 Open Access
 Total Downloads : 249
 Authors : Neenu Mohan N, Fasil V K
 Paper ID : IJERTV6IS040330
 Volume & Issue : Volume 06, Issue 04 (April 2017)
 DOI : http://dx.doi.org/10.17577/IJERTV6IS040330
 Published (First Online): 13042017
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Advanced Control Strategy for Solar PV and Battery Storage Integration using a ThreeLevel NPC Inverter
Neenu Mohan N M.Tech Scholar, Fasil V K
Asst.Professor, Dept. of EEE, Thejus Engineering College
Erumappetty, Thrissur, Kerala 680584
AbstractThis paper introduces a gridconnected solar photovoltaic (PV) system and battery storage, which is implemented using a three level neutralpointclamped (NPC) inverter. A new simplified space vector PWM method for a three phase three level inverter is to be proposed. The number of switching states is a larger number in case of threelevel inverters as compared with a twolevel inverter. In the proposed scheme, threelevel space vector PWM inverter can directly use. This control algorithm will be used to control power delivery between solar PV, and grid. It also has the capability of MPPT. The proposed control topology can generate the correct AC voltage under unbalanced DC voltage conditions by using a vector modulation technique. It can also control the charging and be discharging of battery storage systems in different levels of solar irradiation. In this project area, a three phase threelevel inverter using space vector modulation strategy has been modeled and simulated. Simulations are done using Matlab Simulink.
Index Terms Battery storage, solar photovoltaic (PV), vector modulation

INTRODUCTION
HE most concerns about the environmentfriendly electricity Tgeneration in the modern world has to replace the conventional energy generation. Decreasing the fossil fuels, the renewable energy sources such as solar and wind energy generations have to lead as a green method [1].In the modern world, there has increased in power generation from renewables. Normally renewableenergy sources like photovoltaic (PV) or wind power systems. They are essentially endless and environmentally friendly. Among these renewableenergy sources, solar energy is the most attractive in standalone applications. The green power generation is nature related and cannot control easily. By applying renewable energy resources into the power system, many changes may occur. Advanced power electronic systems are needed to utilize to overcome these crises [2]. In PV and wind energy generation system utilizing maximum power from the source is one of the most important functions of power electronic system. The rapid development of the advance of the power electronics technology has made several changes in static power converter arrangements and industrial
motor drive areas.
The Mainly Generating system requires two types of a converter. Among these converters usually using dc/dc converter is to facilitate the maximum power point tracking (MPPT) of the PV array and also to use DC/AC inverter to produce the appropriate dc voltage [3]. The solar and wind energy systems are unpredictable and fluctuating nature. To overcome this concern the gridconnected renewable energy system has accompanied by a battery energy storage system. Grid connected system required converters for controlling the battery charging and discharging system and another converter required for DC/AC power conversions. Thus here required many converters and system will have higher cost and lower efficiency.
In this paper design and study of a threephase solar PV system and
Battery storage is integrating into the grid and here taking only one threelevel converter which having MPPT capability and acside current control and also have the ability to control the battery charging and discharging. This structure will be better flexibility, reducing the cost, better efficiency and also the power flow control. There are several inverter topologies like transformerbased or transformerless inverters. These inverter topologies can also have different levels of switching pattern. Among this transformerless inverters, the NPC is applying to photovoltaic (PV) panels and the controlling of these inverters. Several types of pulsewidth modulation (PWM) techniques are available. Neutral Point Clamped Multilevel Inverter is a most popular technology in all applications. The NPC can directly connect to the grid. There is no need for any additional converters. Controlling of NPC can be down based on SVPWM technique.The modulation strategies to employ with PV inverters are SPWM and SVPWM. SVPWM has wide range of switching frequencies, and ease to the implementation in multilevel inverters
The rest of this paper is organized as follows. Section II introduces the concept of a threelevel inverter and its capacitor voltage considerations and Concept of a space vector pulse width modulation method for three level invertersThe section III is explain about the proposed topology to integrate solar and battery storage .In section IV describes Simulation and validation of proposed topology

CONCEPT OF A THREELEVEL INVERTER AND ITS CAPACITOR VOLTAGE CONSIDERATIONS

Concept of an equivalent three level inverter
The neutral point clamped (NPC) threelevel inverter was first introduced by A. Nabae, I. Takahashi, and H. Akagi in 1980 and published in 1981 [4]. They have been widely used in several applications, like STATCOM, HVDC, pulse width modulation (PWM) rectifiers, motor drives and renewable energy applications [5], [6]. NPC was also known as Diode Clamped Multilevel Inverter. It is suitable for medium and highlevel voltage applications. As the No.of levels are increased switching frequency will be reduced. NPC can be connected directly to the grid. Fig. 1(a) Shows circuit topology for a three phase threelevel neutralpointclamped inverter. Mainly the converter has two capacitors in dc side to produce the threelevel acside phase voltages [6].
Fig. 1(a) Power circuit of the threelevel diode clamped inverter
The total number of switching states of an " n Level inverter is N3, So the total number of switching states in a 3 level inverter is 33 .Normally 27 switching states in the 3 level inverter but 24 states are active states and 3 zero states.
Fig. 2 Space vector diagram for the threelevel diode clamped inverter
From these Three switching states [1], [0] and [1] can represent the operation of each leg. Fig. 2 shows the space vector diagram for the threelevel neutral point clamped inverter. Here the plane can be dividing into six major triangular sectors (I to VI) by large voltage vectors and zero voltage vectors. There each major sector represents 600 of a fundamental cycle. Within each major sector, there are four minor triangular sectors [12].Table II shows the option of three phase switching states that are characterized by three inverter phases A, B, and C. Here The voltage has four sets[13],[12].

Zero vector – (V1, V2, V3)
which representing three switching states [1 1 1], [1 1 1] and [0 0 0]. The magnitude of V1, V2, and V3 stays Zero.

Small vector (V4 V15) Magnitude is Vd/3.

Medium vectors(V17, V19, V21, V23, V15, V27)
the magnitude is
3 Vd.
3
Fig. 1(b) Phase A of a 3 level inverter Table 1 Switching pattern of a phase A of 3 level inverter


Concept of a space vector pulse width modulation method for three level inverters
In recent years controlling of three level NPC has been a hot topic and some strategies have been proposed in [8], [9].In these various control strategies space vector modulation (SVM) is one of the most commonly used modulation strategies. These have many advantages of low switching frequency and better output voltage quality [10]. Among this SVM method SVPWM is tobe taken here. However
medium vectors will affect the neutralpoint voltage when SPWM applied to the model. It will result to cause low frequency oscillations in the neutralpoint voltage under high modulation conditions [11].
(d) Large vectors(V16, V18, V20, V22, V24, V26), having the magnitude of 2/3 Vd.
Various steps for applying for three level SVPWM inverter.
They are

No of switching states

No of voltage vectors & equivalent voltages.

Sector identification.

Determining the region in the sector.

Calculating the active vectors switching time periods.

Generation of gating signal
Each major sector can be recognized by using space vector phase angle (). is calculated and then find sectors. In which the command vector V* is found. Also it is determined as,If is between 0 < 600, and V* will be in major sector I. If is between 60 < 1200, and V* will be in major sector II. If is between 120 < 1800, and V* will be in major sector


If is between 180 < 2400, and V* will be in major sector IV. If is between 240 < 3000, and V* will be in major sector V. If is between 300 < 3600, and V* will be in major sector VI.
TABLE II
SWITCHING STATES OF 3 LEVEL INVERTER
Switching states
Sa
Sb
Sc
Voltage Vectors
S1
0
0
0
Vo
S2
1
1
1
Vo
S3
2
2
2
Vo
S4
1
0
0
V1
S5
1
1
0
V2
S6
0
1
0
V2
S7
0
1
1
V3
S8
0
0
1
V4
S9
1
0
1
V5
S10
2
1
1
V6
S11
2
2
1
V7
S12
1
2
1
V8
S13
1
2
2
V9
S14
1
1
2
V10
S15
2
1
2
V11
S16
2
1
0
V13
S17
1
2
0
V14
S18
0
2
1
V15
S19
0
1
2
V16
S20
1
0
2
V17
S21
2
0
1
V18
S22
2
0
0
V19
S23
2
2
0
V20
S24
0
2
0
V21
S25
0
2
2
V22
S26
0
0
2
V23
S27
2
0
2
V24

PROPOSED TOPOLOGY TO INTEGRATE SOLAR AND BATTERY STORAGE
New control configurations of a threelevel inverter are integrated to a battery storage and solar PV. A new control will be applied in there and shown in fig. 3 is proposed, there is no other converter is required to connect the battery storage to the gridconnected PV system. It is the major advantage in the medium and high power applications. This model can reduce the cost And improve the overall efficiency of the whole system. In the Fig.3 new proposed configuration of a solar PV is integrated with battery storage system: In these figures, fig (a) represents basic configuration; fig (b) improved the configurations of the model. In this intended system, the power can shift to the grid from the renewable energy source.
As per the same time control of the system will request to allowing charging and discharging of the battery storage system. The suggested control system will able to control the lower capacitor voltage (VC1), and this control method is used for the charging and discharging of the battery storage. This system has an ability to control the sum of the capacitor voltages (VC1 + VC2 = Vdc) to reach the MPPT condition. These conditions are done at the same time of control operations. The total harmonic distortion (THD) relatively low output of the inverter. The outputs of the inverter have the correct voltage waveform. In this system will reduce the total harmonic distortion (THD) current on the AC side even under disturbed capacitor voltages on the dc side of the inverter. The solar PV does not produce any power continuously so the system cannot work correctly. Add up a battery storage system in the model for continues process. Here a single battery storage system cannot be work properly. The improved configuration is shown in the figure that is two
battery connected across the two capacitors. Where the relays are used to connect this battery. When one of the relays is closed, and the other relay is open, the configuration in fig. 3(b) is similar to that in fig. 3 (a)
The accessibility of renewable energy source can be produce power then the battery can be charge or discharge. However, the solar energy is unobtainable the two relays are closed and also allowing to the DC bus to transfer the active or reactive power to the grid, or dc bus is absorbed power from the grid side. It can be noted that these relays are selected to be ON or OFF as essential; there is no PWM control necessity. This PWM control condition it has a flexibility of managing the two batteries. This PWM control condition it has a flexibility of managing the two batteries. The battery to be charging when power is accessible from the renewable energy sources or the grid side. The battery is has connected across the relay, when the battery is fully charged, then the relay is opened while closing the relay the battery is to be charged. The attention needs to the current through the inductor Lbatt must be zero before the opening of any these relays to avoid disrupting the inductor current and avoid of damaging relay.In fig. 3(b), three different relay configurations can be obtained:
1) When the top relay closed; 2) when the bottom relay is closed, and 3) when both relays closed.
Fig. 3 Proposed Configurations for Solar PV and Battery Storage Integration(a) Basic Configuration; (b) Improved Configuration
C. Control topology
In this control topology, the inverter can be controlled by using the SVPWM method. The process of creating gate signals for the NPC inverter can generate after the SVPWM control method. Here primarily find the active and reactive power generation by the inverter to be transferred to the grid. This can be determined using a network supervisory block. This block will obtain based on the solar PV generation, present battery variables also the grid related data. Here figure 4 shows the useful block diagram of thecontrol scheme of "solar PV and battery storage integration system to grid through three phase NPC inverter. The block MPPT used for getting the maximum power from solar PV. To finding the requested active and reactive power generation by the inverter
here using the MPPT algorithms. Then Network supervisory blocks also study about the MPPT state. Using the MPPT blocks to maximum power from solar PV systems. The MPPT algorithms based on the requested active (p) and reactive power (q), and the grid voltage in the dqaxis,vsd and vsq and the requested inverter current in the dqaxis, id and iq can be obtained using
p* Vsd Id Vsq Iq
q* Vsq Id Vsq Iq

v
id * p * vsd q * vsq
vsd
2 2
sq

v
iq* q *Vsd p * vsq
vsd
2 2
sq
Firstly find the reference value based on the balanced operations of the system. The control technique performs based on this value. After approaching the requested reference voltage vector, a suitable sector in the vector diagram can evaluate. The relative errors of capacitor voltages are using for determining which short vectors. Relative errors of capacitor voltages given as,
Fig. 4. Functional block diagram of control scheme.
Also, this value dependents capacitor value and also transient voltages. Due to the practical considerations like size and
evc1
evc 2
vvc1 * vc1
vc1
vvc 2 * vc 2
vc 2
cost,The value of LBAT is preferred to be low and has been chosen to be five mH based our simulation studies
TABLE III
Where Vc1 and Vc2 are the desired capacitor voltages, and VC1 and VC2 are the actual capacitor voltages for capacitor C1 and C2, respectively. The selection of the short vectors will determine which capacitor is to be charged or discharged. To determine which short vector must be selected, the relative errors of capacitor voltages and their effectiveness on the control system behaviour are important. A decision function F, as given, can be defined based on this idea
F G1evc1 G2 evc2
Where G1 and G2 are the gains associated with each of the relative errors of the capacitor voltages G1 and G2 are used to determine which relative error of the capacitor voltages is more important and consequently allows better control of the chosen capacitor voltage. For example, for an application that requires the balancing of the capacitor voltages as in traditional threelevel inverters, G1 and G2 must have the same value with equal reference voltage values, but in the proposed application where the capacitor voltages can be unbalanced, G1 and G2 are different and their values are completely dependent on their definitions of desired capacitor
PARAMETERS OF THE SIMULATED SYSTEM


SIMULATION AND VALIDATION OF PROPOSED TOPOLOGY
c1
voltages By using V *c2 V *dc V
and V *c1 V
selecting G2
Fig. 4. Block diagram of the simulated system.
BAT
much higher than G1 , the PV can be controlled to the MPPT, and C1 voltage can be controlled to allow charging and discharging of the battery. In each time step, the sign of F is used to determine which short vectors are to be chosen. When F is positive, the short vectors need to be selected that can charge C1 or discharge C2 in that particular similarly, when F is negative, the short vectors need to be selected that can charge C2 or discharge C1 in that particular time step. The role of LBAT is to smooth the battery current, especially in the transient condition. A wide range of values is acceptable for the inductor value, however, decreasing its value will increase the current overshoot of the battery.
RESULTS
Simulations performed using MATLAB/Simulink for the proposed system is shown in Fig.5. SVPWM switching strategy applied to the system configuration and the results. Fig.5 (a) shows phase voltage of three level inverter. Fig 5(b) Shows the synchronization between current injected to grid and grid voltage
Fig 5(a)
Fig 5(b)
Fig.5 simulated inverter waveforms (a) Vab, Vbc, Vcaphase to phase inverter voltage (b) grid side response
Fig. 6(a) and (b)requested active and reactive power, and Fig. 6(c) show that PV voltage is controlled and getting the maximum power from the PV module fig 6(d) shows the battery charging and discharging. The power from PV is more than grid power the battery to be charging, and PV power is less than the grid power the battery is starting to discharging Fig. 9(e) shows the inverter acside current, and Fig. 9(f) shows the response of gridside currents with a THD .
Fig. 10 shows the inverter waveforms in the required system model.Fig. 10(a) shows the linetoline voltage Vab , and Fig. 10(b) shows the phase to midpoint voltage of the inverter Vao. Fig. 10(c) and (e) shows Vao, Von, and Van the average value of the PWM waveforms
Simulated results Fig 6 fig (a) Active power injected to the grid. (b) Reactive power injected to the grid. (c) PV module DC voltage.(d) Battery current. (e) Inverter AC current. (f) Grid current.
Fig 7 Simulated inverter waveforms. (a) VabPhase to phase inverter voltage. Fig (b) VaoInverter phase voltage reference to midpoint. (c) Filtered Von Filtered inverter phase voltage reference to midpoint. (d) Filtered Von Filtered midpoint voltage reference to neutral. (e) Filtered VanFiltered phase voltage reference to neutral.
Fig 8 Simulated results for the second scenario. (a) Active power injected to the grid. (b) PV module DC voltage. (c) Battery currents. (d) Grid side currents.(e) Grid side Phase (a) voltage and its current.

CONCLUSION

In this paper, the performance of three level NPC is integrating with renewable energy resource into AC grid was presented. A new control algorithm of space vector modulation has also been accessible to controlling the power flow between solar PV and battery storage system, and grid system, while MPPT operation for the solar PV achieved instantaneously. Threelevel NPC voltage source inverter can integrate both renewable energy and battery storage system. Threelevel vector modulation technique that can generate the correct AC voltage under unbalanced dc voltage conditions has proposed. The system can control acside current, and battery is charging and discharging currents at different levels of solar irradiation. A detailed implementation and the analysis done concerning the application of the SVPWM control strategy on the threelevel voltage inverter presented using MATLAB/SIMULINK and also created to simulate the switching patterns.
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