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This paper presents a grid connected photovoltaic system (PV) with a proposed high voltage conversion ratio DC-DC converter which steps up the variable low input voltages of photovoltaic module to the required DC link voltage. This voltage is applied to an H-bridge inverter which converts DC voltage into AC voltage and a low pass filter is used to filter the output. By adjusting the duty ratio of switches in DC-DC converter, the magnitude of inverter’s output voltage is controlled. The frequency and phase synchronization are ensured by a feedback signal taken from the grid. In this way, inverter is synchronized and connected with the grid to meet the energy demand. The PV system has been designed and simulated.

The demand of electric power is on rise and is anticipated to increase exponentially in future. Among main sources of energy, thermal sources such as fossil fuels contribute to bulk amount of power generation. But these fuels are expensive due to their bulky demand around the world. Also their reserves are limited and running short. Besides this, their combustion generates pollutants gases which affect the environment in various forms. Due to these factors, the renewable sources of energy are getting attention and the photovoltaic systems (PV) are also becoming more prominent during the last few decades. The photovoltaic module provides a low DC voltage with wide range according to various operating conditions [

This paper describes grid-connected photovoltaic system with a proposed high voltage conversion ratio DC-DC converter. This system mainly includes two processing stages: a high voltage conversion ratio converter which converts low DC PV module voltage into the required DC link voltage. The second stage is the inverter which converts it into AC voltage. A low pass filter is designed to filter the inverter output before it is applied to the grid. The DC-DC converter is capable of maintaining the magnitude of inverter’s output voltage and inverter guarantees the phase and frequency synchronization of its output. The proposed system finds applications not only in low power, but can also be extended to some extent to large scale allowing the parallel operation of PV modules.

The grid-connected system is shown in _{1} to S_{4}. The discrete pairs of switches (S_{1}, S_{4}) and (S_{2}, S_{3}) are operated to produce positive and negative half cycles of AC voltage respectively. For the operation of inverter, the pulse width modulation method is used in which a sinusoidal pulse width modulated signal is achieved by the comparison of a sine wave and saw-tooth wave. This sinusoidal PWM signal is then compared with a square wave to get driving signals for the switches (S_{1}, S_{4}). Similarly, the sinusoidal PWM signal is compared with inverted square wave to get driving signals for the switches (S_{2}, S_{3}). The output of inverter comprises of ripples and is filtered out using an LC filter as shown in

The proposed converter is shown in _{1}, capacitor C_{1}, switch S_{a} and diode D_{1} constitute the first stage. A coupled inductor and capacitors C_{2}, C_{3} and C_{4} form the second stage. Switches S_{a} and S_{b} are used to control the operation of first and second stage respectively.

When switch S_{a} is turned on, L_{1} inductor is storing the energy. During this mode, diode D_{1} is reverse biased and capacitor C_{1} supplies current to the second stage which acts as a load. When switch S_{a} is turned off, diode D_{1} becomes on and the current i_{L} flows through the load and capacitor C_{1}. These operating modes are shown in

The voltage conversion ratio of first stage is same as boost converter and is given by

where

_{m} (magnetizing inductance), leakage inductances L_{k}_{1} and L_{k}_{2} on the primary and secondary side respectively and a lossless transformer having n as turn ratio. The switch S_{b} controls the operation of this stage. The steady state waveforms of

second stage are shown in

Mode 1: The switch S_{b} is initially turned on as shown in _{2} becomes reverse biased and the applied voltage causes the current on primary side to increase. In this mode, the current flowing through secondary winding is zero and there is increase in energy on the primary side.

Mode 2: In second mode, switch S_{b} is turned off and diode D_{2} is forward biased which charges capacitor C_{3}. The secondary winding is also receiving some of primary current via capacitor C_{2} during this mode. The energy in magnetizing inductance decreases and there is increase in secondary current. Diode D_{3} remains in off condition. Capacitor C_{3} becomes charged at the end of this mode.

Mode 3: In this mode, switch S_{b} remains off and D_{2} becomes reverse biased. Primary, secondary windings and capacitor C_{2} are series connected across the source. This allows the direct transfer of energy to the output from capacitor C_{2}, coupled inductor and source.

Mode 4: The switch S_{b} is turned on and current on the primary side starts increasing there by storing energy in magnetizing inductance. Diode D_{3} becomes forward biased and capacitor C_{2} is charged by capacitor C_{3}. The current on secondary winding decays to zero and D_{4} becomes reverse biased. The voltage across C_{3} becomes equal to the voltage across C_{3} causing diode D_{3} to become reverse biased at the end of forth mode and the circuit proceeds to its initial condition.

Some assumptions are made for the derivation of voltage conversion ratio. The coupling co-efficient of coupled inductor is unity and the diodes are ideal. Let

During on and off modes the inductor L_{m} voltage is respectively given by:

Application of inductor volt-second balance to Equation (2) and Equation (3) yields:

Generally the relation between primary and secondary voltages is given the following equation:

The inductor L_{m} voltage can be written from Equation (5) in the following form:

During Mode 2, the voltage across capacitor C_{3} is given by the following equation:

Substitution of Equation (6) into Equation (7) leads to the following equation:

As capacitor C_{2} is charged by capacitor C_{3} and at the end of mode 4,

During Mode 3, apply KVL

This is the voltage gain of second stage and the voltage conversion ratio of proposed topology is obtained by the product of voltage gains of both the stages i.e.; the product of Equation (1) and Equation (10) yields:

Equation (11) is used to find the voltage conversion ratio of the proposed DC-DC converter. This equation illustrates that for a selected value of turn’s ratio n, the required DC link voltage can be achieved at lower values of duty ratios.

In order to synchronize the phase and frequency of inverter voltage with the grid, a signal is taken as reference from grid voltage. This feedback signal is converted into square wave in a comparator as shown in _{1}, S_{4}). Similarly the comparison of sinusoidal PWM signal with inverted square wave for half of the period generates gate drive signals for the other pair of switches (S_{2}, S_{3}). The transition from low to high in square will make switches S_{1} and S_{4} to turn on and S_{2} and S_{3} to turn off. Accordingly, the inverter voltage changes its polarity at exactly the same time as the grid voltage. Any shift in grid voltage is followed by the square wave generated accordingly. The same shift is adopted by the gate drive signals and hence the same shift occurs in the output voltage of inverter making it in phase with connected grid.

The inverter in this design operates at constant modulation index (m) and the magnitude of grid voltage and its output voltage should be identical. This is accomplished by changing the duty ratio of DC-DC converter switches (S_{a}, S_{b}) by a µ-controller. The grid voltage is stepped down by a simple voltage divider circuit and a measured value is given to the µ-controller which reads the input voltage (

Equation (11) is used to compute the duty ratio (d). T_{on} and T_{off} are computed from the duty ratio. The frequency of switching signal is taken to be 20 KHz and two timers are used to produce switching signals of this selected frequency and duty ratio for the operation of DC-DC converter.

The system specifications and its design parameters are given in _{in} = 30 V, still it operates excellently to give the required voltage at low value of ~0.45. _{a} has less voltage stress which is about 60 V. The voltage stress across S_{b} is less than 110 V which is far less than

Specification and parameters | Symbols | Values |
---|---|---|

Input voltage | V_{in} | 30 - 60 V_{DC} |

DC link voltage | V_{d} | 350 V |

Output voltage | V_{ac} | 220 V |

Frequency | f | 50 Hz |

Power capacity | P | 500 watt |

Boost stage inductance | L_{1} | 31 µH |

Magnetizing inductance | L_{m} | 188 µH |

Filter inductance | L | 2.5 mH |

Filter capacitance | C | 4 µF |

the output voltage (350 V). These less stresses allow the selection of switches in the design procedure to have low voltage ratings. This lowers the cost of the proposed PV system. Comparison has been made been made in

The required DC voltage can be achieved at low duty ratio of ~0.4 in the proposed converter. The lower value of duty ratio decreases the conduction losses and hence improves the efficiency of the proposed system to some extent. The H-bridge inverter has been also simulated.

This paper proposes the grid-connected photovoltaic system with a proposed topology of DC-DC converter. The converter with a high voltage conversion ratio can easily boost up the lower PV voltage to the required DC voltage at low duty ratio. As the gain is high, this allows the parallel operation of PV modules and also eradicates the need of power transformer to achieve the required grid

voltage. This results in high efficiency and reduced size of this system. The converter has low voltage stresses across the switches with decreased cost of the system. The system is beneficial in terms of unit saving as it can meet the demand of load. It may be implemented for a three-phase system.

Khan, N., Aamir, M., Mehmood, F., Aslam, M. and Arif, M. (2017) Design of Grid-Connected Photovoltaic System. Journal of Power and Energy Engineering, 5, 1-12. https://doi.org/10.4236/jpee.2017.58001