High Voltage Direct Current System Research Articles

Sequence Impedance Modeling and Optimization of MMC-HVDC Considering DC Voltage Control and Voltage Feedforward Control

The dynamic performance of the DC bus significantly influences the impedance characteristics of MMC and the system stability in a high-voltage direct current system. However, most of the existing MMC-HVDC system stability research simplifies the DC side as an ideal voltage source and ignores the impacts of voltage feedforward control, which affects the accuracy and practicability of stability analysis. In this paper, a sequence impedance model considering both DC voltage control and voltage feedforward control is developed, and the necessity of considering DC control and voltage feedforward control for MMC-HVDC stability analysis is illustrated. Then, the impact of control parameters on MMC-HVDC impedance is discussed, and the boundary conditions of control parameters are also derived. Finally, a method of control parameters design and impedance optimization for MMC-HVDC based on the stability boundary is proposed. Compared to the traditional optimization method, the system stability is further improved by the impedance optimization method proposed this paper.

A backup protection scheme for the transmission lines based on the directional traveling wave and waveform similarity

The voltage source converter based high voltage direct current (VSC-HVDC) system is a practical solution to improve renewable energy integration, flexible consumption, and wide-range complementarity. Because the DC line fault must be cleared within a few milliseconds, a reliable and rapid identification of faulty lines and poles is crucial. Unfortunately, the traditional backup protection cannot respond to internal faults fast enough and is sensitive to fault resistance. What is worse is that a double-pole-to-ground fault with unequal positive-pole and negative-pole fault resistances cannot be identified by most of the existing protection methods. The paper proposes a pilot backup protection based on directional traveling wave and waveform similarity for VSC-HVDC transmission lines to compensate for the drawbacks. The fault characteristic of 1-mode directional traveling waves and the waveform similarity of instantaneous currents of DC lines are elaborated theoretically. Based on the analyses, the protection is established, and the four-terminal VSC-HVDC grid is modeled in PSCAD and OPAL-RT. Detailed theoretical analyses and extensive simulation/experiment results have proven that the proposed protection scheme has higher reliability, selectivity, rapidity, and sensitivity. Compared with the existing backup protections, the proposed protection scheme has superiorities in fault resistance and distance tolerance (2,000 Ω and the entire length of DC lines), fault differentiation of double-pole-to-ground fault with unequal positive-pole and negative-pole fault resistances, immunity from signal noise and measurement error interference, and quicker fault judgment (less than 4 ms).© 2017 Elsevier Inc. All rights reserved.

Fault identification and fault location methods for VSC-HVDC transmission lines based on the traveling waveform difference

Due to the low-damping characteristic of the voltage source converter based high voltage direct current (VSC-HVDC) transmission system, the line current will reach the peak value within a few milliseconds after DC fault, threatening the safety of power electronic devices. Therefore, a reliable and rapid protection scheme comprising fault identification and location is necessary. The detection time cost of traditional backup protections is high. Worse still, most existing methods cannot distinguish the double-pole-to-ground fault with unequal positive and negative pole fault resistances. To solve the issues, a backup protection based on the same/different-mode traveling wave, differential forward/backward current, and traveling waveform difference for VSC-HVDC lines is proposed in the paper. The fault characteristics of same/different-mode traveling wave and relationship between fault distance and differential forward/backward current are elaborated. The waveform differences in positive/negative-pole same/different-mode traveling waves and 1-mode differential forward/backward currents are summarized. According to these analyses, the protection is established and the VSC-HVDC is modeled in PSCAD. Based on the sufficient simulation tests, it is verified the proposed fault identification and location schemes have great performance, with superiority in fault resistance and distance tolerance (2,000 Ω and the whole line), strong anti-interference capability, fast fault distinguishment, and accurate fault location.

DC-Side Impedance Estimation of a Modular Multilevel Converter Through System Identification of a Partially Black-Boxed Control System

The stability of a power electronicssystem can be assessed by means of the impedance-based stability criterion. Impedance modeling is a useful tool to analyze the effect of different circuit parameters and control schemes on the behavior of a converter. Modeling the input impedance of a power electronics converter is often successful when having full knowledge of the converter topology, the circuit parameters, and the parameters and implementation of the control system. However, due to the proprietary nature of voltage source converter-based high voltage direct current systems, their exact control structure is often concealed. This complicates the calculation of the impedance of a modular multilevel converter, known for its complex internal dynamics. This paper proposes a method to estimate the impedance of a modular multilevel converter with partially black-boxed converter control. A discussion on partitioning the control system into open and closed parts is made, and the results are verified with simulations in time and frequency domains.

Impact of Wind Power Penetration on Wind–Thermal-Bundled Transmission System

The wind–thermal-bundled transmission system is a feasible way to transmit wind power generation; however, the stability of the system should be paid more attention under high wind power penetration. In this article, the impact of wind energy penetration on torsional vibration and electrical characteristics is investigated for wind–thermal-bundled transmitted by high-voltage direct current system. The torsional vibration responses of generator shaft with different wind–thermal-bundled ratios are analyzed by time and frequency methods. The electrical characteristics of wind–thermal-bundled transmission system are disclosed under the influence of the wind farm penetration. The analysis illustrates that the penetration of doubly fed induction generation makes the torsional vibration of turbo generator and wind turbine shaft present different mode characteristics, and the risks of different modes are different. The current and voltage of wind–thermal-bundled transmission system show regular evolution with the change of wind–thermal-bundled proportion. Analytical results provide the value for the design and control of the wind–thermal-bundled transmitted system, which contributes to the improvement of power system stability.

The Resonant Modular Multilevel DC–DC Converter Adopting Switched-Inductor Cells for High Step-Up Ratio

With the prosperous development of distributed renewable energy power generation, high-voltage direct-current (HVDC) system has evolved into large-scale practical applications from the initial stage, and high step-up conversion dc–dc converters have become a core technology to promote the development of HVDC system. This article proposes a new resonant modular multilevel dc–dc converter (MMDC) based on the conventional boost converter. The single-switch and diode of the conventional boost topology are replaced by submodules (SMs), which are clamped by floating capacitors, and the inductor is replaced by switched-inductor (SL) cells. The phase-shifted pulsewidth modulation (PS-PWM) is applied to regulate the state of SMs, i.e., inserted state or bypassed state, to ensure that there is a constant number of SMs supporting the output voltage at any time. The converter is operated in resonant mode with resonance between capacitors of SMs and the upper arm inductor. The step-up conversion ratio is dependent on the inductors charging ratio, the number of SMs, and SL cells. Hence, more regulation schemes are available. The proposed converter possesses the advantages of MMDC and has the inherent-balancing capability of SM capacitor voltages without additional complex balancing control. The structure of the converter, operating mode analysis, parameter design, and comparison with other MMDCs are presented, which have been verified by simulated and experimental results.

An Inrush Current Suppression Strategy for UHV Converter Transformer Based on Simulation of Magnetic Bias

The Ultra High Voltage (UHV) converter transformer may generate serious inrush current during closing. It will cause commutation voltage distortion and easily result in commutation failure of converters on the same commutation bus. Due to the change of flux amplitude and phase caused by closing resistance in the Ultra High Voltage Direct Current (UHVDC) system, it is difficult to determine the closing time of the converter transformer when considering the influence of residual flux. Therefore, this paper first analyses the existing closing resistance strategy, and constructs the magnetic bias function according to the influence characteristics of closing resistance on the flux and phase. Then an inrush current suppression strategy based on magnetic bias simulation is proposed for star connection and delta connection of UHV converter transformers, which can accurately calculate the ideal closing time of mutual cancellation between the residual flux and bias flux during closing. Finally, the no-load closing model of the UHV converter transformer is established by PSCAD/EMTDC for simulation analysis of the feasibility, real-time verification is carried out using the RTDS closed-loop experimental platform. The results show that the proposed inrush current suppression strategy can effectively suppress the inrush current under different residual flux conditions.

Approach to Inertial Compensation of HVdc Offshore Wind Farms by MMC With Ultracapacitor Energy Storage Integration

Recent studies have shown that the grid connection of offshore wind farms through modular multilevel converter–high voltage direct current (MMC-HVdc) transmission system has brought low inertia issue. It is an urgent requirement for offshore wind farms to compensate for the inertia. After analysis and evaluation in this article, it is found that integrating an ultracapacitor energy storage system (UESS) into an MMC has an advantage comprehensively considering cost, size, and dynamic. Based on the evaluation, this article proposes an approach to inertial compensation of HVdc offshore wind farms by MMC-UESS integration. An ultracapacitor is selected as an energy storage element (ESE) for its high power density since the inertial response is a short-term response and it is hoped that added ESE does not significantly increase the size of an MMC submodule. Batteries with high energy density but low power density are not suitable for such applications. The optimal operating voltage of a UESS and the control scheme between MMC loops and inertial emulation are proposed in this article. Experimental results from real-time simulation and a downscaled prototype are presented to verify the effectiveness of the proposed method.

Study on fault mechanism of LCC HVDC large disturbances under cloud-to-ground lightning with long continuous current

The safe and stable operation of the line commutated converter high voltage direct current (LCCHVDC) system is of great significance to the reliability of electricity supply. In order to clearly reveal the mechanism of LCCHVDC large disturbance fault under the Cloud-to-Ground (C-G) lightning with long continuous current, the HVDC-lightning simulation model considering long continuous current, actual control and protection systems is proposed. The effect of long continuous current on traveling wave protection is analyzed. Transient processes including commutation failure (CF), fault restart and blocking of the DC system under the lighting with long continuous current are simulated. The reasons for CF of LCCHVDC system caused by long continuous current are analyzed, and the influence of long continuous current amplitude and duration on fault restart process is studied. Results show that the long continuous current has little influence on the response characteristics of travelling wave protection, but during the regulation process of control system under lightning disturbance, the DC current control regulation quantity will increase due to the presence of long continuous current. If the amplitude of long continuous current exceeds 80 A, the CF will occur in the DC system. If the duration of long continuous current exceeds the deionization time, the line will fail to restart, and the line can be successfully restarted only if the long continuous current is over before the voltage begins to rise.

VSC-HVDC Fault Location Method Based on Effective Feature and Depth Belief Network

Aiming at the problems that most fault location methods are applied to the high voltage direct current (HVDC) transmission system, such as low location accuracy and vulnerability to noise, a voltage source converter (VSC)-HVDC fault location method based on effective feature and deep belief network is proposed. Firstly, the Karenbauer phase mode transform is used to decouple the original fault current data to eliminate the coupling effect between positive and negative lines. Then, Hilbert Huang (HH) transform is used to obtain six effective features of information from frequency domain and time domain analysis to fully characterize the system fault current signal. Finally, the extracted effective features are input into the depth belief network for learning, so as to estimate the fault location in the transmission line. VSC-HVDC system is built based on the MATLAB platform for experimental analysis. The results show that the fault location obtained by the proposed method has the smallest overall fault location error, and the maximum and minimum errors are divided into about 0.52 km and -0.95 km. Compared with other methods, it has certain advantages in fault location.

Research on the Transient Interrupting Voltage of Mechanical HVDC Circuit Breaker and Its Influences on the Postarc Dielectric Recovery Process

The flexible high-voltage direct current (HVDC) power system develops quickly with the large-scale utilization of new energy and the increase of direct current (dc) loads in recent years. HVDC circuit breakers are very important components in the flexible HVDC power system. The mechanical HVDC circuit breaker including vacuum interrupters is a potential solution for HVDC interruption. After the current interruption, a transient interrupting voltage (TIV) generated from the interaction with the external circuit will be applied to the mechanical HVDC circuit breakers and might cause a failure interruption. However, there are still few studies about the influences of TIV on the postarc dielectric recovery process of mechanical HVDC circuit breakers. In this article, a circuit model based on the simplification of a 10-kV mechanical HVDC circuit breaker is developed for the purpose of studying the characteristics of TIV. Then the effects of the parallel snubber circuit on the TIV are discussed. Moreover, the influence of TIV on the postarc residual plasma dissipation process is simulated by a particle-in-cell (PIC) model. The results show that the TIV with a parallel snubber circuit has little effect on the whole dissipation time compared with that without a parallel snubber circuit. It should be noted that the microdissipation processes of the above two situations are different.

Diode rectifier‐based hybrid high‐voltage direct current converter for offshore wind farms

This paper presents a diode rectifier (DR)-based hybrid converter topology for high-voltage direct current (HVDC) transmission system with offshore wind farms (OWFs). The hybrid converter consists of a DR and an auxiliary converter. A high conversion ratio ac/dc converter is designed as the auxiliary converter, which can form the offshore grid voltage for OWFs to achieve black-start. Under steady state, all the active power output from OWFs is delivered through the DR. Meanwhile, the auxiliary converter is able to compensate the reactive power and harmonic currents of the system. Comparing with the conventional modular multilevel converter (MMC), the number of submodules (SM) of the proposed hybrid converter is significantly reduced, which decreases the construction cost and size of the system. The topology and operation principle of the hybrid HVDC converter are introduced. Then the corresponding control strategy is designed to achieve energy balance and power transmission. Based on the mathematic model, a design method for the parameters of the auxiliary converter is proposed. Finally, the feasibility of the proposed hybrid HVDC converter is verified by both the simulation and hardware-in-the-loop experiment results.

Stability analysis of virtual synchronous generator control in a high-voltage DC transmission system using impedance-based method

Numerous distributed generators (DGs), which often include renewable energy sources, are connected to modern power grids through grid-connected inverters. Conventional linear model approximation methods may not be sufficient to accurately analyze the dynamic response of DGs due to the harmonics and nonlinear distortions generated by power-electronic devices. Therefore, an impedance-based stability analysis method has been adopted and applied in high-voltage direct current (HVDC) power transmission systems as a practical data-driven alternative to the linear model approximation method. However, the presence of multiple AC/DC converters and the non-negligible ground capacitance of DC transmission lines increases the difficulty of analyzing the entire HVDC system. Previous studies have mostly focused on the stability of either the generator-side system or the load-side AC system. In this study, impedance-based analyses were performed on the generator-side AC system, DC transmission line, and load-side AC system to evaluate the entire HVDC system when the generator-side AC system was weak. Additionally, an advanced inverter control method called the virtual synchronous generator (VSG) control method, which can result in grid stabilizing effects, was introduced to the load-side AC system. The analysis results demonstrate that the VSG control significantly enhances the stability of HVDC transmission systems as compared with the conventional power control methods.

A hybrid circuit breaker with fault current limiter circuit in a VSC-HVDC application

A conventional hybrid circuit breaker (HCB) is used to protect a voltage source converter-based high voltage direct current transmission system (VSC-HVDC) from a short circuit fault. With the increased converter capacity, the DC protection equipment also requires a regular upgrade. This paper adopts a novel type of HCB with a fault current limiter circuit (FCLC), and focuses on the responses of voltage and current during DC faults, which are associated with parameter selection. PSCAD/EMTDC based simulation of a three-terminal VSC-HVDC system confirms the effectiveness and value of HCB with FCLC, by using an equivalent circuit modelling approach. Laboratory experimental tests validate the simulation results. The peak fault current is reduced according to the current limiting inductor (CLI) increase, and can be isolated more quickly. By adopting parallel metal oxide arrester (MOA) with the main branch of HCB, voltage stresses across the breaker components decrease during transient and continuous operation, and less energy needs to be dissipated by the MOA. The remnant current for all cases is transmitted to power dissipating resistor (PDR) in the final stage, and the fault current is reduced to the lowest possible value. When the current from the main branch is transferred to the FCLC branch, transient voltage spikes occur, while smaller PDR is required to absorb current in the final stage.

Interval Total Transfer Capability for Mesh HVDC Systems Based on Sum of Squares and Multi-Dimensional Holomorphic Embedding Method

Total transfer capability evaluation is an effective method to analyze the backbone transmission capability among regions. To address renewable energy uncertainties, an interval total transfer capability model based on the multi-dimensional holomorphic embedding method and sum of squares relaxation technique is proposed to solve the regional total transfer capability in meshed high voltage direct current systems. First, the multi-dimensional holomorphic embedding method is used to derive the analytical expressions of regional tie lines. Second, the interval total transfer capability model can be reformulated by two bi-level optimization models. Third, sum of squares relaxation is employed to solve the two optimization problems. Numerical results on a 40-bus European Synthetic System and the Chinese meshed high voltage direct current system validate the effectiveness of the proposed model and method.

A practical impedance modeling method of MMC-HVDC transmission system for medium- and high-frequency resonance analysis

The modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission system can arouse resonance that threatens the safety and stability of the power grid. In this paper, the impedance model and resonance mechanism of an MMC-HVDC transmission system are investigated at medium and high frequencies. First, an MMC-HVDC transmission system equivalent circuit model is proposed based on the partitioning impedance modeling method. Second, the concept of normalized sensitivity index is proposed to quantify the influential effects of crucial factors on the MMC equivalent impedance, including the control gains, control link delay, and circuit parameters; subsequently, the influencing factors are graded, and reduced-order impedance models are proposed for different situations. Then, the resonances in the MMC AC- and DC-side systems are analyzed considering both active and passive resonance, i.e., with or without a harmonic source, to systematically reveal the resonance mechanism of the MMC-HVDC system. Finally, a two-terminal MMC-HVDC simulation model is built to verify the proposed model and resonance analysis.

A novel robust frequency-constrained unit commitment model with emergency control of HVDC

High-voltage direct current (HVDC) transmission systems are widely employed to transmit energy among systems with renewable energy integrated. However, the reduced system inertia and the risk of HVDC failure pose significant challenges to operators. To ensure system reliability especially frequency stability under extreme circumstances, a robust frequency-constrained unit commitment (FCUC) model is proposed in this paper. Unlike previous studies, an extra power-rescheduling procedure is embedded in our model, and consequently the FCUC model is formulated as a three-layer structure, where the day-ahead schedule model, emergency corrective model (primary frequency response), and power rescheduling (secondary frequency response) are co-optimized. Accordingly, an improved tri-level Benders Decomposition (BD) algorithm was developed to solve the problem. By leveraging the optimality cuts of the algorithm, the characteristics of fast power support and flexible regulation capabilities of HVDC system in real-time (RT) market are preemptively considered in DA market. Consequently, the model feasible region is more adaptive to the plausible contingencies The outperformance of our model was validated by the numerical experiments in an asynchronous HVDC interconnected system. The results demonstrate that by formulating the coordination of AC–DC dispatch framework, the global optimal operational cost is further captured when ensuring the system reliability.

An improved reactive power control strategy for LCC-HVDC to mitigate sending end transient voltage disturbance caused by commutation failures

The sending end transient voltage disturbance (TVD) caused by commutation failure of line commutated converter-based high voltage direct current (LCC-HVDC) systems, which is characterized by “first reduce then rise”, has received increasing attention due to its threat to the stable operation of renewable energy integrated modern power systems. To mitigate the TVD, this paper devises an improved reactive power control strategy that takes into account the signal delay between converter stations. Firstly, the mechanism of the TVD is investigated. It is found that the voltage dependent current order limiter (VDCOL) and signal delay between the rectifier and the inverter stations are two vital factors that impact the TVD, while the VDCOL cannot well cope with commutation failure suppression and TVD mitigation at the same time. On this basis, an improved closed-loop reactive power control (CLRPC) strategy is proposed for the LCC-HVDC to mitigate the TVD. The CLRPC can regulate the reactive power demand of the rectifier station by adjusting the DC current order, which replaces the conventional VDCOL during the disturbance. Furtherly, a delay compensation unit is augmented in the proposed scheme to suppress the adverse effect of signal delay. Simulation results conducted in PSCAD/EMTDC verify that the CLRPC can not only alleviate the TVD effectively but also suppress the occurrence of successive commutation failures, thus, enhancing the overall performance of the sending end power system.

Evaluation, Analysis and Diagnosis for HVDC Transmission System Faults via Knowledge Graph under New Energy Systems Construction: A Critical Review

High voltage direct current (HVDC) transmission systems play a critical role to optimize resource allocation and stabilize power grid operation in the current power grid thanks to their asynchronous networking and large transmission capacity. To ensure the operation reliability of the power grid and reduce the outage time, it is imperative to realize fault diagnosis of HVDC transmission systems in a short time. Based on the prior research on fault diagnosis methods of HVDC systems, this work comprehensively summarizes and analyzes the existing fault diagnosis methods from three different angles: fault type, fault influence, and fault diagnosis. Meanwhile, with the construction of the digital power grid system, the type, quantity, and complexity of power equipment have considerably increased, thus, traditional fault diagnosis methods can basically no longer meet the development needs of the new power system. Artificial intelligence (AI) techniques can effectively simplify solutions’ complexity and enhance self-learning ability, which are ideal tools to solve this problem. Therefore, this work develops a knowledge graph technology-based fault diagnosis framework for HVDC transmission systems to remedy the aforementioned drawbacks, in which the detailed principle and mechanism are introduced, as well as its technical framework for intelligent fault diagnosis decision.

A novel current-limiting method for MMC-HVDC by coordinating virtual impedance control and fault current limiter

The modular multilevel converter (MMC) topology is very suitable for high voltage direct current (HVDC) system to achieve feasible power control, favorable black start capability as well as long-distance power transmission. This paper proposes a novel current-limiting method based on the coordination of virtual impedance control (VIC) and resistive fault current limiter (R-FCL) for MMC-HVDC, and it aims to effectively decrease the DC fault current level and assist the DC circuit breaker (DCCB) to remove the fault reliably and smoothly. Firstly, the MMC-HVDC’s topology and control strategy are expounded, and the modeling method of the VIC and the R-FCL is presented. According to the DC fault evolution, the time sequence coordination scheme of the VIC and the R-FCL for the MMC-HVDC is put forward. Then, considering the introduction of the proposed current-limiting method, the fault characteristics of the MMC-HVDC are theoretically deduced, and the capacity design of the VIC and the R-FCL is conducted. Using MATLAB, a typical ±320 kV MMC-HVDC system with VIC, R-FCL, and DCCB is built, and comparative simulations are performed. The results indicate the proposed approach can strongly limit the DC fault current within the acceptable range, and reduce the electrical stress and thermal stress in the MMC and DCCB. The proposed approach possesses better techno-economics than the R-FCL alone to solve the DC fault problem, and a more efficient protection of the MMC-HVDC is realized.