Search

Dielectric testing techniques, in both time and frequency domains, are currently widely used by power utilities for assessment of the condition of transformer oil-paper insulation systems. However, it has been reported that results of these tests are highly influenced by the operating temperature during measurements. The distribution, migration and equilibrium of moisture between oil and paper in a complicated insulation system is highly temperature dependent. It requires adequate experience and proper understanding to interpret the dielectric response results in the presence of temperature variations and thermal instability. Proper analysis of the dielectric test result is only possible with an understanding of the physical behavior of the insulation system in response to temperature. A circuit model, which describes the dielectric behavior of the transformers main insulation system, has been investigated in this paper. The values of the parameters of the model have been identified from the dielectric tests. A correlation has been observed between the operating temperature and the equivalent model parameters that can be used as additional information for better interpretation of the dielectric test results. This paper thus reports a detailed study on the effects of temperature on dielectric measurements of a transformer under controlled laboratory conditions. Some results of practical on-site testing are also presented to demonstrate the possibility of errors that may be introduced in dielectric test results analysis unless temperature effects are taken into consideration.

Knowledge about moisture content in oil impregnated paper insulation is essential when estimating remaining lifetime of power transformers. Direct evaluation of moisture content is rarely possible due to inaccessibility of the internal insulation system in transformers. Therefore, various indirect estimation techniques are utilized. Frequency domain spectroscopy (FDS) measurements of transformer insulation belong to this group. To perform high quality interpretation of results of FDS measurements a good knowledge on dielectric responses in oil impregnated pressboard and paper is required, especially as it refers to their variation with water content and temperature. The aim of this paper is to provide an open access to the frequency domain spectra of oil impregnated paper and pressboard samples, which can then be used in modeling of the results of diagnostic measurements in power transformers.

Recently developed monitoring-systems show the need as also the possibilities for online- and offline-onsite-diagnostics. The limit in sensitivity for electrical pd-measurements according to IEC 60270 which is reached using filters for sinusoidal disturbances and compensation for corona impulses is still not satisfactory. The acoustic partial discharge measurement (pd-measurement) is also a well known and reliable method often used by transformer manufacturers in testing. In laboratory setups a gain in sensitivity of the acoustic in comparison to the electrical pd measurements could be established. This is reached by adapting the sensors to the propagation path, a modern but moderately priced acquisition hardware and the introduction of new signal processing algorithms. The developed sophisticated measuring systems show remarkable results in online acoustic pd-measurements onsite.

Nowadays, there is an increasing interest in using transformer monitoring systems. Some monitoring systems use models to predict the value of certain variables. One of the key variables to be monitored is moisture content in oil, as moisture has a high influence on transformer exploitation. In this paper, the theoretical foundation of a model for monitoring moisture in transformer oil is reported. The moisture model estimates moisture content in transformer oil under certain working conditions. Comparing the value of estimated moisture with the measured one, the model is able to detect failures producing an abnormal water content in the transformer. The model considers the equilibrium relations between paper and oil, the transients that take place until the steady-state equilibrium is attained, and the water increase caused by the insulating paper ageing. In addition, the model can alert to the possibility of oil saturation and the consequent free water formation in the transformer. In this paper, the theoretical foundation of the moisture model is described; in a companion paper, its experimental verification is reported.

For Part I see ibid., p.1417-22, 2005. In the companion paper, a model to detect failures leading to an increase of the water content in transformer oil was proposed. In this paper, the experimental verification of the model is presented. The model was tested in a 1500-kVA test transformer constructed as a reduced scale model of a 60-MVA 220-kV transformer. In order to load the transformer to have the capability of varying load and power factor over a wide range, the opposition method described in IEC 60076-2 Standard was used. Sensors to measure temperature and moisture in oil were installed in the test transformer. The model was adjusted and validated using a set of data corresponding to different operating conditions. The transformer was submitted to an accelerated aging process studying the model response in such a case. In order to verify the model's ability to detect failures, a water ingress in the transformer was simulated.

All electrical utilities deal with the task of determining the residual life of their in-service power transformers. Given the difficulties experienced with the use of first and second generations of markers (carbon oxides and 2-furfuraldehyde), several organizations are now considering the use of methanol for this purpose. Hydro-Quebec, which discovered this approach, uses this molecule on a regular basis to evaluate the state of the cellulose insulation of in-service power transformers and applies it to Electricite de France's nuclear power plant transformers. In this paper, some examples of the application of methanol in the field are presented against the information received from the early marker generations.

Vibro-acoustic measurement on a transformer's on-load tap changer (OLTC) can provide indications on its mechanical condition. Recently, a joint vibro-acoustic and arcing measurement system has been proposed, which can correlate the vibro-acoustic signal to mechanical events of OLTC's operation. However, there are still considerable difficulties in extracting useful information from both vibro-acoustic signal and arcing signal in a synchronized manner without any distortions in the extracted signals. In this paper, Savitzky–Golay filter is introduced to process the signals acquired from a joint vibro-acoustic and arcing measurement system installed on in-service OLTCs. It proves that the Savitzky–Golay filter can process both vibro-acoustic and arcing signals induced by OLTC, extract essential information without any time delay from both types of signals, and retrieve voltage phase information from the arcing signal. The methodologies developed in this paper can improve the visibility of OLTC's mechanical operation for an effective online condition monitoring.

Power-electronic sources, such as Type 3 wind turbine generators and static synchronous compensators (STATCOM), interface to the grid through, partial- or full-scale, and power converters that have inherently fast switching capability to control their output current during short circuits. The short-circuit current is a function of the specific converter control algorithm and differs significantly from the conventional rotating machine sources without converter interfaces. Therefore, if a transmission-line protection scheme is designed for conventional sources, (not taking into account these differences in short-circuit current characteristics), reliability can be at risk. Using real-life short-circuit currents on lines supplied by sources having a power converter interface, this paper illustrates the reliability risk to conventional line protection schemes, in particular, to those which use negative-sequence quantities for the detection of unbalanced faults. This paper discusses the protection schemes, adopted by BC Hydro–a large Canadian Electric Utility, for transmission lines interconnecting Type 3 wind turbine and STATCOM sources. Their application for ground faults is independent of the converter control algorithm as long as the source is interconnected to the grid via a transformer which is a source of zero-sequence current.

For the fast transient study of power transformers, several types of models are available that can be categorized into three groups: white box models, black box models, and gray box models. The identification of their parameters and their usages make the models different. Since white and black box models cannot cover certain cases, such as the failure analysis of a transformer with unknown geometrical data, a new type of model has recently become important for industry and academic researchers. This model, called the gray box model, is intensely analyzed in the current paper, both theoretically and experimentally. The gray box model conceptually lies between the black and white box models. The outcomes of this work have been verified using a suitable test object. The parameter identification of the model is performed by artificial methods. Some useful new ideas, including applying the Weibull Distribution function in inductance estimation and employing an exponential function for the series resistance of windings, are developed in this paper. The number of unknown parameters to be identified in the gray box model is reduced by applying these ideas, which makes the model more simple and feasible.

In conventional HVDC systems where the harmonic filters and shunt capacitors are installed at the ac grid side, the converter transformers inevitably suffer the detrimental impacts caused by harmonic penetration. In this paper, combined with an inductive filtering method, a parallel delta-connected filtering-winding scheme for 12-pulse converter transformer units is proposed to reduce the harmonic currents flowing through the transformers and, thus, to attenuate the harmonic effects on them. This paper describes the presented transformer configuration in terms of the wiring scheme of the 12-pulse converter transformer unit and corresponding flowing paths of different harmonic currents with the orders of $3,6n\pm 1\left(\mathrm{n}=1,3,5,\ldots \right)$ , 11 and 13 in the filtering winding circuit. Then, the mechanism of the proposed scheme on harmonic suppression to the converter transformers is explained theoretically. In an attempt to validate the theoretical analysis, simulations based on PSCAD/EMTDC and experiments in a laboratory prototype system are performed to evaluate the harmonic eliminating performance of the proposed scheme.

Vibration measurement has been adopted in some utilities in Australia, Canada, and several European countries for online condition monitoring of the power transformer's onload tap changer (OLTC). By comparing measured vibration signals at different stages, it might be possible to assess changes of OLTC's condition. However, there are still considerable difficulties in correlating vibration signals to the events of OLTC operation, which may impair the capability of vibration measurement for OLTC condition monitoring. Therefore, arcing measurement is proposed in this paper to complement the vibration measurement. Arcing is provoked when the OLTC switching contact closes at a fixed tap position and can lead to electromagnetic signals flowing through transformer windings, finally reaching earth. The arcing measurement is achieved by using a high-frequency current transducer (HFCT) clamping on the transformer's grounding cable. The joint vibration and arcing measurement can provide a better means for interpreting events involved in OLTC operation and facilitating improved OLTC condition monitoring. Since HFCT measured arcing signals can be coupled with noise, a probabilistic wavelet transform is thus employed in this paper to extract arcing signals from noise. Field measurements on two different types of OLTCs are performed using the joint vibration and arcing measurement system to validate the proposed methodology.

This paper presents challenges and potential impacts of photovoltaic (PV) integration in the low-voltage downtown secondary networks (downtown networks). In the conventional secondary networks, substation feeders are the sole source of electric power and establish unidirectional power to the downtown network. The network protectors prevent the flow of power from inside the network to the upstream feeder by disconnecting the circuit to protect the feeder transformer against upstream faults. The assumption of unidirectional power flow can be violated by PV generation due to the possibility of excess power inside the network. It is shown in this paper that a large number of network protector trips can occur and, thus, voltage collapse may follow even in low PV penetration levels. In addition, it is demonstrated that the reclose action of the network protector relays is adversely affected by the PV power. Other adverse effects of such distributed-generation units, such as voltage profile, line overloads, and flicker, are also briefly discussed. Finally, a solution is proposed, based on differential currents, to upgrade the network protector relays in order to avoid false trips due to excessive PV power. Part of the New Orleans downtown network is modeled and the study is performed through simulations.

This two-part paper is intended to clarify definitions in dual transformer modeling that are vague, provide accurate modeling guidelines, clarify misconceptions about numerical instability, provide a unified dual model for a specific type of transformer, and introduce new paths of research to the power systems/electrical machinery community for low-frequency transients. Part I discussed the topology of duality-based models and some important issues, such as the most common approaches to derive dual models and their variety, the equivalence of negative inductance and mutual couplings to represent the leakage inductance of three-winding transformers, and the numerical oscillations caused by the use of nontopological models. This part of the paper discusses and compares white-, gray-, and black-box models. The paper also reviews hysteresis models (static and dynamic) and highlights the differences between the air-core inductance and saturation inductance. The available dual models for the representation of the transformer tank are then presented. A unified and accurate model of a three-phase core-type transformer adequate for all low-frequency transients is presented. Finally, concrete guidelines are presented for the appropriate selection of the model topology and parameters for different low-frequency transient studies.

The dc cables of the voltage-sourced converter-based HVDC link, including the modular multilevel converter (MMC) configuration, must be protected by surge arresters against transient overvoltages. The selection of the appropriate surge arresters requires a comprehensive understanding of the phenomena that cause the overvoltages and accurate calculation of energy discharged in the arresters. This paper investigates the mechanism that causes transient overvoltages, due to dc-side line-to-ground faults, in a cable-connected MMC–HVDC link. This paper also evaluates the impacts of various system parameters, for example, the prefault power transfer level, the MMC arm inductance, the transformer leakage inductance, the fault location, the ac system short-circuit capacity, the cable inductance, and the MMC blocking delay, on the peak transient overvoltages and the amount of energy discharge in the cable surge arresters. The transient overvoltage studies are also conducted when the dc cable is replaced by an overhead line. The studies are performed in the PSCAD platform. The studies conclude that the prefault power transfer level and the MMC blocking delay have the most significant impacts on the surge arrester energy discharge under dc-side line-to-ground faults in the MMC–HVDC link. The study results also provide essential information for the design of the MMC–HVDC link components.

This paper presents a simple modulation index-dependent Thevenin equivalent circuit model of the voltage-source converters (VSCs) and the associated anti-parallel diode rectifier. The model derivation uses orthogonal transformation and a transformer analogy to transfer impedances to the dc side of the converter. The paper proposes a switched equivalent representation of a three-phase diode rectifier formed due to the antiparallel diodes when VSC switching is inhibited. The proposed model is implemented in Alternate Transients Program–Electromagnetic Transients Program and simulation results are compared with the switched model. The proposed model is shown to under predict the fault currents for pole-ground (PG) faults and to provide a nearly exact match for pole-pole faults. The rectifier equivalent provides increased accuracy in the transient region with the inclusion of the source inductance. A multiterminal dc (MTDC) system model is used to study effects of aggregation which gives increased accuracy of the equivalent circuit. It is shown that the MTDC system simulation with the proposed Thevenin equivalent is about 3 times faster than full-switched circuit simulation for a time step of 5 $\mu$ s. The MTDC system simulation with the Thevenin equivalent circuit is still accurate and 126 times faster when the time step is increased to 750 $\mu$ s.

This paper deals with those measures against overvoltage taken when protecting buildings occupied by people or animals. Such overvoltages can appear in the case of ground fault if the grounding system of the building is located within a potential funnel of a transmission tower's grounding system. This paper presents an entire concept regarding protection against any potential rise within the building. Computations of electrical potential on the grounding system having been performed using data obtained by measurements. This includes equipotential bonding, overvoltage protection of the building, and a transformer substation with metal–oxide varistors and gas-discharge arresters installed on the low-voltage line. A simulation model for the entire overvoltage protection based on known mathematical models has been merged within this paper. Adequate overvoltage protection elements were selected for protection against potential rises in the grounding systems of buildings on the basis of computation results.

This paper presents the analysis of possible transient overvoltages that can be generated during vacuum circuit-breaker (VCB) operation at the connection point of a photovoltaic powerplant. A typical switching event that is related to the VCB concerns energization and de-energization of the unloaded transformer. However, at the typical photovoltaic powerplant, the transformer is fed by an inverter equipped with LC (or LCL) filters that are necessary for limitation of harmonics and ripple in voltage and current. From the point of view of switching operations, the inductance and capacitance of the low-voltage (LV)-side connected filter affect the natural frequency of the transformer, which is reflected by different transient system response during VCB operation. In this paper, laboratory measurement of overvoltages generated during distribution transformer switching by means of VCBs is analyzed. The impact of the LC filter connected at the LV side on the overvoltages suppression was studied. Electromagnetic Transients Program–Alternative Transients Program simulations were conducted in order to verify the possibility of transients elimination by means of the additional series-connected RL choke at the transformer medium-voltage side.

A new wind turbine transformers protection method against high du/dt switching transients taking place during a wind turbine operation is described. High-frequency transients, characterized by a high rate of voltage rise (du/dt) and overvoltage at the wind turbine transformer high-voltage terminals may be a result of operations of the turbine switchgear typically comprising a vacuum circuit breaker. The protection concept described in this paper is a follow up of research activities on a transients suppressing method dedicated for distribution transformers. The principle of operation of the protective device proposed and parameters optimization procedures for windmill transformers are provided in this paper. The performance of the device was verified by means of Alternative Transients Program–Electromagnetic Transients Program simulations. The physical device was built and experimentally verified by full-scale functional tests reflecting a real wind turbine power network.

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.

This paper describes the application of the finite-elements method for determining the distribution of losses over windings and a detailed thermal-hydraulic network model for determining the value of the hot-spot factor and the hot-spot temperature. The influence of nonuniform losses in the conductor of the windings and the influence of nonuniform cooling are clearly separated. A detailed discussion of the components included in the hot-spot factor is given in this paper, enabling proper understanding of this complex and important quantity. The case study transformer was a 100-MVA oil power transformer with oil-directed water-forced cooling. The method was applied within the scope of the project of increasing the power of a hydro power station by 12%. The task was to calculate the temperatures and to recommend whether the reconstruction of the transformer is necessary. The developed calculation method and software were hitherto used for transformer design (in the production or the reconstruction stage of transformers) and it is the first application to give an answer as to whether it is possible to increase the power of an existing transformer. In this specific application, necessary onsite measurements for determining the characteristics of the coolers were performed.

Over the years, there have been a number of cases where users experienced higher noise levels of transformers on-site versus what was measured in the factory acceptance tests. In some of these cases, measurements have shown that a power transformer can have significantly different noise characteristics (both noise level and frequency spectrum) on-site compared to what was measured in the factory. This difference can range from a few decibels to more than 10 dB in some cases. Some of these differences are found to be caused by local environmental factors that influenced the on-site measurements. Others were caused by operating conditions on-site that are different from those of the factory tests. This paper presents results of investigations and measurements performed, over the past several years, on the main contributors to this difference. For each of these contributors, data are presented that demonstrate the impact of this contributor as well as quantify typical magnitudes of this impact. It is hoped that the material presented in this paper will assist users in: 1) understanding some of the pitfalls of on-site measurement of transformer noise; 2) how operating conditions contribute to the noise performance of transformers on-site; and 3) how significantly higher on-site noise levels can, in some cases, indicate an issue with the transformer or any of its components/accessories. It is also hoped that the material in this paper will contribute to industry standards/application guides on noise determination and abatement.

A typical maloperation case of the differential protection utilized in the ultra-high voltage (UHV) voltage-regulating transformer (VRT) under inrush conditions is investigated in this paper. It indicates that the second harmonic restraint principle cannot identify the inrush current reliably under such conditions. This paper proposes a normalized equivalent instantaneous inductance (NEII)-based inrush blocking method, which improves the equivalent instantaneous inductance (EII)-based method in the UHV VRT protection. Although the EII-based method is capable of discriminating inrush currents and internal faults, some factors constrain its application, such as the field limitation of measuring the terminal voltage of the transformer and the difficulty of determining a reasonable threshold due to the diversity of transformer magnetizing characteristics. The proposed NEII-based method overcomes limitations of the EII-based method and can be adopted in UHVVRT protection. A number of simulation tests have verified the effectiveness of the proposed method.

This paper proposes a technique for preventing saturation in series transformers from dynamic voltage restorer (DVR) systems. The method consists in correcting the voltages which are injected through the transformers into the power system to compensate voltage sags. It restricts the compensating voltages during the sag whenever it predicts that a maximum limit for the flux linkage is about to be exceeded. The prediction is carried out at the beginning of a stabilized voltage sag. Moreover, the technique allows a certain level of sag compensation even when the estimated flux is expected to exceed the saturation limit. The voltage sag level and phase are computed through an adaptive recursive least squares (RLS). The RLS estimation incorporates a transient period before it achieves a stable state whenever there is a sag event. The DVR is not supposed to operate in this period. Therefore, this paper also outlines a simple procedure for detecting the RLS estimation stable level. Simulation of different scenarios of voltage sags and the results from the experimental implementation of a DVR show the effectiveness of the method.

This paper presents a practical assessment and validation for the new proposed approach based on the continuous wavelet transform (CWT) to obtain the frequency response from online transient signals for an actual transformer. Apart from the mathematical procedure, an electronic system was also designed and implemented in order to inject superimposed controlled pulses to the power system (50/60 Hz) wave. The results show improved performance of the wavelet transform compared to the Fourier transform, for transient signal analysis to be applied on a nonintrusive transformer monitoring approach. Particularly, this new approach enables overcoming some problems related to the signal filtering and the signal processing in an online frequency-response analysis transformer diagnosis. This paper is presented in three parts. Part I showed the main theoretical basis of this approach. The current Part II shows a practical assessment based on tests performed in a three-phase transformer of 1150/345 V-5 kVA and in a single-phase transformer of 13200/240 V-15 kVA. Part III shows the performance of this new approach on transformers being diagnosed under real conditions.

Inrush currents in transformers can have very disruptive effects, such as voltage sags, false tripping of the protective devices, and mechanical stresses in the transformer windings. This paper shows that there are operating situations that may cause a transformer to draw abnormally high inrush currents. Examples include the normal operation of offline uninterruptible power-supply (UPS) systems, interruptions, voltage sags, and notching. These conditions may produce inrush-like currents of more than twice the value of the “normal” maximum inrush caused by energizing at voltage zero-crossing. For this condition, the term “phase-hop” is used in this paper. Laboratory experiments were performed on four different transformers (1 kVA) with varied characteristics and show the impact of phase-hop in the magnitude of inrush currents. The experiments are also used to validate the Electromagnetic Transients Program model used for the analysis of multiple cases. In addition, the behavior of the magnetic flux in a transformer under phase-hop is investigated and compared with different operating conditions using finite elements. The results of this paper have implications in transformer design and in the operation and design of UPS systems to prevent the damaging effects of phase-hop.

An unbalance voltage between a positive pole and negative pole will exist for a long time in an ungrounded modular multilevel converter (MMC) HVDC transmission system. A zig-zag transformer has been proposed to construct an artificial neutral point and keep voltage balance between the positive pole and the negative pole. An efficient grounding system consists of the zig-zag transformer and the neutral grounding resistor. In this paper, the different impedances, including positive-, negative-, zero-sequence impedance, and dc current impedance, have been analyzed, respectively. Then, the computer simulations have been performed to evaluate the characteristic of the zig-zag transformer under normal voltage, zero-sequence voltage and unbalanced dc voltage, respectively. The simulation results show that 1) the zig-zag transformer can effectively balance the unbalanced voltage in dc side of the MMC under different work conditions; 2) the zero-sequence impedance is low and zero-sequence current is convenient for the detection of a single-phase fault; 3) the power consumption of the neutral grounding resistor is of a light rating, and the cooling of the resistor is unnecessary. The method presented in this paper proves to be an efficient grounding method for the MMC-HVDC transmission system.

This paper presents the concept of complementary measurements with the use of two methods that allow for a more detailed assessment of the mechanical condition of a transformer's active part. These methods are frequency-response analysis (FRA) and vibroacoustic method (VM). The VM was interpreted separately in steady and transient conditions, thus allowing, together with the FRA measurements, the determination of possible source of vibrations. This paper presents a theoretical introduction, experimental measurements performed on a real transformer with introduced controlled deformations and, finally, an example of a practical application.

The geomagnetically induced currents (GICs) produced in power systems during magnetic storms are a function of the electric-field amplitude and direction, and the characteristics of the power system. This paper examines the influence of a number of power system characteristics, which include the resistances and structures of the conductors; the length of the transmission lines; the number, type, and resistances of transformers, the substation grounding resistances, and the topology of the network. It is shown that GIC grows with increasing line length but approaches an asymptotic constant value, and a more relevant parameter than the individual line length is the length of the entire system. This paper also derives the effective GIC for a conventional transformer and an autotransformer, and analyzes the behavior of GIC when the network topology changes illustrated with the GIC-Benchmark Model. The results of these studies provide a guide to estimating GIC impacts on a power network.

During the switching of disconnectors in gas-insulated switchgear (GIS), repeated strikes occur in the contact gap and lead to very fast transient overvoltage (VFTO). Severe VFTO may bring damage to transformers and other equipment. The switching speed of the disconnector is one of the important factors which influences VFTO. There are still different opinions on this factor. Very little quantitative study was done before this paper. In this paper, the whole repeated strike process associated with the switching operation of the disconnector was simulated and analyzed. The polarity and the probability characteristics of breakdown voltage were analyzed. The impacts of the switching speed of the disconnector on VFTO were studied by statistical methods. Comparisons were made between simulated and measured results. The simulation result shows that within a certain range of speed, disconnectors with a lower switching speed generate lower trapped charge voltage after opening operation and lower VFTO magnitude during the opening and closing operations.

This paper presents a physically consistent dual model applicable to single-phase two-winding transformers for the calculation of low-frequency transients. First, the topology of a dual electrical equivalent circuit is obtained from the direct application of the principle of duality. Then, the model parameters are computed considering the variations of the transformer electromagnetic behavior under various operating conditions. Current modeling techniques use different topological models to represent diverse transient situations. The reversible model proposed in this paper unifies the terminal and topological equivalent circuits. The model remains invariable for all low-frequency transients including deep saturation conditions driven from any of the two windings. The proposed model is tested with a single-phase transformer for the calculation of magnetizing inrush currents, series ferroresonance, and geomagnetic-induced currents (GIC). The electromagnetic transient response of the model is compared to the π model and to laboratory measurements for validation.

The key techniques employed in this paper reflect on a comprehensive method for calculating the cost of the electric power and energy needed to supply the life-cycle losses of power transformers. The method is applicable to transformer users who possess their own generation and transmission facilities. The proposed loss evaluation method is based on factors derived from relevant historical and forecasted data that are combined to determine the total ownership cost of power transformers. Finally, in a companion paper, the method is evaluated on a small-scale real system.

This paper proposes a new approach for obtaining parameters of the zero-sequence equivalent circuit of three-phase three-legged three-winding YNynd transformers (i.e., transformers with two wye windings and a third delta stabilizing winding). The methodology proposed in this paper includes a complete set of onsite field measurements using commercial equipment (traditionally used for onsite transformer leakage reactance tests). An easy-to-implement determination of the zero-sequence behavior of the YNynd transformer is achieved by means of separating the equivalent T-circuit into two different circuits, one of them to be used when the delta stabilizing winding is closed and the other when this is open. The effect of the resistive component of the zero-sequence impedances is also analyzed and quantified. Onsite measurements on a 45/16.05/10-kV 25-MVA power transformer and their comparison with high-voltage factory tests are presented to demonstrate and validate the proposed methodology.

The restricted earth fault (REF) relay is one of the protection units of the transformer digital protection system which typically has a fast response and thus is prone to maloperation. Based on measurement results and time-domain simulation results, this paper provides a comprehensive performance evaluation of the REF relay which is an augmented-based adaptive restraint current approach for performance stabilization. The investigation results provide justifications to introduce modification to the relay to improve its performance during transients and, thus, improve the relay security. This paper also introduces and evaluates a scheme to modify the REF relay performance. The proposed scheme has been implemented in the field, and the results indicate performance improvement.

The main purpose of this series of papers is to provide methodology and best business practices to assess the risk posed by geomagnetic disturbances. This paper addresses the methodology used by Dominion Virginia Power to model geomagnetically induced currents. An extreme 100-year geoelectric-field scenario is utilized as a baseline. The impact of topology changes in a large 497-bus system is investigated. Subsequent papers-Parts II and III-will address transformer vulnerability and system voltage stability, respectively.

Thyristor-based onload tap-changing ac voltage stabilizers can replace mechanical tap changers associated with transformers feeding medium-voltage (MV) lines (typically 69 kV primary, 34.5 kV line, 6.3-50 MVA), thus avoiding frequent contacts revision. As shown in a previous paper, the topology using a compensating transformer in the main circuit and a multiwinding coil in the commutating circuit give good technical and economical results independent of the coil operation mode (shared, nonshared, and mixed load). This second paper focuses on this topology and defines the criteria for an optimum design and the synthesis formulae that give the definition parameters of the components as a function of the equipment characteristics. Numerical results are added for a typical 10-MVA stabilizer. One of the best circuit solutions (single switch comb, mixed load, auxiliary short-circuit winding), which does not need a series association of thyristors, is developed in detail. Final synthesis formulae, effective power of the transformers and coil, and summatory of thyristors I·U products for all three operation modes are contrasted in a table as the final step of the iterative analysis and synthesis cost-comparative method followed along the research.

The conventional interphase power controller (IPC) can control the power flow of the transmission line. In this paper, a novel topology for IPC, called the unified interphase power controller (UIPC), is introduced. In this topology, the phase-shifting transformers (PSTs) are replaced by voltage-source converters. In this paper, the transient behavior of UIPC is modeled and investigated. The results are compared with the simulation results of IPC and UPFC. It is shown that UIPC has all of the capabilities of IPC and UPFC, such as power-flow control, fault current limitation, voltage isolation, and local bus voltage regulation simultaneously. The results also reveal that UIPC increases the variations range of control variables of IPC and, as a result, the range of variations of active and reactive powers.

This paper studies the harmonic impact of modern residential loads on the secondary power distribution systems. Several issues of concern to utilities, such as the neutral-to-earth voltage rise, the overloading of service transformers, the impact of compact fluorescent lights (CFL), and the benefits of adopting IEC 61000-3-2 limits, have been investigated. A bottom-up probabilistic harmonic assessment method proposed in the companion paper was applied to conduct the study. Several interesting results have been obtained. For example, the adoption of CFLs based on market trend could result in an increase of voltage total harmonic distortion by about 10% per year over the next couple of years. Adopting the IEC limits on CFLs will drop the growth rate to 2% per year. In addition to the results of interest to utility companies, this paper demonstrates the usefulness and necessity of a bottom-up-based approach to conduct harmonic assessment for residential loads.

This paper presents an analysis of the thermal behavior of a 15-kVA ONAN distribution transformer. Design, performance, and service-life improvements come into view from this study. The mathematical model, conceived as a 3-D solid-oil-environment extended system, considers all modes of heat transfer, including radiation. We computed oil velocity profile and oil and winding temperature distribution, patterns thereafter experimentally validated in accordance with the IEEE-1995 Loading Guide. Since this paper identifies hot-spot and flow patterns more accurately than its predecessors, and reveals a serious malfunction in refrigeration channels, it creates interesting possibilities for new designs in transformers below 75 kVA.

Increased ratio error of capacitor voltage transformers (CVTs) has been recently encountered in several substations in Iran. The results of the performed diagnostic tests indicate that short-circuited capacitor elements, especially in the low-voltage capacitor, have increased the CVT error. In this paper, some simple and effective techniques are suggested to solve this problem. One of the proposed methods to compensate the increased error is utilization of an auxiliary voltage transformer (AVT), which does not require disassembling the CVT. The practical aspects of this method, its effects on the CVT transient response, and the influence of the AVT improper design on the performance of distance relay are scrutinized in this paper. Furthermore, the design requirements of the AVT to avoid distance relay malfunctions due to the AVT ferroresonance are discussed.

The issue of an effective assessment of the technical condition of the mechanical structure of the power transformer core and windings originated the research work discussed in this paper. This publication presents the results which indicate the possibility of developing a new and noninvasive diagnostic method of the transformer active part. The method developed is based on the time-frequency analysis results of the mechanical vibrations measured of the monitored appliance. The registration of vibrations is performed during switching the transformer monitored into idle operation. This paper characterizes the power object under study, the measuring system applied, and discusses a new methodology of assessment of the technical condition of the core and windings using a modified vibroacoustic method. The original measurement results presented in this paper were obtained during switching on a 200-kVA transformer in laboratory conditions. The analysis of the signals measured was carried out for four operating conditions of the transformer under study: 1) with the core pressed by the manufacturer; 2) with the core with loose screws fixing the upper yoke; 3) with the core with loose screws fixing the upper and lower yokes; and 4) with the core with loose screws fixing the upper and lower yokes with a simultaneous unpressing of the windings of the upper voltage side.

This case study shows the application of the calculation method based on the detailed thermal-hydraulic network model. The method is applied for the thermal design of oil power transformer of rated power 750 MVA in oil-directed, air-forced cooling mode; rated powers are also specified for oil-natural, air-forced and oil-natural, air-natural cooling modes. The results of the calculations using developed software for three cooling modes are compared with the results of the heat-run test of the transformer equipped with fiber-optic sensors. In addition to confirmation of the accuracy of the model, this paper demonstrates the full power of the integrated method (inner heating and outer cooling) for thermal calculations. All relevant components (pressure drops, oil flows, oil temperatures, etc.) in different cooling modes are obtained from the model and exposed in this paper.

This paper shows two applications for using vacuum circuit breakers (VCBs) in flexible ac transmission systems in the high-voltage (HV) power grid. This is first a mechanically switched reactor, which is series connected to a thyristor-controlled series compensator in order to enhance the inductive working range. Nevertheless, the switching frequency of VCBs is limited by their mechanical properties. In order to achieve a higher switching frequency, more VCBs are switched in parallel to each other. Second, a device is investigated, which consists of mechanically switched capacitors and a mechanically switched reactor. It is connected to a node in the HV system in order to enhance the voltage quality and to avoid a voltage collapse. Since VCBs are mainly available in the medium-voltage range, a transformer is used to connect the mechanically switched device to the HV system. In this paper, it could be shown that the mechanically switched series reactor is able to enhance the damping progress of power oscillations and that the mechanically switched device using VCBs can stabilize the voltage of the node in case of disturbances, such as faults in the power system. Furthermore, the electrical stress on the VCBs has been assessed for different simulation cases with PSCAD.

This paper investigates the conditions for transformer winding resonances with an emphasis on oscillatory excitation voltages originating from various switching events in the power grids. A section of the Eastern Regional Power Grid in India has been simulated in this paper by using the Alternative Transient Program for the purpose of defining high-frequency transient voltages which may occur during various switching events. The investigation reveals that such oscillatory voltages could excite natural frequencies of a 400/220/132-kV power transformer, which closely matches the terminal excitation frequencies. Short time Fourier transform (STFT) has been used to analyze the frequency contents of the oscillatory terminal excitation as well as the voltage response of the windings. The results of the STFT confirmed the presence of resonant frequency in the oscillatory terminal voltage and in the 400-kV main and tap changer windings of the transformer. It has been demonstrated that the voltage stresses in the windings under certain critical switching operations can exceed the stresses developed under the standard 1.2/50- μs lightning impulse at the transformer's basic insulation level voltage. The reduction in winding voltage stresses under resonance is possible by deploying some remedial measures involving winding design modifications which have also been suggested in this paper.

In this paper, a new equivalent circuit for the leakage inductance of multiwinding (or multisection) transformers is presented. The methods proposed in this paper unify terminal models with models derived from the principle of duality between electric and magnetic equivalent circuits. The new model is identified as the terminal-duality model (TDM). The elements of the circuit in addition to properly representing the transformer behavior at the terminals are physically related to flux paths in the transformer window. The circuit of the TDM consists of a set of mutually coupled inductors available in any circuit simulation program and, in particular, available in all Electromagnetic Transients Program-type programs. The circuit elements of the TDM can be computed in two ways: 1) from the observation of the behavior of the magnetic field in the transformer window (applying the principle of duality) and 2) from measurements on short-circuit tests performed at the transformer terminals. Both methods yield identical results. The 2-D finite-element simulations are used to compute the terminal behavior of a wide variety of winding configurations. Several examples are presented for the illustration of the model capabilities and validation.

The age of the majority of power transformers installed in the western electricity network reaches 30 to 60 years and replacement on short term seems eminent. A technically sound policy concerning the replacement of these assets requires a model that estimates the life expectancies of individual components and from that calculates parameters related to the behavior of a population of assets as a whole. A probabilistic approach is adopted and is applied to thermal degradation of the transformer paper insulation. In this paper, we will focus on the determination of the population reliability from individual reliabilities. These individual reliabilities are based on Arrhenius modeling of paper insulation degradation, including the inherent uncertainty in the parameters involved. A statistical failure model is used to obtain the population reliability figures. The modeling method is demonstrated on two populations of power transformers in The Netherlands to evaluate the different replacement alternatives. Using the model, strategies can be defined to maximize transformer utilization and postpone replacement. The downside is the need to replace the complete fleet in a relatively short time afterwards.

In this paper, design formulas for the calculation of the leakage inductance of toroidal transformers are presented. The formulas are obtained from the analytical integration of the stored energy. The formulas are sufficiently simple and accurate to be introduced in the loop of a design program avoiding expensive finite element simulations. It is found that toroidal transformers naturally produce the minimum leakage inductance possible for medium-voltage power transformers. To limit the short-circuit currents in power and distribution systems, a larger than the minimum leakage inductance is often required. This paper presents two methodologies to increase the leakage inductance of toroidal distribution transformers: selectively enlarging the inter-winding spacing and inserting a piece of ferromagnetic material in the leakage flux region between the windings. Extensive validation with 2D and 3D finite element simulations is performed. Additionally, experimental verification of both formulas and numerical simulations was carried out comparing the calculations against measurements on prototypes.

A power transformer will yield a frequency response which is unique to its mechanical geometry and electrical properties. Changes in the frequency response of a transformer can be potential indicators of winding deformation as well as other structural and electrical problems. A diagnostic tool which leverages this knowledge in order to detect such changes is frequency-response analysis (FRA). To date, FRA has been used to identify changes in a transformer's frequency response but with limited insight into the underlying cause of the change. However, there is now a growing research interest in specifically identifying the structural change in a transformer directly from its FRA signature. The aim of this paper is to support FRA interpretation through the development of wideband three-phase transformer models which are based on three types of FRA tests. The resulting models can be used as a flexible test bed for parameter sensitivity analysis, leading to greater insight into the effects that geometric change can have on transformer FRA. This paper will demonstrate the applicability of this modeling approach by simultaneously fitting each model to the corresponding FRA data sets without a priori knowledge of the transformer's internal dimensions, and then quantitatively assessing the accuracy of key model parameters.

In this paper, a new concept of passive-loop technique called the “high magnetic coupling passive loop” is presented. The proposed shielding technique is characterized by high performances and takes all of the advantages of passive and active shields. The shielding technique can be adopted in different application fields: power lines, line transformer connections, etc. In this paper, the proposed shielding is deeply analyzed. The working principle will be explained and validated by means of simulations and laboratory measurements on some prototypes. Finally, the actual performances of the shield installed on a high-voltage power line are presented, discussed, and compared with the result obtained by means of classical passive loops.

Society's demand for power is constantly increasing. To overcome the strains, substations are more commonly being built within urban areas. This has its shortcomings, since a substation poses a level of risk. For the first time, the development of an evaluation model for costs associated with substation incidents in urban areas was achieved by Toshiba with the cooperation of the Chalmers University of Technology. This paper discusses an evaluation model for quantifying damages in comparison between a gas-insulated transformer (GIT) and an oil-immersed transformer (OIT) in urban areas. The related damage costs associated with transformer failure during its operation are studied within this paper. The findings from the case study, utilizing a hypothetical scenario setting, were that the damage cost of OIT is in the range of U.S.$1M-$4M and that of the GIT is about U.S.$0.5M-$1.2M.