Grid-connected Current Source Research Articles

Developing and Evaluating the Operating Region of a Grid-Connected Current Source Inverter from Its Mathematical Model

Grid-connected power inverters are indispensable in modern electrical systems, playing a pivotal role in enhancing the integration of renewable energies into power grids. Their significance, primarily when functioning as grid-forming inverters, extends to maintaining the grid’s inertia and strength—a distinct advancement over traditional grid-following operations. As grid-forming inverters, these devices emulate the characteristics of synchronous generators and can act as robust voltage sources, providing essential ancillary services. This behavior is particularly relevant when integrating energy storage systems on the converters’ direct current side. Among the various inverter topologies, the current source inverter (CSI) has emerged as a promising yet underexplored alternative for grid-forming applications. CSIs, when paired with their AC output filters, can effectively operate as voltage sources, utilizing control strategies that facilitate the integration of renewable energies into the electrical system. Their design inherently manages output current fluctuations, reducing the need for restrictive current limitations or additional protective measures. This paper examines the operational region of CSIs, obtained through detailed modeling, to explore their advantages, challenges, and potential for enhancing grid-connected systems. Analyzing the operating region from the converter model verifies the limits of where the converter can operate in a plane of active and reactive powers. For a small prototype model operating with 7 amperes in DC and 120 V in AC, it is possible to supply or absorb active power exceeding 1000 W and manage maximum reactive power values around 500 VAr, as determined by its operating region. Simulations also confirm that small changes in the control reference, as little as 5%, towards the region’s right limits cause significant oscillations in the dynamic control responses. This research aims to deepen our understanding of CSIs’ operational capabilities and highlight their unique benefits in advancing grid-connected systems and promoting the integration of renewable energy using this technology.

Power loss model and efficiency analysis of grid-connected seven-switch boost-type photovoltaic current source inverter using two power switches configurations

Power loss model and efficiency analysis of grid-connected seven-switch boost-type photovoltaic current source inverter using two power switches configurations

Conductance Emulating Control Strategy for Three-Phase Current Source Rectifier Under Unbalanced Grid Voltages

Power oscillation is a technical challenge for the grid-connected three-phase current source rectifiers (3ph-CSRs) under unbalanced grid voltage conditions. To manipulate the instantaneous power adapted to different grid requirements under unbalanced grid voltage conditions, a conductance emulating control method is proposed in this brief. The proposed method constructs a fictitious voltage pair with two individually adaptable parameters. One parameter regulates the distribution of oscillating power between active power and reactive power, and another parameter controls the average reactive power. Then, the rectifier imitates variable conductance under the fictitious voltage pair, regulating the average active power. As a result, the power control of the grid-connected 3ph-CSRs becomes flexible and adaptable to various grid requirements. The correctness and effectiveness of the proposed control scheme are verified by experimental results.

Simulation Studies of Control Systems for Doubly Fed Induction Generator Supplied by the Current Source Converter

The control system for a Doubly Fed Induction Generator (DFIG) supplied by a grid-connected Current Source Converter (CSC) is presented in this paper. Nonlinear transformation of DFIG model to the multi-scalar form is proposed. The nonlinear control strategy of active and reactive power of DFIG is realized by feedback linearization. In the proposed control scheme, the DFIG model and CSI parameters are included. Two Proportional-Integral (PI) controllers are dedicated for the control of the respective active and reactive powers. The control variables are the dc-link input voltage vector and the angular speed of the inverter output current. The proposed control approach is characterized by satisfactional dynamics and provides enhanced quality of the power transferred to the grid. In the simulation, evaluation of the characteristic operating states of the generator system, correctness of the feedback linearization and the dynamics of active and reactive power control loops are studied. Simulation results are adequately provided.

Modeling and Simulation of Inverter based Distributed Generators for Renewable Energy Integration

The challenges and potential solutions in modeling and simulation for the new distribution grids with high penetration of inverter interfaced distributed generators including wind and photovoltaic generators are presented in this paper. A dq0 dynamic model for grid-connected Current Source Inverters (CSI) with power control is developed and validated in two different simulation studies. In this way, Electromagnetic transient (EMT) and phasor (PS) simulations are performed for a simple case of a CSI connected to the grid. A modified 13-nodes IEEE distribution test system with photovoltaic and wind based distributed generation is performed to extend the results to a standard distribution test system. Accuracy and reliability problems can be inferred from the simulation results if the network dynamics are neglected, and the need to develop new simulation models for the new emerging distribution grids is shown.

Advanced Control of Grid-Connected Current Source Converter Under Unbalanced Grid Voltage Conditions

There are technical challenges when the grid-connected current source converter operates under unbalanced grid voltage conditions. The negative sequence component of the unbalanced grid voltage would result in the low-frequency ripple at dc side, as well as current harmonics at ac side. It has an adverse impact on the system operation. In order to solve the problem, the system mathematical model is established in this paper. The limitation of the conventional control strategy is clearly clarified. And then, the improved control strategy is discussed from the viewpoint of constant instantaneous active power. In order to further enhance the system performance, a new control strategy is proposed. Instead of the complicated instantaneous power theory, the proposed solution is a direct control strategy. Both the low-frequency dc-side ripple and ac-side current harmonics can be significantly reduced with a simple control structure, due to the direct control concept. Finally, the comparison evaluation of the conventional and proposed control strategies is carried out. The experimental results verify the effectiveness of the proposed solution.

A Grid-Connected Multilevel Current Source Inverter and Its Protection for Grid-Disconnection

Traditionally DC-AC converters are considered with voltage source inverters (VSI); although less studied and discussed, it has started recently to be used current source inverters (CSI). Another possibility for DC/AC conversion is the multilevel configuration. This paper shows experimental operation and simulation analysis of a grid-connected multilevel current source inverter (MCSI), which includes a circuit for equipment safety reasons due to grid disconnections.

Study on output characteristic of bi-direction current source converters

Among the control algorithms of current source pulse-width modulation converter, space vector pulse width modulation (SVPWM) is widely applied for its higher current utilisation factor than sine pulse width modulation (SPWM). However, when the two control methods are applied to active inverters (grid-connected photovoltaic system and current source active power filter (APF) for example), they present a wholly opposite characteristic. The output characteristics are analysed and contrasted in detail between SPWM and SVPWM current source inverters. The whole work is based on energy transfer, area equivalent theorem and different mechanism of the two algorithms when trigger pulses are produced according to the current command. The conclusion is drawn that at the same running condition in inverting operation, SPWM can output higher inverting current than SVPWM. The simulation and test results are presented and a new way of bi-direction current source inverter to obtain higher current utilisation factor is presented.

An Active Modulation Technique for Single-Phase Grid-Connected CSI

Due to the interaction between the rectified voltage appearing on the dc side of the grid-connected current source inverter (CSI) and the dc inductor, the input current on the dc side inherently consists of the second-order harmonic of the grid frequency. This harmonic component will be modulated by the pulsewidth modulation (PWM) signal that results in generating low-order harmonics at the output and thus injecting into the grid. This paper presents an active nonlinear modulation technique for reducing the low-order harmonics. The method requires simple analog circuitry to modify the modulation function of the sinusoidal PWM (SPWM) scheme. It is based on measuring the dc and ac components of the dc inductor current to compensate the modulation effect from the dc side to the ac side of the CSI. Apart from reducing the harmonic content, this method can also reduce the required value of the dc inductor. Modeling, design, and analysis of the method are given. The proposed method has been tested with a 1 kW, 220 V, 50 Hz prototype, powered by a dc voltage of 80 V. The results are favorably compared with the performance of the CSI with the SPWM scheme.