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In this paper, we propose an idea to integrate more renewable energy into the existing China power grid and to provide remote islands with a reliable power supply. The idea is to expand the existing point-to-point line-commutated converter (LCC) HVDC transmission link into a hybrid multiterminal HVDC (hybrid MTDC) system, which contains LCC stations and voltage-source converter (VSC) stations. Accordingly, this paper proposes a novel control strategy for this system called active voltage feedback control, which does not need any high-speed communication system under various disturbances, such as wind speed variations, load fluctuations, faults, etc. This paper also adds secondary frequency regulation to the synchronverter, which is one way of combining the VSC converter with synchronous machine behavior. Then, this paper applies an approach to limit its current under fault conditions. Finally, the entire hybrid MTDC system is modeled in PSCAD/EMTDC. Simulation results show that the improved synchronverter is able to achieve secondary frequency control, and the presented active voltage feedback control works very well when exposed to various disturbances. [ABSTRACT FROM AUTHOR]

The use of VSC-HVDC grids for offshore wind farm integration will require the use of dc breaker systems and presently they require dc reactors to limit the rate of rise of fault current. The introduction of large dc reactors throughout a VSC-HVDC system can have a significant impact on its stable operation and will require additional control. This paper analyzes this problem and proposes a PSS-like control (DCPSS) to aid dc grid stability and cope with this effect. A generalized analytical model for studies on dc voltage control is presented. Key stability and transient performance issues caused by the use of the dc reactors in a multiterminal system are investigated by analyzing poles, zeros, and frequency responses of open-loop and closed-loop models. Design and location identification methods for the DCPSS are provided. An excellent damping enhancement is achieved by this controller. The analytical studies and time-domain simulations in this paper are performed based on two VSC-HVDC models. [ABSTRACT FROM AUTHOR]

A parallel hybrid modular multilevel converter (PHMMC) belongs to the class of modular multilevel converters, which have become potential candidates for high-voltage direct-current (HVDC) transmission systems. Due to the circuit topology of a PHMMC, the dc bus voltage contains low-order harmonics and cannot be fully regulated at a constant dc voltage. The dc bus voltage, if not properly controlled, leads to improper power transfer and increases the magnitude of dc current ripple on the dc transmission line. This paper proposes a zero-sequence voltage injection (ZSVI)-based model predictive control (MPC) strategy to control the dc current/power flow and simultaneously minimize the dc current ripple. The proposed strategy takes advantage of a cost function minimization technique to determine and inject the optimal zero-sequence voltage components into the dc- bus voltage of a PHMMC system. This paper derives a discrete-time dynamic model of the dc transmission-line current and, correspondingly, develops a predictive model. The predictive model is used to inject the appropriate amount of zero-sequence voltage components to the dc bus reference voltage waveform. Compared with the existing triplen harmonics injection method, the proposed ZSVI-MPC strategy improves the performance of a PHMMC system in terms of minimization of the dc current/voltage ripple. Performance of the proposed strategy for a 21-level PHMMC-based HVDC station system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported results demonstrate superior performance of the PHMMC-HVDC station operating based on the proposed ZSVI-MPC strategy, under various operating conditions, as opposed to the existing triplen harmonics injection method. [ABSTRACT FROM AUTHOR]

This paper investigates the applicability of averaged-value models (AVMs) for modular multilevel converters (MMCs) operating in a voltage-sourced converter-based-high-voltage dc (VSC-HVDC) grid. The AVM models are benchmarked by comparison with a detailed electromagnetic transient model of the grid, including a fully detailed MMC model. Analysis results show that the AVM is only effective as long as the capacitors are large enough to maintain nearly constant voltage across each MMC submodule. This paper also shows that previously developed MMC averaged models are not able to accurately simulate the transients under dc fault conditions. This paper introduces topology changes to a previously proposed averaged model that results in much improved simulation for such conditions. This paper also shows that the model can be used effectively to study HVDC grids with significant time savings. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMC) are presently the converter topology of choice for voltage-source converter high-voltage direct-current (VSC-HVDC) transmission schemes due to their very high efficiency. These converters are complex, yet fast and detailed electromagnetic transients simulation models are necessary for the research and development of these transmission schemes. Excellent work has been done in this area, though little objective comparison of the models proposed has yet been undertaken. This paper compares for the first time, the three leading techniques for producing detailed MMC VSC-HVDC models in terms of their accuracy and simulation speed for several typical simulation cases. In addition, an improved model is proposed which further improves the computational efficiency of one method. This paper concludes by presenting evidence-based recommendations for which detailed models are most suitable for which particular studies. [ABSTRACT FROM AUTHOR]

In order to strengthen the onshore transmission network in many parts of the world, voltage-source current–high-voltage direct current (VSC–HVDC) will increasingly be utilized. The effect of the operating point of a VSC-HVDC link and the control strategies employed can substantially affect the electromechanical oscillatory behavior of the ac network as well as the dc-side dynamics. In order for the full, flexible capability of VSC-HVDC to be exploited, further study of the effects of these controllers and their interactions with ac system responses is necessary. This paper addresses this gap. Both modal analysis and transient stability analysis are used to highlight tradeoffs between candidate VSC-HVDC power controllers and to study the electromechanical performance of the integrated ac/dc model. Tests are carried out on both a generic two-area model and a large-scale realistic network with detailed ac generator and HVDC models. [ABSTRACT FROM PUBLISHER]

This paper proposes a model predictive control (MPC) method for modular-multilevel-converter (MMC) high-voltage direct current (HVDC). To control the MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are taken into consideration. The existing MPC methods for the MMC-HVDC system utilize weighting factors to configure the cost function in combinations of the SM capacitor voltage balancing algorithm, ac current control, and circulating current control. Because all combinations of the switch states are considered in order to minimize the cost function, their possible combinations increase geometrically according to the increase of the level of the MMC, which is a significant disadvantage. This paper proposes a new MPC method with a reduced number of states for ac current control, circulating current control, and the SM capacitor voltage-balancing algorithm. The proposed cost functions are divided into three types according to their control purposes. Each cost function determines the minimum number of states for controlling the ac current, circulating current, and SM capacitor voltage. The efficacy of the proposed controlling method is verified through simulation results using PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive analysis of the limitations and the key dynamics of closed-loop active power control systems for voltage-source converter (VSC) HVDC, regarding stability, performance, and robustness. Detailed dynamic models are derived and the controllability and robustness issues for VSC active power control are identified. Limitations imposed by ac system strength, converter operating point, and current control design on the stability and performance of the two leading active power control principles are addressed, using frequency-response analysis and time-domain simulations. The dynamic interactions between the active power control design and the dc voltage droop control are examined. The simulations are performed using average-value VSC models and a high-fidelity modular multilevel converter model. Impacts of the active power control design on dynamic behaviors of multiterminal dc (MTDC) systems are investigated using a four-terminal model. This paper provides a systematic study on the key stability and performance issues associated with the active power control. Furthermore, the methodology offers a framework for the analysis of more complex active power and dc voltage droop controllers for future MTDC systems. [ABSTRACT FROM AUTHOR]

This paper proposes a startup sequence for a modular multilevel converter (MMC) HVDC-connected offshore wind farm (OWF) and demonstrates how it can be simulated using an electromagnetic transient-type tool. The objective is to analyze inrush currents of transformers and cables (HVAC and HVDC). Modeling accuracies and computational performances are examined using three existing modeling methods for the MMCs. [ABSTRACT FROM PUBLISHER]

Protection of HVDC grids against short circuits must act in the order of milliseconds to avoid damage to power electronic components and maintain a stable dc voltage. Therefore, ultra-fast protection algorithms are needed that detect the fault and identify its location. Recently developed communication-less HVDC grid protection algorithms provide high operation speed but are inherently limited in reach to provide selectivity. In this paper, a new method for protecting HVDC grids with long cables is proposed. The method combines protection algorithms, sensors integrated in the cable joints, and proactive hybrid breakers to achieve increased performance compared with the state-of-the-art without adding to the overall fault clearing time. This paper describes the proposed method, provides the theoretical basis for the algorithms, and presents the results of an in-depth analysis using electromagnetic transient (EMT)-type simulations. The simulations were performed in a detailed test system implemented in EMT-type software. Simulation results show that long cables can be protected along their entire length through a combination of the proposed algorithms, although the reach of the individual algorithms is limited. [ABSTRACT FROM PUBLISHER]

This paper presents important results of the European HVDC Study Group founded on an initiative by the German Commission for Electrical, Electronic and Information Technologies in September 2010. The main task of the Study Group has been defining “Technical Guidelines for first HVDC Grids”: 1) to gain a common understanding of basic operating and design principles of HVDC grid systems, and 2) to prepare the ground for more detailed standardization work. The HVDC systems are moving from the stage of point-to-point (PTP) connectors to the stage of the transmission systems interconnecting more than two stations and utilizing multiterminal (MT), multivendor (MTMV) HVDC voltage-source converter (VSC) systems. The development will be modular, for example, in steps, starting with interconnecting PTP connectors and establishing three-pod, radial systems. Then, such simple systems may gradually be interconnected into radial and meshed HVDC–VSC systems comprising more HVDC lines and dc converter stations. This paper describes understanding, specification, and standardization of design and operation principles of the HVDC grids seen as the first needed steps toward such MTMV systems. [ABSTRACT FROM PUBLISHER]

In this paper, a methodology is presented to identify and analyze interaction modes between converters in voltage-source converter multiterminal high-voltage direct current (VSC MTDC) systems. The absence of a substantial level of energy stored in such power-electronics-based systems results in fast system dynamics, governed by electromagnetic phenomena. Moreover, interactions between converters are largely influenced by the control parameters and, in general, an a priori identification of interaction modes based on associated time constants is less straightforward than in ac systems. Furthermore, the extent to which converters interact not only depends on the controller parameters, but is also influenced by the physical characteristics of the HVDC system. The methodology introduced in this paper is based on aggregated participation factors to distinguish between local modes, primarily associated with one terminal, and interaction modes involving multiple terminals. To illustrate the proposed methodology, the influence of droop control parameters, as well as dc breaker inductors, on the system dynamics and the participation of the terminals in system interactions are investigated for a three-terminal MTDC system. [ABSTRACT FROM AUTHOR]

A modeling approach is presented in this paper in order to investigate instabilities originating from dc-network resonances in voltage-source converter HVDC (VSC-HVDC) systems. Through frequency-domain modeling, it is shown that the VSC-HVDC system can be represented as a feedback interconnection of two subsystems. These subsystems are defined in this paper as a dc network impedance connected to a VSC dc-side power-dependent admittance. The conditions in which this admittance has a negative conductance are investigated since they can amplify dc network resonances. It is found that when the VSC that controls the direct voltage injects power into the dc network, the mentioned conductance becomes negative above certain frequencies. In addition, the impact of weak ac grids, control system delays, and other direct voltage controllers on the studied conductance is investigated. The theoretical findings are verified through time-domain simulations. [ABSTRACT FROM AUTHOR]

The high voltage direct current transmission system based on modular multilevel converter (MMC-HVDC) technology is now studied and used widely. But traditional half bridge topology of sub module (HBSM) of MMC can't block dc link fault current because of the freewheeling effect of diodes. In order to solve this problem, firstly, this paper improves HBSM topology, a series connected double sub module (SDSM) is designed and dc link fault current blocking capability is realized without tripping ac circuit breaker, so MMC-HVDC with this improved topology can immediately rebuild dc link voltage and resume power transmission. Secondly, the self starting and fault disposing procedure are also designed. At last, simulations in the software PSCAD/EMTDC are carried out to verify its effectiveness and feasibility. [ABSTRACT FROM AUTHOR]

Negative- and zero-sequence internal currents of a modular multilevel converter (MMC) under unbalanced ac-grid conditions can increase the peak voltage of the submodule (SM) capacitors, which should be kept below a maximum voltage limit. This paper investigates the impact of offset pulsewidth modulation (OPWM) and zero-sequence voltage injection on reducing peak SM capacitor voltages of the MMC during unbalanced ac-grid conditions. It is demonstrated that OPWM extends the operating range of the MMC for a given set of restrictions, as it leads to a reduction of capacitor voltage in every phase by utilizing all the SMs in the arms. Zero-sequence voltage injection further reduces the maximum SM capacitor voltage among three phases under certain conditions. Simulation results from an 800-MVA MMC-based high-voltage direct current transmission system demonstrate the effect of OPWM in the operation of the converters and the efficacy of the method compared to standard modulation methods. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) is regarded as a topology with high potential for high-voltage direct current transmission system. However, the distributed capacitors lead to significant submodule capacitor voltage ripples, which have great influence on the terminal characteristics of MMC. In this paper, an equivalent model considering submodule capacitor voltage ripples and circulating current suppression control is established to explore the terminal characteristics of MMC. In the model, MMC is equivalent to the cascade of buck/boost converter and two-level voltage source converter (VSC) from a terminal perspective. The influence of capacitor voltage ripples is represented by the equivalent series impedances, whose characteristics are investigated in detail with harmonic linearization method. Then, the simplified equivalent series impedances are obtained. Based on the equivalent model, the study of terminal characteristics is greatly simplified by directly applying conclusions of two-level VSC and buck/boost converter to MMC. Furthermore, the control strategy design and impedance modeling of MMC have been carried out to show how the equivalent model can be used in practical applications. Finally, the effectiveness of the proposed model is verified by detailed simulation results. [ABSTRACT FROM AUTHOR]

This paper presents a new method based on piecewise analytical formulas to evaluate the valve losses of the modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. According to the generating mechanism, the total valve losses of the MMC are divided into three parts: 1) conduction losses; 2) essential switching losses; and 3) additional switching losses. Due to the large number of arm submodules in MMC–HVDC links, the harmonic components in the arm voltages and currents are omitted. Therefore, the conduction losses and the essential switching losses can be precisely represented by analytical formulas. The additional switching losses are described by an analytical upper limit. The case studies are performed on a 400-MW\pm 200-kV MMC-HVDC system under 12 typical operating conditions. Both the relative errors of the conduction losses and the essential switching losses between the analytical and the simulated results are within \pm4%. The analytical upper limits cover almost all of the simulated results for additional switching losses. Finally, an acceptable number for the analytical method is described based on the simulation results, which proves the effectiveness of this method and clarifies the potential to take advantage of this method in engineering applications. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) may contain numerous insulated-gate bipolar transistors. The modeling of such converters for electromagnetic transient-type (EMT-type) simulations is complex. Detailed models used in MMC-HVDC simulations may require very large computing times. Simplified and averaged models have been proposed in the past to overcome this problem. In this paper, existing averaged and simplified models are improved in order to increase their range of applications. The models are compared and analyzed for different transient events on an MMC-HVDC system. [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]

This paper analyzes the distance relay performance during power swing conditions for an uncompensated and compensated transmission line with a unified power-flow controller (UPFC). UPFC can control voltage and power flow of the transmission line independently; hence, it has an impact on the apparent impedance seen by the distance relay. To analyze the case, the apparent impedance seen by the distance relay during a power swing is extracted, and the results are synopsized graphically. The impact of UPFC on different transmission-line parameters from an impedance point of view is investigated. Moreover, the impact of different operation modes of UPFC and their reference values on the apparent impedance seen by concentric mho circles as a power swing detection method are also evaluated by detailed simulations. [ABSTRACT FROM PUBLISHER]

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]

This paper presents an accurate steady-state initialization method for electromagnetic transient (EMT) models of modular multilevel converters (MMCs). The proposed method uses steady-state arm voltages and currents to initialize internal electrical variables and control systems of the converter, including capacitor voltage and gating signals of each sub-module. In addition to typical voltage source converter upper level control, MMC specific controls are also considered, including circulating current control, dc ripple control, and capacitor voltage balancing algorithm. EMT simulations on 101-level and 401-level MMC-HVDC systems confirm the accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

The half bridge modular multilevel converter (MMC) coordinated with dc circuit breakers (DCCBs) is one of the most promising solutions for the future multi-terminal HVdc system protection. After the isolation of dc line fault, the electrical quantities of the dc system remain stable and no coupling relationship exists between healthy and faulty poles. Thus, ac adaptive reclose scheme is difficult to be applied in dc transmission systems since the information of the adjacent lines cannot be used to assess whether the line fault is cleared or not. An innovative adaptive reclose scheme applied to the MMC-HVdc system is proposed in this paper by injecting voltage pulses to the faulty line. When the breaker is tripped, the converter and dc faulty line are connected by the main breaker of the hybrid HVdc breaker. On the basis of switching control of solid-state devices in the main breaker, the voltage pulse injection to the dc faulty line is realized. Then, the fault characteristic is identified based on the traveling-wave-based fault location algorithm. A fast restart scheme for the MMC-based bipolar HVdc transmission system is also achieved by this method. The simulation results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, a robust nonlinear control scheme is proposed for voltage source converter based multiterminal HVDC grids, based on the predictive sliding mode control (PSMC). This control is a hybrid of the multivariable nonlinear model predictive control and sliding mode control. The particle swarm optimization method is applied to obtain the optimal control parameters. This control fulfills the objectives of regulating terminal dc voltage, active and reactive power, improved dynamic performance, and superior robust control operation even with plant parameter changes and external measurement disturbances. The performance of the proposed PSMC scheme is compared with the optimal synergetic control and optimal PI. The results confirm the superior transient performance and robustness of the PSMC scheme. [ABSTRACT FROM PUBLISHER]

This paper investigates the dynamic interactions of a modular multilevel converter (MMC) -based HVDC grid and its host ac system during ac-side disturbances. The HVDC grid is composed of five MMC stations that are interconnected through a network of cables and overhead lines. The ac grid includes ten turbine generators. The studies are conducted in the PSCAD platform, including the detailed models of the ac and dc subsystems. The reported case studies indicate that without the HVDC supplementary control functions and based on the conventional dq-frame control structure: 1) each MMC behaves as almost a fixed sink/source of power during the ac-side electromechanical transient oscillations; 2) the MMC-HVDC grid effectively improves dynamic interactions between ac subsystems that are connected only by the HVDC grid; 3) without appropriate provisions, the embedded MMC-HVDC grid can aggravate the transients of the host ac system, for example, interarea oscillations. The observations also contradict the widely accepted notion that even without supplementary controllers, the MMC-HVDC grid necessarily improves the dynamics of the ac system. The study results provide a comprehensive insight into the transient behavior of the MMC-based HVDC-AC grid. Such an understanding is essential to design the protection and control systems of the future HVDC-AC grids. [ABSTRACT FROM AUTHOR]

A new offshore high-voltage dc (HVDC) wind generation scheme is proposed in this paper. The scheme implements a high-voltage hybrid generator (HG) as well as HVDC interconnection and transmission systems. The turbine power train of the proposed system is compared with a typical system installed in a commercial wind farm. The analyses demonstrate improvements in system losses and, hence, efficiency, power-train hardware, including cable system mass and, importantly, a reduction in major component count and installed power electronics in the nacelle and turbine tower, features that lead to reduced capital cost and maintenance. The resulting power conversion system is more simplified and more amenable to higher voltage implementation since it is not constrained by existing state-of-art power-electronic voltage–source converter structures. Voltage control is facilitated via dc/dc converters located away from the turbine tower. To demonstrate the HG operational concept, measured results from a low-power laboratory prototype HG system are compared with analytical results and show good agreement. [ABSTRACT FROM AUTHOR]

The operating region of the modular multilevel converter (MMC) is limited by the maximum-allowable voltage ripple of cells, which is associated with the arm energy variation. Injecting the controlled second-order harmonic component into the circulating current can reduce energy variation of the arm and, thus, extend the operating region of MMC. Care must be taken, however, that the second-order circulating current affects the arm current, and inappropriately injected harmonic current may cause an undesirable arm current polarity change. Therefore, this paper first investigates and proposes the maximum-allowable second-order circulating current for extending the operating region reliably and efficiently. The active and reactive power capability of MMC is further elaborated and illustrated in line with the maximum excess energy capability for two cases, running with: 1) the original operating region and 2) the extended operating region. Finally, the P-Q diagrams for contingent operating conditions with faulty or disabled submodules are presented. Understanding the unique shape and change of P-Q capability with credible submodule failure contingency should be crucial for planning MMC-HVDC lines, and determining the energy requirements and the level of redundancy in submodules properly in order to achieve the envisioned benefits of the new HVDC. The efficacy and accuracy of the research findings are validated for the MMC-HVDC system using PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper focuses on the droop-based dc voltage control design for multiterminal VSC-HVDC grid systems, considering the ac and the dc system dynamics. The droop control design relies on detailed linearized models of the complete multiterminal grid, including the different system dynamics, such as the dc grid, the ac grid, the ac connection filters, and the converter inner controllers. Based on the derived linear models, classical and modern control techniques are applied to design the different controllers, including a multivariable frequency analysis to design the grid voltage droop control. In combination with the droop control, a dc oscillation damping scheme is proposed in order to improve system performance. The control design is validated through simulations of a three-terminal system. [ABSTRACT FROM AUTHOR]

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]

The papers in this special section focus on HVDC systems for large offshore wind power plants. These plants have experienced an important growth in the last decade and are becoming a key energy source worldwide. Offshore wind presents clear advantages due to high and stable wind speeds offshore, enabling the use of larger wind turbines than those commonly used on inland wind power plants. However, transmitting power to load centres also brings a number of technical challenges. Depending on the distance to the shore and the power rating, AC or DC technology may be the preferable option for integrating offshore wind power to onshore grids. There are a number of issues which have to be considered requiring substantial research and development. For DC connections, (Voltage Source Converter) VSC-HVDC technology has shown advantages over (Line Commutated Converter) LCC-HVDC. The development of multiterminal HVDC grids though brings further challenges related to system operation, control, and protection and coordination between HVDC converters, wind power plants, and the onshore system. The development of power electronic switches has reached a point where the size, rating, and the system efficiency can meet the needs of commercial HVDC. Wind power plants connected to HVDC transmission system require specific control and protection coordination between HVDC converters and wind turbines. [ABSTRACT FROM AUTHOR]

This paper proposes an enhanced control method for a high-voltage direct-current (HVDC) modular multilevel converter (MMC). To control an MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are considered. The ac-side current is a fundamental frequency component, and the circulating current is a double-line frequency component. Existing control methods control the ac current and circulating current by separating each component. However, the existing methods have a disadvantage in that the ac-side current must be separated into the positive and negative sequences for control under an unbalanced voltage condition. The circulating current consists of not only negative-sequence components but also positive- and zero-sequence components under an unbalanced voltage condition. Therefore, an additional control method is necessary to consider the positive- and zero-sequence components of the circulating current. The proposed control method has the advantage of controlling not only the ac-side current of the MMC but also the circulating current without separating each of the current components to control each arm current of the MMC. In addition, it can stably control the positive- and zero-sequence components of the circulating current under the unbalanced voltage condition. [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]

This paper proposes a DC/DC current flow controller (CFC) for meshed high voltage direct current (HVDC) grids. The CFC has the advantage of a simplified structure that allows us to use the minimum number of switches when unidirectional current flows through the DC lines are expected. An extended CFC topology that is able to operate with all possible current flows is also presented. Then, the operating principle of the CFC is discussed and its structure is compared with a dual H-bridge analyzing advantages and disadvantages. This paper also details the applied modulation strategy and the control methodology. Finally, a five-terminal meshed HVDC grid is used to validate the CFC by means of simulations. [ABSTRACT FROM PUBLISHER]

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]

An unbalance voltage between a positive pole and negative pole will exist for a long time in an ungrounded modular multilevel converter (MMC) HVDC transmission system. A zig-zag transformer has been proposed to construct an artificial neutral point and keep voltage balance between the positive pole and the negative pole. An efficient grounding system consists of the zig-zag transformer and the neutral grounding resistor. In this paper, the different impedances, including positive-, negative-, zero-sequence impedance, and dc current impedance, have been analyzed, respectively. Then, the computer simulations have been performed to evaluate the characteristic of the zig-zag transformer under normal voltage, zero-sequence voltage and unbalanced dc voltage, respectively. The simulation results show that 1) the zig-zag transformer can effectively balance the unbalanced voltage in dc side of the MMC under different work conditions; 2) the zero-sequence impedance is low and zero-sequence current is convenient for the detection of a single-phase fault; 3) the power consumption of the neutral grounding resistor is of a light rating, and the cooling of the resistor is unnecessary. The method presented in this paper proves to be an efficient grounding method for the MMC-HVDC transmission system. [ABSTRACT FROM AUTHOR]

This paper presents a new control approach for enhancing the fault ridethrough capability of wind farms connected to the grid through a voltage-source-converter-based high-voltage dc transmission line. A controlled voltage drop in the wind farm collector grid is initiated upon the occurrence of fault in the high-voltage grid in order to achieve fast power reduction. In the process, a dc voltage of defined magnitude and duration is injected by the sending-end converter together with the voltage reduction to suppress the dc component of the short-circuit current arising from the voltage reduction. As a result, the electrical and mechanical stress on the wind turbines, especially on the DFIG-based units and their converters, are mitigated. Using a test network, the improvement in the fault ridethrough capability of the system that can be achieved by employing the proposed method has been demonstrated. [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]

The modular multilevel converters (MMCs) are attractive topologies for the high-voltage direct current (HVDC) transmission system. Since the complicated structure of MMC with hundreds sub-modules per arm, building an accurate model to design the appropriate control strategy is one of the main technical challenges. In this paper, the average-value model (AVM) of the MMCs considering the sub-module capacitor voltage ripple is established to explore the control dynamic performance of MMCs. The effect of the sub-module capacitor voltage ripple on the MMC dynamic behavior is explored and the equivalent impedances of the DC and AC sides in the MMC converters are extracted and evaluated. Based on the equivalent impedances, a particular AVM of MMCs, which comprises the equivalent capacitors reflecting the capacitor voltage ripple, is presented. Furthermore, the detailed interaction of the AC side voltages, AC side currents and arm voltages and the coupling relationship of AC output current id and iq can be also precisely described by the introduced AVM. Finally, the effectiveness of the presented model is verified by the PSCAD/EMTDC simulations. [ABSTRACT FROM PUBLISHER]

The large number of switching elements in the modular multilevel converter (MMC) is a challenging problem when modeling the MMC-HVDC systems for the computation of electromagnetic transients. The modeling complexity increases even further when a multiterminal (MT) MMC-HVDC system is used to integrate offshore wind farms (OWFs) with power-electronics-based wind energy converters, such as doubly fed induction generators (DFIGs). This paper compares modeling accuracy and computational performances for various combinations of MMC and OWF models. Onshore and offshore ac fault simulations are performed for an OWF system composed of DFIG-type wind turbines and connected to a practical ac grid through an MT MMC-HVDC system. The OWF system model includes the detailed representation of the offshore collector grid and the associated overcurrent protection. The offshore MMC controls include an offshore fault current limiter and fast OWF power generation reduction-based fault-ride-through function. [ABSTRACT FROM PUBLISHER]

Modular multilevel converter (MMC) based high voltage direct current transmission (HVDC) is an effective solution for large-scale renewable power integration over long-distance. In the overhead MMC-HVDC systems, the high voltage DC circuit breakers (DCCB) are implemented to interrupt the DC fault current. Subsequent to fault isolation, the DCCBs are required to automatically re-close to restore power transmission quickly. However, when the DCCBs are re-closed to permanent faults, they will be tripped again, resulting in a high requirement of interruption capacity for DCCBs and second overcurrent strikes on the HVDC systems. To overcome the drawbacks of the conventional auto-reclosing scheme, this paper proposes an adaptive reclosing scheme based on the active pulse injection from the converter associated with the coordination control of hybrid DCCBs. The single-end adaptive reclosing scheme as well as the two ends adaptive reclosing scheme dedicated to two-terminal HVDC systems and meshed DC grids are presented respectively. By applying this method, the location of faults can also be achieved in the case of permanent faults. In order to verify the effectiveness of the proposed adaptive reclosing schemes, extensive simulations have been conducted under PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

This paper analyzes the control structure of the multiterminal dc (MTDC) system to damp ac system interarea oscillations through active power modulation. A new control structure is presented that maximizes the relative controllability without the need for communication among the dc terminals. In point-to-point high-voltage dc (HVDC) transmission, the active power modulation of the two terminals occurs in opposite directions. In this case, the control direction is given and only needs to be phase compensated to align for maximal damping. In the case of MTDC systems, the control direction interrelates with the active power modulation share of the dc terminals and the relative controllability depends on this. The new control structure eliminates the need for communication between the dc terminals by performing dc voltage feedback loop shaping. This makes it possible to modulate the power in one terminal and let the other terminals react on the dc voltage change. Through loop shaping, where the feedback gain varies in the frequency plane compared to the regular droop design, the control direction is aligned with the direction of highest relative controllability. Loop shaping takes place without influencing the high frequency or steady-state gain. Simulations in the Nordic32 test system show the validity of the proposed controller and its structure. [ABSTRACT FROM AUTHOR]

In this paper, a two-terminal VSC-HVDC system embedded in a weak grid ac environment is considered, emphasizing modeling, controller design, and small-signal stability analysis. Traditionally, the dc cable is modeled by \Pi-sections, implying that care has to be taken when using the model for higher frequencies or in cases of higher cable impedance density, such as submarine cables. Here, a distributed parameter cable model is used to overcome this problem. The VSC-HVDC system can be described as two cascaded blocks. The first block is a transfer function that will differ depending on what input and output variables are considered, but which is in all realistic cases stable. The second block is a feedback loop, where the forward path is a rational function and the return path is a dissipative infinite dimensional function, remaining the same in all cases. The stability is then analyzed, using the Nyquist criterion, in a straightforward manner. Numerical examples are given by the use of MATLAB. The result is that if the VSC-HVDC system using a single \Pi-section cable model is stable, so is the VSC-HVDC system using a distributed parameter cable model. [ABSTRACT FROM AUTHOR]

Due to the rapid development of the voltage-source converter (VSC), the demands for HVDC cables have increased significantly. Although more than ten VSC-HVDC projects are under construction and many more are in operations globally, the testing criteria for HVDC XLPE cables are mainly referenced to test guideline TB 496 recommended by CIGRE. Due to the large stray capacitance of the VSC-HVDC cables, the switching impulse overvoltage stress of the cables is the main electrical parameter concerned, a unified overvoltage factor of 2.1 p.u. being generally accepted. However, the switching impulse overvoltage stress of a VSC-HVDC cable system depends on not only the VSC topology but also the cable parameters and the volt-ampere characteristics of the dc cable surge arresters used. This paper studies the switching impulse overvoltage stresses for the dc cable system of a symmetrical monopole VSC-HVDC transmission system using the modular multilevel converter technology. The studies were performed according to the key technical parameters for a typical 1000-MW/320-kV VSC-HVDC transmission scheme, the XLPE dc cable links being assumed. The results show that a switching impulse overvoltage level of 2.3 p.u. should be applied during the type test of the overvoltage capability of the HVDC cables. [ABSTRACT FROM AUTHOR]

This paper presents a comparison of two forms of cable connection of a distant offshore wind farm to a transmission system: AC and HVDC. The requirements of relevant industry standards in Great Britain (GB) that drive a connection design and, hence, its costs are highlighted along with an analysis of the ways in which AC cable connections might be made to comply while facilitating the export of active power. Dynamic studies investigating responses to grid-side short-circuit faults show that, in the particular scenarios studied, an AC connection of a wind farm in place of a large synchronous generator is marginally detrimental while an HVDC connection is beneficial. A comparison of costs shows that the crossover distance at which HVDC is cheaper than AC for wind farms of different sizes occurs at longer distances than have hitherto commonly been assumed, and AC connections benefit from reactive compensation not only at the point of common coupling and wind farm end, but also at the connection midpoint. [ABSTRACT FROM AUTHOR]

This paper presents the control and operation of a hybrid HVDC system comprising a wind farm-side voltage-source converter rectifier and a grid-side LCC inverter for integrating wind power. The configuration and operation principle of the hybrid HVDC system are described. Commutation failure in the LCC inverter during an ac network disturbance is considered and its impact on the hybrid system operation is analyzed. An enhanced control strategy for the LCC inverter at the grid side and an alternative modular multilevel converter (MMC) topology using mixed half-bridge and full-bridge modules considered for the rectifier at the wind farm side are proposed. Simulation results using Matlab/Simulink are presented to demonstrate the robust performance during LCC inverter commutation failure to validate the operation and recovery of the hybrid system with the proposed control strategy and MMC configuration. [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]

For a large multiterminal HVDC system, it is important for a dc fault on a single branch to not cause significant disturbance to the operation of the healthy parts of the dc network. Some dc circuit breakers (DCCBs), for example, mechanical type, are low cost and have low power loss, but have been considered unsuitable for dc fault protection and isolation in a multiterminal HVDC system due to their long opening times. This paper proposes the use of additional dc passive components and novel converter control combined with mechanical DCCBs to ensure that the healthy dc network can continue to operate without disruption during a dc fault on one dc branch. Two circuit structures, using an additional dc reactor, and a reactor and capacitor combination, connected to the dc-link node in a radial HVDC system, are proposed to ensure that overcurrent risk at the converters connected to the healthy network is minimized before the isolation of the faulty branch by mechanical DCCBs. Active control of dc fault current by dynamically regulating the dc components of the converter arm voltages is proposed to further reduce the fault arm current. Simulation of a radial three-terminal HVDC system demonstrates the effectiveness of the proposed method. [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 feasibility of future multiterminal dc (MTDC) systems depends largely on the capability to withstand dc-side faults. Simulation models of MTDC systems play a very important role in investigating these faults. For such studies, the test system needs to be accurate and computationally efficient. This paper proposes a detailed equivalent model of the modular multilevel converter (MMC), which is used to develop the MTDC test system. The proposed model is capable of representing the blocked-mode operation of the MMC, and can be used to study the balancing control of the capacitor voltages. In addition, the operation of the MMC when redundant submodules are included in the arms can also be studied. A simplified model of a hybrid high-voltage dc breaker is also developed. Hence, the developed test system is capable of accurately describing the behavior of the MMC-based MTDC system employing hybrid HVDC breakers, during fault conditions. Using time-domain simulations, permanent dc-side faults are studied in the MTDC system. In addition, a scheme to control the fault current through the MMC using thyristors on the ac side of the converter is proposed. [ABSTRACT FROM AUTHOR]

Future multi-terminal dc networks are envisioned for the large-scale integration of renewable energy sources into today's power systems. An essential aspect for the reliable operation of these systems is the fast and selective dc-side fault handling and separation of faulted lines. The objective of this paper is the development and evaluation of a multi-terminal dc protection strategy based on fault-blocking converters (e.g., full-bridge based modular multi-level converters) and high-speed switches. Both a suitable fault control and a new high-speed switch design for a fast separation of faulted dc lines are elaborated. The novel concept is intended to reduce the requirements on the high-speed switches compared to dc circuit breakers and to ensure a fast restoration of the active power transmission capability, as well as the dc voltage. The fault-clearing strategy is analyzed using electromagnetic transient (EMT) simulations under variation of the fault location, type, and resistance. The investigations are carried out in a cable-based multi-terminal HVdc system with full-bridge modular multi-level converters in symmetrical monopole configuration. [ABSTRACT FROM AUTHOR]