Current-controlled Converter Research Articles

Transient Stability Analysis of Microgrid Considering Impact of Grid-Following Converter's Current Controller

Transient Stability Analysis of Microgrid Considering Impact of Grid-Following Converter's Current Controller

Grid current quality improvement for solid state transformer with an improved current controller

In this paper, the Solid-State Transformer (SST) is performed by integrating an improved harmonic compensator at the Modular Multilevel Converter (MMC) current controller in order to ensure grid current quality regardless the non-linear load or the unbalance position at the SST stages. The proposed MMC current controller is composed of a high-order harmonic compensator based on a fuzzy logic controller, giving the controller an active filtering feature, a low-order harmonic compensator, and a negative sequence harmonic compensator for unbalanced control. This makes the proposed solution a good alternative for solutions based on extra filters or increase switching frequency. Ensuring power quality independent of loads or unbalanced position in the SST while maintaining reduced switching frequency is the main focus. With the proposed current controller, both low order and high order harmonics are effectively attenuated, thereby consistently reducing the total harmonic distortion (THD) of grid current to levels complying with the IEEE-1547 standard, even when non-linear loads are placed at the LV and at the MV stage of the SST. In addition, the controller eliminates negative-sequence harmonics associated with unbalanced loads in the SST, thus preserving the balance of the grid current. Various results of the proposed current controller are presented.

Application of PI-Notch controller in the control of grid side converter to reduce PCC harmonic current for DFIG wind energy system

This work presents the analysis of harmonic of 5th and 7th orders impacts in the grid voltage in a wind generation system based on DFIG. Therefore, there were simulations of the active and reactive power control of the DFIG using the vector control by orientation of the magnetic stator flux. The results of three simulations are presented. In the first simulation it is presented the analysis of the wind power generation system connected to an electric grid with purely sinusoidal voltage. In the second simulation, the wind generation system is connected to an electric grid with distorted voltage for 5th and 7th harmonic orders. In the third simulation it is shown the results of the reduction of harmonic distortion in the electrical currents at the PCC and at the output of the electronic power converter by applying notch filters in the converter current control loops connected to electrical grid.

Direct Load Current Control in Power Supply Based on DC–DC Converter and AC Current-Controlled Converter in Parallel for Electrolytic Hydrogen Production Test

Direct Load Current Control in Power Supply Based on DC–DC Converter and AC Current-Controlled Converter in Parallel for Electrolytic Hydrogen Production Test

A Detailed Assessment of the Power Quality Improvement of an Islanded AC Microgrid through Upgrading Conventional Grid-Feeding Current-Controlled Converters to Operate as Multifunctional Converters

The decarbonization of the global economy is one of society’s urgent objectives nowadays. Thus, the large-scale adoption of renewable energy sources, like solar and wind energy, seems to be the best pathway to achieving sustainable development. Thus, the microgrid concept has emerged as a solution to address concerns related to the intermittent nature and lack of inertia for such sources. Several studies involving microgrids have been published in the last few years. However, in most of them, power quality (PQ) disturbances and strategies to manage such disturbances are not considered, especially in the islanded operation mode. Hence, in this paper, a detailed assessment of the PQ of an islanded AC microgrid is conducted, considering the coexistence of a nonlinear load, voltage-controlled converters, and a current-controlled converter (CCC). The analyses consider two scenarios, depending on the operation mode of the CCC: (1) the CCC operating as a conventional grid-feeding converter and (2) considering a cost-effective upgrading of the conventional CCC to operate as a multifunctional converter. An experimental test bed is built to validate both scenarios. The presented results provide strong evidence that the AFF significantly improves the microgrid PQ and its suitability for dealing with PQ disturbances in real-world applications.

Analysis of a Multifunctional Inverter Active-Filtering Function Influence on the Small-Signal Stability of Inverter-Based Islanded AC Microgrids

This paper deals with the small-signal analysis of islanded AC microgrids, considering the coexistence of voltage-controlled converters (VCC) operating under droop control law and current-controlled converters (CCC) operating as multifunctional inverters. Thus, the CCCs can deliver power from a non-dispatchable energy source and, depending on the operation mode, they can perform the active filtering function (AFF), which consists of injecting the harmonic currents associated with the local non-linear loads (NLL) resulting in a microgrid power quality (PQ) improvement. However, the transient characteristics following a local NLL step highly depend on the CCC operation mode. When AFF is disabled, the CCC output is almost not affected by the local NLL step. Instead, the harmonic extraction algorithm dynamics affect the CCC output when AFF is enabled. As a result, the overall system dynamic behavior is significantly affected, since microgrids are low-inertia and small power systems. A generic small-signal model is derived to study these effects, and its accuracy is then validated through simulated and experimental results. It was found through time-domain and frequency-domain sensitivity analyses that the AFF significantly affects the CCC PLL and the dynamic interactions between the microgrid network and the inverter's internal control loops.

On the low risk of SSR in type III wind turbines operating in grid-forming control

We have shown in previous work that the risk of subsynchronous resonance (SSR) between wind power plant with Type III wind turbines and a series-compensated transmission line is low when the wind turbines are operated in gridforming mode, instead of the standard grid-following mode. This paper explains the fundamental mechanism behind the improved damping characteristics by modeling the positive- and negative-sequence impedances of Type III wind turbines in gridforming control mode. It is discovered that the grid-forming control naturally acts against the negative resistance behavior of Type III wind turbines at subsynchronous frequencies that results from an interaction between the rotor-side converter current controller and negative slip. The developed sequence impedance models and improved damping behavior are verified using PSCAD simulations of a 2.5-MW Type III grid-forming wind turbine. The modeling predictions are also supported by experimentally measuring the sequence impedance response of a 2.5-MW Type III wind turbine in grid-forming control mode.

Sequence-admittance-based stability analysis for multiple parallel converters

With the development of energy storage system, the voltage-controlled converters (VCCs) that can achieve voltage and frequency support have become hot spots in both theory and practice. Therefore, apart from the traditional multiple parallel current-controlled converters (CCCs), multiple parallel VCCs and multiple parallel hybrid-controlled converters (HCCs) consisting of both the CCCs and VCCs have emerged, which makes stability issues of multi-converter grid-connected systems more complex. To cope with stability concerns related to different kinds of multiple parallel converter systems, this paper analyzes the interactive process of circuit-coupling and frequency-coupling among grid-connected converters and develops equivalent sequence-admittance-based stability analysis models. Based on the proposed models, stability recognition of grid-connected CCCs, VCCs and HCCs are derived. Finally, the effectiveness of the built models and the correctness of the stability analysis results are verified by experiments.

Generalized impedance model and interaction analysis for multiple grid-forming and grid-following converters

The growing use of power electronics converters in the power system leads to emerging stability issues. However, existing impedance models are insufficient for analysis of the converter-induced stability issues in the converter-congregated local area, especially faced with the hybrid integration of both grid-following (GFL) and grid-forming (GFM) converters. In this paper, a generalized impedance model in the d q frame is developed to facilitate the impedance-based analysis of any grid node with both GFL and GFM converters. With this model, closed-loop, internal and external stability performance can be accurately described and predicted. This paper shows that the internal stability of power converters has the feature of transferability. Therefore, to facilitate the internal stability analysis of multiple-input and multiple-output systems, this paper proposes a stability analysis method that utilizes the determinant of total admittance (matrices in high dimension) for direct stability demonstration, based on the recognition of instability pattern. Finally, the procedure for integrating power converters is considered to help with the stability-oriented design. The established impedance model, the proposed stability analysis method and the converter integration procedure are validated with control hardware-in-the-loop results in a study case of a multi-terminal network with current-controlled converters and virtual synchronous generators. • Impedance model describing any local area with GFL and GFM converters is proposed. • The determinant of total bus admittance is used for internal stability demonstration. • Impacts of critical system parameters on internal and external stability are studied. • A three-step procedure is adopted to facilitate the stability-oriented design.

A Feedback-Based Flexible Compensation Strategy for a Weak-Grid-Tied Current-Controlled Converter Under Unbalanced and Harmonic Conditions

Due to ever-increasing power demand and high renewable energy penetration, the power system's harmonics and voltage unbalancing have become significant power quality issues. In this aspect, because of the multifunctioning ability of the power electronic converters, the grid-connected power electronic converter-based distributed generations are expected to provide desired compensation whenever required. Especially in a weak grid environment, the compensation draws more attention due to constricted coupling between the voltage and current at the point of common coupling as a consequence of high grid impedance. Therefore, this article presents a feedback-based flexible compensation strategy for a weak-grid-tied distributed generation (DG) to enhance the compensation of harmonics and voltage unbalance in the system. The main uniqueness of the proposed work is to seamlessly embed a flexible harmonic loop with a fundamental current control loop for achieving three control targets: voltage harmonic compensation, DG line current harmonic rejection, and voltage unbalance compensation. Achieving control targets require voltage feedback and no need to measure nonlinear load current. In order to execute flexible harmonic compensation (i.e., from voltage harmonic compensation to DG line current harmonic rejection) through the harmonic loop, the proposed strategy generates a harmonic reference from voltage feedback and inherent current feedback. The delay introduced by the harmonic compensators is accurately compensated distinctly to improve the performance of the harmonic loop. The voltage unbalance compensation is achieved through the fundamental current control loop by improving the fundamental positive sequence voltage and decreasing negative sequence components concurrently. To enhance the flexibility between two different targets (i.e., flexible harmonic compensation and voltage unbalance compensation), the harmonic and unbalanced compensations are executed on an individual target basis, subjected to the converter's maximum current and system voltage constraints. The proposed compensation strategy offers better harmonic suppression and voltage unbalance compensation than conventional methods. Because it exactly acts upon modulation signals and directly regulates sequence components and voltage harmonics. Furthermore, the proposed control's detailed design and stability analysis is presented, and its superiority is validated through simulations and real-time experimentations.

Settling-Angle-Based Stability Analysis for Multiple Current-Controlled Converters

This letter proposes a settling-angle-based stability criterion, as the extension of determinant-based general Nyquist criterion, to facilitate the stability analysis of nonminimum phase power systems with multiple current-controlled converters. Using only the phase information of converter impedance in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dq$</tex-math></inline-formula> -frame, the proposed stability criterion can accurately demonstrate the system stability and identify resonance sources according to the settling angles of the overall system and its subsystems, which are validated by simulation and control hardware-in-the-loop results.

A current control method for bidirectional multiphase DC-DC boost-buck converter

&lt;span&gt;In the future, more and more electric vehicle (EV) batteries are connected to the direct current (DC) microgrid. Depending on the battery state of charge, the battery voltage can be higher or lower than the DC microgrid voltage. A converter that is aimed to fulfil such function must be capable of working in both charging and discharging regardless the voltage level of the battery and DC microgrid. Battery performance degradation due to ripple current entering the battery is also a concern. In this paper, a converter that can minimize ripple current that entering battery and operate in two power-flow directions regardless of battery and DC microgrid voltage level is presented. A current control method for this kind of converter was proposed. Experiment on a prototype was conducted to prove the proposed converter current control method.&lt;/span&gt;

Optimized Series Dynamic Braking Resistor for LVRT of Doubly-Fed Induction Generator With Uncertain Fault Scenarios

The terminal-connected series dynamic braking resistor (SDBR) is applied to assist the low-voltage ride-through (LVRT) of the doubly-fed induction generator (DFIG). With the fault current and switch-in of the SDBR, the stator voltage oscillates, thus the constant stator voltage drop assumption is invalid and the effect of the converter current control is weakened with the changing voltage-oriented reference frame. In this paper, the xy frame of the point of common coupling is applied to the converter control to avoid oscillation of the reference frame. The analytical expression of fault current with the SDBR and constant converter current control is derived. To evaluate the LVRT effect, the analytical analysis of the LVRT transient is carried out. The resistance of the SDBR is optimized based on an index combining the capabilities of the DFIG to provide the active power support and damp the electromagnetic torque oscillation. The uncertainties of the fault scenario are considered in the optimization algorithm by applying the probabilistic method. Simulation results show that the improved LVRT effect of the DFIG is realized with optimization to the SDBR resistance and its switch-in criterion.

Mathematical modeling of PMSG-based wind power plants (WPPs) for harmonic resonance studies and analytical assessment of wind turbines converters controls on WPP harmonic resonance response

In this paper harmonic resonance analysis of the grid comprising wind power plants (WPPs) with permanent magnet synchronous generators (PMSGs) is presented. Usually, Norton equivalent circuit is used as the wind turbine (WT) model for harmonic resonance studies, in which, WTs are simply modeled as ideal current and voltage sources and impacts of converters controls on the WTs output impedances are neglected. This paper deals with the harmonic resonance analysis of study WPP system taking the impacts of converters controls, and current and voltage measurement filters into account. In this way, harmonic impedance models of the WTs are presented for simple and detailed cases, and then WT harmonic impedance is extracted for the mentioned cases. Next, results of frequency scan and harmonic mode resonance analyses for different cases are given and compared, and then effects of the current and voltage measurements filters and WT converter control on harmonic resonance analysis of the study WPP system are presented. It is shown that increasing the bandwidth of the grid-side converter current control loop can not shift the resonant frequencies, but enhances the system damping, and consequently reduces the driving point impedances in the resonant frequencies.

Performance Evaluation of Effective Multi-Structure Controllers for KY Negative Output Boost Converter

This paper focuses on the design of multi-structure controllers (MSCs) for KY negative output boost converter (KYNOBC) functioned in continuous current mode (CCM) for low-power source applications. KYNOBC is one of the topologies of DC–DC converter that performs the positive DC input supply voltage into a negative output voltage. Dynamic behavior of KYNOBC is nonlinear in nature because of its time-changing operation. The single loop control does not regulate the multi-parameters of KYNOBC during the line and load variations owing to that it produces poor transient and dynamic analyses. With the aim of controlling the multi-parameters of KYNOBC such as inductor current and output voltage, enriching the transient and dynamic analysis, the MSCs are recommended in this paper. In our study, MSCs for KYNOBC consist of two loops such as inner proportional (P) controller inductor current loop for modulating the converter current and outer fuzzy logic controller (FLC)/proportional integral (PI) controller voltage loop for regulating the output voltage. The P and PI values are determined from the mathematical modeling of KYNOBC and the fuzzy rules are framed based on its performance without modeling equations. The behaviors of the KYNOBC with MSCs are validated at various zones by making the MATLAB/Simulink simulations and prototype model. The controller parameters are realized in prototype field programmable gate array (FPGA). The responses are recorded to indicate the dexterous of the MSCs for it.

A grid interface current control strategy for DC microgrids

In this paper, a grid interface current control strategy is presented for a DC microgrid, which aims to reduce the disturbance from PV generation and the load variation to the main grid without a grid interface converter. The grid interface current is directly controlled by a battery DC-DC converter within the DC microgrid. Based on a comprehensive analysis of the battery DC-DC converter and interface current control, the control system has been mathematically modelled. This enabled two transfer functions to be derived that reflect the dynamic response of the inductor current to the duty cycle variation (inner loop), and the dynamic response of the grid interface current to the inductor current variation (outer loop). Experimental study has been done to assess the effectiveness of the proposed control strategy. The experimental results indicate that the proposed control strategy has a good performance to control the grid interface current without an interface converter, regardless the variations of both PV and the load conditions.

Direct Parallel and Hybrid Power Control Scheme of a Low-Power PV and Piezoelectric Energy Harvesting Module

This paper presents a novel direct parallel power control scheme with independent current paths for the proposed hybrid energy-harvesting system that consists of a low-power photovoltaic panel, a piezoelectric harvesting module, and an energy storage system. A battery is connected to store the generated power from the two renewable power sources to provide a continuous power supply for the users. The designed power converter controls both battery state and power supply without any power conditioning system. To achieve this, a low-power DC–DC converter with battery charging function and control algorithm is designed for the hybrid power source. Therefore, in the proposed system, no additional battery charge controller is required. To control the battery, however, an additional current sensor to detect the battery power is simply added to each DC–DC converter. In the proposed power control scheme and controller, each power input, output, and the combined power flow are connected with neither communication protocols nor additional controller. Moreover, the converters can adjust the supplied power for parallel power-sharing. For maximum output current protection in each converter and constant current control for the battery, a hybrid power connection and its related current path are also proposed in this paper. The effectiveness of the proposed hybrid power system and the direct parallel power control scheme was verified through experiments.

Enhanced Fault Ride-Through of Power Converters Using Hybrid Grid Synchronization

Inaccurate phase-angle jump (PAJ) estimation of the grid voltage during faults is one of the major causes for poor fault ride-through (FRT) performance of converters. To overcome this issue and make the converter’s current controller robust, this article proposes a hybrid grid synchronization transition technique. In this concept, a synchronous reference frame-based phase-locked loop (SRFPLL) grid synchronization method is used during normal grid operation and switched to an arctangent-based phase-angle estimation during grid faults. Simultaneously frequency estimation is switched to the arctangent derived frequency. A common transition method, which depends on the phase-angle error between the two-phase estimation techniques, is proposed to ensure a smooth transition between the hybrid phase-angle and frequencies. The transition technique is implemented using the current control of a three-phase voltage source converter in the SRF. The performance of the converter during both symmetrical and asymmetrical grid faults along with the FRT strategies is tested using real-time experiments. It is observed that the transition-based hybrid grid synchronization technique reduces the loss of synchronism duration. Additionally, it offers a more robust converter current control performance compared to the SRFPLL technique over a wide range of voltage sags and PAJs.

Fully-Integrated Dickson Converters for Single Photon Avalanche Diode Arrays

This article presents a novel low EMI technique to regulate the output voltage of fully-integrated switched capacitor Dickson type step-up DC/DC converters for SPAD arrays implemented in the Silterra 0.13 $\mu \text{m}$ HV-CMOS process. SPAD sensors are extremely sensitive to EMI and therefore require careful biasing. This design utilises current sources to limit the current flow into the flying capacitors altering the converter ratio and reduces the discontinuous pulse currents associated with capacitive converters. A variable voltage-controlled reference current enables output voltage regulation. The proposed four stage converter boosts 3.3 V up to an adjustable 15 V output voltage. A typical Dickson converter regulated by frequency control and the proposed current controlled converter were designed and manufactured to compare performance. Measurements confirm that the proposed design reduces the output ripple by more than 2 V when compared to the typical converter. The inclusion of the proposed voltage regulation technique reduces EMI which enables this fully-integrated Dickson charge pump to bias sensor arrays where the noise typically produced by these converters currently restricts or prohibits their use.

Modeling of Interconnected Voltage and Current Controlled Converters With Coupled LC–LCL Filters

Nowadays, more and more converters under voltage control mode with LC filter and current control mode with L / LCL filter are interconnected, forming a more complicated impedance network and resulting in coupled control behaviors. This type of interconnected converter system can be commonly seen in a stand-alone microgrid and some advanced testing benches for power electronics converters. In this article, a system consisting of a voltage-controlled converter with LC filter and a current-controlled converter with LCL filter have been studied. A detailed mathematical model is proposed to describe the stability and coupling characteristics of such system. It is found that the stability problem in this type of configuration may arise in an uncommon way, which is different from the typical resonance in a grid-connected converter system and can be triggered in higher frequency band. Meanwhile, the voltage and current behaviors of interconnected converters are strongly coupled through the complicated filter impedances and control loops in low-frequency band and mid-frequency band.