In any PV based system, the inverter is a critical component responsible for the control of electricity flow between the dc source, and loads or grid. This paper presents a solar PV generation system integrated to the grid. The results of matlab modeling of the system detail the comparative operation of inverter topologies which are the conventional two level inverters and multilevel inverter topology to reduce total harmonic distortions in grid voltage and electromagnetic interference. The proposed control scheme to mitigate the power quality issues for power quality improvement in grid integrated DER simulated using MATLAB/ SIMULINK in power system block set.
Keywords |
| Solar Photovoltaic (SPV), Boost converter, Neutral point clamped (NPC), Multi level inverter (MLIs), Total
Harmonic Distortions (THD), Voltage sources inverter (VSI). |
INTRODUCTION |
| In recent years, there has been an increasing interest in electrical power generation from renewable-energy sources, such as
photovoltaic (PV) or wind-power systems [1], [2]. The benefits of power generation from these sources are widely accepted.
They are essentially inexhaustible and environmentally friendly. Among the different renewable-energy sources possible to
obtain electricity, solar energy has been one of the most active research areas in the past decades, both for grid-connected and
stand-alone applications [3]–[9]. The exponential rate of growth in the worldwide cumulative PV capacity is mainly due to
enhancement in grid-connected inverter topologies. The PV array and the battery are connected to the AC grid via a common
DC/AC inverter. AC output voltage is created by switching the full bridge in an appropriate sequence [10]-[11]. The inverter
topologies can be divided with two types, that are single and multi stage inverter. The single stage inverter has advantage such
as low cost, high efficiency, robust performance, high reliability and simple structure. On the other hand, the multi stage
inverter accept a wide range of input voltage variations, high cost, low efficiency, complicated structured and isolated
topologies with high frequency transformers can extract power from the source even when DC voltage is very low [12]. [13]
present the comparison of expense of power semiconductors and passive components of a (2.3 kV, 2.4 MVA) two-level,
three-level NPC, three-level flying capacitor, four-level flying-capacitor, and five-level series connected H-bridge voltage
source converter on the basis of the state-of-the-art insulated gate bipolar transistors for industrial medium-voltage drives.
And illustrate that in three level inverter topology net total harmonics distortion is reduced in the output wave form without
decreasing in inverter output power [14-18]. |
| Photovoltaic systems require interfacing power converters between the PV arrays and the grid. These power converters are
used for two major tasks. First, is to inject a sinusoidal current in to the grid. And second is to reduce the harmonics content in
the grid injected voltage and current. Normally there are two power converters [19]. The first one is a DC/DC power converter
that is used to operate the PV arrays at the maximum power point. The other one is a DC/AC power converter to interconnect
the photovoltaic system to the grid. As shown in fig.1 |
| The classical single or three-phase two level VSIs is normally used for this power converter type [13]. However, other
topologies have been proposed is the multi level VSI. Multilevel converter topologies are a very interesting choice for
realizing this objective. |
| Multilevel power converters present several advantages over a conventional two level converter such as: |
| · Reducing switching frequency |
| · Output voltage with very low distortion and |
| · Reduced dv/dt stress [14-18]. |
 |
| In order to obtain a galvanic connection between the grid and the PV generator many PV systems use a power transformer,
avoiding that a leakage current may flow through the capacitance between PV generator and ground. Some systems use a
transformer embedded in a high-frequency DC/DC converter. Others use a line frequency transformer at the output of the
inverter as presented in the proposed model. |
| Related to these developments, this paper presents a new power converter structure for SPV systems. The proposed structure
uses twin configured three-phase three levels NPC VSIs and a line transformer. This structure is specially developed to use
with the central -string technology. A control system for the MLI is also proposed. This control system is based on a voltage
oriented controller with a pulse width modulator (PWM). Several experimental results are presented in order to confirm the
characteristics of the proposed system against the conventional two level inverters topology. |
CONTROL STRATEGIES |
| Among multiple functions of grid connected systems, the current control plays one of the most important roles. The
performance of the complete system largely depends on the quality of the applied current control strategy. It has to fulfill
basic requirements, such as low harmonic distortion of the output current, high dynamic response, regulation of the dc-link
voltage and, in a number of cases, provide bi-directional power flow. The desire to propose a current control strategy which
combines most of these requirements has encouraged many researches in the last two decades [12] [14]. |
A. Controller of Proposed dc/dc converter |
| The DC to DC boost converters is shown in fig.2 is an important component to reach the desire voltage level in grid connected
system so as to reduce the complexity and rating of the required PV modules. The voltage source provides the input DC
voltage to the switch control, and to the magnetic field storage element. |
 |
| The control signal or duty cycle is given by open loop switch control that directs the action of the switching element, while the
output rectifier and filter deliver an acceptable DC voltage at the output. The input of 200 volts represented by Vdc sc is given
at the input terminal converter and in the control circuit the reference voltage which is required is given as input signal i.e. 400
volts is represented by symbol Vref as shown in fig.3. These two signals are compared in the PI controller and the controlled
output is given to the signal generator. And required dc output voltage is given by equation (1). |
 (1) |
| VLL = the line to line voltage of the AC-grid |
| M = modulating index |
 |
| B. Controller of Proposed multi level PV inverter |
| In multilevel inverter we will use an fast switching control mechanism i.e. the voltage oriented control approach as shown in
fig.4 we have to use the active power feedback from grid to control the active and reactive power flow in the grid and to
maintain the grid current sinusoidal and within specified limits. In controller the active power from grid is compared with the
reference power and the error signal is then fed to the power controller which extracts the reference current signal in direct
and quadrature axes through synchronous reference frame theory. This again converted into three phase component and this
reference voltage Vref is compared with the direct axis component of the grid voltage Vabc_IB. From which the error signal is
computed and further transform into phased components and the supplied to the PWM generator which will produce the
controlled pulses for controlling the inverter switching state. |
| The control strategy uses a PLL based unit vector template for extraction of reference signal from the distorted output supply.
To get unit vector templates of voltage, the output voltage is sensed and multiplied by gain equal to 1/Vp, where Vp is peak amplitude of fundamental output voltage. These unit vector templates are then passed through a PLL for synchronization of
signals. The unit vector templates for different phases are obtained as follows: |
 (2) |
 (3) |
 (4) |
 |
| C. Controller design for dc/ac two levels inverter |
| The controller plays an important role in controlling and maintaining the proper functioning of the whole system by limiting
the system parameter in specified ranges. Here in fig.5 the controller of grid connected two levels PV inverter is presented. As
shown in the fig.5 we have to use the power feedback from grid to control the active and reactive power flow in the grid and
to maintain the grid current sinusoidal and within specified limits. In controller the grid power is compared with the reference
power and the resultant signal is then fed to the power controller which extracts the reference current signal in dq axes. Which
again converted into three phase component and this reference current is compared with the gird current and the error signal
fed to the hysteresis controller which directly generates the control signals or firing pulse for the inverter to maintain the gird
current within specified limits. |
 |
SIMULATION STUDY OF GRID CONNECTED THREE LEVEL PV INVERTER |
| The role of three level grid connected PV inverter is to improve power quality of grid tied solar powered system by reducing
THDs, increasing the voltage level. |
| A. Proposed grid integrated PV system with three level inverters |
| The base configuration of the proposed grid connected photovoltaic system is presented in Fig.6 This system consists of PV
module as a DC source, DC/DC power converters, a multilevel DC/AC power converter, a power transformer, resistive load
and an AC grid. The input supply is feed through PV module a dc source and this output voltage magnitude is boost to
require level to obtain the line voltage the dc/dc boost converter is employed. The output of DC/DC boost converters is the
DC power supplied to the multilevel DC/AC power converter. |
| In this system the balanced voltage feed to the three phases of grid is given as: |
 (5) |
 (6) |
 (7) |
 (8) |
| Where Vp is the peak amplitude voltage. |
 |
RESULTS AND DISCUSSIONS |
| The three levels inverter generates three-phase voltages and currents which are sinusoidal and balance. The total harmonic
distortion of voltage and current in grid tie three level inverter system in reduced to a large extent than the two level one. And
they provide reactive power compensation at PCC and reduced electromagnetic emission. The results demonstrate the
effectiveness of the proposed MLI based grid connected PV system. |
| Fig.8 (a) shows dc source voltage and current in three levels inverter based grid connected SPV system. Fig.8 (b) shows dc
source voltage and current in two levels inverter based grid connected PV system. And it is clear from both the result that the magnitude of voltage and current generation at the dc boosting stage in two levels that generates 400 volts, 20 amperes is less than the three level one which generates approximate sharp 400 volts, 50 amperes and high distortion it contains. |
 |
| Fig. 9(a) shows the flow of active and reactive power in per unit with three level inverter model. And Fig. 9(b) shows the flow of ‘P’ and ‘Q’ power in p.u with two level inverter models at inverter and grid. The difference is clearly evaluated that the three level inverter based system provides ‘Q’ power compensation to the grid along with high ‘P’ power generation i.e. about |
 |
| Fig.10 (a) & (b) consists three phase voltage and current waveforms of three level & two level inverter and grid. And it is clearly shown from the waveforms of both figures that the three levels levels inverter gives highly sinusoidal waveform of current and voltage with higher magnitude and lower fluctuation level. |
 |
| Fig. 10(a) & (b): Three phase voltage and current of inverter and infinite bus in three levels & two level inverter based gri
connected SPV system. |
| Fig.11 (a) & (b) clarify the difference in THD level of the two proposed system that in three level inverter system it is 0.37% in the inverter output voltage and in two level inverter system the output voltage distortion level is 38.51% which is too high that inversely effects the quality of power in the grid interacted DER’s that is not permissible according to IEEE standards. |
 |
| Fig.12 (a)&(b) the THD content in the grid or infinite bus current in three level inverter based system it is 5.93% while in
two level inverter system it is 197.67% a large difference in the level of distortion at same input provided to the system. |
 |
 |
CONCLUSION |
| The MLIs are very beneficial as the number of switches in inverter increases this increases the level of inverter the harmonics
distortion in AC output voltage and current decreases, it also provide reactive power compensation to the AC grid and
reduction in electromagnetic emissions because they operate on lower switching frequency. |
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