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IEEE Standard 1547 defines grid connection standards for distributed resources (DRs). This standard sets requirements for harmonic performance, which are particularly relevant when converter-interfaced DRs are considered. Specifically, the standard limits the allowable harmonic current injected by a DR, assuming grid voltage at the point of common coupling (PCC) is harmonic free. In this paper, it is demonstrated, both mathematically and experimentally, that the harmonic current injection of a converter-interfaced DR depends highly on the level of PCC background voltage. This paper presents a mathematical formulation that enables the impact of PCC voltage harmonics on the harmonic current injection of a converter-interfaced DR to be determined via experimental identification of the converters frequency coupling matrix. Experimental tests show that an IEEE 1547-compliant solar inverter with measured total demand distortion (TDD) of 0.66% for an undistorted PCC can produce TDD levels in excess of 20% in the presence of 3% voltage distortion. It is concluded that TDD metrics, as employed in IEEE Standard 1547, provide little insight into the field performance of converter-interfaced DRs when background voltage harmonics are present. Improved FCM-based metrics for assessing the harmonic performance of converter-interfaced DRs in the presence of voltage harmonics are provided. [ABSTRACT FROM PUBLISHER]

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]

The development of voltage-source-converter (VSC)-based high-voltage direct current (HVDC) systems has increased significantly. However, the implementation of VSC-HVDC systems is limited for lacking fast, low-loss, and reliable circuit breakers. Hybrid dc circuit breakers (DCCBs) may help to improve this situation. In this paper, an efficient modular multilevel converter (MMC) HVDC system model based on the Zhangbei–Beijing \pm \text500\,\textkV bipolar MMC-HVDC transmission (overhead line) project is first established. Then, the system transient characteristics are investigated involving permanent line-to-ground faults (LGFs), permanent short-circuit faults (SCFs), temporary LGFs, and temporary SCFs with either ac circuit breakers (ACCBs) or DCCBs in service. Finally, the performances, including the fault clearance and recovery, comparison of DCCBs with two different breaking methods, and comparison of ACCBs and DCCBs, in the MMC-HVDC system are presented. This paper provides a deep understanding of LGF and SCF in the MMC-HVDC systems, operation of DCCB, and comparison of faults clearance and recovery under these faults conditions using ACCB and DCCB, respectively. Results also benefit the application of the DCCB and the operation strategies in high-voltage and high-power MMC-HVDC systems with overhead lines. [ABSTRACT FROM AUTHOR]

As the key equipment of ultra-high-voltage direct-current (UHVDC) transmission systems, the converter valve would bear different forms of overvoltage. Consequently, a wideband model for the $\pm $1100 kV UHVDC converter valve is essential for the converter system to make overvoltage analysis, insulation design, and electromagnetic disturbance prediction. This paper proposed a wideband modeling method for the $\pm $1100 kV converter valve. First, the physics-based wideband model for the key component in the converter valve module is built, of which the parameters could be obtained through genetic algorithm. Second, the parasitic capacitances are obtained through a simplified boundary-element method according to the given structure of the $\pm $1100 kV converter valve. By combining wideband models for key components in the converter valve module and the parasitic capacitances of the valve tower, the integrated wideband model for the converter valve could be obtained. The systematic modelling method proposed in this paper could also be of reference value to the modelling of other large-scale complex high-voltage power-electronic equipment. Finally, the experiments for converter valve module and converter tower are carried out, and the corresponding results justified the effectiveness and correctness of the proposed wideband model for the converter valve tower. [ABSTRACT FROM PUBLISHER]

A method is proposed in this paper to determine the harmonic impedances in low-voltage networks in a stochastic way. The consequences of resonances for harmonic propagation and stability of power converters are summarized. By using Monte Carlo simulation, the method includes the uncertainties in customer impedances, specifically due to electronic loads and local generation. The uncertainty in customer impedance is included by considering probability distribution for the resistive, inductive, and capacitive parts of the impedance. The concept of transfer impedance is used for phase-to-neutral connections. A method is developed and applied to two existing low-voltage networks in Sweden. Results show that, for these two networks, the resonant frequencies decrease around 28% once photovoltaic panels are installed. This paper includes a discussion of some of the practical aspects of applying the proposed method. [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 presents the theoretical basis of the control strategy that allows the cell capacitor voltage regulation of the full-bridge modular multilevel converter (FB-MMC) to be controlled independent of its dc-link voltage. The presented control strategy permits operation with reduced dc-link voltage during a permanent pole-to-ground dc fault, and controlled discharge and recharge of the HVDC links during shutdown and restart following clearance of temporary pole-to-pole dc faults. In addition, it allows voltage-source-converter (VSC)-based HVDC links that employ FB-MMC to operate with both positive and negative dc negative dc-link voltages. This feature is well suited for hybrid HVDC networks, where the VSCs operate alongside the line-commutating current source converters, without any compromise to the power reversal at any terminals. The usefulness of the presented control strategy is demonstrated on the full-scale model of the HVDC link that uses FB-MMC with 101 cells/arm, considering the cases of pole-to-ground and pole-to-pole dc faults. [ABSTRACT FROM PUBLISHER]

Having unveiled the problems associated with distance protection of lines connected to full-scale converter-interfaced renewable energy power plants (CIREPPs) due to remote and intermediate infeed in Part I of this paper, Part II describes why the countermeasures currently deployed by commercial relays to tackle these problems in a conventional power system fail in the presence of a CIREPP. Then, a new formula to calculate impedance is devised for the phase element of the relay to prevent maloperation in the event of line-to-line to ground faults. In addition, a communication-assisted method with minimal bandwidth requirement is proposed for balanced and line-to-line faults. The issues that should be taken into consideration for single-line-to-ground faults are also discussed. The solutions presented in this paper are simple and independent of the host system grid code, and can be applied to industrial memory-polarized distance relays. A comprehensive performance evaluation is presented to verify the reliable operation of the proposed methods. [ABSTRACT FROM AUTHOR]

This paper presents an average-value model of a line-commutated converter-based HVDC system using dynamic phasors. The model represents the low-frequency dynamics of the converter and its ac and dc systems, and has lower computational requirements than a conventional electromagnetic-transient (EMT) switching model. The developed dynamic-phasor model is verified against an EMT model of the CIGRE HVDC Benchmark. Simulation results confirm the validity and accuracy of the average value model in predicting the low-frequency dynamics of the ac- and dc-side quantities. Merits and applicability limitations of the average model are highlighted. [ABSTRACT FROM AUTHOR]

This paper demonstrates the stabilization of large power systems using voltage-source-converter-based high voltage direct current (HVDC) links. Based on global power system measurements, a model predictive control (MPC) scheme manipulates the power injections of the HVDC links to damp oscillations in the ac system. The grid controller explicitly accounts for constraints and the expected future behavior of the system. Different scenarios like the loss of production, the loss of consumption and changes to the grid topology are studied in the continental European Network of Transmission System Operators for Electricity (ENTSO-E) system. The simulations illustrate the performance enhancement obtained with a global MPC-based grid controller, compared to a local damping controller and HVDC links with constant reference values. [ABSTRACT FROM PUBLISHER]

Resonance and harmonic phenomena can occur when HVDC and Flexible Alternating Current Transmission System devices are inserted into ac grids. Frequency response analysis and electromagnetic transient (EMT) type simulations of converter stations are essential for analyzing and preventing negative effects. This paper presents two approaches for deriving the high frequency response of a converter station: using simplified analytical representation and accurate EMT-type model. Validations with control parameter impacts are presented. This paper also presents a test case, based on a recent HVdc project, where potential resonances can occur between the converter station and its ac network. It is demonstrated analytically that these resonances are mainly due to control system parameters and ac network configurations. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel bipolar modular multilevel converter (MMC) topology with dc fault ride-through capability. The proposed bipolar MMC topology is a hybrid of two types of arms: the half-bridge submodule (HBSM)-based arms (HBSM-arms) which are connected to the ground pole, and the unipolar-voltage full-bridge submodule (UFBSM)-based arms (UFBSM-arms) which are connected to the positive or negative pole. During a pole-to-ground or pole-to-pole dc fault, the hybrid-arm design offers the proposed UFBSM-based hybrid-arm bipolar MMC (UHA-BMMC) the capability to provide additional reactive power to support the ac grid while blocking the dc fault current. The topology, operation principle, dc fault mechanisms, and dc fault ride-through scheme of the UHAB-MMC are presented in this paper. Compared with other possible bipolar MMC topologies, the UHA-BMMC provides good dc fault ride-through capability with a small cost of power-electronic devices. The validity and features of the proposed UHA-BMMC are verified by simulation results conducted in PSCAD/EMTDC. [ABSTRACT FROM PUBLISHER]

In conventional HVDC systems where the harmonic filters and shunt capacitors are installed at the ac grid side, the converter transformers inevitably suffer the detrimental impacts caused by harmonic penetration. In this paper, combined with an inductive filtering method, a parallel delta-connected filtering-winding scheme for 12-pulse converter transformer units is proposed to reduce the harmonic currents flowing through the transformers and, thus, to attenuate the harmonic effects on them. This paper describes the presented transformer configuration in terms of the wiring scheme of the 12-pulse converter transformer unit and corresponding flowing paths of different harmonic currents with the orders of 3,6n\pm 1\left(\mathrmn=1,3,5,\ldots \right), 11 and 13 in the filtering winding circuit. Then, the mechanism of the proposed scheme on harmonic suppression to the converter transformers is explained theoretically. In an attempt to validate the theoretical analysis, simulations based on PSCAD/EMTDC and experiments in a laboratory prototype system are performed to evaluate the harmonic eliminating performance of the proposed scheme. [ABSTRACT FROM PUBLISHER]

Circulating current can be decomposed into double line-frequency positive-, negative- and zero-sequence components under unbalanced voltage conditions. First, in order to decrease peak/rms value of arm current, double line-frequency circulating current should be eliminated. Second, ac-side current that consists of positive- and negative-sequence components under unbalanced voltage conditions should be regulated to track the reference signal. Furthermore, fundamental frequency circulating current caused by asymmetry of the upper and lower arm should be also removed. To effectively control ac-side current and eliminate all circulating currents, an arm current control strategy based on a decoupler and proportional-integral controller plus the vector proportional integral (VPI) controller is proposed in this paper. The proposed control strategy works well under balanced and unbalanced voltage conditions as well as asymmetry of the upper and lower arm. In addition, this paper proposes a design procedure for the VPI controller to reduce the uncertainty of this process. A three-phase modular multilevel converter simulation model is set up on PSCAD/EMTDC to verify the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) have become the converter topology of choice for voltage-source converter–high-voltage direct-current systems. Excellent work has previously been conducted to develop much needed average value models (AVM) for these complex converters; however, there a number of limitations as highlighted in this paper. This paper builds on the existing models, proposing numerous modifications and resulting in an enhanced MMC-AVM, which is significantly more accurate and which can be used for a wider range of studies, including DC faults. [ABSTRACT FROM PUBLISHER]

Power electronic transformer (PET) composed of modular dual active bridge (DAB) blocks has a promising future in ac–dc hybrid distribution network because of its energy routing ability. The key element that restrains the development of PET is the vulnerability of power electronic devices during faults, especially dc faults. DC faults (mostly line faults) have very short transient process and no obvious fundamental frequency, which means fault location is difficult to be realized for dc lines in distribution network. Considering that fault location plays an important role in realizing fast fault clearance and recovery, this paper analyzed fault characteristics of dc line faults with respect to the basic structure of modular DAB block and proposed a two-terminal fault location method based on transient voltages measured in dc line boundaries. To evaluate the validity of proposed method, numerical simulations with different fault conditions were implemented in PSCAD/EMTDC. The simulation results indicate that the proposed fault location method located the faulted point quickly and correctly. [ABSTRACT FROM AUTHOR]

In this paper, clearing of DC faults in a hybrid multi-terminal HVDC transmission system consisting of line commutated converters (LCCs) and voltage source converters (VSCs) implemented using half-bridge modular multilevel converter (MMC) technology is investigated. While the hybrid HVDC system has several possible configurations, this paper focuses on two of them: 1) a half-bridge MMC-HVDC link piggy-backing on the transmission line of a LCC-HVDC link and 2) LCC-HVDC link tapped by half-bridge MMC inverters. The proposed dc fault recovery strategy employs a high rating series diode valve placed at each VSC inverter pole to block fault currents; AC circuit breakers to isolate the faulty VSC rectifier pole; and force retardation applied at LCC rectifier to extinguish the arc. Detailed simulations demonstrate fast fault recovery performance with the proposed fault recovery procedure. In the case where a single transmission line is shared by the LCC and VSC links, the VSC rectifier is subjected to considerably high current for a period of few hundreds of milliseconds, and the ac side voltage dips momentarily. During a single pole fault, interrupting power flow on the healthy pole of VSC rectifier may be necessary to maintain smooth operation. [ABSTRACT FROM AUTHOR]

Influence of remanent flux on hysteresis curve of the transformer core is addressed in this paper. In addition, its significant negative effect on transformer diagnostic tests is quantified based on experimental studies. Furthermore, a novel approach is proposed to efficiently and quickly eliminate the remanent flux. This approach is evaluated based on simulation studies on a 230/63-kV power transformer. Meanwhile, experimental studies are performed on both 0.2/0.2 and 20/0.4 kV transformers. These studies reveal that the approach not only is well able to eliminate the remanent flux, but also it has various advantages over the commonly used method. In addition, this approach is equally applicable for various power, distribution, and instrument transformer types. [ABSTRACT FROM AUTHOR]

The development of multiterminal voltage-source converters high-voltage direct current (VSC-HVDC) grids poses a new challenge: A fast and selective protection algorithm for such grids must be implemented. Therefore, this paper addresses a novel protection algorithm, which proves to be accurate, fast, and selective for various combinations of fault distance, resistances, and power-flow scenarios in a multiterminal VSC-HVDC environment. It relies on the wavelet transform and principal component analysis to retrieve information from the local DC current measurement, generating a highly informative input for a genetic fuzzy system. Based on previously simulated fault scenarios, the genetic fuzzy system is able to decide if the input signal refers to an internal or external fault, generating the trip signal when needed. Furthermore, the method has the advantage of using local measurements to avoid communication delay. Finally, the algorithm was implemented in hardware and tested in a hardware-in-the-loop simulation, which emphasizes the practical use of the proposed protection scheme. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has become an attractive option for dc/ac conversion in many power system applications, in particular, for HVDC transmission. Due to its parallel arrangement of phase legs, the MMC is known to be a wye-configured architecture. In this paper, the equivalent delta formulation of the conventional wye architecture is introduced. The delta-configured MMC, enabled by series-stacking phase legs, is an alternative realization to the wye-configured MMC and has significantly different operating characteristics. The introduction of the delta MMC enables greater flexibility in converter voltage/current scaling for a given set of submodules, and may be an appealing option for dc/ac applications requiring a power tapoff from a high-voltage dc rail. The single-ended and differential delta topologies are derived, with the latter seen as the preferred topology due to its reduced energy-storage requirements. The principle of operation of the differential delta-configured MMC is described in detail. A dynamic controller is developed that regulates grid power injections while suppressing unwanted circulating ac currents. Comprehensive simulation results validate the differential delta MMC operation and the proposed control scheme. [ABSTRACT FROM AUTHOR]

The large number of voltage levels in a modular multilevel converter (MMC) make staircase modulation a feasible alternative, particularly in high-power applications. However, staircase waveforms do not necessarily mean operation of the MMC submodules (SMs) at the fundamental frequency. This paper presents an analysis of SM switching frequencies in staircase-modulated MMCs and their correlation to the modulation index and load phase angle. A carrier-based pulsewidth-modulation (CB-PWM) equivalent technique is also developed. This analysis demonstrates that CB-PWM techniques provide a similar switching frequency with superior harmonic performance and improved voltage balancing characteristics at all modulation indices compared to staircase modulation. The theoretical analysis is verified with extensive simulation results for MMCs with different SMs and experimental results from a laboratory prototype. [ABSTRACT FROM PUBLISHER]

The three-phase series-connected modular multilevel converter (SC-MMC) is presented as one option for high-voltage direct current (HVDC) tap to reduce the cost of an HVDC station. Compared with conventional three-phase parallel-connected MMC, the number of semiconductor switches in SC-MMC is reduced by one-third to withstand the same dc voltage. First, the paper studies the operating principle of SC-MMC, and then presents an interspacing phase-shifted pulsewidth-modulated method which could improve the waveform of the output ac voltage for a given switching frequency. Next, the control strategy based on the identical real power input of each phase is proposed to balance the dc voltage of each phase under an unbalanced grid condition. The current limit of switching devices is also taken into consideration to protect SC-MMC from overcurrent. Furthermore, a universal zero-sequence current controller is developed as one part of the control strategy. In addition, an innovative method to suppress the third harmonics of ac current is presented. With the third harmonic suppression strategy, the SC-MMC can operate with much smaller SM capacitors without deteriorating the ac current, which contributes to the cost reduction further. Finally, simulation results obtained in PSCAD/EMTDC are provided to validate the proposed control strategies. [ABSTRACT FROM PUBLISHER]

HVDC short-circuit calculation standards will be required to simplify the dimensioning and selection of circuit breakers (CBs) in future HVDC networks. This paper proposes new analytical formulas for the calculation of the ac-side contribution through the blocked half-bridge-based converters during pole-to-ground faults in multiterminal HVDC cable networks. Analytic approximations are derived for the steady-state short-circuit currents and the peak of the transient response for point-to-point HVDC links and extended to multiterminal HVDC cable networks with arbitrary grid topology. The results of the proposed expressions are compared with simulations in PSCAD and provide accurate calculations for the transient peak and steady-state values of the ac-side fault current contribution to the prospective CB current. [ABSTRACT FROM PUBLISHER]

The fault ride-through (FRT) requirement of modern grid codes results in interactions between full-scale converter-interfaced renewable energy power plants (CIREPPs) and the protection systems of high-voltage transmission grids, which normally involve distance elements either as the primary or the backup relay. Such interactions are influenced by the CIREPPs' exclusive fault behavior, and have been left largely unnoticed in relaying literature. Part I of this paper develops a CIREPP model suitable for relaying studies, and highlights the CIREPP properties by which the protection system is endangered the most. Then, the operating scenarios leading to the malfunction of a distance relay that is located at a CIREPP substation and protects the adjacent line are unveiled. These scenarios include in-zone short circuits missed by the relay and incorrect tripping for out-of-zone faults, which would, in turn, neutralize FRT schemes implemented inside CIREPPs. The findings of this study also hold true for the ac lines emanating from voltage-sourced converter-based HVDC connections. [ABSTRACT FROM AUTHOR]

The conventional tripole HVDC system, which uses thyristor converters, has some inherent defects. This paper proposes an improved scheme which uses the full-bridge submodules-based modular multilevel converters to replace the thyristor converters in pole 3, and the converters in pole 1 and 2 are also replaced by the modular multilevel converters based on half-bridge submodules. In order to obtain good performance among three poles, coordination control is proposed. This paper also proposes voltage-inverse control and a control strategy of keeping current zero in pole 3 to suppress overvoltage and to minimize the ground current in transition processes. The performance of the proposed tripole HVDC system is evaluated through PSCAD/EMTDC. Simulation results show that based on the control strategy, the tripole HVDC system can achieve satisfactory response characteristics in transition processes, maintain the balance of capacitor voltages, and minimize the ground current in the meantime. [ABSTRACT FROM AUTHOR]

In order to improve features, such as fault current blocking and capacitor voltage balancing, modular multilevel converter (MMC) topologies incorporating multiport submodules (SMs) are being considered as candidates for HVdc transmission applications. This paper presents high-speed and accurate electromagnetic transient (EMT) models for MMCs composed of such multiport SMs. The approach uses the Schur's complement technique to recursively eliminate internal nodes of the converter structure to create a multiport Norton equivalent that connects to the external network. Thus, the final admittance matrix seen by the EMT solver has a dimension orders of magnitude smaller than that of the unreduced structure. As with previously developed approaches for MMCs with single-port SMs, all internal information, such as individual SM capacitor voltages, is preserved and can be output by the program if needed. This increases the bookkeeping effort, but the overall reduction in matrix size more than compensates for any resulting time penalty. Approximately two to three orders of magnitude speedup over a straightforward implementation in an EMT program is achieved. [ABSTRACT FROM AUTHOR]

The modular multilevel converter-based unified power flow controller (MMC-UPFC) is able to operate under unbalanced grid conditions with symmetric component decoupling. However, the constraint of the voltage limit of UPFC is not considered and no protection schemes are investigated to protect the UPFC from overmodulation under unbalanced grid conditions. To solve this problem, this paper proposes the cascaded control scheme for MMC-UPFC based on voltage limit control and symmetric component decoupling to balance the ac current of the transmission line. With appropriate transformer connections for MMC-UPFC, the negative- and zero-sequence currents are suppressed by the corresponding inner current loops. Considering the voltage limit of MMC, the operating ranges of UPFC-MMC under balanced and unbalanced grid conditions are investigated in both mathematical derivation and point scanning methods. The results are depicted and analyzed in 3-D vision. Based on the analysis of operating regions, the voltage limit control is proposed to protect the MMC from overmodulation and to maximize the controllable region under unbalanced grid conditions. Finally, the final cascaded control structure is constructed. The simulation results obtained in PSCAD/EMTDC are provided to validate the effectiveness and robust performance of the proposed control strategies. [ABSTRACT FROM AUTHOR]

Low frequency ac transmission (LFAC) has been proposed as an alternative to high voltage dc transmission for medium distance (80–150 km) offshore wind farms. Long HVAC cables and their associated low frequency resonance, connected to voltage source converters (VSC), provide technical challenges for the control of the offshore voltage. This paper provides the design of the offshore voltage “grid forming control” to maintain a stable offshore voltage accounting for the connection of a long HVAC cable connected to the VSC. Simulations are performed on an LFAC test system to examine the influence of controller parameters and the associated design tradeoffs between the selection of dq controller time constants and voltage control bandwidth. The LFAC system design and control is then validated in a hardware experiment where the proposed controller operates in a real-time hardware-in-the-loop experiment. [ABSTRACT FROM PUBLISHER]

The generation capacity of renewable energy sources has significantly increased during the last few years. As a consequence, high-power dc transmission systems are key technologies to reinforce the high-voltage transmission system. Regarding the long process of approving for right of way for new lines, a conversion from existing ac into dc lines might be an option. Modular multilevel converters provide the necessary features to avoid technical problems in heavily loaded ac systems; they increase the transmission capacity and system stability very efficiently and assist in preventing cascading disturbances. In this paper, high-voltage direct current transmission systems based on half-bridge topology with and without solid-state breaker and full-bridge (FB) modules are compared. The general behavior during faults on the dc side is described especially with respect to intersystem faults (i.e., faults between the ac and dc systems). Regarding the clearing capability, such disturbances lead to even higher requirements than for pure dc faults, especially the necessary transient interruption voltage (TIV) of the dc breaker design. On the other hand, this adapted design yields some disadvantages with respect to more frequently occurring dc line faults. In contrast, the controllability of the FB solution offers the possibility of an adjustable TIV and does not need to dissipate any energy since the power can be transmitted to the ac side. Therefore, the FB seems suited best for applications in hybrid high-voltage ac/dc systems. [ABSTRACT FROM PUBLISHER]

This paper proposes an evolved capacitor commutated converter (ECCC), embedded with antiparallel thyristors based dual-directional full-bridge module (APT-DFBM), to mitigate the commutation failure of line-commutated converter-based-high-voltage direct-current (LCC-HVdc) transmission and enhance the control flexibility and dynamic response of capacitor-commutated converter-based HVdc (CCC-HVdc). The operating modes and parameter selection approach of the APT-DFBMs are provided, and the coordinated control approach between the APT-DFBMs and converter valves is presented. Then, an ECCC-HVdc system, adopting an LCC station as the rectifier and an ECCC station as the inverter, is developed in PSCAD/EMTDC. The transient responses including commutation failure probability, voltage and current stress, and fault recovery performance are compared under ac fault conditions among the proposed ECCC-HVdc, LCC-HVdc, and CCC-HVdc. The results show that the proposed ECCC-HVdc can effectively reduce the commutation failure risks of LCC-HVdc, and enhance the control flexibility and improve the dynamic responses of CCC-HVdc. [ABSTRACT FROM PUBLISHER]

This paper presents the challenges that the power converters with $dq$ -frame-based control confront when subjected to ferroresonance. This is mainly due to two main properties of the widely used $dq$-frame control systems. It is demonstrated in this paper that regardless of the phase-to-ground or phase-to-phase voltage measurement, the $dq$-frame control only responds to phase-to-phase voltage variations. Furthermore, the PI controllers, which are employed in conjunction with the $dq$ reference frame, can inherently only track dc references and reject dc disturbances. Consequently, a $dq$ -based PI controller can only respond to the positive-sequence phase-to-phase voltage variations, effectively. This, in turn, limits the disturbance mitigation capability of the power converter. This study investigates the impacts of the ferroresonance phenomenon on the control system response and the operating conditions of the power converters. Based on a droop-based $dq$ frame controller, the behavior of an electronically interfaced distribution generation system is studied, under various transient conditions. In spite of a promising performance under the load change and islanding scenarios, the $dq$-frame-based controller of the power converter fails to detect and respond to the voltage fluctuation and excessive overvoltages as a result of ferroresonance. [ABSTRACT FROM AUTHOR]

Recently, a modular multilevel converter (MMC) has become one of the most popular converters in medium/high-power applications. In this paper, a new technique is presented to decrease the computational time of the MMC by using switching state grouping for the predictive controller. The proposed technique has the ability to control the output current, minimize the circulating currents, and balance all capacitor voltages, simultaneously. For a predefined number of MMC submodules, the number available switching states is known. The proposed technique uses two stages of predictive modules. In the first stage, the switching states of the MMC are divided equally into a number of groups, controlled simultaneously by the same number of predictive modules. The optimal number of the switching state groups in the first stage is obtained. In the second stage, only one predictive module is used to obtain the optimal switching state of the MMC without any other complexities or additional sorting controllers. The effectiveness of the proposed control technique has been verified by using MATLAB/SIMULINK software considering steady state and dynamic analysis. The results of the proposed technique are compared with the conventional one in terms of computation time, verifying better performance of the proposed method. [ABSTRACT FROM PUBLISHER]

This paper presents a new model for a stochastic multiperiod optimal power-flow (SMP-OPF) problem which includes an offshore wind farm connected to the grid by a line-commutated converter high-voltage dc link. The offshore wind farm is composed of doubly fed induction generators (DFIGs), and the DFIG's capability curve is considered in order to obtain a more realistic dispatch for wind farms. The uncertainties of wind power generation are also taken into account using a scenario-based approach which can be adopted by the system operator to obtain the optimal active and reactive power schedules for thermal and wind power generation units. To illustrate the effectiveness of the proposed approach, it is applied on the IEEE 118-bus test system. The obtained results demonstrate the capability of the proposed SMP-OPF model for the determination of the optimal operation of power systems. [ABSTRACT FROM PUBLISHER]

The current-source modular multilevel converter (CSMMC) is a good alternative for high-power applications with medium-voltage and high-current requirements. In such applications, to obtain the multilevel current output and for current scaling, a large number of submodules are required in CSMMC. The detailed representation of such systems in electromagnetic transient-type simulation programs brings computational complexity with a large computing burden. This paper presents a computationally efficient equivalent model of CSMMC based on the Norton equivalents. Each arm of the converter is represented by a two-node Norton equivalent in this model, which eliminates the internal intermediate nodes and, hence, significantly reduces computing time. The proposed model is implemented in PSCAD/EMTDC by using the module component coded in FORTRAN and results are validated with the full-size traditional model in PSCAD/EMTDC. Moreover, hybrid models are presented and validated for the submodule level studies such as fail-safe functionality with redundant submodules. This paper also focuses on the control method and dynamic studies of the CSMMC-based back-to-back system. The results of these studies are compared with the proposed equivalent model, and accurate matching of the results confirms the usefulness of the proposed model in such studies. [ABSTRACT FROM AUTHOR]

Periodic preventive maintenance (PPM) is a significant measure to ensure reliable operation of the modular multilevel converter (MMC), but to date has always been neglected in the reliability evaluation. A mathematical reliability model of MMC considering PPM is proposed in this paper. The model is derived rigorously by the reliability function and finally described by the indices equivalent failure rate and the forced outage rate. Two operation modes related to redundant submodules are considered and described by the standby model and k-out-of-n model, respectively. Employing this model, we can analyze the sensitivity of reliability to redundancy and maintenance interval and provide some reliability indices for planning, which is especially valuable for the offshore cases. The necessity of the considered PPM and the effectiveness of the proposed model are verified by case studies. A method to include PPM in reliability modeling is provided. [ABSTRACT FROM PUBLISHER]

This paper presents an enhanced control for HVDC systems employing hybrid modular multilevel converters (MMCs) during asymmetrical dc voltage faults. The characteristics of pole-to-ground (PTG) fault under different system grounding schemes are analyzed. Then, the principle of enhanced control strategy to deal with the PTG fault is presented for a hybrid-MMC-based HVDC system with symmetrical monopolar configuration. A new control variable is proposed on the arm voltages to distribute the power transmitted from the ac side between the upper and lower arms. By adopting the enhanced control, the overvoltage and the fault current can be eliminated when a PTG fault occurs. Furthermore, the hybrid-MMC-based HVDC system can operate continuously with adjustable active power and controllable reactive power by controlling the upper and lower arms of the converter asymmetrically during a fault period. Finally, the feasibility of the proposed control scheme is verified by simulation results in PSCAD/EMTDC. [ABSTRACT FROM PUBLISHER]

This paper proposes a fast, selective, reliable, and local bus primary relaying algorithm for high-voltage direct current (HVDC) grids. The proposed relaying algorithm only uses measurements obtained from current sensors to detect faults and identify their locations. This algorithm is local and requires only communication at the bus level rather than the system level. Furthermore, the algorithm is not sensitive to the system configuration and operating point and can be employed during line or converter outages with limited retuning and adjustments. The proposed algorithm is applied to a four-terminal HVDC grid. Study results show that the proposed algorithm 1) detects faults on both transmission lines and DC buses; 2) identifies the faulted bus or transmission line; 3) requires insignificant computation capacity; 4) distinguishes between faults and other system transients to prevent erroneous circuit-breaker trips; and 5) is not sensitive to measurement noise. [ABSTRACT FROM AUTHOR]

In this paper, a multiagent system-based protection and control scheme is proposed to deal with diverse operation conditions in distribution system due to distributed-generation (DG) integration. Based on cooperation between DG controller and relays, an adaptive protection and control algorithm is designed on converter-based wind turbine DG to limit the influence of infeed fault current. With the consideration of DG control modes, an adaptive relay setting strategy is developed to help protective relays adapt suitable settings to different operation conditions caused by the variations of system topology and DG status. The proposed scheme is tested and validated on a test distribution system in a hardware-in-the-loop real-time testing platform. [ABSTRACT FROM AUTHOR]

This paper presents a real-time implementation of autonomous microgrid protection using agent-based distributed communication. Protection of an autonomous microgrid requires special considerations compared to large-scale distribution networks due to the presence of power converters and relatively low inertia. In this paper, we introduce a practical overcurrent and a frequency-selectivity method to overcome conventional limitations. The proposed overcurrent scheme defines a selectivity mechanism considering the remedial action scheme of the microgrid after a fault instant based on feeder characteristics and the location of the intelligent electronic devices (IEDs). A synchrophasor-based online frequency-selectivity approach is proposed to avoid pulse loading effects in low inertia microgrids. Experimental results are presented for verification of the proposed schemes using a laboratory-based microgrid. The setup was composed of actual generation units and IEDs using the IEC 61850 protocol. The experimental results were in excellent agreement with the proposed protection scheme. [ABSTRACT FROM AUTHOR]

This paper proposes a new method of decoupling and subdividing electrical circuits, containing power-electronics devices, in order to achieve fast and accurate real-time simulation. In this technique, each state variable can be discretized using different discretization methods. Combining implicit and explicit ODE solvers, state-space equations are decoupled while remaining accurate and stable. Unlike most traditional decoupling techniques previously proposed, the proposed one does not require artificial delay or supplementary states to be added in order to decouple the system. Furthermore, this technique is meant to be implemented with commercially available simulation software. By doing so, a large and complex circuit containing several hundreds of state variables can be easily and accurately simulated with minor modification to the existing models. Finally, stability and accuracy of the proposed technique are thoroughly demonstrated in a numerical example during steady state and under faulty conditions. [ABSTRACT FROM PUBLISHER]

The high-power self-commutated voltage-source converter (VSC) and current source converter (CSC) are the key control devices in HVDC, flexible ac transmission systems (FACTS) and distribution FACTS systems. To achieve the expected control objectives, suitable control strategies must be implemented based on the available devices, system, and control techniques. The self-commutated ac/dc converters control the electrical power by generating controllable ac fundamental and dc average outputs. These controllable outputs are controlled by the conducting state combinations of the converter switching devices, driven by their gate signals. The gate signals are specified by fundamental parameters of frequency, amplitude, and phase angle. The converter system model for describing the relation between the system-state variables and the gate signal parameters is essential for the converter system control. This paper (part I) derives the state variable equations for transmission systems using VSC while the second paper (part II) is for the transmission systems using CSC. The self-commutated converter systems provide active and reactive power control flexibility, but their nonlinearity makes their control difficult. The linearized state equations by using feedback linearization are presented to enable the controller design by using linear control theory. [ABSTRACT FROM PUBLISHER]

In last years, the electrical systems are changed from centralized to distributed energy generation. Nowadays, the distributed generation systems consist of many renewable energy sources (RESs) and active loads, connected to the grid by the power electronics converters (PECs). As a result, more often the PECs with novel functionalities like harmonic elimination, voltage sags compensation, grid stability improvement etc. are required. One of the promising solutions for distributed generation system, which allows improve the delivered power is a smart transformer (ST). It is able to replace the classical MV/LV transformer, where additional functionalities are necessary to provide the higher energy quality. Additionally, the ST is characterized by lower weight/volume and provides easy integration of AC and DC sub-grids. Therefore, in this paper, the modelling problem of a modern MV grid with classical and smart transformers have been described. Moreover, the comprehensive simulation analysis of the MV grid was performed, in which different functionalities of ST were considered. The advantages of ST over classical transformer are compared and the impact of ST functionalities on the MV grid is presented, taking into account the currents/voltages distortions in LV side, symmetrization process and voltage support in MV distribution grid as well as the transmission power losses. [ABSTRACT FROM AUTHOR]

This paper proposes an m-port converter structure for high-voltage dc (HVDC) applications to facilitate power-flow routing between k controlled dc buses and m-k independent dc networks. Power-flow control is achieved via the injection of incremental dc voltages between the networks; thus, the converter structure is only rated for a fraction of the rated power and voltage of the connecting dc networks. Unlike previously proposed dc power-flow devices, these incremental dc voltages are generated without requiring power exchange with an external ac network. There are four variants of the structure, each offering unique advantages for deployment. These variants, and the modules that comprise them, are presented. A sample simulation case study is performed to demonstrate three-port bidirectional power-flow control between networks of similar voltage (495/500/505 kV). The proposed converter structure is shown to route power between the networks using modules with megavolt-ampere ratings of approximately 2% of the transmitted power. Thus, the proposed converter structure offers a highly cost-effective means of routing and controlling dc power flows within emerging HVDC grids. [ABSTRACT FROM PUBLISHER]

Multilevel current source converters are a smart choice for the high-power applications with low-voltage and high-current requirements. This paper presents the detailed analysis of a current source modular multilevel converter (CSMMC) and its application as a STATCOM. Various properties of CSMMC are compared with the voltage-source modular multilevel converter (VSMMC). The submodule inductance selection method and a sorting-based algorithm for inductor current balancing are proposed for CSMMC. The validity of the inductance selection method and the inductor current balancing algorithm are confirmed by the simulation of a three-phase CSMMC using PSCAD/EMTDC. The carrier phase-shifted sinusoidal pulsewidth modulation is used as the switching technique. The performance of a converter is analyzed with interleaved and noninterleaved carrier signals for the upper and lower arms. Results show that CSMMCs and corresponding VSMMCs have analogous features. Hence, the development on VSMMCs can be used for reference to CSMMCs and vice-versa. Furthermore, the control designs for CSMMC-based STATCOM are presented and verified using simulations. The obtained results of the proposed STATCOM show that the steady-state and transient performance of the system are quite satisfactory. This makes the CSMMC suitable for HVDC and flexible ac transmission systems applications. [ABSTRACT FROM PUBLISHER]

Newly developed VSC-HVDC systems are reaching power levels of up to 1000 MW. At this power level, the nonminimum phase behavior of the VSC-HVDC systems’ dc plant becomes a threat to the stability of the direct voltage for fast dc control-loop dynamics. This paper presents a novel compensation scheme, called RHP-zero shifting+damping, designed to deal with the nonminimum phase dynamics of the dc plant by adding additional compensation loops to the current controller of the VSC-HVDC system. The compensation scheme can work along with linear controllers and allows the closed-loop bandwidth of the direct voltage controller to be increased without affecting the direct voltage stability of high-power VSC-HVDC systems. As a result, the direct voltage variations are significantly reduced during power changes in the ac or dc network. The performance of the compensation scheme is evaluated through simulations and corroborated in a 1-kW experimental test bed. [ABSTRACT FROM PUBLISHER]

The continuous development of the voltage-source-converter-based multiterminal high-voltage direct-current transmission system has led to a great need for dc circuit breakers to isolate short-circuit faults in the dc side. The operating time of dc breakers is mainly dependent on the mechanical contacts separation time. In this paper, three key units of the ultra-fast operating mechanism, including drive unit, buffer unit, and holding unit were investigated. A prototype of the 40.5-kV ultra-fast vacuum switch has been developed where the Thomson-coil actuator (TCA) is used to fulfill the function of the drive unit and the buffer unit, and a bistable spring mechanism is used as the holding unit. The state equations of the bidirectional TCA used as the drive unit and buffer unit in the operating process are completed based on the equivalent circuit method and finite-element method. The experimental and simulation results of the prototype are compared and they are in good agreement as expected. Therefore, the mathematical model is verified. The factors that influence the drive unit and the buffer unit performances are discussed. The achieved operating time of the prototype is about 2.3 ms. [ABSTRACT FROM PUBLISHER]

With the development of HVdc transmission technology, the isolation of dc faults is becoming increasingly important. As the most effective solution, hybrid HVdc circuit breaker (CB) is not mature enough, which has restricted its engineering application. Accordingly, this paper proposes an auxiliary dc CB scheme suitable for HVdc grid. In this scheme, the traditional half-bridge modular multilevel converter (MMC) is partially innovated to have the ability of auxiliary breaking operation. With the effect of the MMC auxiliary breaking (MAB) operation at both ends of the fault line, the dc fault current can be easily interrupted and isolated by low-cost CB unit installed in the dc transmission line. The equivalent circuit under MAB process is analyzed, and the fault isolation sequences are designed. Furthermore, the economic comparative analysis among three schemes is performed. Finally, a four-terminal bipolar HVdc grid test model is built in PSCAD/EMTDC; in addition, an MMC prototype with proposed scheme is also developed. The simulation and experimental results validate the effectiveness of the proposed scheme. [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 integration of wind power plants introduces new dynamics into power systems, forcing reconsiderations of how they are studied, planned, and operated. High quality models are essential to these studies. Manufacturer-specific electromagnetic transient (EMT) wind turbine models are usually available only as black-boxes, which hinders analysis and research. To overcome this issue, this paper proposes a generic EMT-type model for a specific type-IV wind turbine system, which is validated against field measurements from a wind turbine of the same type. More precisely, it proposes a wind turbine model based on an externally excited synchronous generator system connected to a full converter composed of a six-pulse diode rectifier, a dc–dc boost stage and a two-level voltage source converter. The required control features and internal protection schemes are considered and described. Two different fault ride-through control strategies, in line with existing grid codes, are implemented. A corresponding EMT-type hybrid model representation is also developed based on newly proposed switched equivalent circuits and average models for the considered hardware, control, and power electronics stages. It allows for the use of larger simulation time steps, hence considerably improving computation times. [ABSTRACT FROM AUTHOR]

There are multiple discharges in one breakdown during a lightning incident. A lightning incident usually contains 3–5 return strokes. In this case, the lightning incident will not be well simulated by the first stroke. To date, a single impulse current waveform has been used to simulate the lightning impulse incident in research on the lightning impulse characteristics of soils, leading to some differences with observations of actual lightning discharges. In this paper, a time-delayed control technique and a controlled gaps-group isolation technique were used to produce the multiple high-impulse currents discharge to a soil sample, while nonintrusive X-ray imaging technology was used to obtain images of the discharge channel in the soil. The impulse characteristics of the soil under multiple impulse currents are studied. The differences between the discharge characteristics of the soil under multiple impulse currents and a single pulse are compared. It can be found that there has been a reduction in the effective soil resistivity under multiple impulse currents, and this reduction will be recovered over time. At the same time, a brief analysis of the discharge mechanism of the soil under multiple impulse currents is proposed, based on analysis of X-ray images of discharge channels. [ABSTRACT FROM AUTHOR]