Conventional AC Grid Research Articles

Dynamic Analysis of VSC-HVDC System with Disturbances in the Adjacent AC Networks

VSC-HVDC systems are widely used to integrate wind farms, asynchronous generations and networks operating at different frequencies. The Multi-terminal (MT) and multi-fed (MF) HVDC’s are the system mainly constituted of VSC’s, to integrate renewable sources and transmitting bulk power to conventional AC grids. A sudden change in the steady state even in adjacent networks may create severe disturbances in the operation of such HVDC systems. The disturbances in AC or DC networks directly influence the performance of systems, particularly in MT-HVDC and MF-HVDC systems. However, the HVDC systems are known for their intelligent control in modulating operational states as and when required. This paper presents the dynamic analysis of MF-HVDC system due to load changes, faults and other disturbances in the adjacent AC networks. The result indicates that VSC-HVDC provides decoupled control of active and reactive power with capability in adjusting operational mode during various minor and major disturbances. Based on the results obtained, the paper proposed a novel sensitivity factor indicating percentage coupling among various line parameters during disturbances. Furthermore, the VSC’s injects harmonic signals on both AC and DC sides of HVDC system. These harmonics voltage or currents signals may get amplified to a dangerously high magnitude at resonance frequencies. Thus, the frequency characteristics of different subsystems are also analyzed using FFT. A ±100 kV, 200 MW bipolar MF VSC-HVDC test systems is used to simulated the results in MATLAB/Simulink software.

Transient Stability Improvement of Large-Scale Photovoltaic Grid Using a Flywheel as a Synchronous Machine

The global climate protection policy aimed at achieving a zero greenhouse gas emissions target has led to the fast incorporation of large-scale photovoltaics into the power network. The conventional AC grid was not modeled to be incorporated with large-scale non-synchronous inverter-based energy resources (IBR). Incorporating inertia-free IBR into the grid leads to technical issues such as the degradation of system strength and inertia, therefore affecting the safety and reliability of the electrical power system. This research introduced a new solution to incorporate a flywheel in the rotor of a synchronous machine to improve the dynamic inertia control during a system disruption and to maintain the constancy of the system. The objective of this work is to enhance large-scale photovoltaic systems in such a way that they can avoid failures during a fault. A model of transient constancy with two synchronous generators and a LSPV is established in PowerWorld modeling software. A line-to-ground and three-phase fault are simulated in a system with up to 50% IBR penetration. The outcomes showed that the power network was able to ride through faults (RTFs) and that the stability of frequency and voltage are enhanced because of a flywheel that improved grid inertia and strength.

Multiple objective optimization based on particle swarm algorithm for MMC-MTDC system

Multi-terminal high voltage DC (MTDC) network is an effective technology to integrate large-scale offshore wind energy sources into conventional AC grids and improve the stability and flexibility of the power system. In this paper, firstly, an analytical model of a general applicable MTDC system integrated with several isolated AC grids is established. Then, an improved AC-DC power flow algorithm is used to eliminate the additional DC slack bus or droop bus iteration (SBI/DBI) step of the conventional AC-DC sequential power flow. A multi-objective optimal power flow (MOPF) algorithm is proposed to minimize two optimization targets, i.e., overall active power loss and generation costs of the system. To increase the degree of freedom, adaptive droop control is used in the proposed optimization algorithm in which the voltage references and droop coefficients of the modular multilevel converters (MMCs) are control variables. A multiple objective particle swarm optimization (MOPSO) method is applied to solve the MOPF problem and achieve the Pareto front. A technique for order of preference by similarity to ideal solution (TOPSIS) is incorporated in the decision analysis section and helps the decision maker to identify the best compromise solution.

Adaptive control-based Isolated bi-directional converter for G2V& V2G charging with integration of the renewable energy source

In this research paper, Adaptive control-based Isolated bi-directional converter for modernized electric vehicle system is introduced with control on charge and discharge of the electric vehicle (EV) battery as per the reference value. The EV battery is operated in two modes, such as charge (Grid-to-Vehicle) mode and discharge (vehicle-to-grid) mode. In charging mode, constant current and constant voltage (CCCV) control is adopted with control on charging voltage and current. Whereas in discharging mode, constant current (CC) control is adopted for control on discharge current from battery into the grid. The grid is also interconnected to PV system for renewable energy power sharing. In both the control techniques, ANFIS is integrated for better performance of the battery during charge and discharge conditions. The ANFIS takes input from error generated by comparison of reference and measured value and produces the required duty ratio value. The training of the fuzzy structure is done using data generated from the PI controller input and output. The training uses hybrid algorithm for adapting the data points considered from the PI controller. The EV battery is charged using the conventional AC grid and the renewable energy source in G2V mode, and the EV battery and renewable energy sources are used to supply power to the AC load in the V2G mode, which reducing the load demand on the AC grid. The DC-Link voltage and power to the AC load are kept constant in both modes of operation by using a synchronous reference frame controlled VSC and an ANFIS controlled dual active full bridge converter. The incremental conductance MPPT algorithm is used to control the Photo Voltaic Array (PVA). The proposed work is implemented using MATLAB/Simulink and HIL performance of the proposal system is examined in the RT-LAB OP-5700 real time simulator environment.

A Power Control Method for Hybrid Electrical Accommodation Systems

This paper presents a power control method for a hybrid electrical system, which enhances a conventional main AC grid by adding a DC sub-grid in parallel. In this system, each prosumer connects to the sub-grid via a couple of half-bridge inverters of a device called a power router. All of the power routers connected to the sub-grid are controlled such that they maintain the voltage of the sub-grid equally, which is an essential feature of peer-to-peer power accommodation without synchronization. This paper proposes an adaptive hysteresis current-control technique for controlling sub-grid-connected inverters that maintains the voltage of the sub-grid while accommodating a given power. The proposed control method also balances currents between positive and negative lines of the sub-grid simultaneously. The proposed control method is implemented on a high-speed FPGA-based digital controller and is evaluated on three prototypes of the power router. The experimental results show that the proposed method yields fast and stable responses and enables the power router to accommodate power with a high accuracy.

Finite control set model predictive control integrated with disturbance observer for battery energy storage power conversion system

A typical battery energy storage system consists of a combination of battery packs and a grid-tied power conversion system. The control algorithm of the power conversion system plays an important role when interfacing the DC energy stored in battery packs with the conventional AC grid to generate an obedient bidirectional power flow. Finite control set model predictive control is believed to be one of the most effective choices for controlling power conversion systems. However, the performance of such a control strategy heavily depends on the accuracy of the predictive model. Parameter mismatch in the model leads to prediction error, which deteriorates the overall power quality performance of the power conversion system. Therefore, this paper studies a robust finite control set model predictive control method based on a discrete disturbance observer to eliminate the negative effects caused by model inaccuracy and uncertainty. The stability issue of the additional observer is discussed from the perspective of closed-loop poles. Parameter scan is performed to provide assistance in designing the feedback matrix. Finally, simulations and experimental results obtained from a downscaled prototype rated at 4.2 kVA are conducted as a validation of the presented control algorithm.

Design and Modeling of Hybrid DC/AC Microgrid With Manifold Renewable Energy Sources

The increased demand and depletion of the fossil fuels for power generation led to the need for extracting power from the renewable energy resources (RERs). The microgrids (MGs) are designed with the help of effective power extracted from renewable sources such as solar, wind, tidal, and geothermal. The advent of DC MGs overcomes the conventional AC grids. The hybridization of the AC and DC MGs will provide more advantages for various levels of consumers. This article proposes the design and modeling of a hybrid DC/AC MG with the efficient use of RERs and it can reduce numerous power conversions. The solar energy is extracted through photovoltaic (PV) panels meritoriously using interval type 2 fuzzy logic technique as the maximum power point tracking algorithm. The AC grid is designed using wind energy source and tidal energy. The permanent magnet synchronous generator is used as the wind turbine. Various control mechanisms are employed in order to extract maximum power from the wind and tidal waves at varying conditions. These generated powers can supply the load and are connected to the utility grid. These are executed with the aid of MATLAB/SIMULINK software.

Minimization of AC-DC Grid Transmission Loss and DC Voltage Deviation Using Adaptive Droop Control and Improved AC-DC Power Flow Algorithm

Minimization of the total transmission loss of an interconnected AC-DC grid plays an important role for the economic operation of the AC-DC grid. Different from the conventional AC grid where the transmission loss is usually minimized by reactive power regulation, the transmission loss of a meshed AC-DC grid can be optimized by adjusting the active power exchange between the AC and DC grids. Additionally, smaller DC voltage deviation after grid disturbances is very desirable since it can bring less impact to the operations of AC-DC grid. This paper firstly presents two improved sequential power flow algorithms for modular multilevel converters (MMCs) based AC-DC grid under DC power-voltage droop control. An optimization algorithm is then proposed to minimize the total loss of the AC-DC grid and the overall DC voltage deviation after the change of operating conditions. Adaptive droop control is used in the proposed optimization algorithm in which the power references are control variables solved from the optimal AC-DC power flow. The proposed algorithm is verified in an AC-DC grid consisting of a six-terminal DC grid connected to the IEEE 39-bus AC grid and a classical five-terminal AC-MTDC system.

Decentralized Plug-and-Play Protection Scheme for Low Voltage DC Grids

Since the voltages and currents in dc grids do not have a natural zero-crossing, the protection of these grids is more challenging than the protection of conventional ac grids. Literature presents several unit and non-unit protection schemes that rely on communication, or knowledge about the system’s topology and parameters in order to achieve selective protection in these grids. However, communication complicates fast fault detection and interruption, and a system’s parameters are subject to uncertainty and change. This paper demonstrates that, in low voltage dc grids, faults propagate fast through the grid and interrupted inductive currents commutate to non-faulted sections of the grid, which both can cause circuit breakers in non-faulted sections to trip. A decentralized plug-and-play protection scheme is proposed that ensures selectivity via an augmented solid-state circuit breaker topology and by utilizing the proposed time-current characteristic. It is experimentally shown that the proposed scheme provides secure and selective fault interruption for radial and meshed low voltage dc grids under various conditions.

Maritime DC Power System With Generation Topology Consisting of Combination of Permanent Magnet Generator and Diode Rectifier

The dc grid is emerging as a new electric power distribution concept for several types of vessels due to its higher fuel efficiency compared with the conventional ac grid. Until today, wound rotor generators (WRGs) and diode rectifiers have been applied to the generation part of the onboard dc grid although the efficiency, weight, reliability, and footprint of the WRGs are inferior to those of permanent magnet generators (PMGs). In this article, the possibility of adopting the PMG in the onboard dc grid instead of the WRG is studied. According to the analysis, even though the PMG is inherently incapable of adjusting rotor flux, the rectified dc voltage can be controlled in a certain range under all load conditions. Simultaneously, the diesel engine can be controlled near its optimal operating line (OOL) to minimize fuel consumption. The controllability of the rectified dc voltage of the PMG is verified by computer simulation and small-scaled experimental tests. When comparing the operating trace of the proposed system to the specific fuel oil consumption (SFOC) map of a 350-kW diesel engine, i.e., AD158TI, the fuel-saving by applying the PMG is expected to be more than 10% at light load condition and around 3% average in overall operating conditions.

DC Marine Power System: Transient Behavior and Fault Management Aspects

DC marine vessels with medium-voltage compact dc power systems are dominated by a significant amount of active loads and a finite number of generation sources. In such scenarios, the network configuration of the dc power system is expected to get dynamically altered to fulfill the required generation and load demands for the desired marine mission. Such varying network configurations make the transient responses significantly different from the conventional ac grids and the prospective dc grids. In this regard, this paper performs systematic transient studies to devise fault management strategies for the dc marine vessels. A platform supply vessel (PSV) is taken as an example of the marine vessel, due to its complex operating scenarios and wider applicability in the marine industry. Pole-to-pole short-circuit faults are considered owing to its severity. A novel current-only directional protection for the dc PSV is proposed based on the directional zonal interlocking and short-time Fourier transform. The efficacy of the proposed method is substantiated by confirming against a range of fault impedances initiated at the generator terminals, load terminals, lines, and buses of the dc PSV. All the analyses are conducted in the real-time simulation model of the dc PSV.

Damping Performance Analysis and Control of Hybrid AC/Multi-Terminal DC Power Grids

The voltage-source converter (VSC) based multi-terminal direct-current (MTDC) networks are currently being developed in large-scale power grids for efficient and economical transmission of electrical energy generated from remotely sited renewable energy sources (RESs). As the MTDC network is developed alongside the conventional AC grid, it has transformed the power grid to a hybrid AC/MTDC power network. However, since the MTDC network is controlled via the power electronic converters (PECs), it significantly affects the AC power grid damping and synchronising performance as it decouples the natural dynamics between various electro-mechanical systems in the power grid. This paper proposes a supplementary control scheme to improve the damping performance of the entire hybrid AC/MTDC power grid. The damping torque analysis (DTA) technique is used as the primary technique to develop the supplementary control scheme. The preliminary analysis is carried out using a two-terminal hybrid AC/DC power grid and has suggested a supplementary control loop based on the rotor speed deviation for the DC voltage controller of the VSC to improve the damping performance. With the state-space model, the synchronising and damping torque coefficients are calculated and accordingly, the feedback gain is determined to provide optimal synchronising and damping torque components. Subsequently, the fidelity of the supplementary controller is verified using a four-terminal hybrid AC/MTDC grid. Simulation studies prove that the proposed supplementary controller can improve the hybrid AC/MTDC network damping performance, and it performs very effectively with the master-slave control and the conventional droop control scheme.

HVDC transmission for access to off‐shore renewable energy: a review of technology and fault detection techniques

The ever-increasing demand for electric power has partially been met with access to offshore renewable energy, such as wind and tidal energy. With the development in power electronics, high-voltage direct current (HVDC) transmission is taking over as the primary choice for connecting off-shore generation to on-shore grids. Voltage source converter (VSC)-based HVDC is set to become the backbone of the multi-terminal DC grids replacing the conventional line commutated converter networks. VSC-HVDC networks offer the flexibility for HVDC grids to be connected as conventional AC grids in a meshed network. Advancement of technology has led to the development of the modular multilevel converter which has higher efficiency compared to the two-level VSC configurations. They are gradually becoming a popular choice. Although VSC-based grids offer a varied range of advantages, it is highly vulnerable to DC faults. Many designs of breakers have been patented over the years and various means have been proposed for their control. This study aims to review the available designs of HVDC terminals, the available protection devices and the protection and control methods for HVDC breakers. By comparing the state-of-the-art technologies that are currently available, this study aims to address the research issues and the additional research and development work that needs to be done.

MMC-HVDC 시스템을 위한 부하 전압변동에 강인한 H<SUB>∞</SUB> 제어기 설계

In this paper, load variations in an HVDC system which is connected to an AC grid are considered. As the demand for the introduction of renewable energy increases, stable connections with the conventional AC grid are an important issue. To secure stable connections, a converter system should operate robustly against system noise. Since power systems have much uncertainty and noise, a robust control technique is needed for the adoption of renewable energy sources. First, an MMC based converter system is modeled mathematically. Based on this mathematical model, a linearized model is obtained for the operating point in active and reactive power tracking modes. By using the dq0 coordinate system, a generalized plant is obtained, and a robust HSUB∞/SUB controller for the converter system is designed for voltage disturbances. The designed controller is validated by MATLAB simulation.

Modified adaptive differential evolution based optimal operation and security of AC-DC microgrid systems

The recent advancement in power electronic systems has enabled the formation of AC-DC microgrids by integration of DC microgrids into conventional AC grid. In this paper, an approach for optimal power flow (OPF) problem formulation in AC-DC microgrid networks is proposed. In the formulated AC-DC microgrid OPF problem, the objective functions considered for minimization are the total cost of generation in both the AC and DC networks and the cost of active power transfer from the AC grid to the DC microgrid. The OPF problem is subject to the limits of voltage magnitudes, power flows, and the AC-DC power electronic converter's limits. This paper also presents an approach to reformulate the nonlinear AC-DC OPF problem as an equivalent traditional AC OPF problem. A modified adaptive differential evolution algorithm is proposed to solve the non-convex AC OPF problem, Simulation studies are performed on an IEEE 30-bus and IEEE 39-bus test system connected to 6-bus and 9-bus DC microgrids under both normal operation and network contingency conditions. The results obtained on the test systems by the proposed approach confirm the validity of the developed approach.

Internal converter- and DC-fault handling for a single point grounded bipolar MMC-HVDC system

In order to address challenges caused by an increasing need for power transmission, embedded bipolar HVDC links utilizing Modular Multilevel Converter (MMC-HVDC) technology present a preferred solution as several operational and dynamic advantages compared to conventional ac grid enforcement are provided. In this paper, an approach to quickly detect and handle internal as well as dc-side faults in a full-bridge bipolar MMC-HVDC link is presented. Due to non-technical reasons and environmental constraints, the system is comprised of mixed overheadline-cable (OHL-cable) transmission and single point solid grounding with dedicated metallic return (DMR). To validate and evaluate the proposed concept, detailed electromagnetic transient (EMT) simulations are provided. These highlight effects of mixed transmission system design on transient voltage and current stresses as well as on converter-internal quantities.

DC nanogrid for renewable sources with modular DC/DCLLCconverter building block

Centralised electricity systems are being integrated by a multitude of small distributed generation clusters, called nanogrids, capable of ensuring full utilisation of renewable energy sources, high flexibility, efficiency and improved reliability of the power grid due to redudancy. In this study, small photovoltaic plants combined with energy storage systems represent a promising solution for full exploitation of renewable energies and for effective development of electromobility. The purpose of this study is two-fold: first it deals with a DC nanogrid suitable for small energetic clusters such as residential or office buildings and consisting of photovoltaic arrays, fuel cells, electric vehicle charger stations, loads, then proposes a multifunctional full-bridge DC/DC LLC resonant converter building block. The latter, while accepting large input/output ranges is suitable for photovoltaic and fuel cells energy conversion, electric vehicle recharging stations and other DC applications available in the nanogrid. Proposed power system can be interconnected with conventional AC grid through voltage source converter. After description of proposed nanogrid, some analyses are performed on its core element, i.e. the DC converter building block.

Are Microgrids the Future of Energy?: DC Microgrids from Concept to Demonstration to Deployment

Microgrids are defined as groups of energy resources, both renewable and/or conventional, and loads located and interconnected in a specific physical area that appear as a single entity to the alternating-current (ac) electric grid. The use of distributed resources to power local loads combined with the capability to operate independently of the ac grid makes microgrids a technically feasible option to address the concerns of sustainability, resilience, and energy efficiency. Furthermore, microgrids can operate while completely separated from the grid, representing a lower-cost option to provide electrical power to regions in developing countries where conventional ac grids are not available or are too unreliable. When connected to the ac grid, microgrids appear as controlled entities within the power system that, instead of being a burden to the ac grid power-management system, represent a resource capable of supporting the grid. Energy storage as the element responsible for balancing generation with load is critical to the success of the microgrid concept, and it is more important as larger penetration of renewable resources is present in the microgrid. Accelerated improvements in performance and cost of energy-storage technologies during the last five years are making microgrids an economically viable option for power systems in the very near future (see Figure 1).

직류형 마이크로그리드의 전운전영역을 고려한 협조제어

Recently, one of the main research on the power distribution system is the microgrid. The microgrid is a combination of power sources and loads, which is controllable and has separable connection. The main objective of microgrid is the deployment of the renewable clean energy and the enhancement of load-side reliability. The modern power sources and loads have DC I/O interfaces, which is the major advantage of DC microgrid compared to the conventional AC grid. The components in the microgrid have diverse features, so there is need of proper coordination control. For achieving economic feature, the active power of renewable energy resources is regarded as major control parameter and the whole operation modes of DC microgrid are defined, and the proper operations of each component are described. From the inherent characteristics of DC, there are two control variables: voltage and active power. Through analysis of operation modes, it is possible to determine exact control objectives and optimized voltage & power control strategy in each mode. Because of consideration of whole operation modes, regardless of the number and capacity of components, this coordination control method can be used without modification. This paper defines operation mode of DC microgrid with several DC sources and suggests economic and efficient coordinated control methods. Simulation with PSCAD proves effectiveness.

Stability Analysis of High-Power DC Grids

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> During the last several years, increasing effort has been spent to advance offshore wind power. So far, only small test fields have been realized, and the electrification of offshore wind farms still remains a research topic. Currently, offshore wind farms are equipped with three-phase ac grids, some of them will be connected to the transmission grid via a high-voltage direct-current (dc) link. Consequently, a dc system encompassing the whole wind farm might constitute an alternative to a conventional ac grid. In this paper, the transient stability of such a dc grid is analyzed. Based on traveling wave theory, critical frequencies and cable lengths are calculated analytically. Furthermore, a large dc grid for an offshore wind farm is simulated. The goal is to provide guidelines that support stable operation of an offshore dc grid. </para>