Search

This paper highlights an immediate need for renewable generation developers, manufacturers, utilities and industry standardization institutes to jointly address a challenge of capacitor switching transient immunity in inverter-based wind or solar farms. The need was identified from wide-area operation records which showed partial or full generation loss in coincidence with routine utility-owned capacitor switching. The lack of a clear industry guideline has caused confusion to all involved parties. This paper recommends all entities to address this challenge collaboratively through enhanced equipment specification, design, and test before a well-defined industry guideline becomes available. [ABSTRACT FROM PUBLISHER]

Voltage imbalances are one of the most severe challenges in electrical networks, which negatively affect their loads and other connected equipment. This paper proposes a voltage support control strategy to mitigate the voltage imbalance in inverter-based low voltage distribution networks. The control scheme is derived taking in mind the following control objectives: a) to increase the positive sequence voltage as much as possible, b) to decrease the negative sequence voltage as much as possible, c) to inject the power generated by the primary source, and d) to minimize the output current of the inverter. The innovative contribution of the proposed solution is based on the design of a control algorithm that meets the aforementioned objectives without resorting to communications with other grid components. The theoretical results are experimentally validated by selected tests on a laboratory setup with X/R ratio close to one. [ABSTRACT FROM AUTHOR]

This paper develops a simplified phasor model for predicting harmonics present at the point of common coupling between a voltage-source power converter and the utility network. It employs the frequency coupling matrix (FCM) modeling technique. The FCM is experimentally determined without requiring access to converter internal details, such as control and output filter parameters, which are generally not available for converters used in distributed generation systems. Experimental measurements are conducted on a commercially available two-stage, three phase, grid-tied photovoltaic inverter to assess the accuracy of experimentally determined FCMs to predict the harmonic behavior of grid-connected converters. This paper considers two grid conditions for the experimental analysis: 1) when the grid is stiff and contains only background voltage harmonics and 2) when the grid contains harmonic grid impedances as well as background voltage harmonics. In all cases, a close match between the results predicted by the FCM approach and experimental measurements is achieved, thus demonstrating the practical viability of using experimentally determined FCM models for harmonic predictions. [ABSTRACT FROM PUBLISHER]

Inverter transformers are one of the key components in a PV plant and their operating patterns are considerably different from that of transformers in a substation. Moreover, these transformers experience higher level of harmonics generated by the inverters. Therefore, investigating the ageing profile of the inverter transformer is a timely requirement. In this paper, the ageing of inverter transformers in a medium-scale solar PV plant is studied by utilizing various real-time measurements including load current, top-oil temperature (TOT), ambient temperature, moisture in top oil, solar irradiance and individual current harmonics. An updated moisture-dependent thermal model (MDTMH), with consideration of current harmonics is developed to estimate the hot-spot temperature (HST). The influence of weather conditions, which directly determine the load current of inverter transformers on the loss of life (LOL) of an inverter transformer is also investigated. Furthermore, the effect of current harmonics levels on the inverter transformer's HST and LOL is explored. Finally, comparisons of HST and LOL of inverter transformers connected to PV arrays with three different tracking technologies, i.e., fixed-tilt, single-axis and Dual-axis PV arrays are performed. [ABSTRACT FROM AUTHOR]

Power generators contain control systems that are able to regulate system frequency, but the frequency setpoint values are rarely modified from nominal values. This paper describes design considerations for a communication system from generators to frequency-sensitive distributed energy resources (FS-DER) using changes to frequency setpoint values of generators. Signaling discrete system states by generating off-nominal system frequency values can be used as a novel narrowband unidirectional broadcast communications channel. This paper describes two protocols for utilizing off-nominal frequencies to carry information: First, a protocol for dispatching blocks of FS-DER that is suitable for systems restricted to relatively slow rates of change of frequency (ROCOF). Second, for systems that allow higher ROCOF values, the feasibility of using power generation resources as a power-line communication transmitter is shown. Data from an operating islanded power system with diesel generators are analyzed to demonstrate the feasibility of the proposed communication system in systems fed by rotating machines. The feasibility of the proposed communication system in systems fed by voltage-source inverters is shown with laboratory tests of a 20-kVA inverter. The inverter was found to have a maximum ROCOF of 2.2 Hz/s, sufficient to enable its use as a power-line communication transmitter. [ABSTRACT FROM AUTHOR]

The voltage rise problem due to the reverse power flow is one of the main obstacles to expanding the photovoltaic systems (PVSs) in distribution networks. In this paper, a decentralized control method for coordinated control of on-load tap changer (OLTC) transformers and PV inverters, is proposed for the voltage regulation of radial distribution networks. In this method, the allowable voltage range is divided into several zones, and appropriate corrective actions are adaptively taken according to each zone. To reduce the tension on the OLTC, first, the reactive power capability of the PV inverters is employed for the voltage regulation, and if it is insufficient, OLTC is used as the last solution. In this regard, the unused capacity of PV inverters is utilized for the reactive power compensation, considering the grid codes reactive power requirements. Furthermore, decentralized control based on a signaling method is used for coordination between OLTC and PVSs, which eliminates the need for communication links. Finally, the effectiveness of the proposed method is validated by the simulation results for the IEEE 33-bus distribution network, and the impact of different reactive power control methods on the number of OLTC operation is investigated. [ABSTRACT FROM AUTHOR]

This paper addresses the resonance problem in a parallel-inverter-based grid-connected system. Harmonic interactions between inverters and the grid exhibit various number of complicated characteristics that threatens the system stability and power quality. In this paper, a method of harmonic resonance analysis is proposed to investigate the resonance problem in a multi-inverter grid-connected system based on resonance modal analysis (RMA) and participation factor (PF) calculations. Initially, an impedance-based analytical approach is employed and expanded to an electricity distribution network that is dominated by multiple inverters with LCL filters and proportional resonant controllers. Then, the RMA is introduced to investigate the resonant interactions among inverters and the grid. Specifically, harmonic resonance characteristics are addressed through transfer functions and the proposed method considering variation of inverter number, different inverter combinations, and critical system components. Finally, time domain simulations on the PSCAD/EMTDC platform are presented. The case study results validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Harmonic distortions at a point of common coupling (PCC) are caused by the harmonic sources in both supply system and customer facilities. In order to verify compliance with harmonic limits for an operating facility, it is essential to determine the contribution of a facility to the harmonic distortions measured at the PCC. Over the past many years, a significant amount of research work has been conducted in developing different harmonic analysis methods and especially harmonic contribution assessment methods. A common need of this line of research is consistent test systems that can be used to verify/validate a proposed method or to compare different approaches. To address this gap, a benchmark test system representing a typical medium-voltage/low-voltage network supplying industrial loads is proposed in this paper. This paper also includes examples on how to use the system to assess the performance of harmonic contribution determination methods. [ABSTRACT FROM AUTHOR]

Foil-winding transformers with high currents in low voltage winding normally have copper/aluminum bars as leads of the coils. A practical solution for locating bar leads without increasing the size of the active part is to flatten the coil in the leads zone causing a deformation. Abnormalities in the leakage reactance have been identified in this type of transformers. In order to take this effect into account, this paper proposes a correction to the mathematical formulation for calculating the leakage reactance in transformers with deformations in the low voltage winding. Three methods for calculating the leakage reactance have been compared for four real case studies: the proposed formulation, the commonly used formulation, and detailed simulations with the finite element method in three-dimension (FEM3D). The leakage reactance calculated with the proposed method has been found to have an improved accuracy compared with the one calculated using the formulation without correction in all case studies. The main contribution of this paper is that the proposed correction in the formulation makes it possible to increase the accuracy in the calculation of leakage reactance, avoiding the need to resort to detailed simulations with FEM3D. [ABSTRACT FROM AUTHOR]

This paper proposes a feedback linearization controller for a grid-following inverter (GFLI) that uses a conventional Phase-Locked Loop (PLL) as the synchronization unit. The proposed controller enhances the GFLI synchronization by expanding the PLL domain of attraction to the whole plane that is limited to a small region around the equilibrium point in a conventional PLL, provided that the grid impedance and voltage are known. Linearizing the overall closed-loop system, the proposed controller provides linear system attributes for the PLL, leading to an infinite domain of attraction and only one equilibrium point. Additionally, a state-feedback controller is integrated within the feedback linearization controller so that it enables the system to have an adjustable dynamic response. Finally, it is shown that the system is robust against parametric uncertainties. The performance of the proposed control design is validated in Matlab/Simulink and experiment. It is verified that the proposed controller expands the domain of attraction and enhances the system dynamic response. [ABSTRACT FROM AUTHOR]

This paper extends the previous virtual synchronous generator (VSG) control for voltage-source converters to modular multilevel converters (MMCs). Compared with the ac synchronous generator (SG), the VSG-based MMC is embedded with a power-angle characteristic. Starting from this feature and with reference to the existing ac system coherency-based equivalence methods, this paper proposes a novel coherency equivalence method for MMC used in large-scale renewable energy systems connected to the ac grid. This novel method mainly focuses on the coherency criterion and parameter aggregation methods which have significant differences compared with the ac system methods. The proposed equivalence method is validated on PSCAD/EMTDC and the study results indicate that: 1) the VSG-based MMC model does show the power-angle characteristic similar to the SG model; 2) the proposed coherency criterion can accurately and efficiently classify the paralleled MMC into coherent groups; and 3) the dynamic behaviors of the external systems are preserved after the aggregations of all previous coherency MMC groups. [ABSTRACT FROM AUTHOR]

Unbalanced load currents not only give rise to unbalanced voltages but also adversely affect the performance of the conventional current-limiting mechanisms. The latter might result in overcurrent stress on the distributed energy resources (DERs) or current harmonics. In this paper, a novel decentralized control method is proposed to improve the power quality and protect DERs from overload. The proposed controller makes use of the model predictive control (MPC) technique to minimize the voltage unbalance, improve current limiting, and prevent active power overload. The MPC is combined with the V-I droop method to realize coordinated operation with fast dynamic response. The proposed method is tested on the CIGRE benchmark microgrid. Simulation results demonstrate that the proposed method improves power quality but also allows for operation close to the maximum load capacity without imposing DERs to overload. [ABSTRACT FROM PUBLISHER]

The Sandia frequency-shift method, among all frequency-based active islanding detection methods (IDMs), is one of the most effective methods for inverter based distributed generators (DGs). The nondetection zone (NDZ) of the Sandia frequency shift (SFS) highly depends on its design parameters. If the design parameters are not properly tuned, they may result in failure of the method or deterioration of the system power quality (PQ) through injection of a high amount of harmonics. The main objective of this paper is to propose a new algorithm for SFS method parameter determination, which would add precision to the previous parameter design criterion. In this paper, also a new formula for multi-DG applications is proposed which results in optimally setting of all SFS parameters to eliminate NDZ. Using the new criterion, the negative impact of the conventional criterion on PQ is reduced, and the efficiency of the SFS IDM is increased. The simulation results show that the proposed algorithm results in more accurate detection of the islanding phenomenon in the shortest possible time. All of the simulations and NDZ analysis are carried out in the MATLAB environment. [ABSTRACT FROM PUBLISHER]

This two-part paper presents the review, application, and interpretation of the methods for analyzing the harmonic distortions at the point of common coupling thoroughly. In Part I, two different definitions for the dominant side are discussed regarding the equivalent harmonic “voltage” or “current” source. In addition, common methods are categorized in three general concepts based on the direction of active power flow, the reactive power, and the voltage–current ratio. These three categories as well as their validity are analytically reviewed for both the definitions. In Part II, the application of the methods is illustrated using two common calculation examples. The magnitude and the phase angle of the equivalent harmonic sources are separately varied in the first and second calculation example, respectively. The validity of the methods is investigated using these calculation examples. Afterward, the physical interpretation of the methods is addressed. The difference between the dominant sides regarding the equivalent harmonic “voltage” and “current” source is discussed using the calculation examples. It is shown that if one of the utility and customer equivalent harmonic voltage sources is “much” larger than the other, the dominant sides regarding the equivalent harmonic “voltage” and “current” source will be the same. Some practical aspects such as the tolerance of the methods to errors in input data are discussed. In this regard, nonresonance and resonance conditions are taken into account. This two-part paper provides a comprehensive but clear review of methods for identifying the dominant side. [ABSTRACT FROM AUTHOR]

Diversification of control schemes adopted by inverter-interfaced distributed generators (IIDGs) leads to difficulties in fault current estimation in a microgrid, which might make preexisting protection systems invalid and threaten the safety of power electronic devices. It is therefore important to study fault characteristics of IIDGs. This paper investigates characteristics of fault current of IIDGs caused by both symmetrical and asymmetrical faults. Two kinds of widely used control modes, current control (constant current control and PQ control) and voltage control (V/F control and droop control), are under investigation to provide an intuitive comparison on fault current. In particular, a novel algorithm is proposed to calculate fault current of droop-controlled IIDGs. It is found that different limiters have great impacts on fault response of IIDGs and detailed research works are carried out to identify the effects in this paper. Simulation results based on PSCAD/EMTDC and calculation results based on MATLAB/Simulink verify the correctness of the proposed fault models. [ABSTRACT FROM AUTHOR]

One of the main challenges in the protection of the Inverter-Based Microgrids (IBM) is fault detection. Inverter-based resources have limited fault current; they also adopt different control schemes which make their fault current analysis and estimation more complex. These features make the traditional protection schemes inapplicable in IBMs. In this paper, two parameters are introduced and a protection scheme is proposed based on the introduced parameters to protect feeders and Distributed Generators (DGs) in IBMs. In the proposed scheme, in each sampling window, the last samples of the current waveform are predicted and the difference between the actual samples and the predicted samples are obtained; the voltage phase angle shift between the present and the past sampling windows is also computed; using these parameters and by introducing a new relay characteristic based on Mahalanobis distance the fault is detected in IBM. The effectiveness of the proposed protection scheme is verified through the simulation and experiment results. [ABSTRACT FROM AUTHOR]

The small-signal stability of an interconnected source and load can be analyzed using the frequency-response methods of stability analysis. Existing frequency-response methods of stability analysis introduce artificial conservativeness and do not explicitly address stability robustness against load perturbations, that is, by how much the load can change such that the system will remain stable in the small-signal sense. This paper presents a frequency-response method of stability analysis which provides a less conservative stability condition compared to existing methods, in the sense of having less restriction on the control system; further, the proposed method establishes a robust stability margin in terms of perturbations in load parameters. The proposed method is based on μ analysis, and models the source-load dynamic interaction via a closed-loop system of impedances and admittances. The proposed method is used to examine small-signal stability of an islanded subnetwork extracted from a university campus microgrid, composed of a distributed energy resource (DER) unit feeding a load, providing a study with realistic parameter values. Two case studies are presented to show the features of the proposed method, namely, reduced conservativeness and the establishment of a robust stability margin. Another example is provided to show the application of the proposed stability analysis method in a multi-DER-unit network. The findings of the frequency-domain analysis are illustrated through time-domain simulations around the operating point on the system under study. [ABSTRACT FROM AUTHOR]

Electric line personnel perform energized overhead line work and expose themselves to potential arc flash burns. Arc flash incident energy is calculated to determine proper personal protective equipment to protect line personnel against bodily injuries. As increased amount of distributed energy resources (DERs) are being interconnected to distribution systems, higher arc flash incident energy is encountered. This paper explores the arc flash incident energy calculation method for distribution overhead feeders with the presence of DERs. The impact of DERs on arc flash incident energy is evaluated on nine Dominion energy's distribution feeders that interconnect solar inverters. Various factors, including DER mega volt-ampere (MVA) capacity, DER unintentional islanding run-on time, and DER step-up transformer winding configuration are considered. The effectiveness of neutral grounding resistors and low set instantaneous protection is also explored. [ABSTRACT FROM AUTHOR]

In this paper, a static synchronous compensator (STATCOM) is proposed in which a new modular structure with conventional three-phase voltage source inverters is used to multilevel operation. The inverters are connected to a transformer in an open-end winding configuration and with three independent DC links. In this structure, the inverters will be connected in a cascade configuration. Besides the multilevel operation, the system is also characterized by low required blocking voltages for switch ratings, since the proposed STATCOM can apply the double of the DC capacitor voltage to the transformer windings. The development of the control system for the proposed topology is also presented. The operation of the proposed topology and the performance of the developed control system will be tested through several simulation and experimental tests. [ABSTRACT FROM AUTHOR]

Since the large fault inrush current threatens the safety of power electronic devices, the synchronverter should have the fault ride-through (FRT) capability and should be equipped with a reliable protection system. Therefore, it is important to study the transient fault characteristics of the synchronverter and propose its fault current estimation method. In this paper, by establishing the equivalent model of synchronverter, an asymmetric fault current analysis method using the instantaneous symmetric component method is proposed. In addition, the current components, peak value, peak time, time required for reaching two times of the rated current, and main influencing factors of the asymmetric fault current are also analyzed. It is found that the influencing factors mainly include the control loops of synchronverter, asymmetric fault type, line impedances, and fault occurring moment. Moreover, the time required to reach two times of the rated current is approximately within the range of one hundredth of a microsecond to five milliseconds. These fault current characteristics impose strict requirements on the response speed of FRT control strategies. Finally, the simulation and experiment results verified the correctness of the proposed analysis method. [ABSTRACT FROM AUTHOR]

Large-scale integration of wind generation changes the power swing characteristics of a power system and may result in the misoperation of legacy power swing protection schemes. This paper presents a qualitative study on the impact of wind generation on power swing protection. The objective is to provide an understanding of the problem through case studies and present possible solutions and adjustments in protection schemes to ensure the efficiency of protection under large-scale integration of wind generation. The misoperation of power swing protection functions, namely power swing blocking and out-of-step tripping (OST), as a result of increased wind generation levels, are shown through case studies. It is also shown that the electrical center of a power system may move due to wind generation. In this case, it would be necessary to revise the optimal location of the OST protection. Finally, the impact of various factors, such as wind generator type, control scheme and fault-ride-through function, and wind generation level and capacity are investigated to determine the key features that need to be accounted for in practical protection studies. [ABSTRACT FROM AUTHOR]

Protection against dc faults is one of the main technical challenges for the operation of converter-based HVdc systems. Protection becomes even more challenging when the HVdc systems are expanded to multi-terminal dc (MTdc) grids with more than two terminals/converter stations. Similar to their ac counterparts, proper protection of the MTdc grids necessitates both primary and backup protection schemes in which, in case of malfunctioning/failure of primary protection, backup protection trips as fast as possible. Although several backup relaying algorithms, such as current threshold- and classifier-based methods, have been proposed, none of them offers sufficient speed, robustness, and scalability. In this paper, a novel backup protection algorithm based on quickest change detection technique is proposed, which offers fast and robust backup protection functionality for the primary relay. The performance and effectiveness of the proposed algorithm are evaluated and verified with time-domain simulation studies in the PSCAD/EMTDC environment. The study results confirm satisfactory performance of the proposed algorithm in terms of accuracy, robustness, and speed under various fault scenarios. The proposed method is applicable to different grid configurations and is able to cooperate with different primary protection algorithms and breaker configurations. [ABSTRACT FROM AUTHOR]

The power direction method is a widely used method for harmonic source identification. This method can work with no harmonic impedance information but is not theoretically rigorous. To overcome this theoretical shortcoming of the traditional power direction method, six existence criteria and two dominance criteria are proposed in this paper. These are all sufficient conditions that are mathematically derived based on the widely used utility–customer equivalent circuit. The existence criteria can be used to determine whether some harmonic source exists on the customer or utility side, and the dominance criteria can be used to identify whether the customer side is the dominant harmonic source. These criteria can be regarded as an improved power direction method, and the key improvement is the establishment of a rigorous theoretical basis. These criteria were tested using a MATLAB program, and no counterexample was found. Their practical application for harmonic source identification in a transformer substation is also introduced and discussed. [ABSTRACT FROM AUTHOR]

Microgrids are highly compatible with photovoltaic (PV) sources because of their ability to internally aggregate and balance multiple renewable sources. Traditional grid-connected PV inverter control configurations are basically current sourced and cannot easily control ac voltage or frequency. The PV inverter using the Consortium for Electric Reliability Technology Solutions (CERTS) concepts can control ac voltage and frequency but have a major problem with load transients. During a load transient, the PV microsource becomes overloaded with the possibility of collapsing the dc bus voltage resulting in an ac voltage drop. This paper presents a PV inverter control strategy which enables PV to behave as a voltage source and is capable of maintaining dc bus voltage stability during load transient. With this PV inverter control configuration, it is shown that the PV microsource can operate as a voltage source in the CERTS microgrid. [ABSTRACT FROM AUTHOR]

Although virtually every overhead HVdc line is based on line commutated converter (LCC) technology, hardly any work has been reported on modeling and analysis of the impact of inertia and effective short circuit ratio on control of frequency in weak grids interfacing LCC-HVdc and onshore doubly fed induction generator (DFIG)-based wind farms. This paper develops a comprehensive modeling and stability analysis framework of a weak grid that interfaces an LCC-HVdc station and a DFIG-based wind farm. To that end, a nonlinear averaged phasor model of the system is derived, which is then benchmarked against a detailed switched model. The averaged model is linearized to design a frequency controller for the LCC-HVdc rectifier station. Participation factor and eigenvalue sensitivity measures indicate that the ac system electromechanical mode and the “HVdc PLL” mode are highly sensitive to changes in effective dc inertia constant and effective short circuit ratio, respectively. Root-locus analysis in a progressively weak grid validates these results. A systematic design process of the frequency controller parameters reveals a negative interaction between the “generator speed-HVdc PLL-frequency controller” mode and the “DFIG-GSC controller” mode. [ABSTRACT FROM AUTHOR]

Intentional frequency perturbation by recently researched active islanding detection techniques for inverter based distributed generation (DG) define new threshold settings for the frequency relays. This innovation has enabled the modern frequency relays to operate inside the non-detection zone (NDZ) of the conventional frequency relays. However, the effect of such perturbation on the performance of the rate of change of frequency (ROCOF) relays has not been researched so far. This paper evaluates the performance of ROCOF relays under such perturbations for an inverter interfaced DG and proposes an algorithm along with the new threshold settings to enable it work under the NDZ. The proposed algorithm is able to differentiate between an islanding and a non-islanding event. The operating principle of relay is based on low frequency current injection through grid side voltage source converter (VSC) control of doubly fed induction generator (DFIG) and therefore, the relay is defined as “active ROCOF relay”. Simulations are done in MATLAB. [ABSTRACT FROM AUTHOR]

Microgrid (MG) protection is one of the main challenges in proliferation of microgrids. Due to limited fault current feeding of inverter-based distributed generations (DGs), in islanded operation of MG, protection problems become more complicated; and, therefore, conventional protection strategies cannot be applied. Hence, new protection methods that are applicable in islanded and grid-connected modes of operation are necessary. In this paper, a comprehensive digital-relay based protection is introduced for the protection of MGs. The proposed method includes protection of lines, distributed generations, and the point of common coupling. The structure and graphical schematic of the proposed digital relays are also presented. The proposed method is independent of the MG operation mode and benefits from single-phase tripping. In this method, due to fault isolation from both sides of a line, downstream loads and DGs can operate after fault clearance. To demonstrate the effectiveness of the proposed protection method, numerous simulations are carried out on an MG test system in the PSCAD/EMTDC environment. [ABSTRACT FROM AUTHOR]

Energy policies worldwide are mandating large-scale integration of solar panel (SP) generators with inverters on distribution systems. This causes several SPs to be connected to a distribution transformer. The SP and its interfacing inverter alter the performance characteristics of the transformer. In addition, when new sources and loads are connected to a distribution system, from an asset-management perspective, it is imperative to understand and quantify their effect on distribution system components. This paper presents a two-step study on the effects of SP on distribution transformers via simulation and experiments. In step one, the simulation work quantifies the amount of harmonic distortion caused by SP and associated inverters in distribution transformers considering solar farms and rooftop residential installations. The simulation work uses network topology, load, and generation data of a Canadian utility. Various inverter technologies, output powers, carrier signal frequencies, filtering techniques, numbers of active inverters, and transformer configurations are studied. In step two, level of harmonic distortion observed in simulation is created in a laboratory environment using a commercial inverter for SP applications. A three-phase dry-type transformer is tested to observe the effect of higher harmonic distortion on core and winding temperatures of the transformer. Experimental results conclude that under the worst case loading scenario (i.e., full load with active power flow reversed), the transformer lifetime expectancy is anticipated to decrease by 8.3%. [ABSTRACT FROM PUBLISHER]

Unbalanced distribution network/loads and asymmetrical faults are frequent in power systems. Distributed energy resources employing inverter interfaces need to ride through them to abide by emerging grid codes. The balanced grid operation has been a well-researched topic. However, literature lacks a unified framework to model, analyze, and design controllers for unbalanced interfaces. This work develops a nonlinear time-domain state-space model of inverter interfaced to an unbalanced grid. It includes details of frame transformation, synchronization, inner current controller, and filter dynamics. Its linearization around the steady-state trajectory results in a linear time-periodic (LTP) system. The model is then transformed to the dynamic harmonic domain for ease of analysis, consideration of multiple frequency couplings, and controller design. Precise stability boundaries in parameter space are obtained to understand margins for safe operation intuitively. A stability augmentation using output feedback is presented for increasing these margins. Feedback gain is designed using the generalized Nyquist criterion. Proposed augmentation is shown equivalent to the adaptive active damping of unstable periodic trajectories. Simulation test cases confirm the validity of the proposed design. [ABSTRACT FROM AUTHOR]

In this paper, an improved analytical small signal model of the line commutated converter (LCC) is developed in direct-quadrature-zero coordinates based on the assumption of infinite six-pulse converters. The dynamics of the commutation inductance and transportation delays during commutation are included in the model, and the sampling of the measurement of firing and extinction angles is added to accommodate actual converters with a finite pulse number (e.g., 12-pulse). The model is validated by comparing the frequency response with one obtained by frequency scanning in an electromagnetic transients (EMT) simulation. The model is shown to be effective in investigating system stability when the LCC is connected to an arbitrary external network. To do this, the frequency response of the combined system is plotted, and the generalized Nyquist stability criterion is applied. This stability result is validated by a time domain simulation on an EMT program using the first CIGRE HVDC benchmark system and the IEEE 14-bus system. Compared to the traditional LCC model, the proposed model is shown to have significantly better accuracy in system stability determination. [ABSTRACT FROM AUTHOR]

This paper introduces the control of the parallel operation of two voltage source converter (VSC)-HVdc links interconnecting an offshore wind farm. The aim of the study is to propose and validate a control system that allows the parallel operation of two VSC-HVdc links by controlling the currents injected by the VSC converters. The currents set points are established by a voltage controller in order to maintain constant voltage and frequency in the capacitor of the output filter and therefore within the offshore wind farm (OWF). It is demonstrated that the decoupled control of the d-q component of the voltage at the capacitor allows achieving the direct control of voltage and frequency, respectively. The voltage and frequency control is implemented by orienting the capacitor voltage toward a synchronous axis that is generated within the controller and therefore is not subjected to any grid disturbance. Both converters collaborate therefore in maintaining constant voltage and frequency, achieving in this way the parallel operation of the converters. The validation of this approach is demonstrated by simulation where the OWF and the VSC-HVdc rectifier have been modeled. Simulation results demonstrate that the proposed control system allows the parallelization of the converters while maintaining constant voltage and frequency within the OWF, even during transient faults. [ABSTRACT FROM AUTHOR]

Microgrids’ performance and stability mostly depend on power-flow control strategy. In order to allow for coordinated control while maintaining reliable operation, decentralized control methods based on P and Q droop characteristics have been utilized. Inherently, the power droop control methods have slow dynamics. In this paper, a novel control method based on V-I characteristics is introduced to exploit the flexibility and fast dynamics of the inverter-based distributed energy resources. In the proposed method, the direct and quadrature axis voltage components are drooped with the corresponding currents according to a piecewise linear droop function. Eigenvalue analysis of a sample microgrid shows that the proposed method features faster dynamics and improved damping compared to the conventional droop scheme. Simulation results are presented to verify the efficacy of the proposed method. [ABSTRACT FROM AUTHOR]

Controlled switching technology operates circuit breakers at the minimum arcing time for short-circuit current interruption. However, there is no report to reveal a relationship between the short-circuit current interrupting capability with the minimum arcing time and the opening velocities of vacuum interrupters. The objective of this paper is to reveal a relationship between the short-circuit current interrupting capability with the minimum arcing time and the opening velocities of vacuum interrupters. A 40.5-kV fast vacuum circuit breaker prototype was built up which was equipped with a commercial 40.5 kV–31.5 kA vacuum interrupter and it was driven by an electromagnetic repulsion mechanism. It provided an average velocities ranging from 2.5 to 5.5 m/s. A total of 50 times of 40-kA short-circuit current interrupting tests were carried out in a synthetic circuit. Test results showed that the minimum arcing time of successful interruption decreased with the increasing of the average opening velocity. The minimum arcing gap was determined as 1.8 mm and it kept as a constant as the average opening velocity increased. The experimental results also showed a relationship between the arc energy density and the arc extinguishing gap, which acted a linear boundary between the successful and failure short-current interruptions. [ABSTRACT FROM AUTHOR]

The alternate arm converter (AAC) is an emerging fault-tolerant multilevel converter topology from the same family of multilevel converters as the modular multilevel converter (MMC). Due to the alternate operation of the converter arms, energy balancing in the AAC is not continuous, but restricted to small time intervals. This paper develops a gradient-based current control and energy-balancing method for the AAC. The proposed strategy enforces the dynamic limits on the redundant submodules (SMs) during the overlap period and allocates effectively the maximum available number of redundant SMs to control the circulating current. The choice of the gradient as the circulating current control parameter improves the energy regulation capability of the AAC, and the enforcement of dynamic limitations avoids distortions of the output voltage. Results from an AAC-based HVDC converter model derived from the CIGRE benchmark MMC system demonstrate that the proposed strategy delivers improved energy control and balancing with good harmonic performance compared to existing current control methods for the AAC while also maintaining zero-current switching of the director switches of the AAC arms. [ABSTRACT FROM AUTHOR]

The inductor-capacitor-inductor (LCL) dc/dc converter has been extensively studied for high power and stepping ratio because of elimination of internal transformer, lower footprint/weight, higher efficiency, and most importantly providing dc fault isolation from both dc sides. This paper presents a two-channel, two-layer controller including two inner current loops, which is symmetrical for each bridge of LCL dc/dc. The real-time implementation of the control scheme and its performance under normal conditions and during transient dc faults at both sides are studied on a 30 kW 200 V/900 V 1.7 kHz prototype. The prototype development is presented in some depth. The experimental results show that the converter with closed-loop control operates well at full power and under fast power reversal. Further dc fault testing concludes that there is no need for blocking since the internal voltage and current variables are within the rated values. Detailed study of converter losses is performed and results show that full power efficiency is around 93.4%. [ABSTRACT FROM AUTHOR]

The requisite and redundant submodules (SMs) in modular multilevel converters (MMCs) can ensure the continuous operation of MMC-HVDC systems, but to date, the correlations of the SMs in normal and abnormal operations have always been neglected in the reliability modeling of MMCs. To address the correlation issue, a novel MMC reliability modeling approach is proposed. It is implemented by using the Copula Gumbel Archimedean function that belongs to probability distribution functions. This approach is realized in three steps. First, the reliabilities of the semiconductor switches, capacitors and the grading resistors, the protective switche, and control system of individual SMs are obtained using failure-rate information present in the field statistic data. Second, the reliability of the requisite category SMs is calculated using the series probability model, and the parallel probability model is employed to calculate the reliability of the redundant category SMs. Finally, MMC reliability is calculated using the classic k/N(G) probability model. The effectiveness of the proposed approach is shown using different test cases. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive analysis and a novel solution for the discharge problem on the half-bridge submodules’ (HBSMs’) capacitors in the hybrid modular multilevel converter (MMC)-based high-voltage direct current transmission systems under a long-term (e.g., several minutes or longer) pole-to-ground fault condition. The mechanism of this issue is analyzed, and then a solution based on fundamental-frequency reactive circulating current (FFRCC) injection is proposed. The influence of FFRCC injection is analyzed in terms of the fluctuated arm capacitor energy and the current stress of semiconductors. Moreover, an enhanced arm voltage modulation method based on HBSMs and full-bridge submodules coordinate control is proposed to accelerate the capacitor voltage balance of HBSMs. Finally, the performance of the proposed control strategy is verified by the simulation results. [ABSTRACT FROM AUTHOR]

The undesirable harmonic distortion produced by distributed generation units (DGUs) based on power-electronic inverters presents operating and power-quality challenges in electric systems. The level of distortion depends on the internal elements of the DGUs as well as on the characteristics of the grid, loads, and controls, among others. This paper presents a comprehensive method, focused on power-quality indexes and efficiency for the design of microgrids with multiple DGUs interconnected to the ac grid through three-phase multi-Megawatt medium-voltage pulsewidth-modulated-voltage-source inverters (PWM-VSI). The proposed design method is based on a least square solution using the harmonic domain modeling approach to effectively consider explicitly the harmonic characteristics of the DGUs and their direct and cross-coupling interaction with the grid, loads, and the other DGUs. Extensive simulations and analyses against PSCAD are presented in order to show the outstanding performance of the proposed design approach. [ABSTRACT FROM PUBLISHER]

Ground fault overvoltage can occur in situations in which a four-wire distribution circuit is energized by an ungrounded voltage source during a single-phase-to-ground fault. The phenomenon is well documented with ungrounded synchronous machines, but there is considerable discussion about whether inverters cause this phenomenon and, consequently, whether inverters require effective grounding. This paper examines the overvoltages that can be supported by inverters during single-phase-to ground faults via theory, simulation, and experiment. It identifies the relevant physical mechanisms, quantifies expected levels of overvoltage, and makes recommendations for optimal mitigation. It concludes that under many circumstances, effective grounding of inverters is not necessary to prevent ground fault overvoltage. [ABSTRACT FROM PUBLISHER]

Since photovoltaic (PV) generators have no mechanical inertia, fast solar irradiance changes associated with cloud transients cause instantaneous variations in PV power output, resulting in rapid and possibly significant voltage changes in systems with high penetration of PV generators. In addition to characterizing this emerging power-quality issue, this paper proposes and investigates techniques to mitigate these fast voltage variations caused by cloud transients in distribution system with PV solar farms. The investigated approaches are based on the PV inverter capacity for fast reactive power compensation without the need to oversize the inverter or curtail the active power by using only the available reserve. Real measurement data characterize the issue and time-series power-flow simulations evaluate the mitigation techniques. [ABSTRACT FROM PUBLISHER]

This paper presents a concise frequency-domain interharmonic model for the current-source inverter-fed (CSI-fed) variable frequency drives (VFDs). This model is in the form of a Thévenin equivalent circuit. The equivalent voltage source is determined only by the VFD operating point, while the equivalent impedance is influenced by the system impedance, the dc-link impedance, and motor parameters. This model, verified by simulations and measurements, can be used to predict the interharmonic propagation in a system supplying CSI-fed VFDs. The model also reveals the interharmonic generation characteristics of the VFDs, which will help the proper selection of mitigation options. Based on the model, the analysis of the nature of the interharmonic source of CSI-fed VFD has been conducted. Some applications are demonstrated as well. [ABSTRACT FROM PUBLISHER]

This paper presents a modified power control to integrate inverter-based distributed generation (DG) in distribution networks. This control is designed for mitigating the maloperation of the overcurrent protection coordination, due to the DG current contribution during the fault. The proposed power control introduces a new approach to perform the inverter control by using new orthogonal components of the grid voltages that provide a modified controlled power. Using the proposed methodology, the injected current to the grid during faults is limited to the DG-rated level. By controlling the reference power based on a proposed function of the per-unit terminal voltage, the injected current is limited to a desired value. The results of the simulated system corroborate the control capability to completely limit the inverter-based DG effects on protection coordination in the distribution level. [ABSTRACT FROM AUTHOR]

Voltage source converter based multiterminal HVDC (VSC-MTDC) technology has shown many advantages. However, following a large disturbance, the dc voltage and the output power of the VSC station with fixed droop control may hit their limits, thereby reducing the dc voltage control ability of the whole MTDC system. Here an adaptive droop control scheme is proposed on the basis of both the dc voltage deviation factor and the power sharing factor. It contributes to ensure the dc voltage and the power loading rate of each converter within their limits during large disturbances, and the power sharing capability of the whole MTDC system remain high. The working principle of the proposed adaptive droop control is discussed in details. Case study and simulations are performed based on the PSCAD/EMTDC model of a ±320 kV four-terminal VSC-HVDC system to validate the effectiveness of the proposed adaptive droop method. [ABSTRACT FROM AUTHOR]

Quasi-static models, also known as time-varying phasor models, have been used for many years for dynamic analysis and stability studies in power systems. However, the long track of success of using these models in a broad spectrum of applications resulted in blurring the boundaries between uses and misuses of time-varying phasors. Specifically, one possible misconception is that quasi-static models are always accurate enough when the system dynamics are slow in comparison to the nominal system frequency. This letter shows that in some cases, this assumption is inaccurate and may lead to misleading conclusions regarding the system dynamics and stability. [ABSTRACT FROM AUTHOR]

A correction is presented to the article "A Coherency-Based Equivalence Method for MMC Inverters Using Virtual Synchronous Generator Control."

The fault current of a three-phase inverter-interfaced distributed generator (IIDG) depends on the control strategy employed during grid faults. A new short-circuit current calculation method for IIDGs is developed in this study according to the current control and reactive power support during the low-voltage ride-through operation period. To investigate how the ride-through time relates to the short-circuit current of IIDGs, short-circuit responses under different fault conditions are compared. The short-circuit current of a multi-IIDG distribution network is determined with the proposed technique to consider ride-through time and sequence current control. The correctness of the proposed method is verified by simulation. [ABSTRACT FROM AUTHOR]

Coupling between frequency dynamics of the ac systems on both inverter and rectifier sides of the line-commutated converter HVDC with the rectifier station operating in frequency control is studied, along with the presence of large DFIG-based wind farms on the weak rectifier-side grid. An averaged model with 79 states, which includes dynamic models of grids on the rectifier and inverter sides, phase-locked loop, and the wind farm is derived. To develop a deeper understanding of the frequency dynamics, a simplified four-state nonlinear model is proposed, which, in turn, reveals strong coupling between frequency and ac voltage at the HVDC rectifier terminal. A firing angle correction strategy is proposed to decouple frequency–voltage interactions, thereby improving the frequency dynamics on the rectifier side. The four-state model is linearized to ascertain the interaction between rectifier- and inverter-side frequencies, and an analytical expression for the frequency dynamics in terms of gains of the frequency controller at the rectifier station is derived. Moreover, the proposed reduced-order model shows the implications of frequency droop control of the wind farms in improving frequency dynamics on both rectifier and inverter sides. Expressions for “synchronizing” and “damping torque” contribution from HVDC and wind farm are also established. The analytical expressions and the effectiveness of the proposed strategies are validated through nonlinear time-domain simulations. [ABSTRACT FROM PUBLISHER]

Flat-topped modulation is a member of the family of triplen-series injection techniques that has been extensively utilized in pulsewidth-modulated inverter systems to increase the dc link and, hence, semiconductor utilization. We propose the use of an optimized flat-topped modulation scheme for modular multilevel converter (MMC) control. The optimized flat-topped waveform minimizes the magnitude of the triplen harmonics, particularly compared to the popular space-vector modulation (SVM) technique, while fully utilizing the dc voltage. This has particular advantages if the converter side of the interfacing transformer is earthed. Under such conditions, the zero-sequence earthing current is affected by the triplen series injected into the sinusoidal modulating functions. Therefore, it is critical to minimize the injected triplen harmonics. The operating principle of the flat-topped scheme is presented and the Fourier coefficients are compared with the SVM technique. In addition, the influence of the proposed control scheme on MMC performance is evaluated mathematically. Simulation of a point-to-point HVDC link using the average model demonstrates the effectiveness of the proposed MMC operational schemes. The third harmonic of the flat-topped modulation is reduced by 33%, which lowers the potential zero-sequence current flowing to earth. Compared to conventional sinusoidal modulation, the submodule capacitance is reduced by 25%. This significantly lowers submodule cost, volume, and weight. Station conduction losses are expected to reduce by 11%, yielding higher efficiency and lowering cooling system capacity. In addition to the improvement under normal operation, the proposed control scheme also reduces the fault current by 13.4%. [ABSTRACT FROM AUTHOR]

This letter presents a novel pseudo extinction angle (PEA) for the HVDC inverter, as well as its calculation method. The proposed PEA extends the measured extinction angle (EA) into a real-time calculated value. As a result, the non-measurability of the EA between two adjacent EA updating times, which is likely to cause commutation failures (CFs), can be avoided effectively. Simulation results verify the validity of the proposed PEA calculation method and prove the capability of the PEA to mitigate the CFs. [ABSTRACT FROM AUTHOR]

This letter presents records of unstable operations in grid-connected photovoltaic generation plants. The instabilities involve a wide range of frequencies from tens to thousands of Hertz. Possible causes of the instabilities are discussed based on the literature survey. This letter suggests new industry standards or grid codes for photovoltaic generation integration should consider such practical challenges. [ABSTRACT FROM PUBLISHER]