This study deals with a single-phase single-stage multifunctional grid interfaced solar photo-voltaic (SPV) system. The proposed SPV system is multifunctional as it has MPPT (maximum power point tracking) and it provides harmonics elimination, reactive power compensation and feeding SPV energy into the grid at unity power factor. The PV array is connected at the DC link of voltage source converter. The MPPT controller estimates reference PV voltage and a proportional integral controller is used to maintain the PV string voltage to the reference value. Feed-forward terms are used for SPV array and load components for fast dynamic response. The performance of the system is analysed under abnormal grid (sudden sag and swell) conditions. Simulation studies are performed on MATLAB-based platform. Simulation results are verified experimentally on a developed SPV system. Under steady-state operating conditions, the total harmonic distortion of grid voltage and currents are found well under IEEE-519 standard. A wide range of simulation and experimental results are presented to demonstrate all the features of the proposed SPV system.

This study presents a high-performance photovoltaic (PV)-battery hybrid power conversion system (HPCS) which is integrated to a microgrid using a quasi-Z-source inverter (qZSI). The battery system is directly connected to the intermediate dc link of the qZSI without any extra converter which reduces the size and the cost of the power electronic interface. A controller equipped with a harmonic compensator is designed to control the battery current in the grid-connected mode and to regulate the microgrid voltage in the islanded mode in the presence of unbalanced and non-linear loads. Furthermore, the proposed controller adjusts the shoot-through duty cycle of the qZSI to regulate the PV voltage to its reference value provided through the maximum power point tracking algorithm. Moreover, the proposed controller provides the microgrid with the capability to transfer between the two operation modes. A new strategy is adopted to keep the HPCS operational in case of severe reduction in the available power of the PV modules and its consequent voltage drop. The performance of the proposed control system is evaluated for the control of a typical microgrid simulated in MATLAB/SIMULINK environment.

The prescribed perspective of high accession of distributed generation photovoltaic (DG-PV) units on low-voltage distribution networks has brought up many issues regarding their performance in case of network transient phenomena. In this study, the design principles that DG-PV units must incorporate in order to meet the requirements of low-voltage ride through capability (LVRTC) are investigated through detailed theoretical analysis and calculations. The analysis shows that a realistic over-sizing of interfaced inverters of DG-PV as well as an appropriate selection of the equivalent interconnecting reactance X _DG, in conjunction with high penetration levels, can lead to satisfaction of LVRTC demands without violating the protection limits of the network. Moreover, the scenario of uniform dispersion along the distribution lines is proved rather favourable concerning the demand of LVRTC for voltage selectivity. Finally, considering the derived outcomes, a methodology of optimum design about LVRTC is proposed within the framework of reasonable constraints, which can be applied to any low-voltage distribution network.

A system consisting of a capacitor-excited induction generator (CEIG) with associated power electronic converters has been developed for supplying power to a micro-grid. The power fed to the grid from the CEIG is controlled using a diode bridge rectifier (DBR) and a pulse width modulated (PWM) inverter, connected between the generator terminals and the grid. A simple analog based hysteresis current control (HCC) technique has been employed in which the current control of the PWM inverter alone needs to be carried out by sensing the current and voltage at the grid terminals. The successful working of the system has been demonstrated for three different patterns of feeding power to a micro-grid, by experiments conducted on a three-phase 230 V, 3.7 kW CEIG, with a 100 µF delta connected excitation capacitor bank and employing the controllers fabricated in the laboratory. The grid power is represented as an equivalent load resistance in the steady-state equivalent circuit of the CEIG and the technique of genetic algorithm (GA) has been adopted for the analysis of the proposed system. The predetermined performance characteristics of the system and the results of Matlab/Simulink simulation studies have also been presented.

Renewable energy sources (RESs) such as solar energy are cost-effective to meet part of the energy needs. However, the inherent fluctuation and intermittence of RESs may deteriorate the stability and security of power grids. Energy storage systems can mitigate the problem, but they are very expensive. For this reason, a coordinated control method of virtual power plant (VPP), which includes photovoltaic systems (PVs) and controllable loads, is proposed in this study so that the aggregated power output of the VPP can be flexibly adjusted in a wide range. To achieve this, power output of the PVs and operational modes of controllable loads are coordinated by solving a mixed integer programming (MIP) problem. Meanwhile, with a quadratic interpolation based active power control strategy, each PV can operate in a power dispatch mode and simultaneously estimate its maximum available power, which is an input to the MIP problem. Externally, the VPP can quickly adjust the aggregated power and achieve functions important to power systems with high penetration of distributed energy resources, such as primary frequency regulation. Simulation results validate the effectiveness of VPP in providing frequency support to an island microgrid.

The design of battery storage systems includes technology choices for the batteries and for the inverter. The impact of the inverter design on the optimal design and operation of the storage system has not been analysed before. Therefore four inverter designs are compared with this research. The most basic inverter model assumes only symmetric active power exchange; the most advanced inverter model allows interphase active power transfer and reactive power control. A multi-objective optimisation method is used, to visualise the trade-offs between two technical objective functions for cycling control – voltage regulation and peak power reduction – for a given annual cost. The method is applied to a real-world scenario, based on an existing feeder in a residential part of a city in Flanders, Belgium. Internal losses and losses in the grid are quantified for the different designs. Modelling a battery storage system purely as a finite source/sink of active power in a low-voltage grid, strongly underestimates the potential because of the existing phase unbalance. Counteracting phase unbalance through an inter-phase power transfer capable inverter, even more so than adding reactive power control, improves the performance of battery storage systems.

Energy storage systems have established their capability to overcome the problems caused by intermittent nature of renewable sources when integrated to existing grid. Voltage and frequency control, as well as load shifting can be done using grid level storage systems incorporated with renewable sources. They can effectively serve the system as an energy sink and source. Operational use of energy storage for grid level support of a wind electric generator (WEG) is demonstrated in this study. The battery storage supported WEG along with controllers is modelled. Artificial neural network controller, which is having inherent learning capability, is developed to regulate the power flow between wind generator and utility grid. The proposed algorithm and the corresponding controller are simulated in MATLAB/Simulink and implemented in the DSP processor. The real time data exchange between Simulink and the floating point DSP processor TMSF32028335 is realised using on-board JTAG emulator. The hardware implementation using DSP processor presented in this work establishes the efficacy of the proposed control strategy for real time applications.

Multicell multilevel voltage source power converters are the state-of-the-art and key elements for harnessing and integration of medium-voltage high-power renewable energy resources. This study proposes a novel hybrid topology for double flying capacitor multicell (DFCM) converters that is suitable for grid-tied renewable energy systems. The proposed power converter is realised by cascade connection of one DFCM converter and one full-bridge converter (FBC) comprising a floating-capacitor. The floating-capacitor voltage of the FBC is regulated naturally at its requisite voltage level because of the existence of the abundant redundant charging/discharging states provided by the proposed switching and modulation pattern for the suggested hybrid converter. Hence the proposed control technique not only preserves the natural balancing for the FC voltages of the DFCM converter but also provides natural balancing for the floating-capacitor voltage of the FBC. The main objective of the FBC is to generate additional intermediate voltage steps to double the number of DFCM converter's voltage levels for enhancing power quality significantly. The proposed hybrid DFCM converter is simulated to inject/absorb active/reactive-power provided and harvested from renewable energy resources into/from electric power grid. Furthermore, experimental measurements of the proposed hybrid power converter are also presented to validate its viability, merits, effectiveness and the proposed modulation and control strategy.

This study presents a methodology for siting and sizing static synchronous compensator (STATCOM) devices in the Portuguese transmission system in order to improve system security following severe grid faults. Security issues arise since the Portuguese transmission system incorporates significant levels of wind generation without fault ride through and reactive current injection capabilities during grid faults. As the transmission system operator (TSO) is responsible for assuring system security, the goal of the study is to take advantage of the proved STATCOM ability for injecting reactive current in order to mitigate the disconnection of large amounts of wind farms in case of severe grid faults. The proposed methodology was developed and tested in coordination with the Portuguese TSO and it is based on the formulation of an optimisation problem in order to minimise the installed STATCOM power while ensuring its compliance with the current grid code requirements, namely in what concerns to the system stability and security. Given the discrete and complex nature of the problem, a hybrid approach, combining both a heuristic method and an evolutionary particle swarm optimisation (EPSO) algorithm was developed. Results show the effectiveness of the proposed methodology as well as its robustness regarding the validity of the obtained solutions while facing the most severe operational scenarios.

This study presents the creation of a new simulation tool,MATACDC. It is the first open source program for power flow analysis of high voltage direct current (HVDC) grids and hybrid AC/DC systems and uses state-of-the-art developments in the field of HVDC grids research. MATACDC is based on MATLAB and has been fully integrated with the AC system power flow routines from MATPOWER. The software includes all the models needed to study the steady-state interaction of AC and DC systems for a wide range of converter representations and control functions. Any combination of multiple non-synchronised AC systems and multiple DC systems can be solved. The code is freely available and is intended for researchers and students working in the field of HVDC grid steady-state interactions and HVDC grid operation. MATACDC can also easily be extended with user-defined functionality. The study focuses on the program design and layout, the converter modelling, the practical implementation and the integration with AC power flow routines. Furthermore, different examples of possible user-defined functions show how the tool can be extended to include other control representations to study their effect on overall system interactions. Simulation results demonstrate the viability of the routines to simulate complex hybrid AC/DC systems.

Wind power is one of the most developed renewable energy resources worldwide. To integrate offshore wind farms to onshore grids, the high-voltage direct current (HVDC) transmission cables interfaced with voltage source converters (VSCs) are considered to be a better solution than conventional approaches. Proper DC voltage indicates successive power transfer. To connect more than one onshore grid, the DC voltage droop control is one of the most popular methods to share the control burden between different terminals. However, the challenges are that small droop gains will cause voltage deviations, while higher droop gain settings will cause large oscillations. This study aims to enhance the performance of the traditional droop controller by considering the DC cable dynamics. Based on the backstepping control concept, DC cables are modelled with a series of capacitors and inductors. The final droop control law is deduced step-by-step from the original remote side. At each step the control error from the previous step is considered. Simulation results show that both the voltage deviations and oscillations can be effectively reduced using the proposed method. Further, power sharing between different terminals can be effectively simplified such that it correlates linearly with the droop gains, thus enabling simple yet accurate system operation and control.