Low Frequency AC Transmission Research Articles

Experimental Assessment of a Decentralized Control Strategy for a Back-to-Back Modular Multilevel Converter Operating in Low-Frequency AC Transmission

The Modular Multilevel Converter (MMC) has been widely used in high-power applications owing to its inherent advantages, including scalability, modularity, high-power density, and fault tolerance. MMCs have recently been used in Low-Frequency Alternating Current (LFAC) transmission, particularly in the integration of offshore wind power with onshore grids. However, LFAC applications produce significant voltage oscillations in floating capacitor voltages within the MMC. Early research efforts have successfully established and validated decoupled control strategies for LFAC-based MMC systems. However, validations are usually based on simulations or small-scale prototypes equipped with limited power cells. Consequently, this paper presents a decentralized voltage control strategy based on Nearest Level Control for an MMC-based LFAC system. Experimental results obtained with a 120-cell MMC prototype are presented to validate the effectiveness and operation of the MMC in LFAC applications.

Discharge Characteristics of Needle Plate Electrode in Transformer Oil Under Low-frequency AC Voltage

Low-frequency transformer is the key equipment of flexible low-frequency AC transmission systems for offshore wind power. As the low-frequency voltage is applied, the transformer has significantly different insulation characteristics from those of the conventional transformer, it is necessary to study the typical characteristics of partial discharge of oil-paper insulation under such voltage. In this dissertation, the characteristics of discharge of needle plate electrodes in transformer oil under power frequency to low-frequency voltage are experimentally researched by adopting the pulse current method, and the discharge statistics, discharge atlas as well as feature parameters are analysed. With the decreasing of voltage frequency, the inception voltage (IV) value of partial discharge slightly increases, the extinction voltage (EV) phases of positive half-cycle (PHC) voltage waveform, as well as negative half-cycle (NHC) voltage waveform appear earlier, and the maximum discharge (MD), average discharge (AD), and pulse repetition rate (PRR) all show a downward trend.

Modeling and Control of Modular Multilevel Matrix Converter for Low-Frequency AC Transmission

The modular multilevel matrix converter (M3C) is the core component in low-frequency AC (LFAC) transmission, which is a competitive scheme for offshore wind power integration. In this paper, the M3C control strategy with the reduced switching frequency SM voltage balancing method is proposed. First, based on the conventional αβ0 and dq transformations, the M3C mathematical model is derived. Then, the dual-loop control structure with outer loop and inner loop controllers commonly used in voltage source converters is applied to the M3C. The inner loop controller consists of the current tracking controller in the dq reference frame and the circulating current suppressing controller in the αβ0 reference frame; the outer loop controller is proposed for the offshore wind farm LFAC integration scenario. Additionally, according to the operating characteristics of full-bridge sub-modules (FBSMs), three characteristic variables are defined and a reduced switching frequency SM voltage balancing method based on the nearest level control (NLC) is proposed. Finally, time-domain simulations in PSCAD/EMTDC demonstrate the feasibility of the proposed control strategy.

Grid-forming control strategy for PMSG wind turbines connected to the low-frequency AC transmission system

The inevitable consequence of the rapid transition to high-proportion renewable energy and high-proportion power electronics with the development of power systems is the loss of the traditional high-volume generation of synchronous generators, their inertia, and the accompanying frequency and voltage control mechanisms. This loss of moment of inertia changes the nature of the power system into a low inertia grid, which brings serious stability challenges. The new control technology of the converter promises to address these challenges and replicate the functionality provided so far by the synchronous generator. Firstly, the model of the low-frequency transmission wind turbine being integrated into the power grid is built. Secondly, a data-driven robust control strategy is designed for the machine-side inverter and the grid-side modular multilevel matrix converter (M3C), and the grid-forming control system of the wind turbine is established, so that the low-frequency transmission wind turbine can meet the grid-connection requirements. Finally, the real-time simulation experiment platform of the multi-fan collaborative control system is built. The experimental results verify the effectiveness of the grid-forming controller and the frequency stability of the grid-connected multiple fans.

Electromechanical transient simulation model of low frequency flexible transmission system based on modular multilevel matrix converter

Low frequency AC transmission has broad prospects in the future power system. How to establish its electromechanical transient simulation model and provide basic research means for its adaptability analysis after connecting to the large power grid is an urgent problem to be solved. Based on the existing electromechanical transient simulation methods, the mathematical model of modular multilevel matrix converter (M3C) is simplified, and an electromechanical transient simulation model composed of virtual DC circuit and equivalent AC circuit is proposed. On this basis, the power and voltage controller is designed according to the power decoupling characteristics of M3C. Further considering the operation requirements of the offshore wind power transmission system through low frequency AC transmission, an island control is designed to make M3C have the ability of passive control. Numerical examples verify the effectiveness of the proposed simulation model, which can effectively respond to short-circuit faults and power loss. This model can better meet the calculation requirements of large power grid simulation.

Research on control and protection technology of flexible low-frequency AC transmission project

With the development of offshore wind power development to deep and far sea, offshore flexible low-frequency AC (LFAC) transmission technology has attracted more and more attention from academia and industry in recent years. The control and protection system is the brain of flexible LFAC transmission, which is directly related to the performance, safety and benefit of LFAC transmission. In order to improve its operation reliability and limit the impact caused by any control link fault, in this paper, the control system is divided into system level control strategy, converter station level control strategy and valve level control strategy, which is from high level to low level according to the stratification principle. For the protection system, a specific zoning protection scheme is proposed. Finally, a two-terminal flexible LFAC transmission system is built in PSCAD/EMTDC. The simulation results prove the validity and correctness of the proposed control and protection strategy.

Dual-AC-port modular multilevel converter for hybrid frequency integration of offshore wind power: Modeling and control

Low frequency AC transmission system (LFAC) is a potential solution for deep-sea offshore wind power utilization besides HVAC and HVDC. Multi-port converter is the key device for the interconnection of hybrid frequency wind farms and power grid. An improved multi-port modular multilevel converter with dual AC ports and single DC port (DASD-MMC) is proposed as the key device for the interconnection among HVAC/HVDC/LFAC. The in-depth topology and working principle analysis are presented. The partitioned nearest level modulation method is proposed for high-voltage applications providing submodule capacitor voltage self-balancing ability. The mathematical model and the corresponding decoupling control strategy are presented afterward to realize the free power transfer among the three ports. To solve the output performance issues for hybrid HVAC and LFAC scenarios, an output current distortion suppression strategy is proposed with a novel cross-decoupling harmonic extraction method. Circulating current control strategy is presented with consideration of essential component analysis and calculation. Experiment and simulation validation and discussion are given, which show that the proposed DASD-MMC could provide a high quality hybrid frequency solution for offshore wind power collection and transmission.

Low Frequency AC Transmission Upgrades With Optimal Frequency Selection

The advantages of operating selected transmission lines at frequencies other than the standard 50 or 60 Hz are numerous, encompassing increased power transfer capacity and better utilization of existing infrastructure. While high voltage DC (HVDC) is by far the most well-established example, there has been an emerging interest low frequency AC (LFAC) transmission in applications ranging from offshore wind to railway systems and mining. In this paper, we investigate the use of LFAC as a transmission upgrade and propose models and analysis methods to determine the optimal choice of frequency. The paper first presents an optimal power flow model with frequency as a variable, assuming modular multilevel converters for frequency conversion. Using this model, we analyze LFAC as an embedded upgrade in a transmission system using existing lines. We quantify the system-wide advantages from improved power flow control and frequency reduction and find that an LFAC upgrade achieves similar and sometimes better results compared with HVDC upgrades. Finally, we analyze the factors which determine the optimal frequency for these upgraded transmission lines, and we demonstrate the benefits of changing the frequency in response to different system topologies and operating conditions.

Low Frequency AC Transmission for Offshore Wind Power

This paper presents a low frequency ac (LFAC) transmission system for offshore wind power. The LFAC system is interfaced with the main power grid with a cycloconverter. The wind power plant collection system is dc based, and connects to the LFAC transmission line with a 12-pulse thyristor converter. A method to design the system’s components and controls is set forth. Simulation results are provided to illustrate the system’s performance.

Comparative economic analysis of low frequency AC transmission system for the integration of large offshore wind farms

To fully extract the potential of offshore wind power, the wind farms are built further from mainland and the capacity is growing larger. Low Frequency AC (LFAC) transmission System is a promising solution to the integration of large offshore wind farms in long distance. This paper conducts a comparative economic analysis of LFAC. Compared to 50 Hz High Voltage AC (HVAC) transmission, LFAC can reduce charging current in AC cables to enhance the transmission ability in long distance. Compared to High Voltage DC (HVDC) transmission, LFAC eliminates the large expense of offshore converter station. In addition, the topologies of AC/AC converter in LFAC also influences the economic performance significantly. By designing typical AC/AC converters based on modular multilevel technique in a unified way, this paper performs a full-scale economic comparison in different voltage level, active power and reactive power cases. Furthermore, the economic comparison based on capital cost of HVAC, HVDC and LFAC is comprehensively studied considering different transmission capacity and the AC/AC converter topologies. The economic range of HVAC, LFAC and HVDC application is obtained, and results show that LFAC is suitable for medium distance circumstances (around 80–200 km). The economic range of LFAC expands when power rating increases.

Low frequency AC transmission systems for offshore wind power plants: Design, optimization and comparison to high voltage AC and high voltage DC

Low frequency AC technology has been presented as a configuration that can potentially reduce the total cost of existing transmission systems. Such cost reduction could be achieved by increasing the power transfer capacity of High Voltage AC cables operating at low frequency and removing the offshore converter when compared to High Voltage DC systems. The purpose of this paper is to suggest an optimal design of Low Frequency AC transmission systems. This results in obtaining the optimal Low Frequency AC voltage level, location, number of reactors and the Back-to-Back converter size for an Offshore Wind Power Plants with a defined rated power and distance to shore. Additionally, the Low Frequency AC, High Voltage AC and High Voltage DC equivalent configurations are compared in terms of total cost and the final transmission system is selected. The analysis of a practical case study is presented in detail, followed by an expansion of the methodology application on various configurations. Also, cost sensitivity analyses for two opposed cases are provided to identify the variation of the break-even distance of Low Frequency AC with High Voltage AC and High Voltage DC. Lastly, different non-standard transmission frequencies are analyzed and an optimal frequency is defined for a range of Offshore Wind Power Plants rated powers and distances. The paper presents a methodology that can be applied considering different relevant parameters, e.g. transmission voltages, cable cross sections and cost data. Based on parameters available in the literature, the study results show that Low Frequency AC is not currently viable.

DRU Based Low Frequency AC Transmission Scheme for Offshore Wind Farm Integration

The low frequency AC (LFAC) technology is found to be a competitive solution for offshore wind farm integration. This article proposed an LFAC system with a hybrid onshore back-to-back (BtB) converter, for which the diode-based rectifier unit (DRU) and modular multilevel converter (MMC) based inverter are adopted. Then, this article also proposed corresponding control strategies for offshore wind turbines, where the DC link voltage control is implemented on the machine side converter, and the AC voltage of the offshore grid is controlled by the grid side converter based on the fixed reference frame. The small signal analysis for the whole system was carried out based on the equivalent circuit, and the small-signal stability is proven by calculated eigenvalues. And the system responses to typical operating conditions are investigated by time-domain simulations in PSCAD. Thus, the feasibility of the proposed scheme is verified.

A Novel Control Scheme for Multi-Terminal Low-Frequency AC Electrical Energy Transmission Systems Using Modular Multilevel Matrix Converters and Virtual Synchronous Generator Concept

This paper proposes a new control scheme for the low frequency AC transmission (LFAC) system aiming at extending the point-to-point configuration to form a multi-terminal electrical energy network. The multi-terminal low frequency ac (MT-LFAC) system configuration is based on the use of modular multilevel matrix converters (M3Cs) and virtual synchronous generator (VSG) control. The M3C is the next ac/ac converter generation, which is used as an interface with the conventional AC network and the LFAC electrical energy system. Application of VSG control is proposed to enable proper power sharing, to provide synchronization of each terminal, and frequency stabilization, thus, to offer multiterminal forming capability. Two different operation modes are applied in the system to damp the frequency deviation after a dynamic perturbation, which provides additional stabilization feature to the VSG. Frequency restoration mode and commanded mode of power sharing are applied as dynamic states to validate the robustness of the VSG control system. Besides, to solve the negative impact of low X/R ratio in the LFAC electrical energy system, we enhance the VSG control by proposing a virtual-impedance-based solution, which increases the output total impedance on the low frequency side and prevents the coupling between P and Q. The operation of the proposed system is examined by simulation results with a precise model of M3Cs in the PSCAD/ EMTDC software environment (version 4.2.1, Winnipeg, MB, Canada).

A Dual VSG-Based M3C Control Scheme for Frequency Regulation Support of a Remote AC Grid Via Low-Frequency AC Transmission System

This paper proposes a low-frequency ac (LFAC) transmission system capable of providing frequency regulation support for a remote ac grid aiming at enhancing its frequency stability. The LFAC transmission system using modular multilevel matrix converters (M3Cs) for a direct ac-ac conversion is receiving noticeable attention as an alternative solution for a long-distance transmission system. One of its potential applications is the system connection between a large ac grid and a remote ac grid, whereas the latter usually suffers from a fluctuant frequency due to lack of inertia. The core of this work is addressing a novel control scheme of the remote ac grid-side M3C, which makes both interactions with the LFAC system and the remote ac grid behave like a synchronous generator using a dual virtual synchronous generator (VSG) control scheme. This control scheme can enhance the frequency regulation in the remote ac grid, which the existing control schemes cannot provide. Average dc capacitor voltage versus active power (V ave -P) droop control is proposed to coordinate the power flow between the VSG controls applied in both sides of the M3C. To demonstrate the problem of the existing control scheme, as well as the benefits offered by the proposed control scheme, transient performance including load variation and fault events in the remote ac grid is studied and examined in the EMTDC/PSCAD software environment.

Low-Frequency AC Power Transmission and Distribution for Subsea Application Using Hexverter

Environmental goals set by world leaders to normalize climate changes are quite difficult to achieve without renewable power generation and suitable transmission technologies like low-frequency AC transmission (LFAC). The LFAC is nowadays becoming a popular choice for long-distance power transmission due to its high efficiency and low losses. This research work investigates the feasibility of employing the LFAC system for subsea transmission and distribution of 58 MW power. In this paper, the simulation model of the LFAC-based subsea transmission and distribution system is presented. This model is composed of several parts such as hexverter as a frequency converter, where a novel control strategy to optimize its zero-sequence circulating current is employed. Detailed mathematical modeling based on active, reactive power constraints and DQ transformation is performed to achieve the control strategy for zero-sequence current optimization. An offshore wind farm is proposed to be integrated with the LFAC subsea system to fulfill the compatibility requirements of the system. The control system of both the grid side and the machine-side inverter of the wind farm is designed to eliminate the real-time disturbances such as wind speed fluctuations and harmonics due to heavy inductive load operating at 16 Hz. To drive the subsea pump, a vector control-based variable-speed drive is employed for the heavy induction motor. A 5 MW, 16 Hz RL load is also added in the model to analyze the effect of general-purpose load. Each component of this system is carefully designed to make it as close to real-time as possible. The whole system is designed for 16 Hz and is compared with the standard 50 Hz system to validate this design.

A Hierarchical Energy Balance Control Method for M3C Based on Injecting Output Frequency Circulating Currents

This paper presents a simplified hierarchical energy balance control method for a modular multilevel matrix converter (M3C) operating as a frequency changer (50–50/3 Hz) for low-frequency ac transmission system. The proposed method employs an independent energy balance control for each subconverter of the M3C with only injecting the output frequency circulating current, aiming at balancing the energy among the three arms of the subconverter. Then, the modulation technique is redesigned to balance the energy among n submodules within each arm. The proposed method simplifies the computational complexity of the M3C by considering the subconverter independently, enabling an easy and efficient way to control the M3C. The feasibility of the proposed method is evaluated with normal and dynamic operation, which includes step changes in the output frequency and voltage. The performance and effectiveness of the proposed control strategy is demonstrated by extensive simulation studies and experimentally validated using a scaled-down laboratory prototype.

Performance evaluation of PMSG-based LFAC system for offshore wind power

Nowadays, the practice of Low Frequency AC (LFAC) transmission and grid integration for the multi mega-watt (MW) long distances is an interesting approach. The LFAC approach has the ability to solve the charging current problem in underground AC cables and is an alternative to the conventional offshore transmission schemes. This paper presents the Permanent Magnet Synchronous Generator (PMSG)-based LFAC transmission for offshore wind power to integrate the onshore grid. The PMSG-based offshore wind power is integrated onshore grid via LFAC and High Voltage AC for comparison. The Machine Side Converter extracts the maximum power from the wind and maintains the generator terminal voltage, whereas Grid Side Converter (GSC) maintains the constant DC link voltage by adjusting the dq currents of the grid. The simulation results demonstrate and verify the significant improvement in the power transfer capability, reduced power loss and control schemes performance. The complete work is modelled and analysed using PSCAD/EMTDC.

Configuration and Control of Cycloconverter for Low Frequency AC Transmission System in 500kv Transmission Line

Today, the highest voltage level of the AC power transmission line is 500kV in Myanmar. The highest voltage level encounters thedifficulties of the material and environment issues. If it is a long-distance transmission, its power losses and voltage regulationare increasing. High voltage 50Hz AC transmission system has many limitations, so a high voltage Low-Frequency AC (LFAC) transmission system has been proposed as a new alternative technology for transmission of power between Meikhtila andTaungoo transmission system. The low-frequency AC transmission system is the 500kV transmission system. The low-frequencyAC transmission system utilizes the low frequency for transmission of power in this transmission line. This system would like to bebuilt a low-frequency AC transmission system that used two cycloconverter at the sending end and the receiving end. This paperfocuses on the simulation for three-phase six pulse bridge cycloconverter for low-frequency AC (LFAC) transmission system. Thissystem uses cycloconverter model and cycloconverter control that included voltage control, current limiter, extinction anglecontrol and Virtual Synchronous Generator (VSG) which used the Runge-Kutta Method between Meikhtila and Taungootransmission system. This converter control model used in LFAC system satisfies in long-distance transmission and increasesvoltage level in the transmission line because our country increases in load demands. The MATLAB-SIMULINK model for threephase six pulse bridge cycloconverter has been constructed for a 500 kV LFAC transmission system. LFAC system described that it is nearly 800MW the maximum active power transfer capacity by using the cycloconverter control.

Analysis and Control of the Modular Multilevel Matrix Converter Under Unbalanced Grid Conditions

The modular multilevel matrix converter (M3C) is a promising candidate for direct ac-to-ac conversion systems, particularly for low-frequency ac transmission. This paper is focused on the control of the M3C under unbalanced grid conditions. First, the current control structure of the M3C is improved, which not only enables independent control of common mode and circulating currents with a reduced number of controllers, but also enables a pure dc control system. Then, the arm imbalanced power caused by negative-sequence voltage and current is calculated, based on which a feedforward compensation strategy through circulating current is proposed. Performance and effectiveness of the proposed control and modulation strategies are verified by simulation studies conducted in the MATLAB/Simulink software environment.

Harmonic stability of VSC connected Low Frequency AC offshore transmission with long HVAC cables

Low Frequency AC (LFAC) transmission has been proposed as an alternative to HVDC transmission for the integration of offshore wind. The LFAC offshore grid as a fully power electronic grid with a long HVAC cable provides significant challenges to harmonic stability. This paper presents an impedance based stability analysis to determine the stability of the power electronic offshore system across the harmonic frequency range. The stability analysis is introduced and applied to the LFAC system. The impact of different current and voltage control bandwidths and component sizes on the dynamic impedance of the converters is then examined and their impact on harmonic stability of the LFAC grid is determined. It is found that detailed knowledge of the control parameters and the ability to tune the bandwidths can mitigate significant harmonic instability with the presence of a long HVAC cable. Three phase simulations are then used to validated the impedance based stability technique.