Transformer Secondary Circuit Research Articles

Soft-Switching Resonant DC Transformer for Class-D Switching Amplifier With Symmetric Bipolar Outputs

In this letter, soft-switching resonant DC transformer (DCX) for the class-D switching amplifier with the symmetric bipolar outputs is proposed. The proposed DCX consists of a single transformer primary inverter circuit and two transformer secondary dual series resonant half-bridge rectifier circuits, which reduces the voltage stress of the rectifier diodes. In the proposed DCX, the duty cycle of the primary inverter circuit is 0.5, and the symmetric bipolar output voltage gains only depend on the transformer. By dual series resonance between the leakage inductance and the output resonant capacitors, ZVS turn-on of switches and ZCS turn-off of the diodes can be achieved, which improves the overall efficiency. The characteristic of the DCX is elaborated and analyzed in this letter. Finally, a 60 W prototype is established and the experimental verified the theoretical analysis of the proposed DCX.

STUDY OF THE NATURE OF OVERVOLTAGES IN THE ELECTRICAL NETWORK ARISING FROM VOLTAGE TRANSFORMERS

A non-linear dynamic mathematical model of voltage transformer has been considered and overvoltages arising on the elements of voltage transformer equivalent circuit during transient processes have been investigated. The influence of voltage transformer secondary circuit capacitance on overvoltage multiplicity in the primary circuits and the duration of transients has been determined. The advantages of approximation of nonlinearity of voltage transformers by hyperbolic sine are used. Mathematical expressions determining the nature of changes in the forced and free components of the transient process in an electrical network with a voltage transformer have been obtained. It is shown that with the increase of the electric network capacitance the duration of the transition process damping increases and the frequency of the forced oscillations and the level of overvoltage decrease. It is proved that even small, in comparison with the primary nominal sinusoidal voltage, aperiodic components of the voltage transient process can lead to significant overvoltages during voltage transformer outages. It has been substantiated that both the secondary resistance and the switching torque influence the overvoltage multiplicity arising in the primary winding of voltage transformers. It is shown that the closed secondary winding worsens the disconnection process of non-linear inductance of voltage transformers. The values to which overvoltages increase in this case are determined. According to the results of calculations it is determined that with open secondary winding of voltage transformers the duration of transient process significantly increases. It has been found that the decrease of frequency of forced oscillations, which occurs in this case, is accompanied by an increase of currents in the primary winding of the voltage transformer, which is dangerous in terms of thermal stability of the winding insulation. It is shown, that closing the secondary winding of voltage transformers leads to significant reduction of transient damping time. It is suggested that this algorithm can be used to provide a rapid breakdown (suppression) of ferroresonant processes. The effectiveness of such a measure of stopping of ferroresonance processes as short-term shunting of secondary winding of voltage transformers has been investigated. The correlation of parameters of electric networks (capacity of busbar sections, nonlinearity of characteristics of voltage transformers, disconnection torque, etc.) at which ferroresonance process may occur and consideration of which may allow, in terms of prevention of ferroresonance processes, to identify substations (electric networks) that require more detailed research has been determined. The results of analytical studies were tested in the electric networks of JSC "Kharkivoblenergo" and used in the electricity distribution system for the selection of specific voltage transformers for certain configurations of electrical networks.

Development of universal algorithm of voltage circuits detecting faults of microprocessor protection and automation devices

Reliability, along with response speed, sensitivity, and selectivity, is the key requirement for relay protection (RP) devices. The reliability of modern RP digital terminals is understood as protection operation for all estimated failure modes and failure of protection equipment to operate in all other modes, in which this protection is not provided. Analysis of the secondary voltage schemes and algorithms of detecting faults in these circuits reveals several disadvantages of the implementation of blocking in case of faults in voltage circuits. All existing algorithms and blocking schemes in case of faults in voltage circuits suffer from several disadvantages. In this regard, it is relevant to develop a universal relay protection terminal, which can be used with any types of voltage transformers. The MatLAB Simulink is used to simulate the primary failure of secondary circuits for the connection schemes of the voltage transformer load on phase and electric line voltages. The authors propose the algorithm that allows us to create a typical universal RP terminal, which can be used with any type of voltage transformers. To detect the neutral conductor interruption of the «star» circuits, it is proposed to create an artificial asymmetry at the input voltage circuits of the relay protection and automation (RPA) terminal and develop a group of fault detectors that analyze the position of the offset neutral on the voltage complex plane. Using a simulation model of various types of failure of voltage transformer secondary circuits and the logic diagram of the unit that implements the proposed algorithm, the authors graphically show that the detection of a break in the neutral wire is possible due to monitoring the zero-sequence voltage spike (triggering of the pulsed dU0 fault detector) with neutral displacement to the complex plane area, which is set by the setpoints. The results obtained allow us to implement a universal scheme of blocking faults in voltage circuits. This scheme does not depend on the voltage transformer secondary load connection method and exclude the disadvantages of existing solutions discussed in the article. This algorithm allows us to develop a typical universal RPA terminal. Due to its connection scheme and functional and logical implementation, this terminal can be used with any types of voltage transformer without reprogramming. Also, the proposed algorithm fully complies with the requirements of the current regulatory and legal documents.

Electric arc furnace transformer secondary circuit calculations

This paper demonstrates a procedure for calculating stress and temperature in the secondary high current circuit of a modern electric arc furnace. At the beginning, the separate parameters that form the electric circuit from the furnace to the bushings of the electrodes are determined. The analysis of these parameters allows the selection of the optimum operational point of the furnace. Thermal and stress analysis of the transformer secondary delta closure is further performed. The procedure is given in detail with all the required equations and is illustrated on a real 110-ton furnace with a 110 MVA transformer.

Improvement of algorithms for voltage circuits fault detection in relay protection terminal of 6-35 kV electrical networks

Issues of increase of consumer power supply reliability by prevention of protection device false operation and unreasonable tripping of the protected network element at malfunction of measuring voltage transformer secondary circuits are considered. Options for implementation in relay protection microprocessor terminals of advanced blocking algorithms in case of voltage circuits fault are proposed.

Investigation of transformer failure in Indonesia

Transformer failure investigation is important to obtain necessary actions which prevent similar failure from happening in the future. An investigation was conducted upon the failure of a transformer in Indonesia which damaged substation apparatuses nearby. To determine possible cause of the failure, the investigation was performed by doing visual inspection after failure, further analysis on related historical assessment result data and simulation using related nameplate and actual configuration. Most historical assessment result data showed no early failure indication of the transformer, while sweep frequency response analysis (SFRA) test result showed severe deformation on both primary and secondary winding. A simulation using Transient Analysis Program for internal short circuit between winding and ground was conducted and the output waveform was identical to Digital Fault Recorder (DFR) waveform data during transformer failure. Therefore, the possible cause of transformer failure was internal arcing on primary winding to ground which could not be interrupted by the protection system. According to visual inspection after failure, the possible cause of protection system malfunction was accidental open circuit in Current Transformer (CT) secondary circuit which damaged whole CT wiring.

Автоматическое управление электрическим режимом дуговой сталеплавильной печи по критерию постоянства активной мощности

The paper considers the issues of improving performance of ultra-high power arc steel-making furnaces. It highlights special aspects of 180 t furnace (DSP-180) operation modes and provides a curve of varying active power consumption. It offers a flowchart and operation principle of the ArCOS two-channel power mode control system as well. Its drawbacks mentioned in this paper include the need for regulation of the secondary circuit impedance and a slow system response. Process stages and standard diagrams of controlled parameter changeover are under consideration. A three-channel system may efficiently regulate the process power mode of arch steel-making furnace with automated control of transformer voltage, reactor inductance and secondary circuit impedance. The authors set a challenge to implement this principle at operated electric equipment with the aid of the ArCOS algorithms. Analysis of active power curves helped determining reactor inductances for a certain arch current values at the specified secondary voltage. The authors also propose the formula to calculate the best ratios of transformer secondary voltage and reactor inductance. It enabled determining numbers of transformer and reactor stages that maintain the specified active power consumption. These ratios are recommended for development of the best DSP-180's power modes. Development of power mode control of arc steel-making furnaces with non-contact regulation of transformer voltage and reactor inductance is a promising trend.

DFT based phasor estimation algorithm for numerical digital relaying

A novel phasor estimation algorithm is proposed, by means of an innovative modification of the traditional full cycle discrete Fourier transformer (FCDFT) algorithm. The main idea is to combine the FCDFT outputs for even and odd sample-setsobtained for one cycle data window decimated by two and by four. Its performance was compared with the ones of other full cycle algorithms reported in the late literature on the subject. The obtained results reveal the proposed algorithm leads to the best performance, even considering both the decaying DC component of the fault current and the one caused by the current transformer secondary circuit.

Calibrating Instrument Transformers with Phasor Measurements

Power systems measurements are often performed with current and voltage signals derived from instrument transformer secondary circuits. Errors introduced by these transformers are a recognized component of input data errors in supervisory control and data acquisition and phasor measurements. Correcting these errors by {in situ} calibration is an expensive option; hence, it is seldom employed except in a few special cases. This article proposes a new method for calibrating instrument transformers utilizing phasor measurements by estimating instrument transformers' ratio correction factors and phase angle correction factors. The method focuses on the fundamental frequency component of phasor measurements, and, in contrast with some of the techniques proposed in the literature, it does not require accurate instrument transformer models and can be executed as often as needed to keep the calibration up to date. Two alternative calibration techniques are introduced along with the simulation results to prove the feasibility.

Design and implementation of a low cost fault tolerant three phase energy meter

This paper presents design and implementation of a microprocessor based intelligent three phase energy meter that measures accurate energy, even at the presence of harmonics. Moreover, if a fault occurs in any one of the potential transformer (PT) secondary circuits or the pressure coil in the measuring device, the unit can sense the fault and register the actual energy. Microprocessor provides a simple, accurate, reliable and economical solution of these problems. A framework of hardware circuitry and assembly language program for evaluation of energy values have been given and the problems which should be paid attention to execute the proposed algorithm using microprocessor has been discussed. Illustrative laboratory test results confirm the validity and accurate performance of the proposed method in real-time.

Advanced Control of a Resistance Spot Welding System

This paper deals with a middle-frequency resistance spot welding system. It consists of an input converter, welding transformer, and a full-wave rectifier mounted at the transformer secondary. The welding current at the full-wave rectifier output is normally controlled by the pulse width modulated primary voltage of the transformer supplied by the input converter. The unequal ohmic resistances of the two transformer's secondary circuits and the different characteristics of the diodes of output rectifier certainly lead to the magnetic saturation which, consequently, causes the unwanted spikes in the transformer's primary current and over-current protection switch-off. This disadvantage of classical spot welding systems is completely eliminated by the proposed advanced hysteresis controller (AHC), which keeps transformer iron core saturation within prescribed bounds regardless of how unequal the ohmic resistances and diodes' characteristics in the transformer's secondary circuits are. This is achieved by a combined closed-loop control of the welding current and closed-loop control of the iron core saturation level. The proposed AHC assures a very short rise time of the welding current and the best possible utilization of the transformer iron core.

A Low-Cost Fault-Tolerant Real, Reactive, and Apparent Power Measurement Technique Using Microprocessor

Errors may creep in when measuring power by conventional methods due to the inductance and capacitance of the coils and the induced eddy current in the metal parts of the instruments through the alternating magnetic field of the current coil. Apart from these, if a fault occurs in any of the potential transformer secondary circuits or the potential coil of the measuring equipment, a conventional meter cannot detect it, which results in underregistration. In this paper, a microprocessor-based threephase real, reactive, and apparent power measurement system is developed, which displays the power being fed to a load under both normal and faulty conditions. The microprocessor provides a simple, accurate, reliable, and economical solution to these problems. A framework of the hardware circuitry and the assembly language program for the evaluation of power values is given, and the problems to which attention should be paid to execute the proposed algorithm using the microprocessor are discussed. Illustrative laboratory test results confirm the validity and accurate performance of the proposed method in real-time.

Surge Voltages Induced in Secondary Circuits of 275 kV Full GIS

The study of surge voltage levels induced in low voltage control or monitoring circuits by lightning surges is very important for the rational design of those circuits in substations. To investigate the induction levels in those circuits, we carried out full-scale lightning impulse tests in a new 275kV full GIS substation. The obtained data clarified that the induced surge levels are under the testing levels of related standards. We analyzed the surge waveforms in the Potential Transformer (PT) secondary circuit by EMTP. The analysis indicated that induced surge levels are almost decided by the ratio of winding and the overvoltage level in a main circuit.

Integrated high-quality rectifier-regulators

A new family of AC-DC converters is derived which integrate the functions of low-harmonic rectification, low-frequency energy storage, and wide-bandwidth output voltage control into a single converter containing one, two, or four active switches. These converters utilize a discontinuous conduction mode input inductor, an internal energy storage capacitor, and transformer secondary circuits which resemble the bridge, forward, flyback, or Cuk DC-DC converters. A large-signal equivalent circuit model for this family is presented, which uses the loss-free resistor concept. Design strategies and experimental results are given. High-performance regulation with satisfactory line-current harmonics is demonstrated with conventional duty-ratio control. Further improvements in line current are possible by simultaneous duty-ratio and switching-frequency control.

Summary of the Guide for the Grounding of Instrument Transformer Secondary Circuits and Cases

A summary is presented of ANSI/IEEE C57.13.3, 'Guide for the Grounding of Instrument Transformer Secondary Circuits and Cases'. It is intended to provide consistency in grounding practices for electric equipment that impacts personnel safety and proper performance of relays at power-line frequencies. It includes those practices generally followed today, with revisions where present experience and practice indicate this to be desirable, and it makes recommendations in areas not previously addressed. The aim of this report is to promote an awareness in the industry of the concerns for personnel safety and equipment performance, and to provide an opportunity for written dialogue that could result in improvement in the standard's content and application.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Grounding of instrument transformer secondary circuits

The inadequacies of certain methods of grounding of instrument transformer secondary circuits have been studied by the AIEE relay subcommittee. Multi- and singlepoint grounding have been considered both pro and con. Preferred practices for grounding these circuits are recommended by this subcommittee, which is composed of engineers who have had vast experience with the subject matter.

Grounding of Instrument Transformer Secondary Circuits

Grounding of Instrument Transformer Secondary Circuits