Multiple HVDC Research Articles

Synchronous Condenser Placement for Multiple HVDC Power Systems Considering Short-Circuit Ratio Requirements

Synchronous Condenser Placement for Multiple HVDC Power Systems Considering Short-Circuit Ratio Requirements

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.

Dual-Port Grid-Forming Control of MMCs and Its Applications to Grids of Grids

This work focuses on grid-forming (GFM) control of Interconnecting Power Converters (IPCs) that are used to interconnect multiple HVAC and HVDC subgrids to form a grid of grids. We introduce the concept of dual-port GFM control that leverages the ability of Modular Multilevel Converters (MMCs) to simultaneously form its AC and DC terminal voltage and present two dual-port GFM MMC controls. We provide analytical results and high-fidelity simulations that demonstrate that (i) dual-port GFM control is more resilient to contingencies (i.e., line and generator outages) than state-of-the-art single-port GFM control, and (ii) unlike single-port GFM control, dual-port GFM control does not require assigning grid-forming and grid-following (GFL) roles to the IPC terminals in grids of grids. Finally, we provide an in-depth discussion and comparison of single-port GFM control and the proposed dual-port GFM controls.

Analysis and Prevention of Commutation Failure Caused by Line Coupling Effect in LCC-HVDC

In actual line-commutated converter based high- voltage direct-current (LCC-HVDC) projects, multiple HVDC transmission lines are usually installed on the same tower to save transmission corridors and reduce construction costs. However, electromagnetic coupling effect between adjacent lines can cause commutation failure (CF) in the non-fault poles when a line or pole fails. Hence, this paper deeply investigates the mechanism and the prevention strategy of CFs caused by line coupling effect in single- and double-circuit HVDC systems. The influencing factors of line coupling effect and the coupling characteristics under fault conditions are analyzed first based on the modeling of parallel transmission lines and the traveling wave theory. Then the mechanism of CF caused by line coupling effect is investigated with the consideration of different fault conditions, different line structures, and different system operation modes. Based on the above analyses, prevention strategies for CFs of non-fault poles in single- and double-circuit systems are proposed. Finally, the simulation results verify the correctness of the theoretical analyses and the effectiveness of the proposed control strategies.

Admissible HVDC infeed capacity evaluation of receiving-end power grids

To fulfil the cross-regional bulk power delivery of renewable energy and alleviate the power shortage pressure of load centers, a number of line-commutated converter-based high voltage direct current (LCC-HVDC) transmission projects have been deployed in worldwide. Due to the system-wide operating risk sharply escalated with the intensively infeed of multiple large-capacity HVDC links, a vital knowledge gap exists in understanding the maximal admissible HVDC infeed capacity (AHIC) of receiving-end power grids. To tackle this issue, this paper proposes a novel AHIC evaluation method, which determines the maximal allowed total number and capacity of HVDC links that the receiving-end grid can accommodate while taking into account multi-preconceived fault security requirements. An improved dynamic system strength criterion is developed to assessing the immunity of post-contingency HVDC commutation failures (CF), which creatively considers the support effect from all AC buses based on the voltage sensitivity. The original two-stage model is decomposed into a construction master problem and an operation optimization subproblem, and then being iteratively solved using the generalized Benders decomposition algorithm. The effectiveness of the proposed approach is demonstrated on a modified IEEE RTS-79 system and a practical large-scale system.

Frequency Coordinated Control for the Asynchronous Interconnected Power System With Multiple HVDC Links

The isolation of the HVDC transmission system challenges the frequency stability of asynchronous power grid. Considering the high-capacity power regulation capability of multiple HVDC links, a novel frequency coordinated control method is proposed for the asynchronous interconnected power system. This method firstly establishes a detailed frequency response analysis model by using widearea measurement data. Then, for the lowest frequency control after disturbance, based on the frequency response analysis model, the functional relationship between the lowest frequency and DC power support is deduced. Finally, considering the influence of multiple HVDC power support on node voltage, power flow of line and frequency variation of the HVDC opposite-end grids, an optimal model of frequency coordination control with minimal influence is established. By solving the model, an multiple HVDC coordinated power support scheme can be obtained to achieve the lowest frequency control target after disturbance. A typical IEEE system is improved to an asynchronous interconnected power grid for case simulation. The accuracy and effectiveness of the proposed frequency coordination control are verified by comparison with traditional method.

Multi-feed System Partitioning Method Based on N-1 Preconceived Fault Set and Local Expansion Theory

In view of the fact that the AC system with multiple LCC-HVDC feeds is prone to simultaneous commutation failure, this paper proposes the scheme of grid division for the multiple feed system to isolate the coupling between multiple HVDC and reduce the risk of occurring simultaneous commutation failure. First, N-1 scan is performed on the multi-feed system to find the set of lines that can cause simultaneous commutation failure of multiple HVDC. The line with high line load rate is removed from the line set. Then, on the basis of local expansion theory, the nodes in the system are aggregated to form the initial partition. In this process, the calculation method of voltage/reactive sensitivity matrix considering PV node and LCC-HVDC is firstly given; moreover, the corresponding indexes were calculated by the node aggregation function and the partition fitness function. Finally, the results of the partition are modified by considering the circuit set and the partition short circuit ratio, and the flow chart of the partition scheme is given. The partitioning process is validated with an IEEE 39-node system as an example. The results show that the partitioning method is reasonable and solves the problem of the failure of commutation effectively.

Subsea cable key challenges of an intercontinental power link: case study of Australia\u2013Singapore interconnector

The high potential for renewable energy generation in Australia, in particular solar and wind, and the high carbon content of Southeast Asian electricity and projected demand growth create favourable conditions for a HVDC power link between Australian and Southeast Asia. Such an interconnector would link predominantly solar farms located in northern Australia, known for its highest insolation levels in the world, to Singapore given its central location within Southeast Asia, high reliance on natural gas for its power generation, high demand growth and limited renewable potential and land surface. The current paper presents a holistic view of the key challenges of an Australia–Singapore power link related to its length, in the order of 3200 km, the water depth of sections crossing the Timor Trough and Indonesian waters, up to 1900 m, and the manufacturing and logistic issues of extensive length of cable to be deployed in a part of the world distant from the main manufacturing facilities. This very ambitious project will require a unique integrated contracting strategy involving multiple HVDC cable suppliers, marine heavy transport companies and cable installation contractors to effectively deliver this project within a sensible timeframe.

Multiport Modular Multilevel Converter for DC Systems

Multiport HV DC-DC converters are required to facilitate future HVDC infrastructure with the ability to interconnect and manage power flow between multiple HVDC networks. Existing topologies offer limited modularity and scalability, making them difficult to implement in the fast-growing HVDC industry. In this paper, a multiport modular multilevel converter (MP-MMC) is proposed as the first truly modular multiport HV DC-DC converter. The MP-MMC is made up of multiple subconverters that can be controlled individually with de-centralized controllers, allowing easy reconfiguration and high reliability of the converter power circuit. The MP-MMC is compared with other prominent multiport HV DC-DC converters based on modularity, reliability, semiconductor effort, magnetics requirement, and fault-blocking performance. Operation and performance of the MP-MMC are verified by simulation.

Virtual Synchronous Power Strategy for Multiple HVDC Interconnections of Multi-Area AGC Power Systems

Automatic generation control (AGC) in multi-area interconnected power systems is experiencing several adaptions due to increasing level of power converter based components in the system. The concept of virtual synchronous power (VSP) to simulate the dynamic effects of virtual inertia emulations by HVDC links for higher level control applications is introduced and reflected in the multi-area AGC model. By using this proposed combination in the AGC model, the dynamic performance of the studied system shows a significant improvement. The proposed formulation is generalized for multi-area systems with multiple HVDC links. The active power loop control in VSP-based HVDC links has a second-order characteristic, which makes a simultaneous enabling of damping and inertia emulations into the system. Trajectory sensitivities are also used to analyze the effects of VSP's parameters on the system stability. The effectiveness of the proposed concept on dynamic improvements is tested through MATLAB simulation of a four-area system.

Study on the stability mechanism of the sending‐side three‐machine‐group system after multiple HVDC commutation failure

Study on the stability mechanism of the sending‐side three‐machine‐group system after multiple HVDC commutation failure

Impact of HVDC line on the convergence property of AC/DC power flow calculation

This paper explores the impact of HVDC line on the convergence property of AC/DC power flow calculation. Firstly, the model of AC/DC power flow computation is constructed, including unified method and alternate iterative method. Then, an improved Levenberg–Marquardt (LM) method with line search technique is developed to solve the nonlinear equations of AC/DC power flow. To test the convergence property of the proposed method, LM method with trust region technique and Newton method are also employed. The researches on the impact of heavy DC transmission power and multiple HVDC lines on the convergence property of AC/DC power flow calculation are made on a modified IEEE 118-bus AC/DC hybrid test system. Simulation results show that the developed method under the model of alternate iterative method is superior to other methods under different models in convergence property. For the influence of heavy DC transmission power on the convergence property, it is improved by the new parameter configuration of DC transmission line firstly and adding proper new shunt capacitor on the weak nodes secondly; for the influence of multiple DC transmission lines on the convergence property, it can also be improved by the change of control mode firstly and adding new shunt capacitor secondly, which are based on the least square solution obtained by proposed method under the model of alternate iterative method.

高压直流输电系统中次同步振荡研究综述

由于高压直流输电(HVDC)技术有着损耗小、线路造价低等优点，在我国电力系统中被广泛采用。但HVDC与汽轮发电机组轴系相互作用可能产生次同步振荡(SSO)，影响系统的安全稳定运行。本文以直流输电系统对次同步振荡的影响和抑制方法为立足点，对高压直流输电系统中次同步振荡研究进行综述。首先，比较了几种常用的HVDC数学建模方法。然后，总结了换流器触发方式、控制模式及控制参数等对次同步振荡的影响，并介绍了多直流运行、串补或其他电气设备与HVDC联合运行时的系统的次同步振荡特性。接着指出在HVDC整流侧的定电流控制器里附加次同步阻尼控制是一种最常用的抑制SSO的策略，并比较了常见SSDC的设计方法。本文还介绍了新型直流输电VSC-HVDC以及风电加入系统后对次同步振荡的影响。最后，结合电力系统的发展要求对高压直流输电系统的次同步振荡研究方向做出展望。 HVDC has been applied widely in power systems because it has some merits such as low loss, low line cost and so on. However, interaction between HVDC converters and turbine generator shaft system may cause subsynchronous oscillation (SSO) which threats the safety and stability of power systems. This paper is intended to make a survey on SSO in HVDC by proceeding from the influence of HVDC on SSO and corresponding suppression strategy. The common methods of HVDC mathematical modeling are compared first. Secondly, the paper summarizes the effect of HVDC converter trigger modes, control modes and control parameters on SSO. The SSO characteristics of multiple HVDC and HVDC combined operation with series compensation or other electrical equipment is also introduced. Then the most common suppressing strategy that is installing a supplementary subsynchronous damping control (SSDC) in the constant current controller of rectifier is pointed out. And the design approaches of SSDC are compared. Besides, the influences of VSC-HVDC and HVDC system including wind power on SSO are presented. Finally, vistas of research are projected into the future.

Multiport DC–DC Autotransformer for Interconnecting Multiple High-Voltage DC Systems at Low Cost

This paper proposes a multiport dc–dc autotransformer (multiport dc auto) that is used to interconnect multiple HVDC systems with different voltage levels. The multiport dc auto is able to reduce 50–80% the converter cost compared with conventional dc–ac–dc technology. Different from the conventional dc–ac–dc technology using magnetic coupling at the ac sides of the converters, there is direct electrical connection between the interconnected dc systems in the multiport dc auto. Such direct electrical interconnection significantly reduces the used power converters in the multiport dc auto. Taking interconnecting a ±250, ±320, and ±400 kV dc system with the rated exporting/importing dc power at each of the dc system being 500, 1000, and 1500 MW as an example, the conventional multiport dc–ac–dc technology requires a total of 3000 MW power converters while only 775 MW power converter is required in the multiport dc–dc autotransformer. The required power converter in the multiport dc–dc autotransformer is only 26% of the converter used in the conventional multiport dc–ac–dc technology. Cost and operating power loss is therefore significantly reduced.

Analysis of the sending-side system instability caused by multiple HVDC commutation failure

Analysis of the sending-side system instability caused by multiple HVDC commutation failure

Modeling and Simulation of Cascaded STATCOM Based on RTDS

In 2013, China Southern Power Grid (CSG) installed three ±200Mvar STATCOMs on the 500kV substations which are adjacent to the receiving ends of multiple HVDC lines. Based on the project of STATCOM test in CSG, the ±200Mvar cascaded STATCOM simulation model is established in the real-time digital simulator (RTDS) ,the aim is to study the modeling technology of H Bridge valve group by using small time-step and main control strategy .Amounts of tests has been done to verify the validity of the cascaded STATCOM model in different control modes. The RTDS STATCOM model also tested in the closed-loop CSG power network and the transient state response characteristics has been compared with real STATCOM control and protection devices for engineering prototype to verify the feasibility of the model. The constructed model has been successfully applied on the analysis of impacts on AC/DC paralleled CSG with STATCOM using RTDS.

Optimal Coordination of a Multiple HVDC Link System Using Centralized and Distributed Control

This paper presents both offline and online optimization techniques for the control of a multiple high voltage direct current link power system. A frequency control scheme based on classical proportional-integral-derivative controllers is proposed and optimally tuned offline using particle swarm optimization. The performance of this scheme is compared with the performance of a centralized model predictive control (MPC) scheme, and a distributed MPC scheme that uses only local communications. The results illustrate that a significant performance improvement can be achieved using distributed MPC instead of classical control, illustrating the potential of distributed MPC for use in future power networks.

Assessment and Choice of Input Signals for Multiple HVDC and FACTS Wide-Area Damping Controllers

This paper proposes a hybrid method to assess and select suitable control inputs for multiple HVDC and FACTS supplementary wide-area damping controllers (WADCs) of large-scale interconnected power system. This method can not only reduce the interaction among different WADCs, but also get good performance of damping multiple inter-area oscillations. The residue analysis method based on observability/controllability index is used to preselect the input signal candidates for each WADC. Then, the relative gain array (RGA) analysis, including the steady-state values and the dynamic behavior, is carried out to evaluate and determine the suitable input signals for multiple WADCs. Since the undesired interaction among multiple WADCs is reduced significantly by the proposed method, we can design each WADC independently without considering the coordination of controllers, which can reduce the design difficulty of multiple WADCs. A detailed case study validates the proposed method, and indicates that the multiple WADCs adopting the selected wide-area signals can effectively damp multiple inter-area oscillations at different operating scenarios (e.g., line fault, line outage, and load shedding).

Effect of non-standard operating frequencies on the economic cost of offshore AC networks

The effect of choosing a non-standard operating frequency on the equipment and infrastructure costs of an offshore AC network is investigated. The offshore AC network considered is similar in design to the European SuperGrid “SuperNode”. It is designed to connect several large wind arrays to multiple HVDC converters through which power may be transmitted to shore. As the offshore AC network is isolated from onshore networks by the use of HVDC links, it may be operated unsynchronised at any desired frequency. The cost associated with operating the network at a fixed frequency in the range 20–120 Hz is investigated, focusing on the frequency-cost scalings of electrical devices (such as cables, transformers and reactive compensation) and offshore infrastructures. A case study is presented based upon Tranche A area of Dogger Bank, UK, where a minimum point in the total cost of the offshore network is found at 93 Hz.

Coordination of a multiple link HVDC system using local communications based Distributed Model Predictive Control

AbstractAs the complexity of power networks increases, the installation of devices such as High Voltage Direct Current links (HVDC) and Flexible AC Transmission Systems (FACTS), and the use of advanced control techniques, can be used to improve network stability. Model Predictive Control (MPC) is an example of such an advanced control technique. However, it is often impractical to implement this technique in a centralised manner, as often the problem can be too computationally complex or several independent controllers may be responsible for different subsystems. Distributed approaches use communication between a number of controllers to approximate control of a centralised system. In this paper it is proposed to use distributed MPC for controlling a multiple link HVDC system using local communications only.