In PV applications, possibility of unbalance in DC voltages is more. This needs to be mitigated by balancing the voltage through adaptive PWM techniques. (d) When the system operates at lower modulation index, Number of levels is less hence the system needs to be designed to operate at optimum modulation index. (e) Since the CHB inverter operates only when all the DC links are isolated from each other, this cannot be used in systems where negative grounding for PV arrays is mandatory. In such cases, a DC-DC converter along with high-frequency isolation transformers may be required. Size, cost, and complexity may increase due to additional components.

Even though the number of H-Bridges is more, the Number of levels at the output is less when the system operates at lower modulation index Failure in One H-Bridge may halt the entire system

Possible Solution

The following are the challenges involved in the design.

Control architecture needs to have One Master controller and Independent Slave controllers System needs to be designed to operate at optimum Modulation Index Using Redundant H-bridge in each Phase and by using Bypass switch in each module Adaptive MPP techniques

5.4. Challenges involved in the design of PV-STATCOM based on CHB MLI

Phase-Shifted PWM technique is ideally suitable for this application. Modifications need to be carried out in PWM techniques to achieve DC link voltage balancing.

Conclusion

From the presented results, it is observed that the active and reactive power through a CHB inverter can be controlled independently which enables the operation of PV-STATCOM through a CHB Inverter. Hence Reactive power can be regulated in addition to independent MPP tracking through a CHB based PV-STATCOM. Due to the modular construction of the system, the power and voltage ratings of PVSTATCOM can be upgraded to any level by incorporating additional HBridges in cascade.

Rotation of Switching Sequence Modified PWM techniques to be adapted

5.3. Feasibility of CHB MLI as PV STATCOM

Uneven utilization of H-Bridges in case of Level Shifted PWM. Phase shifted PWM is suitable but Unbalances in DC link voltages may cause voltage stresses in components Communication between H-Bridges is necessary for synchronizing the reference signal and carrier signals

the sum of grid and inverter currents is always equal to the load current as shown in Fig. 19(d). The changeover of current from Grid to the inverter is shown in Figs. 19(d) and (e) when there is a step change in the reference inverter power from Zero to the local load requirement. When the power reference is made equal to double the local load requirement, then the inverter supplies current equal to double the local load requirement which is shared amongst the load and the grid. In this case, the direction of grid current changes after the changing over of reference inverter power as shown in Fig. 19(f), hence the additional current supplied by the inverter is fed to the grid which will be consumed by the load connected to the grid.

By bypassing the H-Bridge module through internal switches or through bypass switch, fault tolerant operation can be achieved. Since P & O oscillates around MPP, a modified Incremental conductance method needs to be developed for better performance. Accuracy of the control depends on the performance of voltage detection algorithms.

Renewable and Sustainable Energy Reviews 78 (2017) 138–156

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Renewable and Sustainable Energy Reviews 78 (2017) 138–156

V. Sridhar, S. Umashankar

In this work, PV-STATCOM using CHB MLI configuration is presented to minimize the drawbacks of conventional system. The concept of PVSTATCOM and its controls is reviewed and the working of the system is explained with the help of simulation studies. It is observed that by adding on to the technology of a Two-Level PV STATCOM, it is possible to design a Multi-level PV-STATCOM for high power applications. A review on multilevel inverters suitable for grid connected applications is carried out and it is concluded that CHB configuration is best suited for the proposed application. Basic operation and PWM techniques are explained briefly. Fault redundant and tolerant operations of CHB inverters are analyzed and switching strategy to bypass the faulty Hbridge is presented. Studies on CHB inverter for PV applications is reviewed and observed that the independent MPPT control is possible through the CHB inverter as the inverter operates with isolated DC links. Hence the extraction of power from the PV arrays can be improved by using CHB inverter. Due to modular construction, the design of the system for higher power ratings is simpler as the power rating can be increased by cascading additional H-Bridges to the existing system. For better utilization factor of the system, the feasibility study on CHB inverter for PV-STATCOM applications is carried out. As part of the feasibility study, a brief review of power flow controls of CHB inverter for Grid applications is carried out. After a thorough review, it is concluded that, by adding on to the technology of Two-Level PV STATCOM, it is feasible to design a CHB inverter based PV-STATCOM for large scale systems. By Using CHB MLI for PV-STATCOM, The Filter Size can be reduced, Power extraction from PV Cells can be maximized and utilization factor can be improved through the feature of reactive power compensation. Based on the discussions and the feasibility study presented in this paper, the system can be extended further for Grid Energy storage in PV applications and for micro-grid applications. The salient points in the present work and CHB based PV-STATCOM design considerations are summarized in Table 4.

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