Explain the different stages in a PV power conversion circuit and their function

Question

Explain the different stages in a PV power conversion circuit and their function. The "MPPT tracker, " the "DC-DC converter", the "PWM converter" and the "filtering" stage. Draw generic current-voltage and power-voltage curves for a solar panel at two different levels of solar insolation. Use these charts to illustrate the need for the DC-DC converter and the MPPT tracker. What are the 3 different kinds of PWM converters that can be used and what output do they produce? What are their advantages and disadvantages? What is the output waveform of the filtering stage?

Explanation / Answer

The PV Power Conversion System (PCS) plays an important role in renewable energy source based power generation. The PCS converts the available DC supply from the source into required AC voltage (415 V, 50 Hz) as specified by the utility grid by means of power electronic converter and is feed into the grid. This chapter focuses on the components of a PV grid connected system and its characteristics. The various power conversion systems are studied in literature and compared based on efficiency, cost and volume from which a suitable power converter for PV power conversion system is suggested to obtain high reliability and high efficiency with low cost. Then a brief literature review of transformerless grid connected system is carried out to identify the setbacks of conventional transformerless inverter. Finally synchronization methods of grid connected system are discussed. 2.2 SYSTEM DESCRIPTION Figure 2.1 shows the block diagram of a grid connected PV system. It consists of PV plants, MPPT controller, PWM controller, power conditioner (inverter), and filter. PV plant converts the sunlight into DC power, and a power conditioning unit that converts the DC power to AC power. The 17 generated AC power is injected into the grid and/or utilized by the local loads through the filter. In some cases, the PV system is combined with storage devices which improve the availability of the power. The subsequent section provides more details about various components of the PV system. MPPT Controller PWM Controller DC to AC Inverter 3Ø PV Filter Grid Array PLL Figure 2.1 Block diagram of PV grid connected inverter 2.3 PV ARRAY MODEL The PV plant consists of PV cells and it is arranged in series and parallel combination to supply the desired DC voltage and current. Normally PV cell is made up of silicon semiconductor and each silicon cell generates 0.6V. The commercially available PV module consists of 36 or 72 cells connected in series to form a PV plant. The typical PV module used for simulation parameters are listed in Table 2.1. 18 Table 2.1 Electrical parameters of BP 3235T PV panel at standard test condition G=1000W/m2 C PV Parameters Ratings Maximum output Power (PMax) 235 W Power Tolerance (PT) [0 to 5] W Percentage Efficiency of Module ( ) 15.0% Voltage at maximum power point (VMPP) 30.0 V Current at maximum power point (IMPP) 7.84A Open circuit voltage (Voc) 37.44 V Short circuit current (Isc) 8.83 A The simple equivalent circuit of the PV cell model is shown in Figure 2.2. It consists of ideal current source in parallel with the diode. The practical model of PV cells consists of series resistance (Rs) and parallel resistance (Rp) and is shown Figure.2.3. + D VD ISc Vpv ID I + - - Figure 2.2 Basic equivalent circuit of PV cell From the basic model, output current (I) is represented in Equation (2.1) and diode current is given in Equation (2.2). sc D I = I - I (2.1) 19 qvd kT d 0 I =I e -1 (2.2) + - D VD Iph Vpv ID I + - Rs RP Figure 2.3 Practical equivalent circuit model of PV cell The voltage current (Vg Ig) relation of the PV plant is given in the literature Rauschenbach (1980). s s ph 0 s t sh V+IR V+IR I=I -I exp -1 - n V R (2.3) where V - Module voltage I - Module current Iph - Photon generated current I0 - Dark saturation current respectively Vt - Junction thermal voltage, Rs is the series cell resistance, and Rsh cell shunt resistance ns - number of cells connected in series Is - saturation current, K - Boltzmann constant 1.38×10-23 J / K 20 A - solar cell ideal factor of the diode, q - electron charge 1.6×10-19 C t KTA V = q The photo current mainly depends on the solar irradiance and cell temperature (Tsai et al 2012). ph sc i c ref I =(I +k (T -T ))G where Isc - Shortcircuit current of PV cell at 25°C G - Solar insolation (kW/m2 ). = (1 kW/m2 ), ki - Temperature coefficients T-ref- Reference temperature of the cell. 2.3.1 Characteristics of PV Module The voltage-current (V-I) characteristics and power-voltage characteristics (P-V) of the PV plant are illustrated in Figures 2.4 (a) and (b) respectively and it shows that both V-I and P-V curves are nonlinear because PV panel is made of semiconductor material. The Maximum Power Point (MPP) curve is shown in Figure 2.4 (c). The output power of PV plant is maximum at certain value of voltage (VMPP) and this point is called as maximum power point. The corresponding voltage (VMPP) and current (IMPP) at Maximum Power Point (MPP) are denoted in Figure 2.4 (c).

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