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The alternative energy resources, like solar, are always complementary due to environmental changes. Energy generation with the sources such as solar and wind is gaining importance and of that photovoltaic conversion is the main focus of researches due to its promising potential as the electrical source. This paper presents the constant voltage method of control where the output of the converter is maintained constant irrespective of the variations in the irradiance with the high step-up isolated efficient single switch DC-DC converter for the solar PV systems. Constant voltage method of control uses the array of photovoltaic systems as its energy source. The output of the Solar PV systems is nonlinear and has its dependency on temperature and irradiance by which the panel voltage and current varies with the variation in irradiance. Constant voltage control method always operates in such a way that the converter voltage is tried to be maintained constantly to the reference voltage which is set by the user. The system used here utilizes high step single switch isolated DC-DC converter and monitors the voltage continuously by varying the duty cycle to maintain the converter voltage always constant. As a way of improving the performance, both the open and closed loop analysis is done where the closed loop analysis uses the PI controller for its performance. The model is implemented in MATLAB and it accepts the irradiance as the input and outputs the constant voltage from the converter and the feasibility of the proposed converter topology is confirmed with experimental results of the prototype model.

Multi-output DC-DC converters have become very popular recently and they are particularly used in many portable and handheld consumer applications. Portable devices use sub-modules which have different voltage requirements. Their small-size and light- weight make them very attractive and the cost is also optimized. Traditionally, isolated transformer-based multi-output DC-DC converters were widely employed to provide multiple output voltages. However, they are relatively bulky due to the presence of the reactive components. There is a single maximum power point in the I-V and P-V characteristics of the solar PV systems and it is the point where the panel can deliver the maximum power [

is given from the I-V characteristics of the panel and it can be set according to the load requirement. Here the load is considered to be any static load.

The High step-up isolated converter has played a vital role especially in renewable solar PV energy applications. The conventional boost converters due to its high boost ratio will have high switching losses and high voltage stress across the switch [_{c}) connected across the switch to reduce the stress in it. It is also provided with the clamp diode (D_{c}) and the passive clamp circuit is used for recycling the energy from the leakage inductance [_{s}), secondary Inductor (L_{s}) and the freewheeling diode (D_{f}). The topology is modeled with transformer Inductance with the turns ratio of N, the leakage inductance (L_{k}), the magnetizing inductance (L_{m}). There are six modes of operation with this high step-up isolated single switch converter where I_{c} is the clamp capacitor current; I_{dc} is the clamp diode current; I_{dr} is the reverse recovery diode current. The modes of operation are explained below.

The performance of proposed high step single-up switch efficient isolated converter circuits has been explained in six operating modes as below.

In the first mode switch (S) is turned ON. The diodes D_{f} and D_{c} are reversed biased.

The magnetizing inductance (L_{m}) charges with the input source V_{in}. The magnetizing current i_{lk} increases by which the inductor L_{k} resonates along with the switching capacitor (C_{s}). The inductors L_{p} and L_{s} are connected through the diode D_{r}. L_{k} is used to control the current flow in the secondary circuit and the current i_{lk} is given by

When the switch is turned off, the current I_{c} will start to decrease. The capacitor voltage will be appearing across the switch. L_{k} resonates with the switching capacitor C_{s} and the clamp capacitor C_{c} by which the leakage current through the inductor gets decreased linearly. L_{k} is discharged by the V_{c} _{ }

The secondary circuit becomes zero in this mode. The voltage of the switching capacitor and the clamp capacitor stays then the switching capacitor resonates with the leakage inductor. The switching capacitance decreases along with the leakage inductor also decreases. There will cause an increase in the secondary circuit current.

The switch is turned off and the secondary transformer current is maximum at this stage. The voltage across the switch is very high. The magnetizing inductance discharges and gives its energy to the load. The inductor L_{k} gets discharged through the coupled inductor. The value of the current i_{k} is given as below.

In Mode-V, the active power switch (S) remains in the OFF state. The magnetizing inductor is discharged and the secondary inductor gets discharged linearly and the energy is given to the load. In this interval, the leakage inductor current falls to zero. The magnetizing current is given by

The switch (S) is turned ON. This switching is done under Zero Current Switching condition where V_{ce} across the switch is high. The Leakage inductance increases from zero and the leakage current is controlled by the leakage inductor. The relationship between the leakage inductance and the voltage gain is given by

The voltage of the switched capacitor is given by

The inductor L_{m} gets discharged when the switch (S) is turned OFF. The voltage is expressed by

The voltage gain of the high step-up isolated single switch DC-DC converter is given by,

From Equation (9), it is clear that the voltage gain increases with increase in the turns ratio (N). If the converter is designed with the high value of N then the voltage gain increases. The voltage stress of the clamp and the switch diode is given by

The proposed converter is designed with low conduction losses [

The Initial duty cycle for the proposed converter is given by the calculation below

The voltage gain of the converter is given as,

The proposed converter is a high boost converter which gives a voltage gain of 10. This high voltage gain is achieved with the use of switched capacitor and the clamp capacitor. The passive lossless circuit helps in attaining better efficiency by recovering the energy from the clamp and the switching capacitor. The zero current switching condition greatly helps to reduce the switching losses [

Parameter | Values |
---|---|

Input Voltage (V_{in}) | 32.5 V |

Output Voltage (V_{out}) | 321 V |

Switching Capacitor (C_{s}) | 6.25 ´ 10^{−6} F |

Clamped Capacitance (C_{c}) | 6.25 ´ 10^{−6} F |

Capacitance (C_{0}) | 180 ´ 10^{−6} F |

Magnetizing Inductance (L_{m}) | 23 ´ 10^{−6} H |

Isolation Transformer (Turns Ratio) | 1:4 |

Switching Frequency | 10 KHz |

diagram of High step-up isolated single switch DC-DC converter.

The constant voltage method of control here means maintaining the output voltage of the converter to be constant irrespective of the change in irradiations and the temperature. This method compares the reference voltage that is the voltage for which the converter output has to be maintained constant.

with the instantaneous voltage that gets varied with the change in irradiations. The reference voltage is taken as V_{out} and the instantaneous voltage from the panel is considered as V_{in}. The output voltage of the converter is maintained to be constant by modifying the duty cycle according to V_{out} and V_{in}. The formula gives the duty cycle calculation for maintaining the output voltage to be constant. The constant voltage control block will generate the duty cycle as per the instantaneous and the reference output. As the irradiation changes, the voltage V_{in} changes simultaneously and the duty cycle also will be generated for the converter by which the output of the converter will be maintained constant which constitutes the open loop system. The simulation diagram of the constant voltage control method is shown in

generation in constant voltage control method.

The change in irradiance is made using the signal builder block where the irradiations are changed as shown in

In order to obtain the constant voltage from the converter, the open loop method of constant voltage method is implemented and the duty cycle generation is made where it gets automatically adjusted as per the changes in the irradiance. But due to worst case

of change in irradiance, the simulation response of the converter presents the oscillations with the voltage dip of −6 V and swell of +5 V. The reference voltage for which the converter voltage is to be maintained and the converter output voltage is given as the second input to the PI controller. The generated duty cycle (ΔD) is added to the initial duty cycle (D_{i}).

_{P} and K_{i} are chosen as 0.25 and 0.001 by the method of trial and error.

The comparison made clearly depicts that there is an improvement in rise time and the voltage dip as well. And thus proves to be an efficient control method of the oscillations.

In order to confirm the feasibility of the proposed topology of prototype converter

Parameters | open loop system | Closed loop system |
---|---|---|

Time (sec) | Time (sec) | |

Rise time | 0.23 | 0.16 |

Voltage Rise | 1.3% | 1.3% |

Dip in voltage | 1.8% | 1.6% |

model was built and implemented with PIC Microcontroller and driver circuits to obtain the same results as discussed in Section 6. The experimental setup is depicted in

The switching frequency of the converter is considered as 10 kHz. The gate pulse and the output voltage waveforms for proposed boost converter mode are measured and observed as shown in

For instance, the input voltage of the given prototype for boost mode is 5 V and obtains 45.2 V as output it is nearly ten times greater than the given input voltage is observed as shown in

This paper shows that a PV system with the high step-up single switch isolated DC-DC converter with the constant voltage control block is capable of maintaining the output

Input Voltage | Theoretical output voltage | Output voltage obtained from MATLAB simulation | Output voltage obtained from hardware |
---|---|---|---|

2 | 20 | 19 | 18.5 |

4 | 40 | 36 | 35.4 |

5 | 50 | 47 | 45.2 |

of the converter to be constant irrespective of the irradiation changes by changing the duty cycle of the converter. But due to the worst case change of irradiance, the simulation response of the converter presents the oscillations with the voltage dip of −6 V and swell of +5 V. The response of the closed loop system with the PI controller is also presented to give the PV system output with better response comparing the open loop system.

Later, as a future scope, the decentralized PV systems can be considered where each PV system can have its own converter and the constant voltage control block that paves way for the sizing of panels as per the load requirement. In addition the switching scheme can also be designed to switch the PV system according to the load requirement.

Alagammal, S., Rathina Prabha, Dr.N. and Aarthy, I. (2016) Centralized Solar PV Systems for Static Loads Using Constant Voltage Control Method. Cir- cuits and Systems, 7, 4213-4226. http://dx.doi.org/10.4236/cs.2016.713346