Simulation Studies of the SEPIC Converter Based PV System

3.6.1

Development of Dynamic Models for Simulation

To design a complete dc-dc converter incorporating the feedback system, a dynamic model of the switching converter is needed. The model is implemented using a dynamic system software simulator – in this case, the SimPowerSystems of Matlab/Simulink. To use component models that approximate the physical behaviour of the devices, the non-ideal component models of the IGBT-based power switch and the power diode of the SimPowerSystems software are used as shown in Figs 3.6 and 3.7. Such models provide physical insight into the switching transitions, switching losses, instantaneous voltage and current stresses, and responses to load or input transients.

Figs 3.6 and 3.7 integrate the two-panel PV array model developed earlier in Chapter 2, as the dc source, and the 800W SEPIC converter designed in Section 3.4 for the simulation models. The use of the PV array model is necessary to capture the effects of changing environmental variables on system performance. While Fig. 3.6 depicts the open-loop model, Fig. 3.7 shows the closed-loop model. The feedback loop of Fig. 3.7 incorporates the designed PI controller discussed in previous section, the unit delay to model the calculation delays associated with the PWM process and analogue-to-digital conversion, and the PWM block for the generation of the pulse train used to drive the IGBT-based power switch.

55 The variable output voltage of the two-panel solar array is boosted up to generate a regulated dc voltage of 180V – as indicated by the reference value, Vref. The regulated output voltage

serves as the input to the other subsystems discussed in subsequent sections – the bidirectional dc/dc converter, and the dc-ac converter. To achieve voltage regulation, the voltage mode rather than the current mode technique is used in this simulation studies. This technique is used in this section to show its capability. However, a superior control technique using a multi-loop approach integrating the current mode control and the maximum power point tracking technique is discussed and implemented in Chapter 4.

Fig. 3.6: Simulation model for open loop operation: SEPIC converter with integrated two- panel PV array in Simulink/SimPowerSystems software.

Fig. 3.7: Simulation model for closed-loop operation: SEPIC converter with integrated two- panel PV array in Simulink/SimPowerSystems software.

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3.7

Simulation Results and Discussions

The responses of the PV power conversion system in both open-loop and closed-loop systems are investigated through simulations as discussed below.

3.7.1

Effect of Duty Cycle Variations on Open-loop Operation

The three major parameters that affect the output voltage of any dc power system – input voltage variation, load variations, and changes in the duty cycle values – are investigated for operation in the open-loop mode, Fig. 3.6. Specifically, Fig. 3.8 shows the variation in output voltage of the PV system with changes in the duty cycle. It is observed that increasing the duty cycle increases the output voltage, and similarly decreasing the duty cycle decreases the output voltage. This is the same situation with variations in input voltage. The essence of Fig. 3.8 is to show that without a controlled feedback loop, the system’s performance in terms of voltage regulation and stability cannot be guaranteed.

3.7.2

Output Voltage Regulation

The closed-loop operation of Fig. 3.7 is simulated with a reference voltage of 180V, as shown. With the parameters of the PI controller tuned and the switching frequency of 25 kHz, the output voltage is shown in Fig. 3.9 for an input voltage of 44.2V at 1000W/m2 and 25oC (standard test condition) from the PV solar array – first subplot. An overshoot of approximately 240V occurs during the transient response when the simulation is first started

Fig. 3.8: Variation of output voltage with duty cycle for open loop operation.

0 1 2 3 4 5 6 7 8 9 10 0 50 100 150 200 250 300 350 400 Time (s) O ut pu t V ol ta ge ( V ) Duty cycle = 0.4 Duty cycle = 0.6 Duty cycle = 0.7 Duty cycle = 0.8

57 Fig. 3.9: Regulated output voltage of the SEPIC converter for a given PV array voltage. Top subplot shows the output of the PV array; bottom subplot shows the regulated output voltage of the converter.

(system start-up). The simulation shows that it takes approximately 1.1 seconds for the output voltage to reach ±2% of the desired output voltage of 180V. Thus, the steady-state error is almost eliminated. Also, comparison with the open-loop response of Fig.3.8 shows that the closed loop systems apart from providing the desired regulated voltage has a faster transient response, and hence attains the steady state value faster following system start-up.

3.7.3

Robustness Analysis: Input Voltage Fluctuations.

The two major environmental variables that affect the output of a PV array are temperature and irradiation. The response of the controlled converter to changes in the output voltage of the PV array occasioned by changes in the ambient temperature is considered in this section. The topmost subplot in Fig. 3.10 shows a drop in the ambient temperature from 25oC to 15oC, at step time 5seconds. This drop in temperature results in an increase in the output voltage of the PV array from 44.2V to 45.8V – shown in the middle subplot. For this drop in temperature and the associated rise in the output voltage of the PV array, the output voltage of the SEPIC converter (bottom subplot) is regulated at the desired output voltage of 180V.

0 1 2 3 4 5 6 7 8 9 10 43 43.5 44 44.5 45 45.5 Time (s) V o lt a g e ( V )

Output voltage of the PV array@ 1000W, 25 deg Centigrade

0 1 2 3 4 5 6 7 8 9 10 0 50 100 150 200 250 Time (s) V o lt a g e ( V )

58 The transient response and the steady-state error are within the desired ranges. This shows the robustness of the PI controlled PV system.

Fig. 3.10: Regulated output voltage for rapid drop in the ambient temperature. Topmost subplot is the drop in ambient temperature; middle subplot shows accompanying rise in PV output; bottom subplot is the regulated response of the converter to these changes.

3.8

Conclusions

Effectively converting the “raw energy” from sunlight into usable electrical energy requires a dc-dc converter. This chapter has presented the design and modelling of such a converter based on the SEPIC topology. The designed converter is integrated with a PV array model to study the performance of a PV power plant under open-loop and closed-loop controls. The open-loop PV power plant is seen to be sensitive to input disturbances – input voltage variations, system load changes, duty cycle variations and component variations – and lacks the ability to correct these disturbances. On the other hand, the closed-loop system compensates for these disturbances. The closed-loop offers the PV power plant greater accuracy, and the transient response and the steady-state error are more conveniently controlled using a compensator – a PI controller in this simulation. Moreover, any adjustment of the PI compensator parameters required to achieve a desired response can be made by changes in software. 0 1 2 3 4 5 6 7 8 9 10 15 20 25 Time (s) T e m p e ra tu re ( o C )

Ambient Temperature Variations

0 1 2 3 4 5 6 7 8 9 10 44 44.5 45 45.5 46 Time (s) V o lt a g e ( V )

Output Voltage of the PV array.

0 1 2 3 4 5 6 7 8 9 10 0 50 100 150 200 250 Time (s) V o lt a g e ( V )

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