Black Start Capability Research Articles

Black-Start Capability of DFIG Wind Turbines Through a Grid-Forming Control Based on the Rotor Flux Orientation

The increasing penetration of renewable energies in the power system is demanding new services from wind farms. In particular, system operators are concerned about system restoration after a black-out and new grid codes are under consideration that demand the participation of wind farms in the restoration process. Black start requires the wind farms to be capable of controlling voltage and frequency in an isolated grid. This paper proposes the use of a novel grid forming control for doubly fed induction generators (DFIG) for restoring the system after a black-out. The proposed grid forming control uses the rotor flux orientation principle, that allows to generate an internal electromotive force for controlling in this way the terminal voltage and frequency. Then, it is demonstrated that the rotor flux magnitude and frequency can be controlled by means of the rotor voltage applied the rotor side converter of the DFIG. The paper also proposes a wind turbine control system for balancing the power demanded by the load at the wind turbine mechanical system. Black start capability of the proposed control system has been validated through a comprehensive simulation. The simulated system consists of a wind farm with two DFIG-based wind turbines connected in parallel to a point of common coupling that supplies an induction motor through a feeder. Also, the synchronization of the isolated system to a main grid has been simulated, demonstrating the ability of the proposed control system for successfully completing the restoration process.

Offshore Wind Farm Black Start Service Integration: Review and Outlook of Ongoing Research

A review of the ongoing research on black start (BS) service integrated with offshore wind farms (OWFs) is presented in this paper. The overall goal is to firstly gain a better understanding of the BS capabilities required by modern power systems. Subsequently, the challenges faced by OWFs as novel BS service providers as well as an outlook on the ongoing research which may provide solutions to these are presented. OWFs have the potential to be a fast and environmentally friendly technology to provide BS services for power system restoration and, therefore, to ensure resiliency after blackouts. As a power electronic-based system, OWFs can be equipped with a self-starter in the system in order to perform BS. The self-start unit could be a synchronous generator (SG) or a power electronic unit such as a grid-forming (GFM) converter. Preliminary BS studies performed in PSCAD/EMTDC are presented in a simplified OWF system via an SG as the self-start unit. Consequently, technical challenges during the BS procedure in an OWF benchmark system are outlined via theoretical discussions and simulations results. This is useful to understand the threats to power electronics during BS. Finally, the most relevant GFM strategies in the state-of-the-art literature are presented and their application to OWF BS is discussed.

The black-start capability improvement of VSC-based HVDC transmission system using fuzzy-adaptive PI controller

ABSTRACT The High Voltage DC Transmission (HVDC) transmission system commissioned in 1954 was planned for meeting the objectives like economy, faster recovery rate, frequency conversion, bulk power transmission, rapid controllability from sending and receiving end and so on. These objectives are difficult to achieve easily with the HVAC system. Among these, quicker reccovery rate, which is also called black start capability during fault and maintenance, is considered in this paper. This is achieved effectively when real power is controlled from sending end, while the dc link voltage is controlled at the other end. For this, better control schemes from both sending and receiving end, fast acting phase locked loop (PLL) and faster and intelligent adaptable controllers play a vital role. Hence, new PLL, fuzzy-adaptive PI with faster and effective control schemes are proposed to improve the black start rate of HVDC links. The performance of the proposed system is verified with traditional PI with conventional technique and with fuzzy-adaptive PI controller. The results are verified using MATLAB/SIMULINK software.

Analysis of Black Start of a Microgrid with PV, DG, and BESS

Different combinations of operating scenarios for a microgrid with distributed energy resources and energy storage system is considered to understand the operation of a microgrid. An operational strategy analysis of a microgrid system consisting of photovoltaics, diesel generator, and battery energy storage system during a black start in islanded mode is considered in this paper. BESS under study consists of a bidirectional converter and a battery system. BESS is assumed to be active as a solution provided in all the scenarios. The various capabilities of BESS in a microgrid system is also discussed. Microgrid system provides reliable power supply and hence black start capability for such a system is essential in keeping intact the advantages of a microgrid. Performing a black start requires a sequential process to be followed to avoid fluctuations in bus voltage, frequency, and protecting the fuses/ contactors from blowing. To black start the system under study, the DC breaker connecting the battery and the bidirectional converter needs to be closed. Closing the DC breaker results in high inrush current from batteries at the DC output of the bidirectional converter. A DC arrangement is envisaged in the battery energy storage system to avoid arcing due to high inrush current.

On Integration of Wind Power into Existing Grids via Modular Multilevel Converter based HVDC Systems

Due to the advent of advanced solid-state power devices and converters, it is now promising to transmit huge quantities of renewable energies to the main electrical grids. One of the major renewable energy resources is the wind power plant, which has the benefits of very low generation cost, simple infrastructure, lesser operating staff, lesser erection times, and negligible environmental pollution. Various far located power stations, including offshore and mainland wind power plants, as well as big industrial loads can be joined together with the existing AC grids by implementing multiterminal HVDC transmission systems. This recently established strong feature of HVDC systems is making them more feasible for future grids. The current source converter (CSC) is an established topology used in HVDC systems, worldwide. Rigorous research is being carried out on the voltage source converters (VSCs), especially on Modular Multilevel Converters (MMCs) based HVDC systems that are distinguished by their benefits of lower harmonics, lower maintenance, almost absence of bulky DC link capacitors and AC filters, black start capability, smooth control of powers, and addition of renewable energy using DC cables with the prevailing AC grids. In this paper, technical features associated with wind power plants and the interconnection of such plants via new MMC based HVDC systems with the present AC grids are discussed in detail. Mathematical modeling and performance analysis of advanced MMC technology is carried out by performing simulations in Matlab/Simulink ® . The deduced technical outcomes remarkably show the effective use of this converter in the near future grids.

Temporary emergency power supply using medium-voltage islanded microgrids – a modular concept for distribution system operators

Due to the continuous increase of distributed generation (DG) units at the distribution grid level, temporarily islanded microgrid (TIM) operation in case of a blackout becomes energetically feasible. DG units with black start capability and additional MVA-scale generation units in TIMs are a strong option for a fast intermediate supply. They create possibilities for a longer-term emergency supply for a large number of customers until the overall grid restoration (GR) is completed. To develop generalised concepts that are applicable for more than one individual medium-voltage grid, general requirements and conditions have to be analysed thoroughly and defined as generic as possible. Therefore, a modular concept is developed comprising individual modules for GR, energy management (EM), control, protection, communication and operation. The focus of this study is to present the developed modular concept with a special focus on EM and control approaches for TIMs. As a result, the exemplary application of the modular concept is successfully conducted. Finally, the modular concept provides recommendations for project planning and engineering for distribution system operators, which can easily be extended. Special requirements of the distribution grid operation can be defined and considered by this approach.

PMSG-Based Black-Start Technology and Its Field Tests

It is of great importance for power grids to have black-start capability for rapid recovery, and there is great theoretical significance and practical application value in studying how to use wind farms as the black-start power supply source for power grids with large-scale renewable energy generation. In this paper, a black-start scheme using a permanent-magnet synchronous generator (PMSG)-based wind farm as black-start power supply source is formulated. First, a diesel generator is used as an external supporting power supply for the self-start of a wind power unit (WPU). Then, after all the planned WPUs operate normally, the wind farm with the diesel generator and static var generator (SVG) is used to black start the simulated auxiliary load of a thermal power plant. A field test of the proposed black-start scheme is carried out on an actual wind farm in Jiangsu Province (China). The results of the field test show that wind farms can act as a black-start power supply source for the grid after appropriate technological transformation.

Closed loop control of voltage source converters during faults in high voltage DC transmission system

High voltage DC transmission is very cost effective alternate solution for high voltage AC transmission. High voltage DC transmission towers occupy less space and poles carry more current compared to high voltage AC system. High voltage DC transmission lines are emerging as an alternate solution for flexible AC transmissions (FACTS) technology. Earlier either thyristor valves or mercury valves were used in converters. Voltage source converter has replaced conventional converter in high voltage DC system because of their flexible control, black start capability, and they can work with weak AC system. Switching at higher frequency is also an added advantage. In voltage source converter-based high voltage DC transmission, both real and reactive power transmitted can be controlled whereas in conventional line commutated converter-based high voltage DC transmission system, only the real power transmitted was controlled. There may be chances of grid faults in sending end and receiving end stations. The typical faults are single line fault, double line to ground fault and three phase fault. The faults in one high voltage DC station should not affect the other end station. Necessary control should be initiated to take care of AC faults on both the sides that are before rectification in the sending end station and after inversion in the receiving end station. To handle such faults, the voltage source converters should be controlled to regulate the power transmitted through the line in such a way that the fault in one grid will not affect the parameters of the other grid. This paper presents closed loop control of both voltage source converters to take care of line to ground fault. The controller uses PQ theory. The behavior of high voltage DC system during fault without and with controller is presented in this paper. This simulation study is used to educate electrical transmission and faults detection to the trainees, they can easily understand.

A Study on the Black Start Process of a real Distribution Network with CHP plants and BESS

The power system experiences many changes, which are leading to new challenges. The electric energy will no longer be generated only by large power plants, but rather by numerous decentralized energy resources that are primarily installed in distribution grids. In particular, power system restoration processes after a blackout will face new challenges. However, new opportunities by the widespread use of renewable energy resources arise. In this paper, the black start capability of a real distribution grid in Germany, operated by SWW Wunsiedel, will be investigated, as it has a high share of decentralised energy resources and a BESS with black start capability. For this purpose, the medium voltage level of the distribution grid is investigated with static load flow simulations to evaluate the best black start strategy. The driving forces of this black start concept are an 8 MW BESS unit and a 13.5 MW CHP plant.

Optimal generation units start-up sequence during restoration of power system considering network reliability using bi-level optimization

During recent years power systems are operated near the nominal capacity of system equipment with low stability margin. Operation of power systems in this condition is extremelyrisky and power systems lose their stability with any intense failure of system equipment. In bulk power system, it is more critical and may cause a partial or overall blackout. The capability of black start to bring back system to a normal condition in the case of partial or overall shut down is very important in each power system. When a shutdown occurs, power plants with black start capability supply cranking power for non-black start power plants, pick up critical loads and energize required transmission lines. These actions should be done in minimum time interval to maximize system provided energy during the restoration process. Most important decision making during restoration process is the determination of start-up sequence of generation units. During the restoration process, units with black start capability (BS units) start at the beginning of the process to provide cranking power for non-black start units (NBS units). Hence the determination of NBS units is decision variable in the restoration problem. In this paper, this problem has been described as a bi-level optimization problem which in upper level determines the optimal start-up sequence of NBS units by using a Teaching-Learning Based Optimization algorithm and in lower level determines the optimal transmission path with minimum number of switching and maximum reliability between any two necessary buses using the searching path graph-based algorithm. The proposed approach has been implemented successfully on IEEE 24-bus RTS and IEEE 118-bus test systems.

Controlled transition bridge converter: Operating principle, control and application in HVDC transmission systems

This paper employs an amplitude modulation with sinusoidal plus third harmonic injection instead of trapezoidal modulation to operate a controlled transition bridge (CTB) converter as ac/dc and dc/ac converter terminals. With such an operation, the CTB converter may require small ac filters; thus attractive for high-voltage direct current (HVDC) transmission systems. To facilitate ac voltage control over a wide range and black-start capability, the injected 3rd harmonic allows the cell capacitor voltages of the CTB converter to be regulated independent of the modulation index and power factor. The insertion of 3rd harmonic into modulating signals achieves two objectives: extends the regions around voltage zeros so that the total voltage unbalanced can be distributed between the cell capacitors, thereby exploiting the bipolar capability of the full-bridge cells in each limb; and to ensure that each limb can be clamped to the positive and negative dc rails every half fundamental period independent of the modulation index to allow recharge of the cell capacitors from the active dc link. The suitability of the CTB converter for HVDC type applications is demonstrated using a two-terminal HVDC link that employs a 21-cell CTB converter, considering normal operation and ac faults.

Hybrid HVDC (H2VDC) System Using Current and Voltage Source Converters

This paper presents an analysis of a new high voltage DC (HVDC) transmission system, which is based on current and voltage source converters (CSC and VSC) in the same circuit. This proposed topology is composed of one CSC (rectifier) and one or more VSCs (inverters) connected through an overhead transmission line in a multiterminal configuration. The main purpose of this Hybrid HVDC (H2VDC), as it was designed, is putting together the best benefits of both types of converters in the same circuit: no commutation failure and system’s black start capability in the VSC side, high power converter capability and low cost at the rectifier side, etc. A monopole of the H2VDC system with one CSC and two VSCs—here, the VSC is the Modular Multilevel Converter (MMC) considered with full-bridge submodules—in multiterminal configuration is studied. The study includes theoretical analyses, development of the CSC and VSCs control philosophies and simulations. The H2VDC system’s behavior is analyzed by computational simulations considering steady-state operation and short-circuit conditions at the AC and DC side. The obtained results and conclusions show a promising system for very high-power multiterminal HVDC transmission.

A new optimal contingency-based design for placement of gas turbines to enhance black-start capability

A new optimal contingency-based design for placement of gas turbines to enhance black-start capability

A Parallel Restoration for Black Start of Microgrids Considering Characteristics of Distributed Generations

The black start capability is vital for microgrids, which can potentially improve the reliability of the power grid. This paper proposes a black start strategy for microgrids based on a parallel restoration strategy. Considering the characteristics of distributed generations (DGs), an evaluation model, which is used to assess the black start capability of DGs, is established by adopting the variation coefficient method. Thus, the DGs with good black start capability, which are selected by a diversity sequence method, are restored first in parallel under the constraints of DGs and network. During the selection process of recovery paths, line weight and node importance degree are proposed under the consideration of the node topological importance and the load importance as well as the backbone network restoration time. Therefore, the whole optimization of the reconstructed network is realized. Finally, the simulation results verify the feasibility and effectiveness of the strategy.

Quasi-dynamic model of VSC-HVDC transmission systems for an operator training simulator application

This paper proposes a simplified quasi-dynamic model of two terminal voltage-source-converter (VSC)-HVDC transmission systems based on the existing full-order models for application in an operator training simulator (OTS). The simplified model is verified against the full-order model for some verification scenarios. The model stability and behaviour is assessed to check if the model meets the grid code requirements as stated in the ENTSO-E Network Code. These features include black-start capability, set point change, active power reversion, frequency contributions and reactive power-voltage regulations. The proposed linearized model is presented in state-space equations and is stabilized by state space feedback using the linear quadratic regulator (LQR) design. The model has sufficient accuracy, requires less input data compared to full-order model, is tuneable by any vendors and is easily implementable in an environment such as OTS.

Blackstart capability planning for power system restoration

Blackstart capability is essential for power system restoration following a blackout. System restoration planners determine the restoration sequences to provide cranking power from blackstart units (BSUs) to non-blackstart units (NBSUs), pick up critical loads, and energize the necessary transmission paths. This paper proposes a new algorithm for optimization of the restoration actions. An optimal search algorithm is proposed to determine the plan to crank NBSUs through the selected paths of transmission lines. Assuming that the generation capability of a BSU is constant, the method is used to optimize the overall system MWHr generation capabilities from NBSUs. To reduce the computational complexity of system restoration planning, a new generator model is proposed that results in a linear integer programming (IP) formulation. Linearity of the IP problem formulation ensures that the global optimality is achieved. The optimal power flow (OPF) is used to examine the feasibility of planned restoration actions. Test cases from the IEEE 39-bus system, ISO New England system, and Duke-Indiana system are used to validate the proposed algorithm. Numerical simulations demonstrate that the proposed method is computationally efficient for real-world power system cases.

基于MAS的校园微电网黑启动策略

利用MAS技术，以某高校校园微电网为模型，我们设计了一种微电网孤岛运行时黑启动策略。以单轴微型涡轮机为主启动电源，燃料电池为参考电源，具有黑启动能力的分布式电源同时启动，接入各自的受保护负荷，形成多个子微电网。再根据参考电源的电压、频率，各子网络同步后并联运行。黑启动最后阶段无黑启动能力的分布式电源从低压电网吸收功率启动，各分布式电源在微电网中央管理层的调配下改变控制方式，协调电源的输出功率，使微电网持续、稳定、高效地运行。通过Matlab/Simulink平台上搭建仿真模型，仿真结果验证了该黑启动策略能独立自主、稳定快速地实现微电网故障瘫痪后的黑启动。 A strategy of black-start based on MAS is proposed for isolated microgrid, which builds in a uni-versity campus. The SSTM is used as chief start-up power; fuel cell power provides reference and voltage. Distributed power with black-start capabilities starts at the same time; every source sup-plies the power to the load which they protect. According to the voltage and frequency from the fuel cell, every subnetwork runs synchronously. At last, the distributed power without black-start capabilities gets power from the low-voltage grid to start-up. Every distributed power changes their control method under the control of MGCC agent, the MGCC coordinates the power to make microgrid continuously, stably and efficiently. Based on the Matlab/Simulink platform, built the simulation model, the simulation results verify that the black-start strategy can achieve the goal of black start independently and stably after micro-grid fails.

The New Black Start: System Restoration with Help from Voltage-Sourced Converters

The very first commercial high-voltage dc (HVdc) transmission system, commissioned in 1954, had black-start capability. This system links the island of Gotland in the Baltic Sea with the Swedish mainland. Gotland had very few generating units, and the HVdc system was required to start up and deliver power without any generation operating on Gotland. Because the HVdc stations used line-commutated converters (LCCs), synchronous condensers were necessary to start up the dc system with a dead receiving network. This complicated the start-up sequence for system restoration.

Evaluation of a payments proposal for blackstart services in the Colombian electricity system: A system dynamics approach

All electrical power systems require some contingency arrangements to enable a restart in the unlikely event that all or a key part of the system shuts down. The process of restoring the power system is commonly referred to as blackstart. Not all power generators have, or are required to have blackstart capability. We propose a market-based scheme for blackstart services in the Colombian electricity sector where it has been an unregulated ancillary service. In this study, a system dynamics model is developed to better understand and analyze the behaviour of such a market in the long term. The developed simulation model uses a 20-year planning horizon and the analysis covers most elements of the proposed market for provision of blackstart services, including price behaviour, investment, installed capacity, and potential shortfalls. The simulation results show that the proposal for a dynamic market for blackstart services is stable and viable over the time period investigated.

New Breed of Network Fault-Tolerant Voltage-Source-Converter HVDC Transmission System

This paper proposes a new breed of high-voltage dc (HVDC) transmission systems based on a hybrid multilevel voltage source converter (VSC) with ac-side cascaded H-bridge cells. The proposed HVDC system offers the operational flexibility of VSC-based systems in terms of active and reactive power control, black-start capability, in addition to improved ac fault ride-through capability and the unique feature of current-limiting capability during dc side faults. Additionally, it offers features such as smaller footprint and a larger active and reactive power capability curve than existing VSC-based HVDC systems, including those using modular multilevel converters. To illustrate the feasibility of the proposed HVDC system, this paper assesses its dynamic performance during steady-state and network alterations, including its response to ac and dc side faults.