Nameplate Rating Research Articles

Impact of turbine availability and wake effect on the application of dynamic thermal rating of wind farm export transformers

Dynamic thermal rating allows transformers to operate beyond the nameplate rating according to the actual weather and loading conditions. This paper proposes a methodology to improve the application of this technology in the design of new transformers or in the operation of existing transformers connected to wind farms by accurately predicting their load profiles, accounting for the influence of wake effect and turbine availability. Specifically, the variation of turbine availability due to the intermittent wind is considered in the load profile estimation. Additionally, a correction method, which can be incorporated into any wake model, is proposed to improve the accuracy of wake loss computation. A case study shows that the wake effect and the changing turbine availability shorten the time that the transformers maintain at full load, thereby reducing the aging rate of the wind farm export transformers. The findings suggest that considering these two factors in the DTR application can benefit the longevity and efficiency of wind farm exported transformers.

A Thermally-Based Dynamic Approach to the Load Management of Distribution Transformers

The future demand for electric vehicle (EV) charging power is expected to increase the peak loading of existing distribution transformers. In the absence of appropriate solutions, this may lead to the replacement of transformers with higher-rated types, incurring significant expense. Therefore, a novel approach is proposed to improve the management of transformer EV charging loads, avoiding or postponing the need for a transformer upgrade. It allows for the determination of whether loads that exceed transformer nameplate ratings are acceptable in terms of power level, time and duration. This is achieved with knowledge of the maximum charging duration, transformer thermal time constants, current loading levels, voltage regulation limits, oil pressure and temperature limits, and desired transformer lifetime requirements. The results of a case study are presented based on a future EV car park scenario for which an EV charging load is added to the existing base load on a 300 kVA transformer. The results show the proposed approach has advantages over the traditional approach of limiting loading to nameplate levels. These include reduced waiting time and increased levels of charging power, which are achieved without compromising the desired transformer lifetime target.

Monthly energy yield assessment of solar photovoltaic system under uniform irradiance and partial shaded condition

The energy yield of an installed solar photovoltaic (PV) system depends on the environmental and climatic conditions of the location. Since an installed PV module experiences different conditions of temperature, irradiance, wind speed, solar spectrum, etc., therefore how much energy a PV module will generate in the field is not fully understood by its nameplate rating. This is because PV manufacturers rate the modules at only standard test conditions (STC) of 1000 W/m2 and 25oC. Estimating PV energy yield is of practical importance, especially from the point of view of solar project developers and end customers. Moreover, partial shading conditions which is an inevitable situation for the PV panels installed in the field can lower the energy yield. The main objective of the present work is to assess the energy yield of the photovoltaic system under uniform irradiance and partial shading condition. The work highlights the adverse impact of partial shading on energy yield. For this purpose, an experimental setup of a PV system of polycrystalline silicon of rating 135 Wp has been established in the National Institute of Solar Energy, Gurugram, India. An artificially generated scenario of inter-row or self-shading of the modules has been considered for this work. I-V data has been collected in real operating conditions to estimate the PV maximum output power at different conditions of irradiance and temperature under uniform irradiance as well as partial shading condition. Further, weather data for one complete year for the location is also obtained. Using the obtained set of data, monthly PV energy yield is estimated under uniform irradiance and inter-row partial shading condition. It is found that maximum energy is generated in March and least in July under uniform as well as considered partial shading condition. But under long-term considered partial shading condition, the monthly energy yield is reduced to more than 54% every month.

Optimal sizing of the wind farm and wind farm transformer using MILP and dynamic transformer rating

An increase in electricity demand and renewable penetration requires electrical utilities to improve and optimize the grid infrastructure. Fundamental components in this grid infrastructure are transformers, which are designed conservatively based on static rated power. However, load and weather change continuously and hence, transformers are not used most efficiently. For this reason, new technology has been developed: Dynamic transformer rating (DTR). Applying DTR makes it possible to load transformers above the nameplate rating without affecting their lifetime expectancy. This study uses DTR for short-term and long-term wind farm planning. The optimal wind farm is designed by applying DTR to the power transformer and using it as an input to a Mixed-Integer Linear Programming (MILP) model. Regarding the transformer thermal analysis, the linearized top oil model of IEEE Clause 7 is selected. The model is executed for 4 different types of power transformers: 63 MVA, 100 MVA, 200 MVA and 400 MVA. As a result, it is obtained that the net present value for the investment and the capacity of the wind farm increase linearly with respect to the size of the transformer. Then, a sensitivity analysis is carried out by modifying the wind speed, the electricity price, the lifetime of the transformer and the selected weather data. From this sensitivity analysis, it is possible to conclude that wind resources and electricity price are critical parameters for the wind farm’s feasibility.

Optimal Dispatch With Transformer Dynamic Thermal Rating in ADNs Incorporating High PV Penetration

Under the high penetration of distributed photovoltaic (PV) resources, distribution networks face great transfer capacity and voltage rise challenges, which drives the transformation to active distribution networks (ADNs). Transformers are critical and costly components that are generally hard to reinforce or replace. Dynamic thermal rating (DTR) is an effective strategy that can enhance the transfer capacity of existing transformers according to the thermal condition of the equipment. In this article, an optimal dispatch model of ADNs incorporating transformer DTR and active management strategies is proposed. The thermal ratings of transformers are determined by the hot-spot temperature and loss of life (LOL) instead of the nameplate rating. Both the operation cost and transformer LOL are cooptimized in the model to avoid considerable insulation damage while enhancing the transfer capacity. The optimization model is transformed into a mixed-integer second-order cone programming (MISOCP) that can be easily solved. Numerical results demonstrate that the proposed model can significantly promote distributed PV accommodation by the synergy of transformer DTR and active management strategies and effectively address the tradeoff between higher utilization of transfer capacity and increased transformer insulation damage.

Evaluation of dynamic loading capability for optimal loading strategies of power transformers

Increasing needs for operational flexibility encourage concepts of using thermal operational limits of grid equipment instead of nominal limits, e.g., “dynamic line rating” for transmission lines. Using the available loading flexibility of complex and valuable power transformers requires a comprehensive approach. The thermal state and condition of a power transformer are among the most considerable impact factors for planning overload capability and lifetime management in the long term. This paper investigates the dynamic loading capability of power transformers by using a dynamically calculated initial thermal state, set thermal limits and a dynamic thermal model as proposed in the IEC 60076-7:2017. Different thermal limits can be set depending on the operation state, e.g., long-term emergency or insulation-friendly. The suggested thermal limits also consider ageing and dielectric properties of insulation. Thermal limits of equal ageing velocity for different moisture content in solid insulation can be used to operate the transformer with respect to long-term asset management strategies. Using the proposed reverse calculation of the thermal model’s steady-state loading limits, e.g., nameplate rating, the permissible short-term loading beyond the steady-state limits can be obtained for given timespan. The dependency of the steady-state loading limit from ambient temperature is approximated as a polynomial function with linear, quadratic and cubic components. For a safe loading of the power transformer the steady-state limits of the subsystems have to be compared. The study shows that the hot-spot steady-state limits for normal and long-term emergency loading are lower than the top-oil steady-state limits. Power transformers are usually subject to variations in the loading profile. The current thermal state becomes essential for the estimation of the thermal reserve and loading capability, especially when permissible overloading for a time of up to 60 minutes is scheduled. When determining the possible overload for a longer time span, the validity of the results is decreasing due to growing uncertainties. Moreover, the paper proposes a system to enhance the model by a moisture migration model for mineral-oil-immersed power transformers, decision-making under uncertainties assistance and transformer’s intelligent condition assessment system.

Demand Side Management Effects on Substation Transformer Capacity Limits

In high penetrations, demand side management (DMS) applications augment a substation power transformer’s load profile, which can ultimately affect the unit’s capacity limits. Energy storage (ES) applications reduce the evening peaking demand, while time-of-use rates incentivize end-users to charge electric vehicles overnight. The daily load profile is further augmented by high penetrations of photovoltaic (PV) systems, which reduce the midday demand. The resulting load profile exhibits a more flattened characteristic when compared to the historical cyclic profile. Although the initial impact of PV and ES applications may reduce a unit’s peak demand, long-term system planning and emergency conditions may require operation near or above the nameplate rating. Researchers have already determined that a flattened load profile excessively ages a unit’s dielectrics more rapidly. The focus of this research was to identify an approach for establishing new transformer capacity limits for units serving flattened load profiles with a high harmonic content. The analysis utilizes IEEE standards C57.91 and C57.110 to develop an aging model of a 50 MVA SPX Waukesha transformer. The results establish a guideline for determining transformer capacity limits for normal operation, long-term emergency operation, and short-term emergency operation when serving systems with high penetrations of DSM applications.

Reliability considerations and economic benefits of dynamic transformer rating for wind energy integration

An increasing share of renewable energy on the electricity market creates the need for economic and efficient production, operation and integration technologies, associated with the specific behavior of renewable energy sources (RES). Dynamic rating (DR) provides a possibility to apply improvements to the system both during planning and operation stages. The DR benefits are well described in various literature sources. However, DR is often focused on more efficient exploitation of power lines, not power transformers. Power transformers are costly equipment and their efficient usage and planning can have drastic effect on total costs.Our analysis focuses on the dynamic transformer rating (DTR) for wind energy applications. The main objective is to study reliability effects of DTR from the component perspective. We utilize existing knowledge about transformer heat balance models from IEC and IEEE standards to obtain information on the loss of life (LOL) of the transformer under investigation and propose possible improvements for the system in question. The method can be employed for identifying the appropriate transformer size by taking into account ambient temperature and load variations and then overloading the transformer beyond nameplate ratings. The reliability of the proposed application is ensured by calculating the risk of overloading the transformer for each day of the year. A risk of overloading is quantified as LOL of the transformer. The risk is presented as a function of ambient temperature and duration of an overload. The final step consists of an economic analysis, which demonstrates economic benefits of DTR application.

A Probabilistic Approach for Forecasting the Allowable Current of Oil-Immersed Transformers

The overall growth of electrical energy consumption and the spread of distributed generation pose severe problems to system planners and operators, mainly due to the difficulties in expanding and upgrading electrical distribution systems. These problems can be partially overcome through the smart operation of power system components. In particular, under the new smart grid paradigm, lines and transformers could be loaded beyond their nameplate rating in favorable thermal conditions, without loss of rated life or breakdown. This paper deals with the loading of oil-immersed distribution transformers, and it proposes a probabilistic approach based on the monitoring of electrical and environmental conditions. The approach consists in forecasting the predictive distribution of the transformer dynamic rating, and then in forecasting the transformer allowable current. The probabilistic approach takes into account the unavoidable uncertainties involved in the thermal modeling of the transformer; indeed, it selects the allowable current through an index that takes into account both the probability of the allowable current to be higher than the predicted dynamic rating and the corresponding expected load curtailment. Numerical applications are performed on real data to evaluate the effectiveness of the proposed procedure.

Performance of Mn-Zn ferrite magnetic fluid in a prototype distribution transformer under varying loading conditions

The temperature distribution performance of prototype transformer has been evaluated for varying loads. A 3 KVA single phase transformer is prototyped for this purpose and thermocouples were inserted at various places for the study. A novel temperature sensitive magnetic fluid (TSMF - Mn-Zn ferrite magnetic fluid) is proposed to improve the cooling performance. Winding temperature reduces when prototype transformer is submerged in TSMF compared to the same experiment performed with transformer oil, for all loading (normal load, under load, overload). The reduction in temperature with TSMF attributes to the thermo-magnetic convection, and it is the maximum when the Curie temperature of the magnetic fluid is comparable to the hot spot temperature of the transformer. The experimental results match with numerical calculation and the existing simulation. The maximum cooling is observed with TSMF for overload condition and increases normal life expectancy nine times. In case of planned overload condition normal life expectancy doubles for all situations. Study leads to the conclusion that TSMF works as a better transformer coolant under all conditions, delivers more power than its nameplate rating without using any external accessories, and improves normal life of transformer. This will reduce the basic investment cost and maintenance of transformer.

Modelling and Simulation of Single-Phase Series Active Compensator for Power Quality Improvement

A single-phase active series compensator is proposed in this work to reduce harmonic currents at the ac mains and to regulate the dc link voltage of a diode bridge rectifier (DBR) that acts as the front end converter for a voltage source inverter feeding an ac motor. This ac motor drive is used in any of the domestic, commercial or industrial appliances. Under fluctuating ac mains voltages, the dc link voltage of the DBR depicts wide variations and hence the ac motor is used at reduced rating as compared to its name-plate rating. The active series compensator proposed here provides dual functions of improving the power quality at the ac mains and regulating the dc link voltage thus averting the need for derating of the ac motor.

Compendium of photovoltaic degradation rates

AbstractPublished data on photovoltaic (PV) degradation measurements were aggregated and re‐examined. The subject has seen an increased interest in recent years resulting in more than 11 000 degradation rates in almost 200 studies from 40 different countries. As studies have grown in number and size, we found an impact from sampling bias attributable to size and accuracy. Because of the correlational nature of this study we examined the data in several ways to minimize this bias. We found median degradation for x‐Si technologies in the 0.5–0.6%/year range with the mean in the 0.8–0.9%/year range. Hetero‐interface technology (HIT) and microcrystalline silicon (µc‐Si) technologies, although not as plentiful, exhibit degradation around 1%/year and resemble thin‐film products more closely than x‐Si. Several studies showing low degradation for copper indium gallium selenide (CIGS) have emerged. Higher degradation for cadmium telluride (CdTe) has been reported, but these findings could reflect a convolution of less accurate studies and longer stabilization periods for some products. Significant deviations for beginning‐of‐life measurements with respect to nameplate rating have been documented over the last 35 years. Therefore, degradation rates that use nameplate rating as reference may be significantly impacted. Studies that used nameplate rating as reference but used solar simulators showed less variation than similar studies using outdoor measurements, even when accounting for different climates. This could be associated with confounding effects of measurement uncertainty and soiling that take place outdoors. Hotter climates and mounting configurations that lead to sustained higher temperatures may lead to higher degradation in some, but not all, products. Wear‐out non‐linearities for the worst performing modules have been documented in a few select studies that took multiple measurements of an ensemble of modules during the lifetime of the system. However, the majority of these modules exhibit a fairly linear decline. Modeling these non‐linearities, whether they occur at the beginning‐of‐life or end‐of‐life in the PV life cycle, has an important impact on the levelized cost of energy. Copyright © 2016 John Wiley & Sons, Ltd.

Prevention of hot spot temperature in a distribution transformer using magnetic fluid as a coolant

A Mn–Zn ferrite magnetic fluid (TCF-56) having 5.6 mT fluid magnetization and high pyromagnetic co-efficient, (∂M/∂T = 177 A/m K) has been investigated as a coolant in a 3 kW prototype transformer for overloading condition (167%). The winding temperature of a transformer submerged in magnetic fluid reaches at 396.8 K after 3 h of overloading, which is 20 K lower, when the same experiment was carried out with pure transformer oil. Similarly, core and top oil temperature also decrease by 14 and 21 K, respectively, when TCF-56 is used. This cooling performance of TCF-56 attributes to the thermo-magnetic convection, which sets up due to the significant change in magnetization of the fluid with increasing temperature. This can be explained using the Rayleigh number. The normal life of a transformer under 167% overloading condition is calculated for pure oil and TCF-56. The result shows nine times increase in normal life expectancy in TCF-56 fluid compared to use of pure oil. The study leads to the conclusion that Mn–Zn ferrite magnetic fluid (TCF-56) used in a transformer can deliver more power than its nameplate rating with an improved normal life.

ИССЛЕДОВАНИЕ ИЗМЕНЕНИЙ МЯСНОГО СЫРЬЯ В ПОСОЛЕ С ИСПОЛЬЗОВАНИЕМ АКУСТИЧЕСКИ АКТИВИРОВАННОГО РАССОЛА

Research on the use of ultrasonic cavitation in the technology of meat products proves high efficiency in regard to the acceleration of technological processes. It is possible to obtain objective data on the forms of integration of moisture with the material applying the method of NMR relaxation using the device “Minispek PC-120” by Bruker Corporation (Germany) in conjunction with other research methods. Cooled low-fat pork, which is subjected to salting with saturated sodium chloride solution (1:3 ratio) after ultrasonic exposure by the frequency of 20 kHz and nameplate rating of 40 % in the production unit such as mercury-vapor console street lamp with piezoceramic radiator, is used as an object of research. Histological studies have shown that when salting meat the microstructural changes in the muscle fibers and connective tissue occur gradually in a certain sequence. Muscle fiber cross banding weakens gradually, the nuclei undergo pycnosis and become homogeneous. Destructive changes in muscle fibers are characterized by the gradual formation of transverse micro cracks and chinked spaces. When exposing the raw meat in the brine containing 3 % of sodium salt, the brine penetrates to a depth of 1.5 cm already on the first day; on the fourth day the brine spreads to the entire muscle depth. Salting brine containing 4% penetrates to a depth of 2.0 cm on the first day of salting, on the fourth day the brine spreads to the entire depth of the muscles. Results of the study of relaxation characteristics of water with the use of NMR method shows that the bulk of the water in the meat is distributed among the factions with the times T12 and T22, and the amount of water with the time T2b is small, and according to the results of many studies, is little subjected to changes. Conducted research has revealed that the use of the activated brine has a positive effect on the kinetics of salting and allows significantly reducing the duration of exposure of whole muscle raw materials in salting by increasing the intensity of the diffusionosmotic processes.

DC Arc Flash Studies for Solar Photovoltaic Systems: Challenges and Recommendations

A dc arc flash hazard exists in solar photovoltaic (PV) power systems, but there is no widely accepted methodology for characterizing the severity of the hazard. Calculation methods have been proposed, and most rely on the nameplate $I$ – $V$ characteristic of the PV modules at standard test conditions to determine the worst case incident energy. This paper proposes to consider other factors in performing a dc arc flash hazard analysis, including possible weather conditions and variations of PV module characteristics from the datasheet ratings. It is recommended to consider two conditions when determining the worst case incident energy from a PV system: 1) the failure of all protective devices to trip within 2 s due to insufficient current and 2) the array output power exceeding the nameplate rating due to technological and environmental factors.

Sizing and placement of distribution substations considering optimal loading of transformers

Summary In recent decades, many researches have been focused on the distribution planning, mostly on the medium voltage (MV) level. However, a considerable part of distribution system losses is pertained to low voltage (LV) networks, and the costs of MV networks are comparable with those of LV networks. Therefore, it is necessary to study this subject. In this paper, a new method is presented to determine the optimal placement of distribution transformers. This method determines the optimal location, size, and number of these transformers. A transformer loading above the nameplate rating is allowed without any influence on the relative thermal aging rate. This is carried out during peak hours to benefit from low load currents during off-peak hours. The proposed approach is applied to a test system consisting of 42 electric load points. It is observed that the proposed planning approach considerably reduces the total cost of the distribution transformer placement as well as the costs of the associated capital and losses. Copyright © 2014 John Wiley & Sons, Ltd.

Real-Time Distributed Control for Smart Electric Vehicle Chargers: From a Static to a Dynamic Study

At high penetrations, uncontrolled electric vehicle (EV) charging has the potential to cause line and transformer congestion in the distribution network. Instead of upgrading components to higher nameplate ratings, we investigate the use of real-time control to limit EV load to the available capacity in the network. Inspired by rate control algorithms in computer networks such as TCP, we design a measurement-based, real-time, distributed, stable, efficient, and fair charging algorithm using the dual-decomposition approach. We show through extensive numerical simulations and power flow analysis on a test distribution network that this algorithm operates successfully in both static and dynamic settings, despite changes in home loads and the number of connected EVs. We find that our algorithm rapidly converges from large disturbances to a stable operating point. We show that in a test setting, for an acceptable level of overload, only 70 EVs could be fully charged without control, whereas up to around 700 EVs can be fully charged using our control algorithm. This compares well with the maximum supportable population of approximately 900 EVs. Our work also provides engineering guidelines for choosing the control parameters and setpoints in a distribution network.

A Novel and Fundamental Approach Toward Field and Damper Circuit Parameter Determination of Synchronous Machine

In this era of advanced computing where complex algorithms and expensive approaches are used to determine the machine parameters of a synchronous machine (SM), this paper proposes a novel, economical, and yet fundamental approach toward estimation of the d- and q-axis fields and damper circuit parameters of a low-/medium-power wound-field SM. The proposed novel methodology employs fundamental voltage, current, and flux linkage relationships of the three-phase wound-field SM in a-b-c reference frame theory. First, the proposed methodology has been explained in detail using analytical equations and then employed to determine the aforementioned parameters of a small laboratory SM. Other equivalent circuit parameters have been determined using conventional tests. Further validation of the proposed methodology was performed using two other larger machines with different nameplate ratings. Moreover, the aforementioned parameters of the larger machines were also experimentally determined using IEEE standard tests. Finally, a comparison of the results obtained employing the conventional and the proposed methodologies was performed, and the proposed methodology has been established to be valid as the results are in close agreement.

Assessment of transformer mineral oil condition based on dc and ac conductivity

Due to the increase of electrical energy consumption, the power transformers are loaded beyond nameplate rating. The degradation of their insulation systems is intensified and the dielectric properties are worsened. Therefore, the interest of asset managers, regarding the condition of their transformers is a very important topic. Electrical conductivity is strongly influenced by the high operating temperatures and catalysis products (water, gases, acids) that appear as a result of oil and paper degradation reactions during transformer operation. The present paper is focused on an experimental study regarding the influence of water, acids and gases (hydrogen, acetylene, ethylene, methane etc.) resulted from degradation processes on electrical conduction phenomena of mineral oil. For that, the frequency/time variation curves of ac/dc conductivity for different concentrations of water, acids and gases are drawn. Finally, the correlation between ac and dc conductivity and the possibility to assess the ageing state of the mineral oil based on these quantities are analyzed.

STC power for 15 MW of PV comparing nameplate, initial power and power after 4 years

To date, the majority of quality controls performed at PV plants are based on the measurement of a small sample of individual modules. Consequently, there is very little representative data on the real Standard Test Conditions (STC) power output values for PV generators. This paper presents the power output values for more than 1300 PV generators having a total installed power capacity of almost 15.3 MW. The values were obtained by the INGEPER-UPNA group, in collaboration with the IES-UPM, through a study to monitor the power output of a number of PV plants from 2006 to 2009. This work has made it possible to determine, amongst other things, the power dispersion that can be expected amongst generators made by different manufacturers, amongst generators made by the same manufacturer but comprising modules of different nameplate ratings and also amongst generators formed by modules with the same characteristics. The work also analyses the STC power output evolution over time in the course of this 4-year study. The values presented here could be considered to be representative of generators with fault-free modules. Copyright © 2012 John Wiley & Sons, Ltd.