Numerical Prediction of Maximum Magnetizing Inrush Current in Power Transformers Based on Design Parameters

Numerical Prediction of Maximum Magnetizing Inrush Current in Power Transformers Based on Design Parameters

1Atul C. Talati 2Prof. A. M. Patel

PG Student, Gujarat Technological University, Asst. Prof. Electrical Engg. Dept.

Ahmedabad, Gujarat, India BVM Engineering College, Anand-388120

Abstract This paper presents an analytical method to predict the maximum inrush current for three phase power transformer. The calculation of transformer design parameters and the method for calculation of peak inrush current is proposed. The magnitude of maximum inrush current by analytical method and the results obtained from simulation in PSCAD/EMTDC are in agreement with one another. This method is very useful to estimate the maximum inrush current before manufacturing of the power transformer.

Keywords:- Power transformers, Magnetizing inrush currents, Measurements, Design parameters, PSCAD/EMTDC simulation.

1. INTRODUCTION

   The reliability of power transformers directly determines whether the power system works safely or not. According to the statistical reports, most of power transformer faults are internal faults. At present, the main protection scheme of internal faults of the power transformer is the differential protection. The sum of the differential circuit currents equals to the exciting current of the power transformer. When the unloaded power transformer is connected to the power system or the power transformer is reconnected to the power system after the fault cleared, it will usually produce magnetizing inrush current, which often causes the differential relay acting wrongly. Therefore the precise simulation of the magnetizing inrush current is essential for designing differential protection relay and setting operating range correctly.

2. INRUSH CURRENT

   Transformer energization at no load may result in a very high inrush current as the peak value of the inrush current is a function of the switching instant of the terminal voltage, the characteristics of the magnetization curve(residual and saturation fluxes), the primary winding resistance and the inherent primary winding air core inductance. Inrush currents are originated by the high saturation of the iron core during switching in.

   The driving force of the inrush current is the voltage applied to the primary of the transformer. The voltage may drive the flux to build up to a maximum theoretical value of twice the steady state flux plus any residual flux. This super-saturation

   of the core may lead to an inrush current hundreds of times larger than the normal excitation current and many times larger than the rated current.

   Following are the adverse effects of the magnetizing inrush currents of the transformer:

   1. The protective devices for overloads and internal faults may falsely operate and may disconnect the transformer.

   2. The windings are exposed to mechanical stresses that can damage the transformer[12]-[14].

   3. Power quality problems may arise; high resonant over voltages[15] and voltage sags[16].

   4. Special problem occurs if large number of transformers are being switched on grid after break in electric energy supply because of appearance of higher harmonics.

   5. Mutual effect occurs if transformer is being switched on bus where other transformers are already switched.

   Disturbances caused

   by switched transformer are transferred to other transformers and consumers in the area.

3. INRUSH CURRENT CALCULATION

   In this regard, some numerical and analytical methods have been proposed in the literatures. Analytical expressions for the magnetic fluxes of no-load power transformers are presented that can be used for inrush current calculation. In this paper, an analytical method to predict the maximum inrush current for power transformers is presented. The magnitude of maximum inrush current by analytical method and the results obtained from simulation in PSCAD/EMTDC are compared.

   Table 1 : Technical Specification of 10 MVA power transformer

   KVA capacity : 10000 HV voltage : 66000

   LV voltage : 11000 Vector Group : Yd1 Et : 42.35

   Net area : 1238.45 Core diameter : 420.6 HV turns : 1356

   LV turns : 150 Another formula utilized is :

   Table 2 : Transformer Coil Dimensions

   2B

   i mp

   • Br

   2.03A H

   c

   w

   o max

   o Aw N1

   Fig 1 : Dimension of 10 MVA power transformer

4. ANALYTICAL CALCULATION

   Spect , Holcomb , Bertangoli have derived analytical formulae for inrush current measurement.ABB also have given rigorous formula to estimate Inrush current in a transformer.

   Here, we have used following two formulae to calculate inrush current.

   First formula finds the inrush peak for the first cycle as:

   Where ,

   Ac = Area of core

   Aw = area inside the mean turn of excited winding

   H w = height of energized winding.

5. Simulation

   Fig 2 : Modelling circuit of transformer on NO Load in PSCAD/EMTDC

   io max

   K2V

   X s

   2 1

   • cos

   Where V = rms value of the applied voltage K2=correction factor for the peak value=1.15

   = saturation angle which corresponds to

   the instant at which the core saturates,

   1 Bs Bmp Br

   Fig 3: Flux wave form along with the voltage when switched on at energization angle 0o

   K1 cos

   B

   mp

   Here, Bs

   = saturation flux density,

   Bmp = peak value of designed steady state flux density in the core.

   Br = residual flux density.

   Fig 4: Magnitude of the flux wave form is almost double or more when the voltage is switched on at energization

   angle 0o.

   Fig 5: Magnetization Inrush current in all the three phases of 10 MVA power transformer.

   Fig 6: Inrush current in A Phase of 10 MVA power transformer.

   Fig 7: Inrush current in B Phase of 10 MVA power tansformer.

   Fig 8: Inrush current in C Phase of 10 MVA power tansformer.

6. RESULTS AND DISCUSSION

   Inrush currents are computed in this paper with time-domain simulations using the PSCAD/EMTDC software. The switching angle was selected to be at 0o so that flux builds in the direction of the residual flux. Therefore, we always obtain twice the normal flux plus any residual flux on the leg.

   Table 3 : Comparison of Results

   Analytical Results	First formula	358.66 A
   	Second Formula	364 A
   Simulation Results	A Phase	-366.29 A
   	B Phase	355.65 A
   	C Phase	274.47 A

   Table 4 : Comparison of Maximum Inrush Currents

   Analytical Results	First formula	A	358.66
   	Second Formula	A	364
   Simulation Results	Maximum |x| =		366.29 A

   As shown in figure 9, we have used CSMF (continuous system model function) control component to find maximum inrush current out of the three phases of the power transformer.

   Fig 9: Use of CSMF control Block to find maximum Inrush current in 10 MVA power tansformer.

   Fig 10: Simulation f Maximum Inrush current in 10 MVA power tansformer.

   The differences between the analytical approach and simulation are as follows:

   1. Here, residual flux were omitted, since we are concerned with first time energization without any residual flux.

   2. Positive leakage inductance were introduced to prevent numerical instabilities with non-linear inductors.

   3. The formulae of analytical approach are for single phase transformers. Hence, they are converted to three-phase considering the type of transformer connections. The inrush current value is divided by

   3 for the star connected primary and multiplied with 0.557 for delta connected primary.

7. CONCLUSION

This study enabled the numerical prediction of maximum magnetizing inrush current of single phase and three phase transformers on the basis of design parameters available from the manufacturer.

The magnitude of maximum inrush current by analytical method and the results obtained from simulation in PSCAD/EMTDC are in agreement with each other. This method is very useful to estimate the maximum inrush current before manufacturing of the power transformer.

ACKNOWLEDGEMENT

The authors would like to thank .Dr. Nilesh G. Chothani of ADIT for his help with the practical usage of PSCAD/EMTDC program. The author would also like to thank Mr. Chirag Parekh of Atlanta Electricals for providing Technical specification of 10 MVA power transformer. The authors would also like to thank the reviewers of this paper for their thorough reviews.

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