Transformer Energization Research Articles

A Wavelet-Based Transformer Differential Protection: Internal Fault Detection During Inrush Conditions

This paper proposes the boundary discrete wavelet transform-based differential protection for detecting internal faults during inrush conditions in order to enhance both the safety and reliability of the differential relay during transformer energizations. The proposed method was assessed by using representative simulations of transformer energizations (with and without internal faults) as well as sympathetic inrush conditions. The performance of the proposed method was compared with a conventional technique and a wavelet-based scheme, and good results were obtained.

Reducing the Number of Tests Cases for Reliability Assessment of Protective Relays

This paper presents a methodology to perform application tests in protective relays. It is based on using the Gabriel graph to select, from a large number of events generated by a Monte Carlo-based procedure, a smaller subset that is able to predict the relay performance without compromising the quality of the estimator. The proposed methodology is successfully applied to test the behavior of differential protective relays in two situations: turn-to-ground faults and transformer energizations. In both cases, the proposed methodology significantly reduces the number of tests required to predict the success rate of the tested relays for a desired confidence level and variance. It is observed that the efficiency of the method increases as the estimated success rate of the relay increases. This means that if the estimated success rate is high, the proposed methodology leads to larger reduction in the number of events that are actually necessary to test the relay. The validity of the proposed methodology is demonstrated by means of tests performed with an actual protective relay.

Evolving neuro-fuzzy network for real-time high impedance fault detection and classification

This paper concerns the application of a neuro-fuzzy learning method based on data streams for high impedance fault (HIF) detection in medium-voltage power lines. A wavelet-packet-transform-based feature extraction method combined with a variation of evolving neuro-fuzzy network with fluctuating thresholds is considered for recognition of spatial–temporal patterns in the data. Wavelet families such as Haar, Symlet, Daubechie, Coiflet and Biorthogonal were investigated as a way to provide the most discriminative features for fault detection. The proposed evolving neuro-fuzzy classification model has shown to be particularly suitable for the problem because the HIF environment is subject to concept changes. Different from other statistical and intelligent approaches to the problem, the developed neuro-fuzzy model for HIF classification is not only parametrically, but also structurally adaptive to cope with nonstationarities and novelties. New neurons and connections are incrementally added to the neuro-fuzzy network when necessary for the identification of new patterns, such as faults and usual transients including sag, swell and spikes due to the switching of 3-phase capacitors and energization of transformers. Experimental evaluations compare the proposed classifier with other well-established computational intelligence methods, viz. multilayer perceptron neural network, learning vector quantization neural network and a support vector machine model. Results have shown that the evolving neuro-fuzzy system is effective and robust to changes. The system is able to maintain its detection and classification accuracy even in situations in which other classifiers exhibit a significant drop in accuracy due to gradual and abrupt changes of the fault patterns. Fuzzy rules are useful for interpretability purposes and help to enhance model credibility for decision making.

Integral-based stabilization of generator differential protection

This paper presents a new approach to stabilization of generator differential protection with use of the integral principle. An additional relay stabilization is needed especially for the cases of close external faults with CT saturation, energization of the near-by transformers or start-up of supplied motors. Most of contemporary produced generator protection devices do not employ any stabilization or may use the second harmonic restraint that is well known from transformer protection. The proposed here integral principle is simple and exhibits superior performance against other commonly applied algorithms. The theoretical investigations are followed by testing with simulation runs as well as recorded signals.

Energization and Start-Up of CHB-Based Modular Three-Stage Solid-State Transformers

This paper discusses the energization and start-up process of modular three-stage solid-state transformers (SSTs) based on a cascaded H-bridge topology. A key element of the SST is the auxiliary power supply (APS) feeding the auxiliary circuitry. Redundancy and simpler implementation can be achieved by powering the APSs from the dc-link capacitors of cells, but at the expense of a more challenging start-up process of the SST. The first step of this process must be performed before APSs are operative. This can result in undesired transient events (inrush currents, voltage sags in the capacitors voltages, etc.), which could prevent a proper start-up of the SST or even jeopardize the power devices or other elements. A simple strategy for the energization and start-up of SSTs using this APS structure is proposed. The method is valid for both grid-forming and grid-feeding operations of the SST, regardless of the energization port and without the addition of any extra circuitry. Simulation and experimental results are provided.

The Impact of Transformer Energization Levels on Powerline Communication Signal Integrity

There are advantages in implementing “through-transformer” powerline communication (PLC) on distribution grids. Research in distribution transformer characteristics has produced PLC strategies that take advantage of resonance phenomena due to shunt capacitive effects, which result in reduced high-frequency impedance at certain frequencies. However, the effects of phenomena associated with energized transformers on PLC signals has been largely overlooked. This paper outlines an experimental study of the PLC channel characteristics of two single-phase 11-kV/230-V transformers rated at 15 and 1 kVA, under various energization levels. Results show that distribution transformers can significantly influence the integrity of PLC signals impinging on their terminals by introducing noise, and signal amplitude and phase variation effects. This paper further identifies the phenomena behind these effects, and reveals how they can impact the quality of “through-transformer” PLC.

Mitigation of Electric Arc Furnace Transformer Inrush Current Using Soft-Starter-Based Controlled Energization

The energization of power transformers is a frequent operation in electric arc furnaces (EAFs). Magnetization inrush currents with 10–50 times the rated transformer current can flow during several line cycles. The magnitude of the inrush currents depends on parameters like the point on wave of voltage energization, residual flux in the core, transformer size, and utility systems characteristics. This momentary high current results in system disturbances and reduction of the transformer life cycle. In this paper a scheme to mitigate large inrush currents using controlled voltage ramp soft-starter-based circuit is presented. Simulation and experimental results prove the effectiveness of the soft-starter-based controlled energization.

Analisis Karakteristik Arus dan Tegangan pada Inisiasi Feroresonansi Transformator Tegangan Rendah

Ferroresonance is an interaction phenomenon between non-linear inductance and capacitance that initiates abnormal waveform of current and voltage. This phenomenon is very sensitive to system parameter and initial condition. In this paper, ferroresonance initiation involved a basic circuit of series ferroresonance with variation of source voltage magnitude on a single phase low voltage transformer. The scheme of experiment was based on the case of transformer energizing. The trends of current and voltage waveform on the primary windings were investigated and transformed by using Fast Fourier Transform (FFT). The harmonics spectrum was then employed to identify the feroresonance mode and calculate the Total Harmonics Distortion (THD) of current and voltage. The experimental results showed that the higher source voltage magnitude caused the ferroresonance responses to be seen more clearly. It was indicated by the more distorted waveform, the higher odd harmonics frequency amplitude especially for 150 Hz, and the greater THD. Based on the harmonics spectrum and wave period, the ferroresonance responses were identified as fundamental mode. The effects of initial conditions, such as residual flux of transformer and capacitor charge, were also seen on the distorted wave peak during transient condition. In addition, the difference of waveform between normal and ferroresonance condition could be more easily observed on the voltage than the current. The waveforms of current and voltage during experiment had been verified by using simulation.

Prototyping and hardware‐in‐loop verification of OCR

This study presents the design and development of a soft-core prototype of an overcurrent relay (OCR). The prototype is implemented on the field programmable gate array and its functionality has been verified using hardware-in-loop testing on the real-time digital simulator. Computational efficiency and memory requirement of the OCR is improved by using integer arithmetic Verilog design platform. Extreme inverse and very inverse characteristics of the OCR based on standard inverse-time characteristics as per IEEE standard C37.112-1996 is used and tested in the proposed design. Digital design of the proposed algorithm, emulation results, complete hardware setup and experimental test results are presented in this study. The performance of the OCR is evaluated for a large range of characteristic parameters (pick-up current and time-dial setting) and various operating conditions which validate the operation of the relay in real time with the power network. The designed relay has the ability to differentiate between fault and inrush current to avoid any mal-operation during energisation of transformers.

A Wavelet-Based Transformer Differential Protection With Differential Current Transformer Saturation and Cross-Country Fault Detection

The current transformer (CT) saturation phenomenon has been one of the main problems for the power transformer differential protection, leading to incorrect current measurements and relay misoperation. This paper proposes a fast and efficient transformer differential protection scheme with additional differential CT saturation and cross-country fault detection modules after the external fault detection, all of them based on the differential wavelet coefficient energy with border distortions in order to stabilize the relay during external faults and distinguish accurately CT saturation from cross-country internal faults. The proposed method was assessed by using representative simulations of internal faults, transformer energizations, and external faults with CT saturation followed by cross-country internal faults, and good results were achieved.

Transformer Energization From Low-Voltage Side With Limited Generation—Power System Constraints and Protection Considerations—A Case Study

A large existing delta-wye step-down transformer was proposed to be temporarily used as a step-up transformer and energized from the low-voltage wye winding by multiple small diesel generators (DGs) to accelerate commissioning activities by utilizing existing temporary generation. This paper is a case study that discusses the associated power system constraints created by this proposed modified power system configuration. The modified power system configuration resulted in an ungrounded high-voltage (HV) system created by the transformer's delta winding, which required HV system modifications to create a grounded system. Additionally, the existing power system had generation and load flow constraints and had protection and coordination constraints that needed to be maintained to supply the existing loads. Also, the inrush current resulting from the transformer energization needed to be evaluated and compared with the generator's short time current rating. Finally, modifications to the protection and coordination settings resulting from the modified power system configuration and confirmation that the arc flash incident energy levels were acceptable after implementation of modified settings were required.

Fast Discrimination of Transformer Magnetizing Current From Internal Faults: An Extended Kalman Filter-Based Approach

Differential protection is the most common type of protection in power transformers. However, inrush current due to transformer energization may appear as fault current to the protective relay, causing a false trip of the transformer. Therefore, discrimination of the transformer inrush current from internal faults is necessary to improve the security of the protection scheme. In this paper, a new method for discrimination of the transformer inrush current from an internal fault current is proposed. First, the nonlinear state-space model of a real single-phase transformer is derived, which incorporates the nonlinear phenomena of hysteresis and magnetic saturation. Based on the derived model, an Extended Kalman Filter (EKF) is used for estimation of the primary winding current. A residual signal is defined as the difference between the measured and estimated currents. When a healthy transformer is energized, the EKF perfectly estimates the primary winding current and, hence, the residual signal is almost zero. However, when the transformer is faulty, the EKF cannot effectively estimate the current due to the existing large model mismatches. Consequently, a large residual signal is created. The superiorities of the proposed algorithm are demonstrated using different experimental scenarios.

Análisis de transitorios electromagnéticos en el Sistema Nacional Interconectado (S.N.I.) a nivel de 230 kV y 500 kV usando el simulador digital en tiempo real HYPERsim

El presente trabajo utiliza el Simulador Digital en Tiempo Real HYPERsim para poder modelar los componentes del Sistema Nacional Interconectado (S.N.I.), en los niveles de voltaje de 230 kV y 500 kV, y realizar simulaciones en valores instantáneos para transitorios electromagnéticos (EMT) de maniobra. De esta forma, se analizan sobrevoltajes de frente lento originados por maniobras de energización de líneas de transmisión y transformadores. Para esto, se plantea una metodología que permite modelar los elementos del SNI en HYPERsim, sobre la base de un apropiado despliegue conceptual de los modelos de cada componente. El desarrollo del presente trabajo constituye un insumo fundamental para realizar estudios eléctricos en HYPERsim que permitan analizar el comportamiento de transitorios del SNI. Este aspecto es aún más significativo, considerando que el aplicativo HYPERsim no ha sido usado en el país de forma previa. De esta forma, se realizan simulaciones en tiempo real completamente digitales, lo cual permite exaltar las potencialidades del trabajo desarrollado, y permitirá a CENACE realizar estudios especializados futuros de sistemas de potencia, por ejemplo, calibración de relés, diseño de esquemas de protección, pruebas de controladores, etc.

Improving Pulp and Paper Plant MV Transformer Protection

In pulp and paper plants, power transformers play a critical role in process continuity. These transformers are subject to internal short circuits, external short circuits, and abnormal operating conditions. The following protection challenges to power transformers will be explored, and methods to improve the protection are provided. First, remanence in a current transformer (CT) may cause misoperation of phase differential protection due to compromised CT performance. Heavy through-faults, sympathetic inrush, and recovery inrush all cause high current. This, combined with high remanent flux, can create a security issue. IEEE CT performance calculations will be used to support the use of dual slope differential characteristics to promote secure differential protection operation when challenged with unequal CT performance. Second, on transformer energizing, the second harmonic current has been traditionally used as a means to prevent phase differential misoperation. Certain transformers may not exhibit high enough the second harmonic, causing a dependability issue if the restraint is set too low. The use of the second and fourth harmonics for inrush detection will be shown to enhance the reliability during energizing inrush situations. Third, overexcitation can occur from abnormal operation of the utility system or the plant's excitation control. Causes of overexcitation will be outlined and the use of volts per hertz protection is explored. With overexcitation occurring from system voltage rise, the phase differential protection has been traditionally blocked using the fifth harmonic restraint. This may cause an undesired nonoperation of the phase differential protection if an internal fault occurs while the transformer is overexcited, causing a delay in tripping and severe damage to the transformer. A technique using adaptive phase differential pickup value will be illustrated to overcome this challenge. Finally, on resistance-grounded power transformers, phase differential protection sensitivity for ground faults near the neutral is decreased. The use of ground differential protection will be explored and the increased sensitivity gained will be demonstrated.

Assessment of Sudden Voltage Changes and Flickering for a Grid-Connected Photovoltaic Plant

In this paper, sudden voltage changes studies are carried out for a 20 MW grid-connected photovoltaic plant to assess the impact of energization of the step-up transformers, which may cause a voltage dip that could be a nuisance to the point of common coupling (PCC) of the transmission system. In addition, the flicking performance of the plant is checked. At the design stage, these studies are necessary to verify compliance of the photovoltaic plant and to design its connection with regulatory grid codes and international standards. The photovoltaic plant is compliant with the FICHTNER standard for long-term flicker severity and with the short-term flicker severity requirements of Engineering Standard P28. Based on the study results, the main breaker which is connected to the PCC should be ordered with 150 Ohm Pre-Insertion Resistor to make the Photovoltaic plant comply with grid code requirements of sudden voltage changes.

Controlled switching strategies for transformer inrush current reduction: A comparative study

Transformer inrush current can be efficiently mitigated by controlled switching. For energization of three-phase transformers, several strategies have been developed that depend on whether the residual flux can be measured and whether independent-pole-operated or three-pole-operated circuit-breakers are used. In this paper, a comparative analysis of these strategies has been carried out. Residual flux uncertainty and dispersion of the characteristics of the circuit-breaker can significantly affect the performance of the controlled switching. These issues have been taken into account in the analysis of each strategy since they can influence the performance in different ways. The study has resulted in the selection of the most appropriate strategy for each case.

Power Transformer Differential Protection Using the Boundary Discrete Wavelet Transform

Currently, the differential function is the most used for the power transformer protection, leading to a reliable discrimination between internal faults and other events. However, the conventional phasor-based differential protection function presents difficulties in the detection of some internal faults and its performance depends on the harmonic restraint and blocking functions in order to avoid relay misoperation during inrush currents. However, internal faults and other disturbances present transient, which can be properly detected by using the wavelet transform. This paper recreates the phase current and the negative-sequence current differential elements by means of the boundary wavelet coefficient energy. The proposed method was evaluated by using representative simulations of internal faults, external faults, and transformer energizations in two different power systems. By using the boundary wavelet coefficient energy instead of phasor estimation, the proposed method was quite fast, accurate, was not affected by inrush currents in transformer energizations and fault clearance, and could be used in a real-time application with low computational burden. In addition, the proposed method presented no failure in fault with overdamped transients, was scarcely affected by the choice of the mother wavelet, presented no time delay associated with the wavelet filtering, and was not affected by typical noise.

Study on the Unusual Misoperation of Differential Protection During Transformer Energization and its Countermeasure

The differential protection of the transformer, generator, or transmission line under normal operating conditions may misoperate when the adjacent transformer is switched-on. To research the cause of the misoperations, impacting factors, such as magnetizing inrush, sympathetic inrush, and current-transformer saturation are analyzed based on the field-recorded fault data and the simulation results in this paper. It is disclosed that the CT transient or CT local transient saturation, which are caused by the long-lasting dc component from the magnetizing inrush current, will lead to misoperation. To solve this problem, a new CT saturation detection algorithm based on the dc component in secondary current is proposed to prevent this sort of misoperation, and the effectiveness of the proposed algorithm is proved by the field-recorded fault data and simulation results obtained under different conditions.

Assessment of Temporary Overvoltages During Network Lines Re-Energization

Power system blackouts are very infrequent, but they have a Brobdingnagian effect on the system performance and devices. The present research work offers remarkable techniques for the assessment of temporary overvoltages all through the re-energization of network lines. The main goal of this research work is to first-rate and reenergize the network lines for the purpose of restoration. In the later stage, the magnitudes and durations of the Temporary Overvoltages (TOVs) that occurred during the energization of unloaded transformer are estimated.The assortment and re-energization of network lines is done on the basis of Data Envelopment Analysis (DEA) and conceptual method respectively. The assessment of TOVs is done on the basis of MATLAB/Simulink and Feed Forward Neural Networks (FFNNs). The proposed models are verified on IEEE 30 bus test system for the analysis purpose. The Mean Absolute Percentage Error (%MAPE) obtained through various forecasting methods is examined to check the robustness of the proposed approaches. The simulation and FFNN results presented in this research work helps in designing the exact withstand voltage rating for various network components employed at the moment of re-energization.

Setting-less transformer protection for ensuring security and dependability

The “setting-less” protection approach is an innovative protection scheme with increased dependability and security. This method is based on dynamic state estimation, which continuously extracts the operating condition of a protection zone from real-time measurements and the protection zone dynamic model. The dynamic state estimation computes the probability that the measurements fit the dynamic model of the protection zone within the measurement accuracy via the Chi-square test. The probability represents the goodness of fit of measurements to the dynamic model of the protection zone. We refer to it as the confidence level that the protection zone is healthy. High confidence levels imply that the protection zone operates normally, while low confidence levels indicate that internal abnormalities/faults may have occurred. Note that the proposed method does not require coordination with other protection schemes which sometimes compromise security and/or dependability. In this paper, the setting-less protection approach is described and applied to transformer protection. The feasibility of the setting-less transformer protection is verified with numerical experiments of several case studies (e.g., transformer energization, external faults, internal faults, and high impedance faults). The method has been verified in the laboratory and there are plans for field testing.