A Novel Approach for Grid Integration of Cascaded H-bridge Multilevel Inverter Under Partial Shading Condition

Received Apr 24, 2018 Revised Jul 3, 2018 Accepted Jul 5, 2018 A modular cascaded H-bridge PV inverter based system is presented in this paper. It helps in obtaining the maximum output power of PV system along with increase the overall efficiency of the whole system. Moreover to utilize the system up to the best a distributed MPPT controller is attached with each PV panel. As partial shading causes power imbalance at the converter output that leads to imbalance grid current, a control technique called the modulation compensation is adopted in such a way that if three phase unbalanced voltage varies directly according to unbalanced power, the injected zero sequence voltage components make the grid current balanced. Keyword:


INTRODUCTION
From the last couple of years renewable energy sources are growing extensively to fulfill the energy demand of consumers. Among the different energy sources, solar energy is the most reliable and sustainable one [1]- [3].With the wide development of power electronics technology it becomes easier to integrate various PV inverters with the utility grid. A PV-inverter is the main component for integration in a grid-connected application. It converts the DC power available from solar panel into AC power that is fed into an utility grid. PV panels are connected either in series or parallel or both to obtain the required voltage and power. This arrangement is called as centralized PV inverter. But the requirement of high voltage DC cable between PV panel and inverter limits its usage.
Till now different types of PV inverters are developed based on PV module connected method, utilization of transformer and number of power stages [4]. Among the various inverter toplogies the multilevel inverter configuration is paying the attention of the researchers worldwide for large scale applications. Because of its multilevel structure the output waveform obtained in the grid side must be sinusoidal with less harmonics content [5]. It does not require any bulky low frequency transformer for its integration with the grid. Moreover,because of the requirement of individual DC source for each H-bridge configuration,the voltage control can be done individually. A single phase seven level CHB comprises of three H-bridges each fed by multiple strings of PV panels through their own individual H-bridge is presented in Figure 1. In such arrangement, the individual MPPT , called distributed MPPT technique is applied to track the extreme power of each PV panel [6].

Modular cascaded H-bridge multilevel inverter
A seven level CHB inverter produces the output voltage values (levels) with respect to negative terminal of the capacitors. If n represents the number of step of phase voltage with respect to the negative terminal of the inverter , then the number of stages k between two phases across load is represented as, Figure 1. Configuration of single phase grid-connected PV system [7] k = 2n+1 (1) If the DC-link voltages are equal, at the load side 7-level voltage can be obtained. The output voltage can be represented by the upcoming equations: where represents the switching state or it can be ON, 0 or OFF. P 1 and P 2 represent the discrete values having the values of -1,0,+1. For linearization, continuous switching functions 1 and 2 [-1,1] are taken in place of the functions P 1 and P 2 [8] [9]. Equations (3) represent the dynamic behavior of the system that is represented as: Due to the presence of isolated DC source in case of CHB, independent MPPT control can be possible and utmost power can be obtained from the entire system more efficiently. Nevertheless, since the possibility of independent voltage control through each H -bridge, there is a power imbalance. This problem can be classified into two classes : 1) interphase (clustered) power imbalance, when power generated by each phase is not equal; and 2) inter bridge (individual) power imbalance, which happens when bridges connected to the same phase leg generates an unequal power. If this system cannot be considered seriously, dc-link voltage will float leading to voltage distortion at the grid side. Because of the inadequate irradiance obtained from sun, varying temperature and usage of the PV panels from long time, there is an unbalanced power supply to the CHB , causing unbalanced current at the grid side. Therefore this issue is considered seriously with the designing of suitable controller so that at the grid side balanced current is achieved [10] [11].

CONTROL STRATEGY
PV mismatches is very hazardous problem in case of large PV system. Henceforth, for utmost power extraction from the system a control scheme is essentially needed. Generally two control scheme is proposed in this paper (i) MPPT control (ii) Inverter control.

MPPT control
To show the requirement of MPPT , 185W PV panel is considered that is connected to each Hbridge of level multilevel inverter of a three phase system. The P-V and I-V curves for 185W panel is simulated in MATLAB/SIMULINK environment and are shown in Figure 2.The various plots at different radiations are shown. Each CHB inverter is connected to 185W solar panel and for a single phase system n-CHB inverters are connected in series. Likewise the whole three phase system is integrated with grid through inductor L that is used to minimize the switching current harmonics at the grid side. In order to resolve the PV mismatch issue distributed MPPT control technique is adopted.

Distributed MPPT control
Various MPPT techniques were developed in order to extreme power from a PV panel [12]. In this paper, Incremental conductance MPPT method is adopted to generate the dc-link voltage reference for individual PV panel. Here the MPP point goes on adjusting its position irrespective of the environment changes. In order to track MPP, Incremental Conductance (IC) method compares the ratio of incremental conductance to instantaneous conductance value of PV module. Based on this value, the slope of P-V characteristic is varied and according to the change in slope, duty cycle of converter can be generated [13][14][15][16].
For MPP tracking, applying IC algorithm the following steps are taken into consideration: a. When = 0 , the error is zero and can be found out .
b. When > 0 i.e > − ,the MPP drags towards the left side of the curve (having a positive error).
c. When < 0 i.e < − ,the MPP drags towards the right side of the curve (having a negative error).
After the MPPT tracking the dc link voltage reference is found to be 36.4V as shown in Figure 3.

Inverter control
It comprises of control of both active and reactive power. It has two different control loops : one is called the voltage control loop that is used to set the capacitor voltage across each dc link and other one is called the current control loop that is required for the generation of the sinusoidalgrid current. Figure 4. shows the control scheme for the entire system. The sum of dc link voltage 1 to are compared with respective voltage reference to produce the sum of error that is passed through the total voltage controller loop to generate the reference current . As reactive power compensation is not taken into account in this paper, hence is adjusted to be zero. The phase angle of grid voltage θ is determined by a three phase locked loop (PLL). The grid currents , and are passed through abc to dq converter circuit to generate the , current coordinates. These current coordinates are again feed to a PI controller to generate the modulation index in dq coordinates and with the help of dq to abc controller modulation indices in abc coordinates are generated. For simplicity, take phase a into consideration. Due to individual MPPT control, the generated voltages 2 to are controlled through n-1 loop. Each voltage controller gives the modulation index proportion of one H-bridge module in phase a. After multiplied by the modulation index of phase a, n-1 modulation indices can be generated. The calculation for phase b and c are also approachable in the same way to determine the modulation indices. A phase shifted modulation scheme is used for switching each Hbridge inverter.

Phase shifted modulation scheme
There are different modulations techniques proposed for switching of multilevel inverters [17]- [21]. Among which PS-PWM is most popularly used to produce PWM control signals for switching of the inverters [22] [23] . As partial shading phenomenon causes a serious issue in case of switching of cascaded inverter, hence the modulation index of each cell must be properly updated to consider unbalanced conditions at dc side by producing different modulating signals. In order to achieve a balanced condition at grid side, cell with high power is assigned with high modulation index and with low power is assigned with low modulation index and hence each cell is modulated individually using the PWM, in order to provide an even power sharing among the cells. In PS-PWM all the triangular carriers are have the same frequency and peak-peak amplitude and a phase shift is made among any two immediate carrier waves. For m Voltage levels, there is need of (m-1) carrier signals and they are phase shifted with an angle of θ = (360°/m-1).

Power balance control
Partial shading is a common problem generally occurs in a large PV system that results in unequal power generation by the PV modules connected to each H-bridges , leading to unbalanced grid current as shown in Figure 5(a). However , the addition of zero sequence voltage in each phase, manages in distribution of balanced power among the three phases so that at the grid side current is balanced as shown in Figure 5(b) [24]. If * is the output voltage of the inverter during partial shading condition then with the injection of ,the output voltage of the inverter modified as, * + = (where i= a,b,c for three phase voltages). It is based on the concept that if three phase unbalanced voltages at the grid side varies directly according to unbalanced power, the injected zero sequence voltage makes the grid current balanced [25]. Figure 6. shows

RESULTS AND ANALYSIS
A seven level three phase cascaded multilevel inverter is initially simulated with 185W of PV panel as dc source in MATLAB/SIMULINK environment. All PV panels are operated at 1000W/ 2 and at 25 0 C. At time t=0.8 sec due to partial shading panel 1 and 2 of phase a get the radiation of 600 W/ 2 .From Figure  7(a). it shows that the MPP voltage obtained from panel 3 of phase a is 36.4 while due to partial shading panel 1 and 2 give the MPP voltage of 36V.The MPP voltage obtained in phase b is shown in Figure 7(b). It shows that partial shading effect the MPP voltage of phase a only ,whereas the other two phases are not affected by the environmental issues. Phase current obtained of phase a under distributed MPPT is shown in Figure 7 Power obtained from each phase is shown in Figure 7(d). From time t=0 to t=0.8 sec, power extracted from three phase are approximately 535W and at t=0.8 sec because of change in radiation, power extracted from phase a is 340W causing a three phase unbalanced power. For this purpose, the modulation scheme with zero sequence voltage injection is adopted. After zero sequence voltage injection the inverter output voltage is shown in Figure 7 (e). It shows at t=0.8 sec, due to zero voltage injection, voltage at the grid side becomes imbalance that causing a balanced grid current as shown in Figure 7 (f).

CONCLUSION
A three phase cascaded H-bridge multilevel PV inverter with grid integration is proposed in this paper. In order to reduce the complexity and increase the efficiency distributed MPPT is applied to each PV panel. As partial shading is an important phenomenon to be considered in a large scale PV system that has direct impact on grid current balancing, a modulation compensation scheme is proposed. It helps in balancing the grid current by injecting a zero sequence voltage in proportion to power imbalnce so that maximum power is extracted.