VSC-based HVDC Transmission Research Articles

A novel frequency constrained unit commitment considering VSC-HVDC's frequency support in asynchronous interconnected system under renewable energy Source's uncertainty

The increasing share of renewable energy source (RES) poses a challenge to the frequency security of the power system. Frequency constrained unit commitment (FCUC) serves as an effective measure to address this challenge at the operational level. This paper introduces a novel FCUC model applicable to the asynchronous interconnected system connected by voltage source converter based HVDC (VSCHVDC), fully taking into account the frequency support capability of VSCHVDC in order to reduce the demand for synchronous generators' inertia and reserve while ensuring frequency security, thereby lowering operating costs. Additionally, the uncertainty of RES is also considered. The constraint expressions for three frequency indicators are derived based on a system frequency response model that includes frequency support from VSCHVDC. The proposed model optimizes unit commitment, generation and reserve dispatch and VSCHVDC transmission power and frequency response parameters, while addressing RES uncertainty using the distributionally robust chance constrained approach. In response to the highly nonlinear characteristics of the maximum frequency derivation (MFD) constraints, we propose an intelligent sampling-based support vector machine to convexify the MFD constraints and introduce a two-stage decomposition algorithm for solving the model. The effectiveness of the proposed model is demonstrated based on a modified IEEE RTS-79 system.

Performance assessment of VSC-based HVDC system in asynchronous grid interconnection: Offline and real-time validation of control design with symmetric optimum PI tuning

Asynchronous interconnection is essential for integrating AC networks operating at different frequencies, typically 50 Hz and 60 Hz. This need arises from distributed power generation methods, including offshore renewable sources and diverse regional grid configurations. Advanced strategies are required to overcome these frequency differences and ensure uninterrupted power transfer.High-Voltage Direct Current (HVDC) transmission systems facilitate efficient power exchange, enhancing grid reliability and stability. This study focuses on optimizing the Proportional-plus-Integral (PI) controller parameters within a 20 MVA Voltage Source Converters (VSC)-based HVDC system to enable asynchronous interconnection between offshore and onshore AC networks. The offshore VSC regulates active and reactive power, while the onshore VSC controls DC voltage and reactive power. A vector control approach with symmetric optimum PI tuning is proposed for a comprehensive performance assessment of the VSC-based HVDC transmission system.The effectiveness of the tuned PI controller parameters is evaluated through four test cases using MATLAB/Simulink for offline simulation and Typhoon HIL604 for real-time validation. These cases involve abrupt changes in reference active and reactive power for the offshore VSC; and in reference reactive power and DC voltage for the onshore VSC. Results demonstrate rapid and satisfactory dynamic performance across all test cases, as evidenced by offline simulations and real-time validation.The validation highlights the effectiveness of the proposed control design with symmetric optimum PI tuning, confirming its ability to enhance the overall performance of the HVDC transmission system for efficient asynchronous interconnection.

Experimental study on fault ride-through capability of VSC-based HVDC transmission system

Experimental study on fault ride-through capability of VSC-based HVDC transmission system

Design and Implementation of Advanced Output Feedback Control for VSC-HVDC Transmission System

This paper introduces a backstepping control scheme incorporating a nonlinear observer for Voltage Source Converter-based High Voltage Direct Current (VSC-HVDC) systems. The proposed strategy relies on observer design to estimate previously inaccessible states within the system, including remote states along the HVDC cable and parameters of the power grids. The strategy requires only the measurement of line currents for implementation. Simulation results demonstrate the efficacy of the developed approach. Furthermore, a semi-experimental implementation, utilizing a Process-in-Loop (PiL) test, is conducted to validate the feasibility of the proposed methodology in practical scenarios. The outcomes of this study contribute to the advancement of control strategies for VSC-HVDC systems, offering valuable insights for improving system stability and performance.

New Functions of VSC Based HVDC Transmission Systems that Can Effectively Support the Future South-East European Grid with Low Inertia and Large Amount of Res Generation

Abstract The energy mix in South-East Europe is nowadays mainly composed by hydro and fossil fuels generation almost equally distributed in the share of electricity generation. Wind and solar PV generation play a very marginal role playing cumulatively less than 7% of the total generation, while fossil fuels exceed 52% of the total generation. A Multi-Terminal HVDC grid between the different countries in South-East Europe may provide various operational and socio-economic gains. In this paper are presented some grid specific challenges and explored some new functions of HVDC transmission systems based on VSC technology that can effectively support the grid with large amount of RES. The paper aims to explore how the VSC converters can improve the system’s voltage stability and provide the necessary frequency control when the inertia in the grid is very low and how the HVDC transmission system can accomplish transferring the primary and secondary reserves solving the rising problems related to dispatchability and inertia response in RES based power grids.

Nonlinear Observer-Based Controller Design for VSC-Based HVDC Transmission Systems Under Uncertainties

Nonlinear Observer-Based Controller Design for VSC-Based HVDC Transmission Systems Under Uncertainties

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.

A waveform-similarity-based protection scheme for the VSC-HVDC transmission lines

The protection for the voltage source converter based high voltage direct current (VSC-HVDC) transmission lines must detect the faulty DC lines and faulty poles within a few milliseconds, because the fault current rises quickly. The traditional protections are easily influenced by fault resistance and signal noise. What is worse, the double-pole-to-ground (DPG) fault with its positive-pole fault resistance unequal to negative-pole fault resistance cannot be detected accurately. To solve the problems, a non-unit protection scheme based on the waveform similarity and wavelet transform modulus maximum (WTMM) theories for the VSC-HVDC lines is proposed in this paper. A four-terminal VSC-HVDC power network is modeled in PSCAD to verify the protection. Extensive simulations of various fault situations prove that the protection scheme can detect high-fault-resistance, DPG, and remote faults. Moreover, it is insensitive to the sampling frequency, signal noise, smoothing reactor, and start delay. Furthermore, it can meet the rapidity requirement. Therefore, the proposed protection can identify the DC faults accurately, which minimizes the disturbance to power grids and improves system security and stability.

Inertia Emulation and Fast Frequency-Droop Control Strategy of a Point-to-Point VSC-HVdc Transmission System for Asynchronous Grid Interconnection

This article proposes a novel inertia emulation and fast frequency-droop (IEFF) control strategy for a point-to-point voltage source converter-based high voltage direct current (VSC-HVdc) transmission system. It allows asynchronous ac grid synchronization in forms of inertial and fast frequency-droop responses provided to “both” sides of the VSC-HVdc interconnected grids, different from the historically reported schemes only providing these responses to one grid. Under the IEFF scheme, the power-regulating VSC (PR-VSC) station emulates a synchronous generator directly through its active power control, whereas the dc-voltage-regulating VSC (DR-VSC) station employs a dc voltage/frequency droop control to convey its connected grid frequency communication-free to the remote PR-VSC for inertial and fast frequency-droop oriented power variations. To avoid the clash of the PR-VSC IEFF and DR-VSC IEFF control under simultaneous frequency events in the asynchronous grids, a novel frequency support mode selection scheme is designed to effectively prioritize the IEFF support with a higher frequency gradient. The key parameters of the IEFF controller are optimized through small signal stability analysis. The effectiveness of the proposed IEFF scheme is verified through controller hardware-in-the-loop experiments. The results show that the proposed IEFF scheme effectively promotes synchronization and enhances overall frequency stability for the two interconnected asynchronous grids.

Short-term generation capacity expansion planning considering multi-terminal VSC[sbnd]HVDC links using a linear programming framework improved by shift factors

This paper describes the modelling of short-term generation capacity expansion planning (GCEP) for multi-terminal VSC-based HVDC transmission systems. The GCEP formulation is based on a mixed-integer linear programming (MILP) framework where transmission losses are considered by piecewise linearisation. As opposed to existing methods, the present GCEP formulation is based on shift factors which enable the representation of the hybrid AC/DC grids including VSC stations. This way new power generation investments can be efficiently determined at any of the VSC-connected AC systems. This new method lies in sharp contrast with the so-called classic GCEP method where the system nodal power balances are formulated using the voltage phase angles as decision variables thus increasing the model complexity. Indeed, to determine the GCEP investments, the present model aimed to multi-terminal HVDC links only uses AC/DC transmission line losses, power generations and projected generating power plants as decision variables. The new GCEP formulation is compared to the classic method using two compelling multi-terminal VSCHVDC test systems for validity and applicability purposes. Results show that the proposed formulation reduces the simulation running time by more than 80%, compared to the standard model, accompanied by acceptable accuracy of results since errors inferior to 5% are obtained.

4-Pole Hybrid HVDC Circuit Breaker for Pole-to-Pole (PTP) Fault Protection

Numerous problems have emerged with the development of HVDC transmission technology. One of them is the fault current that occurs when there is an issue in the line. In particular, in the case of a PTP (pole-to-pole) fault, a DCCB design suitable for the direction is required because the fault current flows through the positive and negative poles. In the case of PTP fault, a fault current of the same value occurs in the opposite direction, and in the case of DCCB, the breaking efficiency may be reduced or even impossible to break due to a mistake in the emission direction or a wrong design. In particular, when designing using several unidirectional DCCBs, it is cheaper and consumes less area than typical DCCBs, so it is economical, but this problem may be more prominent. To solve this problem, this paper proposes a 4- pole Hybrid HVDC circuit breaker to solve this problem. This circuit optimizes the DCCB internal components through quantitative analysis of the fault current to reduce the influence of the residual fault current as well as the previously most important parameter, Zero-Cross Time (ZCT). We also verified the circuit’s energy dissipation process to increase reliability. The circuit in this paper is simulated based on the VSC-based HVDC transmission link. Physically analyze the derived results and described the circuit mechanism.

Design and Tests of a 160-kV/1-kA DC Superconducting Fault Current Limiter

DC Superconducting fault current limiter (SFCL) can be used to reduce the fault current of voltage source converter-based high-voltage direct current (VSC-HVdc) transmission systems, thereby significantly reducing the manufacturing difficulty and cost of dc circuit breakers. The authors aim to develop a 160-kV/1-kA resistive type dc SFCL, and apply it in the VSC-HVdc transmission system in Nan'ao of China. In order to verify the feasibility of the transmission voltage dc SFCL, the research is divided into two stages, and the results of the first stage are presented. The rated voltage and current of the developed dc SFCL is 160 kV and 1 kA, respectively, and the current limiting resistance is no less than 2 Ω within 10 ms, which is 1/4 of the second stage prototype. The used superconducting material is stainless steel reinforced yttrium barium copper oxide (YBCO) conductor produced by Shanghai Superconductor. A three terminal structure is presented to meet the different operation parameters. A 160-kV dc SFCL prototype has been fabricated, and the prototype has passed the steady-state current test of 1 kA, the dc withstand voltage test of 252 kV within 1 min, the lightning impulse test of 550 kV/1.2 μs/50 μs, and the operating impulse test of 450 kV/250 μs/2500 μs. The average operating thermal loss of the device at rated current is 652 W. Power switching test results show that the developed SFCL could generate the desired limiting resistance of about 2.2 Ω in less than 10 ms, and the maximum fault current duration time could be as high as 60 ms.

Design and performance tests of a 160 kV/1.0 kA DC superconducting fault current limiter

Superconducting fault current limiter (SFCL), which is used in voltage source converter based high voltage direct current (VSC-HVDC) transmission systems to limit fault currents and protect the grid together with DC circuit breakers, could improve system safety and stability. The first transmission voltage DC SFCL is developed in China, and it has been applied in the 160 kV VSC-HVDC transmission system in Nan’ao. The electromagnetic design as well as insulation analysis of the current limiting unit is presented, where the current limiting unit is composed of 96 bifilar coils made of stainless steel reinforced YBCO tapes. The liquid nitrogen at the temperature of 77 K is used as the cooling and insulation medium, and the liquid nitrogen is cooled by a closed-loop cryogenic system consists of 3 G-M refrigerators. The 160 kV DC SFCL prototype has been fabricated, and the prototype has passed the steady-state current test of 1 kA, the DC withstand voltage test of 252 kV within 1 min, the lightning impulse test of 550 kV/1.2 µs/50 µs and the operating impulse test of 450 kV/250 µs/2500 µs. Power switching test results show that the developed SFCL could generate the desired limiting resistance, and the maximum fault current duration time could be as high as 60 ms. The developed prototype has been installed in the Nan’ao VSC-HVDC transmission system, and put into operation on August 17, 2020.

A Zero Crossing Hybrid Bidirectional DC Circuit Breaker for HVDC Transmission Systems

With the increasing demand for renewable energy power generation systems, high-power DC transmission technology is drawing considerable attention. As a result, stability issues associated with high power DC transmission have been highlighted. One of these problems is the fault current that appears when a fault occurs in the transmission line. If the fault current flows in the transmission line, it has a serious adverse effect on the rectifier stage, inverter stage and transmission line load. This makes the transmission technology less reliable and can lead to secondary problems such as fire. Therefore, fault current must be managed safely. DC circuit breaker technology has been proposed to solve this problem. However, conventional technologies generally do not take into account the effects of fault current on the transmission line, and their efficiency is relatively low. The purpose of this study is to introduce an improved DC circuit breaker that uses a blocking inductor to minimize the effect of fault current on the transmission line. It also uses a ground inductor to efficiently manage the LC resonant current and dissipate residual current. DC circuit breakers minimize adverse effects on external elements and transmission lines because the use of elements placed on each is distinct. All of these processes are precisely verified by conducting simulation under 200 MVA (±100 kV) conditions based on the VSC-based HVDC transmission link. In addition, the mechanism was explained by analyzing the simulation results to increase the reliability of the circuit in this paper.

State Space Modeling and Stability Analysis of a VSC-HVDC System for Exchange of Energy

Nowadays, relevant actors are searching for solutions to produce energy with a low impact on the environment. Indeed, transmitting power via large distances with maintaining low losses is one of the main challenges. To improve electricity communication between countries and offshore wind, a new interconnections line must be built. Therefore, Voltage Source Converter High Voltage Direct Current (VSC-HVDC) transmission is incoming as the exceptive technology in order to address the challenges related to the integration of future offshore wind power plants. In spite of its many advantages, VSC-based HVDC transmission systems can experience unexpected instability and interaction phenomena: Small disturbances that occur continually in VSC-HVDC transmission systems due to the complex VSC based interconnections and an important number of components with non-linear nature that may cause failure. Thus, before installing the HVDC system, there is a significant need for studying a hybrid AC-DC system to guarantee the reliable and stable operation. This paper deals with the stability of a VSC-HVDC system by the use of a small signal stability method; such procedure enables to study the stability of a linearized VSC-HVDC system through state-space modeling and eigenvalue-based stability analysis.

A traveling-wave-based protection scheme for the bipolar voltage source converter based high voltage direct current (VSC-HVDC) transmission lines in renewable energy integration

The voltage source converter based high voltage direct current (VSC-HVDC) technology is a feasible solution to realize wide-area complementarity and flexible consumption of the renewable clean energy resources but DC line faults are crucial problems. The traditional traveling wave protection for HVDC transmission lines are easily affected by the fault resistance and signal noise. Besides, it cannot detect the double-pole-to-ground fault. To overcome these problems, a single-ended protection scheme based on the phase-mode transformation and wavelet transform modulus maximum (WTMM) theories for DC lines in the VSC-HVDC transmission system is proposed in this paper. The protection scheme includes the protection startup unit, the internal fault identification unit, and the faulty pole detection unit. Extensive simulation tests carried out in PSCAD/EMTDC verify that the protection scheme has high selectivity in the high fault resistance faults, remote faults, and close-by faults with different fault types. And it does not rely too much on data synchronization. Furthermore, sensitivity analysis and comparisons with other methods indicate that the protection is insensitive to signal noise and suitable for different operational modes. And the effectiveness of the protection is independent of the high sampling frequency. Moreover, it can identify the double-pole-to-ground fault correctly.

The black-start capability improvement of VSC-based HVDC transmission system using fuzzy-adaptive PI controller

ABSTRACT The High Voltage DC Transmission (HVDC) transmission system commissioned in 1954 was planned for meeting the objectives like economy, faster recovery rate, frequency conversion, bulk power transmission, rapid controllability from sending and receiving end and so on. These objectives are difficult to achieve easily with the HVAC system. Among these, quicker reccovery rate, which is also called black start capability during fault and maintenance, is considered in this paper. This is achieved effectively when real power is controlled from sending end, while the dc link voltage is controlled at the other end. For this, better control schemes from both sending and receiving end, fast acting phase locked loop (PLL) and faster and intelligent adaptable controllers play a vital role. Hence, new PLL, fuzzy-adaptive PI with faster and effective control schemes are proposed to improve the black start rate of HVDC links. The performance of the proposed system is verified with traditional PI with conventional technique and with fuzzy-adaptive PI controller. The results are verified using MATLAB/SIMULINK software.

Novel travelling wave directional pilot protection for VSC‐HVDC transmission line

As known, voltage source converter-based high-voltage direct current (VSC-HVDC) transmission system is widely utilised in actual power grid. It is suitable for asynchronous networking, such as the connection of offshore wind farm to main power grid. Generally, as primary protection principle, travelling wave (TW) protection is adopted based on the technologies of ABB and SIEMENS, of which is only transient quantity based protection principle instead of true TW protection principle and has no fault direction discrimination ability. Besides, because of the large capacitances applied on converter station, the voltage TW (VTW) information cannot be detected by the protection units of two terminals. Therefore, this study proposes a novel TW directional pilot protection based on current TW (CTW) and direct current voltage. In the beginning, this study the unusual propagation characteristic of VTW or CTW along with the VSC-HVDC transmission line. Afterwards, this study proposes a novel fault direction discrimination criterion using the integral of instantaneous TW power. Eventually, using PSCAD/EMTDC, a typical VSC-HVDC transmission system simulation model is constructed to verify novel protection's performances.

DC Fault Protection in Multi-terminal VSC-Based HVDC Transmission Systems with Current Limiting Reactors

Multi-terminal VSC-based HVDC transmission system is the recent interest for grid integration of large-scale offshore wind farms. Protection of multi-terminal voltage source converters (VSC)-based HVDC transmission systems against DC faults is challenging. This paper presents a single-ended protection scheme for DC faults in a three-terminal VSC-HVDC transmission system. The under-voltage criterion is used to distinguish the DC faults from the transient and normal conditions. The rate of change of DC voltage and current as well as the variation of transient energy is used to discriminate the internal faults from the external faults. The DC fault current has very high value within a few milliseconds during the transient phases such as the capacitor discharging and diode freewheeling stages. Therefore, current limiting reactors are introduced in series with the DC circuit breaker to maintain the DC fault current within the breaker capacity. The single-ended protection scheme is tested with the three-terminal VSC-HVDC transmission system with current limiting reactors for various DC fault conditions. The DC fault data is generated from PSCAD/EMTDC simulation and the protection scheme is tested in MATLAB environment. Test results show that the proposed protection scheme gives reliable protection for the DC faults in a three-terminal VSC-HVDC transmission system.