Heat Run Test Research Articles

Dynamic Thermal Behavior of Natural Ester Liquid-Filled Transformer for Offshore Wind Turbine in Cold Climate

The current study presents a dynamic thermal model (DTM) which accounts for variation of liquid properties with temperature to predict the development of temperature pattern of the entire transformer for offshore wind application during energizing process. The DTM is benchmarked against dynamic heat run test which is applied on a natural ester filled transformer designed for offshore wind application. The transformer is equipped with fiber-optic sensors (FOS) to observe the temperature distribution of the winding and the liquid during heating up and cooling down processes. The investigations involve in-depth analysis of the thermal behavior of the winding and liquid at different locations to give physical interpretation of the dynamic thermal response of different transformer components during dynamic loading. The results are further validated with transient computational fluid dynamics (CFD) simulations. Eventually, the study gives guidelines and recommendations of dynamic loading of transformer filled with natural ester in cold climate.

Investigating and calculating the temperature of hot-spot factor for transformers

This article explores the measurement of temperature in transient states, utilizing the principles of heat transfer and thermal-electrical metaphor. The study focuses on the nonlinear thermal resistances present in various locations within a distribution transformer, while taking into account variations in oil physical variables and temperature loss. Real-time data obtained from heat run tests on a 250-MVA-ONAF cooled unit, conducted by the transformer manufacturer, is used to verify the thermal designs. The observations are then compared to the loading framework of the IEC 60076-7:2005 system. The findings of this research provide a better understanding of temperature measurement in transient states, particularly in distribution transformers, and can be applied to the design and development of more efficient and reliable transformer systems.

インホイール駆動システム向けダイレクト油冷技術の基礎評価

We are developing a small and lightweight direct-drive system toward realization in-wheel electric vehicles (EVs). To increase the power density of the motor, we developed a direct-cooling motor that immersed core, coils, and magnets in cooling oil and improved cooling efficiency. Moreover, we performed measurements of frictional loss, measurement of pressure drop, and continuous heat-run test on the test bench. The results demonstrated that direct cooling is effective and continuous operation can be achieved. Additionally, it was found that the coil temperature can be controlled by the flow rate of the cooling oil. In this paper, we report the concept of direct cooling and the measurement results.

Synchronous PWM Control With Carrier Wave Phase Shifts for Permanent Magnet Synchronous Motor

Permanent magnet synchronous motors (PMSMs) are utilized in a wide range of applications as high-power-density and highly efficient drive sources. However, a PMSM generates electromagnetic vibration and loss caused by space and time harmonics. In this article, we propose a synchronous pulsewidth modulation control method for shifting the carrier wave phase against the modulation wave phase. As the proposed control matches the vibration frequencies caused by space and time harmonics, the pulsation caused by the space harmonics is canceled out by the pulsation caused by the time harmonics with the phase shift of the carrier wave. Experiments on the proposed control demonstrate that vibration at a specific frequency was reduced by 46% at 7900 rev/min. In addition, to change the carrier phase with small <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> -axis current harmonics, magnet eddy-current loss was reduced by shifting the carrier wave phase. The temperature of a magnet was measured in a heat-run test, and the temperature rise of the magnet could be suppressed by 5 K.

CFD SIMULATIONS OF OIL IMMERSED AND DRY TYPE TRANSFORMERS

At Siemens the in-house CFD code UniFlow is employed to investigate fluid flow and heat transfer in oil immersed and dry type transformers as well as transformer components like windings, cores, tank walls, and radiators. We outline its physical models and numerical solution methods. As an oil transformer application of the method, the simulation of oil flow and heat transfer in 5 windings of a prototype transformer with ONAN/ONAF cooling mode is described. It corresponds to a heat run test with the total losses. Furthermore, we outline an application to an AFWF cast resin transformer prototype operated at ships in an enclosure. The ventilator driven air flow is cooled by sea water. In addition to the LV and HV windings the core is simulated. Here also the heat radiation makes a significant contribution to the heat transfer.

Emulating Full-Load Testing of Air-Cooled Nanocrystalline IHT at Zero Power

High-power high-frequency air-cooled induction heating transformers, mostly used as current multiplier and isolation purposes, are custom designed. For their reliable performance and long life, the thermal evaluation at rated load is necessary. Creating an equivalent load as a test facility for reliability testing of such type of transformer is difficult. Characteristics of such loads drastically change after Curie temperature. Moreover, prolonged heating could increase the nearby ambient temperature and, more importantly, the traditional heat run test wastes large amount of energy. Whenever the coil is energized, windings of transformer draw respective rated current; even at no-load condition, the copper loss is at rated value. While both windings drawing rated current at a desired frequency, using the concept of localized eddy current loss as well as excess eddy current loss, this article proposes a novel method to inject requisite core loss to the magnetic circuit to emulate the characteristics of full-load condition but the power drawn from the transformer would be zero. The proposed idea would be validated where only a 200-W resonant inverter would be used to inject power loss equivalent to full-load condition of 30-kW transformer to emulate the heat run test.

Determination of local losses and temperatures in power transformer tank

Paper presents research of local losses and temperature rise in transformer steel tank. First experimental method based on initial rate of rise of temperature is presented. This is a direct method for determining distribution of losses in transformer structural steel parts. Technique relies on the fact that after a body has settled at a steady state temperature and the internal heat source is suddenly removed or applied, the initial rate of temperature change at any point is proportional to heat input (loss density) at that point. To test applicability of sensors and instrument for the local loss measurement method, measurement system was tested on conductors (strips) and magnetic steel rings. Second part of experimental work consisted of investigations on model for tank local overheating. The model consisted of excitation windings that were sources of magnetic field. Existence of three separate windings gave the possibility to change value and position of magnetic field source inside the tank. Local losses in the tank were evaluated by proposed method of initial rate of rise of temperature. Heat-run tests were made on the model and local temperatures on the tank were measured. Measured local losses and local temperatures were used for determining local heat transfer coefficients on tank – oil interface. It was concluded that heat transfer coefficients can be presented as function of heat flux from tank to oil. Finally, temperatures in transformer tank were calculated by finite element method. Losses calculated by electromagnetic calculation represented heat sources in thermal numerical model. Heat transfer equations were solved in solid domain (tank) while cooling conditions were defined by heat transfer coefficients checked experimentally. Calculated temperatures were compared to measured temperatures and gave good agreement.

Dynamic thermal model of liquid-immersed shell-type transformers

The paper presents the dynamic thermal model of shell-type transformers. The data from heat run test and field operation of single-phase shell-type transformer 512 kV/13.8 kV/13.8 kV, 100/50/50MVA, with oil directed air forced (ODAF) cooling, and 9 built-in fibre-optics sensors, are used for the development and testing of the model. Starting model was standard IEC model with parameters determined from heat run test and transformer field operation data. For the computations supporting software in C# was developed, whereas Microsoft Solver Foundation (MSF) library was used for the estimation of the parameters from the recorded values during thermal transients. The following improvements of the IEC model were introduced: a)the dependence of the losses on the temperature, b)correction of top oil temperature rise based on ambient temperature and c)correction of hot-spot to top oil temperature difference depending on top oil temperature. The paper proposes a method based on the application of the safety margins, which guarantees the measured temperatures will be lower than calculated in high temperature region. The testing of the method was performed on one-year data set from transformer operation.© 2017 Elsevier Inc. All rights reserved.

Recent developments in engineering design for the quasi-axisymmetric stellarator CFQS

A quasi-axisymmetric stellarator, the CFQS, has been designed as a joint project of the National Institute for Fusion Science and Southwest Jiaotong University to prove intrinsic advantages of quasi-axisymmetry. Principal parameters of the CFQS are as follows: the major radius is 1 m, the magnetic field strength is 1 T, the aspect ratio is 4, and the toroidal periodic number is 2. The magnetic field configuration is designed based on that of the CHS-qa. Enhanced confinement properties within the context of neoclassical theory are achieved by its quasi-axisymmetric configuration. In the entire radial range, the magnetic well is retained to keep favourable stability features in the magnetohydrodynamic equilibrium. A magnetic field coil system was designed for the CFQS, which consists of 16 modular coils, 12 toroidal field coils, and 4 poloidal field coils. The supporting structure is designed to withstand strong electromagnetic force under 1 T operation, maintaining enough space for heating and diagnostic systems. The mock-up modular coil with the most complicated shape was constructed by Hefei Keye Electro Physical Equipment Manufacturing Co., Ltd. to check manufacturability and the achieved accuracy. A heat-run test was performed to check the temperature rise of conductors, and the capability of 1 T operation was confirmed. After various tests for the mock-up coil, construction of actual modular coils and the vacuum vessel has begun.

A Thermal Network Approach for a Quick and Accurate Study of Multiple Connected Switchgears

In the presented paper a thermal network approach was proposed for a quick and accurate study of the thermal performance of two interconnected and unsymmetrically energized switchgears. The method base on the full computational fluid dynamics (CFD) simulation of a single standalone switchgear which was used to provide unknown input parameters for a constructed thermal network. Next, a thermal network was compared with CFD simulation results to confirm the correctness of the network setup. Finally the network was used to build a model of two adjacent switchgears which was verified by performed heat run test. As a result of the studies it was found that proposed thermal network approach can be successfully applied for a quick and accurate study of a multiple connected switchgear systems.

Evaluation of the DC bus link capacitors and power transistor modules in the qualification testing of PV inverters

AbstractTo achieve useful, fair, and cost‐effective design qualification and type approval testing for PV inverters, two procedures were developed and demonstrated to evaluate the bus link capacitors and power transistor—and where applicable, its module—for long‐term reliability. We evaluate the inverter under simulated use conditions with heat run tests to find the highest operating temperatures reached by these components. The procedures are demonstrated in this study with a 4‐kVA class string inverter. The maximum temperature of the DC bus link capacitors was determined to be 76.4°C when the ambient temperature was at the inverter's maximum rated temperature of 60°C with derating occurring. On the other hand, the power transistor module's maximum temperature using the heat sink temperature as the index, 92.6°C, occurred in an ambient of 40.8°C when derating was not occurring. The conditions found for upper temperatures exhibited by these components are, respectively, proposed for implementation in the dry heat and thermal cycling tests in the IEC 62093 “Power conversion equipment for photovoltaic systems – Design qualification testing” international standard draft so that the levels applied in the tests are in proportion to those experienced in the field. As a result, fairer evaluation of inverter reliability is realized.

Experimental and Numerical Investigation of the Internal Temperature of an Oil-Immersed Power Transformer with DOFS

To accurately detect and monitor the internal temperature of an operating power transformer, the distributed optical fiber sensor (DOFS) was creatively applied inside an oil-immersed 35 kV transformer through high integration with the winding wire. On the former basis, the power transformer prototype with a completely global internal temperature sensing capability was successfully developed and it was also qualified for power grid operation through the ex-factory type tests. The internal spatially continuous temperature distribution of the operating transformer was then revealed through a heat-run test and the numerical simulation was also applied for further analysis. Hotspots of windings were continuously located and monitored (emerging at about 89%/90% height of low/high voltage winding), which were furtherly compared with the IEC calculation results. This new nondestructive internal sensing method shows a broad application prospect in the electrical equipment field. Also, the revelation of transformer internal distributed temperature can offer a solid reference for both researchers and field operation staff.

Algoritam i softver za skraćenje i obradu rezultata tipskog ogleda zagrevanja uljnih energetskih transformatora

One of the type tests on power transformers is heat run test, which checks that the characteristic winding and oil temperatures are below allowed values. This is necessary to prevent accelerated ageing and shortening of the transformer life if the temperatures are higher than the allowed values. In addition to the user's clear interest, manufacturers are also interested in the results of heat run tests, as they can verify and improve the accuracy of calculation methods and software [1]. Then they can increase the accuracy and reduce the safety margins of calculation methods, so they can change transformer construction and achieve temperatures that are much closer to the allowed ones. As the result of that, they can reduce material and production costs. The minimal test is defined by the IEC standard [2]. Due to the great mass and consequent great thermal capacity of the transformer parts, heat run tests take a very long time. The paper presents an algorithm, and its software implementation, which reduce the occupancy of test station, energy consumption and the total cost of performing heat run tests.

Use of Alternative Fluids in Very High-Power Transformers: Experimental and Numerical Thermal Studies

The very high-power transformer lifespan depends mainly on the temperature that cellulose insulation reaches during its operation. Traditionally, its cooling has been carried out using mineral oil as coolant. Nowadays, alternative ester-based liquids are under study as substitutes due to their better environmental and fire-safety properties. This paper compares the cooling capacity of two ester-based fluids with that of a mineral oil using a 3D numerical model of a 100 MVA low voltage winding of a power transformer with axial cooling system and ONAN cooling mode. Heat-run test results with mineral oil have been used to validate this model. As a first approximation, according to the comparison developed, ester-based fluids could replace mineral oils in this type of transformers when they are going to work in a range of powers close to the rated one.

Numerical Estimation and Experimental Verification of Optimal Parameter Identification Based on Modern Optimization of a Three Phase Induction Motor

The parameters of electric machines play a substantial role in the control system which, in turn, has a great impact on machine performance. In this paper, a proposed optimal estimation method for the electrical parameters of induction motors is presented. The proposed method uses the particle swarm optimization (PSO) technique. Further, it also considers the influence of temperature on the stator resistance. A complete experimental setup was constructed to validate the proposed method. The estimated electrical parameters of a 3.8-hp induction motor are compared with the measured values. A heat run test was performed to compare the effect of temperature on the stator resistance based on the proposed estimation method and the experimental measurements at the same conditions. It is shown that acceptable accuracy between the simulated results and the experimental measurements has been achieved.

Experimental Analysis of Performance and Thermal Capability of Three Phase Squirrel Cage Induction Motor Using Plastered Composite Conductors

Background: This article deals with the analysis on improved performance and efficiency of induction motor by using nano composites for stator winding. Methods: The nanocomposites are added with different enamel. Enamel is mostly preferred for induction motors’ winding, due to three main reasons: adhesion, infusion and plaster. To predetermine the plaster and nanocomposite conductor’s behavior when they are used for transmitting AC currents and developing AC magnetic field, a numerical analysis is performed. The total heat losses are determined by the heat run test. Open circuit and short circuit tests are used to analyze the performance and efficiency of the proposed induction motor. Results: The AC losses of composite and plaster conductors having good accord are compared with previous solid and hollow conductors. Analysis of the coil by a composite and plaster conductor shows that the AC losses in low current are lower than the coil, which is wrapped by a solid, and hallow conductors. Due to this reason, composite and plaster conductors are considered advantageous for low and medium power motors. Conclusion: Adding nano composites with the plaster material will help to improve electrical, thermal and mechanical characteristics. The property of enamel can change the lifetime of induction motor. The induction motor winding makes use of nano composites SiO2 and TiO2 with enamel coated.

Improved Dynamic Thermal Model With Pre-Physical Modeling for Transformers in ONAN Cooling Mode

In order to dig out more potential of design parameters besides nameplate information, a set of pre-physical modeling methods was proposed and integrated into the dynamic thermal modeling procedure, aiming to optimize the key thermal parameters individually. In this method, a global oil momentum model was created considering the flow resistance, and for the problem of heat transfer between windings, oil flow, and radiator, a whole oil flow thermal model was established based on heat transfer principle and energy conservation. Then, the double logarithmic linear regression was applied to quantify the value of the exponents. According to the overload heat run tests, the temperature calculated by the existing models during multiple overloads is obviously different from the measured temperature, while the improved dynamic thermal modeling procedure with optimized thermal parameters attains estimates more accurately.

Experimental Research on the Characteristics of Radiator Batteries of Oil Immersed Power Transformers

This paper presents the results of experimental research on radiator batteries for cooling oil immersed power transformers. A special test setup was built, consisting of a tank and heaters, simulating the active part of the transformer, and a radiator bank, with the pump in the pipe connecting the tank and the radiators. Fans were mounted below (vertical blowing) and on the side (horizontal blowing). The main parameters measured include temperatures, oil flow, and pressures in characteristic positions. The motivation behind the experimental research lies in the somewhat higher discrepancy between oil flows through the radiators calculated using detailed thermal-hydraulic model from the results of heat run tests in the case of forced oil flow and the application of the radiators for the outer cooling. Since the calculated oil flows were higher than the values obtained from the heat run test, it was obvious that there are some additional pressure drops along the oil path. The hypothesis is that these pressure drops appear in the top and bottom head of the radiators. Corresponding measurements, analysis, and steps taken to fine tune the detailed thermal-hydraulic model are presented in this paper.

A 3-D Electrothermal Simulation of the Outdoor Medium-Voltage Circuit Breaker

This paper presents a three-dimensional electrothermal numerical model of the medium voltage outdoor circuit breaker. The model was validated by heat run test performed under the conditions required by IEC 62271-1 and IEC 62271-100 standards, as well as some other specific requirements related to the breaker overload under low ambient temperature conditions and natural cooling. The purpose of this study was to build a numerical model of the medium-voltage circuit breaker that could be successfully applied for the redesign of the breaker for more demanding operating conditions with minimum physical test iterations. The numerical results obtained were found to be in a good agreement with experimental values having the average discrepancy of 3 K with the maximum below 5 K on busbars.

Oil exponent thermal modelling for traction transformer under multiple overloads

To quantify the non-linear variation of top-oil temperature with load current, and further investigate the key parameters in the thermal model, a model for calculating the oil exponent is proposed in this study. First, a global oil momentum model was established based on the fluid resistance characteristic. Then, based on the heat transfer coupling relationship between the winding, the oil flow, and the radiator (outside air), a set of control equations describing the oil temperature and the oil flow rate was established by using energy conservation. Simultaneously, the top-oil temperature was recorded from a field traction transformer to verify the physical part of the proposed model. The regression model parameters were identified with the ordinary least-square estimation so the oil exponent can be calculated naturally. The calculated oil exponent of the traction transformer at a wide range of load was 0.7308, so the accuracy was improved by 9.47% compared with the IEEE/IEC recommended value. Newly updated oil exponent was also verified through a dynamic overload heat run test. It is expected that the proposed oil exponent model can help in estimating top-oil temperature with more convenience and accuracy, especially in frequent overload conditions.