A. Vijayakumari

Dynamic grid impedance calculation in D-Q frame for micro-grids

This paper presents a method for online dynamic measurement of the grid impedance that may prevail in an electrical network at any instant of time. Here the grid resistance and inductance are measured during the operation using a non-characteristic frequency current continuously injected into the grid, and subsequently calculating the impedance using discrete Fourier transforms. The proposed measurement is implemented in d-q reference frame so that the computational overhead on the microcontroller is relatively reduced. The continuous injection of non-characteristic current at 75 Hz avoids the injection of inter-harmonics into the grid during measurements. The resulting grid impedance value can be used for many purposes viz. knowing the status of the independent generations in micro-grids, controller tuning in converter systems, islanding detection etc. The proposed dynamic measurement of grid impedance is tested through simulation studies and experimentally. The experiments are conducted with the proposed controller on a scaled down laboratory model of micro-grid with a 1 kVA solar inverter, and the consolidated results of both simulation and experiments are presented.

A dual mode control for bridgeless SEPIC power factor correction rectifier

This paper presents the design and development of a dual mode controlled bridgeless SEPIC unity power factor correction rectifier for a typical low power application like an LED driver. The proposed topology has reduced component count with reduced conduction losses because of no front end diode rectifier. The bridgeless SEPIC converter is designed to operate in discontinuous current mode so as to obtain a natural zero current turn off in output diode and zero current turn on in switches. This will reduce the complexity in control and provides voltage following capability for the source current. The proposed dual mode control circuit gives a supplementary control capability to attain sinusoidal current even in the presence of harmonics in source voltage. A harmonic detection is carried out on the source voltage and the control is directed according to the results either to voltage follower or average current control. The second control uses an extraction technique to get a sinusoidal current reference and maintains UPF even with the presence of harmonics in the source voltage. The proposed converter with its associated control is developed in MATLAB/Simulink for a power rating of 10W a practical rating of an LED load. Beside the normal operating conditions, the circuit is tested under varying source voltage, frequency and also with harmonic source voltage. The source current is found to be sinusoidal under all the conditions and the current THD is found to be well below 5% with unity power factor.

Extraction of Photovoltaic (PV) Module’s Parameters Using Only the Cell Characteristics for Accurate PV Modeling

This paper presents a method for extraction of the model parameters pertaining to a commercially available PV module, which can be used to develop an accurate PV model that can serve as a source for simulation studies of both grid-connected and stand-alone PV systems. All the physical parameters required for the PV model are determined only from the published V–I characteristics of the module and the standard cell equation. A curve fitting based extraction technique is proposed to obtain the parameters pertaining to a particular PV module available in the market. The extracted parameters are used and a simulation model of the PV module is developed in MATLAB/Simulink. The inputs to the model are irradiance, ambient temperature, number of series and parallel modules in an array as required in an application. A 115 W commercial module S115 is taken for verification, and its physical parameters are extracted and its simulation model is developed. The characteristics of the developed model with the extracted parameters, for several G and T C are presented and compared with the published data of S115 and compliance confirmed. The model has been tested for sudden changes in irradiance and temperature using a maximum power point tracker.

Power Angle Control of a Single Phase Grid Connected Photovoltaic Inverter for Controlled Power Transfer

This paper focuses on the design and development of a 500 W, single phase single stage low-cost inverter for the transfer of direct current (DC) power from the solar photovoltaic (SPV) panel to the grid while meeting the standards for interconnection. The power transfer from SPV to grid is facilitated by controlling the angle between the inverter voltage and the grid voltage i.e. the power angle or phase angle (δ). Simulation of the single stage inverter for a power rating of 500 W is implemented in MATLAB/Simulink. The synchronization of the inverter with the grid was carried out by monitoring the grid frequency continuously and updating it. The power transferred to the grid is made equal to the available power by monitoring and updating δ. A pulse-width modulation (PWM) control is implemented using PIC16F877 microcontroller for the inverter so as to deliver the available power from the solar panel to the grid, as well as, make the frequency of the output voltage of the inverter equal to the grid frequency for sustaining the synchronization.

Effect of Grid Impedance Variation on the Control of Grid Connected Converters with Synchronous Reference Frame Controllers in Micro-Grids

This paper presents an investigation on the effect of grid impedance variation on the control of grid connected voltage source converter when they are controlled using synchronous reference frame (SRF) current controllers in micro-grid applications. Mutual coupling terms introduced between the d and q control loops in SRF PI controller which are normally decoupled using the grid impedance through a feed-forward control to achieve independent control of active and reactive powers. But if the configurations change in systems like micro-grids due to intermittent nature of renewable energy sources then the feed forward decoupling becomes inadequate. A practical micro-grid and its parameters are taken for the investigation and the analysis is extended through simulation studies. The effectiveness of decoupling and the system stability under configuration change in micro-grids were investigated. The loss of independency of power control and stability analysis is presented to justify the need for a dynamic intelligent controller.

Renewable generators' capacity optimization for a micro-grid in rural feeder using HOMER — A case study

Utility grid outages can be tackled by the recently advocated micro-grid concepts, which is envisaged to escalate the use of regionally available renewable energy sources. The objective of this paper is capacity optimization of RES in a practical 5 bus micro-grid considering different operating conditions of both load and sources. The micro-grid consisting of wind turbine, solar panel, hydro and battery and HOMER software is used for the optimization process. The micro-grid considered is located in a rural feeder at Amrita University, were load shedding are commonly occurring for prolonged periods. This optimization process considers the load shedding periods and actual load profile of the University so as to arrive the best optimized capacity to independently work even under grid outages. A peak demand of 1 MW along with the predefined architecture of the micro-grid is fed as input to HOMER along with the load shedding periods. The optimization results are subjected to sensitivity analyses in order to account for wind speed and solar irradiance variations to finally result best optimized capacities for the RES present in the micro-grid.

Grid connected wind driven permanent magnet synchronous generator with high frequency solid state transformer

This paper presents the control of grid connected wind driven permanent magnet synchronous generator through solid state transformer with a high frequency multistage converter interconnect. Maximum power point tracking is carried out in the high frequency generator side converter stage where as the grid side converter takes care of the synchronization. The model parameters for the high frequency transformer are obtained by analytical calculation and the same is used in the simulation. A push pull converter with incremental conductance algorithm on the generator side tracks the MPP. It also converts the dc output into high frequency wave so that it can be directly applied to the primary of the high frequency transformer. The rectified secondary voltage is grid synchronized through the grid side inverter with synchronous reference controller. The entire system including the high frequency transformer is simulated in MATLAB/Simulink for a 2.5 kW PMSG.

A reduced converter count solid state transformer for grid connected Photovoltaic applications

This paper presents a Solid State Transformer (SST) interface for grid connected Photovoltaic (PV) system. A new method is proposed for Maximum Power Point Tracking (MPPT) through a single phase inverter with phase displacement control. The inverter control variable is redefined such that it can be used for MPPT. Due to this control technique one converter stage for power tracking is eliminated, leading to a reduced component count for the overall system. The high frequency transformers model parameters are designed for an operating frequency of 20 kHz for simulation studies. The grid side converter is controlled for synchronization and power flow using a Synchronous Reference Frame (SRF) current controller. The overall system is tested in simulation for a panel rating of 1.15 kWp under various insolation conditions and power balance is verified and presented. Testing is carried out under step change in the insolation to evaluate the performance of the SST interface under transient conditions. The proposed system is tracking the maximum power available from the panel and delivering it into the grid with high efficiency. The overall system efficiency and the high frequency transformer efficiencies are presented under various operating conditions.