Articles written in Sadhana

    • A controller design method for 3 phase 4 wire grid connected VSI with LCL filter

      Anirban Ghoshal Vinod John

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      Closed loop control of a grid connected VSI requires line current control and dc bus voltage control. The closed loop system comprising PR current controller and grid connected VSI with LCL filter is a higher order system. Closed loop control gain expressions are therefore difficult to obtain directly for such systems. In this work a simplified approach has been adopted to find current and voltage controller gain expressions for a 3 phase 4 wire grid connected VSI with LCL filter. The closed loop system considered here utilises PR current controller in natural reference frame and PI controller for dc bus voltage control. Asymptotic frequency response plot and gain bandwidth requirements of the system have been used for current control and voltage controller design. A simplified lower order model, derived for closed loop current control, is used for the dc bus voltage controller design. The adopted design method has been verified through experiments by comparison of the time domain response.

    • A novel approach for electrical circuit modeling of Li-ion battery for predicting the steady-state and dynamic I–V characteristics


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      A novel approach for electrical circuit modeling of Li-ion battery is proposed in this paper. The model proposed in this paper is simple, fast, not memory intensive and does not involve any look-up table. The model mimics the steady-state and dynamic behavior of battery. Internal charge distribution of the battery is modeled using two RC circuits. Self-discharge characteristic of the battery is modeled using a leakage resistance. Experimental procedure to determine the internal resistance, leakage resistance and the value of RC elements is explained in detail. The variation of parameters with state of charge (SOC) and magnitude of current is presented. The internal voltage source of the battery model varies dynamically with SOC to replicate the experimental terminal voltage characteristics of battery. The accuracy of model is validated with experimental results.

    • Battery impedance spectroscopy using bidirectional grid connected converter


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      Battery impedance can provide valuable insight into the condition of the battery. Commercially available impedance measurement instruments are expensive. Hence their direct use in a battery management system is not justifiable. In this work, a 3-kW bi-directional converter for charging and discharging a batterybank has been implemented with the capability of impedance measurement. The converter is grid connected and controlled to operate at unity power factor. Additional requirements on filter design and control structure of battery converter for impedance measurement are discussed. An algorithm has been developed to measure impedance by frequency sweep, avoiding transients. The measured impedance has been compared to that from a commercially available impedance measurement equipment and is shown to have a good match.

    • Auxiliary subsystems of a General-Purpose IGBT Stack for high-performance laboratory power converters


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      A PWM converter is the prime component in many power electronic applications such as static UPS, electric motor drives, power quality conditioners and renewable-energy-based power generation systems. While there are a number of computer simulation tools available today for studying power electronic systems,the value added by the experience of building a power converter and controlling it to function as desired is unparalleled. A student, in the process, not only understands power electronic concepts better, but also gains insights into other essential engineering aspects of auxiliary subsystems such as start-up, sensing, protection, circuit layout design, mechanical arrangement and system integration. Higher levels of protection features are critical for the converters used in a laboratory environment, as advanced protection schemes could prevent unanticipated failures occurring during the course of research. This paper presents a laboratory-built General-Purpose IGBT Stack (GPIS), which facilitates students to practically realize different power converter topologies. Essential subsystems for a complete power converter system is presented covering details of semiconductor device driving, sensing circuit, protection mechanism, system start-up, relaying and critical PCB layout design, followed by a brief comparison to commercially available IGBT stacks. The results show the high performance that can be obtained by the GPIS converter.

    • Impedance estimation of photovoltaic modules for inverter start-up analysis


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      Starting-up of photovoltaic (PV) inverters involves pre-charging of the input dc bus capacitance. Ideally, direct pre-charging of this capacitance from the PV modules is possible as the PV modules are current limited. Practically, the parasitic elements of the system such as the PV module capacitance, effective wire inductance and resistance determine the start-up transient. The start-up transient is also affected by the contactor connecting the PV modules to the inverter input dc bus. In this work, the start-up current and voltages are measured experimentally for different parallel and series connections of the PV modules. These measurements are used to estimate the stray elements, namely the PV module capacitance, effective inductance and resistance.The estimation is based on a linear small-signal model of the start-up conditions. The effect of different connections of the PV modules and the effect of varying irradiation on the scaling of the values of the stray elements are quantified. The System model is further refined by inclusion of connecting cable capacitance and contactor resistance. Dynamics of the resulting fifth-order model are seen to be consistent with those of the simplified third-order model. The analysis of this paper can be used to estimate the expected peak inrush current in PV inverters. It can also be used to arrive at a detailed modelling of PV modules to evaluate the transient behaviour.

    • Design and comparative study of discrete and module-based IGBT power converters


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      This paper discusses concepts of a 20 kVA power converter design and key differences between discrete IGBT and module-based design approaches. Module-based power converters have been typically employed in academic and research institutes for power levels of 10 kVA and more. However, with advancement in IGBT technologies and the growing need to minimize system size and weight, designs based on discrete devices are now an attractive alternative for such power levels. A simple procedure is presented for power converter design that includes power loss evaluation, heat-sink thermal characterization, thermal model of overall system and sizing of DC link capacitor. Using the same, a state-of-the-art discrete device and modulebased power converters are designed. A comparison is subsequently made, where it is shown that discrete approach yields a compact and economic design up to a power level of 20 kVA. A key objective of this work is to lay emphasis on laboratory design of power converters. This enables a graduate level student to build a converter from start and in the process gain insights into the underlying engineering design aspects.

    • Linearised model for PV panel power output variation with changes in ambient conditions


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      In closed loop control of PV systems it is important to model the small signal variation of PV panel array output with ambient conditions, namely irradiation and temperature. Changes in these conditions act as a disturbance to the system, but this disturbance needs to be reflected in terms of the quantity being controlled,which can be the PV panel current or the real power. In this work a linearised model is derived to relate the change in system input, namely: irradiance and temperature, with its output, namely: array current and power. The proposed model is experimentally verified with tests run on PV panels, when they are subjected to varying irradiation and temperature conditions in the laboratory. The experimental results confirm the accuracy of the linearised PV panel model.

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