Meshed HVDC Grids Research Articles

Combined Data-Driven and Model-Driven Location of Lightning Strikes: Application to Meshed HVDC Grids

This paper addresses the low-delay location of faults due to lightning strikes using single-ended measurements in HVDC grids. A combined data-driven and model-driven approach is used to estimate the fault location iteratively. No prior knowledge of the voltage evolution at the fault location is required. Once the first wavefront due to a fault is detected at some observation location of the HVDC grid, the measurements are fed to a model of the propagation of transient waves along the lines. The model is parameterized by the estimated fault location. As a result, a model of the waveform samples starting from the second wavefront is obtained. The measured and modeled waveform samples are then compared to update the estimate of the fault location. The performance of the proposed approach is evaluated via simulations, including field measurements of lightning currents. Typical location accuracy of 300 m is obtained by considering observations performed at 1 MHz over an observation time interval of less than 1.5 ms.

Deep Learning-Based Robust DC Fault Protection Scheme for Meshed HVDC Grids

Deep Learning-Based Robust DC Fault Protection Scheme for Meshed HVDC Grids

A Hybrid Modular Interline Current Flow Controller for Meshed HVDC Grids

In meshed high-voltage direct current (HVdc) grid, the current flows through the lines cannot be controlled with sufficient freedom without additional power electronics-based devices, namely current flow controller (CFC). In this article, a novel hybrid modular interline CFC is proposed based on H-bridge submodules and thyristor valves. Due to its modularity, the proposed CFC is particularly suitable for applications requiring high voltage and large power capacity. The circuit structure, operation principle, and parameter design are presented. In addition, a control strategy is developed for the proposed CFC. Simulations are carried out using a four-terminal meshed HVdc grid to verify the effectiveness of the topology and its control strategy. A downscaled prototype is built, which further validates the proposed CFC.

DC-side fault current estimation approach for HVDC circuit breaker sizing

Selection of the interruption capabilities of HVDC circuit breakers (DCCB) requires an estimation of the maximum DC-side fault current and absorbed energy. However, estimating these requires iterating over a large range of scenarios. At present, this task is performed for a specific grid using extensive time domain simulations in Electromagnetic Transient (EMT)-type software. These simulations require a large computational effort, especially for large-scale meshed HVDC grids. In this paper, an alternative DC-side fault current estimation approach based on numerical inverse Laplace transform is proposed to reduce the computational burden in estimating the DCCB parameters without the need for extensive EMT simulations. The proposed approach includes the pre-fault conditions, stages of the converter fault response, DCCB operation and detailed transmission line model. The paper shows that the fault current interruption and absorbed energy in the DCCB, can be approximated accurately for the DCCB operating times within the first two stages of the converter fault response, while an upper limit of these quantities can be estimated for DCCBs operating within the subsequent stages. The proposed approach allows for investigating a large range of scenarios in large-scale meshed HVDC grids with acceptable computational effort, sufficient accuracy and adaptability to system configuration/parameter changes.

Voltage control of common capacitor and self-faults analysis of DC current flow controllers for meshed multi-terminal HVDC grids

In this paper, the control methodology of the common capacitor voltage and the DC current flow controller (DC CFC) under self-fault conditions are investigated with the following two contributions. (1) A selection method of the common capacitor voltage based on various specific control requirements with explicit control margins of the DC line currents is proposed with analytic derivations of two-line/three-line DC CFCs in the meshed three-terminal/four-terminal high-voltage direct current grids (HVDC), respectively. (2) The impact of different self-faults of the DC CFC on the control performance of the DC line currents is analyzed with a simplified topology with unidirectional current flow presented. Case studies are conducted in the time-domain simulation environment PSCAD/EMTDC and the results justify the effectiveness of the theoretical analysis. The proposed common capacitor voltage control method and self-fault analysis would be useful for practical applications of the DC CFC under both steady and transient states.

Fast Fault Identification in Bipolar HVDC Grids: A Fault Parameter Estimation Approach

The protection of meshed HVDC grids requires the fast identification of faults affecting the transmission lines. Communication-based methods are thus not suited due to the transmission delays. Many approaches involving a model of the transient behavior of the faulty line have recently been proposed. Nevertheless, an accurate description of the traveling wave phenomenon in multi-conductor lines such as overhead lines requires complex computations ill-suited for fast fault identification. This paper presents a single-ended fault identification algorithm using a closed-form parametric model of the fault transient behavior. The model combines physical and behavioral parts and depends explicitly on the parameters that characterize the fault, namely the fault distance and impedance. When a fault is suspected, the fault parameters are estimated so that the model fits best the received measurements. The confidence region of the estimated fault parameters is used to decide whether the protected line is actually faulty or not. The proposed algorithm is tested on a 4 station grid simulated with EMTP-RV software. The method is able to identify the faulty line using a measurement window of less than 0.5 ms. This allows ultra-fast fault clearing and can hence improve the overall reliability of future HVDC grids.

Impact of Meshed HVDC Grid Operation and Control on the Dynamics of AC/DC Systems

The efficacy of long-distance and bulk power transmission largely depends on the efficient control and reliable operation of a multiterminal high-voltage direct current (MT-HVdc) grid, more precisely, a meshed HVdc grid. The capability of enduring the dc grid fault eventually enhances the reliability and improves the dynamic performance of the grid. This article investigates the operation and control of an AC/multiterminal dc (MTDC) system with bipolar topology incorporating the dc grid protection schemes. Based on the scale of a circuit breaker's operating time, the performance of three different protection strategies is compared and analyzed using DIgSILENT PowerFactory. Simulation results explicitly reveal that the dynamic performance of the MTDC grid significantly deteriorates with the slow functioning of the protection schemes, followed by a dc grid fault. Besides, prolonged recovery time causes a substantial loss of power infeed and affects the ac/dc grid's stability. Finally, to assess the frailty of the MTDC grid, a transient energy stability index is proposed considering the voltage variation in the prestate and poststate fault clearing interval. Relevant case studies are performed on the MTDC grid using an analytical approach and nonlinear simulation studies to validate the effectiveness of the proposed index.

DC Fault Management Strategy for Continuous Operation of HVDC Grids Based on Customized Hybrid MMC

Successful deployment of high-voltage direct current (HVdc) grids necessitates effective dc fault handling strategies, which can minimize the severe consequences caused by dc faults on the ac and dc sides of the HVdc grids. Therefore, this article investigates the enhanced dc fault performance of the customized hybrid modular multilevel converter (CH-MMC), in which a limited number of full-bridge submodules (FB-SMs) are added into the arms of the conventional MMC in an effort to significantly extend the timespan between fault inception and fault clearance, thus allowing the use of relatively slow and cheaper dc circuit breakers (DCCBs). Based on this converter, a dedicated dc fault handling strategy for CH-MMC-based HVdc grids is proposed, which aims to improve the fault resiliency and security of HVdc grids for pole-to-pole faults. Moreover, the proposed dc fault management strategy guarantees the continuous operation of the grid during pole-to-ground dc faults, including full reactive power provision from the converter stations. The performance of the strategy is demonstrated using comprehensive electromagnetic transient (EMT) simulation studies conducted on an illustrative four-terminal meshed HVdc grid, which considers a range of scenarios with different fault current limiting inductors and DCCB operation times.

Multiport Interline Current Flow Controller for Meshed HVDC Grids

In meshed high voltage direct current grids, the current of each line cannot be controlled independently, since it depends on the resistances between nodes. Additional devices, such as current flow controllers (CFCs), may be needed to avoid bottlenecks or line overloads. This work presents a multiport dc-dc-based CFC topology to be connected to n lines with unidirectional current flows. The device is able to control the dc lines' currents to the desired value by inserting variable voltage sources in series. First, the modeling of the generic n-port topology is presented and then, its modulation and control strategy are described. In the first case study, the concept is validated considering a 5-port CFC by means of dynamic simulations using different control methods. Finally, in the second case study, a 3-port CFC prototype is built and tested in an experimental platform in the laboratory considering different control modes.

Nyquist stability analysis and capacitance selection method of DC current flow controllers for meshed multi-terminal HVDC grids

Controllability of DC current/power flow is essential in multi-terminal HVDC (MTDC) grids, particularly for the MTDC grids in a meshed topology. In this paper, considering meshed MTDC (M 2 TDC) grids with the installation of two-line/multi-line DC current flow controllers (CFCs), a small-signal model of the DC CFCs integrated M 2 TDC grids is derived, studying the impact of the power losses of the DC CFC and their influence on the analysis of energy exchanges. The system stability analysis is analysed using the Nyquist diagram, which is more suitable for analyzing complex nonlinear systems with more compact and reliable indicators of stability in comparison with gain/phase margins shown in the Bode diagram. In addition, a selection method of the interconnected capacitor of the DC CFC is proposed under different operating conditions. The impact of the switching frequencies of the DC CFC on the control ranges of the DC current flows is analyzed. The effectiveness of the Nyquist analysis and the capacitance selection method is verified by simulation studies using PSCAD/EMTDC. The obtained control ranges of the DC CFC with different switching frequencies and capacitances would be useful for practical engineering applications.

Transient-based fault identification algorithm using parametric models for meshed HVDC grids

This paper addresses the problem of fault identification in meshed HVDC grids once an abnormal behavior has been detected. A parametric single-ended fault identification algorithm is proposed. The method is able to determine whether the line monitored by a relay is faulty or not using a very short observation window. When a fault is suspected, the proposed algorithm estimates the fault distance and impedance using a parametric model describing the voltage and current evolution just after the fault occurrence. This model combines phenomenological (knowledge-based) and behavioral (black-box) parts to represent the fault propagation and to account for ground effects and various losses. The identification of the faulty line is then based on the size of the confidence region of the obtained estimate. The performance of the algorithm for a three-node meshed grid is studied using Electro-Magnetic Transient (EMT) simulations. On the considered grid model, the current and voltage need to be observed during less than 0.2 ms to get a sufficiently accurate estimate of the fault characteristics and identify consistently the faulty line.

Components Sharing Based Integrated HVDC Circuit Breaker for Meshed HVDC Grids

DC fault isolation is one of the main challenges of the HVDC grid. DC fault current can be interrupted within several milliseconds by employing hybrid HVDC circuit breaker. However, this scheme is not cost-effective, especially in meshed HVDC grid. In this paper, a novel integrated HVDC circuit breaker by sharing components is proposed to overcome these drawbacks. Suppose that a converter in a meshed HVDC grid is connected to m DC lines. The integrated HVDC circuit breaker consists of m + 1 ultra-fast disconnectors (UFDs), m + 1 load commutation switches (LCSs), m + 1 mechanical disconnectors (DSs), and only one main breaker (MB) and surge arrester (SA). The behavior of the proposed integrated HVDC circuit breaker is similar to that of the conventional hybrid HVDC circuit breaker. These two kinds of breakers have the same operating time, which means the integrated HVDC circuit breaker is fast enough to clear DC faults. Besides, the IGBT modules in the MB and LCSs of the proposed integrated HVDC circuit breaker are connected in one direction, and the m DC lines share a single MB. Therefore, this scheme is more cost-effective compared to the conventional hybrid HVDC circuit breaker. Moreover, the adjacent lines can still be used to transmit power after DC bus faults. Simulation results on a three-terminal HVDC grid are presented to verify the effectiveness and feasibility of the proposed scheme.

A novel HVDC circuit breaker for HVDC application

Hybrid high voltage direct current circuit breakers (DCCBs) are capable of interrupting fault current within a few milliseconds, but this technology has high capital cost, especially in a meshed HVDC grid. To increase the economic competitiveness of hybrid DCCBs, this paper proposes a capacitor commutated dc circuit breaker (CCCB). The CCCB mainly comprises an auxiliary branch with a fast dis-connector in series with semiconductor devices and the main branch with the series connection of a dc capacitor and diode valves. This paper provides a detailed depiction of the CCCB. The topology and operating principles are discussed. The impact of snubber circuits and stray inductances on the commutation process is analyzed. The general sizing method for the main components in the CCCB is detailed. Reclosing to transmission lines with different operating conditions is studied. Several extended topologies are proposed to further reduce the semiconductor cost and on-state operation power loss. The power loss and cost of CCCB are assessed. Extensive simulations on PSCAD/EMTDC verified the dc fault isolation and reclosing of the CCCB.

Droop Control for a Multi-Line Current Flow Controller in Meshed Multi-Terminal HVDC Grid Under Large DC Disturbances

The security, stability, and reliability of power supply are essential and must be guaranteed during operation of multi-terminal HVDC (MTDC) grids, particularly under disturbances, such as a line or converter loss, on the dc grid side. In this paper, a multi-line current flow controller (CFC) is expanded from the existing two-line CFC for meshed MTDC grids with two main contributions: a droop control scheme for a multi-line CFC, which is capable of coordinating dc line current/power sharing under large dc disturbances is proposed and a selection method for the droop gains is proposed. The multi-line CFC with the droop control scheme proposed can achieve full controllability of the currents among multiple dc lines and the security and continuity of power supply of the unfaulty dc lines can be guaranteed. The stability and dynamic performance of the droop controller can both be satisfied with the proposed selection method. The effectiveness of the proposed scheme and selection method is justified by RTDS simulations. The proposed scheme is simple and easy to implement. The proposed dynamic simulation-based selection method has the flexibility suiting both online and offline calculation tools and the consideration of system nonlinear characteristics and control limits/constraints.

Fault mode operation strategies for dual H-bridge current flow controller in meshed HVDC grid

Current flow controllers (CFCs) can remove grid bottlenecks or extend grid operation area by changing amount of power flowing through DC transmission lines. This study focuses on behavior of interline H-bridge CFC in a DC grid in fault condition. In addition to the CFC circuit level fault studies, non-linear and linearized simplified models are developed for system level analysis. The analysis and fault study shows that the interline H-bridge CFC cannot survive during DC transmission line and bus faults due to an overvoltage occurring in its capacitor. Further investigation figures out that this overvoltage cannot be avoided even in presence of fast HVDC circuit breakers. Hence, an improved control system together with circuit level modifications are proposed to improve the CFC post-fault operation and to retain its components from possible damages.

Experimental Validation of Dual H-Bridge Current Flow Controllers for Meshed HVdc Grids

The current-carrying capability of dc lines is limited by their thermal and electric stress limits. Thus, the line current must be maintained within the permissible operational region to protect the lines from damage. In a dense dc grid, control over each line current cannot be achieved without including additional control devices. In this paper, a dual H-bridge current flow controller (2B-CFC) is used to manage the dc grid line power flow by providing dc voltage compensation in series with dc lines. A centralized hierarchical control system is proposed to coordinate the operation between multiple CFCs. A novel voltage-sharing control scheme is demonstrated. It is shown that such a scheme reduces the workload on a single CFC by sharing the required control voltage between multiple CFCs, and, in addition, can be used to avoid control conflicts among active CFCs during communication failure. An experimental platform consisting of a three-terminal dc grid and small-scale 2B-CFC prototypes has been developed to validate the concepts. For completeness, the CFC performance has been analyzed for overload conditions and when no communication exists. Small-scale dc circuit breakers have been developed to study the CFC performance under a pole-to-pole fault.

Current Flow Controlling Hybrid DC Circuit Breaker

This paper proposes a new device by combining features of an interline dual H-bridge current flow controller with the core idea of a hybrid HVdc circuit breaker for meshed HVdc grid application. The proposed device can substitute two dc circuit breakers at a dc bus with at least two adjacent transmission lines. In addition to the current interruption action, the current in one of the adjacent lines can be controlled by the embedded current flow controller. The system-level behavior of the proposed current flow controlling hybrid dc circuit breaker is similar to that of the typical hybrid dc circuit breaker and the interline dual H-bridge current flow controller. The operation principles of the proposed device are introduced and analyzed in this work. The component ratings are compared to the existing solution, and the functionality of the proposed device is verified by simulation.

Modelling and Control of an Interline Current Flow Controller for Meshed HVDC Grids

This paper focuses on the modelling and control of an interline current flow controller (CFC) for meshed HVDC grids. The operation states of the CFC are presented, and an average model is derived. The average model is used to perform steady-state analysis on a 3-terminal meshed grid, showing the current change capabilities and the benefits on the operation area. The converter control is designed using a linearized model. The system performance of the CFC is tested by means of simulation in a 3-terminal grid and in a 5-terminal grid.

Test Circuits for HVDC Circuit Breakers

High voltage direct current circuit breakers (HVDC CBs) are necessary for reliable and safe operation of future multi-terminal meshed HVDC grids. So far no commercially mature products of such equipment exist. A few industrial concepts as a result of advances in power electronics have been proposed and prototypes have been built. However, performances of these concepts have never been demonstrated under realistic operation conditions. Since these tests require construction of high power DC sources incurring significant investment costs, alternative test circuits providing equivalent stresses must be considered. In this paper various test methods and circuits used for testing HVDC CBs are investigated and their performance is evaluated against the stresses in a conceptual future HVDC grid. A novel method of testing an HVDC CB using existing installations in a high power ac test laboratory is proposed and its prospective fault current and voltage is experimentally demonstrated. It is concluded that adequate test circuits need to maintain the supply voltage not only up to the current zero in the main interrupter unit of the HVDC CB, but also during the entire energy dissipation phase of the interruption process.

Mixed AC high‐voltage direct current benchmark test system for security constrained optimal power flow calculation

Mixed AC-HVDC systems enable new perspectives in the fields of operation, control and security of AC transmission systems. Although a promising vision, meshed HVDC grids do not exist in a real setting. Research on HVDC grid operation is therefore in need of valid test systems for static and dynamic aspects. Besides the CIGRE test system, which focuses on offshore factors and which is consequently not an overlay HVDC grid, a publically available test system does not currently exist. Recent publications in this field adapt established standard AC test systems to their case-related requirements of an HVDC extension. This work introduces and provides a mixed AC-HVDC test system enabling the evaluation of optimal power flow and security constrained OPF algorithms. State-of-the-art literature for HVDC-OPF applications is assessed, applied AC standard test systems are evaluated and key indices are derived from the findings. With respect to this, the proposed test system’s validity and the AC grid’s reference to reality is ensured. The proposed system, comprising 67 AC nodes, 102 AC branches, 9 VSCs and 11 HVDC branches, contains 13 critical contingencies which can be removed by either preventive or corrective measures. The data describing the proposed system is provided.