Thiophene-benzothiadiazole-thiophene (D-A-D) based polymers : Effect of donor/acceptor moieties adjacent to D-A-D segment on photophysical and photovoltaic properties

New push-pull copolymers based on thiophene (donor) and benzothiadiazole (acceptor) units, poly[4,7-bis(3-dodecylthiophene-2-yl) benzothiadiazole-co- thiophene] (PT3B1) and poly[4,7-bis(3-dodecylthiophene-2-yl) benzothiadiazole-co-benzothiadiazole] (PT2B2), are designed and synthesized via Stille and Suzuki coupling routes respectively. Gel permeation chromatography shows the number average molecular weights are 31100 and 8400 g mol-1 for the two polymers, respectively. Both polymers have shown absorption throughout a wide range of the UV-vis region, from 300 to 650 nm. A significant red shift of the absorption edge is observed in thin films compared to solution of the copolymers; the optical band gap is in the range of 1.7 to 1.8 eV. Cyclic voltammetry indicates reversible oxidation and reduction processes with HOMO energy levels calculated to be in the range of 5.2 to 5.4 eV. Upon testing both materials for organic field-effect transistors (OFETs), PT3B1 showed a hole mobility of 6.1 × 10-4 cm2 V-1 s -1, while PT2B2 did not show any field effect transport. Both copolymers displayed a photovoltaic response when combined with a methanofullerene as an electron acceptor. The best performance was achieved when the copolymer PT3B1 was blended with [70]PCBM in a 1:4 ratio, exhibiting a short-circuit current of 7.27 mA cm-2, an open circuit voltage of 0.85 V, and a fill factor of 41% yielding a power conversion efficiency of 2.54% under simulated air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm-2). Similar devices utilizing PT2B2 in place of PT3B1 demonstrated reduced performance with a short-circuit current of 4.8 mA cm -2, an open circuit voltage of 0.73 V, and a fill factor of 30% resulting in a power conversion efficiency of roughly 1.06%.

A new method of interfacing battery/supercapacitor energy storage systems for distributed energy sources

Battery-supercapacitor hybrid energy storage systems can achieve better power and energy performances compared to their individual use. These hybrid systems require separate dc-dc converters, or at least one dc-dc converter for the supercapacitor bank, to connect them to the dc-link of the grid connecting inverter. However, the use of such dc-dc converters introduces additional cost and power losses. Therefore, the possibility of direct connection of energy storage systems, to the dc-link of a diode clamped 3-level inverter is investigated in this paper. Even though the proposed topology does not use dc-dc converters, a vector selection method is proposed to produce a similar control flexibility that is found in the separate dc-dc converter topology. The major issue with the proposed system is the imminent imbalance of the neutral point potential. A PWM technique with modified carriers is used to solve this problem. Simulations are carried out using MATLAB/SIMULINK to verify the efficacy of the proposed system.

A novel dc-link voltage regulation method for single source hybrid multilevel inverters

This paper presents a novel dc-link voltage regulation technique for a hybrid inverter system formed by cascading two 3-level inverters. The two inverters are named as “bulk inverter” and “conditioning inverter”. For the hybrid system to act as a nine level inverter, conditioning inverter dc link voltage should be maintained at one third of the bulk inverter dc link voltage. Since the conditioning inverter is energized by two series connected capacitors, dc-link voltage regulation should be carried out by controlling the capacitor charging/discharging times. A detailed analysis of conditioning inverter capacitor charging/discharging process and a simplified general rule, derived from the analysis, are presented in this paper. Time domain simulations were carried out to demonstrate efficacy of the proposed method on regulating the conditioning inverter dc-link voltage under various operating conditions.

A Bit-Stream based space vector modulator

Bit-Stream based control, which uses one bit wide signals to control power electronics applications, is a new approach for controller design in power electronic systems. Bit-Stream signals are inherently high frequency in nature, and as such some form of down sampling or modulating is essential to avoid excessive switching losses. This paper presents a novel three-phase space vector modulator, which is based on the Bit-Stream technique and suitable for standard three-phase inverter systems. The proposed modulator simultaneously converts a two phase reference to the three-phase domain and reduces switching frequencies to reasonable levels. The modulator consumes relatively few logic elements and does not require sector detectors, carrier oscillators or trigonometric functions. The performance of the modulator was evaluated using ModelSim. Results indicate that, subject to limits on the modulation index, the proposed modulator delivers a spread-spectrum output with total harmonic distortion comparable to standard space vector pulse width modulation techniques.

A dual inverter with integrated energy storage for wind power systems

This paper explores the possibility of using grid side inverter as an interface to connect energy storage systems. A dual inverter system, formed by cascading two 2-level inverters through a coupling transformer, is used as the testing model. The inverters are named as “main inverter” and “auxiliary inverter”. The main inverter is powered by the rectified output of the wind generator while the auxiliary inverter is attached to a Battery Energy Storage System (BESS). If there is a surplus of wind power compared to the demand, then that would be stored in BESS while if there is a deficit in wind power then the demand will be satisfied by supplying power from the BESS. This enables constant power dispatch to the grid irrespective of wind changes. Novel modulation and control techniques are proposed to address the problem of non-integer, dynamically-varying dc-link voltage ratio, which is due to random wind changes. Furthermore, a maximum power tracking controller for this unique system is explained in detail. Simulation results verify the efficacy of proposed modulation and control techniques in suppressing random power fluctuations.

Design of a wind turbine - battery energy storage scheme to achieve power dispatchability

Battery energy storage system (BESS) is to be incorporated in a wind farm to achieve constant power dispatch. The design of the BESS is based on the forecasted wind speed, and the technique assumes the distribution of the error between the forecasted and actual wind speeds is Gaussian. It is then shown that although the error between the predicted and actual wind powers can be evaluated, it is non-Gaussian. With the known distribution in the error of the predicted wind power, the capacity of the BESS can be determined in terms of the confident level in meeting specified constant power dispatch commitment. Furthermore, a short-term power dispatch strategy is also developed which takes into account the state of charge (SOC) of the BESS. The proposed approach is useful in the planning of the wind farm-BESS scheme and in the operational planning of the wind power generating station.

Connecting two wind turbine generators to the grid using only one three level NPC inverter

This paper explores the possibility of connecting two Wind Turbine Generators (WTG) to the grid using a single three level inverter. In the proposed system the rectified output of one WTG is connected across the upper dc-link capacitor of a standard diode clamped three level inverter. Similarly the rectified output of the other WTG is connected across the lower capacitor. This particular combination has several advantages such as, direct connection to the grid, reduced parts count, improved reliability and high power capacity. However, the major problem in the proposed system is the imminent imbalance of dc-link voltages. Under such conditions conventional modulation methods fail to produce desired voltage and current waveforms. A detailed analysis on this issue and a novel space vector modulation method, as the solution, are proposed in this paper. To track the Maximum power point of each WTG a power sharing algorithm is proposed. Simulation results are presented to attest the efficacy of the proposed system.

Dual inverter based battery energy storage system for grid connected photovoltaic systems

This paper explores a new breed of energy storage system interfacing for grid connected photovoltaic (PV) systems. The proposed system uses the popular dual inverter topology in which one inverter is supplied by a PV cell array and the other by a Battery Energy Storage System (BESS). The resulting conversion structure is controlled in a way that both demand matching and maximum power point tracking of the PV cell array are performed simultaneously. This dual inverter topology can produces 2, 3, 4 and 5 level inverter voltage waveforms at the dc-link voltage ratios of 0:1, 1:1, 2:1 and 3:2 respectively. Since the output voltage of the PV cell array and the battery are uncorrelated and dynamically change, the resulting dc-link voltage ratio can take non-integer values as well. These noninteger dc-link voltage ratios produce unevenly distributed space vectors. Therefore, the main issue with the proposed system is the generation of undistorted current even in the presence of unevenly distributed and dynamically changing space vectors. A modified space vector modulation method is proposed in this paper to address this issue and its efficacy is proved by simulation results. The ability of the proposed system to act as an active power source is also verified.

A dual inverter based supercapacitor direct integration scheme for wind energy conversion systems

This paper presents a new direct integration scheme for supercapacitors that are used to mitigate short term power fluctuations in wind power systems. The proposed scheme uses the popular dual inverter topology for grid connection as well as interfacing a supercapacitor bank. The dual inverter system is formed by cascading two 2-level inverters named as the “main inverter” and the “auxiliary inverter”. The main inverter is powered by the rectified output of a wind turbine coupled permanent magnet synchronous generator. The auxiliary inverter is directly connected to a super capacitor bank. This approach eliminates the need for an interfacing dc-dc converter for the supercapacitor bank and thus improves the overall efficiency. A detailed analysis on the effects of non-integer dynamically changing voltage ratio is presented. The concept of integrated boost rectifier is used to carry out the Maximum Power Point Tracking (MPPT) of the wind turbine generator. Another novel feature of this paper is the power reference adjuster which effectively manages capacitor charging and discharging at extreme conditions. Simulation results are presented to verify the efficacy of the proposed system in suppressing short term wind power fluctuations.

Controller design for variable-speed permanent magnet wind turbine generators interfaced with Z-source inverter

Increased awareness of environmental concerns has caused greater interest in developing power sources based on renewable technologies, such as wind. Due to the intermittent nature of the wind speed, output voltage and frequency of the direct driven permanent magnet synchronous generators (PMSG) are normally unsteady. Recently proposed Z-source inverter has been considered as a potential solution for grid interfacing wind power generators, thanks to buck-boost function that the single stage Z-source inverter can offer. Two control methodologies, namely unified controller for isolated operation and a multi-loop controller for grid interfaced operation are investigated in this paper. Theoretical analysis of these two control schemes is presented and experimental results to verify the effectiveness of the control method are also included.

Space vector modulated cascade multi-level inverter for PMSG wind generation systems

Modulation and control of a cascade multilevel inverter, which has a high potential in future wind generation applications, are presented. The inverter is a combination of a high power, three level “bulk inverter” and a low power “conditioning inverter”. To minimize switching losses, the bulk inverter operates at a low frequency producing square wave outputs while high frequency conditioning inverter is used to suppress harmonic content produced by the bulk inverter output. This paper proposes an improved Space Vector Modulation (SVM) algorithm and a neutral point potential balancing technique for the inverter. Furthermore, a maximum power tracking controller for the Permanent Magnet Synchronous Generator (PMSG) is described in detail. The proposed SVM technique eliminates most of the computational burdens on the digital controller and renders a greater controllability under varying DC-link voltage conditions. The DC-link capacitor voltage balancing of both bulk and conditioning inverters is carried out using Redundant State Selection (RSS) method and is explained in detail. Experimental results are presented to verify the proposed modulation and control techniques.

A cascade multilevel STATCOM for wind generation systems

This paper presents a novel STATCOM configuration for voltage quality improvement in wind power generation systems. The proposed STATCOM is formed by cascading two 3-level inverters, `bulk inverter' and `conditioning inverter', through a coupling transformer. Both inverters are powered by dc-link capacitors and they are charged by a small amount of active power drawn from the grid. To minimize switching losses, the high power bulk inverter operates at low frequency while low power high frequency conditioning inverter is used to suppress harmonic content produced by the bulk inverter output. With only 24 switches this topology can synthesize a nine level inverter, if the dc-link voltage ratio is maintained at 3:1. Modulation and control techniques have been developed to meet this requirement. Reactive power of the STATCOM is controlled to mitigate voltage sags or swells caused by sudden wind changes. Simulation and experimental results are presented to verify the efficacy of the proposed modulation and control techniques used in the STATCOM.

Energy storage systems in distributed generation schemes

An overview is given of the various energy storage technologies which can be used in distributed generation (DG) schemes. Description of the recent photovoltaic DG initiative in Singapore is included, in which several of the storage systems can find ready applications. Schemes pertaining to the use of solid oxide fuel cell for power quality enhancement and battery energy storage system used in conjunction with wind power generation are also described.

Z-source converter based grid-interface for variable-speed permanent magnet wind turbine generators

A Z-source inverter based grid-interface for a variable-speed wind turbine connected to a permanent magnet synchronous generator is proposed. A control system is designed to harvest maximum wind energy under varied wind conditions with the use of the permanent magnet synchronous generator, diode-rectifier and Z-source inverter. Control systems for speed regulation of the generator and for DC- and AC- sides of the Z-source inverter are investigated using computer simulations and laboratory experiments. Simulation and experimental results verify the efficacy of the proposed approach.

On the design of power buffer and its applications

A power electronics-based buffer is examined in which through control of its PWM converters, the buffer-load combination is driven to operate under either constant power or constant impedance modes. A battery, incorporated within the buffer, provides the energy storage facility to facilitate the necessary power flow control. Real power demand from upstream supply is regulated under fault condition, and the possibility of voltage or network instability is reduced. The proposed buffer is also applied to a wind farm. It is shown that the buffer stabilizes the power contribution from the farm. Based on a battery cost-benefit analysis, a method is developed to determine the optimal level of the power supplied from the wind farm and the corresponding capacity of the battery storage system.