Adaptive hysteresis band current control of grid connected PV inverter

Received Jul 18, 2020 Revised Nov 23, 2020 Accepted Dec 19, 2020 In this paper, adaptive hysteresis band current controller is implemented to control the current injected into the grid. Initially it was implemented by B.K Bose for control of the machine drive. Now it is implemented for the grid connected PV inverter, to control the current injected into Grid. It is well suitable for the distribution generation. The adaptive hysteresis band controller changes the bandwidth based on the modulating frequency, supply voltage, input DC voltage and slope of the reference current. Consequently, the controller generates pulses to the inverter. It is advantageous over the conventional hysteresis controller, as the switching frequency is maintained almost constant. Thereby quality of grid current is also improved. It is verified in time domain analysis of simulation using MATLAB.


INTRODUCTION
Due to the development of power electronic devise in power system generates the harmonics in voltage as well as current power high frequency which effect the performance of the loads. These are crucial role in power quality issues. There was a technical improvements are there in HVDC and FCTS device to reduce the generation of harmonics [1]. The VSC based converter are used in medium and low level power transmission for high level power SCR based HVDC system is used [2][3][4][5]. HVDC DC light is made of fully controlled switch such as IGBT or MOSFET along with PWM technique. It reduces the hormonic generation, minimizes the filter requirements [6,7]. The HVDC system is a highly efficient alternative for transmitting large amounts of electricity over long distances and for special purpose applications. As a key enabler in the future energy system based on renewables, HVDC is truly shaping the grid of the future [8]. This hierarchical control structure is discussed which is important tool to maintain the dynamics, stationery performance of the micro grid with respective economical aspects [9]. Different control schemes are discussed in DC micro-Grid to mitigate the issues in it with the goal of providing control design guild lines [10,11]. The high bandwidth hierarchical control structure for AC micro grid in distribution level. Conventionally to control Grid voltage and frequency were done by hierarchical linear control loops. they suffer from slow response with parametric effects. So the model predictive control technique is used to realize with heigh band width to control the multiple VSC. Furthermore, droop control and virtual impedance methods were employed to share active and reactive power. Based on a large-signal dynamic model of a micro grid, a linear parameter varying based state estimator that is localized in each distributed generation (DG) unit serves as an alternative communication role and obtains the dynamics of the other DG units independently. A linear matrix inequality formulation for pole placement condition was derived for the state estimator design. A decentralized secondary voltage controller is, thus, able to achieve accurate reactive power sharing and average voltage restoration without any additional communication links [12]. A novel system operation strategy with energy management system was proposed in DC micro grid with different energy storage system where high penetration of renewable energy source. In both islanded and grid connected operation strategy of the micro grid, the ultra-capacitor unit has vital role to support the dc link. The micro-grid was controlled to deliver/absorb certain amount of power to grid. No power is dispatched to the grid in island mode [13].
In hysteresis controller, the error band is normally fixed to a certain value. Consequently, the switching frequency varies within a band because peak to peak of ripple current must be controlled at all points of the fundamental frequency wave. Adaptive hysteresis band current controller changes the hysteresis bandwidth according to the reference current to make switching frequency of inverter nearly constant and further reduces the THD of supply current. Adaptive hysteresis band current controller was used as current control method [14]. In section 2 modeling of the PV system with MPPT technique, hysteresis and adaptive hysteresis current control of the grid tied inverters with detailed modelling is presented. In Section 3 simulation results of hysteresis and adaptive hysteresis band controllers are inspected and compared in terms of % THD in grid current.

DESIGN AND IMPLEMENTATION OF THE SYSTEM 2.1. Design of the PV array
PV array is the combination of the PV modules. The primary element is PV cell, which converts the solar power to electrical power. Different modelling of the PV cell is given in literature survey such as single diode model, two diode models, and multiple diode models. In this paper two diode model of the PV cell is considered is shown in Figure 1. Ipv is the photo current. D1 and D2 are two anti-parallel diodes, Rp is leakage resistance and Rs is contact resistance. V is the output voltage of single cell and the output current of cell 'I' is given in (1) [15][16][17][18][19].
Here λ is solar irradiation, Iscr is short current of PV cell, Tk and Trefk are the actual and standard temperatures respectively. K is temperature coefficient of short-circuit current (A/ K), A1 and A2 are the ideality factors of the diodes.' q 'is the charge of the electron, Io1 and Io2 are the reverse saturation currents of diodes D1, D2 respectively. As shown in (1) is modified for PV module as given in (5).
Here Ns and Np are the No. of cells are cascaded and shunted in module. These modules are connected in cascade and shunt to meet the voltage and power rating. PV array current is given in (7).
Where Nss and Npp are the number of series connected modules and parallel connected series strings in PV array respectively. The standard KC200GT data sheet parameters are used for PV array simulation are given  [20][21][22]. Generally, a DC-DC boost converter is used to extract the maximum power from the PV array. It is shown Figure 2. The inductor stores the energy when switch is ON, the stored energy is released to the load when it is OFF.
The design the parameters of the boost converter is given in (8).
Where, Vs=Source voltage, Vo=Output voltage, D=Duty cycle, =Ripples in current, =Ripples in output voltage.

Perturb and observe method
In this work the DC link is derived from the PV array, due to uncertainties in the climatic conditions. The power from PV panel is variable. To extract the maximum power from the PV panel, different MPPT methods are proposed in literature. Such as fixed duty cycle method, beta method, Incremental conductance method, perturb and observe method, constant voltage current method, and fractional voltage method. Each method has its own advantages and disadvantages. In this paper perturb and observe (P&O) method is used due to the easy implementation [23].

Operation of 3-phase inverter
Currently 3-phase inverters are predominantly used in both AC/DC Microgrid operations. A 3-phase inverter consist of six switches as shown in the Figure 3. When upper device is 'ON', the load terminals are connected to the positive voltage and when lower device is 'ON', the load is connected to the negative terminal voltage. Applying proper pulses to the switches three phase voltage is generated, that is applied to the grid through a filter. To filter out the harmonics in the current, different filters 'L', 'LCL', 'LC' are available in literature [24]. 'L' filter was used to filter the harmonics in the current [25][26].

Hysteresis band controller
In this current control technique, actual grid current is compared to reference current as shown in Figure 4. If error current is more than the fixed band then the switches are turned 'on'. If it is less than the lower limit, then lower switches in the respective phases are turned 'on'. Thereby the grid current is restricted within the set limits. The harmonics presented in the grid current depends highly on the selected band for controller, type of the filter and its value. One of the main disadvantages of this controller is variable switching frequency due the fixed bandwidth. It is difficult to design the filter at variable frequency. This is overcome with a novel hysteresis current controller. Its design and implementation is discussed in the next section of this paper. The circuit diagram of the system with this controller is shown in Figure 3 [27].

Adaptive hysteresis band current controller
The switching pattern of the single phage of the 3-ph. inverter is described below. Conventional hysteresis current controller.

IINV ≥ (IR-HB ) Then T1 is ON &T4 is OFF (9) IINV≥ ( R + HB) Then T1 is OFF & T4 is ON
The switching frequency of the hysteresis fixed band controller depends on the how fast inverter current changes from the upper limit to the lower limit and vice-versa. Slope of the inverter current depends on the filter value and DC input voltage of the grid connected inverter. Accordingly, the switching frequency varies [28].
Bandwidth of the hysteresis controller allows the inverter current error. By varying the bandwidth, variation in switching frequency can be achieved. As the switching frequency increases, the switching loss of the inverter and subsequently, electromagnetic interference (EMI) increases. So while selecting the bandwidth for a hysteresis controller, the above mentioned parameters are compromised. Hysteresis controller is simple to implement among PWM converters. It has inherent peak current limiting control and it does not require any system information. However, it suffers from variable frequency. Improved version of this technique is implemented for shunt active filters and drives [29][30][31]. In case of the adaptive hysteresis band controller, the band width of the hysteresis controller will be updated depending on the DC voltage, slope of the reference current, and filter values and grid voltage. as detailed in (10)- (19).
where; fC : Switching frequency m : Slope of the inverter current. Adaptive hysteresis band current controller changes the bandwidth depending on the slope of the inverter current and DC voltage. It reduces the influence of current distortion on the reference wave. The expression for the HB is given in the equation. The simple block diagram of the adaptive hysteresis band current controller is given in Figure 6.

SIMULATION RESULTS
Simulation waveform of PV array voltage is shown in Figure 7, where the output voltage is 380 V upto 0.5 Sec. Due to change in temperature at 0.5 sec, voltage raises to around 480 V. The current and power supplied by the PV module is shown in Figure 8 and Figure 9 respectively. It is observed in Figure 9. that the current as well power supplied by the PV also changes at 0.5 sec. It is due to the constant reference current to the PV inverter.
Output voltage and current of the inverter are illustrated in Figure 10. It can be noticed that 3-ph. inverter output is changed at 0.5 sec. This is due to change in DC link voltage. Reference current is taken as 4 A. THD of the grid current with adaptive hysteresis controller is shown in Figure 11. THD of the grid current with Hysteresis controller is depicted in Figure 12. It is observed that THD is improved to 2.34% with adaptive hysteresis controller in comparison with 3.19 with hysteresis controller.

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THD of the inverter current for the different reference currents values are tabulated in the Table 2. without changing the system parameters, the adaptive hysteresis band controller has given the less THD of the current, compare to the hysteresis controller. The voltage across switch is shown Figure 13. with hysteresis current controller. The time period of the switch is not constant and more ever the switch frequency is low. The adaptive hysteresis band controller the switching time low, means the switching frequency is heigh, where which requires the low value of the filter, is also constant shown Figure 14.   Figure 13. Voltage across a switch with hysteresis controller Figure 14. Voltage across switch with a novel hysteresis controller

CONCLUSION
In this paper both hysteresis and adaptive hysteresis band current controllers are implemented to the PV inverter tied to grid. In case of the adaptive hysteresis band current controller, the switching frequency is maintained almost constant. The adaptive hysteresis band controller changes the bandwidth based on the modulating frequency, supply voltage, input DC voltage and slope of the reference current, switching frequency is all most all constant. % THD of the grid current is also low with the adaptive hysteresis band controller. This technique can be suggested to control the power in case of micro grid application where current control is required.