Power configuration for PV systems PV Cells Configuration Central Inverter 10 kW -250 kW, 3 phase several string in parallel High efficiency, low cost, low reliability, not optimal MPPT Used for power plant String inverters 1.5 – 5 kW, typical residential applications Each string has its own inverter enabling better MPPT The strings can have different orientations Three phase inverter for power < 5 kW Module inverters 50 – 180 W, each panel has its own inverter Lower efficiency, difficult maintenance Higher cost / kW p
Power Configurations for PV Systems Without boost /with boost of dc voltage Galvanic isolation necessary some places LF/HF transformer (Cost-Volume issue) The optimal topology is not matured yet as for drives Transformer less topologies having higher efficiency are emerging
Equivalent circuit for a PV cell Vpv : PV array output voltage (V) Ipv : PV array output current (A) Ns : number of cells connected in series np : number of strings connected in parallel q: charge of electron k : Boltzmanconstant A :pnjunction ideality factors (2.15) T : Cell temperature Irs : Cell reverse saturation current Iscr : cell short circuit current at reference temperature and radiation ki: short circuit current temperature coefficient s : solar radiation Using this PV array model it is possible to simulate the dynamic performance of the power and control systems and MPPT strategy in response to the radiation and temperature step changes
V-I characteristic under different irradiance conditions Behavior of system under different irradiance value, bigger irradiance value the bigger value of V-I
Generic Single Phase H-bridges Converter a. Generic single phase string converter b. Generic single phase H-bridges converter
Cascaded Multilevel Inverter Features can generate output voltage with extremely low distortion and lower dv/dt. circuit layout has more flexibility. compared to other multi level topologies, CMC requires least number of components, because there is no need for clamping diodes and flying capacitors. The number of output voltage levels can be easily adjusted by adding or removing the H-Bridges cells.
Cascaded Multilevel inverter System Overview
Inverter topology and carrier shifting control scheme
Switching Pattern of 11-level cascade inverter
PWM Control features pros To balance power sharing between the modules, this switching scheme gives a very balanced averaged power sharing between all modules. Cons In reality, inter cycle difference of power still exists in each module.
General control methodology of Converter Shows the block diagram of generalized voltage control for any k-thmodule.
Voltage Control Loop of Each module Describe how to generated DC voltage of each module of PV module. The corresponding digital filtered voltages Vdckare sent to next stage where the power control is carried out.
Power Control Loop Power Control loop output is Vht, whose Vht that must be synthesized by the converter.
Phase locked loop (PLL) configuration Typical PLL algorithms include inverse park-based PLL, hilbert transformer-based PLL, and transport delay-based PLL
Simplified output circuit and unitary factor power operation Power flow controlled according to
MPPT algorithms Instead of sensing the individual panel voltages (depend on temperature and irradiance), MPPT algorithms determines the optimal point of operation of the panel by calculating the output power and phase angle variation. It monitor output voltage and current parameters by making small changes on phase angle and looking at the power variation In this PV configuration a perturbation and observation (P&O) algorithms has been used.
Conclusions This project uses string or multi string array configuration, and multilevel converter without DC-DC converter and without isolation (transformer less). This project uses 11-level PWM H-bridge cascade PV converter, which uses PLL and MPPT with separate solar DC panel DC source to enable grid interaction. Each PV array will have its specific MPP. And MPPT algorithms using P&O algorithms. Present new control strategy for CMC to gain the best performance of the systems under all environmental conditions.
Future Works Implementation of MPPT algorithms each solar array. Implementation Of PWM control. Finding best filter configuration for inverter output. Implementation Of PLL to inverter. Making general controls methodology working properly.