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Budget constraints may cause delays in the development of transmission expansion projects. These delays increase the probability of congestion in power systems. The congestions preclude a non‐discriminatory access of Generation Companies (GenCos) to transmission network, thus leading to financial losses to certain GenCos. On the other hand, a higher degree of risk is involved in transmission investment, and a long time period is required to return the capital cost. It would, therefore, be effective to offer structures to promote merchant investments in order to lighten the financial burden on the transmission service provider to avoid lengthy delays. This paper proposes a structure to involve the GenCos in merchant transmission investment. The rationale for engaging in such projects is that, as well as the profit from the merchant investment, the GenCos could make further profit from energy production by gaining improved access to load areas. The merchant investment cost of the transmission project is returned through long‐term firm contracts together with revenues from financial transmission right. The most profitable plan of the GenCo is identified by conducting a cost‐benefit analysis among expansion plans specified by the transmission system operator. [ABSTRACT FROM AUTHOR]

The increased penetration of distributed energy resources (DERs) installed in distribution networks via power electronic converters reduces the overall rotating inertia of the power system, causing faster frequency dynamics after a large disturbance. However, these DERs coupled with energy storage systems (ESSs) can be managed to provide valuable grid support functions such as fast frequency response (FFR) and assist the transmission system operator (TSO) in frequency control. In this study, single‐ and three‐phase clusters of electric vehicles (EVs), acting as mobile‐ESSs, are grouped together as virtual power plants (VPPs) to centralise the provision of FFR for the distribution system operator (DSO) whilst considering network unbalance. Furthermore, the parameters of the EV clusters within each VPP are optimised to minimise the inertia‐weighed maximum frequency deviation following a disturbance whilst adhering to network security and power quality constraints. To this aim, a variant of the metaheuristic optimisation algorithm called improved harmony search is used for the optimisation process. Finally, the merits of the proposed methodology are shown with an illustrative example of an unbalanced three‐phase transmission and distribution network modelled in DIgSILENT PowerFactory. The results show that clusters of EVs grouped as VPPs effectively improve frequency support via the TSO–DSO interaction. [ABSTRACT FROM AUTHOR]

An asynchronous heterogeneous decomposition (async‐HGD) algorithm for integrated transmission and distribution networks coordinated optimisation considering communication conditions is proposed. Compared with traditional synchronous HGD (sync‐HGD), a partial synchronisation mechanism is introduced instead of the full synchronisation to alleviate the influence of communication delays. Under this mechanism, the transmission system operator does not need to wait for the slowest distribution system operator to complete its subproblem and update boundary parameters before the next iteration can proceed. Also, a bounded delay is introduced under this mechanism to ensure sufficient freshness of all the updates. Further, the previous value strategy (PVS) and linear regression strategy (LVS) are applied to handle non‐instantaneous interruptions in the communication networks as an improvement of the async‐HGD. Numerical experiments show that considering communication delays in the actual operation; the async‐HGD takes less time to converge compared with the sync‐HGD under appropriate settings. In addition, PVS and LVS can both enhance the robustness of the overall algorithm by achieving approximate solutions, and the LVS can achieve more accurate results compared with the PVS. [ABSTRACT FROM AUTHOR]

This article describes a novel power system architecture and operational concept which is founded on the idea of organising sets of distributed generators, loads, storages and multimodal interfaces at distribution level in supervised and controlled grid subsections called smart power cells, which are controllable entities intended to enforce the stability of the transmission network and participate in its operation. Further, the article addresses a control scheme which can be implemented within a smart power cell to control its active and reactive power exchange with the transmission network following set points provided by a superimposed control system, and thus participate in the operation of the transmission grid (e.g. participation in congestion management, system optimisation, balancing of generation and load). The dynamic behaviour of a future power system under consideration of the proposed system architecture including several smart power cells controlled with the developed control scheme is demonstrated through time‐domain simulation using a combined transmission–distribution power system model. [ABSTRACT FROM AUTHOR]

As the coupling between transmission systems and distribution systems is significantly enhanced, transmission–distribution coordinated optimisation becomes more and more necessary. Considering that the objectives of the transmission system operator (TSO) and the distribution system operator (DSO) are usually significantly different in the type and dimension, a synthesised‐objective collaborative model is proposed in this study, by normalising different objectives, introducing a weight factor to reflect the importance degree difference of multiple objectives and constructing a common objective. Then, a modified heterogeneous decomposition (M‐HGD) algorithm is proposed to solve the model. In the M‐HGD, two modifications on the update of iterative variables and the stop criteria are proposed, which effectively relieves the fluctuation of the iterative variables and restrains the tail effect of the convergence curve. The proposed model and algorithm can avoid privacy issues between the TSO and the DSO, achieve Pareto optimality solutions, and have powerful flexibility. Numerical experiments validate the effectiveness of the proposed model and demonstrate that the M‐HGD has high accuracy and has better convergence and efficiency than traditional distributed optimisation methods. [ABSTRACT FROM AUTHOR]

To provide continuity of balancing demand and generation, renewable sources will be more active than today in near future due to the tendency of massive investment on renewable energy sources (RESs) by countries. However, due to the uncertain and intermittent nature of RESs, RESs would create problems on power system operations such as power quality, efficiency, stability and reliability. Owing to having problems with RESs integration, virtual power plant (VPP) has introduced to make this integration smooth without compromising the grid stability and reliability along with offering many other techno‐economic benefits. This study reviews structures, types, architecture and operations of VPP along with the status of present implementations worldwide. The types of VPP are introduced in details with the optimisation algorithm used with each type. In addition, VPP is linked with the most of the components in power systems such as distributed generation, active prosumers, transmission system operator and distribution system operator, grid services such as fault ride through, reactive power control as well as with the help of technology such as communications, control and optimisations. This study gives a comprehensive outline of transforming microgrid to VPP that is useful for researchers, consumers, prosumers and utility operators. [ABSTRACT FROM AUTHOR]

The solar power installed capacity all over the world is growing and this renewable energy technology is playing an important role to build a clean energy future. The solar forecast is a necessary tool for the transmission system operator in order to maintain the electricity network safety and reliability. This study aims to report the experience of R&D Nester in the solar forecast at national level in Portugal, forecasting for all solar plants connected to the very high voltage (400, 220 and 150 kV) and high voltage (60 kV) networks, providing this information to the Portuguese transmission system operator, REN. A novel approach to calculate cloud index from images from a sky camera is presented. [ABSTRACT FROM AUTHOR]

The increased adoption of renewable energy generation is reducing the inertial response of the Great Britain (GB) power system, which translates into larger frequency variations in both transient and pseudo‐steady‐state operation. To help mitigate this, National Grid, the transmission system operator in GB, has designed a control scheme called enhanced frequency response (EFR) specifically aimed at energy storage systems (ESSs). This study proposes a control system that enables the provision of EFR services from a multi‐electrical ESS and at the same time allows the management of the state of charge (SOC) of each ESS. The proposed control system uses a Fuzzy Logic Controller to maintain the SOC as near as possible to the desired SOC of each ESS while providing EFR. The performance of the proposed controller is validated in transient and steady‐state domains. Simulation results highlight the benefits of managing the SOC of the energy storage assets with the proposed controller. These benefits include a reduced rate of change of frequency and frequency nadir following a loss of generation as well as an increase in the service performance measure which renders into increased economic benefits for the service provider. [ABSTRACT FROM AUTHOR]

This study provides an overview of the different techno‐commercial aspects of a wind energy conversion system. The review includes the grid codes provided by the transmission system operator of different countries supplemented by various communication protocols, explanation of different types of conventional and advanced wind generators, discussions on different maximum power point algorithms along with various conventional and advanced pitch angle control systems. The study also describes a variable speed system using different types of power electronic converters at the generator and the grid side to maintain power quality, grid synchronisation and unity power factor operation along with various cooling techniques utilised. In a subsequent part, a clear overview of different control topologies such as field‐oriented control, direct torque control, voltage‐oriented control and direct power control has been provided along with the recent advancement of each and every topology. The study also provides the challenges of the prevailing technology along with the already available and possible ways of minimising these technological gaps. Finally, a techno‐commercial comparison of different commercially available wind systems by different companies has been presented along with their market share based on the current scenario. [ABSTRACT FROM AUTHOR]

The most common approach to risk assessment for power systems is based on the N−1 principle. Nevertheless, the economic rationale suggests its relaxation in cases where the consequences are relatively minor and exacerbation when they are large. This study addresses this need by proposing an alternative operational risk assessment methodology that is based on both probabilities and costs of possible contingencies. The foundations are built on listing all possible contingencies that may be considered by the transmission system operator (TSO). As only a subset of these contingencies can be examined in reasonable time, the upper and lower risk boundaries are introduced to quantify the risk underestimation. The ratio of those limits is used as an accuracy indicator, which – according to the desired level – may help the TSO to identify the required number of contingencies that have to be analysed. Furthermore, several approaches to improve the reduction of the number of simulated contingencies are discussed and the results obtained basing on the dynamic IEEE39 model are presented. [ABSTRACT FROM AUTHOR]

To better assess the contribution of wind power plants (WPPs) during disturbances, Hydro‐Québec TransÉnergie (HQT), the main transmission system operator in the Quebec Interconnection, uses on‐line monitoring to record data at the point of common coupling of each WPP. This data is used to analyse their performance during voltage and frequency disturbances. Until recently, very few WPPs were able to provide an inertial response (IR) for under‐frequency events. On 28 December 2015, a generation loss of 1700 MW caused a frequency nadir of 59.08 Hz on the system, to which most WPPs required to provide IR contributed significantly. The analysis of the event showed that the behaviour of the WPPs had a significant effect on the recovery of the system frequency. This study presents the performance analysis of WPPs during this under‐frequency event. The analysis is based on data collected from 25 large‐scale WPPs in operation and equipped with the IR feature as required by HQT. Field measurements and simulation results are used to emphasise the effect of the active power reduction during the recovery phase. Impacts of the IR from the WPPs as used on the Hydro‐Quebec system are discussed. [ABSTRACT FROM AUTHOR]

This study discusses synthetic inertia from the perspective of a transmission system operator and compares it to fast frequency response based on frequency deviation. A clear distinction of the meanings between these concepts is discussed, the basis of which is a description of their characteristics. A contribution and the purpose is the clarification of these concepts in addition to share the perspectives of a transmission system operator. The frequency response of a power system based on the Nordic system is examined for future scenarios with large amounts of wind power. Conclusions are drawn regarding the benefit of synthetic inertia compared with fast frequency response based on frequency deviation. [ABSTRACT FROM AUTHOR]

Implementing sophisticated voltage controls of wind farms may provide manifold benefits such as reducing losses of the distribution network (DNET) and harvesting network (HNET), increasing the wind hosting capacity of transmission networks (TNETs) and reducing wind curtailments. However, it represents an enormous challenge since wind energy is dispersed increasing the complexity of the required centralised control systems. This study contains a comprehensive state‐of‐the‐art review of the huge research that is available in the literature in order to foster the participation of on‐shore wind energy in voltage control. The current situation is described by the actual technological developments of control devices and the regulatory framework of a set of European countries with high wind penetration figures. A classification of key wind voltage control concepts will be proposed in order to understand and organise the available literature references. The state‐of‐the‐art review covers the integration of wind energy through DNETs, smart grids and dedicated HNETs into the TNET, covering research from a steady state and also dynamic perspective. The new requirements of theTransmission system operators tend to suggest that wind farms should have a similar performance compared with conventional plants. Thus, in this study, special attention will be given to the literature review of HNETs since it has been the most promising alternative of integrating wind voltage control into the existing transmission grid hierarchical control schemes. A set of relevant field experiences and international projects will be described, showing some effort to implement academic research into practice. [ABSTRACT FROM AUTHOR]

With integration of a battery energy storage system (BESS) into the wind power system, the wind power variation can be mitigated to dispatch a constant power to the grid. To effectively supply the constant power dispatch to the grid with a limited BESS, the power dispatch capability should be defined primarily at beginning of each dispatching interval to cooperate with the transmission system operator. This study introduces a method to determine the power dispatch capability of a wind‐battery hybrid power system with the minimal battery capacity. The power dispatch capability is decided under the condition that the battery power is maintained below its rating and that the state of charge of the battery is guaranteed within a safe range. For economic BESS operation, the battery capacity, including the power and energy ratings, is also determined cost‐effectively to obtain the constant power dispatch. Based on the long‐term wind power profile, the authors develop the determination process to minimise the battery capacity which ensures that the BESS sufficiently mitigates wind power variation to dispatch the constant power into grid under all wind conditions. To evaluate the proposed determination method, a case study is carried out with a 3‐MW wind turbine generator and real wind speed data measured on Jeju Island in Korea. [ABSTRACT FROM AUTHOR]

This study presents a methodology for siting and sizing static synchronous compensator (STATCOM) devices in the Portuguese transmission system in order to improve system security following severe grid faults. Security issues arise since the Portuguese transmission system incorporates significant levels of wind generation without fault ride through and reactive current injection capabilities during grid faults. As the transmission system operator (TSO) is responsible for assuring system security, the goal of the study is to take advantage of the proved STATCOM ability for injecting reactive current in order to mitigate the disconnection of large amounts of wind farms in case of severe grid faults. The proposed methodology was developed and tested in coordination with the Portuguese TSO and it is based on the formulation of an optimisation problem in order to minimise the installed STATCOM power while ensuring its compliance with the current grid code requirements, namely in what concerns to the system stability and security. Given the discrete and complex nature of the problem, a hybrid approach, combining both a heuristic method and an evolutionary particle swarm optimisation (EPSO) algorithm was developed. Results show the effectiveness of the proposed methodology as well as its robustness regarding the validity of the obtained solutions while facing the most severe operational scenarios. [ABSTRACT FROM AUTHOR]

The drive towards renewable energy sources (RES) is dramatically changing the dynamic electrical characteristics of Generators, which are traditionally the dominant dynamic component in Power Systems. Non‐synchronous generation (NSG) approaching or even exceeding control area demand is now happening on a rapidly expanding scale. The study identifies from National Grid's perspective as Great Britain transmission system operator, the key challenges during periods of operation with high proportion of RES relative to demand. It raises some major questions. What determines system strength in an HVAC system largely without synchronous generators and what is adequate for stability? What levels of %NSG can be expected in the future energy scenarios? What are the financial consequences of constraining RES production if the technical capabilities create an upper limit of %NSG which the system can be operated at? The study finally proposes closer collaboration across the industry to find the optimal missing solutions. [ABSTRACT FROM AUTHOR]

Several research efforts are conducted towards the impact of wind power variability on the operating reserve requirements of power systems. In most systems, reserve capacity is determined by means of a heuristic or statistical analysis of the different drivers for system imbalances. This contribution puts forward a dynamic reserve approach in which the operating reserve capacity is modified on hourly basis, based on expected wind power conditions. This would improve the current methodologies where the capacity is fixed for longer periods, avoiding expensive overestimations. By means of a decentralised approach, the suggested methodology conforms with an unbundled and liberalised market framework. Results show how the average upward and downward capacity reductions are reduced with at least 36% and 16%, compared to the corresponding static reserve strategy. System simulations integrating this dynamic reserve strategy reveal substantial reductions in operational costs. The average cost of withholding reserve capacity is observed to decrease from 4.5, 5.1, 17.1 and 51.2 to 3.1, 4.8, 7.9 and 14.6 €/MWh wind energy injected, respectively, for an installed capacity representing 6, 12, 18 and 24% of the annual electricity demand. In conclusion, the results of this contribution encourage transmission system operators to evolve towards dynamic reserve strategies. [ABSTRACT FROM AUTHOR]

This paper presents a literature review on the coordination between transmission and distribution grid operators in power systems, with particular emphasis on the provision of balancing and congestion management services. Firstly, a brief historical background is presented in order to highlight the ongoing changes current power systems are experiencing and that call for greater coordination between transmission system operators (TSO) and distribution system operators (DSO). These include the growth in distributed energy resources (DER), new roles to be adopted by both system operators (SO), and the integration of new agents such as independent aggregators. Next, the paper places its focus on a particular topic which, based on the review presented, is presumably considered as the most relevant issue requiring tighter cooperation mechanisms, that is, the procurement of balancing services from DER by TSOs whilst DSOs procures the same DER flexibility to manage local congestions. Four key elements are identified for this coordination to take place, namely (a) DER flexibility integration, (b) coordination schemes, (c) transmission‐distribution optimization, and (d) data exchange. For each one of them, the paper presents a literature review and identifies the main existing technical, economic, and regulatory barriers. These barriers include, among others, the limitation for small DER flexibility to participate in grid service markets, the challenges for independent aggregators, the technical barriers for optimizing a transmission‐distribution grid, and the lack of maturity in coordination schemes and TSO‐DSO data exchange models.This article is categorized under: Energy Systems Analysis > Systems and Infrastructure Energy Systems Economics > Economics and Policy [ABSTRACT FROM AUTHOR]