Search

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 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]

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]

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]

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]

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]

The half-bridge modular multilevel converter (MMC) is a voltage source converter (VSC) with high efficiency, controllability, and modularity. The topology is weak to dc side faults unless bipolar submodules are used, but this results in decreased efficiency. Power groups (PGs), a thyristor augmented multilevel structure, have been proposed as a way to reduce the power-loss increase arising from achieving dc-fault-tolerance. This paper investigates whether the PG concept can also achieve significant efficiency improvements in VSCs that are not required to be dc fault tolerant. A single submodule voltage (SSMV) method of controlling the turn-on/turn-off of the thyristor assembly within each PG structure is presented, and the differences with the previously detailed dual submodule voltage (DSMV) technique are described. Two thyristor-based PG structures for use in non-dc-fault-tolerant MMCs are proposed, one using SSMV and the other using DSMV. A comparison is made considering the required semiconductor device count, the impact on thyristor snubber design, and the overall power-losses achieved. A further, simplified, variant using a diode bypassed PG structure is presented, which results in power-loss reductions during rectifier mode only. Results show that power-loss reductions of $\sim$ 20%–25% can be achieved by using the proposed PG structures to augment a half-bridge MMC. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive and fundamental analysis of the steady-state voltage and power stability of single-infeed voltage-source converter (VSC) HVDC systems. The classical stability concepts developed for such analysis of line-commutated converter (LCC) HVDC systems are also shown to be applicable to VSC-HVDC systems as well. The voltage-support capability and power delivery capacity of the VSC-HVDC system to a weak receiving ac system are also shown in relation to the LCC-HVDC system. These contributions are shown to be particularly effective for the VSC-HVDC system having a typical reactive ac power control but supplemented in this work with an ac voltage modulation control. Time-domain dynamic simulations on a single-infeed VSC-HVDC system also validated the fundamental derivations in this work. [ABSTRACT FROM AUTHOR]

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 meshed High Voltage Direct Current (HVDC) grids, Current Flow Controllers (CFC) may be required to avoid bottlenecks or line overloads. The present paper proposes the concept of installing Distributed Current Flow Controllers (DCFCs) in different nodes of a meshed HVDC grid that are operated selectively. This concept allows to use simplified DCFC devices to prevent the overloads, with a reduction of the required CFC voltage compared with a single CFC in the grid. First, a methodology to investigate the effect of installing DCFCs in a generic meshed HVDC grid is presented. Several case studies with a 3-terminal and a 7-terminal meshed HVDC grids are conducted and the most suitable topology for the DCFCs is derived based on the previous analysis. Then, this work analyses the increase in the operational area brought by installing DCFCs in a 3-terminal HVDC grid and it is compared with the increase achieved by a Dual H-bridge CFC in one node. The CFC losses and voltage requirement depending on the location of the DCFCs are investigated. Finally, the operation and control of the DCFCs are designed and tested in a 3-terminal meshed HVDC grid using dynamic simulations. [ABSTRACT FROM AUTHOR]

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

Peak fault current and energy dissipation in high-voltage direct current (HVdc) circuit breakers (CBs) are very important parameters that impact dc grid protection development. This paper analyzes a hybrid DCCB (HCB) control that reduces peak current and energy dissipation by regulating the voltage across contacts of the ultrafast disconnector (UFD). This is achieved by manipulating the number of inserted surge arresters while contacts of the UFD are moving apart. The controller is seamlessly integrated with the current controller of HCBs. An analytical model for current and energy calculation is presented, verified, and employed for parametric studies. A PSCAD simulation with 320-kV, 16-kA test circuit confirms that the proposed voltage control reduces the peak current and energy dissipation by around 20%–30%. A 900-V, 500-A HCB laboratory hardware is described and the experimental results are shown to corroborate simulation conclusions. [ABSTRACT FROM PUBLISHER]

Modular multilevel converters (MMCs) are currently used in a number of high-voltage dc (HVdc) projects connecting very remote offshore wind power plants to shore. For this application, the offshore HVdc voltage-source converter provides the reference for voltage and frequency, that is, grid-forming operating mode. In fault ride through situations, however, the converter reactive current increases and has to be limited to protect the equipment. At the same time, the injection of maximum currents into the offshore grid is advantageous for fault detection and isolation by circuit breakers and to maintain the voltage level. State-of-the-art current limitation might underuse the converter capability in the grid-forming operating mode in specific unbalanced conditions. This paper proposes an extended limitation method for the converter current references for these conditions. It is demonstrated that the proposed method improves the voltage and current profile. In addition, the impact on the internals of the MMC is outlined. Dynamic simulations are conducted to validate the proposed extended limitation for two operation scenarios. The results demonstrate an enhanced utilization of the MMC capability, a better voltage profile, and improved transient behavior of the MMC for the proposed extended limitation compared to the normal limitation during specific unbalanced conditions. [ABSTRACT FROM PUBLISHER]

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 studies protection and control methods for a large dc grid based solely on full-bridge modular multilevel converters (MMCs). An initial theoretical study concludes that dc circuit breakers (CB) energy dissipation will depend on inductance and square of fault current, but not on protection operating time. Using differential protection is proposed because of robust selectivity and since it operates well with small series inductors. The analysis of dc CB dissipated energy leads to new protection logic design that delays tripping signals until local current reduces to low values. Low-speed dc CB with very small inductors are adequate. The design of controllers for MMC converters should be coordinated with dc grid protection and the study derives values for current controller references. The fault recovery time is found to depend on current reference settings at MMC terminals, and optimal values are derived. The conclusions are confirmed using Electromagnetic Transients Program (EMTP) simulation on a 400-kV, 4-MMC dc grid considering two topologies: with five overhead lines and with five dc cables. [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]

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]

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]

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]

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]

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]

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 presents an enhanced independent pole control scheme for hybrid modular multilevel converter based on a full-bridge submodule and half-bridge submodule. A detailed analysis of the power distribution between upper and lower arms under asymmetrical dc pole voltages is presented. It is found that the fundamental ac currents in the upper and lower arms are asymmetrical. To enable operation under asymmetrical dc pole voltages, an enhanced independent pole control scheme is proposed. The controller is composed of two dc control loops, two ac control loops, and circulating current suppression control based on current injection. Six modulation indices are presented to independently control the upper and lower arms. With this controller, the dc voltage operating region is significantly extended. To ride through a pole-to-ground dc fault without bringing dc bias at the neutral point of the interface transformer, a pole-to-ground dc fault ride through strategy is proposed. The feasibility and effectiveness of the proposed control scheme are verified by simulation results using PSCAD/EMTDC. [ABSTRACT FROM PUBLISHER]

This paper investigates the effect of control-loops interactions on power stability limits of the voltage-source converter (VSC) as connected to an ac system. The focus is put on the physical mechanism of the control-loops interactions in the VSC, revealing that interactions among the control loops result in the production of an additional loop. The open-loop gain of the additional loop is employed to quantify the severity of the control-loop interactions. Furthermore, the power current sensitivity, closely related to control-loops interactions, is applied to estimate the maximum transferrable power of the VSC connected to an ac grid. On that basis, stability analysis results show that interactions between dc-link voltage control and phase-locked loop restrict the power angle to about 51° for stable operation with no dynamic reactive power supported. Conversely, the system is capable of approaching the ac-side maximum power transfer limits with alternating voltage control included. Simulations in MATLAB/Simulink are conducted to validate the stability analysis. [ABSTRACT FROM PUBLISHER]

Bipolar HVDC transmission systems typically offer up to 50% transmission redundancy in case of single pole-to-ground line faults. Continuous operation during permanent multiconductor faults, however, is not permitted, emphasizing the need for accurate fault type discrimination in such applications. For bipolar schemes with a dedicated metallic return (DMR) conductor, discrimination of pole–ground faults and pole–DMR faults represents a challenging task due to their similar electrical characteristics. In this paper, several approaches to detect DMR fault involvement are identified and analyzed, comprising both noninvasive methods based on measurement data and invasive methods based on DC current control strategies of full-bridge VSC-MMC stations (Modular Multilevel Converter based on Voltage Source Converter technology). An exemplified point-to-point HVDC system is modeled in PSCAD/EMTDC and extensive simulation studies are carried out. Based on the obtained results, a comprehensive fault type discrimination concept for permanent line faults is developed, which can be incorporated into fault handling schemes. Thus, successful fault type discrimination can be performed for all relevant fault types, even under exceptional fault conditions. [ABSTRACT FROM PUBLISHER]

This paper presents the design, development, control, and supervision of a hardware-based laboratory multiterminal-direct-current (MTDC) testbed. This work is a part of the TWENTIES (transmission system operation with large penetration of wind and other renewable electricity sources in networks by means of innovative tools and integrated energy solutions) DEMO 3 European project, which aims to demonstrate the feasibility of a dc grid through experimental tests. This is a hardware-in-the-loop dc system testbed with simulated ac systems in real-time simulation; the dc cables and some converters are actual, at laboratory scale. The laboratory-scale testbed is homothetic to a full-scale high-voltage direct-current (HVDC) system: electrical elements are the same in per unit. The testbed is supervised by a supervisory control and data acquisition system based on PcVue. A primary control-based droop control method to provide dc grid power balance and coordinated control methods to dispatch power as scheduled by a transmission system operator are implemented. Since primary control acts as converter level by using local measurements, a coordinated control is proposed to manage the dc grid power flow. The implemented system is innovative and achievable for real-time real-world MTDC-HVDC grid applications. [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]

Existing local dc node voltage control characteristics for HVDC grids are dedicated to special control modes of converters with a limited variability/flexibility. Thus, also an adaptation of those characteristics according to special grid requirements is highly restricted. This paper proposes a new characteristic which is an explicit and thus continuous function covering the entire allowed operation area of a converter. This new formulation allows us to emulate all existing characteristics but also to continuously adapt the control characteristics shape towards (changing) grid requirements. A method to parameterize this characteristic according to the degree of availability of dc balancing power from the ac grid by close by units is described as well as parameterization rules to emulate existing characteristics. [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 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]

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]

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]

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]

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]

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 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]

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]

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]

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]

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]