AC Transmission Research Articles

Transient characteristics and protection coordination strategy of hybrid AC / DC system based on modular multilevel converters under AC/DC transmission lines touching fault

Rapid development of hybrid AC/DC transmission systems has increased the probability of the touching faults between AC and DC overhead lines. The mechanism of coupling interaction is so complex that the adaptability of the existing hybrid system protection is affected. Previous studies mainly focus on the line-commutated converter based high-voltage direct current (LCC-HVDC) system and AC system, this paper concentrates on the fault coupling characteristics analysis and coordination protection of modular multilevel converter based on high voltage direct current(MMC-HVDC) and AC transmission systems. The analytical expressions of fault current are deduced under the different operating states of the converters and the valve-side AC circuit breaker. Subsequently, the fault current coupling characteristics and generation mechanism are revealed. On this basis, the adaptability of existing hybrid system protection is assessed. Further, by comparing and analyzing the influence and risk of mal-operation of AC transmission line protection, the coordination reclosing protection strategies for hybrid system is proposed. Finally, the simulation tests were carried out based on PSCAD/EMTDC. The simulation results show that the proposed coordination of AC and MMC-HVDC system reclosing strategy is effective for improving the reliability of hybrid system.

HVDC Transmission Placement Study to Increase Critical Clearing Time Using Sensitivity Analysis

Stability is the ability to go back to ordinary conditions after an interruption. Power transfer using an AC transmission system has some stability issues. Fast modulation of the HVDC transmission can escalate the transient stability of interconnected power systems thereby increasing the CCT. To calculate load flow sensitivity, use the load flow sensitivity analysis command in Powerfactory 15.1. The variable considered in this research is dφ/dP. In this paper, we will discuss the use of sensitivity analysis to specify the location of the HVDC transmission to increase the CCT value. By taking several samples that have high sensitivity values,namely channels 8-9 to 7-8 with an average cct change value of 0.555 s, mediumvalues, channels 5-8 to 9-6 have an average cct change of 0.234 s, and low fromchannels 6-4 to 9-4 with an average change of 0.109 s. The data above shows thata channel with a large sensitivity value will provide a higher increase in cct valuecomparedwith a low sensitivityvaluewhen paired with an HVDC.

REACTIVE POWER COMPENSATION TECHNOLOGIES

This article provides an overview of the current state of reactive power compensation technologies. The principles of operation, design characteristics and examples of the use of VAR compensators on thyristors and self-commutated converters are given. VAR static generators are used to improve voltage regulation, stability and power factor in AC transmission and distribution systems. Also described are examples obtained from the respective annexes describing the use of reactive power compensators implementedusing the new static VAR technologies.

Simulation and calculation of maximum transmission power for offshore wind plants accounting for the Electro-Magnetic transient process

The offshore wind power industry is witnessing a shift towards larger scale and longer offshore distances, pushing the transmission capacity and transmission distance of submarine cables to their maximum limits. Consequently, it is essential to evaluate the maximum transmission limit of offshore wind power. At the same time, the larger scale of the wind farm will make the Electro-Magnetic transient (EMT) processes have a greater impact on the system stability. To address these issues, this paper firstly established a wind generator (WG) model through a permanent magnet synchronous generator (PMSG) model with averaging parameters, as well as a cross-linked polyethylene (XLPE) submarine cable model to construct an offshore wind power system via AC transmission. Subsequently, a continuous EMT simulation method is proposed to calculate the maximum transmission capacity and transmission distance limit. Real-time transient simulations were conducted to check the transient stability constraints during the calculation process. Finally, through several case studies on different types of cables, the simulation and calculation results are presented to provide useful references for engineering applications.

Voltage Profile Improvement Using Static Synchronous Compensator STATCOM

Static synchronous compensator (STATCOM) is a regulating device used in AC transmission systems as a source or a sink of reactive power. The most common use of the STATCOM is in improving the voltage stability of the transmission system. A voltage regulator is a custom power device used to correct the voltage disturbance by injecting controllable voltage as well power into the system. This paper implement Nruro-Fuzzy controller to control the STATCOM to improve the voltage profile of the power system The strategy of STATCOM controller is to keep the voltage in the nominal range. The controller has been simulated after some kinds of disturbances and the results show improvements in voltage profile of the system.

Overview of Various Voltage Control Technologies for Wind Turbines and AC/DC Connection Systems

Wind power generation is one of the mainstream renewable energy resources. Voltage stability is as important as the frequency stability of a power system with a high penetration of wind power generation. The advantages of high-voltage direct current (HVDC) transmission systems become more significant with the increase of both installed capacity and transmission distance in offshore wind farms. Therefore, this study discusses various voltage control methods for wind turbines and HVDC transmission systems. First, various voltage control methods of a wind farm were introduced, and they include QV control and voltage droop control. The reactive power of a wind turbine varies with active power, while the active power from each wind turbine may be different owing to wake effects. Thus, QV and voltage droop control with varying gain values are also discussed in this paper. Next, the voltage control methods for an HVDC transmission system, such as power factor control, voltage control, and Vac-Q control, are also summarized and tested in this study. When a three-phase short circuit fault occurs or a sudden reactive power load increases, the system voltage would drop immediately. Thus, various voltage control methods for wind turbines or HVDC can make the system’s transient response more stable. Therefore, this study implemented the simulation scenarios, including a three-phase short circuit fault at the point of common coupling (PCC) or a sudden increase of reactive power load, and adopted various voltage control methods, which aim to verify whether additional voltage control methods are effective to improve the performance of transient voltage. The voltage control method has been implemented in PSCAD/EMTDC, and the simulation results show that the QV control performs better than the droop control. In addition, when applying the voltage control technique during a three-phase fault, transient voltage nadir can be improved through either an HVDC transmission system or an AC transmission system.

An Analytical Fault Diagnosis Method: Component Relation and Event Relationship Description Matrices

There are large amounts of status, switch, and protection information in ac transmission and distribution networks. Due to the complex relationship of information and high overlap of recognition logic, it is difficult to find the exact homeomorphic mapping between a fault and information. This article proposes an analytical fault diagnosis method by the use of a matrixed relationship description and operation to build a certain mapping between the fault and certain related information.

Transient Stability Improvement of Multi Machine Power System Including DFIG Wind Farm Using HVDC Link

In this paper, transient stability analysis of a detailed model multi-machine power system was carried out to focus on the impact of HVDC link on system stability by comparing the critical fault clearing times of two operational scenarios, namely an AC transmission system with and without parallel HVDC transmission. The advantage and performance of AC-DC transmission for the improvement of response to transient instability caused by phase to ground fault was researched by detail simulation carried out in MATLAB/SIMULINK.

Data-Driven Optimization of Transmission Section Limits and Their Adaptability for New Power Systems With High Penetration of Renewable Energy

In traditional power systems, the transmission section limits are often set to predetermined values according to the typical peak-to-valley demand modes, which may be difficult to adapt to the complex and diverse operation modes of the new power systems with high penetration of renewable energy, and it cannot consider the coupling characteristics of AC and DC transmission sections also. In order to make better use of the efficiency of the transmission sections, a mathematical model for optimizing the adaptability of the transmission section limit is firstly proposed, in which a data-driven typical demand mode extraction method based on K-means clustering is proposed according to the load forecast, renewable energy forecast and market transaction data. Then, the adaptability assessment of the section limit and the transmission requirements are carried out. Finally, the power perturbation method is used to calculate the influence factor matrix of associated transmission section limit and stability margin of its dominant fault. And the influence factors are optimized according to section limit adaptability and stability margin influence, and the best adaptable limit of the associated transmission sections are obtained. The calculation and optimization results of the actual case of the interconnection corridor between Xinjiang and the northwest main power system of China verify the effectiveness and practicability of the proposed limit optimization model, which is able to improve the flexibility and adaptability of the actual power system operation.

An AC active power regulation method for MMC by employing the internal energy

Similar to the onshore wind power integrated through AC transmission, the offshore wind power integrated through the modular multilevel converter (MMC) based HVDC also lays remarkable impacts on the stability of the power system. According to grid codes, offshore wind power should be capable of adjusting its active power fast. This paper proposes a fast regulation method at the onshore point of common coupling for offshore wind power by utilizing the energy stored in the distributed capacitors in the MMC. By carefully designing the curve of capacitor energy, the active power can vary expectedly. Meanwhile, no disturbance is placed on the DC-side active power or DC voltage. The effectiveness of the proposed method is verified by electromagnetic simulation.

An advanced HBNP multi-level inverter for the performance enhancement with reduced components

ABSTRACT This article proposed an advanced three-phase multilevel inverter called the H-Bridge Neutral Point (HBNP) Inverter, with a minimum number of components. This inverter is designed to generate five-level output voltage using a single solar system in each phase, making it suitable for low-, medium-, and high-power applications like AC drives, grid integration of renewable energy sources, electric vehicles, and high voltage DC and AC transmission systems, etc. The proposed circuit configuration has reduced gate driver circuits, less capacitors, fewer input sources, modularity in structure, and better THD. Furthermore, the hybrid PWM control technique provides switching signals and ensures that all switches have equal conduction intervals. All of the switches have the same temperatures and voltage sharing, indicating a simpler thermal design and DC bus voltage balancing. The proposed and existing topologies are validated using MATLAB Simulink, and the proposed topology prototype is implemented in the laboratory.

ADMM-based Distributed Active and Reactive Power Control for Regional AC Power Grid with Wind Farms

A distributed active and reactive power control (DARPC) strategy based on the alternating direction method of multipliers (ADMM) is proposed for regional AC transmission system (TS) with wind farms (WFs). The proposed DARPC strategy optimizes the power distribution among the WFs to minimize the power losses of the AC TS while tracking the active power reference from the transmission system operator (TSO), and minimizes the voltage deviation of the buses inside the WF from the rated voltage as well as the power losses of the WF collection system. The optimal power flow (OPF) of the TS is relaxed by using the semidefinite programming (SDP) relaxation while the branch flow model is used to model the WF collection system. In the DARPC strategy, the large-scale strongly-coupled optimization problem is decomposed by using the ADMM, which is solved in the regional TS controller and WF controllers in parallel without loss of the global optimality. The boundary information is exchanged between the regional TS controller and WF controllers. Compared with the conventional OPF method of the TS with WFs, the optimality and accuracy of the system operation can be improved. Moreover, the proposed strategy efficiently reduces the computation burden of the TS controller and eliminates the need of a central controller. The protection of the information privacy can be enhanced. A modified IEEE 9-bus system with two WFs consisting of 64 wind turbines (WTs) is used to validate the proposed DARPC strategy.

A Novel Excitation Approach for Power Transformer Simulation Based on Finite Element Analysis

Power transformers play an indispensable component in AC transmission systems. If the operating condition of a power transformer can be accurately predicted before the equipment is operated, it will help transformer manufacturers to design optimized power transformers. In the optimal design of the power transformer, the design value of the magnetic flux density in the core is important, and it affects the efficiency, cost, and life cycle. Therefore, this paper uses the software of ANSYS Maxwell to solve the instantaneous magnetic flux density distribution, core loss distribution, and total iron loss of the iron core based on the finite element method in the time domain. In addition, a new external excitation equation is proposed. The new external excitation equation can improve the accuracy of the simulation results and reduce the simulation time. Finally, the three-phase five-limb transformer is developed, and actually measures the local magnetic flux density and total core loss to verify the feasibility of the proposed finite element method of model and simulation parameters.

Location-based uncertainty management of off-shore wind farms: A multiple radius robust decision making

Increasing the penetration of renewable energies creates various operational and even security challenges for power systems, due to the variability of their output power and inability to provide system inertia. These issues are even more inevitable in power systems which are connected to off-shore wind farms (WFs) in different locations, creating more complexity in decision making process. This paper, proposes a decision making model to increase the robustness of optimal power flow in presence of off-shore WFs. Information gap decision theory (IGDT) is used as a decision making tool for dealing with the uncertainty radius of multiple WFs, by definition of weighting factors for the uncertainty of each WF. Voltage source converter (VSC)-based high voltage DC (HVDC) transmission links are considered to connect the off-shore WFs to the main AC grid. The proposed optimization model, which includes both AC transmission and HVDC systems constraints, is implemented on the IEEE 118-bus standard test system and solved by general algebraic modeling system (GAMS) optimization software. The simulation results show the economic effect of location factor in definition of uncertainty for WFs.

Suppressing Voltage Spikes of MOSFET in H-Bridge Inverter Circuit

Power electronics devices are made from semiconductor switches such as thyristors, MOSFETs, and diodes, along with passive elements of inductors, capacitors, and resistors, and integrated circuits. They are heavily used in power processing for applications in computing, communication, medical electronics, appliance control, and as converters in high power DC and AC transmission in what is now called harmonized AC/DC networks. A converter’s operation is described as a periodic sequencing of different modes of operation corresponding to different topologies interfaced to filters made of passive elements. The performance of converters has improved considerably using high switching frequency, which leads to a significant improvement in a power converter’s performance. However, the high dv/dt through a fast-switching transient of the MOSFET is associated with parasitic components generating oscillations and voltage spikes having adverse effects on the operation of complementary switches, thereby affecting the safe operation of the power devices. In this paper, the MOSFET gate-driver circuit performance is improved to suppress the H-Bridge inverter’s voltage spikes. The proposed technique is a simple improvement to the gate driver based on the IR2112 driver (IC) by adding a capacitor to attenuate the effect of parasitic components and the freewheeling current, suppressing the negative voltage spikes. This paper’s main contribution is to improve the gate driver circuit’s capability for suppressing the voltage spikes in the H-Bridge inverter. The improved gate driver circuit is validated experimentally and is compared with the conventional gate driver. The experimental results show that the proposed technique can effectively suppress the MOSFET’s voltage spikes and oscillations.

Design of an Optimal Controller in a Voltage Regulation System When Powering Submersible Electromechanical Equipment

The paper addresses the issue of designing an optimal controller using the measured state variables of a nonlinear multi-dimensional object with transient disturbances – power supply of submersible electromechanical equipment with AC transmission via a cable power line. We present an approach to the design of an adjustable PI-controller enabling to determine its closed-form parameters with the linearized model of a complex control object. A numerical example for simulation of a power supply highlighting capability of the design algorithm for the optimal controller with the quadratic criteria is provided. The obtained results can be used to tackle problems of adaptive control of remotely powered electromechanical objects without sensors.

Analysis of the DC Fault Current Limiting Characteristics of a DC Superconducting Fault Current Limiter Using a Transformer

Recently, a lot of interesting research has been conducted to solve the fault current problem of the DC system. In long-distance transmission, DC transmission is more economical than AC transmission. The connection of power grids with a DC system can also better control the power flow and provide high stability. However, the control of the fault current in a DC system is more difficult to handle than in an AC system because the DC system does not make a zero point, unlike the AC system. In addition, there is a disadvantage, in that an arc occurs when a circuit breaker operates. In this paper, a new type of DC superconducting fault current limiter (SFCL) is proposed. This new type of SFCL is composed of two superconducting elements, a current limiting resistor/reactor (CLR), and a transformer. With the proposed SFCL, the DC fault current limiting experiments were performed and the DC fault current limiting characteristics of this SFCL due to the component of the CLR were analyzed.

Three-Port Small Signal Admittance-Based Model of VSCs for Studies of Multi-Terminal HVDC Hybrid AC/DC Transmission Grids

Multi-terminal high voltage direct current (HVDC) systems, together with AC transmission systems and voltage source converters (VSCs), form hybrid AC/DC grids with complex dynamic and transient interactions. VSCs characterization taking into account DC- and AC-side dynamics in order to study these interactions is not yet well solved. This paper presents a three-port transfer admittance-based matrix model of VSCs that can be applied for such purpose. It is derived from dq-complex space vectors and characterizes both AC- and DC-side dynamics by relating AC- and DC-side current and voltages in a three-dimensional admittance transfer matrix which considers the VSC outer control loops. The paper also proposes a systematical procedure for studying multi-terminal HVDC hybrid AC/DC transmission grids by the Norton admittance method, where the proposed VSC model can be easily included in the Norton admittance matrix. This procedure allows the study of grid dynamics using impedance-based stability criteria. The proposed model and procedure are applied to a stability study in a multi-terminal HVDC hybrid AC/DC transmission grid. PSCAD/EMTDC simulations are used to validate the application.

Harmonic interaction of a static Var compensator with AC power system containing multiple non-linear loads

In this paper a study of the impact of the harmonics generated by a static Var compensator (SVC) is presented. The SVC is modeled, in the harmonic domain, as a coupled current source by using the complex Fourier transforms. Then, this model is converted to polar form to be integrated into the harmonic power flow program. This approach has been carried out on the IEEE 14 bus test power system, in order to show its effectiveness in evaluating the impact of harmonics, injected by the shunt compensating devices, and its interaction with the AC transmission system, in meshed power networks. Since the SVC consists of a thyristorcontrolled reactor (TCR) and a fixed capacitor, the harmonic currents are functions of the TCR thyristors firing angles. The variation of the total voltage harmonic distortion as function of firing angle changes and location of nonlinear loads is clearly presented and discussed.

A smart bi-objective two-stage algorithm for optimal transmission switching without islanding

In a deregulated environment, optimal transmission switching (OTS) plays an important role in coordination among power generating companies (GENCOs) and transmission company (TRANSCO) by minimizing operating costs. This paper solves ac OTS (ACOTS) in two different stages sequentially with ac optimal power flow (ACOPF) and transmission switching through ac power flows (TS-ACPF) for optimal power generation and efficient power transmission, respectively. The mutual dependency along with the coordination among these optimal problems are treated intelligently by adopting a metaheuristic based two-stage algorithm with a multi-objective approach to achieve an effective solution through maximum utilization of power in the existing system or for future expansion planning. Production cost optimization problem is taken care of in the first stage while transmission loss is optimized in the second stage with successive iterations of the proposed algorithm. MATLAB simulations to optimize both transmission loss and production cost for both the IEEE-30 bus system and the Indian Utility 62-bus real system are experimented. Results are provided to validate the proposed technique which satisfies several cases of load variations, N − 1 standards and other combinations of stressed security constraint conditions. The detailed investigation of the results prove the effectiveness of the proposed intelligent two-stage method over the existing methods in terms of automatic and cost-effective bi-objective solution relying on the flexibility of switching strategy and security by avoidance of islanding without any manual interaction in between.