Transformer Energization Research Articles

Application of machine learning methods for the classification of vibroacoustic signals measured at the moment of transformer energization with varying degrees of core and winding defect complexity

Application of machine learning methods for the classification of vibroacoustic signals measured at the moment of transformer energization with varying degrees of core and winding defect complexity

Power Transformer Inrush Current Minimization During Energization using ANFIS based Peak Voltage Tracking Approach

Energizing the power transformer at no load causes inrush current flow. The value of this current depends on main three factors, the residual and saturation flux of the transformer core, the rating of the transformer, and the switching instant. Inrush current may decrease the life of the transformer and causes mall function of the protective relays. Many efforts were done for limiting the inrush current using a current limiter or improve the core material to reduce residual flux. Other treating is to control energizing instance. This paper focused on controlling the instant of the transformer energization switch using fuzzy logic inference system. A new technique depends on adaptive seeking the crest of the voltage waveform. By this method there is no need to zero-crossing technique or phase looked loop. At this point, the flux of the core reaches the minimum value. Simulation and laboratory results show the success of this technique in reducing the inrush current. This technique gives the freedom to the operational engineering for energizing the power transformer at any time.

Real-Time Implementation of Controlled Energization of Coupled Power Transformer Using Multi-Stepped PIR, Part-1

Real-Time Implementation of Controlled Energization of Coupled Power Transformer Using Multi-Stepped PIR, Part-1

A power transformer differential protection method based on variational mode decomposition and CNN‐BiLSTM techniques

AbstractPower transformers play a critical role in the performance of power systems. This equipment is costly due to significant copper and iron prices and manufacturing costs. Therefore, maintenance and protection of such equipment is essential. Despite its robust performance, maloperation of differential protection (DP) in transformers may cause operational challenges to power system operators. The differential relay may operate incorrectly after the transformer energization leads to an inrush current (IC) and the relay identifies the event as an internal fault, and consequently issues the trip command. The other case of maloperation includes, but not limited to, a moment when the current transformer saturates due to an external fault. In this paper, a novel approach for DP is proposed, that is based on signal processing methods. In this paper, variational mode decomposition (VMD) and the deep neural network are implemented by using the convolutional neural network (CNN) and bi‐directional long short‐term memory (BiLSTM) models. The VMD decomposes differential current signal (DCS) to intrinsic mode functions with corresponding narrow‐band property frequency spectrums, which provides more detailed information about signal characteristics in different frequency bands. At the next stage, an effective feature for the BiLSTM is extracted by the CNN with the convolutional layers to classify events and proper discrimination. Extensive simulations on a 500 MVA transformer in MATLAB demonstrate the effectiveness of the proposed protection approach to differentiate ICs from internal and external faults with 99.8% accuracy in less than 1/8th of a power cycle.

Investigation on ferroresonance due to power transformer energization in high voltage 400 kV transmission grid

Investigation on ferroresonance due to power transformer energization in high voltage 400 kV transmission grid

An Experimental Set-up Involving Low-cost Digital Controller to Study the Magnetizing Inrush Current in a Transformer using Point-on-Wave Switching Technique

Abstract Generally in under-graduate studies, magnetizing inrush current (MIC) is discussed theoretically without giving much practical exposure. This paper presents the development of a low cost experimental set-up using a digital controller to study the MIC and the different parameters which can affect the same. This also helps to show how the inrush current can be minimized. This set-up also provides a hands-on experience of MIC and its control in under-graduate study, which can help an upcoming practitioner in industry as well as in further research. This paper presents a brief description of MIC, followed by a short analysis. Here, a pair of anti-parallel thyristors are connected in series with the primary winding of a single-phase power transformer. The turningon instant of this switch, with respect to the zero-crossing instant of the input supply voltage, may be adjusted through a firmware, in a PIC18F4620 from Microchip Technology microcontroller development board from Microchip Technology to control the transformer energisation instant. The firmware is developed in MPLABX-IDE from Microchip Technology, and the scheme is verified via simulations in Proteus simulation software. A suitable circuit to support the microcontroller development board to achieve the above function is designed and fabricated.

The problem of the actual insulation coordination for 500 kV with overvoltage in substations and the needed for more surge arresters in high voltage grid

The new extra high voltage overhead lines and substations have increased the overvoltage phenomenon in the electrical interconnected system, especially in high voltage circuit breakers operate in a failure condition. This research develops a new methodology for transitory analysis, due to limits in the insulation coordination, available in the international standards IEC 60071, however it is not enough, more surge arresters should be required with the new overhead lines caused by overvoltage. The influence by lightning, circuit breaker failure (50BF), grounding system, towers and insulation chains are affected, in three-phase reclosing, maneuvers and transformer energization, finally a new assessment of the performance develops an accurate calculation of the overvoltage. Our findings are in the three-phase reclosing, the maneuvers have introduced overvoltage through the transmission line, higher values ​than 2.3625p.u. However, the insulation coordination is less than 2.32p.u.; compared with the limit of 2.28p.u. Finally, the overvoltage has increased from 160% to 258% according to the distance in the overhead line in 500 kV, with the needed of surge arresters, with the new equipment the overvoltage has decreased from 258% to 100%.

Power Differential Protection for Transformer Based on Fault Component Network

Current differential protection has been widely used as the primary protection of transformers. However, inrush currents during transformer energization can cause misoperation of the protection when the second harmonic restraint algorithm fails. This paper proposes a power differential protection scheme based on the fault component network (FCN), which is free from inrush detection and computationally cheap. First, the fault component differential power (FCDP), defined as the differential active power of a transformer in the FCN, is analyzed under different conditions. Second, a transformer protection scheme is presented based on the FCDP and the traditional differential power. A removal algorithm of decaying direct current (DDC) offset is developed to accurately estimate the FCDP. The performance of the scheme is tested under various conditions, including internal faults, external faults and transformer energization. The simulations using PSCAD, and Real Time Digital Simulator (RTDS), experiments in the Electrical Power Dynamic Laboratory (EPDL) and real recording data validate that the proposed method can protect the transformer reliably.

Sympathetic Transformer Inrush Detection and Relay Blocking During Power System Restoration

A classic inrush current occurs during normal energization of reactive grid elements like transformers. It can simulate short-circuit currents. Hence inrush currents have to be detected and protection relays need to be blocked for avoiding their overfunction. Conventional detection criteria are the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2^{\textrm {nd}}$</tex-math></inline-formula> harmonic content and some kind of current waveform analysis. A classic inrush can evolve into a sympathetic inrush event, if the energization of a single transformers drives other transformers into magnetic saturation. The risk of facing a sympathetic inrush current is particularly high during a power system restoration process. Its characteristic waveform differs from the classic inrush. While the classic inrush has unipolar current peaks, the sympathetic inrush is characterized by bipolar peaks. The conventional detection criteria tend to fail in this case. This paper proposes a new approach to detect sympathetic transformer inrush currents. The approach includes the detection of classic inrush currents as well as the distinction against short-circuit currents for ensuring the relay blocking. First it introduces the basic physical mechanism behind the evolution of a classic to a sympathetic inrush. The higher risk of facing a sympathetic inrush during the restoration process is shown by the physical mechanism as well. The proposed detection method is then introduced in detail. Three new differentiation criteria will be applied and merged together to a final decision. The paper closes with practical lab tests of the new approach on simulated sympathetic and measured classic inrush currents. All lab tests showed good results and promise a practical implementation in the grid.

Transformer and line energisation via grid forming converter based on multi-loop droop control

Transformer and line energisation via grid forming converter based on multi-loop droop control

Transformer Inrush Current Mitigation Techniques for Grid-Forming Inverters Dominated Grids

The use of inverters-based resources (IBRs) is rising rapidly in power networks due to increased renewable energy penetration. This requires revisiting of classical network operation standards. For instance, high transformer energization inrush current has been studied extensively under the classical network paradigm. Whereas this paper investigates transformers' energization techniques in the context of inverters dominated grids, where inverters with limited short-circuit current are expected to utilize their grid-forming capabilities for black-start. Common transformer energization techniques such as controlled switching and soft energization are first analyzed with a new perspective aiming to assess their feasibility when used with grid-forming inverters and existing network assets. Parameters influencing soft energization voltage ramp-up time ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T_{ramp}$</tex-math></inline-formula> ) are investigated, and a new <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$T_{ramp}$</tex-math></inline-formula> estimation framework for transformer energization from IBRs is introduced. Due to the variability of available point-on-wave circuit breakers (CBs) in distribution networks, controlled energization using single-pole and three-pole CBs is investigated for various configurations and their application limits are identified. A comprehensive case study is then presented using a test network with multiple transformers to benchmark the performance and requirements of each technique when the network is energized from an IBR, followed by a set of practical recommendations.

Analysis and Explanation of Resonant Phenomena Involving EHV Transformers during Power System Restoration Tests

This paper deals with the simulation and the experimental confirmation of electromagnetic events that could interfere with the successful formation of the restoration path during the power system restoration procedure. The studied phenomena are more relevant for bulk power systems characterized by a low short circuit power as the restoration backbone. In particular, two case studies have been simulated and analyzed: one related to a transformer energization during the formation of the restoration path, and the other one occurred after the de-energization of some transmission lines and one autotransformer belonging to the restoration path. From the simulation results, it emerged that such events are related to the resonant effects between the supplying transformer and the restored network. Such resonances could have negative effects on the restoration if they are not effectively managed. In order to evaluate the impact of such phenomena in real networks, the measurement recordings of on-field tests were compared with the simulation results. It is worth noting that the performed analyses require the knowledge of several parameters that were not always available in practice. Hence, the exact magnitude of the described resonant phenomena was not easy to foresee for the restoration of real networks. The performed comparison confirms the preliminary simulation results and highlights that detailed electromagnetic models are particularly important to support the power system restoration management, in particular the planning of recovery procedures.

A Waveform-Based Approach for Transformer Differential Protection Under GIC Conditions

Harmonic blocking (HB)-based differential relays, generally employed in the transformer protection, are likely to fail to interrupt internal short-circuit faults that occur under geomagnetically induced current (GIC) conditions. This paper proposes a waveform-based approach that enhances the differential protection dependability under the GIC conditions by detecting the GIC and overriding the HB signals for the internal faults. A novel algorithm is developed for the GIC detection, which employs three-phase instantaneous differential current waveforms and recognizes a GIC condition if the waveforms are all asymmetric in one direction. The proposed waveform-based approach also utilizes trip request signals of the differential relay's fault detection module to unblock the relay for the internal faults during the GIC. The results show that the proposed waveform-based approach (i) can successfully unblock the differential protection for the internal faults under the GIC conditions, (ii) does not malfunction for the external faults during the GIC, (iii) discriminates transformer energization cases from the GIC conditions, and (iv) is immune to the measurement noise. The current transformer (CT) saturation is also considered in the studies. The proposed approach's effectiveness is verified for different transformer types, including three-phase and single-phase transformers.

THE NUMERICAL CALCULATION OF LOW FREQUENCY ELECTROMAGNETIC TRANSIENT PHENOMENA IN POWER TRANSFORMERS

In this article, a transformer model is presented that is applicable to low frequency electromagnetic transient phenomena of up to 1 kHz. An example of the energization of a no-load transformer is analyzed. A simplifed analytical approach is presented first. Due to the limitations of this approach, a numerical approach is introduced for the solution of the stiff differential equations that describe the transient phenomena. In both cases, algorithms have been developed for generating the waveforms of the state variables. The results of both algorithms are compared to the results of the MATLAB/Simulink/Power System Blockset for the analysis of electromagnetic transient phenomena in electrical energy systems. Developed algorithm with the introduced numerical approach can be used successfully in other low frequency transient phenomena where the main subject of analysis is the nonlinear character of the transformer: ferroresonance, load switch-off, transformer faults etc.

A superimposed current based busbar protection scheme using slope degree of grey incidence analysis model

Superimposed current components contain effective information, which can be employed to enhance the dependability and security of busbar protection relays. This study aims to investigate the use of the slope degree of grey incidence analysis model to design a methodology for busbar protection. A grey-based busbar protection criterion has been established to inspect the similarities of the superimposed currents measured by nearby CTs inside busbar protection zone. It is concluded that fault is internal if all of the superimposed currents are similar, otherwise, fault is external. It is shown that in all of the simulated cases, the authors’ methodology can make reliable discrimination between internal and external faults. Moreover, the results demonstrated that the protection indexes of the proposed scheme including security, dependability, sensitivity, and selectivity, are satisfactory for different conditions including fault type, fault resistance (to 500 Ω), fault inception angle, simultaneous faults, evolving faults, faults during transformer energization, CT saturation and normal operating switchings and white noise. Besides, the improvements noted in our algorithm are achieved with a low computational burden and sampling rate. Also, the scheme can be implemented in digital relays. It means that the existing busbar protection scheme can be replaced by the proposed scheme.

REDUCTION OF THE INRUSH CURRENTS OF THREE-PHASE POWER TRANSFORMERS

Inrush currents that occur during the energization of a three-phase power transformer can reach very high values and cause many problems in an power system. A mathematical model of a transformer has been developed for the investigation of this problem, taking into account the effects of saturation, hysteresis and remanent magnetic flux. An algorithm has been described for the determination of the optimal instant of energization, with the goal of reducing inrush currents. As a method for measuring remanent magnetic flux, integration of the secondary voltage during the previous deenergization is proposed. A circuit model has been prepared of a system for the controlled energization of a three-phase transformer in which the simulation results are confirmed by measurement.

Power Quality Problems Due to Transformer Inrush Current

Transformer energization can produce a large nonsinuoidal inrush current which contains both odd and higher order harmonic components that can put transformer winding under mechanical stress. Additionally, they can cause irregular tripping of harmonic protection relays. Furthermore, in relatively weak power systems, such as is the Bosnian system, the superposition of harmonic components with system resonance frequencies may produce temporary overvoltages (TOV). Transformer winding failures and metal oxide surge arrester (MOA) energy stresses can occur due to TOV. The paper demonstrates a case study of an energization of a 220/110 kV transformer and power quality problems that can appear due to higher harmonics. Energy stresses of MOA provoked by transformer energization are considered in the paper.

Minimum Voltage-Current Characteristic for Calculating Surge Arrester Energy Dissipation in Temporary Overvoltage Conditions

Abstract--A method is described for estimating required energy capability of surge arresters with respect to temporary overvoltages (TOVs). The method makes use of surge arrester catalogue data on thermal energy rating and TOV capability for estimating minimum voltage-current characteristics suitable for energy calculations by electromagnetic transient simulation tools. The need to recalculate composite TOV events for comparison with TOV capability curves is thereby avoided; this makes the method especially useful in conditions of resonant TOVs comprising harmonics, e.g., from energization of transformers in transmission networks with a large share of underground cables.

Normalized Dynamic Time Warping Distance Based Auxiliary Restraint Scheme for Restricted Earth Fault Protection of Converter Transformers

Converter transformers are one of the most important and costly equipment in High Voltage Direct Current (HVDC) transmission systems. They require sensitive and reliable protections. However, the maloperation of restricted earth fault (REF) protection of converter transformers caused by unloaded energization of converter transformers has been reported. Aiming at this problem, the saturation and measurement distortion of the current transformer on neutral line (CTN) and the resulting maloperation mechanism of REF protection are analyzed firstly. Then, an auxiliary restraint scheme of REF protection of converter transformers is proposed exploiting the similarity between the waveforms of the zero-sequence current on neutral line and the calculated zero-sequence current from measured phase currents under magnetizing inrush and fault conditions. The proposed auxiliary restraint scheme is based on normalized dynamic time warping distance (DTWD), which has strong robustness against CT saturation. The proposed auxiliary restraint scheme is verified by simulations and practical data recorded on-site.

Mechanism analysis of unbalanced three-phase inrush current of Y0D transformer

During the energization of transformer with high-impedance (HT), the zero-sequence overcurrent protection (ZSOCP) often malfunction with the inrush current of primary winding (iP-inrush) shows a large unbalance. In order to clarify the cause of the misoperation and prepare for a solution, the imbalance characteristic is studied in this paper. The imbalance is visually displayed by dynamic phasor diagram. Through the method of numerical simulation, it can be obtained that the peak value of the zero-sequence current always appears near characteristic opening and switching angles (COSAs). At this time, the two phases are deep saturated and one phase is not saturated. The iP-inrush of the saturated phase is large, and that of the unsaturated phase is numerically the same as the circulating current (iD). According to the parameters characteristics of HT, the iD is small, so iP presents a large imbalance. The research conclusions provide a theoretical basis for zero-mode inrush current suppression.