Current source inverter

ABSTRACT The growth of the world’s population leads to increased energy needs. The use of fossil energy leads to challenges. The main reason for this is that the use of fossil fuels has a negative impact on global climate change, and the world is heading towards running out of fossil fuels in a few years. Renewable energy sources like photovoltaic and wind power plants are good solutions for those challenges. For the generation of clean energy, batteries are not necessary. Most renewable energy sources are intermittent in nature, which is why battery energy storage systems could be useful. There are mainly two types of converters for converting direct current to alternating current. They are voltage-source inverters and current source inverters. A voltage source inverter is well-known and widely used in many industrial applications. But a little research has been done about current source inverters due to some problems such as open circuits and large inductances on the DC side. The project deals with the Current Source Inverter for Battery Energy Storage System. The main objective of this project is to model, design, control, and simulation a current source inverter and analyse the advantages and disadvantages of both converters, with focus on the current source inverter. In this thesis, a cascaded control structure is used for controlling three-phase grid-connected CSI. Current and voltage control design and simulation of CSI will be the focus. The proportional integral controller (PI-controller) is used to control the inverter. To verify the theoretical study, a simulation model in MATLAB/Simulink is built and verified. Keywords Current source inverter, voltage source inverter, battery energy storage system, pulse-width modulation, proportional integral control and grid-connected

Power quality improvements of single-phase grid-connected photovoltaic systems

PhD ThesisThe number of distributed power generation systems (DPGSs), mostly based on photovoltaic (PV) energy sources is increasing exponentially. These systems must conform to grid codes to ensure appropriate power quality and to contribute to grid stability. A robust and reliable synchronization to the grid is an important consideration in such systems. This is due to the fact that, fast and accurate detection of the grid voltage parameters is essential in order to implement stable control strategies under a broad range of grid conditions. The second-order generalized integrator (SOGI) based phase-locked loop (PLL) is widely used for grid synchronization of single-phase power converters. This is because it offers a simple, robust and flexible solution for grid synchronization. However, the SOGI-PLL is affected by the presence of a dc offset in the measured grid voltage. This dc voltage offset is typically introduced by the measurements and data conversion process, and causes fundamental-frequency ripple in the estimated parameters of the grid voltage (i.e. voltage amplitude, phase angle and frequency). In addition to this ripple, the unit amplitude sine and cosine signals of the estimated phase angle (i.e. unit vectors), that are used to generate reference signals in the closed-loop control of grid-connected PV converters will contain dc offset. This is highly undesirable since it can cause dc current injection to the grid, and as a consequence, the quality of the power provided by the DPGSs can be degraded. To overcome this drawback, a modified SOGI-PLL with dc offset rejection capability is proposed. The steady-state, transient and harmonic attenuation performance of the proposed PLL scheme are validated through simulation and experimental tests. The overall performance demonstrates the capability of the proposed PLL to fully reject such dc current injection as well as to provide a superior harmonic attenuation when compared with the SOGIPLL and two other existing offset rejection approaches. It is shown that, the proposed PLL scheme can enhance the overall total harmonic distortion (THD%) of the injected power by 15% when compared to the conventional SOGI-PLL. In addition to the synchronization, grid-connected PV systems require a current control scheme to regulate the output current. Due to the simple implementation, proportional-integral (PI) controllers in the stationary reference frame are commonly used for current controlled inverters. However, these PI-controllers exhibit a major drawback of failure to track a sinusoidal reference Abstract ii without steady-state error, which may result in low-order harmonics. This drawback can be overcome if the PI-controllers are implemented in direct-quadrature (dq) rotating reference frame. In single-phase systems, the common approach is to create a synthesized phase signal orthogonal to the fundamental of the real single-phase system so as to obtain dc quantities by means of a stationary-to-rotating reference frame. The orthogonal synthesized signal in conventional approaches is obtained by phase shifting the real signal by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. This thesis proposes an alternative way of implementing such PI-controllers in the dq reference frame without the need of creating such orthogonal signals. The proposed approach, effectively improves the poor dynamic of the conventional approaches while not adding excessive complexity to the controller structure. The results show that, in addition to its ability to regulate the current and achieve zero steady-state error, the proposed approach shows superior dynamic response when compared with that of conventional delay-based approach.Libyan Governmen

Modelling and optimisation of solar voltaic system using fuzzy logic

There is considerable increase in residential solar grid connected installations with many advantages offered by solar energy. As more solar panels are connected to grid, the Solar Inverter between solar panels and grid have to perform at optimum levels. Modern Inverters consist of DC-DC Converter and DC-AC Inverter. One problem associated with Inverter design is voltage fluctuation, this defect lies in the DC-DC converter Maximum power tracking (MPPT) algorithms responsible for extracting maximum power from the solar panels. The defect is due to large sampling number required for conventional MPPT algorithm. This thesis has proposed a new MPPT algorithm based on Mamdani Fuzzy logic. In research we use 5 parameter one diode model for solar cell modelling. The P-V/I-V characteristics curve is generated. The P-V characteristics curves sectioned and input membership and output membership functions is created. And unique fuzzy rules is used to optimize fuzzy controller output. Mamdani Fuzzy logic algorithm is compared to traditional PI controller hill climbing method. When small sampling number is used hill climbing method response is slow and good at tracking. When big sampling number is used hill climbing method response is fast and not good at tracking. The voltage also fluctuates when sampling number is big. Fuzzy logic provides a compromised solution with best response time and moderate tracking accuracy compared to hill climbing method. Fuzzy Logic based DC-DC converter together with PLL and Recursive Discrete Fourier Transform (RDFT) DC-AC inverter synchronization algorithm is employed and simulated in matlab. The MPPT simulation is conducted for a realistic 2.5KW solar panels in a 8 x 2 Matrix. In addition the MPPT algorithm is analyzed to see if it performs under power quality and voltage level tolerance of utility grid requirements. The Fuzzy Logic MPPT is excellent at tracking power. When temperature is fixed and irradiance is varied, the maximum tracking error is 5.2% in all scenarios with one exception. When irradiance is fixed and temperature varied, the maximum tracking error is 1.98%. Furthermore the Fuzzy Logic MPPT meets the power quality and voltage level tolerance requirements of utility grid for irradiance over 600 W/m2. Power quality and voltage level tolerance requirements for irradiance under 600 W/m2 is not critical as this is outside twilight conditions. Out of all the Synchronization algorithm identified in this Thesis, RDFT achieves synchronization very quickly and in addition it suppresses harmonics and noise. The possibility of future study to extend MPPT is also briefly discussed. The extension of future study is using Takagi-Sugeno fuzzy logic. Takagi-Sugeno uses more sophisticated inference and rule evaluation mathematics

Energy Storage as Enabling Technology for Smart Grid

Awareness about human impact on mighty climatic changes is radically changing our concept of energy. The thoughtless use of energy slowly leaves our habits and good use of energy is certain the way of a better future. CO2 emission reduction and carbon fossil fuel limitation are the main targets of governmental actions: this is possible thanks to technology improvement as efficient generation from renewable sources and good management of the electricity network. In recent years distributed generation, also of small size, grew up causing new management problems, indeed production from renewable energy sources (RES) is intermittent and unprogrammable. Energy storage systems can be a solution to these problems and pave the way to completely active users, grid parity and smart grid, moreover can be an useful tool to increase electricity access in rural areas. Research on energy storage is intrinsically a multidisciplinary field: storage types, power stages, technologies, topologies, weather, forecast, control algorithms, regulatory, safety and business cases to mention the most importants. In the present work is described the whole design of an energy storage system. First chapters are dedicated to a description of energy storage context, chapters 1 and 2; indeed, it is a matter of fact that in the last years, energy storage became more and more interesting from explicit mention as a tool against climatic changes to first options on the market. The general approach was the realization of a modular energy storage system for residential application, hardware and software design steps are deeply described in chapters 3 and 4. Simulations and tests on the prototype are reported in chapter 5. Finally conclusion and future works are given. At the end of the document some appendices are included to cover specific aspects touched during the work thesis

Wind turbine type IV simulation with grid-following and grid-forming control

The demand for renewable energy is rapidly increasing for several reasons, including global warming, environmental effects, and energy independence for nations that now import the ma- jority of their energy needs. Europe, as a whole, imports more fossil fuels than any other region in the globe, but the continent made the bold choice to become carbon neutral by 2050. Utilizing as much renewable energy as possible, including wind and solar, will be crucial to meet that goal. In many European nations, wind energy is one of the major sources of electricity, and its proportion will rise. Since wind power over the oceans and seas is more plentiful and powerful than wind power on land, off-shore wind farms are one of the keys to advancing the higher penetration of renewables. Repairs and maintenance in offshore wind farms can be complex, in addition to the difficulties brought on by activities on the sea and the problems caused by humidity. The wind turbine Type IV is typically utilized in off-shore applications to reduce maintenance concerns since it may be gearless and may use a permanent magnet generator and full-scale converter. As the penetration of renewables with power converters grows, new control approaches must be considered, and the benefits and drawbacks of each control structure must be weighed. A wind turbine Type IV model with grid-following and grid-forming control was built in Simulink and PSCAD during this project, and their behavior in an IEEE 9-bus system was evaluated at the completio

Seamless Transition of a Microgrid Between Grid-Connected and Islanded Mode

This thesis focuses on improving the behavior of inverters during transition periods from islanded mode to grid-connected mode (GC) and vice-versa. A systematic approach is presented to add smart features to inverters to enhance their capability to cope with sudden changes in the power system. The importance of microgrids lies in their ability to provide a stable and reliable source of power for critical loads in the presence of faults. For this purpose, a design is proposed consisting of a distributed energy resource (DER), battery energy storage system (BESS) and a load connected through a bypass switch with the main utility distribution substation. The BESS is connected to the AC distribution feeder through a smart inverter that is controlled in both modes of operations. The system was tested using MATLAB/Simulink models and the results show proof of the seamless transition between the two modes of operation. The cost of building the software system was unnoticeable due to the availability of a MATLAB license but the real cost of the hardware needed to build the system will be moderate though the importance will be significant

Circuit Structure and Control Method to Reduce Size and Harmonic Distortion of Interleaved Dual Buck Inverter

A new circuit structure and control method for a high power interleaved dual-buck inverter are proposed. The proposed inverter consists of six switches, four diodes and two inductors, uses a dual-buck structure to eliminate zero-cross distortion, and operates in an interleaved mode to reduce the current stress of switch. To reduce the total harmonic distortion at low output power, the inverter is controlled using discontinuous-current-mode control combined with continuous-current-mode control. The experimental inverter had a power-conversion efficiency of 98.5% at output power = 1300 W and 98.3% at output power = 2 kW, when the inverter was operated at an input voltage of 400 V-DC, output voltage of 220 V-AC/60 Hz, and switching frequency of 20 kHz. The total harmonic distortion was < 0.66%, which demonstrates that the inverter is suitable for high-power dc-ac power conversion.11Ysciescopu