Coordinated control strategy for a PV-storage grid-connected system based on a virtual synchronous generator

Due to the characteristics of intermittent photovoltaic power generation and power fluctuations in distributed photovoltaic power generation, photovoltaic grid-connected systems are usually equipped with energy storage units. Most of the structures combined with energy storage are used as the DC side. At the same time, virtual synchronous generators have been widely used in distributed power generation due to their inertial damping and frequency and voltage regulation. For the PV-storage grid-connected system based on virtual synchronous generators, the existing control strategy has unclear function allocation, fluctuations in photovoltaic inverter output power, and high requirements for coordinated control of PV arrays, energy storage units, and photovoltaic inverters, which make the control strategy more complicated. In order to solve the above problems, a control strategy for PV-storage grid-connected system based on a virtual synchronous generator is proposed. In this strategy, the energy storage unit implements maximum power point tracking, and the photovoltaic inverter implements a virtual synchronous generator algorithm, so that the functions implemented by each part of the system are clear, which reduces the requirements for coordinated control. At the same time, the smooth power command is used to suppress the fluctuation of the output power of the photovoltaic inverter. The simulation validates the effectiveness of the proposed method from three aspects: grid-connected operating conditions, frequency-modulated operating conditions, and illumination sudden-drop operating condition. Compared with the existing control strategies, the proposed method simplifies the control strategies and stabilizes the photovoltaic inverter fluctuation in the output power of the inverter.     

A review on distributed generation impacts on electric power system

In 2021, the world's total installed capacity of generation units based on renewable energy sources (not including hydropower) amounted to about 1674 GW: over 825 GW and 849 GW of wind and solar power plants were installed respectively. The growing of the installed capacity of these distributed generators is a response to the increasing the power consumption, global environmental issues and has also become possible due to the development of technology in field of power semiconductor devices. However, on the way of large-scale implementation of distributed generators based on renewable energy sources, traditional electric power system meets new challenges to ensure the reliability and sustainability of new electric power systems with renewable energy sources. In particular, distributed generators change processes in the electric power system, impact to the parameters and power balance, change the magnitude and direction of power flow and short-circuit current, which determines the need to update the settings of the relay protection and automation systems of traditional electric power system and to coordinate their operation with automatic control systems of installed distributed generators. The above-mentioned tasks form a number of scientific research directions, one of which is a task of determining optimal size and location of distributed generators. The main purpose of this optimization task is to reduce power losses, operating and total electricity cost, improve the voltage profile, etc. In addition, the correct and reasonable placement of distributed generators defines an effective planning of the operating modes of electric power system and power plants (especially based on renewable energy sources, the operating modes of which depend on weather conditions and can be sharply variable).The paper highlighted the impacts of distributed generators on power losses, the voltage level, maintaining the power balance and the possibility of participating in the frequency regulation, and short-circuit current in power system. The optimization criteria, the main limiting conditions, as well as methods for solving this optimization problem are considered. This review will help the System operators and investing companies, especially in Russia, to form the main aim, objective function and constraints that will aid to meet their load demand at minimum cost and to choose from the options available for optimization of location and capacity of distributed generators.     

VSC-based direct torque and reactive power control of doubly fed induction generator

This paper proposes a novel direct torque and reactive power control (DTC) for grid-connected doubly fed induction generators (DFIGs) in the wind power generation applications. The proposed DTC strategy employs a variable structure control (VSC) scheme to calculate the required rotor control voltage directly and to eliminate the instantaneous errors of active and reactive powers without involving any synchronous coordinate transformations, which essentially enhances the transient performance. Constant switching frequency is achieved as well by using space vector modulation (SVM), which eases the designs of power converter and ac harmonic filters. Simulated results on a 2 MW grid-connected DFIG system are presented and compared with those of the classic voltage-oriented vector control (VC) and traditional look-up-table (LUT) direct power control (DPC). The proposed VSC DTC maintains enhanced transient performance similar to the LUT DPC and keeps the steady-state harmonic spectra at the identical level as the VC strategy when the network is strictly balanced. Besides, the VSC DTC strategy is capable of fully eliminating the double-frequency pulsations in both the electromagnetic torque and the stator reactive power during network voltage unbalance.     

Reactive power control of wind farms for voltage control applications

In the last few years, there is a strong trend towards decentralised production and supply, leading to a situation where a growing number of small and medium size producers will be connected to energy networks. But at the same time, the power quality of the generation must be ensured and this means that the electrical parameters of the distribution network have to be maintained within their upper and lower limits. Therefore, new problems related to the management and operation of energy transfer and distribution and to the efficient distribution of renewable energy in the grids are actually arising. Hence, it is reasonable to think that dispersed generation (wind energy generation in this paper) should start to take part in the control of electric variables, and in particular, in reactive power control which is directly related to the voltage level control of distribution networks. This paper presents a control strategy developed for the reactive power regulation of wind farms made up with double fed induction generators, in order to contribute to the voltage regulation of the electrical grid to which farms are connected.     

Evaluation of intermittent-distributed-generation hosting capability of a distribution system with integrated energy-storage systems

The penetration rate of distributed generation is gradually increasing in the distribution system concerned. This is creating new problems and challenges in the planning and operation of the system. The intermittency and variability of power outputs from numerous distributed renewable generators could significantly jeopardize the secure operation of the distribution system. Therefore, it is necessary to assess the hosting capability for intermittent distributed generation by a distribution system considering operational constraints. This is the subject of this study. An assessment model considering the uncertainty of generation outputs from distributed generators is presented for this purpose. It involves different types of regulation or control functions using on-load tap-changers (OLTCs), reactive power compensation devices, energy storage systems, and the reactive power support of the distributed generators employed. A robust optimization model is then attained It is solved by Bertsimas robust counterpart through GUROBI solver. Finally, the feasibility and efficiency of the proposed method are demonstrated by a modified IEEE 33-bus distribution system. In addition, the effects of the aforementioned regulation or control functions on the enhancement of the hosting capability for intermittent distributed generation are examined.     

Trends of distributed generation development in Lithuania

The closure of Ignalina Nuclear Power Plant, impact of recent global recession of the economy, as well as changes and problems posed by the global climate change require significant alterations in the Lithuanian energy sector development. This paper describes the current status and specific features of the Lithuanian power system, and in particular discusses the role of the distributed generators. Country's energy policy during last two decades was focused on substantial modernisation of the energy systems, their reorganisation and creation of appropriate institutional structure and necessary legal basis. The most important factors stimulating development of distributed generation in Lithuania are the following: international obligations to increase contribution of power plants using renewable energy sources into electricity production balance; development of small (with capacity less than 50 MW) cogeneration power plants; implementation of energy policy directed to promotion of renewable energy sources and cogeneration. Analysis of the legal and economic environment, as well as principles of regulation of distributed generation and barriers to its development is presented.     

Control of variable speed wind turbines equipped with synchronous or doubly fed induction generators supplying islanded power systems

A control method for variable speed wind turbines (VSWTs) supplying islanded parts of electrical networks is presented. Active power/frequency and reactive power/voltage droops are applied in order to determine the active, reactive power production, thus downscaling to the VSWTs the conventional control concepts of the power plants. Two types of VSWTs comprising doubly fed induction generators or synchronous generators are considered. Electrical, aerodynamic and structural detailed dynamic models were developed and combined with the proposed control strategies ensuring fast regulation of the frequency and the voltage in the islanded mode of operation. The obtained models are used for the simulation of a representative simplified distribution network supplied by VSWTs.     

Distributed photovoltaic generation and energy storage systems: A review

Currently, in the field of operation and planning of electrical power systems, a new challenge is growing which includes with the increase in the level of distributed generation from new energy sources, especially renewable sources. The question of load redistribution for better energetic usage is of vital importance since these new renewable energy sources are often intermittent. Therefore, new systems must be proposed which ally energy storage with renewable energy generators for reestablishment of grid reliability. This work presents a review of energy storage and redistribution associated with photovoltaic energy, proposing a distributed micro-generation complex connected to the electrical power grid using energy storage systems, with an emphasis placed on the use of NaS batteries. These systems aim to improve the load factor, considering supply side management, and the offer of backup energy, in the case of demand side management.     

An approach for the integration of renewable distributed generation in hybrid DC/AC microgrids

This paper presents an investigation of a hybrid DC/AC integration paradigm to establish microgrids (MGs) by using a conventional three-phase local power delivery system. This approach adds an additional DC power line to the local power distribution system in order to collect energy generated by distributed domestic renewable sources. The local renewable distributed generation (DG) works in conjunction with the conventional grid utility to reduce the power draw from the grid. Researchers designed an energy conversion station to mix energy from the local DGs with energy from the grid utility. This approach, therefore, uses a continuous energy mixing strategy for DC integration of local generation and grid energy to supply energy to MG consumers via the conventional three-phase power distribution system. Thus, local distributed renewable generators do not have to contend with AC integration problems, such as AC stability and line synchronization. This approach can facilitate the transformation of conventional local power distribution systems into reliable MGs in an affordable way for stakeholders and it is a step towards construction of future smart grids.     

Study on electrical energy and prospective electricity generation from renewable sources in Australia

Nowadays renewable sources are being used as clean sources to generate electricity and to reduce the dependency on fossil fuels. The uses of renewable sources are being increased in electricity generation and contributed to reduce the greenhouse gas emission. The function of any electrical power system is to connect everyone sufficiently, clean electric power anywhere and anytime of the country. This can be achieved through a modern power system by integrating electrical energy from clean renewable sources into the nation's electric grid to enhance reliability, efficiency and security of the power system. The paper on the status of review the driving force of the generation of renewable energy and proposing electrical energy generation from renewable sources to be ensured at least 20% of total energy of Australia. This paper has been studied the existing electricity generation capacity of Australia from renewable and non-renewable sources. Optimal electricity generation from renewable sources has been examined. The environmental impact of electricity generation from renewable sources has been considered. Under this paper the yearly average wind data of past 20 years and above for some meteorological stations of Australia have been used. The prospective electricity generation from wind turbines and solar photovoltaic panels has been proposed in the paper that will increase electrical energy of the power grid of Australia. It was estimated the capital cost of prospective electricity generation farms from wind and solar PV sources.     

Freely Customized virtual generator model for grid-forming converter with hydrogen energy storage

Currently, there is a large-scale penetration of renewable energy sources (RES) in power systems around the world, most of which are based on the power converter technology. The result of such penetration is the emergence of frequency and voltage regulation problems in power systems with RESs. An effective way to solve the mentioned problems is the use of energy storage systems (ESS), among which hydrogen ESSs are promising. However, the quality of such solution mainly depends on the approach to the power converters control. One of the most effective is a grid-forming power converter control strategy. Within this strategy, many scientific groups have proposed the concept of a virtual synchronous generator (VSG). However, the existing VSG models use a series structure with a stiff direction of signals, which completely mimics conventional synchronous generators. In this regard, the fundamental problems inherent in such a structure arise, e.g., a coupling between active and reactive power control. This paper describes a freely configurable VSG model (FC-VSG), in which the structural control blocks can be transferred from one level to another, and the levels themselves can be installed not only in series, but also in parallel. The case studies carried out made it possible to identify the configuration of the FC-VSG, which excludes the fundamental problems inherent in the conventional VSG structures. At the same time, the most efficient and reliable frequency and voltage regulation is provided, which has been proved by qualitative and quantitative comparison, including with the conventional VSG structure.     

UK scenario of islanded operation of active distribution networks with renewable distributed generators

This paper reports on the current UK scenario of islanded operation of active distribution networks with renewable distributed generators (RDGs). Different surveys indicate that the present scenario does not economically justify islanding operation of active distribution networks with RDGs. Anti-islanding protection schemes currently enforce the renewable distributed generators (RDGs) to disconnect immediately and stop generation for grid faults through loss of grid (LOG) protection system. This greatly reduces the benefits of RDG deployment. With rising RDG penetration, much benefit would be lost if the RDGs are not allowed to island only due to conventional operational requirement of utilities. For preventing disconnection of RDGs during LOG, several islanding operation, control and protection schemes are being developed. Technical studies clearly indicate the need to review parts of the ESQCR (Electricity Safety, Quality and Continuity Regulations) for successful islanded operations. Commercial viability of islanding operation must be assessed in relation to enhancement of power quality, system reliability and supply of potential ancillary services through network support. Demonstration projects under Registered Power Zone and Technical Architecture Projects should be initiated to investigate the usefulness of DG islanding. However these efforts should be compounded with a realistic judgement of the associated technical and economic issues for the development of future power networks.     

Engineering practices for the integration of large-scale renewable energy VSC-HVDC systems

With the continuous development of power electronic devices, intelligent control systems, and other technologies, the voltage level and transmission capacity of voltage source converter (VSC)-high-voltage direct current (HVDC) technology will continue to increase, while the system losses and costs will gradually decrease. Therefore, it can be foreseen that VSC-HVDC transmission technology will be more widely applied in future large-scale renewable energy development projects. Adopting VSC-HVDC transmission technology can be used to overcome issues encountered by large-scale renewable energy transmission and integration projects, such as a weak local power grid, lack of support for synchronous power supply, and insufficient accommodation capacity. However, this solution also faces many technical challenges because of the differences between renewable energy and traditional synchronous power generation systems. Based on actual engineering practices that are used worldwide, this article analyzes the technical challenges encountered by integrating large-scale renewable energy systems that adopt the use of VSC-HVDC technology, while aiming to provide support for future research and engineering projects related to VSC-HVDC-based large-scale renewable energy integration projects.     

基于IDA-PBC的混合储能虚拟同步发电机功率协调控制

随着可再生能源在电力系统中的渗透率不断升高,虚拟同步发电机(VSG)技术广泛应用于现代电力系统的调频控制中。为了模拟同步发电机的惯性与阻尼特性,VSG需配备储能单元。针对VSG中蓄电池与超级电容混合储能系统(HESS),提出了基于互联阻尼分配的无源控制策略(IDA-PBC),通过控制超级电容快速补偿VSG惯性模拟环节引入的功率变化,蓄电池响应VSG一次调频相对缓慢的功率需求,减少蓄电池功率波动。最后,在Matlab/Simulink环境下仿真验证了所提控制策略的可行性。     

虚拟同步发电机技术控制的MMC型固态变压器

针对在大容量直流负荷和高渗透率分布式电源接入固态变压器低压侧时,高压并网接口易呈惯量低、阻尼特性差的问题,提出一种虚拟同步发电机技术控制的模块化多电平换流器（MMC）型固态变压器。首先,分析虚拟同步发电机原理并推导MMC与虚拟同步发电机的等效数学模型,并将虚拟同步发电机技术融入到输入级的控制中,使并网接口的惯性与阻尼增强,在输出端功率变化时对上级电网呈现出友好的柔性缓冲能力。其次,为提升MMC型固态变压器对上级电网的频率支撑能力,在低压直流环节配置储能装置,通过改变充放电功率主动响应一次调频。然后,通过输入级的无功控制环节,验证其具备一定的调压能力。最后建立仿真模型验证了所提控制策略的有效性与可行性。     

含高渗透率海上风电电网的DSSC优化配置与运行控制策略

随着海上风电并网规模的不断增大,高渗透率风电引起的线路潮流阻塞问题已成为制约风电并网规模的重要因素之一。针对高渗透率海上风电区域的输电线路阻塞问题,文章提出一种含高渗透率海上风电电网的分布式静态串联补偿器(DSSC)优化配置与运行控制策略。该策略包含两个阶段:第一阶段为考虑多场景下DSSC的优化配置,以线路阻塞程度最小为目标,确定DSSC的配置地点与容量;第二阶段为DSSC的运行控制,利用DSSC的潮流控制能力,优化调度系统中的发电机,降低系统运行成本,并考虑系统在出现风电水电高出力场景下DSSC的运行控制,从而提高系统可再生能源的消纳。仿真结果表明,与采用静态串联补偿器(SSSC)和不采用柔性交流输电(FACTS)设备相比较,所提策略可降低系统的运行成本,提高可再生能源的消纳水平。     

可再生能源交流孤网转并网运行中的电压控制策略

柔性直流远距离输送有仅通过直流送出模式和交直流混联送出模式。两种送出模式下,系统的运行点存在较大差异,直接切换会对电网产生较大冲击。文章首先分析了两种送出模式下系统的调压控制特点,指出并网点电压幅值偏差是切换过程的关键点,设计了以可再生能源集群并网点电压偏差最小为主目标,换流站综合无功裕度最大为次目标的柔性切换策略。最后,基于某柔直电网可再生能源送端规划系统进行仿真,验证了所提策略的有效性。     

Coordinated control of TCPS and SMES for frequency regulation of interconnected restructured power systems with dynamic participation from DFIG based wind farm

Among the several wind generation technologies, variable-speed wind turbines utilizing doubly fed induction generators (DFIG) are gaining momentum in the power industry. Increased penetration of these wind turbine generators displaces conventional synchronous generators which results in erosion of system frequency. With this assertion, the paper analyzes the dynamic participation of DFIG for frequency control of an interconnected two-area power system in restructured competitive electricity market. Frequency control support function responding proportionally to frequency deviation is proposed to take out the kinetic energy of wind turbine for improving the frequency response of the system. Impacts of varying wind penetration in the system and varying active power support from DFIG on frequency control have been investigated. The presence of thyristor controlled phase shifter (TCPS) in series with the tie-line and Superconducting Magnetic Energy Storage (SMES) at the terminal of one area in conjunction with dynamic active power support from DFIG results in optimal transient performance for PoolCo transactions. Integral gains of AGC loop and parameters of TCPS and SMES are optimized through craziness-based particle swarm optimization (CRPSO) in order to have optimal transient responses of area frequencies, tie-line power deviation and DFIG parameters.     

Control of permanent-magnet generators applied to variable-speed wind-energy systems connected to the grid

Wind energy is a prominent area of application of variable-speed generators operating on the constant grid frequency. This paper describes the operation and control of one of these variable-speed wind generators: the direct driven permanent magnet synchronous generator (PMSG). This generator is connected to the power network by means of a fully controlled frequency converter, which consists of a pulsewidth-modulation (PWM) rectifier, an intermediate dc circuit, and a PWM inverter. The generator is controlled to obtain maximum power from the incident wind with maximum efficiency under different load conditions. Vector control of the grid-side inverter allows power factor regulation of the windmill. This paper shows the dynamic performance of the complete system. Different experimental tests in a 3-kW prototype have been carried out to verify the benefits of the proposed system.     

A topological reconfiguration procedure for maximising local consumption of renewable energy in (Italian) active distribution networks

Distribution networks (DNs) are facing great changes, due to the strong increase in distributed generation (DG), often driven by renewable energy sources. Designed to deliver electrical power from the transmission system to the final consumers, they are now becoming active and may inject power into the transmission network. In case of large DN, a portion of the system can be absorbing power from the transmission grid, while another portion injects power into it. In order to satisfy the power balance as much as possible at the local level, the distribution system operators are interested in the minimisation of the power exchange with the transmission network, maximising the local consumption of DG energy. This paper presents a topological reconfiguration procedure, based on the branch exchange technique, for the maximisation of the local consumption of renewable energy. A case study is presented, based on a real DN located in northern Italy.