S. K. Sahoo

Single-Phase Inverter Control Techniques for Interfacing Renewable Energy Sources With Microgrid—Part I: Parallel-Connected Inverter Topology With Active and Reactive Power Flow Control Along With Grid Current Shaping

In this paper, a novel current control technique is proposed to control both active and reactive power flow from a renewable energy source feeding a microgrid system through a single-phase parallel-connected inverter. The parallel-connected inverter ensures active and reactive power flow from the grid with low-current total harmonic distortion even in the presence of nonlinear load. A p-q theory-based approach is used to find the reference current of the parallel-connected converter to ensure desired operating conditions at the grid terminal. The proposed current controller is simple to implement and gives superior performance over the conventional current controllers, such as rotating frame proportional-integral controller or stationary frame proportional resonant controller. The stability of the proposed controller is ensured by direct Lyapunov method. A new technique based on the spatial repetitive controller is also proposed to improve the performance of the current controller by estimating the grid and other periodic disturbances. Detailed experimental results are presented to show the efficacy of the proposed current control scheme along with the proposed nonlinear controller to control the active and reactive power flow in a single-phase microgrid under different operating conditions.

Single-Phase Inverter-Control Techniques for Interfacing Renewable Energy Sources With Microgrid—Part II: Series-Connected Inverter Topology to Mitigate Voltage-Related Problems Along With Active Power Flow Control

In this paper (Part II), a control strategy for a single-phase series-connected inverter with the microgrid is proposed to interface ac loads not only to regulate the load voltage under voltage disturbances, but also to control the load power drawn from the microgrid. The inverter compensating voltage works in such a way that, irrespective of any type of disturbances in the microgrid voltage (such as sag, swell, or harmonic distortions), the load voltage is maintained at its rated voltage level with low total harmonic distortion (THD) in voltage. The proposed control strategy also facilitates a specific amount of active power flow (from renewable energy source) to the load irrespective of the microgrid voltage condition. The rest of the load power is supplied by the microgrid. To facilitate this control strategy, a spatial repetitive controller (SRC) is proposed and implemented in microgrid phase (θ) domain to make the controller independent of the microgrid frequency. The proposed controller ensures dynamic stability of the system even if there is a sudden change in the microgrid frequency. Detailed experimental results are presented to show the efficacy of the proposed series inverter system along with the controller under different operating conditions.

A Lyapunov Function-Based Robust Direct Torque Controller for a Switched Reluctance Motor Drive System

A novel Lyapunov function-based direct torque controller for minimization of torque ripples in a switched reluctance motor (SRM) drive system is reported in this paper. SRM magnetization characteristics are highly nonlinear, where torque is a complex and coupled function of the phase currents and rotor position. The direct torque control (DTC) scheme avoids the complex process of torque-to-current conversion as required in indirect torque control scheme. The traditional DTC scheme uses a hysteresis-type torque controller and it leads to large amount of torque ripples when implemented digitally. The proposed controller is intended to take care of the nonlinear system dynamics of magnetic characteristics associated with accurate torque control using DTC scheme for the SRM drive system. In the Lyapunov function-based controller, the feedback gain is varied using a heuristic technique. The stability of the proposed controller is ensured by the direct method of Lyapunov. Experimental results for a 1-hp, 4-phase SRM are provided to demonstrate the efficacy of the proposed torque control scheme.

Indirect torque control of switched reluctance motors using iterative learning control

This paper proposes the use of iterative learning control (ILC) in designing a torque controller for switched reluctance motors (SRMs). The demanded motor torque is first distributed among the phases using a torque-sharing function. Following that, the phase torque references are converted to phase current references by a torque-to-current converter and the inner current control loop tracks the phase current references. SRM torque is a highly nonlinear and coupled function of rotor position and phase current. Hence, the phase current references for a given demanded torque can not be obtained analytically. Assumption of linear magnetization characteristics results in an invertible torque function. However, the nominal phase current references obtained using this torque function will lead to some torque error as motor enters into magnetic saturation. For a constant demanded torque, the error in the phase current references will be periodic with rotor position. Hence, we propose to use ILC to add a compensation current to the nominal phase current references so that torque error is eliminated. Similarly, current tracking for the nonlinear and time-varying system is achieved by combining a simple P-type feedback controller with an ILC controller. The proposed scheme uses ILC to augment conventional feedback techniques and hence, has better dynamic performance than a scheme using only ILC. Experimental results of the proposed scheme for an 8/6 pole, 1-hp SRM show very good average as well as instantaneous torque control.

Application of Four-Switch-Based Three-Phase Grid-Connected Inverter to Connect Renewable Energy Source to a Generalized Unbalanced Microgrid System

In this paper, a four-power-semiconductor-switch-based three-phase inverter is proposed for renewable energy source integration to a generalized microgrid system. The proposed topology b-4 of three-phase inverter is investigated to make the commercial microgrid system to be cost effective and hardware optimized. A simple sine-pulse-width-modulation-based (SPWM) control strategy is proposed for the b-4 inverter topology instead of the traditional complex four-switch-based space vector techniques. The overall control structure is implemented using the Lyapunov function-based nonlinear controller to track the inverter current directly in the a-b-c frame so that a specific amount of active and reactive grid power flow to the grid can be controlled in a decoupled manner along with low total harmonic distortion of grid currents in the presence of nonlinear load at the point of common coupling (PCC). A novel technique of using the spatial repetitive controller (SRC) is also proposed to eliminate the effect of midpoint voltage fluctuation of the dc link even in the case of asymmetrically split dc-link capacitors without any extra voltage or current sensors unlike conventional methods. Detailed experimental results are provided to show the efficacy of the proposed hardware system for grid-connected applications in the microgrid.

Lyapunov Function-Based Current Controller to Control Active and Reactive Power Flow From a Renewable Energy Source to a Generalized Three-Phase Microgrid System

In this paper, a novel current control technique, implemented in the a-b-c frame, for a three-phase inverter is proposed to control the active and reactive power flow from the renewable energy source to a three-phase generalized microgrid system. The proposed control system not only controls the grid power flow but also reduces the grid current total harmonic distortion in the presence of typical nonlinear loads. The control system shapes the grid current taking into account the grid voltage unbalance, harmonics as well as unbalance in line side inductors. The stability of the control system is ensured by the direct method of Lyapunov. A SRC is also proposed to improve the performance of the current controller by estimating the periodic disturbances of the system. The proposed control system (implemented digitally) provides superior performance over the conventional multiple proportional-integral and proportional-resonant control methods due to the absence of the PARKs transformation blocks as well as phase lock loop requirement in the control structure. A new inverter modeling technique is also presented to take care of unbalances both in grid voltages and line side inductors. Experimental results are provided to show the efficacy of the proposed control system.

Iterative learning-based high-performance current controller for switched reluctance motors

Switched reluctance motors (SRMs) are being considered for variable speed drive applications due to their simple construction and fault-tolerant power-electronic converter configuration. However, inherent torque ripple and the consequent vibration and acoustic noise act against their cause. Most researchers have proposed a cascaded torque control structure for its well-known advantages. In a cascaded control structure, accurate torque control requires accurate current tracking by the inner current controller. As SRM operates in magnetic saturation, the system is highly nonlinear from the control point of view. Developing an accurate current tracking controller for such a nonlinear system is a big challenge. Additionally, the controller should be robust to model inaccuracy, as SRM modeling is very tedious and prone to error. In this paper, we have reviewed various current controllers reported in the literature and discussed their merits and demerits. Subsequently, we have proposed and implemented a novel high-performance current controller based on iterative learning, which shows improved current tracking without the need for an accurate model. Experimental results provided for a 1-hp, 8/6-pole SRM, demonstrate the effectiveness of our proposed scheme.

A Plug and Play Operational Approach for Implementation of an Autonomous-Micro-Grid System

In this paper, a plug and play type autonomous-micro-grid system formation is proposed. Multiple distributed generating sources and loads interaction is considered pertaining to the stability of the micro-grid. The proposed method enables communication-less operation of each of the elements of the micro-grid system. It is also considered that the sources are of different power capacity as can be seen in a typical autonomous micro-grid system. Spatial Repetitive Controller (SRC) is proposed to control each of the distributed generating sources in a decentralized manner to stabilize the overall micro-grid system. The proposed system considers sudden change in load as well as other distributed generators conditions like a true plug and play operation. A novel signature frequency voltage injection method is proposed to identify the presence of special distributed generator and operation of backup distributed generators. The implemented control architecture also ensures stability of the micro-grid fundamental frequency even in the case of dynamic conditions unlike the traditional droop-control method. A detailed experimental study is carried out and the experimental results presented show the efficacy of the proposed system.

Derivation of instantaneous current references for three phase PV inverter connected to grid with active and reactive power flow control

In this paper, inverter reference current generation for a three phase grid connected PV inverter under generalized grid voltage conditions is proposed. The proposed method facilitates high bandwidth grid active and reactive power flow control along with minimum DC link ripples with grid current THD control facility by finding suitable current reference directly in the a-b-c frame. The proposed method is much faster than the conventional methods because of the absence of phase lock loop (PLL) and Parks transformation requirements. The proposed method is also independent of grid voltage fundamental frequency and capable of rejecting the grid voltage harmonics. These advantages make it more suitable for micro-grid applications. The proposed method is validated with rigorous experiments and test results presented depict the efficacy of the proposed system.

A Lyapunov function based current controller to control active and reactive power flow in a three phase grid connected PV inverter under generalized grid voltage conditions

In this paper, a novel control system is proposed to control the active and reactive grid power flow in a three phase grid connected PV inverter. The control system not only controls the grid power but also reduces the grid current THD in the presence of typical non-linear loads in parallel with grid at the point of common coupling (PCC). The proposed control system also takes care of not only the grid voltage unbalance but also the unbalance in the connecting line side inductors. The stability of the proposed control system is ensured by the direct method of Lyapunov. The proposed control system is not only simple to implement in the digital form but also provides superior performance over the conventional multiple PI or resonant control methods. A new grid connected inverter modeling technique is also proposed to take care of unbalances in the inverter system. Experimental results are provided to show the efficacy of the proposed control system.