Generator Step-up Transformer Research Articles

Recommendations on Selecting the NPP Generator Transformer VA Rating

The specific features of selecting the VA rating of a nuclear power unit step-up transformer are analyzed. The analysis is based on the provisions of the international standard IEEE C57.116-2022, which is a guide for transformers connected to turbine generators, taking into account the definition of the term “transformer VA rating” according to Russian and European standards (GOST 16110-82 and GOST 30830-2002). The definition of this term in this guide differs from that adopted in Russia and the EU member states. An example of analytical calculation of the generator step-up transformer steady-state apparent power in the reactive power generation and absorption modes (turbine generator inductive and capacitive operation modes) is considered. The calculation is confirmed by the simulation results in the ETAR software package. The inexpediency of selecting the generator step-up transformer VA rating from the standardized range of rated capacities according to GOST 17544-85 and GOST 9680-77 is substantiated. An adverse effect of overestimating the transformer VA rating on the maximum three-phase short-circuit fault current in the generator voltage circuit (in the main electrical connection circuit section between the turbine generator and step-up transformer) of a nuclear power plant unit is shown.

On the Development of the Theory of Resonant Processes in Power Transformer Windings. Part 3. Transients in a Circuit with Two PI Sections

The development of electrical networks and widely used technical solutions lead to more frequent winding insulation failures in power transformers operating together with cable lines or long busbars, which are caused by resonant overvoltages. Recently, there have been cases of failures of 110–750 kV generator step-up transformers, 10–110 kV distribution transformers, and wind turbine transformers for voltage levels up to 35 kV. In the first and second parts of the article, an attempt is made to develop the theory of resonant processes in power transformer windings. The results of theoretical studies of the frequency responses in a homogeneous chain circuit containing two and four PI section were presented, and conclusions about the voltage resonance occurrence conditions were drawn. The third part of the article presents the results of theoretical studies of transients in a homogeneous chain circuit containing two PI sections. Analytical expressions are obtained for the transient voltage under the impact of a step voltage impulse and a voltage impulse with an exponentially rising front. The influence of the impulse front duration on the free transient voltage component amplitude is analyzed. Analytical expressions for the transient voltage in the considered equivalent circuit under the influence of undamped and damped sinusoidal voltage are given. Expressions for estimating the maximum resonant voltage amplification ratio under the influence of a damped sinusoidal voltage are obtained.

A Novel Approach for Evaluating Eddy Current Loss in Wind Turbine Generator Step-Up Transformers

A Novel Approach for Evaluating Eddy Current Loss in Wind Turbine Generator Step-Up Transformers

Prognostic and Health Management of Active Assets in Nuclear Power Plants

This paper presents the development of diagnostic and prognostic capabilities for active assets in nuclear power plants (NPPs). The research was performed under the Advanced Instrumentation, Information, and Control Technologies Pathway of the Light Water Reactor Sustainability Program. Idaho National Laboratory researched, developed, implemented, and demonstrated diagnostic and prognostic models for generator step-up transformers (GSUs). The Fleet-Wide Prognostic and Health Management (FW-PHM) Suite software developed by the Electric Power Research Institute was used to perform diagnosis and prognosis. As part of the research activity, Idaho National Laboratory implemented 22 GSU diagnostic models in the Asset Fault Signature Database and two wellestablished GSU prognostic models for the paper winding insulation in the Remaining Useful Life Database of the FW-PHM Suite. The implemented models along with a simulated fault data stream were used to evaluate the diagnostic and prognostic capabilities of the FW-PHM Suite. Knowledge of the operating condition of plant asset gained from diagnosis and prognosis is critical for the safe, productive, and economical long-term operation of the current fleet of NPPs. This research addresses some of the gaps in the current state of technology development and enables effective application of diagnostics and prognostics to nuclear plant assets.

Identifying hot spots in GSU power transformers using multiple methods

A routine using multiple methods is proposed for detecting and locating hot spots in power transformers based on thermographic measurements and advanced computational and analytical techniques. The proposed method identifies oxide and hydrocarbon gases dissolved in the insulation oil of power transformers and use the well-established key gas method to provide information about the cause of the hot spots, such as partial discharge, electric arc, and overload heating. The method then uses infrared mappings and temperature measurements to find the approximate location of the hot spots and, finally, the finite element method and an analytical formulation are employed to refine the location of the hotspots and the estimation of their temperature. The proposed method was developed during the design and manufacturing of a commercial 120 MVA generator step-up transformer, produced by a well-renowned manufacturer. The authors conclude that a method is a resourceful tool for identifying hot spots and could be used by manufacturers to identify and predict design and manufacturing failures.

New adaptive coordination approach between generator-transformer unit overall differential protection and generator capability curves

This paper presents a new method to increase the reliability of the generator-transformer unit overall differential protection with use of the capability charts. The main objective of proposed method is preventing the false tripping under both heavy external solid-faults associated at Generator Step-Up Transformer (GSUT) HV Side and Unit Auxiliary Transformer (UAT) LV Side, load rejection either house load rejection or zero load rejection, and synchronization. Also, the proposed method simultaneously detects both faults at low voltage side of UAT where the fault current is very small and near to normal load current comparing to generator capacity and single-line-to-ground faults at GSUT Low voltage side (generator terminal) due to the transformer delta connection that isolates the zero-sequence component from the network side. The presented method is based on using adaptive setting (three selected settings) for the generator-transformer unit overall differential protection to raise the characteristic setting under the external fault, load rejection and synchronization and decrease the characteristic setting under internal high-impedance fault. The suitable setting is selected according to the location of the generator operating point inside the generator capability curves. The generator capability curves are divided into four operating regions, where each one has a different differential characteristic setting having dual-rate of change of the differential current. The four operating regions are normal load operation and high impedance internal fault; heavy external and internal faults; load rejection and synchronization; and at under excitation operation without faults, respectively. The real dynamic simulation of the power station has been conducted by ATP/EMTP software for the large steam turbine driven synchronous generator. Extensive simulation case studies for internal faults and system disturbances are investigated as well as practical recorded signals.

Low Energy Degradation Triangle for power transformer health assessment

Dissolved gas analysis (DGA) remains one of the best non-intrusive methods of assessing transformer health and internal faults. However, few approaches identify consistently the incipient faults initiated by low energy degradation processes. The Low Energy Degradation Triangle (LEDT) method formulates three low energy dissolved gases hydrogen (H2), methane (CH4) and carbon monoxide (CO) into a triangular representation to identify early state changes from normal to unhealthy. The approach has been applied to records of DGA from over 200 generator step-up transformers, to large transmission transformers and industrial units. A degradation parameter derived from the LEDT gave early prediction of the failure of more than 90% of transformers that failed or were removed from service. The approach appears to have benefits for transformer health assessment. It also illustrates an association in some transformers between degradation and geomagnetic disturbances. Additionally, the LEDT indicates there is little benefit from oil treatment after low energy degradation has commenced.

Coupled 3D Transient Magneto-Mechanical FEM Simulation of a Short Circuit Test on a Mock-up of a 570 MVA Transformer Unit

Large generator step-up (GSU) transformer units are particularly important components of the power generation network. In order to ensure a reliable functioning of the power network, GSU transformers have to be able to withstand a wide variety of stresses, of which Lorentz forces generated during an external short-circuit can be potentially dangerous for the transformer. In this case, a reference mock-up unit designed to withstand similar mechanical stress underwent the actual short circuit testing. Upon the demonstration of short-circuit withstand capability of the mock-up unit, a destructive test was undertaken in order to demonstrate the safety margin of the design. This was achieved by gradually increasing the current until mechanical deformations occurred. This paper describes the process of model generation, calculation, numerical behaviour and the results of a 3D transient magneto-mechanical FEM simulation of the winding assembly of the mock-up transformer unit. This unit underwent short circuit testing with the standard short circuit test currents. This type of simulation allows for a deeper insight into the actual generation and distribution of the magnetic forces and the resulting mechanical stresses during a short circuit which cannot be achieved by presently used simulation and calculation methods used in the calculation of the short circuit withstand capability of power transformer.

Wideband modeling of a 45-MVA generator step-up transformer for network interaction studies

A stable and passive six-terminal model is developed for a 45-MVA generator step-up transformer based on frequency sweep measurements in the range 5Hz–10MHz and curve fitting with rational functions. The modeling employs separate treatment of the zero sequence system in combination with a new variant of the fast residue perturbation method for passivity enforcement. For use in system level simulations, the model can be employed in PSCAD, EMTP-RV and ATP. With ATP, the representation with an equivalent electrical circuit requires some care to prevent the occurrence of near zero circuit elements. Comparison with measured time domain responses at reduced voltage shows that the model can accurately reproduce transient voltage transfer between the windings while taking into account the loading effect of the connected system. The model also reproduces the transient recovery voltage (TRV) when interrupting short-circuit currents. As an application example, the model is demonstrated for simulation of resonant voltage transfer from the high-voltage side to the low-voltage side. In this setting, it is shown that the transformer input impedance can at high frequencies greatly influence the shape of the impinging overvoltages, thereby giving a self-protective effect against incoming overvoltages.

Forensic Examination and Failure Analysis of a 220-MVA Step-Up Transformer Fire

The paper presents the results of investigation and engineering analysis by Exponent of the failure of a 220-MVA generator step-up transformer in Southern California. The incident was caused by a dielectric breakdown from the high-side C-phase bushing to the side wall of the transformer. The ensuing fire continued for 27 hrs, fueled by approximately 11,000 gal of dielectric HYTRANS 61 oil. The paper will discuss the investigation conducted as per the IEEE C57.125 “Guide for Failure Investigation, Documentation, and Analysis for Power Transformers and Shunt Reactors,” [1] and the engineering review and analysis to identify the likely failure scenario(s).

Eliminating Measurement Cable Effects From Transformer Admittance Measurements

Admittance frequency-sweep measurements is an accepted procedure for characterizing transformer terminal behavior for the purpose of frequency-dependent black-box modeling. The errors introduced by the measurement cables is with one existing practice mitigated by removal of the associated shunt capacitance effect. In this paper, it is shown that the accuracy can be greatly improved by using transmission-line representation of the measurement cables with parameters obtained from standard cable data or from S-parameter measurements. The procedure is demonstrated for the modeling of a 45 MVA generator step-up transformer requiring cable lengths of six meters and an upper frequency limit of 10 MHz.

Cold Start of a 240-MVA Generator Step-Up Transformer Filled With Natural Ester Fluid

A 240-MVA, 165-GRDY/20-kV generator step-up transformer was insulated with Envirotemp FR3 fluid. The transformer was energized during a cold temperature period in mid-December 2010. This is believed to be the largest generator step-up transformer filled with a natural ester fluid that was energized at low temperatures. The lack of cold start experience of natural ester-filled power transformers created questions and at least some uncertainty as to the outcome. This paper describes the instrumentation used and data collected during the cold startup procedure. In addition, a computational fluid dynamics analysis of this transformer design was performed that provided calculated temperatures and elapsed times until steady-state temperatures were achieved. Comparisons were made between the actual and calculated values. The results indicate that the cold startup procedures for a power transformer are the same whether filled with mineral oil or natural ester fluid of similar characteristics as used for this paper.

Improvement of thermal performance of generator step-up transformers

Generator step-up transformers have high losses in structural parts due to high current leads. As distribution of magnetic field is nonuniform it may easily cause high local losses and high temperature rises. This paper studies impact of aluminum shields on losses and temperature rise. Using a 3D finite element method (FEM) software, calculations for five different leads arrangement are done on a model of a 220 MVA transformer with phase current of 4.4 kA. Influence on results of non-linear magnetic property and non-linear thermal parameters are studied as well. Finally, calculation results are verified by measurements. Experiment has confirmed that aluminum shield is a correct measure for reducing tank local overheating.

Short circuit verification for a 570 MVA, 420 kV single-phase GSU-transformer by SC-withstand tests on a mock-up unit

Large generator step-up transformers in power generation plants are exposed to several kinds of stresses under service conditions. One kind of stress is caused by forces at short circuits, which can even endanger and affect the entire reliability of a power plant. According to the international standards, there are three methods to demonstrate the ability to withstand the dynamic effects of a short circuit. The first one relies only on calculation and design checks with the manufacturer’s design rules for short circuits. The second one observes the performance during a conventional full-scale short circuit test and the last one involves a design comparison to a similar reference transformer which has successfully passed a short circuit test. EDF and Siemens decided to evaluate the short circuit capability of a 570 MVA single-phase GSU-transformer by comparison to a similar reference mock-up transformer designed, manufactured and short circuit tested for this purpose. The reference transformer was designed to undergo equal or similar mechanical stresses as in the real full-scale unit during a short circuit. In this paper the most significant stress types in windings are analysed, described and evaluated. Similarity criteria and constraints regarding radial and axial winding stresses are discussed in detail. The short circuit tests on the mock-up transformer were performed in two test sequences. The first one was a successfully performed “customer acceptance test” where nominal short circuit currents and stresses were applied to demonstrate the short circuit stresses in the real unit. The second test series was a “destructive test”. Currents shots were increased step by step until the unit experienced a notable change in the winding reactance. The purpose of the second test was to identify the critical mechanical stresses which caused permanent winding deformations and loss of stability on the mechanical structure. Results from a visual out–of tank active part inspection and some failure modes detected after winding dismantling are presented in this paper. Frequency Response Analysis (FRA) measurements performed before and after the short circuit test series show high correlations to detected mechanical movements. A not well-known failure mode of helical windings, the so called “spiralling effect”, where compressive forces tighten up the winding turns towards the inner support, are analysed. Corrective measures to prevent such effects in transformer windings are suggested. The paper presents an experimental as well as a theoretical design evaluation and calculation approach to verify the short circuit withstand capability of large power transformers unable to be tested in high-power laboratories.

Influence of sympathetic inrush on voltage dips caused by transformer energisation

Energising a transformer with other adjacent transformers in service may induce sympathetic inrush that could lead to long duration voltage dips. This study addresses this issue through analysing voltage dip events observed on a 400/132 kV grid when energisations of several generator step-up transformers were performed. With the help of a number of field measurements of voltage dips involving sympathetic inrush, a detailed network model in alternative transients program/electromagnetic transients program has been developed and validated. Based on the network model, comprehensive simulation studies have been performed. In addition, because of the nature of multiple controlling parameters, sensitivity assessment has been carried out to identify the key influential parameters. It is found that the sympathetic inrush, although does not affect voltage dip magnitude, can prolong voltage dip duration by 143% under the worst energisation condition. The sympathetic inrush occurring in substation transformers gives rise to a network-wide sympathetic interaction, resulting in further prolonged voltage dips on the 132 kV grid. Finally, it is proven that application of static var compensator is able to speed up the voltage dip recovery.

Online Monitoring of Plant Assets in the Nuclear Industry


 
 
 Today’s online monitoring technologies provide opportunities to perform predictive and proactive health management of assets within many different industries, in particular the defense and aerospace industries. The nuclear industry can leverage these technologies to enhance safety, productivity, and reliability of the aging fleet of existing nuclear power plants. The U.S. Department of Energy’s Light Water Reactor Sustainability Program is collaborating with the Electric Power Research Institute’s (EPRI’s) Long- Term Operations program to implement online monitoring in existing nuclear power plants.
 Proactive online monitoring in the nuclear industry is being explored using EPRI’s Fleet-Wide Prognostic and Health Management (FW-PHM) Suite software, a set of web-based diagnostic and prognostic tools and databases that serves as an integrated health monitoring architecture. This paper focuses on development of asset fault signatures used to assess the health status of generator step-up transformers and emergency diesel generators in nuclear power plants. Asset fault signatures describe the distinctive features based on technical examinations that can be used to detect a specific fault type. Fault signatures are developed based on the results of detailed technical research and on the knowledge and experience of technical experts. The Diagnostic Advisor of the FW-PHM Suite software matches developed fault signatures with operational data to provide early identification of critical faults and troubleshooting advice that could be used to distinguish between faults with similar symptoms. This research is important as it will support the automation of predictive online monitoring
 
 

Well-being analysis of GSU transformer insulation incorporating the impact on power generation using fuzzy logic

With the prevailing power scenario, every watt-second of electrical energy has its own merit in satisfying the consumer demand. At the state of such a stringent energy demanding era, failure of a power generation equipment compounds the energy constraints which will not only result in a huge loss of generation but also have an impact on capital revenue. The unexpected failure of generator step-up (GSU) transformer is especially a major disturbance in the power system operation and leads to unscheduled outages with power delivery problems. The time lag in bringing back the equipment in service after rectification or replacement may increase the criticality as the process involves mobilization of spares and maintenance professionals. Hot atmosphere existing in the vicinity of thermal power stations running round-the-clock with more than 100% plant load factor (PLF) increases the thermal stress of the electrical insulation which leads to premature failure of windings, bushings, core laminations, etc. The healthy state of the GSU transformer has to be ensured to minimize the loss of power generation. As the predication related to failure of a GSU transformer is associated with some uncertainties, a fuzzy approach is employed in this paper along with actual field data and case studies for the well-being analysis of GSU transformer.

Moisture in cellulose insulation of power transformers - statistics

The article presents the results of extensive investigations aimed at determining the moisture content in a large population of power transformers (161 units). The project was carried out during the period of 2003-2011 in Poland, Germany, and Sweden. All results were obtained by means of the Frequency Response Spectroscopy (FDS) method. Among the transformers tested were grid transformers, generator step-up (GSU) transformers, and distribution transformers. According to the classification rules established by IEEE, 30% among the investigated transformers could be classified as “wet” (2-4% moisture content). The observed moisture variation with age was found to depend on transformer design and operating conditions. It is suggested that, based on the presented statistical results and knowledge concerning transformer type and age, one may discriminate between probably dry transformers and transformers that require actual moisture analysis and therefore prioritize further condition assessment.

Postmortem Investigation of Power Transformers—Profile of Degree of Polymerization and Correlation With Furan Concentration in the Oil

In this paper, postmortem aging investigations on insulation material of power transformers are presented. A methodology for taking paper samples in a systematical manner and a scheme for calculating an average degree of polymerization (DP) for the entire transformer out of the DP values of the individual paper samples will be shown. The typical DP profile in the axial direction of transformer coils will be explained using calculated distribution of losses within the coils of a 740-MVA generator stepup transformer. Further, correlations between the furan (2-FAL) concentration in the oil and the average DP of the investigated German GSU transformers and grid transformers are derived and compared with existing correlations.

Statistical analysis of online ultrahigh-frequency partial-discharge measurement of power transformers

This article presents an overview of UHF PD measurements of a generator step-up transformer over a three-year period. Several PD sources are active in the transformer, and fast changes in the PD pattern and levels are observed. An approach for the evaluation of measurement data is introduced, and the long-term development of PD is discussed in the article. The aim is to discriminate different PD sources from the PRPD pattern and classify them in terms of their level and frequency of occurrence on a long-term basis. Thus, a classification of different sources and their severity is attempted.