This paper focuses on the design of the inverter power stage connected with PV-grid which supports the contrived PV system. The increased number of grid connected photovoltaic (PV) inverters gave rise to problems concerning the stability and safety of the utility grid, as well as power quality issues. The proposed systems can overcome these issues and improve standard regulation methods for gird connected PV inverter. The maximum available voltage in the PV string is tracked by the power stage which has been planned and designed in such a way. The tracked voltage is boosted then. The important components to voltage source inverter (VSI) are boost inductor and input capacitor which are calculated. To get a clear sinusoidal output phase voltage of 230 V from a DC capacitance bus projected to deal with 400 V , t he important inverter stage parameters have been planned and modeled in Mat lab. Each block stage of the converter is easily understandable by the Simlink of the dual stage DC-AC converter explanation. The control schemes which have been proposed would compromise with the inverter power stage which forms the neat grid system. The existing renewable energy sources in the laboratory are integrated by the proposed control.
According to temperature and irradiance, the characteristics of solar affect the open circuit voltage. The PV-grid connected systems’ standards to model and install are discussed which helps to contrive the PV system. Proposed a grid connected photovoltaic fly back inverter operation in DCM, with combinations of fractional short circuit current and hill climbing [
DC-DC converter topologies
In day to day usage, the role of DC-DC converters is ceaselessly increasing. DC power supply is one of the paramount uses of converters. It is enumerated that there are three basic types of DC/DC converters and the product of these converters are Cuk converters and Full bridge converters. Probably the inverters comprise of the boost converter, buck converter respectively as a step-up converter and step down converter in order to have higher output and lower output and a buck-boost converter is either to reduce or to increase the voltage ratio with a unit gain for a duty ratio of 50%. The grid connected PV inverter’s power stage uses full bridge switch mode DC-DC converter. Half-Bridge Converter is another configuration for providing high voltages. Provided with higher efficiency it has only two switches wit simpler structure but it produces half output voltage of the Push-Pull Converter. The sensitivity to the load variations is another main drawback of this design. In order to put up the rapid change of the voltage ratio a more complex control circuit is needed. Attaining regulated output within desired constraints is very difficult and losing switches with current changes are other main problems on this topology. Circuit symmetric results in a more complex control circuit design moreover it is difficult to achieve.
Discussing different converter topologies in detail, DC-DC converter is felt feasible. Unstable and low voltage from the PV array through the input capacitor PV C to a constant 400 V DC voltage at the capacitor link, DC C is converted by the DC-DC converter in the DC-DC input stage.
Full bridge inverter is used in this proposed circuit; DC power is converted into AC power in this DC-AC stage at preferred output voltage and frequency. The 400 V DC output voltage of the full bridge converter is converted to the grid voltage of 230 V AC ? 240 V AC at 50 Hz/60 Hz frequency by the designed power stage. Accessing same input voltage, the full-bridge inverter produces an output power twice that of the half-bridge inverter. Four switching devices are shown in
Input and DC-link capacitors
The means of electrolytic capacitor enables DC link capacitor to decouple power. After
several years, electrolytic capacitor technology has been chosen by design engineers to use it as a bus link capacitor in inverter designs.
Electrolytic capacitors are the eminent technology for hard switched inverter bus link capacitors. The technology of electrolytic capacitor remains same for several years. The electrolytic capacitors costs low that the technology gets the center of attraction for choosing it. The DC link capacitor plays a vital role in the long life of the converter and it is substituted with film capacitor and kept as possibly small. Intended to reduce DC-link capacitance of inverters, many toils have been taken that film capacitors have been proposed to replace electrolytic capacitors. Were compared and explained.
Earlier sections explained the significance and inevitability of the input parameters which parameters are boost inductor or DC inductor PV L and input capacitor PV C. On estimating these parameters direct formulae have been used. There are two strings in the system which has been set in a series for having voltage of 40 V and it is connected in parallel for attaining 10 a current. V-I characteristics of the NTNU PV array is affected by irradiance and temperature. MPPT performance is affected by these changes so it has to be minded in the design. 15% of the solar radiation is converted into electricity by PV cells and it is hard to attain the preferred 420 V from the PV array. Considering the reasons, 100 V is taken as minimum available voltage at any time. Duty ratio is calculated with the usage of minimum voltage because it renders maximum switching current. This duty ratio D = 0.68 is noteworthy to the full bridge converter. 85% is assumed as the efficiency of the converter at any worst case.
Many designs show the inductor in a certain range provided. Calculating the boost inductor directly is advised when data sheet is not available. 20% to 40% of the output current is calculated as the inductor ripple current. In this assumed design input voltage is estimated to be VPV = 100 V and the estimated switching frequency is f = 10 kHz. The value of projected inductor ripple current can be found by using equation. On assuming the continuous conduction mode, (CCM) of the boost converter stage, the boost inductor is estimated using equation. by assuming the cause of 20% approximation and is established to be, LPV = 0.2 mH.
On obtaining the boost inductor, the input capacitance also can easily be obtained. The continuous conduction mode (CCM) of the boost converter is assumed by the analysis. The capacitance see the voltage ripple from the PV array MPP as ΔVpv according to the variation of temperature and irradiance. Since temperature changes from −20˚C to 40˚C, the open circuit voltages changes. Consequently the open circuit of the PV string voltage change is by, ΔVpv = 5 V. Thus, PV input capacitance is significant to voltage source inverters (VSI). The model of the PV modules is converted and it becomes voltage source to the inverter. It maintains the voltage stable and decreases power fluctuation at the input.
230 V is the mentioned output AC voltage of the inverter as VSI single phase. According to this value, the estimated voltage value of 400 V at the DC bus is attained. At the DC link capacitance this voltage is viewed and it is called as power coupling capacitor. Using Equation (2), the DC link voltage is assumed as the bus voltage, bus V. 10% from the bus voltage or link voltage is the ripple voltage in this design. Sticking to the reason, 10% is given and the approximated DC bus voltage of 400 V is attained. Besides the benefits discussed already, the paramount function of the. The net nominal power from the strings is assumed as PPV = 1 kW at the input voltage VPV = 220 V is to be the output and it is to be delivered to the DC bus. On account of substituting the grid frequency of 50 Hz the value of DC link capacitor CDC = 100 μF is contrived. The summary of the calculated values are given in
Control strategies of the inverter power stage
In order to control voltage source, VSI power stage, various techniques are mentioned. Including hysteresis band, predictive, and sinusoidal pulse width modulation (SPWM) control, it is three important output current control techniques for the single phase VSI. Various techniques of PWM are explored in which one is concentrated and it will be considered the very apt one to control the inverter power stage. On the technique Pulse Width Modulation (PWM), The DC-AC inverters are usually operated.
Parameter | Value |
---|---|
Duty ratio | 0.68 |
Switching frequency | 10 kHz |
Input inductance, Lpv | 0.2 mH |
Input capacitance, Cpv | 170 µF |
DC-Link capacitance, CDC | 100 µF |
Ripple voltage, ∆V | 40 V |
This modulation technique has multiple numbers of output pulses per half cycle and the pulses have different width. According to the amplitude of a sine wave calculated at the centre of the same pulse, the width of each pulse is varying in proportionately. Having compared a sinusoidal reference signal with a high frequency triangular signal, the gating signals are generated. The inverter output voltage is determined by the reference signal frequency. According to the SPWM generating techniques, the triangle waveform tri V is at switching frequency s f; this frequency restricts the speed at which the inverter switches are turned on and off. The control signal control V is applied to modulate the switch duty ratio and has a frequency 1 f. This is the basic frequency of the inverter output voltage. Switching frequency affects the output of the inverter that results in harmonics at the switching frequency.
The power stage inverter is controlled and switched by IP cores. Generating 10 kHz switching frequency FPGA DDS Triangle Gen IP.vi is used. On comparing the phase sine wave signal from the grid, the pulses are obtained. This comparison output is fed into the inverter power stage for scheming opening and closure of the switches. The signals of the switches SW 1 to SW 4 of the DC to DC stage open/close switches TSW 1 to TSW 4 and signals of the switches s 1 to s 4 of the DC to AC stage open/close switches TS 1 to TS 4 are restricted by SPWM with the help of voltage switching. Each leg is controlled autonomously of the other. 0.68 is the set duty ratio to maintain the ratios of the pulses to the switches. The switches SW 3 and SW 4 are closed and opened by the pulses. PWM switching pulses as simulated in mat lab.
The designed dual power stage is shown in
The implemented inverter power stage its circuitry design is designed by Matlab. The circuits are controlled by MATLAB and it is made as a suitable user interface. The generated IPWM from the IP cores controls output voltage of the inverter. The different
control function blocks are illustrated clearly in
The output waveform of the inverter with power stage is shown in Figures 6-8. It can be seen that the phase output voltage with pure sinusoidal waveform.
This paper is focused on modeling and MPPT technology device grid connected Photovoltatic system. A causal information method and its proprieties are permitted to determine all elementary models and to calculate the PV-side and grid-side controllers. In this way, an MPPT controller is used to take out the most favorable photovoltaic power. A current and a dc link voltage regulator are used to transfer the photovoltatic power and to synchronize the output voltage with the grid system. The simulated model and
the results obtained for standard operating conditions are shown the performances of the grid connected photovoltaic.
Kirubasankar, K. and Kumar, Dr.A.S. (2016) Inverter Power Stage Connected with PV-Grid. Circuits and Systems, 7, 4113-4123. http://dx.doi.org/10.4236/cs.2016.713339