Multi-infeed HVDC Systems Research Articles

Non-Euclidean Grid-Partitioning to Mitigate Cascading Risk in Multi-Infeed HVDC System

Non-Euclidean Grid-Partitioning to Mitigate Cascading Risk in Multi-Infeed HVDC System

Evaluation Approach and Controller Design Guidelines for Subsequent Commutation Failure in Hybrid Multi-Infeed HVDC System

Due to the difference in output characteristics between the line-commutated converter-based high-voltage direct current (LCC-HVDC) and voltage-source converter-based high-voltage direct current (VSC-HVDC), the hybrid multi-infeed high-voltage direct current (HMIDC) presents complex coupling characteristics. As the AC side is disturbed, the commutation failure (CF) occurring on the LCC side is the main factor threatening the safe operation of the system. In this paper, the simplified equivalent network model of HMIDC is established by analyzing the output characteristics of VSC and LCC. Hereafter, based on the derived model and the control system of LCC-HVDC, the dynamic equations of the extinction angle are deduced. Consequently, by applying the phase portrait method, the causes of CF occurring in the HMIDC system as well as the impacts of control parameters on the transient stability are revealed. Furthermore, the stabilization boundaries for the reference value of the DC voltage are obtained via the above analysis. Finally, the theoretical analysis is verified by the simulations in the PSCAD/EMTDC.

Iterative Searching Method for Closest Saddle-Node Bifurcation Point in Multi-Infeed HVDC Systems

Iterative Searching Method for Closest Saddle-Node Bifurcation Point in Multi-Infeed HVDC Systems

Sensitivity analysis of commutation failure in multi-infeed HVDC systems: Exploring the impact of ac system representations and fault types

This paper investigates commutation failure (CF) in line commutated HVdc converters within multi-infeed HVDC systems and thereby provides valuable insights for the operation and design of HVdc systems. CF susceptibility across various operational scenarios, fault types, and ac system representations at different frequencies is explored. An Electromagnetic Transients (EMT) model is constructed and parameter changes are applied using Monte-Carlo Simulation. The process is fully automated with the use of a controlling program written in Python, which varies parameter values and facilitates result retrieval and post-simulation analysis. A typical analysis results in several hundred thousands of simulation runs, requiring the procedure to be implemented on a parallel computing platform. The results show that two-phase faults are the most critical CF triggers, an aspect often overlooked in previous literature focused on three-phase or single-phase-to-ground faults. Furthermore, the frequency dependent characteristics of the ac network can result in very different CF susceptibilities.

Research on emergency DC power support coordinated control for hybrid multi-infeed HVDC system

Research on emergency DC power support coordinated control for hybrid multi-infeed HVDC system

Research on Reactive Power Coordination Control Strategies of Multi-Infeed Line-Commutated Converter–High-Voltage Direct Current Systems

For a receiving-end power grid with multi-infeed LCC-HVDC systems, simultaneous commutation failures may seriously threaten the safe and stable operation of the system. To evaluate the impact of commutation failure and improve the voltage stability of the commutation buses in multi-infeed HVDC systems, this paper proposes a method for evaluating the voltage stability of commutation buses and a reactive power coordination control (RPCC) method for commutation failure of multiple HVDC systems. Firstly, three indicators and the entropy weight method are adopted to comprehensively evaluate the voltage stability of commutation buses. Then, an RPCC method is proposed to resist commutation failure. The proposed RPCC method uses the voltage interaction factor (VIF) to screen out DC systems that are strongly related to dynamic reactive power compensation devices and activates the devices to provide RPCC to the DC systems through an auxiliary controller. Finally, the effectiveness of the proposed method is verified through a practical example of the Jiangsu power grid.

Research on protection adaptability and optimization scheme of VSC/LCC DC parallel hybrid system

With the continuous development of high-voltage direct current (HVDC) technology and the relative concentration of power load and power, the HVDC parallel hybrid system will inevitably appear again. Because conventional DC transmission (LCC HVDC) and flexible DC transmission (VSC HVDC) in existing hybrid multi-infeed HVDC systems operate independently and are not coordinated with each other, and there is a gap in the research on the protection coordination relationship between VSC/LCC parallel hybrid systems and their near-area AC systems, it is necessary to study them systematically. This paper studies the protection coordination relationship between the VSC/LCC DC parallel hybrid system and its near area AC system, simulates and analyzes the typical faults of the hybrid multi-feed DC system, and puts forward a protection optimization scheme suitable for the VSC/LCC DC parallel hybrid system, which improves the safe and stable operation ability of the power grid, eliminates the hidden dangers of subsequent operation and maintenance, and provides a reference basis for further research.

Research on operation characteristics and control strategy of hybrid multi-feed HVDC system

Flexible DC transmission technology based on voltage source converter (VSC-HVDC) and high voltage DC transmission technology based on grid commutation converter (LCC-HVDC) have complementary characteristics, so the hybrid DC technology combined with LCC-HVDC and VSC-HVDC can make them complement each other and become the development trend of power grid technology in the future. In the existing hybrid multi-infeed DC system, conventional DC transmission (LCC-HVDC) and flexible DC transmission (VSC-HVDC) operate independently and are not coordinated with each other, but there is a blank in the research on the protection coordination relationship between VSC/LCC parallel DC hybrid system and its near area AC system. Therefore, it is necessary to theoretically analyse and study the operation characteristics, control strategy and protection coordination relationship between near-area AC systems of hybrid multi-infeed HVDC systems. Due to the limited space, this paper mainly analyses the operation characteristics and control strategy of hybrid multi-infeed HVDC system, and verifies the correctness of the theory through simulation research. In the next step, the corresponding protection is configured in the above simulation model, and the protection adaptability analysis and protection optimization scheme of hybrid multi-infeed HVDC system are obtained.

Harmonics Interaction Mechanism and Impact on Extinction Angles in Multi-Infeed HVDC Systems

Harmonics interaction is an important factor in causing concurrent commutation failures (CCF) of multiple converter stations in multi-infeed HVDC (MI-HVDC) systems, but it was not studied sufficiently in existing literature. In this paper, the mechanism analysis and quantitative calculation of harmonic interaction are investigated. Firstly, the generation and interaction process of harmonic during AC fault events in the MI-HVDC systems with single-terminal connection and hierarchical connection on receiving-end are illustrated, respectively. Then, the common characteristics of harmonic interaction in generalized MI-HVDC systems are concluded. Moreover, to evaluate the impact of harmonic interaction on commutation process quantitatively, an equivalent circuit model of harmonic transfer in MI-HVDC system is established. Based on this equivalent model, the harmonic voltage transfer coefficient is deduced, and the extinction angle of inverters considering harmonic interaction in MI-HVDC systems can be calculated. Finally, the calculation accuracy based on equivalent model is verified via the electromagnetic transient simulation of MI-HVDC system. Simulation results show that harmonic interaction will eventually cause the formation of harmonic source in each converter station of MI-HVDC systems. These harmonic components will change the extinction angle of remote inverters to increase the risk of CCFs.

The Operation Strategy of the MIDC Systems for Optimizing Renewable Energy Integration of Jeju Power System

The power system on Jeju Island currently faces challenges due to an imbalance between power supply and demand owing to increasing renewable energy (RE) generation. Consequently, the multi-infeed high-voltage direct-current (MIDC) systems are expected to play an important role in reducing RE curtailment and increasing RE penetration. However, despite the balance between power supply and demand, transmission line (TL) overloads on Jeju Island, which has a low permissible rating, may lead to RE curtailment. We propose an operational strategy for the MIDC systems to mitigate TL overloading and RE curtailment. This strategy was aimed at managing the possible congestion of TLs, maintaining a power supply–demand balance, and increasing the penetration level of RE. In this paper, we present a scheme for calculating the operating points of the MIDC systems and allocating RE curtailment using optimization with security constraints to mitigate TL overloading. The proposed optimization model simplifies solving the mathematical equations involved by employing a sensitivity factor called the power transfer distribution factor (PTDF), using which linear programming (LP) optimization can be performed. To verify the feasibility of the operational strategy, case studies are conducted using an actual Jeju Island power system with a power system analysis tool—Power System Simulator for Engineering (PSS/E). The optimization problem was solved using MATLAB.

Research on the Voltage Supporting Capability of Multi-VSC-HVDC Subsystems Operation Strategy to Receiving-end LCC-HVDC Network in Weak AC Grid

For the hybrid multi-infeed HVDC system in which the receiving-end grid is a strong AC grid including LCC-HVDC subsystems and multiple VSC-HVDC subsystems, it has higher voltage support capability. However, for weak AC grid, the voltage support capability of the multi-VSC-HVDC subsystems to the LCC-HVDC subsystem (voltage support capability-mVSCs-LCC) can resist the risk of commutation failure. Based on this consideration, this paper proposes an evaluation index called Dynamic Voltage Support Strength Factor (DVSF) for the hybrid multi-infeed system, and uses this index to qualitatively judge the voltage support capability-mVSCs-LCC in weak AC grid. In addition, the proposed evaluation index can also indirectly judge the ability of the LCC-HVDC subsystem to suppress commutation failure. Firstly, the mathematical model of the power flow of the LCC and VSC networks in the steady-state is analyzed, and the concept of DVSF applied to hybrid multi-infeed system is proposed. Furthermore, the DVSF index is also used to qualitatively judge the voltage support capability-mVSCs-LCC. Secondly, the influence of multiple VSC-HVDC subsystems with different operation strategies on the DVSF is analyzed with reference to the concept of DVSF. Finally, the indicators proposed in this paper are compared with other evaluation indicators through MATLAB simulation software to verify its effectiveness. More importantly, the effects of multi- VSC-HVDC subsystems using different coordinated control strategies on the voltage support capability of the receiving-end LCC-HVDC subsystem are also verified.

An Analysis on the VSC-HVDC Contribution for the Static Voltage Stability Margin and Effective Short Circuit Ratio Enhancement in Hybrid Multi-Infeed HVDC Systems

Over the years, high voltage transmission of large energy blocks over long distances has widely developed from the Line Commutated Converter (LCC) technology. However, the continuous expansion of the AC network and the increase in renewable energy penetration leads to the weakening of Electric Power Systems (EPS), causing operational problems for the LCC-HVDC. This paper evaluates the degree of contribution of the VSC-HVDC on the LCC-HVDC dynamic performance when the infeed is in a hybrid multi-infeed HVDC system. Through a steady-state mathematical framework, the new concept of Hybrid Power Voltage Interaction Factor (hPVIF) is proposed to assess the transient stability improvement of LCC. Additionally, this article introduces two key contributions from hPVIF: the complementary Improved Commutation Failure Immunity Index (iCFII), as a measure of the effective short circuit enhancement, as well as the apparent line length, which emulates the line lengths adjusting the power dispatch in the VSC-HVDC. PSCAD/EMTDCTM time-domain dynamic simulations are performed to assess the indexes, and the Matlab® software will be used to assist in mathematical operations.

Grid Strength Assessment for Inhomogeneous Multi-infeed HVDC Systems via Generalized Short Circuit Ratio

Grid Strength Assessment for Inhomogeneous Multi-infeed HVDC Systems via Generalized Short Circuit Ratio

Commutation failure suppression method considering chain reaction in multi-infeed LCC-HVDC systems

In multi-infeed HVDC (MIDC) systems, in which HVDC means high-voltage direct current, the complex interactions between AC/DC systems and different line-commutated converter-based high-voltage direct current (LCC-HVDC) produces a special commutation failure (CF) chain reaction phenomenon. The existing methods are mostly aimed at the transient process of the single LCC-HVDC, which reduces the CF suppression effect but may become the driving force of the CF chain reaction. In view of this situation, this study proposes a new CF suppression method based on the CF chain reaction security region. First, the CF chain reaction process of multiple LCC-HVDC systems is analyzed, and the boundary of the CF chain reaction is characterized. Second, by establishing the CF chain reaction security region, the control method of the coordinating DC current and firing advance angle to suppress the CF chain reaction is proposed. Simulation results show that the proposed method avoids the subsequent CF of the LCC-HVDC while maximizing the suppression effect of the successive CF in the adjacent LCC-HVDC, thereby blocking the cascading reaction of CF that usually caused large-scale security problems.

Interaction Mechanism Analysis and Additional Damping Control for Hybrid Multi-Infeed HVDC System

With the emergence of the hybrid multi-infeed high voltage direct current (HMIDC) system, there is complex dynamic interaction between the AC/DC system and converter stations, while the interaction mechanism of the HMIDC system is unclear. For this purpose, this paper establishes the HMIDC linearized model based on the motion equation concept. Then, after decoupling and reconstructing the interaction path of the HMIDC system, the damping coefficients are derived. The AC system strength and voltage source converter (VSC) terminal voltage controller parameters are selected to analyze the impacts on damping coefficients, and the interaction mechanism of the HMIDC system is analyzed. In order to solve the potential instability issue, an additional damping controller in the VSC terminal voltage loop is proposed to enhance the voltage stability of HMIDC system, which is realized by adjusting the reactive power/amplitude coupling damping coefficient. Finally, the RT-LAB-based hardware-in-the-loop experiments verify the accuracy of the interaction mechanism analysis of the HMIDC system and the effectiveness of the proposed controller.

Coordinated Control Strategy for Cascading Commutation Failure Mitigation Considering Reactive Power Interaction

Cascading commutation failure (CF) caused by the reactive power interaction between HVDC is one of the new forms of cascading failures in multi-infeed HVDC systems. However, the concept and mechanism of cascading CF have not been explained clearly, and few studies have been devoted to control strategy for cascading CF mitigation considering reactive power interaction. In this paper, the cascading CF is defined and novel coordinated control strategy for cascading CF mitigation is proposed. Firstly, the mechanism of cascading CF in multi-infeed HVDC systems is analyzed. Then influence degree of HVDC on the power system stability is evaluated considering both active power impact and reactive power interaction, based on which novel coordinated commutation failure prevention as well as voltage dependent dc current order limiter is developed to mitigate cascading CF. Simulation results based on CIGRE HVDC benchmark model in PSCAD/EMTDC verify the effectiveness and advancement of proposed strategy on cascading CF mitigation.

Analytical Evaluation Approach of Complex Inter-Inverter Interactions in Emerging Hierarchical-Infeed LCC-UHVDC Systems

Compared to traditional multi-infeed HVdc (MIDC) systems with inverters coupling only in the ac-side, more complex inter-inverter interactions exist in emerging hierarchical-infeed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCC</i> -UHVdc (HIdc) systems with inverters coupling in both the ac- and dc-sides. However, the effectiveness and accuracy of the earlier interinverter interactions evaluation approach in HIdc systems are limited since it is established by the empirical reasoning without rigorous theoretical basis. Thus, in this article, the hierarchical-infeed voltage interaction factor (HVIF) index is first used to quantify the complex inter-inverter interactions in HIdc systems. The salient feature of the HVIF is that both the voltage magnitude and angle couplings at various inverter buses are considered in its definition. Hence, the definition of the HVIF enables the proposed analytical evaluation approach to be more effective than the earlier approach. Moreover, with various dc control modes comprehensively considered, the inverter power-voltage dependence terms in the quasi-steady-state power-flow model of HIdc systems are derived based on the rigorous theoretical analysis. This lends the fundamental mathematical basis to subsequently develop the analytical calculation method of the HVIF, which makes the proposed approach more accurate than the earlier approach. Finally, both the simulated and experimental results sufficiently validate the proposed approach.

Analytically Quantifying the Impact of Strength on Commutation Failure in Hybrid Multi-Infeed HVdc Systems

Analytically Quantifying the Impact of Strength on Commutation Failure in Hybrid Multi-Infeed HVdc Systems

Analytically Assessing the Effect of Strength on Temporary Overvoltage in Hybrid Multi-Infeed HVdc Systems

In hybrid multi-infeed HVdc (HMIDC) systems comprising both voltage-source and line-commutated converters’ inverters, the strength has a significant effect on the temporary overvoltage (TOV). However, this effect has always been assessed by the simulated approach in earlier works. It is unable to provide the sufficiently theoretical perspectives and also time-consuming as onerous electromagnetic transient simulations are required. Therefore, in this letter, the mathematical expression of the TOV is first derived as the analytical function of the hybrid multi-infeed interactive effective short-circuit ratio (HMIESCR) strength index. This derivation is achieved by using the quasi-steady-state analytical model of HMIDC systems with the interinverter interactions considered. Then, the derived expression is utilized for analytically assessing the effect of the HMIESCR on the TOV versus various system parameters and operation variables. Compared to the simulated approach, the proposed analytical approach here can offer more sufficiently theoretical perspectives and is also more efficient as simulations are no more required. Finally, the experimental results of a hybrid tri-infeed HVdc test system based on the hardware-in-the-loop platform validate the analytical approach.

Operation and Challenges of Multi-Infeed LCC–HVDC System: Commutation Failure, AC/DC Power Flow, and Voltage Stability

This paper presents a detailed analysis of commutation failure, AC/DC power flow, and voltage stability of multi-infeed high-voltage direct current (HVDC). The use of HVDC power transmission technology has become common in modern power systems. During the past two decades, HVDC technology has been extensively used for long-distance bulk power transmission to remote areas. Throughout the world, the demand for power has drastically increased in recent years due to industrialization; such situations make HVDC an economic candidate because the distance between power generation plants and load areas is significantly very long. The line-commutated converter (LCC) technology-based HVDC system is well more mature than other available conversion schemes (i.e., voltage source converters), and it is widely used in high-power projects. China had approximately 50 HVDC–LCC links in 2020, and a single LCC-based link with the highest capacity is 12 GW. The installation of several HVDC links in an existing power network has led to a situation where two or more HVDC links terminate in the electric vicinity of each other’s AC network or even in same AC busbar. Such scenarios are termed multi-infeed HVDC system. Multi-infeed HVDC systems bring various challenges related to voltage stability, local and concurrent commutation failure, and AC/DC power flow. Here, the literature available on these phenomena of LCC-based HVDC is discussed for future research. The assumptions and drawbacks of various techniques used for investigating the mentioned phenomena are also highlighted.