Photovoltaic Emulator for Grid-connected Quasi-Z-Source Inverter

Received Jun 6, 2018 Revised Jul 5, 2018 Accepted Oct 23, 2018 The performance of PV panel is very much dependent on the amount of sun light as well as the temperature of the surrounding environment which normally hard to be predicted. The use of PV emulator in the investigation of solar inverter especially at a lab scale platform helps to mitigate the inconsistency factors due to this uncontrollable variation. This work discussed on the design and development of a PV emulator for the gridconnected quasi-Z-source inverter which has different topology and control method compared to the conventional voltage source inverter. The I-V characteristics of the PV panel is modelled from the commercially available product and through circuit analysis the relation between capacitor voltage control and the PV terminal voltage is established, thus realizing the MPPT operation. Results from both simulation and experimental verification demonstrated that the PV emulator successfully able to produce power for the inverter according to the requirement. Keyword:


INTRODUCTION
Penetration of photovoltaic (PV) based distributed generation system in the energy grid becomes very significant nowdays. Data from the annual report produced by the International Energy Agency as in Figure 1 shows that the PV installation all over the world reaches 98 GW for the year 2017, which is 29% growth from the previous year with China leading the way by 32% of the total installed capacity [1].
Along with this rapid development, so as the research activities on the energy conversion aspect of the PV generation system to achieve high efficiency operation. In the design and development of power electronics circuitries for the PV energy conversion, having a stable condition over wide range of input power is important to verify the maximum functionality of the system. Although the sun light during the day is something which is abundantly available, testing a system under certain condition can be very challenging as the wheather and temperature can vary unexpectedly. For that, the use of PV emulator especially for the laboratory scale experimental platform is preferable. Many works have been discussed in the literature among others such as in [2]- [4]. All these works in common, use a lookup data generated from the commercially available PV panel I-V characteristics as a reference. As the PV source can be modelled as a current source input to the system, the PV is then simulated by means of current control using power electronics circuit such as the DC-DC buck converter which is simple and effective. As a result, the power produced by the PV emulator can be specifically controlled to emulate the real environment condition. From the perpective of integration between the PV source and the grid-connected inverter, the DC-DC boost converter is preferably used to increase the voltage across the inverter and to implement the the maximum power point tracking (MPPT) to gain maximum power from the PV source [5]- [6]. The Z-source inverter [ZSI] proposed in [7] which then evolves into many other types of impedance source inverters such as the quasi-Z-source inverter (qZSI) [8] which have been proven as an alternative to conventional voltage source inverter (VSI) for the grid-connected PV system with equivalent performance as discussed in the previous works such as in [9]- [12]. Compared to the conventional VSI based PV inverter system, the qZSI eliminates the necessity to have the DC-DC boost converter in between, thus an advantage in term of reducing the number of components as well as on the control complexity. From the point of real PV source implementation however, not very much have been discussed especially on realizing and evaluating the operation and effectiveness of the MPPT, which requires the use of PV emulator as discussed above. For that, this paper is filling the gap by specifically discussing on the implementation of the PV emulator for the case of the qZSI. It is expected that the results from this implementation can be applied to other type of impedance souce as well.

RESEARCH METHODOLOGY 2.1. Modelling of the PV Source Emulator
Power produced by the solar panel in general varies based on environmental factors such as the amount of sunlight and the temperature. The use of PV source emulator makes it easier to reproduce and maintains the same condition for investigating the performance of the PV inverter system. The PV emulator basically consists of two parts; a lookup table which consists of PV array I-V curve data and the DC-DC buck converter used to generate the current according to the lookup table data provided based on the voltage sensed at its output terminal. Figure 2 shows the structure of the DC-DC buck converter based PV emulator. A general PV array model can be defined from the following Figure 3 which shows the equivalent circuit of the PV cell. The relationship between the output current I and the terminal voltage V is given as [13],   Table 1 shows the definition of parameters involved in the above equations  Figure 4 shows the graphs voltage-current I-V curve characteristic and the responded power curve calculated based on solar PV panel commercial product BP MSX 120 from BP Solar [14] which is used as the base for the configuration of the PV emulator output voltage, current and power. From the circuit and structure of the PV emulator shown in Figure 2, according to the voltage across the output of the DC-DC buck converter, the corresponding reference current is obtained from the lookup table which is produced based from the MSX 120 product in Figure 4. The current controller adjusts the PWM duty ratio so that the average inductor current matches to the reference current. By controlling the average inductor current, the output power of the PV emulator can be controlled to the desired level.

Integration of PV Emulator with Grid-connected qZSI
As shown in Figure 2 the voltage and current output from the PV emulator is connected to the quasi-Z-source network which consist of asymmetrical LC network L 1 , L 2 , C 1 , and C 2 plus a diode D 1 . There are two states of operation in the conventional voltage source inverter (VSI); the active states when a non-zero voltage exists across the bridges and the zero states when either all upper and lower transistors are in ON or OFF condition (S 1 S 3 S 5 /S 4 S 6 S 2 000 or 111) to produce a zero voltage condition across the bridges.
In the qZSI, a shoot through condition (short-circuit of inverter switches) is purposely introduced during the zero states. Figure 5 shows the equivalent circuit of qZSI during both shoot-through and nonshoot-through operation. The shoot through interval is defined as T 0 , the non-shoot-through interval as T 1 and the switching period as T s where T s =T 0 +T 1 . The shoot-through duty ratio d is defined as d=T 0 /T s . When the qZSI is in a shoot through condition for a duration of T 0 from switching cycle of T s , using the KCL, KVL, the following equations can be defined.
Then when the qZSI is in an active states condition for duration of T 1 from switching cycle of T s , the following equations (5) can be defined.
Considering the average voltage of the inductors and average current of the capacitors are zero over one cycle in steady state, from (4) and (5), From the above equations the capacitor voltage and inductor current can be obtained as, and the DC link peak voltage across the inverter can be derived as It is shown from (7) and (8) that the capacitor voltage and the DC link peak voltage across inverter can be boosted from v in by varying the shoot-through duty ratio d accordingly. As for the case the input voltage comes from the output of PV arrays (v in v pv ), if the capacitor voltage v C1 can be controlled to be constant, the input voltage v pv increases when the shoot-through time is decreased, and decreases when the shoot-through time is increased as in equation (9).
Considering this fact for the case of MPPT implementation, the maximum power point voltage (V mpp V pv ) can be tracked by adjusting the shoot-through duty ratio d.  Figure 6 shows the overall configuration of the system with the grid-connected qZSI. It consists of the PV emulator as the main power supply to the inverter system which replicates the behaviour of the actual PV array I-V characteristic, the quasi-Z-source inverter, the local load and the 3-phase 415V grid-connection via a transformer. Figure 6. Overall block diagram of the grid-connected qZSI PV inverter system

The Maximum Power Point Tracking Alghorithm
As the capacitor voltage V C1 is controlled constantly, the shoot-through duty ratio d is controlled by the MPPT algorithm to regulate the voltage across the PV terminal V pv to achieve the maximum power point according to equation (9). Figure 7 shows the algorithm of the pertube and observe MPPT method (P&O) [15] used to control the d value. i is the time sequence V(i) and I(i) are the voltage and current at the PV terminal measured by the sensor and the produced power P(i) is computed. Based on the computed P(i) and the current voltage V(i), decision is made whether to increase or decrease the shoot-through duty ratio d.  Table 2 shows the parameters and specifications used for both platforms. For the 5 kW system, the PV source configuration is set at Vmpp 235.9 V with the capacitor voltage VC1 controlled at 680 V. This makes it possible for direct connection to the 3-phase 415 Vl-l grid. The 0.5 kW system is the scaled down version of the 5 kW system, designed with the purpose for comparison with the results from the experiment works presented in the next sub-section. The PV source is configured at Vmpp 141.1 V, with capacitor controlled at 200 V and connection to the 3-phase 415 Vl-l grid is made through a transformer.  Figure 8 shows the I-V curve data used in the simulation for 5 kW configuration based on the actual PV panel product MSX-120 from BP Solar. From the calculation and the power curve shown below, it is expected the maximum power of approximately 5 kW can be obtained at approximately V mpp 235.9 V and I mpp 21.36A at 25ºC. MPPT operation of the qZSI PV inverter is verified by gradually increasing and decreasing the insolation level between 0.2 to 1.0 kW/m 2 as shown in Figure 9. Variation in insolation value directly affects the power produced by the PV array, and the MPPT operates to find the V pv value that will produce a maximum power. In Figure 9, V pv changes in the range 220 V to 245 V through the adjustment in the shootthrough value d between 0.39 to 0.41. The PV emulator produces the output current I pv based on I pv_ref in the lookup table which corresponds to the controlled V pv as shown in Figure 10 where the finally produced power P pv is proportional to the given insolation value between 1 kW to 5 kW. The successful operation of the MPPT depends on the ability to control the capacitor voltage V C1 which is shown in Figure 11 at around 680 V with the produced voltage across the inverter switches V dc ranges between 1110 V pk to 1140 V pk . Figure 12 shows the output phase current of the inverter where the value between 2.5 A pk-pk to 9.5 A pk-pk at 339.4 V phase-pk is proportional to the power produced by the PV emulator P pv .

Simulation Results at 0.5 kW
The I-V curve data used in the simulation at 0.5 kW power level based on commercial product MSX-120 from BP Solar is shown in Figure 13. The maximum power of approximately 0.5 kW is expected at V mpp 141.1 and I mpp 3.6 A at 25ºC. MPPT operation of the qZSI PV inverter is verified through the same manner as for 5 kW power level and the results are shown in Figure 14. The insolation level is gradually increased and decreased between 0.3 to 1 kW/m 2 at 25ºC. The MPPT algorithm operates to vary the V pv between 120 V to 145 V to achieve maximum power P pv at each insolation value ranges between 130 W to 530 W. The results show a good agreement with the I-V curve characteristic data in Figure 13.
In order to realize the successful operation of the MPPT, the capacitor voltage is controlled at 200 V, shown in Figure 15 (top) and the shoot-through duty ratio d is adjusted between 0.2 to 0.3. This produces a peak voltage across the inverter V dc ranges between 257 V to 275 V in Figure 15 (bottom) which is a good agreement with the theoretical values in Table 2.  Figure 16 shows the close-up of the PV emulator unit. A digital programmable DC power supply XDC600-10 from Xantrax with maximum voltage of 600 V and current of 10 A is used as the power supply. A single IGBT IXEN60N120 from IXYS is used for the switching with breakdown voltage of 1200 V and current capacity of 100 A. For the gate driver, a gate driver unit based on VLA502-01 from POWEREX is used to drive the IGBT with 5 kHz PWM signal. The DC-DC buck converter is designed to operate at maximum 2 kW power level, with V in 600 V, V out_max 200 V and output current I out_max 10 A with maximum ripple of 20%. Worst case sampling time which is also the switching frequency f sw is made at 5 kHz and duty cycle D max 0.35. In the experimental setup an inductor of 20 mH is used together with two electrolyte capacitor of 2200 uF/450V connected in series to create a 1100uF capacitor value.

Hardware Experimental Results
Figures 17 to Figure 18 shows the results when insolation value is gradually increased and decreased at various level between 0.2 to 1.0 kW/m 2 to produce approximately 100W to 500W power from the PV emulator. In all cases capacitor C 1 voltage V C1 is well controlled at 200V to enable the MPPT operation. The maximum power point is achieved through the increment and decrement of V pv within a range of 120V to 145V by varying the shoot-through duty ratio d between 0.2 to 0.3. Inductor current I pv is successfully controlled according to the values I pv_ref referred by the I-V lookup table. The power produced by the emulator goes into the inverter and output current of the inverter I inv is proportional to the input power from the emulator output in Figure 19 (a)-(d). Based on the value of inverter phase output current of 6.43 A pk-pk and 10.25 A pk-pk in Figure 19 (a) and (b), the amount of power produced by the emulator is roughly match the power released by the inverter.

CONCLUSION
The paper presents the design and development of PV emulator for use with the grid-connected quasi-Z-source inverter system. The integration with the qZSI is unique with the elimination of the DC-DC boost converter as normally applied in the conventional VSI topology. The PV emulator is modelled and the I-V characteristics are obtained from the commercially available PV panel product. The overall system of the grid-connected qZSI is configured at two distincts power levels of 5 kW and 0.5 kW to show functionality over wide range operation. Based on both simulation and experimental verification results, the PV emulator satisfactorily operates as a current source for the inverter with the MPPT functions well as expected.