Low Frequency AC System Research Articles

Selection of an Optimal Frequency for Offshore Wind Farms

Offshore wind power has attracted significant attention due to its high potential, capability for large-scale farms, and high capacity factor. However, it faces high investment costs and issues with subsea power transmission. Conventional high-voltage AC (HVAC) methods are limited by charging current, while high-voltage DC (HVDC) methods suffer from the high cost of power conversion stations. The low-frequency AC (LFAC) method mitigates the charging current through low-frequency operation and can reduce power conversion station costs. This paper aims to identify the economically optimal frequency by comparing the investment costs of LFAC systems at various frequencies. The components of LFAC, including transformers, offshore platforms, and cables, exhibit frequency-dependent characteristics. Lower frequencies result in an increased size and volume of transformers, leading to higher investment costs for offshore platforms. In contrast, cable charging currents and losses are proportional to frequency, causing the total cost to reach a minimum at a specific frequency. To determine the optimal frequency, simulations of investment costs for varying capacities and distances were conducted.

Design and control of modular multilevel matrix converter with symmetrically integrated energy storage for low frequency AC system

AbstractIntegrating energy storage units (ESUs) into part of sub‐modules (SMs) enables the decoupling active power control for the modular multilevel matrix converter (M3C). The low frequency AC (LFAC) system based on M3C with symmetrically integrated energy storage (SI‐AM3C) can provide functions such as renewable power fluctuation smoothing and other power grid auxiliary services, which is attractive in engineering practices. This paper focuses on the design and control of SI‐AM3C. For the converter design, an evaluation method for the number of required active SMs (ASMs) is presented. On this basis, the influences of different system operating conditions on the ASM requirement are analyzed in case studies, and the effects of specific circulating injection strategies are also investigated. The control system targeted at decoupled AC side active power for SI‐AM3C is introduced. A modified inner‐arm capacitor voltage balance strategy is introduced to reduce the capacitor voltage deviation between ASMs and passive SMs (PSMs). The conclusions related to the presented estimation method and converter control strategy are verified by time‐domain simulations in PSCAD/EMTDC.

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.

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 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.

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.

Provision of ancillary services by renewable hybrid generation in low frequency AC systems to the grid

Wind energy high penetration levels in power systems lead to continuous power imbalance due to the intermittent nature of wind power. This paper proposes and investigates different methods to enable a hybrid generation system to provide frequency support to the grid. The hybrid generation is 100% renewable and composed of a wind farm and hydropower plant (HPP) of comparable generation capacities, and they are interconnected through a Low Frequency AC system (LFAC). The grid–tie is composed of a Voltage-Source Converter based High-Voltage, Direct Current (VSC-HVDC) junction that acts as frequency changer to maintain the grid nominal frequency. The HPP provides two types of ancillary services: wind power smoothing and frequency drops mitigation to avoid the use of thermal generation and battery energy storage. The paper offers different control methods to provide the two AS with improved coordination between the different controls in the hybrid generation system and complying with the common requirements of Grid Codes. The results obtained show that the frequency at the LFAC can tolerate mild drops to provide frequency support to the grid. The controllers’ parameters have a clear impact on the frequency response at both systems. Simulation environment is MATLAB and Simulink.

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.

A State of Art Review on Offshore Wind Power Transmission Using Low Frequency AC System

Offshore wind power generation, transmission and integration around the world are becoming higher, and it provides technical and economic challenges for future practitioners and industries to make an alternative transmission system to an existing system in reduction of cost. In most of countries, e.g. The German government planned to install 25000MW of offshore wind farms by 2030. The greater part of offshore wind farms are integrated with High Voltage AC (HVAC) transmission to the onshore grid. Offshore wind farms are integrated with High Voltage DC (HVDC) transmission for long distances (>50km) to the grid because of capacitive cable current in HVAC. The major challenge in HVDC transmission is the installation, operation and maintenance (O&M) of the Voltage Sourced Converter (VSC) HVDC substation in the offshore climate. Currently, in case offshore wind farm the research have been fastened to reduce the complexity with increasing reliability and minimizing cost. This paper gives a comprehensive review of integration of offshore wind farm via Low Frequency AC or Fractional Frequency AC (LFAC or FFAC) transmission. LFAC transmission is adopted from HVAC and it is operated at one third of nominal frequency (16.67 Hz). As compared to HVDC the main advantage of LFAC is an absence of the offshore converter station, hence system complexity and cost reduced. In design considerations, especially the offshore transformer is one of the challenges. This paper presents a comprehensive review on components and its design considerations of offshore novel LFAC transmission. The offshore wind turbine considerations, collector network and different types of onshore frequency converters explained in detail.

Steady-State Analysis and Performance of Low Frequency AC Transmission Lines

The steady-state performance of low frequency AC (LFAC) systems for bulk power transmission is proposed and investigated in this paper. It is demonstrated that the LFAC is superior to the conventional 60 Hz AC system in terms of power transfer capability and voltage stability. An existing power system can increase its power transfer capability by up to more than eleven times if it operates at 5 Hz. In low operating frequency conditions, the transmission overhead line reactance is significantly lowered and thus results in less voltage drop along the line. If the system operating frequency is in the range of 1 to 10 Hz, the voltage profile along a transmission line is comparable to the surge impedance loading at the no-load condition or approaches that of the HVDC system at the full-load condition. More importantly, the V-P and Q-V characteristic curves show that the dependency of voltage on the real and reactive power flow is greatly reduced. In other words, the sensitivity of voltage on reactive power variations is diminished and results in a more stable voltage and a higher stability margin for the LFAC system compared to the conventional 60 Hz system.

A Proposal on Low Frequency AC Transmission as a Multi-Terminal Transmission System

This paper is focused on the discussion and comparison of characteristics and behavior of three low frequency ac (LFAC) transmission system configurations operating under the same control scheme and conditions to identify the most promising operation system for LFAC. Merits of LFAC over high voltage direct current (HVDC) are mentioned first. By changing power flow direction without auxiliary switches in multi-terminal application and easiness of short circuit protection are explained. The three configurations of LFACs are described and applied by the control scheme with the aid of the tool of the PSCAD/EMTDC software to consider the behavior of each LFAC system on line frequency and low frequency sides. For two-phase system, no fluctuation occurs on the line frequency side, which is the advantage over single-phase system. Furthermore, current rating on thyristor devices during operation and number of devices that used in each type of LFAC are calculated and compared. These results can lead to determine the most suitable transmission system for the LFAC system operation.

Economic Assessments of LFAC and HVDC Transmissions for Large Offshore Wind Farms

Abstract Offshore wind farms extend a distance from an onshore grid to increase their generating power, but long distance and high power transmissions raise a lot of cost challenges. LFAC (Low Frequency AC) transmission is a new promising technology in high power and low cost power transmission fields against HVDC (High Voltage DC) and HVAC (High Voltage AC) transmissions. This paper presents an economic comparison of LFAC and HVDC transmissions for large offshore wind farms. The economic assessments of two different transmission technologies are analyzed and compared in terms of wind farm capacities (600 MW and 900 MW) and distances (from 25 km to 100 km) from the onshore grid. Based on this comparison, the economic feasibility of LFAC is verified as a most economical solution for remote offshore wind farms. Keywords: LFAC, HVDC, Offshore, wind farm I. INTRODUCTION In recent years, energy systems based on wind power have rapidly enlarged their application areas, especially towards large offshore wind farms (over 100 MW) and micro grid systems. The conventional onshore wind farms have small power generation and short distance power transmission to a power grid. However, for a large remote wind farms, a new power transmission system is required to provide high energy density and low loss power transmission characteristics with low investments. So, how to connect large remote wind farms to the onshore micro grid with low power losses and economic benefits is the prime concerns of researchers, and its economic power system and wind farm layouts for transmitting high power and long distance has gained more attentions. The conventional HVAC (High Voltage AC) system consists of wind generators, transformers, transmission cables and reactive power compensators, and the generated power is converted to a very high voltage (154 kV or 345 kV) by transformers. The HVAC power system transmits the power through 3 cores XPLE cables through underwater, but the transmission distance of the HVAC power system is the most critical factor against power transmission capability because reactive power losses are proportional to the distance. Therefore, HVAC transmission system is not adequate to long distance large offshore wind farms. Recently, new technologies for power systems have been reported [1]-[15] to provide alternative ways to maximize the power transmission capability. The most outstanding technology is HVDC power system, which has high economic benefits for long distant applications because HVDC power system has no limitation of the transmission capability. HVDC Transmission does not suffer from the reactive losses found in the transmission of HVAC system. However, in order to transmit DC power from a remote wind farms the generated AC power must be converted to the DC power and must be converted back to the AC power for a grid connection. Converting the AC power into the DC power requires an expensive AC to DC converter station to be installed at the remote wind farm area as well as a DC to AC power converter station at a receiving end, prior to the grid. An alternative technology is LFAC (Low Frequency AC) transmission system. LFAC transmission system uses lower frequency (50/3 Hz or 60/3 Hz) than a grid frequency (50 or 60 Hz) and requires no offshore power converter stations but an onshore frequency converter station. LFAC transmission system has an ability to extend a transmission distance and capability rather than the conventional HVAC transmission system. However, LFAC system can generate some audible noises and have transformer saturation and size problems. To adopt a best power system topology for large remote offshore wind farms, an economic analysis about HVDC and LFAC system should be performed. However, economic investments are directly dependent on a power system configuration, a distance and transmission capability. Therefore, this paper proposes HVDC and LFAC power system configurations and presents an economic assessments and comparison of LFAC and HVDC transmissions for large offshore wind farms. The economic assessments of two different transmission technologies are analyzed based on the proposed power configurations and compared in terms of wind farm’s capacities (600 MW and 900 MW) and distances (from 25 km to 100 km) from the onshore grid. From the comparison, the economic feasibility of LFAC is verified as a most economical solution for the large offshore wind farms.

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.

低周波ケーブル送電方式の提案

This paper proposes a novel transmission concept of Low Frequency AC (LFAC) transmission system, which is suitable for a long distance and high power cable transmission. An HVdc system is conventionally adopted for a long distance power cable transmission by Oil Filled or Solid cables. An XLPE cable shows preferable performance as a power cable in the conventional AC system for relatively short distance transmission. But it has difficulty of operation in an HVdc system, for space charges accumulate in the insulating dielectrics under high dc electric field stress, and which may cause breakdown. A low frequency ac system can avoid this difficulty as a conventional utility frequency ac system does. In the proposed LFAC system, frequency converters are installed at the sending and receiving terminals. Then, the presened system is expected to control the power flow of the interconnection as an HVdc does. This paper shows the advantages and disadvantages of the presented LFAC system and its basic control strategy. They are confirmed through digital computer simulations using a simplified model.