International Journal of Engineering Trends and Technology (IJETT) – Volume 67 Issue 10 - Oct 2019 Power quality issues and minimization of Magnetizing Inrush Current by Controlled Switching in Three Phase Transformers Renukadevi S. M. Department of Electrical &Electronics Engineering. Musaliar College of Engineering& Technology, Pathanamthitta Abstract — Three phase power Transformers are key Index Terms: Inrush current, power quality, equipment in power systems and power controlled switching plants.Security,power quality and stability of three phase transformers are both important and necessary I. INTRODUCTION to system operation Energization of unloaded Electric power transformers play an important role in transformer results in magnetizing inrush current very the stable operation of power systems. A power often with high amplitude ,harmonic rich currents transformer functions as a node to connect two generated when transformer cores are driven into different voltage levels. Therefore, the continuity of saturation .These currents have many unfavorable transformer operation is of vital importance in effects, including operation failure of transformer maintaining the reliability of power supply. The major differential protection, deterioration of the insulation concern in power transformer protection is to avoid the and mechanical support structure of windings and false tripping of the protective relays due to the reduced power quality of the system. The inrush misidentifying the magnetizing inrush current. currents are always unbalanced among three phases. The amplitude of the magnetizing current depends In this paper we propose transformer-based solutions to mainly on two factors; the residual flux in the several power quality problems. Elsewhere we have magnetic core and the transient flux produced by the shown how to solve the acoustical noise emission integral of the sinusoidal supply voltage. To satisfy the problems. In this paper we introduce toroidal principle of the flux steadiness, it is necessary to build transformers with reduced inrush currents, an equalizing flux with the same magnitude, but transformers with reduced electromagnetic emissions, opposite polarity to the prospective flux. Inrush and other transformers useful to reduce the transfer of currents from transformer and reactor energization harmonics. With the low-inrush transformers we are have always been concern in power industry. So it is able of eliminating (or reducing) voltage sags and needed to find simpler and low cost scheme to limit nuisance service interruptions caused by false operation these currents. Independent power producers are of breakers and fuses. The low-stray emissions especially interested in such techniques. Different transformers are used for reducing the electromagnetic methods are used for minimizing the transient current. interference (EMI) caused by stray fields emitted from In this Simultaneous closing of circuit breaker and the transformer. Electromagnetic noise reduction sequential closing of circuit breakers are used for transformers (NRT) are used reduce the harmonic reduce this large current Electric utilities and end pollution problem. users of electrical power are becoming increasingly Because of their construction tape-wound toroidal concerned about the quality of electric power. A transformers are more efficient and produce less neutral resistor could provide some damping to the acoustical and electromagnetic noise than standard E-I currents. The idea is further improved by introducing transformers. Therefore, properly designed and built controlled energization of each phase of the toroidal transformers are smaller and/or work cooler. transformer. The performance and characteristics of The power density (per volume or weight) is larger. the proposed scheme is investigated using MAT LAB The sole disadvantage could be that standard toroidal simulations. transformer designs produce larger inrush currents. We have solved the inrush problem right from the transformer design without resorting to external inrush ISSN: 2231-5381 http://www.ijettjournal.org Page 58 International Journal of Engineering Trends and Technology (IJETT) – Volume 67 Issue 10 - Oct 2019 limiting circuits. Therefore, we increase the reliability because the transformer will always have some residual of the overall system while simultaneously reducing flux density and when the transformer in re energized cost. We have also reduced the electromagnetic field the incoming flux will add to the already existing flux emissions even below the already reduced fields which will cause the transformer to move into emitted by standard toroidal transformers. In addition, saturation. This transient current is called inrush we have invented a transformer with a narrow current. frequency bandwidth to limit the transfer of harmonics The phenomenon of transient transformer inrush from primary to secondary or vice versa currents [11] was published by Fleming in 1892. Anyhow, up to 1988 the only method to reduce inrush Inrush currents are instantaneous currents flowing in currents was the installation of pre-insertion resistors. the transformer primary circuit when it is energized. Transient transformer inrush currents can exceed the Uncontrolled energization of large power transformers nominal current and may achieve the rated value of the may result in large dynamic flux and saturation in one short-circuit current of the power transformer. The or more cores of the transformer. The saturation result amplitude is decaying very slowly and reaches its in high amplitude magnetizing inrush current that are steady magnetizing current after some seconds. Decay rich in harmonics and have DC component. They are rate of inrush current is determined by the ratio of normally of short duration, usually of the order of resistance to inductance of primary winding. The milliseconds. Sometimes it may reach up to 10-20 current wave form is completely offset in the first few times the rated current. In the case of three phase cycles. transformers, these currents are highly unbalanced. They are found to be interfering with the normal Inrush Current is in the form of over-current that operation of the power systems. Some of the problems occurs during energization of a transformer and is a caused by inrush currents are operation failure of large transient current which is caused by part cycle transformer differential protection, deterioration of the saturation of the magnetic core of the transformer. For insulation and mechanical support structure of power transformers, the magnitude of the first peak of windings and reduced power quality of the system. inrush current is initially several times the rated load Without controlled switching the energization may current but slowly decreases by the effect of oscillation occur at any time on the voltage wave producing high damping due to winding and magnetizing resistances inrush current peak when the transformer core is of the transformer as well as the impedance of the driven into saturation. system it is connected to until it finally reaches the Transformer inrush current due to flux saturation in normal exciting current value. This process typically the core is a transient phenomenon. Normally takes several minutes. As a result, inrush current could transformers are designed to operate below the knee of be mistaken for a short circuit current and the the saturation curve. But when switched on no load, transformer is erroneously taken out of service by the flux builds up to a high value; thereby falls in the over - current or the differential relays. Therefore, it is saturation region and this causes the current to important to have an accurate calculated value of the increase. It has been found that in cores having certain magnitude and other parameters of inrush current in amount of remanant flux, the inrush current is many order to design the relaying to properly differentiate times higher than that in cores having no remanance. between inrush and short circuit incidents. These currents can cause false operation of protective Uncontrolled energization of large power transformers relays and fuses.. Closing resistors have been used to may result in large dynamic flux and saturation in one reduce the magnitude of inrush currents. Controlled or more cores of the transformer. The saturation results closing, or controlling the point on the power in high amplitude magnetizing inrush current that are frequency voltage wave where energization occurs, has rich in harmonics and have a high direct current also been employed to reduce these inrush transients component. The amplitude of the magnetizing current and thus improving power quality 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. II. INRUSH CURRENT When energizing a transformer at zero Inrush current or input surge current refers to the crossing of the sinusoidal voltage the prospective maximum, instantaneous input current drawn by an magnetizing current and the flux have their maximal electrical device when first turned on. When values, and delay by 90 electrical degrees. To satisfy transformer energizes a transient current much larger the principle of the flux steadiness, it is necessary to than the rated current flow several cycle. This is caused build an equalizing flux with the same magnitude, but ISSN: 2231-5381 http://www.ijettjournal.org Page 59 International Journal of Engineering