Reactive Current Injection Research Articles

Research on Low Voltage Ride through Strategy for Power Electronic Transformer

In view of the operating characteristics for voltage sags of AC side of the power electronic transformer(PET), a low-voltage ride through(LVRT) strategy adapted to bidirectional power exchange of PET is proposed for the purposes of maintaining the system stability, assisting the system voltage recovery and protecting PET safety. During the asymmetric voltage sag, the negative sequence current of PET is eliminated to ensure the symmetry of the injected current. According to the degree of positive sequence voltage sag, the reactive current injection is provided to assist in voltage recovery. According to the PET active power condition before the voltage sag, the level and direction of which are maintained as far as possible without exceeding the limit, for which the disturbance to the AC and DC grids is reduced. Finally, the effectiveness of the proposed LVRT strategy is verified by simulation model.

A Novel Wind Farm Control Strategy to Mitigate Voltage Dip Induced Frequency Excursion

In this paper, a novel wind farm control strategy is proposed to mitigate voltage dip induced frequency excursion in wind integrated power systems. In the proposed control strategy, conventional fixed reactive/active current injection priority is replaced by an optimum adaptive priority based reactive and active current injection. The control problem is formulated as a binary integer optimization problem, which simultaneously minimizes active power shortfall from wind generation during fault and complies with grid code requirements. Detailed wind farm model and wake effect are taken into consideration to achieve realistic response from the wind farm, while IEC 61400-27-1 wind turbine generator model is adopted for wind turbine modeling. Dynamic model of the test system has been developed in DIgSILENT PowerFactory, while the static model is also developed in MATLAB, and cosimulation platform has been used to test and validate the proposed control strategy. The proposed control strategy is applicable to variable speed wind turbine-based wind farm.

PI‐based controller for low‐power distributed inverters to maximise reactive current injection while avoiding over voltage during voltage sags

In the recently deregulated power system scenario, the growing number of distributed generation sources should be considered as an opportunity to improve stability and power quality along the grid. To make progress in this direction, this work proposes a reactive current injection control scheme for distributed inverters under voltage sags. During the sag, the inverter injects, at least, the minimum amount of reactive current required by the grid code. The flexible reactive power injection ensures that one phase current is maintained at its maximum rated value, providing maximum support to the most faulted phase voltage. In addition, active power curtailment occurs only to satisfy the grid code reactive current requirements. As well as, a voltage control loop is implemented to avoid overvoltage in non-faulty phases, which otherwise would probably occur due to the injection of reactive current into an inductive grid. The controller is proposed for low-power rating distributed inverters where conventional voltage support provided by large power plants is not available. The implementation of the controller provides a low computational burden because conventional PI-based control loops may apply. Selected experimental results are reported in order to validate the effectiveness of the proposed control scheme.

Optimized Demagnetizing Control of DFIG Power Converter for Reduced Thermal Stress During Symmetrical Grid Fault

Direct linking between the stator of a doubly fed induction generator (DFIG) and the power grid makes this type of generator sensitive to disturbances in the grid voltage, which may lead to high voltage and current on the rotor side. Moreover, modern grid codes, which specify stringent requirements on reactive power compensation, challenge fault ride-through operation even more. In this paper, based on conventional demagnetizing current control, the capability of a DFIG rotor-side converter to ride-through a symmetrical grid fault is calculated in accordance with its current and voltage ratings. Afterwards, an optimized demagnetizing coefficient is designed to guarantee the same rotor current amplitude between the instants of the fault occurrence and the reactive current injection. A reduction of the junction temperature of the power device can thereby be achieved. It is concluded that, regardless of the rotor speed, the demagnetizing coefficient is related only to the dip level. Compared with traditional vector control, a simulation of 2 MW DFIG system agrees with the reduced thermal stress during the fault period, and experimental results in a down-scale DFIG system verify the feasibility of the proposed control strategy as seen from the electrical characteristics.

On Full-Bridge Bipolar MMC-HVDC Control and Protection for Transient Fault and Interaction Studies

Bipolar high voltage direct current (HVdc) links present one reasonable solution to realize bulk power transmission schemes. Especially if full-bridge modular multilevel converter topologies (FB MMC) are utilized, a broad variety of operational advantages exists. To completely unveil intrinsic technological potentials, comprehensive control and protection concepts to discriminate and handle ac, dc, as well as internal converter faults are essential. The proposed scheme consists of a fast and selective multizonal fault detection paired with decoupled ac and dc current controls. As a result, actively controlled dc fault current interruption as well as ac fault reactive current injection are incorporated. It is demonstrated that internal converter quantities remain within feasible technical boundaries, interactions are limited or even fully mitigated, and protection thresholds can be chosen to enable selective tripping. To validate and evaluate the proposed concept, simulations for characteristic faults are performed in electromagnetic transients software.

Voltage Support Experimental Analysis of a Low-Voltage Ride-Through Strategy Applied to Grid-Connected Distributed Inverters

In recent decades, different control strategies have been designed for the increasing integration of distributed generation systems. These systems, most of them based on renewable energies, use electronic converters to exchange power with the grid. Capabilities such as low-voltage ride-through and reactive current injection have been experimentally explored and reported in many research papers with a single inverter; however, these capabilities have not been examined in depth in a scenario with multiple inverters connected to the grid. Only few simulation works that include certain methods of reactive power control to solve overvoltage issues in low voltage grids can be found in the literature. Therefore, the overall objective of the work presented in this paper is to provide an experimental analysis of a low-voltage ride-through strategy applied to distributed power generation systems to help support the grid during voltage sags. The amount of reactive power will depend on the capability of each inverter and the amount of generated active power. The obtained experimental results demonstrate that, depending on the configuration of distributed generation, diverse inverters could have different control strategies. In the same way, the discussion of these results shows that the present object of study is of great interest for future research.

Placement of measurements for electrical distribution network state estimation

The paper is concerned with the problem of placement of the minimum number of smart meters to ensure either the observability of all state variables in distribution network or the observability of voltage magnitudes. Voltage control is important in the distribution network with distributed generation sources which adoption can lead to unpredictable overvoltage exceeding admissible values. The algorithm for smart meters including measurements of voltage magnitudes and active and reactive current injections is similar to the algorithm of choosing the minimum number of phasor measurement units to ensure topological observability. Optimal control of the active distribution network operation requires monitoring to be based on a classical linear state estimation procedure. The results of the research demonstrate the effectiveness of the proposed approaches and are illustrated by example of a test distribution network.

Performance Comparison of Grid-Faulty Control Schemes for Inverter-Based Industrial Microgrids

Several control schemes specifically designed to operate inverter-based industrial microgrids during voltage sags have been recently proposed. This paper first classifies these control schemes in three categories and then performs a comparative analysis of them. Representative control schemes of each category are selected, described and used to identify the main features and performance of the considered category. The comparison is based on the evaluation of several indexes, which measure the power quality of the installation and utility grid during voltage sags, including voltage regulation, reactive current injection and transient response. The paper includes selected simulation results from a 500 kVA industrial microgrid to validate the expected features of the considered control schemes. Finally, in view of the obtained results, the paper proposes an alternative solution to cope with voltage sags, which includes the use of a static compensator in parallel with the microgrid. The novelty of this proposal is the suitable selection of the control schemes for both the microgrid and the static compensator. The superior performance of the proposal is confirmed by the analysis of the quality indexes. Its practical limitations are also revealed, showing that the topic studied in this paper is still open for further research.

Combined Reactive Power Injection Modulation and Grid Current Distortion Improvement Approach for H6 Transformer-Less Photovoltaic Inverter

In this paper, a combined reactive power modulation and grid current distortion improvement approach is proposed for an H6 transformer-less full-bridge single-phase photovoltaic (PV) grid-connected inverter. H6 transformer-less inverters with traditional modulation and control strategies may not satisfy the requirement of reactive power compensation or may result in more severe zero-crossing current distortion. Therefore, contrary to the traditional modulation, a novel reactive power injection space vector pulse width modulation (SVPWM) is proposed, which consists of two operation stages—inverter modulation and reactive power modulation. The implementation of SVPWM for reactive power modulation using a digital signal processor is also investigated. Furthermore, to suppress the current zero-crossing distortion in the reactive power injection mode, a global sliding mode function based on the proportion-integration-resonance current controller is designed, and the control law of the global sliding mode is derived. Using the segment modulation and grid current distortion improvement approach, the function of reactive current injection is implemented in commercial PV inverters, and the total harmonic distortion of the grid current is decreased significantly by more than 5% in the low-power segment, under the operating conditions of a lagging or leading power factor of 0.95. The effectiveness and feasibility of the proposed approach are verified through simulation and experiment using a 5-kVA prototype.

Design of an inverter-side current reference and controller for a single-phase LCL-based grid-connected inverter

Summary This paper presents the design of a current reference used in an inverter-side current controller for a grid-connected single-phase transformerless inverter with an Inductive-Capacitive-Inductive of LCL filter. The idea is to indirectly control the inverter-side current, by directly acting on the grid-side current. Furthermore, it can be shown that the simple proportional term guarantees damping injection. However, the design of the current reference for such a controller is a more involved process, which turns out to be even more challenging in the case where the grid voltage signal is subject to harmonic distortion. In this case, the distortion propagates into the converter through the LCL filter. Therefore, an inverter-side current reference must be proposed to be as distorted as necessary to assure indirect tracking of the grid-side current towards its corresponding reference, which is built as a sinusoidal with a given phase displacement to allow reactive current injection. The inverter-side controller must then include a harmonic compensation mechanism, to assure perfect tracking of the inverter-side current towards its distorted reference, and thus, achieve an adequate performance under harmonic distortion. Experiments are performed in a prototype to evaluate the performance of the proposed controller.

Peak Current Limitation for Grid Side Inverter by Limited Active Power in PMSG-Based Wind Turbines During Different Grid Faults

Grid faults constitute the main challenge for distributed generation, especially permanent magnet synchronous generator (PMSG)-based wind turbines connected to the grid via back-to-back (BTB) converters. On the other hand, grid codes enforced wind turbines to aid network and accomplish low-voltage ride-through (LVRT) requirements during grid faults. DC-link overvoltage suppression without external devices and reactive current injection to the grid in symmetrical and asymmetrical faults were the main required subjects in different grid codes. Reactive current injection may lead to overcurrent in one or two phases during asymmetrical faults. This paper presents solutions to accomplish LVRT requirements during grid faults, including dc-link overvoltage suppression by improving BTB converter's controllers and the design of active power limitation to maintain the peak current of the grid side inverter in a safe range during different asymmetrical grid faults. In fact, by controlling generation of PMSG active power directly with grid side inverter, the peak current remained in the safe limit and required reactive current, including negative-sequence component can be injected to the grid. Furthermore, the grid side active power oscillation and dc-link voltage ripple can be suppressed by using the proposed controller. Simulation results showed the validity and efficiently of the proposed control approach in different conditions.

최대 전압 강하에 비례하는 무효전류 공급 루프를 이용한 DFIG 풍력단지의 계층전압제어

최대 전압 강하에 비례하는 무효전류 공급 루프를 이용한 DFIG 풍력단지의 계층전압제어

Hybrid modular multilevel converter based multi‐terminal DC/DC converter with minimised full‐bridge submodules ratio considering DC fault isolation

An isolated high-power multi-terminal DC/DC converter is studied in this paper, based on hybrid MMC configuration consisting of full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs). To decrease the investment and power losses, a reduced arm FBSMs ratio (less than 0.5) scheme is adopted. A detailed analysis on the relationship of the DC/DC converter inner AC voltage and the arm FBSMs ratio under reduced DC voltage is presented. Based on this, a control strategy during DC fault is proposed which continues operating the converter connected to the faulty DC side with reactive current absorption. Under the same arm FBSMs ratio, compared to the conventional strategy of blocking the faulty side converter during a DC fault, the proposed unblocking method with reactive current injection can not only achieve greater DC fault current declining rate, but also ensure maximum power transfer between the interconnected healthy DC grids by maintaining a higher inner AC voltage in the DC/DC converter. The two strategies are compared and validated by simulations using PSCAD/EMTDC under different arm FBSMs ratio.

Probabilistic Wavelet Fuzzy Neural Network based reactive power control for grid-connected three-phase PV system during grid faults

This study presents a reactive power controller using Probabilistic Wavelet Fuzzy Neural Network (PWFNN) for grid-connected three-phase PhotoVoltaic (PV) system during grid faults. The controller also considers the ratio of the injected reactive current to meet the Low Voltage Ride Through (LVRT) regulation. Moreover, the balance of the active power between the PV panel and the grid-connected inverter during grid faults is controlled by the dc-link bus voltage. Furthermore, to reduce the risk of over-current during LVRT operation, a current limit is predefined for the injection of reactive current. The main contribution of this study is the introduction of the PWFNN controller for reactive and active power control that provides LVRT operation with power balance under various grid fault conditions. Finally, some experimental tests are realized to validate the effectiveness of the proposed controller.

3상 계통 연계형 인버터의 역상분 전류 주입을 이용한 계통 등가 임피던스 추정 기법

A new algorithm is proposed for the estimation of equivalent grid impedance at the point of common coupling of a grid-tie inverter output. The estimated impedance parameter can be used for the improvement of the performance and the stability of the distributed generation system. The estimation error is inevitable in the conventional estimation method because of the axis rotation due to PLL. In the conventional estimation error, the d-q voltage and current are used for the calculation of the impedance with active and reactive current injections. Conversely, in the proposed algorithm, the negative sequence current is injected, and then the negative sequence voltage is measured for the impedance estimation. As the positive and negative sequence current controller is independent and the PLL is based on the positive sequence component only, the estimation of the equivalent impedance can be achieved with high accuracy. Simulation and experimental results are compared to validate the proposed algorithm.

Current Harmonics From Single-Phase Grid-Connected Inverters—Examination and Suppression

Environmental conditions and operational modes may significantly impact the distortion level of the injected current from single-phase grid-connected inverter systems, such as photovoltaic (PV) inverters, which may operate in cloudy days with a maximum power point tracking, in a nonunity power factor, or in the low-voltage ride-through mode with reactive current injection. In this paper, the mechanism of the harmonic current injection from grid-connected single-phase inverter systems is thus explored, and the analysis is conducted on single-phase PV systems. In particular, the analysis is focused on the impacts of the power factor and the feed-in grid current level on the quality of the feed-in grid current from single-phase inverters. As a consequence, an internal model principle-based high-performance current control solution is tailor-made and developed for single-phase grid-connected systems to produce high-quality currents in different operation conditions, where a design procedure is also provided. The developed current controller in this paper can achieve a minimum steady-state error while maintaining a relatively fast transient response, and also being feasible in other single-phase applications as a promising harmonic mitigation solution. Experiments on single-phase grid-connected systems have verified the correctness of the relevant analysis and also the effectiveness of the tailor-made control solution in terms of good harmonic mitigation.

Enhanced Dynamic Voltage Control of Type 4 Wind Turbines During Unbalanced Grid Faults

The fully rated converter of type 4 wind turbines is capable of providing dynamic voltage control during grid faults by injecting controlled reactive currents. This paper describes three different dynamic voltage control options during unbalanced grid faults: 1) the positive sequence voltage control with only a positive sequence reactive current injection and suppression of the negative sequence current; 2) the positive sequence voltage control with limitation of the positive sequence reactive current injection and suppression of the negative sequence current; and 3) the positive and negative sequence voltage control with both a positive and a negative sequence reactive current injection. These different control options are compared in simulations of a wind power plant connected to a meshed power system, including synchronous generators. It is shown that both the positive sequence voltage control with limitation and the positive and negative sequence voltage control can overcome the voltage rise and voltage distortion that can occur with pure positive sequence voltage control without limitation. Both of these options have a distinct fault response, where the positive and negative sequence voltage control results in a fault response that resembles the fault response of a synchronous generator with higher fault current contributions in the faulted phases.

Sizing and siting static synchronous compensator devices in the Portuguese transmission system for improving system security

This study presents a methodology for siting and sizing static synchronous compensator (STATCOM) devices in the Portuguese transmission system in order to improve system security following severe grid faults. Security issues arise since the Portuguese transmission system incorporates significant levels of wind generation without fault ride through and reactive current injection capabilities during grid faults. As the transmission system operator (TSO) is responsible for assuring system security, the goal of the study is to take advantage of the proved STATCOM ability for injecting reactive current in order to mitigate the disconnection of large amounts of wind farms in case of severe grid faults. The proposed methodology was developed and tested in coordination with the Portuguese TSO and it is based on the formulation of an optimisation problem in order to minimise the installed STATCOM power while ensuring its compliance with the current grid code requirements, namely in what concerns to the system stability and security. Given the discrete and complex nature of the problem, a hybrid approach, combining both a heuristic method and an evolutionary particle swarm optimisation (EPSO) algorithm was developed. Results show the effectiveness of the proposed methodology as well as its robustness regarding the validity of the obtained solutions while facing the most severe operational scenarios.

Reactive current injection protocol for low‐power rating distributed generation sources under voltage sags

Voltage sags are one of the main problems in transmission and distribution networks. This study proposes a voltage support control scheme for grid-connected low-power rating inverters under voltage sags. Voltage support capability is provided thanks to reactive current injection. The main objective is to inject the maximum rated reactive current during the voltage sag. Second, to raise the higher phase voltage to a predefined maximum boundary, thus preventing over-voltage. Moreover, with this strategy the phase voltages can be equalised. The first objective can be always accomplished during voltage sags. Achieving the second objective depends on the grid characteristics, the sag profile and the power rating of the inverter. Selected experimental results are reported to validate the effectiveness of the proposed control.

Novel Fault Ride-Through Scheme and Control Strategy for Doubly Fed Induction Generator-Based Wind Turbine

This paper presents a novel modulated series dynamic braking resistor (MSDBR) control strategy for enhancing the fault ride-through (FRT) of doubly fed induction generator-based wind turbines. The proposed cost-effective protection scheme introduces a voltage booster that offers series voltage compensation capability and provides a means of power evacuation to mitigate the power imbalance during grid faults. To attain flexible and robust control solution for handling both balanced and unbalanced grid faults, the proposed scheme employs a modulated pulse width modulation (PWM) switching technique to control the stator phase voltage individually. The proposed transient management scheme allows the MSDBR to mitigate the impact from different types of grid faults and to fulfill with the recent grid code requirement. Also, reactive current injection capability during faults is also investigated with the proposed voltage reference algorithm. For the controller design, small-signal modeling is utilized with consideration of measurement dynamics for the tuning of controller parameters in order to ensure the system robustness and stability. Finally, the simulation results demonstrate the satisfactory performance of the MSDBR with its preferred allocation for enhancing the FRT performance against both balanced and unbalanced faults.