Abstract — Three phase power Transformers are key equipment in power systems and power plants.Security,power quality and stability of three phase transformers are both important and necessary to system operation Energization of unloaded transformer results in magnetizinginrush current very often with high amplitude ,harmonic rich currents generated when transformer cores are driven into saturation .These currents have many unfavorable effects, including operation failure of transformer differential protection, deterioration of the insulation and mechanical support structure of windings and reduced power quality of the system. The inrush currents are always unbalanced among three phases. The amplitude of the magnetizing current depends mainly on two factors; the residual flux in the magnetic core and the transient flux produced by the integral of the sinusoidal supply voltage. To satisfy the principle of the flux steadiness, it is necessary to build an equalizing flux with the same magnitude, but opposite polarity to the prospective flux. Inrush currents from transformer and reactor energization have always been concern in power industry. So it is needed to find simpler and low cost scheme to limit these currents. Independent power producers are especially interested in such techniques. Different methods are used for minimizing the transient current. In this Simultaneous closing of circuit breaker and sequential closing of circuit breakers are used for reduce this large current Electric utilities and end users of electrical power are becoming increasingly concerned about the quality of electric power. A neutral resistor could provide some damping to the currents. The idea is further improved by introducing controlled energization of each phase of the transformer. The performance and characteristics of the proposed scheme is investigated using MAT LAB simulations.
Therefore, the relay is restrained from tripping if the phase voltages are above a certain threshold . The algorithm described in  uses voltages rather than current harmonics for the restraint function. The authors in  suggest the possibilities of using the transformer flux as a restraining quantity. If the flux could be estimated correctly, then it would provide a sound discriminate for external as well as magnetizinginrush conditions. Although the voltage at the transformer terminals shows severe distortions, the flux levels during these periods are high. Consequently, the uncertainties associated with the windows of voltage magnitude for restraining function no longer exist when the flux is used as a restraining quantity. Authors in  present method based on the ratio of voltage and fluxional differential current to overcome flux-restraint limitation. When the transformer has internal faults, the ratio is usually small even to zero in one cycle. While, in magnetizinginrush currents in the transformer, the ratio is very big in one part of one cycle and very small in the other part
The value of the transformer inrush current is a function of various factors, such as the switching angle of the terminal voltage, the residual flux of the core, the transformer design, the power system impedance, and others. Holcomb  proposes an improved analytical equation for the inrush
Warning of electrical failure and can prevent catastrophic losses. It can minimize damages and enhanced the reliability of power supply. Accordingly, high expectations are imposed on power transformer protective relays. Expectations from protective relays include dependability (no missing operations), security (no false tripping), speed of operation (short fault clearing time) and stability. Differential relaying principle is used for protection of medium and large power transformers. This superior approach compares the currents at all terminals of the protected transformer by computing and monitoring a differential (unbalance) current. The value of differential current greater than no-load value indicates an internal fault. Magnetizinginrush occurs in transformer at the time of large change in
On WT, there are a number of sharp spikes during the period of inrush current transient. Several sharp spikes occur immediately following the fault inception time, these sharp spikes rapidly decay near zero within ten cycles, whereas those spikes associated with inrush current, are attenuated at the most during a 10 cycle period for small transformers, lasting for 1 min for large units. This difference can be effectively used as a key feature to discriminate an internal fault current from magnetizinginrush current.
The power transformer is one of most important equipment in electrical power system. Due to its immense importance its protection is also very important. Relays currently used for protection of power transformer are differential current based and uses filters to restrain the second harmonic component and sometimes even fifth harmonic component for avoiding false tripping against the magnetizing currents . However harmonic component can be reduced by using proper magnetic material for manufacturing transformer core . Some researchers have used artificial-neural-network (ANN) based protection technique to differentiate between magnetizinginrush from internal faults in power transformers . Also large number of training data samples, slow convergence during training, and a tendency to over fit data are the limitations of ANN-based schemes.  Proposed a decision making method based on wavelet transform for discriminating internal faults from inrush currents but over excitation conditions have not considered by them.
Transformers are one of the most important components in power systems. Security and stability of transformers are both important and necessary to system operation. The steady-state magnetizing currents of transformer may be one to five percent of the rated current, but anytime the excita- tion voltage applied to a transformer is changed, a magne- tizing inrush current flows whose first peak may reach sev- eral times as large as the rated current. Although magnetiz- ing inrush is typically considered to occur when a de-energized transformer is energized, magnetizinginrush can also flow after system voltage dips and during post fault voltage recovery. Such inrush currents may last from tens of milliseconds to tens of seconds before the steady-state con- dition is reached. The decay time of the inrush current is dependent on the time constant of the system.
Magnetizinginrush current in transformers results from any abrupt changes of the magnetizing voltage. Energizing an unloaded transformer, occurrence of any external fault, voltage recovery the external fault have been cleared and out-of phase synchronizing of connected generator causes magnetizinginrush current in transformer. Because the amplitude of inrush current could be as high as a short circuit current, a detailed analysis of the magnetizinginrush current under various conditions is required for the concerns of a protective system for the transformers. The magnitude of the inrush current may cause voltage dips in the local grid, which can result in poor power quality. The magnetizinginrush currents are rich in harmonic content, they usually have a high direct current component, and it might erroneously trigger transformer over current protections. Much work has been conducted in making the protective relays recognize the difference between over currents at a fault and a magnetizinginrush current.
ABSTRACT: Very often it is noticed that complete black out takes place in an industrial plant for occurrence of an abnormal condition in the power distribution system. Generally restoration of power is done through available Emergency Diesel Generator (EDG) sets in the plant to ensure power supply to the critical loads. It is observed that during initial load pick up phase switching inrush of the distribution transformers creates enormous difficulty in synchronisation of EDG sets resulting in secondary collapse which may again delay the entire process of emergency power restoration. This delay may create irreversible damage to the power sensitive critical loads. Similar such situation was faced in a large chemical plant and was studied further to overcome the situation through cost effective measures. In these paper mitigation measures of magnetizinginrush current due to switching of downstream distribution transformers at 6.6 KV level has been devised to avoid synchronisation failure of large EDG sets during emergency power restoration.
Power Transformers are energized depending on load requirement by closing of circuit breakers which generate asymmetrical flux and result into saturation of transformer core. Due to which huge transient magnetizinginrush current is generated. Inrush current can be as high as ten to fifteen times its rated current. The magnitude of inrush current depends upon residual flux, angle of voltage during energization of transformer, source strength, and leakage impedance. This inrush current will further increase if a feeder containing multiple transformers is energized. DC-component of inrush transient currents of the incoming transformer generates additional saturation in the already connected transformers.
These converters need a minor magnetizing inductance and work with extensive switching frequency deviations, due to which the converter optimization becomes complicated. To overcome this hurdle, a LLC converter having wide input and wide output (WIWO) voltage with load range is recommended. The suggested topology of LLC resonant converter includes a circuit having two resonant tanks having two transformers, resonant capacitors including two auxiliary switches which are introduced into the secondary side
Ferroresonance in three phase systems can involve large power transformers, distribution transformers, or instru- ment transformers. The general requirements for fer- roresonance are an applied or induced source voltage, a saturable magnetizing inductance of a transformer, a capacitance, and little damping. The capacitance can be in the form of capacitance of underground cables or long transmission lines, capacitor banks, coupling capacitances between double circuit lines or in a temporarily-un- grounded system, and voltage grading capacitors in HV circuit breakers. Other possibilities are generator surge
In no load condition, by using the proposed structure, when the inrush current reaches the pre-defined value ( I s = 20 A ), the proposed structure operates (at t s = 0.5071 s ) and the semiconductor switch turns off. So, the resistor enters the line current pass. Considering Fig. 7, in this condition, the proposed structure not only reduces inrush current in an acceptable value, but also, decrease time constant of line current (see Eq. (8)). Of course, considering rated power of the transformer, inrush current decay time constant is valued 0.2s .
Transformer inrush currents that may occur on a power system can be categorised as energisation inrush current, recovery inrush or sympathetic inrush. The recovery inrush is said to occur when the transformer voltage is restored after having been reduced by a system disturbance whereas sympathetic inrush current occurs when an un-energised transformer is switched on and the transformers that are already in operation go into saturation . The quantification of the effect of inrush current on voltage dips during transformer energisation as well as the assessment of the probability distributions of voltage magnitudes and durations were reported by Peng in . As only one transformer is involved in the investigations reported in this paper, sympathetic inrush currents are not of prime interest.
The transformer modeling is simulated using PSCAD/ EMTDC. All the design parameters including the calculated positive sequence leakage reactance, no load losses and copper losses are used in the transformer design. As the same rated transformer is available in the laboratory, the normal inrush at the same switching angle with the lab work is carried out. Here, the single phase 16 kVA transformer is energised, and the decay of the inrush is observed. It is known that it depends on the L/R ratio of the circuit. Figure-6 shows the decay of the inrush transients and flux for the transformer. The current peak is at 175 A at 60 o phase angle.
As the power network, consist of large numbers of transformers which are affected by inrush current drawn due to sudden loading. Hence inrush current across the transformer also create the voltage sag which is not addressed in any voltage sag compensator. A system to mitigate the inrush current is required to minimize the voltage sag in the network. Hence proposed mitigation technique reduces the inrush current in distribution power system.
Abstract: The Electrical Power Transformer is the essential part in Power System and this Power Transformer is protected by the relay with main circuit breaker. In this Almost there is no possibility of external fault condition to it other than the internal faults like open circuit faults, winding short circuit faults. The Inrush current occurs in the transformer during switching on of a transformer and it may be rising up to 10 times the normal load current of an electrical power transformer during switching on operation. Then it is considered normal operation. Electrical Transformer is often tripped during inrush current flows in the system causing many problem in operation of transformer and customer disturbance. The technique Point on wave switching method is used to reduce the inrush currents of a transformer initially connected to the supply. In this method the energising of three phases are controlled by the residual flux which remains in the power transformer. It is necessity to discriminate the inrush currents and fault currents. For this a different method of discrimination of inrush and fault current considering current waveforms with the help of Fuzzy logic controller technique is considered.
Therefore, it cannot only perform the power transfer during an entire switching period but also achieve the high power factor. And when the switch is off the proposed converter perform the forward operation regardless of the input voltage, the magnetizing inductor offset current, core loss and transformer size can be minimized to achieve high efficiency.
Now that the block diagram for the simulation has been discussed, the parameter sensitivity results will be revealed. FL-IOL is the most sensitive to parameter change. FL-IOL is sensitive to errors in rotor and stator resistances, magnetizing inductance, and rotor and stator self inductances. This is demonstrated in the Jacobian shown in (14). The reason for this is that in order to totally decouple the input and output, all parameters must be known accurately; if not, the scheme does not work correctly.