High Voltage Direct Current System Research Articles

An Intelligent Method for Fault Location Estimation in HVDC Cable Systems Connected to Offshore Wind Farms

Large and remote offshore wind farms (OWFs) usually use voltage source converter (VSC) systems to transmit electrical power to the main network. Submarine high-voltage direct current (HVDC) cables are commonly used as transmission links. As they are liable to insulation breakdown, fault location in the HVDC cables is a major issue in these systems. Exact fault location can significantly reduce the high cost of submarine HVDC cable repair in multi-terminal networks. In this paper, a novel method is presented to find the exact location of the DC faults. The fault location is calculated using extraction of new features from voltage signals of cables’ sheaths and a trained artificial neural network (ANN). The results obtained from a simulation of a three-terminal HVDC system in power systems computer-aided design (PSCAD) environment show that the maximum percentage error of the proposed method is less than 1%.

Cooperative Voltage and Frequency Regulation with Wind Farm: A Model-Based Offline Optimal Control Approach

Converter blocking is a serious malfunction encountered in high voltage direct current (HVDC) transmission systems. During sending-end converter blocking, the resultant active power and reactive power surplus in the sending-end power system lead to a severe increase in bus voltage and grid frequency. Consequently, this poses a substantial threat to the stability of the power system. Traditional control techniques generally control the frequency and voltage separately, which makes it challenging to regulate them jointly. This research paper introduces a collaborative approach for optimal control of voltage and frequency to address this issue. State space models for converter bus voltage and grid frequency prediction are developed using the bus voltage sensitivity matrices and system swing equation. The regulation of the converter bus voltage and grid frequency are intrinsically integrated using the explicit model predictive control (EMPC). When blocking occurs and results in an increase in the converter bus voltage and grid frequency, the EMPC controller regulates the output of active power and reactive power from the wind farm to realize the cooperative regulation of the converter bus voltage and grid frequency. The applicability and effectiveness of this strategy have been confirmed through simulation studies.

Analysis of Secondary Controller on MTDC Link with Solar PV Integration for Inter-Area Power Oscillation Damping

Integration of renewable energy sources is important in limiting the continuous environmental degradation and emissions caused by energy generation from fossil fuels and thus becoming a better alternative for a large-scale power mix. However, an adequate analysis of the interaction with the alternating current (AC) network during network disturbance, especially during inter-area power (IAP) oscillations is needed. Insufficient damping of oscillations can significantly impact the reliability and effective operation of a whole power system. Therefore, this paper focuses on the stability of the modified Kundur two-area four-machine (MKTAFM) system. A robust secondary controller is proposed and implemented on a line commutated converter (LCC)-based multi-terminal high voltage direct current (MTDC) system. The solution consists of a local generator controller and the LCC MTDC (LMTDC) system, voltage-dependent current order limiter, and extinction angle controller. The proposed robust controller is designed for the LMTDC systems to further dampen the inter-area power oscillations. Three operational scenarios were implemented in this study, which are the local generator controller and double circuits AC line, local generator controller with LMTDC controllers, and local generator controller with LMTDC controllers and secondary controller. The simulation result carried out on PSCAD/EMTDC recorded better damping of the inter-area power oscillation with LMTDC. A considerable improvement of 100% damping of the IAP oscillations was observed when a secondary controller was implemented on the LMTDC.

An Improved Power-Sharing Method for a Multi-Terminal HVDC Transmission System Based on Adaptive Voltage Droop Control

The prerequisite for the normal operation of a flexible high-voltage direct current (HVDC) transmission system is the maintenance of the stability of the direct current (DC)-side voltage, and droop control has a good dynamic regulation capability. In this paper, we first study the operating characteristics of droop control and derive its equivalent circuit, as well as the power distribution equation for droop control with a four-terminal system as an example. Then, based on this, an improved droop control method is proposed so that the droop factor can be adaptively adjusted according to the power change and provide corresponding characteristics under different operating conditions to enhance the power regulation capability of the controller under high power fluctuations. Finally, a power systems computer-aided design (PSCAD) electromagnetic transient model of the four-terminal, flexible, high-voltage DC transmission system was established and verified by simulation results.

Dynamic Performance of High-Voltage Direct Current Systems for Offshore Wind Farm Based on Modular Multilevel Converter

The modular multilevel converter (MMC), which is the foundation of voltage-source converter (VSC)-high- voltage direct current (HVDC), has received significant attention over the past ten years. As a result, the MMC has undergone extensive technical and operational improvements, making it an appealing option for achieving efficient renewable energy harvesting, particularly for offshore wind farms. This paper discusses the state-of-the-art control algorithms that are most effective for simulating large HVDC systems, including offshore wind farms. Moreover, a test system is suggested to show how well the selected techniques perform in practical scenarios. Overall, this work will serve as a helpful shortcut to relevant material that pertains to this research topic.

Impact Mechanisms of Commutation Failure Caused by a Sending-End AC Fault and Its Recovery Speed on Transient Stability

A sending-end AC fault may lead to commutation failure (CF) in a line-commutated converter high-voltage direct current (LCC-HVDC) system. In this paper, a theoretical analysis of the impact mechanisms of a CF and its recovery speed on the transient stability of a sending-end power system (TSSPS) is performed. Firstly, the models of the sending-end power system and DC power of CF are established; the ramp function is utilized to characterize the DC power recovery process. Secondly, the swing direction of the relative rotor angle caused by a sending-end AC fault is discussed, and the DC power flow method is employed to theoretically analyze the impacts of CF and its recovery speed on TSSPS. Next, the mathematic relations between parameters of the voltage-dependent current order limiter (VDCOL) and DC power recovery speed are further derived. It is concluded that the impacts of CF and its recovery speed on transient stability are related to the swing direction caused by a sending-end AC fault, the inertia of generators, and the location of the rectifier station. Finally, the theoretical analysis is validated by Kundur’s two-area system and IEEE 68-bus-based AC/DC asynchronous interconnection test power systems, respectively.

Control of an Offshore Wind Farm Considering Grid-Connected and Stand-Alone Operation of a High-Voltage Direct Current Transmission System Based on Multilevel Modular Converters

This work presents a control strategy for integrating an offshore wind farm into the onshore electrical grid using a high-voltage dc transmission system based on modular multilevel converters. The proposed algorithm allows the high-voltage DC system to operate in grid-connected or stand-alone modes, with the second case supplying power to local loads. In either mode, the modular multilevel rectifier works as a grid-forming converter, providing the reference voltage to the collector network. During grid-connected operation, the modular multilevel inverter regulates the DC link voltage while the generating units are controlled to maximize power extracted from the wind turbines. Conversely, in the event of grid disconnection, the onshore modular multilevel converter takes over the regulation of the AC voltage at the point of connection to the grid, ensuring energy supply to local loads. Simultaneously, the generator controller transitions from tracking the maximum power of the wind turbines to regulating the DC link voltage, preventing excessive power injection into the transmission DC link. Additionally, the turbine pitch angle control regulates the speed of the generator. Mathematical models in the synchronous reference frame were developed for each operation mode and used to design the converter’s controllers. A digital model of the wind power plant and a high-voltage dc transmission system was implemented and simulated in the PSCAD/EMTDC program. The system modeled includes two groups of wind turbines, generators, and back-to-back converters, in addition to a DC link with a rectifier and an inverter station, both based on modular multilevel converters with 18 submodules per arm, and a 320 kV/50 km DC cable. Aggregate models were used to represent the two groups of wind turbines, where 30 and 15 smaller units operate in parallel, respectively. The performance of the proposed control strategy and the designed controllers was tested under three distinct scenarios: disconnection of the onshore converter from the AC grid, partial loss of a wind generator set, and reconnection of the onshore converter to the AC grid.

Modular multilevel converter‐based voltage source converter high‐voltage direct current valve operating test circuit

AbstractIt is very important to obtain high reliability in the high‐voltage direct current (HVDC) system operation because of extremely high and huge power capacity. To guarantee the reliability IEC‐62501 standard requires valve test for the voltage source converter (VSC) HVDC system. This paper proposes a test circuit for modular multilevel converter (MMC)‐based HVDC valve. The proposed scheme has good features of adequate test current generation, considerably reduced power supply voltage, simplified test circuit topology, etc., compared to the conventional test circuits. The validity of the proposed scheme is verified through experimental results.

A Modification in Thyristor-Based Hybrid DC Circuit Breaker for DC-Side Fault Identification in Auto-Reclosing of MMC-HVDC Systems

Safe reclosing of high voltage direct current (HVDC) systems after DC-side fault current interruption is essential. Otherwise, a second strike can endanger the components of DC circuit breakers (DCCBs) and modular multilevel converters (MMCs). Thus, it is necessary to identify the fault type prior to auto-reclosing of MMC-HVDC systems. This paper proposes a method that enables an MMC-HVDC system equipped with the thyristor-based hybrid DCCB introduced by Alstom to identify and localize the DC-side fault. In the proposed method, adding some capacitors with low capacitance parallel to the thyristors in one part of an auxiliary branch enables the hybrid DCCB to inject active pulses. To clarify, by turning a specified number of the thyristors on, a desirable number of capacitors parallel with the off thyristors are applied, and a second-order circuit is formed, resulting in the injection of active pulses with controllable amplitude and width. According to the traveling wave theory, the difference in the reflection of the injected pulse determines whether the fault is permanent or temporary. The efficacy of the proposed method is validated by an MMC-HVDC test system built in PSCAD/EMTDC platform. Furthermore, generating active pulses with the modified hybrid DCCB is verified by a scaled-down prototype.

VSC control strategy for HVDC compensating harmonic components

This paper proposes a voltage source converter (VSC) control strategy for the high voltage direct current (HVDC) transmission systems compensating the harmonic components. For VSC-HVDC AC system accounted for a relatively large share of the 5th, 7th, 11th, 13th and other low harmonics. A selective harmonic current control strategy based on vector proportional integral (VPI) regulator in dq coordinate system is proposed on the basis of PI control. In which, PI is used to control the DC component of the current error, while VPI is used to suppress the multiplicative fluctuations of the current error. Unlike the proportional-integral resonant (PIR) regulator, the VPI contains a second-order numerator that achieves the ideal 0° phase delay of the closed-loop transfer function of the control system at the set resonant frequency point, so its control accuracy for harmonic currents is better than that of the PIR. A two-terminal VSC-HVDC system is built using Simulink software, and current compensation simulations are performed for three control methods: conventional PI, PIR and PI parallel VPI schemes. The superior harmonic suppression performance of VPI is verified by comparing the harmonic content.

DC Admittance Model of VSCs for Stability Studies in VSC-HVDC Systems

High-voltage direct current (HVDC) systems linked to AC grids with converters are promising energy transmission systems. These systems present complex AC- and DC-side dynamic interactions. Impedance-based stability studies have recently been proposed to assess DC-side dynamics from DC-side characterization of voltage source converters (VSCs) considering AC-side dynamics. However, the existing approaches used for stability studies in VSC-HVDC systems do not completely model VSCs because they do not consider together the VSC delay, the grid voltage feedforward filter, and all the d- and q-reference current controls. Moreover, these approaches are analytically characterized from dq-real space vectors (less related to circuit theory than dq-complex space vectors), and some work with simple AC grids. The main contribution of this paper is a detailed and complete DC admittance model of VSCs from dq-complex space vectors, which considers the VSC delay, feedforward filter, and d- and q-reference current controls, and also a general AC grid. The proposed model can be used for DC-side stability studies in VSC-HVDC systems considering AC grid dynamics. The capabilities and drawbacks of impedance-based stability methods for DC-side stability assessment were analyzed, and the positive-net-damping criterion was validated as a robust approach. The model was validated by PSCAD/EMTDC simulations and applied to a stability study in a VSC-HVDC system.

One-terminal traveling wave-based transmission line protection for LCC-HVDC systems

This paper proposes a one-terminal traveling wave (TW)-based transmission line protection for line commutated converter (LCC) of high-voltage direct current (HVDC) systems. The method requires the first and second wavefronts to reach the local bus and considers the boundary conditions of the system. A detailed mathematical analysis of the sampling frequency effects, as the basis for a number of innovations of practical interest, is presented here. Firstly, a definition of a minimum sampling frequency is formulated. This is crucial when dealing with the high sampling frequencies traditionally needed by TW-based methods. Secondly, mathematical expressions of protected, unprotected, and uncertainty zones on the transmission line are defined. Thanks to these calculations, the method is also capable of distinguishing faults at the line terminals from faults on the protected transmission line. Thirdly, the non-requirement for the TW propagation speed estimation is proven. Some TW-based protection elements require knowledge of the TW propagation speed, which is a source of errors. The proposed function presented good dependability and was able to operate below 15 ms for a transmission line of 2900 km in length.

Resonance stability analysis of MMC-based DC grid with coupled multiphase DC lines

The resonance stability of modular multilevel converter (MMC) based high-voltage direct current systems has been extensively researched using the impedance-based stability theory. However, when there are coupled multiphase DC lines (CMDCLs), the DC side stability issues would become more complicated and harder to be analyzed by many well-established stability criteria. To address this challenge, this paper proposes a resonance stability analysis method for MMC-based DC grid with CMDCLs, based on the nodal admittance matrix analysis. First, the DC side impedance models of MMCs with typical control modes are established using the harmonic state-space theory. The parameter matrices of CMDCLs are also introduced and their elements are transformed into the s-domain. Second, the nodal admittance matrix of the DC grid when there are CMDCLs is derived in detail. The resonance mode of the DC grid can then be obtained by solving zeros of the determinant of the s-domain nodal admittance matrix. Finally, the resonance stability characteristics of a bipolar four-terminal MMC-based DC grid are investigated. Simulations in PSCAD/EMTDC verify the effectiveness of the analysis results.

Harmonic Modeling of the Series-Connected Multipulse Rectifiers Under Unbalanced Conditions

With the promotion of high voltage direct current (HVDC) system construction around the world, the series-connected multi-pulse rectifiers (SCMPRs) have been widely used in high-power rectifying occasions in the power system. Since the certain order of harmonics cannot be effectively eliminated under the non-ideal conditions, many different phenomena, such as the unbalance of supply voltage and line/transformer parameters, the influence of background harmonics, and the non-equidistance of thyristor triggering pulses, may cause the non-characteristic harmonics (NCHs) of SCMPR. However, the existing frequency-domain harmonic model cannot be used to analyze the harmonic characteristics of the SCMPRs under non-ideal conditions. In this paper, a general frequency-domain harmonic coupling matrix model (HCMM) is proposed. The proposed HCMM can consider most of the above imbalance factors and the model can explicitly illustrate the harmonic coupling relationship between the SCMPRs supplied voltage and the generated harmonic currents. The generality of the SCMPR model under non-ideal conditions is improved accordingly. The HCMM is simplified by comparing and analyzing the effects of the transformer structure and parameters. Moreover, by analyzing the harmonic generation characteristic of the negative-sequence component, the HCMM can be further simplified. Finally, time-domain simulation and lab experimentation are performed to verify the accuracy and efficiency of the frequency-domain HCMM of SCMPRs. The results prove the generality of the proposed model and also illustrate the harmonic generating influence of unbalanced parameters from the transformer and supply voltage.

Study of Cyber Attack’s Impact on LCC-HVDC System With False Data Injection

Against the crippling assault of power grid, the impact of cyber attack on line-commutated converters based high voltage direct current (LCC-HVDC) system is investigated in this paper when the false data injection (FDI) attacks its control loops. Based on the basic control strategy of LCC-HVDC, the impact on each HVDC control mode is investigated when FDI occurs, including the rectifier side constant DC current control and supplementary damping control, the inverter side constant DC voltage control and constant extinction control. Through the theoretical analysis, it is found that the rectifier side FDI attack would limit the power transmission and increase HVDC’s reactive power demand, which will finally lead to instabilities, where the damping ratio could also be decreased if the rectifier supplementary damping control is attacked. Another important fact is that the inverter side attack could have much more severe impact when the false data is injected because the inverter extinction angle may be decreased and causes commutation failures. The above analysis and conclusions are validated by the simulations finally.

Application of Time-Frequency Analysis with Selection of Suitable Mother Wavelets for Transient States of HVDC Transmission Lines

Recent developments in power semiconductor devices have made High Voltage Direct Current (HVDC) transmission systems the most crucial technology for future power systems. Today several HVDC projects have been commissioned around the world. The primary concern for such an advanced HVDC grid is to follow standard grid codes to maintain the safety and stability of the electrical equipment during transient and dynamic circumstances. Although the HVDC system has been recognized as the most prominent technology to transmit electric power, some issues must be addressed, such as selective and quick detection of faults. For selective fault detection, it is essential to look into the several fault detection strategies that fortify the HVDC system with time-frequency signal processing techniques. This research uses a time frequency-based method to identify the different HVDC transmission line faults. The most suitable mother wavelet for various transient states is selected by analyzing fault voltage and current signals based on variance, standard deviation, and range parameters. An experimental setup of a thyristor-based HVDC transmission system has been developed in the MATLAB/Simulink environment, and multiple fault scenarios have been analyzed. The simulation outcomes reveal that the most effective mother wavelet for detecting DC line, symmetrical and unsymmetrical faults occurs at the 6th level of fault signal decomposition. By examining current and voltage signals, our work has determined the best mother wavelet for various faults. Using more than one mother wavelet to detect faults is more reliable than using a single mother wavelet to detect all faults. Additionally, it has been noted that each fault's voltage and current signals have two distinct mother wavelets.

High-frequency oscillation mechanism analysis of wind farm-side MMC station considering converter transformer stray capacitance

High-frequency oscillation is one of the critical issues threatening the stability of modular multilevel converter (MMC) based high-voltage direct current (HVDC) system. A new 2 kHz high-frequency oscillation has occurred in the wind farm-side MMC station of Rudong wind farm integrated MMC-HVDC project of China during no-load charging process. The mechanism of this high-frequency oscillation is different to previous ones. Apart from the time delay of MMC, the stray capacitance of converter transformer makes a significant contribution to the high-frequency oscillation. This paper investigates the high-frequency oscillation mechanism of wind farm-side MMC station during no-load charging process. First, the harmonic state space (HSS) model of a wind farm-side MMC station is established. In addition to the leakage inductance, the stray capacitance is considered in the converter transformer model. To reveal the significance of the converter transformer stray capacitance in this new high-frequency oscillation, the root loci of the system are calculated with varying stray capacitance. Then, the state variables with high degree of participation in the high-frequency oscillation modes are determined by the participation factor analysis. A reduced-order model suitable for the high-frequency oscillation analysis is further established based on the participation factor analysis results. Next, the high-frequency oscillation mechanism is revealed by impedance analysis. Finally, the effects of system parameters are analyzed by the combination of eigenvalue analysis and impedance analysis. The analysis results are validated by electromagnetic transient simulations.

Interconnecting Power Converter Control Role Assignment in Grids With Multiple AC and DC Subgrids

In emerging Multi-Terminal High Voltage Direct Current (MT-HVDC) systems interconnecting different Alternating Current (AC) asynchronous areas by means of Interconnecting Power Converters (IPC) different challenges arise. One of them is the control role assignment of IPCs present in these systems. Broadly speaking, the IPC controls can be classified into grid-forming or grid-following schemes for both AC and DC terminals of IPCs. This paper presents a methodology to optimize the control role assignment of the IPCs considering (i) small-signal stability and, (ii) control performance for different combinations of IPC roles and network power flows. The method considers multiple power flows, determines the steady-state deviations, and small-signal stability following selected events. While presenting this methodology different results on the small-signal and steady-state analysis of these systems are analyzed. Using high-fidelity linearized dynamical models, an optimization problem is formulated to assign control modes to IPCs to ensure stability and performance of the system for a set of given power flow scenarios.

Analysis and Design of MMC-Based High-Power DC–DC Converter With Trapezoidal Modulation

The modular multilevel converter (MMC) based dc-dc converters (MMDC) are essential for voltage matching, galvanic isolation, renewable integration and other applications in high voltage direct current (HVdc) systems. The trapezoidal modulation in these converters has been a proven advantage owing to reduced capacitive requirement and high-power density. However, the existing short overlap (SO) trapezoidal modulation technique presents significant dc link current ripples requiring bulky dc link filter. This paper introduces an extended overlap (EO) modulation technique in MMDC to mitigate the need of dc link filters. The change in overlap angle alters the conduction states, component sizing, harmonic spectrum and other converter parameters. Therefore, a detailed mathematical analysis of MMDC with both SO and EO modes of trapezoidal modulation is carried out in this paper. The effect of change in overlap duration on size of sub-module (SM) capacitor, dc link filter is analyzed by simulation and analytical studies. A single stage three phase MMC experimental setup is built to verify the converter operation in both SO and EO modulations.

A simplified model design of MMC-HVDC transmission system for steady state and transient stability analyses

A simplified model of a modular multilevel converter (MMC) implemented on a high voltage direct current (HVDC) transmission system is presented in this study for the analyses of steady-state and transient stabilities. The simplified model design’s goal is to reduce the complicated modeling of the MMC circuit as well as the simulation time of the analysis. The validity of the model design has been examined using the PSCAD/EMTDC simulation program, where the simulation outcomes of the detailed model and the simplified model are contrasted. It has been established that the simplified model is accurate enough and that simulation time can be substantially decreased. It is concluded that the suggested model design is very significant as a simplified model for analyzing the steady state and transient stabilities of MMC-HVDC transmission system.