Real Transformer Research Articles

Model of irreversible finite-heat capacity heat reservoir absorption heat transformer cycle and its application

An irreversible four-temperature-level absorption heat transformer cycle model with variable-temperature heat reservoirs is established, which considers the heat resistances between the heat reservoirs and the working fluid, the internal irreversibility due to internal dissipation of the working fluid, and the heat leakages between the heat reservoirs and the surrounding. The general relations between the heating load and the coefficient of performance are derived, and the general performance characteristic and the optimal performance characteristic are obtained using numerical examples. Moreover, the cycle model and the derived general relations are confirmed by comparing the prediction results of the model and engineering analysis results for real absorption heat transformer, and the cycle performance characteristic are discussed. The results obtained herein can provide some guidance for the optimal design of absorption heat transformer.

Recognition of power transformer winding movement and deformation using FRA

PurposeThe purpose of this paper is to present a frequency domain technique for the recognition and location of winding movements and mechanical deformations in power transformers. Both, axial and radial coil movements, as well as changes in clamp pressure will be studied.Design/methodology/approachThe developed algorithm is based on using a distributed s‐domain parameter equivalent network, which is cascaded with other sections to constitute a generalized model. The frequency domain transfer admittance is used as the main criterion to recognize, identify and locate the winding movement and deformation.FindingsIn order to reduce the number of failures due to winding movements and deformations, it is suggested to measure and store the transfer admittance over frequency range 0‐10 MHz region at normal operation as saved data for comparison with the corresponding data during failure. With the proposed technique, the type and the location of eventual displacement, as well as the number discs involved can be identified.Research limitations/implicationsAlthough the simulation was based on real experimental data adopted from the literature, the algorithm should be tested for real transformers. Several checks have been conducted in order to validate the mathematical model.Originality/valueA relatively simple mathematical model has been presented. It can be used for any transformer, to check the integrity of the winding in order to meet certain degree of equipment safety and reliability.

Exploring Possibilities for Characterization of Power Transformer Insulation by Frequency Response Analysis (FRA)

This paper explores the possibility for using frequency response analysis (FRA) technique to characterize the quality of transformer insulation. For this purpose, a single-phase and a three-phase lumped parameter circuit models were developed. The single-phase model represents a transformer without a laminated core, whereas the three-phase model represents a real transformer with core. Calculations of frequency dependent circuit parameters are presented and the effects imparted on the frequency response by different loss mechanisms (in insulation, in windings and in core) are discussed. The simulations are compared with results of measurements on real objects. It is shown that FRA could be used for insulation diagnostics if the measurements were of high enough quality.

Accelerated ageing of wood Kraft insulating paper by means of a pulsed KrF excimer laser radiation

We report on the successful use of a KrF excimer laser as a highly promising tool to investigate the accelerated ageing of wood Kraft papers used as insulating components in high-voltage power transformers. The effects of varying both the laser repetition rate (5 to 180 Hz) and the laser energy density (0.02 to 0.11 J cm-2) on the degradation of wood Kraft paper in air were systematically investigated. During the laser irradiation experiments, the temperature of the paper samples was real-time measured by means of an IR camera. It is shown that the ageing temperature of the paper can be controlled very precisely by simply adjusting the laser parameters. Indeed, levelling-off temperatures in the 25–320 °C range can be reached after only 3 s of laser exposure, by either varying the repetition rate at a fixed energy density or changing the laser fluence at a given pulse rate. By performing both scanning electron microscopy (SEM) observations and measurements of the degree of polymerization (DPv) of the laser irradiated paper samples, we were able to demonstrate that the laser irradiation induces a very rapid degradation of the paper samples (typical laser exposure times are around 35 s to be compared with ∼10 days for standard dynamic heating based ageing tests). Both the DPv of the irradiated paper samples and the average width of the corresponding cellulose fibers as measured from SEM micrographs, are shown to reduce by about 50% following their laser heating at ∼320 °C for 35 s of exposure time. It is pointed out that the observed laser-induced degradation is predominantly a photothermal process. Finally, by using gas chromatography/mass spectrometry (GC/MS), we were able to show that low molecular weight compounds such as acetaldehyde, methyl formate, acetone, and methanol are produced not only by the laser ageing process but also in real on-field transformers that underwent a severe short-circuit.

Optimal performance for Tesla transformers

The previous work related to finding improved performance for Tesla transformers is shortly reviewed. The possibilities to reach the optimal working point by modifying the main components are discussed from a practical standpoint. A methodology for maximizing the secondary voltage by regulating the tuning ratio T and the coupling coefficient is examined in particular. It is shown that its results are valid only if primary and secondary inductor values remain unchanged, and the secondary capacitor value is decreased. All in all, the best improvement from the typical condition of T=1 increases the secondary voltage of only 18% and requires tide coupling. This, in turn, imposes severe engineering problems to avoid dielectric breakdown between the primary and secondary coils, and makes the practical utility of this result someway questionable. In a real Tesla transformer, the most practical mean to perform tuning is to move the tap feeding the primary coil, rather than rewinding the secondary coil or redesigning the secondary top terminal. The resonant circuits are not undamped and it is crucial to reach the maximum voltage at the secondary in the shortest time, to minimize losses. It is shown that, in order to achieve optimal performance, a better strategy is to tune the primary coil to achieve T=1 and then to increase the coupling coefficient as much as possible, aiming at one of the values selected from a given table.

Suppression of secondary explosions in transformer rooms

Large-scale explosion tests in a 50 m 3 room have been performed to simulate real transformer room explosions. The scale and the room layout were chosen as realistic as possible, with a moderate vent area. Prior to ignition hydrogen gas and transformer oil was sprayed into the room. Reference tests were carried out to investigate how gas concentration (i.e. energy deposited in the electric arc) and amount of oil mist would influence the explosion strength. Thereafter 3 manufacturers were invited to mitigate and suppress the explosions. The manufacturers were Atex (superheated water), Fike (powder) and Kidde (water suppression with salts added). In the initial tests showed that the amount of gas is a very important parameter for the explosion load, an implication of this is that a good circuit breaker in the transformer is important. The amount of oil mist was found to influence the explosion loads less. The suppression tests showed that for weak explosions, with average gas concentrations about lower explosion limit, all concepts reduced the explosion loads significantly. For higher gas concentrations (twice as high, still far from worst-case), only a moderate pressure reduction, or no reduction at all, was seen. Some concepts reduced the pressure impulse and flame exposure time significantly. For the strong explosions, it is assumed that performance of most systems could be improved, and suggestions for improvements for all concepts are proposed.

Modeling the polarization spectrum in composite oil/paper insulation systems

The determination of the polarization spectrum in the low frequency region of the dissipation factor, commonly has been used in practice to diagnose insulation conditions. The recovery voltage method (RVM) is one of the methods used to estimate the polarization spectrum. Nevertheless, current analysis of the RVM tests do not use all the information available from the test. In this paper, a mathematical model of the polarization spectrum is proposed. It is shown that, from the derived model, some of the hidden RVM peaks can be identified. Since the difficulty to derive a model from RVM data increases with the number of the peaks to be identified, a simpler model is always preferable. A scheme for determining the parameters of an equivalent circuit is also given, The composite oil/paper insulation is considered which is extensively used in cables and transformers. Cellulose paper, such as the Manila-Kraft type, which is used as insulation, exerts a major influence on the life of the equipment because of its hygroscopic nature. The importance of detecting and eliminating moisture from paper is crucial in oil-impregnated HV insulation. Analysis of the polarization spectrum has been used by several utilities to determine moisture in the paper. Recently, some test results indicated that the polarization spectrum may not clearly show important peaks corresponding to local moisture ingress and thus can mislead experts in the field, The developed model has been validated using RVM data of real power transformers.

The coefficient of performance of a multi-temperature-level absorption heat transformer at maximum specific heating load

An endoreversible cycle model of a multi-temperature-level absorption heat transformer is set up and used to analyse the performance of the heat transformer affected by the irreversibility of finite-rate heat transfer. The key performance parameters, such as the coefficient of performance, specific heating load, temperatures of the working fluid in the heat exchangers, heat-transfer areas of the heat exchangers and so on, are optimized. Some new results which are conducive to the optimal design and operation of real heat transformer systems are obtained and several special cases are discussed in detail. Moreover, the endoreversible cycle model is generalized to become an irreversible cycle model. The important results describing the optimal performance of a multi-temperature-level absorption heat transformer affected simultaneously by the internal and external irreversibilities can be obtained simply and conveniently from the corresponding formulae of the endoreversible cycle model because of the introduction both of the equivalent temperatures and of the equivalent overall heat-transfer coefficients.

溶剤洗浄方式による柱上変圧器構成材からのPCB除去に関する実証的研究

In order to establish a method of disposing a trace of PCB contained in reclaimed insulating oil in pole transformers (50 mg PCB/kg or less) in a safe manner, a full-scale insulating oil removal test by means of solvent cleaning was conducted using a real transformer not containing PCB. Test conditions were determined to prepare for the decontamination test for transformers containing PCB, and safety of the facilities was confirmed. Major results obtained are shown as follows: (1) Satisfactory removal performance was obtained by breaking the transformer with the low-temperature crushing technique and sorting the members for cleaning. (2) It was estimated that residual PCB in ferrous materials, copper materials and ceramics could be redacted to the level below a preset limit after primary cleaning of five minutes and secondary cleaning of ten minutes, and that inpieces of paper and wooden chips could be reduced to the level of 0.1 mg PCB/kg members or below after primary cleaning of 60 minutes and secondary cleaning of 600 minutes or less. (3) It was confirmed that the facilities could be operated safely, and enviromental and working safety could be ensured.

The influence of multi-irreversibilities on the performance of a heat transformer

A general cycle model including several major irreversibilities existing usually in real heat transformer systems is established and used to investigate the optimal performance of a heat transformer affected by the irreversibilities of finite-rate heat transfer between the working fluid and the external heat reservoirs, heat leak losses of the heated space, and internal dissipation of the working fluid. The coefficient of performance and the specific heating load of the cycle system are used in turn as objective functions for optimization. The maximum values of these parameters are calculated for a given total heat-transfer area of heat exchangers. The dimensionless specific heating load versus coefficient of performance curves describing the general performance characteristics of heat transformers are presented. The practical operating region of the heat transformer is discussed.

Thermodynamic analysis of the performance of a solar absorption heat transformer at maximum coefficient of performance

It is proven that a solar absorption heat transformer affected by the irreversibility of finite-rate heat transfer may be modelled as an equivalent combined system consisting of a solar collector and an endoreversible absorption heat transformer, the latter being further treated as a combined cycle having an endoreversible heat pump driven by an endoreversible heat engine. The maximum coefficient of performance of the system is determined, based on the linear heat loss model for solar collectors and the general optimum relation for endoreversible absorption heat transformers. The optimality problems concerning the primary performance parameters of the system are discussed. The results obtained here may serve as a good guide for the evaluation of existing real solar absorption heat transformers or provide some theoretical bases for the optimal design of future solar absorption heat transformers. © 1997 by John Wiley & Sons, Ltd.

A neural diagnostic system for the monitoring of transformer heating

A new method to monitor and estimate heating of the transformers in electrical power systems is proposed. It is based on neural identification techniques and was tested for a real transformer operating under typical environmental and load conditions. Finally, some considerations on the applicability of the method to real protection and control systems in electrical power networks are also made.

Optimal choice of the performance parameters of an absorption heat transformer

The optimal performance of an absorption heat transformer, i.e. a type II absorption heat pump, is investigated by using the cyclic model with continuous flow. The effect of thermal resistances between the heat transformer and the heat reservoirs is considered in the model. A general expression related to the rate of heat-pumping, the coefficient of performance and the overall heat transfer area of the heat transformer is derived. The expression is used to optimise the main performance parameters of the heat transformer. The maximum rate of heat-pumping and the corresponding coefficient of performance are calculated. For a given overall heat transfer area of the heat transformer, the optimal relation of the heat transfer areas of the heat exchangers is obtained. The problems concerning the optimal choices of other performance parameters are discussed. The results obtained here can not only enrich the theory of finite time thermodynamics, but also provide some new theoretical bases for the optimal design and operation of real absorption heat transformers.

Overlay of cellular CDMA on FSM

The spectrum sharing capability of a CDMA PCN (48 MHz bandwidth), overlayed on the fixed service microwave (FSM) band (1850-1990 MHz), is investigated. This overlay may cause two types of interference: 1) interference from the overlayed PCN on the FSM systems, 2) interference from the microwave system to the PCN users. In the first case, both analog and digital systems are considered, because they are both in operation in the above frequency band. Furthermore, for the digital microwave system performance degradation, most of the modulation methods employed for digital transmission are considered. In the second case, the performance degradation of both the forward and the reverse PCN links are investigated, and, for the forward link, two options are considered: 1) incorporating power control, and 2) a no-power-control scheme. Next, real Fourier transformers and notch filters in the PCN receiver and transmitter are employed, in order to minimize both types of interference, and the above analysis is repeated. >

Evaluation of electrical tests on transformer board

In electrical tests on insulating materials for transformers, general, in-model test arrangements varying the parameters of electrical properties are established. These values cannot be transferred to real transformer insulation without further consideration. However, from these individual test results with their differing statistical variations of parameters of electrical proportions, statements about the electrical strength of the insulation are drawn and evaluated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optimal real and reactive power control using linear programming

This paper develops an improved real and reactive power control technique using linear programming (LP) for an integrated power system. The problem is decomposed into two subproblems comprising real ( P) and reactive power ( Q) modules, and, using a unified approach, the real power generation, voltage magnitude, and transformer tap settings are optimized. The objective function is the fuel cost which is minimized in both the P and Q modules, subject to the operating constraints. The P- Q decomposition combined with the LP formulation improve the computation speed. The paper has another advantage of using the same cost objective function for both modules, unlike other conventional methods which use the power loss function for the Q module. The LP formulation is used for both the P and Q optimization modules, utilizing the revised simplex method which is normally available in a mainframe computer.

The dependence of transformer ratio on the shaper weighting function in transformer-coupled low noise preamplifiers

The optimum turn-ratio of transformers used for coupling large capacitance detectors to amplifiers is demonstrated to be dependent on the actual weighting function of the following shaper. As an example, the optimum finite cusp shaping with a flat top is considered. For this case, the optimum turn-ratio is found to be dependent on the flat top duration. As finite magnetizing inductance in real transformers produces diverging noise with unipolar shaping, flat-top tripolar zero-area shapers are also considered.

DC chopper with energy recovery transformer

In DC choppers and inverters transformers are employed to recover the trapped energy in the commutating inductors in order to reduce the overvoltage of the commutating capacitors. It is known that the leakage factor of real transformers keeps the overvoltage substantially high. The present work shows that a finite Q-factor of real transformers significantly reduces the overvoltages, resulting in lowering of voltage and current ratings of the components. The proposed modification in the connection of the transformer leads to additional recovery of the trapped energy, resulting in further reduction of overvoltage and commutation loss. This energy recovery process must be completed in the OFF-interval to avoid spikes in the output voltage. Graphs for overvoltages, peak currents, per-unit loss and maximum permissible frequency factor have been plotted for the original and the modified scheme of the DC chopper. Experimental results justify the theoretical analysis.

Heat transformer cycles—II. Thermodynamic analysis and optimization of a single-stage absorption heat transformer

In the present paper the results of a detailed analysis of a real one-stage absorption heat transformer working with ammonia-water are discussed. Irreversibilities due to temperature differences, pressure drops and efficiencies for the absorber and the liquid pumps are taken into account and also the performance of the rectification column is considered. Heat fluxes are delivered or removed by fluids like water or humid air. The working temperatures of the process are varied. From the analysis losses of exergy are detected and an exergy flow diagram of the whole process is presented.

Limitations in transformer coupling between radiation detector and head amplifier

Transformer coupling between a nuclear radiation detector and a low noise head amplifier is discussed as a method of improving the signal-to-noise ratio for large capacitance detectors. Such an improvement, however, is considerably lower than expected according to the ideal model because of several limitations and intrinsic noise sources of real transformers. Theoretical and experimental work has been carried on to investigate the effects that limit the performances of real transformer couplings. The way in which their behaviour depends on the type of shaping employed is discussed in detail.