Distribution Transformer Aging Research Articles

Assessment of Electric Vehicle Charging On Distribution Transformer Aging, Aswan City as a Case Study

Assessment of Electric Vehicle Charging On Distribution Transformer Aging, Aswan City as a Case Study

Assessment and mitigation of the influence of rising charging demand of electric vehicles on the aging of distribution transformers

The world is currently moving towards more environmentally friendly modes of transport. As a result, the sales of electric vehicles (EVs) are growing exponentially, and the new charging-associated loads are influencing the operation of the power distribution networks and their elements. Given the stochastic nature of the additional load from EVs, it is difficult to predict such a load with analytical methods. In this regard, a fuzzy-logic-based tool to assess the influence of the charging load of EVs on the transformers’ aging was developed in the MATLAB-Simulink environment, and the impact of different EV penetrations was simulated and assessed. The fuzzy-logic-based model incorporates the influence of ambient temperature, power quality, and overloads. It comprises a diagnostic part, which warns a user about possible issues by providing actual information about the transformer's state, and a tuning part, which is aimed at optimizing the transformer's load level and the power factor at the secondary voltage side. Moreover, the effectiveness of photovoltaic generation units, shunt capacitor banks and battery energy storage systems, installed at the secondary voltage side, for distribution transformers’ service life extension were analyzed. The results show that, for high EV penetration levels, a combined grid reinforcement strategy allows to reduce the loss of life of the transformer by around 3–5 times.

Impact of photovoltaic generation on the allowed revenue of the utilities considering the lifespan of transformers: A Brazilian case study

This paper investigates the effect of distributed photovoltaic (PV) generation on the aging of overhead distribution transformers and on the allowed revenue of the utilities that defines tariff readjustments. A methodology based on the calculation of the depreciation rate of assets and replacement of transformers as a function of transformers’ aging is proposed. Different penetration levels of PV generation are considered by means of a probabilistic load flow using the Monte Carlo simulation method. Randomness of the load, solar irradiation, location of the PV systems and ambient temperature are modeled. Three indexes are defined to represent the effects and conduct the discussion. The network hosting capacity is analyzed in relation to overvoltage and overcurrent impacts. A realistic case study based on representative feeders of a distribution utility in the southeast of Brazil is conducted by computer simulations. Results indicate that the effect of distributed PV of increasing revenue readjustments occurs at PV penetrations higher than 40%. Overvoltages can limit the amount of distributed PV only at very high penetration levels and overcurrents are not a major problem.

Automated energy storage and curtailment system to mitigate distribution transformer aging due to high renewable energy penetration

The increase of distributed generation units in low voltage distribution grids, stimulated by different mechanisms (mostly economic), like the Energy Performance of Buildings Directive in Europe and the associated nearly-Zero Energy Building concept, stresses in-depth research on the possible impacts that such generation may impose on fundamental grid equipment such as distribution transformers.Taking this into consideration, this paper has two main objectives: i) to analyze the impacts of distributed generation on a non-residential building supplied by a dedicated distribution transformer when this building is converted to a nearly-Zero Energy Building; and ii) to develop a Transformer Anti-Aging Protection System (TAAPS) that mitigates existing negative impacts in order to reduce transformer aging.The present study is based on 1-year, 1-min resolution, real electricity demand and weather data and uses the standard IEC 60076-7 (Loading Guide For Oil-immersed Power Transformers) to model the transformer aging. The collected results show that the introduction of distributed generation increases transformer aging and that the proposed protection system (TAAPS) fulfills its objectives preventing the excessive aging. An economic analysis, related with the proposed system, is also provided in this paper.

Vehicle-Directed Smart Charging Strategies to Mitigate the Effect of Long-Range EV Charging on Distribution Transformer Aging

The recent introduction of affordable long-range electric vehicles (EVs) has the potential to trigger more widespread adoption of EVs with higher charging needs. Increased EV charging can have a detrimental effect on the distribution grid, especially by causing accelerated aging of transformers. Although many EV smart-charging strategies have been proposed to mitigate this problem, centralized and distributed smart-charging strategies require numerous new infrastructure components and, thus, take time and money to implement. This article proposes the term vehicle-directed smart charging to describe strategies that individual EVs can use to charge in a more intelligent way and, thus, lessen grid impact. This article proposes a new vehicle-directed smart charging concept, random-in-window (RIW), which has fixed-rate and variable-rate variants. The RIW strategy allows for random charging start times within a specific time window after the residential peak load has reduced. The RIW strategies are compared to other strategies using the real-world logged driving data from 150 drivers for one week using long- and short-range EV models. A transformer aging model indicates that the RIW strategies are approximately as good as a fully controlled centralized smart-charging algorithm at EV penetration rates up to 60% for long-range EVs and 70% for short-range EVs.

Distribution Transformer's Loss of Life Considering Residential Prosumers Owning Solar Shingles, High-Power Fast Chargers and Second-Generation Battery Energy Storage

In this study, the effect of the electric vehicle high-power fast chargers in the residential sector on the aging of distribution transformers is probabilistically quantified. This work takes into consideration the presence of the traditional rooftop solar photovoltaic panels and the most recent technology namely rooftop solar shingles used in residential prosumers (power producers and consumers). Moreover, the effectiveness of the second-generation residential home and community battery energy storage, installed in the residential prosumers and in the electric utilities’ premises, in preserving the distribution transformer's lifetime is discussed and conclusions are drawn.

A case study on the impact of nearly Zero-Energy Buildings on distribution transformer aging

This paper addresses the impacts introduced on Distribution Transformer (DT) aging due to the integration of nearly Zero-Energy Buildings (nZEBs) into existing Low-Voltage Distribution Grids. The study considers a neighborhood in the Portuguese municipality of Évora where buildings are set to become nZEBs using solar Photovoltaic systems. This transition is reproduced by simulation using real pre-transition electricity demand and climate data. Following the IEC 60076-7 standard to model the aging process of the considered DT, the collected results show that nZEBs can have a positive impact on the analyzed aging process. However, when relatively high nZEB integration levels are considered, this study also reveals that the subsequent reverse power flows can achieve relatively large magnitudes, especially around noon, thus accelerating the DT's aging into prohibitive values. Given the utmost importance of the nZEB concept for the European building stock, this study identifies nZEBs operation related issues that can result from excessive coincident on-site generation and presents a battery based power export limitation strategy to mitigate such impacts on DT aging.

Optimal Design of Community Battery Energy Storage Systems With Prosumers Owning Electric Vehicles

This paper presents a novel approach that aims to assist a distribution system operator to intelligently design the community battery energy storage systems considering high penetration of prosumers equipped with rooftop solar photovoltaics and electric vehicles. The design problem is mathematically formulated after incorporating the battery storage system's cost model. The results show the capability of the proposed approach to optimally design the community battery energy storage systems. This results in improving the voltage profile, reducing the power loss, and mitigating the distribution transformer aging while attaining a profit from the energy arbitrage. Also, the obtained results show a positive net present value of using the community battery energy storage systems, which demonstrates the cost-effectiveness of the proposed optimal design approach. The proposed approach may represent an effective tool for future disturbed energy resources management systems.

Co-Optimization of Distribution Transformer Aging and Energy Arbitrage Using Electric Vehicles

The advent of electric vehicles (EVs) will bring forth increases in power transmitted over longer periods of time through the distribution power grid. Such an effect will result in accelerated loss-of-life of distribution grid assets including pole-top transformers. As preventive and corrective measures, the charging of the set of EVs connected to a particular pole-top transformer can be centrally managed (e.g., by a distribution system operator or independent aggregator). This paper proposes a centralized model to co-optimize the transformer loss-of-life with the benefits for EVs’ owners on charging/discharging management. The proposed model is compared against a decentralized optimization model in which EVs’ owners optimize their benefits, while ignoring the effect on the transformer. Results show the benefit of the centralized strategy in maintaining the grid assets, while modestly reducing consumers’ arbitrage benefits.

Optimal design of secondary distribution system considering electric vehicles high‐power residential fast chargers

This study presents a new approach for optimally designing a secondary distribution system (SDS) considering electric vehicle (EV) charging demand and in particular the high-power fast chargers in residential dwellings. The mathematical formulations of the optimal SDS design problem, which minimises the total annual cost per customer ( TACCu ) taking into consideration the voltage and the distribution transformer (DT) aging constraints, are developed. The proposed design approach, considering different house sizes and types, are also discussed. The obtained designs are then tested and verified through an IEEE 34-test distribution system and a real distribution system. The results have shown that the proposed optimal design approach was able to accommodate the EV charging demand without any violations of voltage and a DT aging constraints.

On the impact of single-phase plug-in electric vehicles charging and rooftop solar photovoltaic on distribution transformer aging

This study investigates the impact of single-phase plug-in electric vehicles charging on increasing the rate at which center-tapped distribution transformers experience aging. Distribution transformer aging is investigated considering varying rooftop solar photovoltaic generation penetration rates. Monte Carlo methods are used to probabilistically estimate the transformer’s loss of life considering the effect of time-of-use (TOU) pricing. The results of applying the proposed method have revealed that plug-in battery electric vehicle charging impact on both transformer aging and neutral current is largest in the case that vehicles charge based on time-of-use pricing methods. Further application has shown that while rooftop solar photovoltaic generation reduces transformer aging, no significant reduction in neutral current is observed.

On the Impact of High Penetration of Rooftop Solar Photovoltaics on the Aging of Distribution Transformers

This paper investigates the effect of increasing the penetration of Rooftop Solar Photovoltaics (PVs) on a Distribution Transformer's (DT's) Loss of Life (LOL). The modeling of the secondary distribution system is introduced and the Markov chain Monte Carlo is used to probabilistically estimate the hourly loading on a DT while the LOL is estimated based on the DT's thermal model. The results have shown that the minimum impact on the DT's LOL may be only achieved at 60% penetration of PVs that can be considered as the maximum permissible solar PVs penetration level.

Consideration of the Impacts of a Smart Neighborhood Load on Transformer Aging

Smart grid solutions with enabling technologies such as energy management systems (EMSs) and smart meters promote the vision of smart households, which also allows for active demand side in the residential sector. These technologies enable the control of residential consumption, local small-scale generation, and energy storage systems to respond to time-varying prices. However, shifting loads simultaneously to lower price periods is likely to put extra stress on distribution system assets such as distribution transformers. Especially, additional new types of loads/appliances such as electric vehicles (EVs) can introduce even more burden on the operation of these assets, which is an issue that needs special attention. Such extra stress can cause accelerated aging of distribution system assets and significantly affect the reliability of the system. In this paper, the impact of a smart neighborhood load on distribution transformer aging is investigated. The EMS of each household is designed to respond to prices and other signals emitted by the responsive load serving entity within the relevant demand response strategy. An optimization framework based on mixed-integer linear programming is presented in order to define the EMS structure. Then, the equivalent aging of the distribution transformer is examined with a thermal model under different scenarios. The case studies that are presented indicate that the integration of EVs in residential premises may indeed cause accelerated aging of the distribution transformers, while the need to investigate the efficiency of dynamic pricing mechanisms is rendered evident.

Effects of distributed PV generation on California’s distribution system, Part 1: Engineering simulations

Deployment of high-penetration photovoltaic (PV) power is expected to have a range of effects – both positive and negative – on the distribution grid. The magnitude of these effects may vary greatly depending upon feeder topology, climate, PV penetration level, and other factors. In this paper we present a simulation study of eight representative distribution feeders in three California climates at PV penetration levels up to 100%, supported by a unique database of distributed PV generation data that enables us to capture the impact of PV variability on feeder voltage and voltage regulating equipment. We find that feeder location (i.e. climate) has a stronger impact than feeder type on the incidence of reverse power flow, reductions in peak loading and the presence of voltage excursions. On the other hand, we find that feeder characteristics have a stronger impact than location on the magnitude of loss reduction and changes in voltage regulator operations. We find that secondary distribution transformer aging is negligibly affected in almost all scenarios.

Impact of Second-Generation Plug-In Battery Electric Vehicles on the Aging of Distribution Transformers Considering TOU Prices

This paper investigates the impact of second-generation (SG) plug-in battery electric vehicles (PBEVs) on distribution transformers insulation life considering the time-of-use (TOU) prices. The effect of dual Level 2 charging, 3.7kW, and 6.6kW on the loss of life (LOL) of a distribution transformer is also studied. The LOL results demonstrating the effectiveness of SG PBEVs charging as two clusters versus one cluster are discussed and conclusions are drawn.

Reducing the Impact of EV Charging Operations on the Distribution Network

A key assumption made in this paper is that electric vehicle (EV) battery charging profiles are rectangular. This requires a specific and new formulation of the charging problem, involving discrete action sets for the EVs in particular. The considered cost function comprises of three components: 1) the distribution transformer aging; 2) the distribution energy losses; and 3) a component inherent to the EV itself (e.g., the battery charging monetary cost). Charging start times are determined by the proposed distributed algorithm, whose analysis is conducted by using game-theoretic tools such as ordinal potential games. Convergence of the proposed algorithm is shown to be guaranteed for some important special cases. Remarkably, the performance loss with respect to the centralized solution is shown to be small. Simulations, based on realistic public data, allow one to gain further insights on the issues of convergence and optimality loss, and provide clear messages about the tradeoff associated with the presence of the three components in the considered cost function. While simulations show that the proposed charging policy performs quite similarly to existing (continuous) charging policies such as valley-filling-type solutions when the non-EV demand forecast is perfect, they reveal an additional asset of rectangular profiles in presence of forecasting errors.

General Smart Meter Guidelines to Accurately Assess the Aging of Distribution Transformers

Smart grid technology has enhanced the ability to monitor numerous aspects of the distribution system for improving system performance and reducing system losses. One benefit is the continuous assessment of insulation degradation within distribution transformers, which is commonly referred to as a loss of life assessment. Calculating the expended life of a transformer consists of determining its winding hotspot temperature, which fluctuates with customer demand, ambient temperature, and cooling characteristics. Most residential transformers in service today do not possess the ability to connect to a smart grid communication system for continuous monitoring; therefore, they rely upon fusing for protection against extreme loading conditions. Smart meters can provide utility companies with the information required to identify distribution transformers that are experiencing higher than rated losses that can ultimately reduce their expected life. The research presented in this paper defines the smart meter functions required to accurately assess the aging of distribution transformers according to IEEE Std C57.91 and C57.110. To establish general accuracy guidelines, transformer loading indices were developed to evaluate acute excessive loading, long-term excessive loading, and excessive loading due to harmonics. Metering functions evaluated included power factor, harmonic demand, and ambient temperature.

Estimating the Impact of Electric Vehicle Smart Charging on Distribution Transformer Aging

This paper describes a method for estimating the impact of plug-in electric vehicle (PEV) charging on overhead distribution transformers, based on detailed travel demand data and under several different schemes for mitigating overloads by shifting PEV charging times (smart charging). The paper also presents a new smart charging algorithm that manages PEV charging based on estimated transformer temperatures. We simulated the varied behavior of drivers from the 2009 National Household Transportation Survey, and transformer temperatures based an IEEE standard dynamic thermal model. Results are shown for Monte Carlo simulation of a 25 kVA overhead distribution transformer, with ambient temperature data from hot and cold climate locations, for uncontrolled and several smart-charging scenarios. These results illustrate the substantial impact of ambient temperatures on distribution transformer aging, and indicate that temperature-based smart charging can dramatically reduce both the mean and variance in transformer aging without substantially reducing the frequency with which PEVs obtain a full charge. Finally, the results indicate that simple smart charging schemes, such as delaying charging until after midnight can actually increase, rather than decrease, transformer aging.