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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]

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]

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]

Conservation voltage reduction (CVR) and distributed-generation (DG) integration are popular strategies implemented by utilities to improve energy efficiency. This paper investigates the interactions between CVR and DG placement to minimize load consumption in distribution networks, while keeping the lowest voltage level within the predefined range. The optimal placement of DG units is formulated as a stochastic optimization problem considering the uncertainty of DG outputs and load consumptions. A sample average approximation algorithm-based technique is developed to solve the formulated problem effectively. A multiple replications procedure is developed to test the stability of the solution and calculate the confidence interval of the gap between the candidate solution and optimal solution. The proposed method has been applied to the IEEE 37-bus distribution test system with different scenarios. The numerical results indicate that the implementations of CVR and DG, if combined, can achieve significant energy savings. [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]

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]

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]

When a transmission line close to points of common coupling (PCCs) experiences a short-circuit (SC) fault, the fast reactive power control of voltage-source converter-HVDC (VSC-HVDC) is likely to affect the protective relay operation of transmission lines. To study the performance of distance relays on an ac grid with an offshore wind HVDC network, this paper presents an apparent impedance calculation method, which utilizes the bus impedance matrix (Zbus) to calculate the impedances viewed by distance relays during a three-phase SC fault. The proposed method is used to identify the potential miscoordinated Zone 2 relays in the proposed combined ac/dc system. The analysis is verified by software simulation results. It is shown that the proposed method results in accurate impedances viewed by distance relays. It also identifies the protective device settings on the ac grid that need to be adjusted due to HVDC control of offshore wind generators. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) is a potential candidate for medium/high-power applications, specifically for high-voltage direct current (HVDC) transmission systems. One of the technical challenges associated with the control of an MMC is to carry out the submodule (SM) capacitor voltages balancing task without imposing any unnecessary switching transition among the SMs. This paper develops a general framework for the capacitor voltage balancing of an MMC. Based on the developed framework and with the objective of reducing the unnecessary switching transitions among the SMs, three capacitor voltage-balancing strategies are proposed and investigated: 1) a slow-rate balancing strategy based on the conventional sorting method; 2) a hybrid balancing strategy which combines a predictive method with the conventional sorting method; and 3) a fundamental-frequency balancing strategy. Performance of the proposed strategies for a point-to-point 21-level MMC-based HVDC system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported study results demonstrate the capability and tradeoffs of the proposed strategies to reduce the number of switching transitions and, consequently, the MMC losses, under various conditions. [ABSTRACT FROM AUTHOR]

In this paper, a method for fault locating in HVDC transmission lines is proposed which only uses the voltage signal measured at one of the line terminals. The postfault voltage signal, in a relatively short-time window, is considered and the corresponding fault location is estimated based on the similarity of the captured voltage signal to existing patterns. In this approach, the Pearson correlation coefficient is used to measure the similarity. Despite simplicity and low complexity of the proposed fault-location method, it does not suffer from the technical problems which are associated with the traveling-wave-based methods, such as the difficulty of identifying traveling wavefronts or the strong dependency of accuracy on the sampling frequency. Numerous training and test patterns are obtained by simulating various fault types in a long overhead HVDC transmission line under different fault location, fault resistance, and prefault current values. The accuracy of the proposed fault-location method is verified using these patterns. [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]

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]

This article deals with dc voltage dynamics of multi-terminal HVdc grids (MTDC) with energy-based controlled modular multilevel converters (MMCs) adopting the commonly used power-voltage droop control technique for power flow dispatch. Special focus is given on the energy management strategies of the MMCs and their ability to influence on the dc voltage dynamics. First, it is shown that decoupling the MMC energy from the dc side causes large and undesired dc voltage transient after a sudden power flow change. This occurs when this energy is controlled to a fixed value regardless of the dc voltage level. Second, the virtual capacitor control technique is implemented in order to improve the results. However, its limitations on droop-based MTDC grids are highlighted. Finally, a novel energy management approach is proposed to improve the performance of the later method. These studies are performed with detailed MMC models suitable for the use of linear analysis techniques. The derived MTDC models are validated against time-domain simulations using detailed electro-magnetic transient MMC models with 400 sub-modules per arm. [ABSTRACT FROM AUTHOR]

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

The modular multilevel converter-based high voltage direct current transmission (MMC-HVDC) technology has become an effective integrating method for large-capacity offshore wind farms. However, if AC side faults occur between the MMC and the wind farm, conventional current differential protection methods based on phasor measurements might not detect the large fault current at the wind farm and the MMC, resulting in poor protection performance. In the worst case, MMC blocking may occur during faults near the MMC station, causing relay refusal. The fault currents provided by both converters are analyzed in detail. Due to the fault-ride-through control objectives, it is proved that the singularity of fault current between wind farms and MMC is different. Therefore, a novel singular value decomposition (SVD) based pilot protection is proposed to solve this problem. A Hankel matrix is established for the short-circuit current at each end of the AC line. The Hankel matrix rapidly detects the singularity of the signal and its derivative, and the SVD method uses a short time window to extract the waveform features. This approach enables the detection of the fault features before MMC blocking in the worst-case fault scenario and prevents the incorrect fault identification caused by the minor amplitude of fault current provided by the converters. A detailed model of an offshore wind farm connected to the MMC-HVDC system is established in the real-time digital simulator (RTDS). A protection device compliant with industry standards is used to implement the SVD-based protection algorithm. Hardware-in-loop dynamic experimental tests are conducted to verify the performance of the proposed protection method. The experimental results demonstrate that the SVD-based protection method has excellent performance during various fault scenarios. [ABSTRACT FROM AUTHOR]

A methodology of fast voltage-balancing control based on average comparison and a fast numerical simulation model for the modular multilevel converter are proposed. In each control cycle, the average voltage of all the submodules (SMs) in an arm is calculated. The switching state of each SM will be determined by comparing the capacitor voltage of each SM with this average value. Little sorting of the capacitor voltages is required; therefore, the calculation burden on the controller is significantly reduced. The previous switching state of each SM will be mostly retained. Two variables \Delta U {C\sort} and \Delta U {C\\max} are introduced to adjust the equivalent switching frequency of each SM. Simulation results verified the effectiveness of the proposed voltage-balancing control in normal and fault conditions. A fast numerical simulation model for MMC on PSCAD/EMTDC is also proposed. The dynamics of each SM are sufficiently represented in the proposed model. A hybrid simulation model is further developed to incorporate the detailed switching dynamics of specific interested SMs. The simulation results show that the proposed models give almost identical results as the detailed switching models while the simulation speed of the proposed model is approximately 5000 times faster than the detailed switching model. [ABSTRACT FROM PUBLISHER]

This letter proposes a new intelligent semiconductor transformer (IST) which consists of a bidirectional ac/dc resonant converter, dc/dc converter, and dc/ac converter. This structure provides good voltage-balancing performance on the high-voltage side and a simple control method for bidirectional power flow. The proposed IST was verified though building and testing a single-phase 2 kVA (1.9 kVrms/127 Vrms) prototype. A three-phase transformer can be implemented with three units of the proposed IST. [ABSTRACT FROM PUBLISHER]

A new control method for a line-commutated converter-based (LCC) high-voltage direct-current (HVdc) system is presented and compared to a conventional strategy. In the proposed method, both the dc voltage and current of an LCC HVdc system are regulated to increase the short-term operating margin of dc power transfer and improve transient responses to dc power references. In particular, an increased operating margin of dc power transfer is achieved via the dc voltage regulation method. To verify the effectiveness of the proposed method, a state space model of an LCC HVdc system is developed considering dc voltage and current references as input variables and analyzed for various values of the dc line inductance and converter controller gains. The state space model can be used for time-efficient analyses of the dynamic characteristics of an LCC HVdc system. Simulation case studies are performed using MATLAB, where the state space model of the Jeju–Haenam HVdc system is implemented as a test case and compared to its comprehensive PSCAD model. The case study results suggest that the proposed method increases the short-term operating margin and speeds up the transient response of the HVdc system. Therefore, it will effectively improve real-time grid frequency regulation. [ABSTRACT FROM PUBLISHER]

The sparse modular multilevel converter (MMC) is a new type of high-voltage ac/ac MMC topology suitable for high-power applications. It is based on an alternative configuration of half/full-bridge submodules, and voltage unfolder stages on each side of the converter. This topology has fewer components compared to conventional approaches and as an additional benefit, more than half of the switches operate under the soft switching condition. A comprehensive control strategy is proposed to ensure capacitor voltage balancing while exploiting the full power capability of the converter. A modified unfolder is also suggested to eliminate the inherent zero-crossing circulating current. The effectiveness of the proposed control strategy is confirmed by simulation and experimental results. [ABSTRACT FROM PUBLISHER]

The change rate of the dc reactor voltage with predefined protection voltage thresholds is proposed to provide fast and accurate dc fault detection in a meshed multiterminal HVDC system. This is equivalent to the measurement of the second derivative of the dc current but has better robustness in terms of electromagnetic-interference noise immunization. In addition to fast dc fault detection, the proposed scheme can also accurately discriminate the faulty branch from the healthy ones in a meshed dc network by considering the voltage polarities and amplitudes of the two dc reactors connected to the same converter dc terminal. Fast fault detection leads to lower fault current stresses on dc circuit breakers and converter equipment. The proposed method requires no telecommunication, is independent of power-flow direction, and is robust to fault resistance variation. Simulation of a meshed three-terminal HVDC system demonstrates the effectiveness of the proposed dc fault detection scheme. [ABSTRACT FROM PUBLISHER]