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This paper proposes an adaptive self-tuned current controller design for an LCL-filtered LC-tuned single-phase shunt hybrid active power filter (HAPF), which offers adaptability and flexibility of the control system to filter parameters changes. System modelling and online identification of parameters are presented. Then, a systematic self-tuned controller (STC) design is proposed to calculate the controller parameters and obtain the final control law adaptively. The exact HAPF parameters design is described and the overall closed-loop stability evaluation with considering digital control delay is discussed in this paper. To confirm the theoretical achievements, simulations and experimental tests on a real prototype system are performed. The results are in good accordance and demonstrate the effectiveness and superior performance of the suggested control technique. Furthermore, a comprehensive comparison between the proposed control technique and other controllers is provided to show the superiority of the proposed system from different aspects. [ABSTRACT FROM AUTHOR]

The penetration of inverter-based resources is growing rapidly around the globe. During a fault, the behavior of inverter-based resource is very different from a traditional synchronous machine based generation. As the penetration of inverter-based resources increases, there is an urgency for the industry to understand their behavior so that these resources can be accurately modeled and their impact on various protection schemes can be studied in detail. This path begins with standardizing the response of inverter-based resources to a fault. Additionally, this paper discusses issues with modeling inverter-based resources in a short circuit model which is developed for systems dominated by synchronous machines and finally a need for further research to understand impacts on traditional protection schemes as inverter-based resources continue to displace traditional generation. This paper does not offer solutions however, it is intended to encourage further study by industry professionals toward development of solutions. [ABSTRACT FROM AUTHOR]

As high-speed railways develop rapidly worldwide, the problems of neutral section and power quality are becoming increasingly prominent. These problems affect the normal operation of the power grid and the electric locomotives. The co-phase power supply schemes were proposed by researchers, which can solve these problems well. Among them, the MMC-based scheme (MMC-5 L) is a representative one. The existing scheme, when applied to the auto-transformer (AT)-fed system which is usually adopted by high-speed railways, needs to install an auto-transformer on the output side. To eliminate the auto-transformer and to obtain the more suitable and superior co-phase power supply schemes to AT-fed system, two improved schemes are proposed in this paper based on the novel MMC-based scheme (MMC-6 L) we proposed before. While establishing the mathematical model, creating the control system, and carrying out simulation verification of the proposed schemes, the proposed schemes are compared with MMC-6 L and MMC-5 L in terms of topological characteristics, control system complexity, power loss, and functionality. The results show that while the proposed schemes can fulfill the functions of the co-phase power supply scheme well like the MMC-5 L and MMC-6 L, they are more suitable and superior for the AT-fed system in terms of control system complexity and power loss. Among them, MMC-5L1CB performs best in power loss, and MMC-4L2CB performs best in control system complexity. [ABSTRACT FROM AUTHOR]

Most microgrid protection schemes found in published literature suffer from a lack of generality in that they work well for the assumed topology, including type and placement of sources. Other generic protection schemes tend to be too complicated, too expensive, or both. To overcome these drawbacks, a topology-agnostic, scalable, and cost-aware protection based on fundamental principles that work in the presence of high penetration of inverter-based resources (IBRs) is developed and tested in this paper. The protection system also implements stable automatic reconfiguration of the healthy sections of the system after clearance of fault, thus increasing resilience by self-healing. To achieve this ambitious goal, stable inverter models are developed that operate in unbalanced networks in grid-connected and islanded modes, even with 100% IBRs, share power without conflicting controls, and can ride through faults while limiting fault currents. The scheme is tested for primary and backup protection and reconfiguration on the IEEE 123-node feeder in grid-connected and islanded modes with 15 IBRs connected to the system. [ABSTRACT FROM AUTHOR]

The high-frequency resonances (HFRs) around 700 Hz and 1.8 kHz have occurred in Yu′E project. Mechanism studies indicate that HFRs are caused by the interactions between the negative damping inductance characteristics of the MMC and the capacitive characteristics of the AC system. In this paper, two optimized methods regarding the current control loop are presented to suppress the HFRs. The first one is to use a round(x) function inserted into voltage feed-forward loop instead of low pass filter (LPF). The proposed method has less negative impact on transient characteristics of an MMC. The second one is to employ a second-order damping controller, including the parameters design, to reshape the MMC impedance around the multiple risky regions. The core of HFRs suppression is that the real part of the MMC impedance near the multiple intersection frequencies should be positive according to impedance stability theory. Combined with the proposed control methods and core of HFRs suppression, the simplified model of MMC is adopted to design the parameters using the analytical expressions. Moreover, the general steps of parameters design are presented. Finally, the optimized methods to suppress HFRs are verified by time-domain simulations. [ABSTRACT FROM AUTHOR]

This paper proposes a new passive Islanding detection approach for the Inverter Interfaced Distributed Generators (IIDGs) operating under different control modes. First, a novel index is formulated using Root Mean Square (RMS) value of the voltage and the frequency information, acquired at the Point of Common Coupling (PCC) of the target DG. The index is then post-processed by estimating the number of peaks and valleys $(N_{p_{k}-v})$ in a cycle using the sliding window concept, which indicates the occurrence of an event. Finally, the nature of the event, i.e., Islanding or non-Islanding, is decided by exploiting the two statistical features viz., Square of RMS $(S_{RMS})$ of $N_{p_{k}-v}$ and Average Crest Factor $(ACF^{m})$ of the proposed index. The performance of the proposed method (P-IDM) is demonstrated through comprehensive studies in PSCAD/EMTDC for the single-bus and IEEE 34-bus Active Distribution System (ADS). Also, the real-time validation is performed for the IEEE 34-bus using Controller Hardware-in-the-Loop (CHIL) setup comprising of Real-Time Digital Simulator (RTDS) and dSPACE1104. The simulation and hardware results for different test systems indicate that the proposed scheme, being generic in nature, sustains its efficacy under various interface controls of IIDGs. Finally, a comparative assessment with the other existing Signal Processing (SP) based passive methodologies shows the supremacy of the proposed method in terms of accuracy and faster detection. [ABSTRACT FROM AUTHOR]

This paper presents a combined control scheme that consists of two complementary control strategies, namely hierarchical control and autonomous control to handle contingencies in a multi-terminal high voltage direct current (MTDC) system. The proposed hierarchical control considers three optimal targets, i.e., equal or proportional power sharing, minimizations of transmission loss and total DC voltage variation. The primary control layer implements the droop control for voltage source converters (VSCs). While in the secondary control layer, voltage reference setpoints of the VSCs in droop control scheme are calculated by the proposed optimal power flow method with the equal or proportional power sharing as equality constraints. As the hierarchical control relies on communication system, the autonomous control for power sharing can be activated during communication system malfunctioning or rapid power variation caused by renewable energy sources. The combined control strategy can greatly increase the reliability of the MTDC system. The simulation results of a five-terminal MTDC network conducted in the MATLAB/Simulink software verify the performance of the proposed combined control strategy. [ABSTRACT FROM AUTHOR]

Traditional single-ended fault location method is influenced by the fault resistance, and the remote in feed, which reflects as the two equations built by fault phase have four unknowns. And in the grid-connected converter system, the weak feedback characteristic further reduces the performance of the method. To solve it, a single-ended fault location method based on control and protection coordination is proposed in this paper. First, using the high controlled-ability of the converter, the control strategy for fault ride-through (FRT) operation is divided into two stages by switching control modes. Then, according to the electrical quantities under two stages, four uncorrelated equations can be established, and the equations still have four unknown parameters. The fault distance can be calculated by solving them. The simulation results show that the proposed method has high accuracy and is immune to fault resistance and fault location, it solves the problem of insufficient equations in principle. [ABSTRACT FROM AUTHOR]

This paper develops a new control strategy for voltage source converter (VSC) based multi-terminal high voltage dc (MT-HVDC) grids for enhancing the mutual frequency support between MT-HVDC interconnected asynchronous ac systems. The existing droop control structure of VSCs is augmented with an additional feedback voltage-based loop that generates a voltage shifting term to be added to the original dc voltage reference. The addition of this integrator-based controller nullifies the difference between voltage errors seen at adjacent converter stations. This action helps to overcome inaccuracies in the mutual frequency support caused by the uneven voltages across a dc system. The proposed strategy uses only local variables for the continuous-time control; thus, it does not rely on a high-bandwidth communication network, and its performance is not affected by the change of ac systems’ parameters. Modal analysis is carried out to investigate the stability of the proposed controller. Finally, comprehensive simulation studies are used to verify the proposed strategy’s performance and compare it with recently reported alternative controllers. [ABSTRACT FROM AUTHOR]

This paper presents a control of the parallel operation of diode rectifier based high-voltage direct current (DR-HVDC) and voltage source converter based HVDC (VSC-HVDC) links. The two links are connected to offshore wind farms (OWFs). The VSC-HVDC controls the voltage and frequency of the offshore AC-grid needed for the OWFs start-up. The proposed grid forming control is based on controlling the OWFs voltage by controlling the active power balance at the VSC-HVDC AC-bus. When the OWFs inject more power than a certain active power reference established for the VSC-HVDC, the OWFs voltage increases and the DR-HVDC starts conducting. Then, the offshore voltage automatically accommodates the required value for transmitting the OWFs power through the DR-HVDC link while the system frequency is controlled by the VSC-HVDC through satisfying the reactive power balance. The power transmitted through the VSC-HVDC can be set by adjusting the corresponding power command while the DR-HVDC will automatically close the power balance. The main advantage of the proposed control system is that the VSC-HVDC can be sized just to support the system energization; while once the OWFs are in operation, the DR-HVDC will be loaded automatically. Simulation results verify the proposed control system. [ABSTRACT FROM AUTHOR]

On March 31, 2019, a 500 kV air-core, shunt reactor at a large generating station in BC Hydro was damaged shortly after four unit transformers were simultaneously energized from a 500 kV series and shunt compensated line source. Forensic investigations revealed that the controlled switching system, controlling the instant at which each breaker phase closes during energization to minimize inrush current, had failed. Uncontrolled closing led to under-damped, high-magnitude and sub-synchronous currents. Protection tripped and isolated the four transformers, followed shortly by shunt reactor protection tripping. Before the reactor could be isolated by circuit breakers, a damaging internal fault developed. This paper presents forensic investigation describing sequence of events leading up to reactor damage and shares the lessons learned with the peer utilities having similar installations.    [ABSTRACT FROM AUTHOR]

This paper presents a control strategy to mitigate subsynchronous oscillations (SSOs) in doubly-fed induction generator (DFIG)-based wind farms integrated into series-compensated transmission systems. The strategy has two parts: in the first one, a compensation filter based on the motion-induction amplification concept is proposed to increase the damping of the DFIG machine in the subsynchronous frequency range; in the second one, a proportional-integral (PI)-like controller is designed using an optimal quadratic technique to minimize the control effort and the additional rotor voltage required by the SSO damping action. The SSO mitigation strategy acts locally on the DFIG control system reducing the negative resistance the rotating machine presents to the grid at subsynchronous frequencies; this approach reduces the control dependence on the topology and resonance frequencies of the network. The control strategy is validated with a case study based on the Argentinian power system and evaluated in a wide range of operating conditions, showing that the DFIG control system can be enhanced to mitigate poorly damped SSOs and increase the penetration level of wind power in the system. [ABSTRACT FROM AUTHOR]

In high power applications, 3-phase ac/dc Voltage Source Converters (VSCs) are paralleled to increase the overall aggregated converter power rating, effective switching frequency and control bandwidth. For this, interleaved Pulse Width Modulation (PWM) and current limiting paralleling inductors are required. Interphase Transformers (ITRs) are often used for the converter paralleling because they provide large magnetizing inductances for the currents flowing between the converters while introducing negligible leakage inductances for the currents flowing from the converters to the ac grid. This large asymmetry in the inductance values makes it difficult to independently control active power flows in the individual converters. Thus, if the converters dc buses are not mutually interconnected, instabilities in active control of balance of converter dc bus voltages can occur in certain operational conditions. This paper presents a theoretical analysis of the active power flows produced by the currents flowing among the converters and explains origins of potential instability in the active control of balance of dc bus voltages in multi-converter systems with floating dc buses, paralleled using the ITRs. [ABSTRACT FROM AUTHOR]

Multi-port solid-state transformer (SST) characterized by high scalability is expected to be widely used in AC/DC hybrid microgrid. However, the DC bus voltage deviation and dynamic response speed are two key issues in its application. Therefore, a model predictive direct power control (MPDPC) of the three-port SST (TPSST) is proposed. Compared with the traditional PI-based method, the oscillation of the DC bus voltage is inhibited, and the steady-state performance is improved with the MPDPC method. Moreover, a direct power path is incorporated into the control system in order to integrate the rectifier and dual active bridge (DAB) stage. This power path delivers the power required by the load to the DAB stage and the rectifier stage for power control, thereby improving the dynamic response of the system in the process. Besides, since the MPDPC strategy is a control scheme for the whole system, the design of the control system is simplified. Finally, a low power prototype has been built and tested, and the experimental comparison with the traditional voltage control (TVC) and model predictive voltage control (MPVC) method verifies the efficiency improvement of the proposed control strategy. [ABSTRACT FROM AUTHOR]

This work focuses on grid-forming (GFM) control of Interconnecting Power Converters (IPCs) that are used to interconnect multiple HVAC and HVDC subgrids to form a grid of grids. We introduce the concept of dual-port GFM control that leverages the ability of Modular Multilevel Converters (MMCs) to simultaneously form its AC and DC terminal voltage and present two dual-port GFM MMC controls. We provide analytical results and high-fidelity simulations that demonstrate that (i) dual-port GFM control is more resilient to contingencies (i.e., line and generator outages) than state-of-the-art single-port GFM control, and (ii) unlike single-port GFM control, dual-port GFM control does not require assigning grid-forming and grid-following (GFL) roles to the IPC terminals in grids of grids. Finally, we provide an in-depth discussion and comparison of single-port GFM control and the proposed dual-port GFM controls. [ABSTRACT FROM AUTHOR]

This work proposes the Hybrid Control Converter (HCC), a new kind of VSC control. This method aims to have one single converter mimic the behavior of two individual, virtual converters working in parallel (one with grid-following and the other with grid-forming control). This way, the hybrid converter can show behavioral characteristics of both kinds of converter control at the same time. Two possible applications for the HCC are presented. The first one is related to synthetic inertia capability of wind turbines. Results show that the HCC presents superior performance when compared to conventional techniques based on grid following converters, resulting in a smaller grid frequency drop during transients. The second application shows that the HCC allows for autonomous connection of multiple distinct AC systems using a meshed DC transmission system. This enables automatic frequency support and generation and load sharing between the AC grids without requiring any specific action by the system operator. [ABSTRACT FROM AUTHOR]

This article proposes a comprehensive coordinated suppression strategy for DC overvoltage to achieve the receiving-end fault ride-through of DC grid connected large-scale wind power. For the monopolar DC overvoltage, by reasonably switching the control mode of bipolar modular multilevel converter (MMC), the power transfer capability of the healthy pole MMC can be improved while maintaining DC voltage stability, and the precise load-shedding control of wind farm and its coordination scheme with dissipative resistors are designed, which can effectively reduce the input capacity of dissipative resistors and the risk of wind turbines being cut off. For the bipolar DC overvoltage, the load-shedding control strategy and its parameter selection principles are proposed, which can make wind farm and capacitors in each MMC absorb excess power together, to reduce the change rate and amplitude of DC voltage. Then its cooperation scheme with the dissipative resistors is designed to achieve fault ride-through in the full power range of wind farm. Finally, based on the simulation model of four- terminal DC grid, the effectiveness of the proposed coordinated control methods is verified. [ABSTRACT FROM AUTHOR]

This article proposes a new control structure for grid-interfaced converters to enhance ancillary services from emerging electric vehicle (EV) charging stations (CS) under unbalanced and distorted grid conditions. Compared to existing methods, the proposed control structure is relatively simple yet effective for both regulating powers and minimizing harmonics under unbalanced and distorted grid conditions. A simple direct predictive control methodology is proposed where the positive fundamental component of currents is controlled for power regulation while other negative sequence or harmonic current components are set to zeros. Consequently, the charging station operation is enhanced with efficient ancillary services to the electric grid. The novelty originates from a simple predictive control structure in an abc-natural frame without adding any decoupled or separated current-component controllers. Comparative studies on both Matlab/Simulink platform and OPAL-RT-based real-time system among proportional integral (PI) control, finite predictive control, and the proposed method are conducted to verify the efficacy of the proposed structure. The comparative results show that the control structure improves the control performance and significantly enhances the support for the local loads under unbalanced, harmonic distorted grid conditions. [ABSTRACT FROM AUTHOR]

Small-signal models (SSM) of modular multilevel converter (MMC) based HVDC grid under unbalanced grid conditions in recent literatures do not consider interactions between MMC and dc network. Therefore, it is necessary to correctly model the double-fundamental-frequency zero-sequence circulating currents (DFF-ZSCC) existing under unbalanced grid conditions, although they can be suppressed to avoid penetrating into dc side. In view of that, a general and modular small-signal modeling method of MMC-HVDC grid under unbalanced grid condition is developed. Firstly, a general d-q transformation process for multi-sequence and multi-frequency components is introduced to build the state-space model of MMC electric part in d-q frame. Secondly, the SSM of dc network, which is divided into dc- and double-fundamental-frequency parts, is developed based on the existing frequency-dependent-cascaded-pi (FDC-PI) cable model. Then, to interface MMC stations with dc network under unbalanced grid conditions, a general method to synchronize the variables of double fundamental frequency at dc side of each MMC station is introduced to compose the SSM of the entire HVDC grid from SSMs of each MMC and dc network. Finally, taking a three-terminal MMC-HVDC grid as an example, two SSMs of the MMC-HVDC grid with FDC-PI cable model and single-PI cable model are developed in Matlab, respectively. The small-signal dynamics and stability of both SSMs of the grid under unbalanced grid conditions are compared to validate the accuracy and stability of the proposed SSM of the whole HVDC grid. The simulation results validate the correctness and effectiveness of the proposed modular modeling method. [ABSTRACT FROM AUTHOR]