Delta-configured Cascaded H-Bridge (DCHB) topology is a suitable alternative for Static Synchronous Compensator (STATCOM) applications. However, under unbalanced voltage and/or current conditions, ...zero-sequence current needs to be injected to guarantee dc-link capacitor voltage balancing. The control loop of this zero-sequence current is analyzed in this paper, in order to determine the requirements that the implemented controller must fulfill. Considering these requirements, appropriate transient response and stability margin indicators are defined to quantify and evaluate the performance of different controllers that could be employed —Proportional-Resonant controller (PR), PR controller with delay compensation (PRd), and Vector Proportional-Integral controller (VPI) are proposed and analyzed for this application in this paper. Based on the defined indicators, the VPI is the most appropriate among the studied control techniques. Experimental results validate the analytical model of the controllers and their performance.
•Identification of the particularities of the i0 control loop in a DCHB STATCOM.•Definition of appropriate indicators to quantify the performance of the controllers.•Comparison of the PR, PRd and VPI controllers.•VPI preferable option to the PR and PRd in the scenario under study.•Experimental validation in a 100 kVA set-up.
Multilevel Voltage-Source Converters (VSC) based on modular structures are envisioned as a prominent alternative for grid and industry applications. Foremost among these are the Cascaded H-Bridge ...(CHB) and the Modular Multilevel Converter (MMC). In this context, depending on the application and the power conversion structure, unbalanced operating conditions can be asked to the converter. Previous investigations regarding the operation and the solutions for modular structures under unbalanced conditions have already addressed this topic, but information is dispersed over a wide number of sources. This paper identifies, classifies, and analyzes the intercluster active power balancing strategies for the adequate operation of the most commonly used modular structures in some typical unbalanced operating scenarios: the Static Synchronous Compensator (STATCOM) under unbalanced voltage and/or current conditions, the unequal power generation in large-scale photovoltaic (PV) power plants, and the uneven power distribution in a battery energy storage system (BESS). Each of the applications has been independently studied so as to provide a comprehensive analysis of the alternative techniques found in the specialized literature, clearly explaining their respective strengths and drawbacks. Several future challenges have been identified during the study, which will involve greater research effort in this key research topic.
It is getting more common every day to install inverters that offer several grid support services in parallel. As these services are provided, a simultaneous need arises to know the combined limit of ...the inverter for those services. In the present paper, operational limits are addressed based on a utility scale for a real inverter scenario with an energy storage system (ESS) (1.5 MW). The paper begins by explaining how active and reactive power limits are calculated, illustrating the PQ maps depending on the converter rated current and voltage. Then, the effect of the negative sequence injection, the phase shift of compensated harmonics and the transformer de-rating are introduced step-by-step. Finally, inverter limits for active filter applications are summarized, to finally estimate active and reactive power limits along with the harmonic current injection for some example cases. The results show that while the phase shift of the injected negative sequence has a significant effect in the available inverter current, this is not the case for the phase shift of injected harmonics. However, the amplitude of the injected negative sequence and harmonics will directly impact the power capabilities of the inverter and therefore, depending on the grid-side voltage, it might be interesting to design an output transformer with a different de-rating factor to increase the power capabilities.
Having a method for analyzing and designing regulators of controls that contain many current loops such as active filters is not a trivial task. There can be many parameters of regulators and filters ...that must be carefully selected in order to fulfill certain desired requirements. For instance, these can be stability, dynamic response, robustness under uncertainty of parameters, and rejection capability to switching harmonics. Hence, this paper provides general analysis guidelines for designing current control loops by using mathematical models in an αβ reference frame. Then, by using the proposed modeling tool, a multi-objective tuning algorithm is proposed that helps obtain all the control loops’ regulator and filter parameters, meeting all the desired requirements. Thus, the proposed analysis and design methodology is illustrated by applying it to three different controls conceived in a dq rotating reference frame with PI (Proportional Integral) regulators. The first control presents two current loops (simple dq current control), the second control uses four current loops (dual vector control, for unbalanced loads), while the third control presents eight current loops (active filter controlling current harmonics). Several experimental and simulation results show the effectiveness and usefulness of the proposed method. Since the mathematical model employed is in the αβ reference frame, it can also be easily applied to controls conceived in a αβ reference frame using resonant regulators, providing also a common comparative framework.
Soft Open Point and Shore-to-Ship Power applications permit to deploy smart grid infrastructure into distribution and port networks. Both are based on power electronics converters that require an ...optimal integration to maximize capacitive reactive power limits at minimum impact over costs and efficiency. To meet this objective, the paper defines a methodology to calculate the optimal connection voltage of power converters used in these applications. The proposed methodology is based on three novelties related to capacitive reactive power limits definition. Firstly, all affecting variables from positive and negative sequences are jointly considered completing partial approaches identified in previous works. Secondly, the impact of negative sequence affecting multiple converter terminals is considered, adding accuracy to reactive power limit calculation of Soft Open Point converters. Thirdly, the influence of two main constraints affecting modulation limits in real converters is described, quantifying its impact over maximum capacitive reactive power limits. These constraints are related to semiconductor characteristics and to digital implementation; usually not considered for dimensioning purposes at application level, but which can have a key impact on capacitive operation. At this point, the authors provide explicit details of industrially available converter designs to support calculations and evaluate its impact over the final solution. Considering the above, the proposed methodology is implemented on a real 20 MW 6.6 kV medium voltage converter design and results are quantified and compared from a reactive power capacity, efficiency and cost perspective.
With an increasing capacity of inverter-based generation and with a 100% renewable energy power system on the horizon, grid forming converters have the potential to become the prevalent control mode ...in the grid. Thus, the correct performance of these devices is going to be crucial for system stability and security of supply. Most research related to the grid-forming control is focused on normal operating conditions, although significant effort has been devoted to current limitation strategies to ensure Low Voltage Ride Through (LVRT) capability. However, most contributions usually consider only balanced faults. This paper, proposes a new current limiting method based on the well-known threshold virtual impedance (TVI) that keeps the voltage source behaviour associated to the grid forming (GFM) capability, even when the current limit is reached, while reducing the voltage unbalance according to user-defined settings.
Grid forming inverters are expected to play a key role in future power grids, replacing synchronous generator-based plants. However, the limited current capability of power electronics makes a ...difference when facing fault induced voltage sags. This work provides a comprehensive review of strategies to handle low voltage ride through events in grid forming inverters. A key contribution of this work is to differentiate between current limiting and transient stability enhancing strategies. Current limiting strategies are classified into voltage and current-based approaches according to the inverter behaviour during the fault. Their performance is evaluated attending to three criteria: (1) transient current limitation capability, related to the self-preservation of the device; and (2) fault current management and (3) transient synchronization stability, key aspects to meet grid code requirements. The modifications that are required to support the grid under asymmetrical faults are also described, focusing on the management and prioritization of positive and negative-sequence fault currents. Transient stability enhancing strategies are classified according to their implementation into (1) power synchronization loop-based and (2) current saturation-based approaches. Their main characteristics are highlighted and compared, whereas their compatibility with previous current limiting strategies is also discussed. Finally, identified open issues and challenges are covered.
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•Fault induced voltage sags will lead to overcurrents in grid forming inverters.•Current limiting strategies are classified into voltage and current-based strategies.•Transient current, current contribution and stability will depend on the strategy.•Transient enhancing strategies are used to ensure the stability during faults.•Current limiting strategies can prioritize contribution during asymmetrical faults.
High penetration of renewable energy resources in the power system results in various new challenges for power system operators. One of the promising solutions to sustain the quality and reliability ...of the power system is the integration of energy storage systems (ESSs). This article investigates the current and emerging trends and technologies for grid-connected ESSs. Different technologies of ESSs categorized as mechanical, electrical, electrochemical, chemical, and thermal are briefly explained. Especially, a detailed review of battery ESSs (BESSs) is provided as they are attracting much attention owing, in part, to the ongoing electrification of transportation. Then, the services that grid-connected ESSs provide to the grid are discussed. Grid connection of the BESSs requires power electronic converters. Therefore, a survey of popular power converter topologies, including transformer-based, transformerless with distributed or common dc-link, and hybrid systems, along with some discussions for implementing advanced grid support functionalities in the BESS control, is presented. Furthermore, the requirements of new standards and grid codes for grid-connected BESSs are reviewed for several countries around the globe. Finally, emerging technologies, including flexible power control of photovoltaic systems, hydrogen, and second-life batteries from electric vehicles, are discussed in this article.
Selective harmonic elimination-pulse width modulation (SHE-PWM) is a widely used low switching frequency modulation technique for medium-voltage high-power converters. This approach is able to adjust ...the converter fundamental component while eliminating low-order harmonics. However, some applications such as active power filters (APFs) require regulating simultaneously, both the fundamental and low-order harmonics in amplitude and phase. This article presents a novel selective harmonic control-PWM (SHC-PWM) modulator, valid for APFs, based on artificial neural networks (ANNs) and sequential quadratic programming (SQP). A new offline search methodology, based on a hybrid metaheuristic-numerical algorithm, is defined to calculate the solution space when both the fundamental and a low-order harmonic are controlled in phase and amplitude. The solutions obtained are used to train the ANNs offline. Afterwards, the ANN + SQP calculation method is used to solve the SHC-PWM problem in real-time (RT). Experimental results are provided for a three-level converter to verify the effectiveness of the proposed RT control method.
This paper presents an overview of Back-to-Back-VSC type power links as a promising technology, yet little deployed, to enhance the operational flexibility of MV distribution networks. It provides a ...technical insight of four cornerstones that Distribution System Operators could consider from a planning perspective: power electronics, backhaul network characteristics, dispatching control strategies and protection requirements. Details on power electronics and protections are given with an industrial approach, differential with information available in academic works and generic reviews. The paper showcases figures of multi-megawatt size converters and uses a real 30kV distribution network model to assess converter capacity selection and protection requirements. Additionally, different control strategies of Back-to-Back VSC power links are introduced showing the influence that backhaul network can have over them. Seen from a dispatching perspective, an overview of different control strategies is presented describing options available for MV grids that account for different digitalization levels. The overview given in this paper allows to have a clear vision of technology readiness, integration requirements and technical limitations of Back-to-Back VSC power links for its deployment in MV distribution networks.
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