Subtransmission System Research Articles

Investigation of DG units influence on 66 kV sub-transmission system network considering region load growth: a case study

Abstract Voltage breakdown in sub-transmission networks resulting from an increase in load demand can be addressed by incorporating distributed generation (DG) units such as hydro, wind, photovoltaic, and geothermal in some buses. This approach can lower power losses and the price of the primary generated energy while increasing network dependability and efficiency. Feasibility of dispersed generation integration & its effect on sub-transmission system operation were investigated using Ben Walied 66 kV sub-transmission network in Libya as a state. This study presents modified particle swarm optimization (MOPSO) technique to select the best option with dispersed generation units under various operating conditions. The Ben Walied 66 kV sub-transmission network has been subject of effects research on both normal operation and load growth scenarios. The goal of this study was to find best way to adjust penetration level of the three DGs in response to changes in the network loading. Three DG units have been refitted to study the effects on test networks for sub-transmission, which consist of 30 and 51 buses. A comparison analysis shows that while the ideal locations of DG units remain constant with load expansion, the optimal solution for the penetration level percentage of three DG units was boosted by new optimal sizes. Research has been done on how the power factor of distributed generators (DGs) affects the practical performance of 66 kV networks in steady-state conditions. The integration of DG units in regulating these losses and voltage variations is demonstrated by the results, which indicate that the ideal solution for DG unit’s power factor was at a value of 0.87 lagging.

The consequence of ignoring load modeling on the circuit breaker selection in densely-meshed sub-transmission systems

In this paper, the selection of circuit breakers for a densely-meshed sub-transmission system with relatively short transmission lines is investigated. The results highlight the significance of modeling the load at the end of lines for accurately calculating the transient recovery voltage (TRV) following the short-circuit current interruption. According to IEEE standards, assuming load modeling at the end of the transmission lines can result in a maximum discrepancy of 20 % in the calculated TRV. Supposing that the load modeling leads to a moderating effect on the calculated TRV, the difference is considered as a safety margin, and load modeling is ignored. Nevertheless, the results presented here illustrate that this assumption strongly depends on characteristics of the studied network and the load type. In a network possessing the characteristics outlined in this paper, even a passive load as small as 15 % of the nominal capacity can significantly moderate the calculated TRV, with impacts reaching up to 38 %. Conversely, modeling an active load may result in a more severe TRV than if it were disregarded. The simulation results indicate that for networks with transmission lines longer than 70 km, neglecting to model passive loads on the transmission lines still yields sufficiently accurate calculations of TRV.

Proposal of a predictive and preventive maintenance plan in electrical substations through the use of thermography

The objective of this study was to make a proposal for a predictive and preventive maintenance plan in the electrical substations La Propicia, Las Palmas, and La Pradera belonging to CNEL-EP Esmeraldas Business Unit through the use of thermography. The methodology was of a qualitative approach, under the type of documentary-bibliographical research. The search for information was carried out online in scientific and academic databases. It was found that the information on the maintenance plans in the subtransmission system of CNEL EP Esmeraldas Business Unit, is meager, preventive and predictive maintenance actions are carried out such as revision, washing, readjustment and replacement of elements in poor condition, but no information was found on the analysis technique used leading to the maintenance activities of electrical equipment and installations. The results of the consulted investigations agree that the thermographic technique and its tools such as drones, infrared cameras and software programs are very useful for the diagnosis, collection and processing of data in electrical systems to determine maintenance requirements. Finally, the investigation yielded substantial information that allowed the preparation of a maintenance plan for the aforementioned La Propicia, Las Palmas and La Pradera electrical substations of CNEL-EP Esmeraldas Business Unit through the use of infrared thermography, to assist in their better operation. and performance by allowing the evaluation of the state of the equipment that makes up the electrical system of the aforementioned substations.

Assessment of most critical project delays on a multi-stage transmission expansion plan using particle swarm optimization

Transmission reinforcement projects of a Transmission Expansion Plan can suffer implementation delays that can impact the short-term power system operation in unpredicted ways. These delays may lead project managers to paying particular care to the development of those plans proved to be most critical from this point of view. This paper proposes a multi-objective dynamic-programming based method to evaluate the impacts of such delays in a short-term transmission plan with respect to load shedding, reliability and losses. It uses the Binary Pareto-Optimal Particle Swarm Optimization as optimization tool. For the trade-off analysis and the final decision to be made, the paper proposes indices to rank the projects in the plan from most to least critical, should they be delayed, based on the severity of the impacts and the probability of occurrence, both on an annual basis and for the whole planning horizon. The proposed method has been tested on an actual Brazilian 40-bus subtransmission system for a 3-year and a 6-year planning horizon, and with the 24-bus IEEE RTS reliability test system for a 3-year planning horizon. To assess its accuracy and efficiency, the results are compared to those using the Non-dominated Sorting Genetic Algorithm II method (NSGA-II) and to the exhaustive search that considers every delay configuration, showing the same accuracy as the exhaustive search and better convergence properties than NSGA-II. The most critical projects are accurately ranked helping the project manager to decide which projects should receive higher attention to guarantee its construction on the planned date. The method obtained a reduction in computational time of 35% for the Brazilian system and of 42% for the IEEE RTS in relation to the exhaustive approach, demonstrating its efficiency.

Generation dispatch with large-scale photovoltaic systems

ABSTRACT The objective of this paper is to explore the role of generation dispatch strategy and spinning and non-spinning reserve methods in solving challenges posed by grid-connected large-scale PV systems as opposed to the commonly used approaches such as voltage, rotor angle and frequency control methods. The investigation covers the issues associated with the inherent intermittent of PV output, which results in additional operational challenges such as load-following requirements, spinning reserve requirements, load frequency excursions and system stability. A detailed performance evaluation of generation dispatch with large-scale PV systems is carried out in this research work based on the IEEE 14-bus test system using the MATLAB-based Power System Analysis Toolbox (PSAT). An analysis is conducted by using the power flow analysis methods such as power flow (PF), continuation power flow (CPF) and time domain (TD) analysis to gain an understanding of the influence that large-scale PV systems have on sub-transmission and transmission networks. The outputs of the study can be used to work out the appropriate penetration level of large-scale PV systems that can minimize their negative impacts on sub-transmission and transmission systems.

Providing Distributed Flexibility for Curative Transmission System Operation Using a Scalable Robust Optimization Approach

Managing flexibility across transmission and distribution system boundaries unlocks degrees of freedom for efficient system operation. It is attractive to use this freedom for curative transmission system operation, i.e., an operation that reliably reestablishes safe states after contingencies while enabling higher grid utilization in the normal state. However, the computational complexity of coordinating the fast-growing number of renewables becomes challenging. For this reason, we develop a fast and robust optimization approach to calculate flexibility potentials of individual assets and aggregated flexibility potentials across the grid interconnections. The proposed method guarantees that the flexibility-providing grid always remains in a safe state while providing maximum flexibility. The method is successfully tested on a real European sub-transmission system. As part of a future distribution grid control function, the method can be used to provide flexibility to transmission system operation and provide the necessary aggregation methods to handle thousands of distributed assets.

Optimal subtransmission switching using a reliability simulation-based multi-objective optimization model

The growth of subtransmission network aims at satisfying load growth, maintaining a contingency level, and providing a high quality and reliable electricity service. Utilities direct the investments to reinforce this system and thus a meshed network with multiple-point feeding to the transmission system arises. At this point, an efficient alternative to achieve these objectives is to carry out a diagnosis of the network architecture and, taking advantage of the switching capability, to plan the switching of the subtransmission lines. An optimal subtransmission switching approach is proposed based on constrained multi-objective optimization that deals with energy losses and reliability, in addition to using information on the characteristics of loads and generation. A simulation-based optimization framework is constructed using the non-dominated genetic classification algorithm NSGA-II in the optimization phase and reliability assessment during simulation phase. As a result, a set of non-dominated solutions approximating the Pareto front is obtained, which allows the planner to make decisions based on its priorities and needs. The performance of the proposal is assessed with a real subtransmission system of an Ecuadorian power utility. This approach to the operational planning of a meshed subtransmission network constitutes a powerful decision-making tool that could be adopted by distribution utilities.

Optimal distributed generation penetration considering relay coordination and power quality requirements

Although the electric energy supply from distributed generation (DG) resources is increasing, these sources also face limitations. Protection and power quality constraints are the most important issues that limit the maximum DG penetration in power systems. Changing the short circuit level due to DGs' presence—especially synchronous-based (SDG) type—may lead to miscoordination of the protective relays. Also, standard harmonic limits may be violated due to harmonic injection of inverter-based DGs and non-linear loads. A protection coordination problem is solved for the basic configuration, and the optimal settings are applied. Then, using a proposed approach and a new objective function, the location, size, and type of DGs are optimized to maximize the DG penetration and the speed of protection system, while reducing power losses. Finally, the protection setting of SDGs is optimized and applied. In these optimizations, the protection coordination constraints and the power quality requirements are considered and met. IEEE 14-bus system and a real sub-transmission system are simulated to evaluate the proposed approach and the new objective function. The obtained results show significant power loss reduction and protection speed improvement for different scenarios and case studies.

Allocation of HV/MV Substations in Iraqi Power Grid Using Delaunay Triangulation

In provincial areas, allocation of the HV/MV substations aims to assign a proper set of numbers and locations of substations to supply loads spatially distributed over a large geographic area with continually emerging towns. This requires identifying the sparse load centers, the existing substations, and the new substations required to cover the potential extension. This paper proposes a method to allocate the main substations to the targeted area based on the lines’ length which are assigned by Delaunay triangulation and geographical coordinates. The proposed configuration is identified by a criterion that is tailored to the Iraqi sub-transmission system in which the standard distances among the substation follow specific patterns. The standard distances that are identified by the proposed methods are represented by the edges of Delaunay triangles. This paper aims to improve the voltage regulation of the distribution grid and avoid problems of overloaded feeders by assigning the proposed locations that reduce the lines’ lengths. The proposed method is verified using simulation tests on Diyala 9-bus system which is a part of the Iraqi 132 kV power system. The results show that the proposed configuration of the substations is reasonably valid and quite close to the real locations of the towns of the system with efficient lengths for the lines.

Transmission-distribution long-term volt-var planning considering reactive power support capability of distributed PV

High penetration of grid-edge, inverter-based photovoltaic (PV) can cause significant voltage fluctuations not only at the distribution but also at the sub-transmission levels due to PV output intermittency. This paper proposes a reactive power planning tool for sub-transmission systems to mitigate voltage violations and fluctuations caused by high PV penetration and intermittency with a minimum investment cost. The planning tool considers all existing var assets in both sub-transmission and distribution systems to reduce the need of new equipment. It also closely coordinates with an optimization-based volt-var operational tool for selecting a set of scenarios with voltage violations due to PV intermittency and testing the final investment decision. The planning tool obtains an investment need for each scenario based on a proposed optimal power-flow framework. Because of a high number of discrete variables in this optimization problem, an efficient technique to recover the feasibility for the solution of the relaxed optimization problem is included. After obtaining separated solutions for all scenarios, two options are provided for making final planning decision: i) a conservative direct combination of investment need solutions and ii) a machine learning-based selection of representative investment needs at most time steps. The final investment decision options are verified using a realistic large-scale sub-transmission system and 5-min PV and load data. The results show a significant voltage performance improvement with a lower investment cost for additional var equipment compared to conventional approaches.

COORDINACIÓN ADAPTATIVA DE LA PROTECCIÓN DE DISTANCIA SOBRE LÍNEAS DE SUBTRANSMISIÓN CONSIDERANDO LA INCORPORACIÓN DEL EFECTO PRODUCIDO POR LA GENERACIÓN ADYACENTE

ABSTRACT: This article describes an algorithm for an adaptive distance protection in a sub-transmission system considering operating conditions with changes in the topology due to an abnormal condition, electrical failure or a natural disaster, includes the effect of adjacent generation in the distance relay protection zones, with the incorporation of renewable energies in mixed participation. Two scenarios are proposed, in the first case occurred a contingency, and the transmission systems changes the initial topology. Protections are automatically reparametrized. The second case includes the income of renewable energy generation in a specific point of power electrical Systems. Protections are automatically recalculated. The objective of an automatic coordination permit to operator to restore the sensitivity, selectivity and reliability the electrical protection system in the lowest time. Keywords: Adaptive distance relay, renewable energy, distance protection, infeed effect..)

Distributed Synchronous generation ride-through enhancement by anti-islanding protection blocking

Fault and other perturbations in the distribution or subtransmission system can change the system voltage and frequency, leading to improper operation of anti-islanding protections. This causes disconnections of distributed generations (DG) and, consequently, financial losses and degradation of Bulk Power System performance. This paper proposes a blocking scheme for an islanding detection function, based on impedance and rate of change of frequency. Several methods have been recently proposed for islanding detection in synchronous generators. However, commercial relays still use functions such as Rate of Change of Frequency. Even though it poses problems that are explored in this paper, it is consolidated and easy to implement. Thus, the proposed method is aimed at improving the performance of DG anti-islanding protection. The method has been tested through simulations using the IEEE 34 node test system. The results show that the proposed method is able to discriminate islanding from other transients, and it has operated properly in all non-islanding events, which include single-phase and three-phase short-circuits, load and capacitor switching, and outages of other DGs. Tolerance to transients allows the use of a small frequency threshold which ensures a small non-detection zone and faster disconnection in case of islanding.

An agent-based Stackelberg framework for joint expansion planning of privately Owned regional energy systems and sub-transmission grid

Traditionally, a regional electric company (REC) should supply electricity demand of consumers at sub-transmission level in special geographic areas. As a promising alternative for traditional centralized planning, in this paper, a Stackelberg game approach is employed to solve the investment and operational expansion planning problems in the presence of privately owned Regional Energy Systems (RGESs) and multi-agent systems are used to ease the participation of private ownerships in planning model by preventing agents to disclose their private properties. The RGES's and the REC's agents are the active players of the proposed method and in the form of a semi-decentralized framework, the information exchange among them is fully decentralized. Also, an Independent Coordinating Agent (ICA) is centrally responsible for Stackelberg Nash equilibrium point extraction. Further, a bi-stage Stackelberg game approach is proposed for coupled expansion planning program of Sub-transmission network and privately owned RGESs. To validate the feasibility and efficacy of the suggested model, a modified electric sub-transmission system (located in Guilan Province, Islamic Republic of Iran) is employed as a realistic test case system for simulations. And, the obtained results of simulations by the proposed method are compared with conventional expansion planning of sub-transmission systems.

Techno-Economic Analysis of Increasing PV-Wind based DG Penetration in Sub-Transmission System

This paper deals with techno-economic impact of increasing photovoltaic and wind power distributed generation in sub-transmission system. This paper suggests a new approach to interpret a multi-objective problem of integrating different types of distributed generation using an improved elephant herding optimization method. The main aim of this paper is to minimize the power losses and elevate the voltage profile of the electrical system. The effectiveness of improved elephant herding optimization had been tested on IEEE 30 bus sub-transmission system and is compared with results obtained with Particle Swarm Optimization method. In this paper, three cases have been considered where different types of distributed generation are integrated into the system and the best results have been obtained for case 3 where both photovoltaic and wind power are penetrated in system network. The optimal solutions obtained are compared with particle swarm optimisation and base case. The comparison shows that the proposed optimization method is favourable. The economic analysis of the corresponding distributed generation has been presented in this paper.

GIS-Based Substation Expansion Planning

This article proposes an innovative approach based on analyzing a raster map in geographic information system to perform substation expansion planning for a very large-scale real network. This approach provides an irregular planning miniregion to decompose the planning area into irregular miniregions to tackle such a very large-scale real network expansion planning. Meanwhile, the article develops a suitable model regarding the concept of a system of systems to bring about collaborative interactions between miniregions to preserve the integrity of the distribution network. This model has been established using the proposed index called the maximum permissible radius of load feeding. Computer simulation is performed for a very large-scale subtransmission system of Mashhad, consisting of 1 359 410 consumers spread in an area of 11 450 km2, to demonstrate the efficiency of the proposed methodology.

Techniques for Power Control of PV Systems: A Review Study

The traditional power systems comprise of the generation system which consists of huge central power stations, the transmission and sub-transmission systems to deliver power from remote generating plants to the location near load centres, and therefore the distribution system to serve all end users like residential, commercial and industrial loads. the facility flow is hence, unidirectional, that is, from generation to the load centres. With the ever-increasing demand of electricity that has been raising important power grid operational issues like voltage and frequency instability, the mixing of distributed energy resources into the fashionable power systems became very fashionable since previous couple of decades. The fast depletion of fuel reserves and environmental concerns have provided greater incentive to integrate renewable energy-based Distribution Energy Resources (DER) like solar, wind and biomass in modern power systems. DERs are usually connected to the facility distribution system. during this paper, we evaluate different techniques of power control for PV systems, and evaluate the simplest techniques which will be used for an equivalent purpose.

On-Line Control of DERs to Enhance the Dynamic Thermal Rating of Transmission Lines

The increasing penetration of variable distributed generation causes the transmission lines to operate closer to their design thermal limits. In this context, Dynamic Thermal Rating is a very promising technique, since it permits a better exploitation of the real capability margins of the infrastructures and eliminate network congestions. In this vein, the article proposes a novel control strategy that allows maintaining the conductor temperature of a given line within its thermal limit through the real-time curtailment of distributed energy resources in the network. The impact of weather volatility and measurement uncertainty on the dynamic response of the controller is evaluated. A comprehensive case study, based on a real-world Italian sub-transmission system and measurement data serve to illustrate the dynamic behavior of the proposed controller. The effect of measurement noise and delays is also discussed. Finally, the performance of the proposed control strategy is compared with a conventional robust optimal power flow approach.

Sub-Transmission Network Expansion Planning Considering Regional Energy Systems: A Bi-Level Approach

In order to facilitate the transformation of the existing generation and transmission networks’ structure into a scalable and competitive grid structure, this paper introduced regional energy systems (RGESs) that have the role of aggregating distributed energy resources (DERs) and flexible loads. The economic justification for the expansion of sub-transmission networks in the presence of regional energy systems was also investigated. To achieve this goal, multi-criteria optimization solutions were employed to find techno-economic solutions. While solving the proposed multi-criteria optimization problem, a Pareto front was determined to show the tradeoff between the criteria examined. In addition, fuzzy satisfying and the max-min method were used for finding equilibrium point. In order to demonstrate the performance and effectiveness of the proposed model, a realistic sub-transmission system in Guilan Province, Iran, was used as a test system and the results were compared to those from a traditional sub-transmission expansion planning model.

Individual Load Model Parameter Estimation in Distribution Systems Using Load Switching Events

There currently exists a mature literature on modeling the aggregate load of a distribution feeder by making use of measurements at its feeder-head at substation. The primary application of such feeder-aggregated load models is in sub-transmission or transmission system analysis. However, there is a growing need in practice also to model each individual load across the feeder. If available, such individual load models have applications in power distribution system analysis, e.g., to better integrate distributed energy resources or to improve power quality and reliability. Motivated by this observation, in this paper, we propose a new method for individual load modeling in power distribution systems. It works by using the measurements only at the feeder-head. It takes an innovative approach to analyzing the load switching events across the distribution feeder itself, instead of or in addition to relying on upstream voltage events that are commonly used in feeder-aggregated load modeling. By tracking the downstream load switching events, the proposed method can make a robust estimation of the ZIP load model parameters for all individual loads. The proposed method is examined on small illustrative test-feeders as well as the IEEE 33-bus test system under various operating scenarios. The adverse impact of errors in measurements and system parameters are also investigated on the performance of the developed load modeling method.

Deterministic aggregated harmonic source models for harmonic analysis of large medium voltage distribution networks

Modelling of harmonics in large medium voltage (MV) distribution networks has so far been a challenge due to the presence of a large number of harmonic sources contributing to total harmonic voltages. This study proposes a deterministic methodology of modelling harmonics in large real MV distribution networks using aggregate harmonic source models parameterised based on measurements and results from the literature. In this research, background harmonic voltages from the sub-transmission system are parameterised based on measurements in the analysed network. Aggregate harmonic current emission of different customer categories in low-voltage (LV) networks (households and small commercial customers) parameterised based on several measurements in LV networks was dominated by residential or office customer type configurations. The harmonic current emission of industrial customers is parameterised based on the results from published literature. Two methods for modelling harmonic sources are used and compared: (i) modelling according to the IEC 61000-3-6 summation law and recommended summation exponents and (ii) modelling using complex phasors. The results of the models show a good match with the measurements from power quality monitors installed in the analysed MV network. Based on these results, the method according to the IEC 61000-3-6 is recommended for large MV distribution networks.