Download Inrush Current in a Transformer

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REDUCTION OF THE INRUSH CURRENT IN ELECTRICAL
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TRANSFORMER USING VOLTAGE DOWN-CHIRP
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TECHNIQUE
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Somkid Suttisak* and Teerapong Chimphet
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Department of Electrical Engineering, Faculty of Engineering,
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Rajamangala University of Technology Srivijaya, Muang Songkhla, 90000, Thailand
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* Corresponding author, e-mail: [email protected]
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Abstract
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This study aims to examine the reduction of the inrush current on a single phase
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transformer when there is no power transmission by using a voltage Down-Chirp
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technique. The transformer is initially pressured with a frequency higher than the
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frequency range. Then, such frequencies are downgraded in an equal to the frequency
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range of the transformer in a short span of time, resulting in the reduced size of the
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magnetic flux on the magnetic axis and line of the transformer. The design of the
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process and simulation are relying on MATLB SIMULINK software. The results of
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stimulation of the 1 2 0 0 VA transformer showed that the inrush current reduced when
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the default frequency of the voltage is equal to 90Hz.
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Keywords: inrush current, transformer, Down-chirp techniques.
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Introduction
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Inrush current in a transformer has a higher rated electrical currents, and the voltage is
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asymmetric, resulting from the instantaneous electricity distribution to the transformer which
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cause a momentary electric current with the increased amplitude values up to 10 times of the
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current primary coil of the transformer with no power transmission. [1] An occurrence of the
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inrush current of a transformer in transmission lines, distribution systems, or coordinate low
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voltage transformer is an important concern for a protective device as a protective equipment
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may view the exceeded as the short circuit current and may cause the circuit breaker of
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transformer.
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transformer coils as well.
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Reviewing such existing issues, the researcher conducted this study to provide ways to reduce
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the amount of inrush current occurring on the transformer by employing the Down-Chirp
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method with the voltage of the supply to reduce the amount of excess flow. The results of the
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study provide guidelines for further development of the inrush current reduction.
The frequent occurrence of the inrush current may directly damage the
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Notations
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v (t ) : Transformer supplied voltage
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Vmax : Maximum voltage of supply
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max : Maximum magnetic flux
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N P : Turns of wiring
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 : Angular frequency
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f: Ordinary frequency
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3
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Inrush Current in a Transformer [2]
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The connection between the transformer and the supply result in a momentary overload
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currents or inrush current in the primary circuit that is higher than the rated current because of
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the change of the magnetic flux on transformer cores. The current quantity depends on the
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form of the coil, transformer size, the structure of the axis, the residual magnetic flux on
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transformer cores, and the phase sequence of the supply. The characteristic pattern of the
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inrush current of the transformer is presented in Figure 1.
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The change of the magnetic flux on transformers according to the phaser of supply affecting
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the amount of the inrush current can be explained in Figure 2, which shows the direction of
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the magnetic flux occurring on the axis of the transformer when the transformer has the
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maximum voltage. It was determined from the following equations
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v(t )  Vm sin( t   ) V
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The maximum flux height reached on the first half-cycle of the applied voltage depends on
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the phase of the voltage at the time the voltage is applied. If the initial voltage is
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v(t )  VM sin( t  90 )  VM cos  t V
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and if the initial flux in the core is zero, then the maximum flux during the first half-cycle
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will just equal the maximum flux at steady state, that is
max 
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Vmax
N p
(1)
(2)
(3)
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This flux level is just the steady-state flux, so it causes no special problem. However, if the
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applied voltage happens to be
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v(t )  VM sin  t V
(4)
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the maximum flux during the first half-cycle is given by
 /
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1
NP
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Integrating gives
 (t ) 
max 
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
0
VM sin  t dt
2Vmax
 NP
(5)
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This maximum flux is twice as high as the normal steady-state flux in the core and results in
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an enormous magnetization current that look like a short circuit, and a very large current
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flows, that shown in Figure 1.
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Equation (1) to (5) represent the factors that affect the magnetic flux values occurring on the
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transformer while receiving voltage. The phaser position of the supply is significant and
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directly affect the quantity of magnetic flux. The changes of flux values result in the
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relationship between the magnetic flux with the magnetomotive force (mmf) by considering
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from the transformer magnetization curve. If the magnetization curve of the transformer
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increased, the inrush current will increase correspondingly. Therefore, if the quantity of
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magnetic flux is controlled and reduced, the amount of the inrush current on the transformer
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can be controlled and reduced as well.
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The Reduction of the Inrush Current in Electrical Transformer
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An effective way to reduce or limit the inrush current in electrical transformer while it is
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running with power is to control the quantity of magnetic flux on the transformer axis so that
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the value has decreased. The equation 4 presents a variable that defines the quantity of
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magnetic flux such as the maximum voltage of the supply (Vmax ) number of the cycle of the
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transformer’s coils ( N P ) , and the angular frequencies  . This research, therefore, focuses
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and places an attention on the way to reduce or limit the inrush current from the supply
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frequency   2 f to change the performance of the angular frequency and the values of the
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magnetic flux.
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Changing the frequency of the supply to a value higher than the frequency range while there
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is the electrical power in the transformer causes the magnetic flux values to decrease due to
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the increase of the angular frequency  . As a result, the magnetization curve of the
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transformer gets smaller. The values of inrush current are a direct variable in the change of
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magnetization curve. Characteristics of the changes of magnetization curve and the
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magnetization current according to the frequency of the supply are presented in Figure 3.
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Figure 3(a) demonstrates the change of the magnetic flux in the transformer when
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transformer’s coils receive the voltage with different frequency values. It can be noted that if
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the frequency of the supply has a higher value, the quantity of magnetic flux is reduced,
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resulting in the changes of the magnetization curve of the transformer. In addition, the
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saturation of the transformer core occurs quickly as shown in Figure 3(b). The performance
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of the magnetization current of a normal frequency is different from the sine signal. However,
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when the transformers get a higher frequency value, the magnetization current changes its
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shape and becomes similar to a sine signal as shown in Figure 3(c). From this process, it can
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be explained that to increase the frequency of the voltage until it is higher than the frequency
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range of a transformer is a method that can be used to reduce the amount of the inrush current
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in the transformer.
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Down-Chirp Technique [3]
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The Down-Chirp of the supply voltage to reduce the inrush current in a transformer refers
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to the change in the frequency of the voltage supplied to the transformer from high frequency
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to a frequency range. That is, a transformer starts working with the high-frequency value
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voltage for a while before turning back to operate with a frequency range. The pattern of
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voltage supply with the adaptive Down-Chirp model is shown in Figure 4, which presents the
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voltage supplied to the transformer. The frequency of a signal is changed in form of a linear
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equation as shown in the equation
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f f 
f (t )  f 0   1 0  t
 f1 
(6)
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When the default frequency is f 0 , f1 is the last frequency. It was observed that the frequency
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of the voltage supplied to the transformer had a high value at the beginning, resulted in the
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saturation of the magnetic flux and reduction of the inrush current in the transformer.
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Operational Testing
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In testing the inrush current, this research study simulated the operation using MATLB
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software. The coordinated information and important characteristics of the transformer used
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in the test are presented in Table 1 , the performance of the inrush current in the 1200VA
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50Hz can be tested and simulated as shown in Figure 5, which shows the comparison of the
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simulation results obtained from the MATLB SIMULINK software with inrush current
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values which were actually measured from oscilloscopes. It was found that the values of
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inrush current were approximately ten times greater than the normal currents in the
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transformer. Afterwards, the circuit was designed to reduce the values of inrush current by
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Down-Chirp the supply voltage. The simulation of the circuit is shown in Figure 6.
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Results
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In the simulation of the initial configuration, the default frequency and the transformer’s
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value of the frequency range of the Chirp Signal are controlled by comparing results from the
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default frequency value of four samples: 60Hz, 70Hz, 80 Hz, and 90Hz respectively. The
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results of the experiment determine the performance of the inrush current in the transformer
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as presented in Figure 7, its shows that the supply’s default frequency of 60 Hz, 70Hz, 80Hz,
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and 90Hz can reduce excess flow down to 3.25A, 2.7A, 2.4A, and 1.85A respectively. This
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research study suggests that the amount of excess currents can be reduced up to 50 percent of
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the actual inrush current which were measured.
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8. Conclusion
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Down-Chirp the frequency of the supply is a method that can be effectively used to reduce
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the inrush current of the transformer. The quantity of the inrush current depends on the
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default frequency of the voltage supplied to the transformer. Practically, in using such
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technique, the operation of an equipment which has the capability to control the frequency of
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voltage and the ability to modify the default frequency value with the transformer.
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References
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Taylor D. I., Law J. D., Johnson B. K. and Fischer N. "Single-phase transformer inrush
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current reduction using prefluxing," IEEE Transactions on Power Delivery, Vol. 27,
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S. G. Abdulsalam, Wilson Xu, W. L. A. Neves, and X. Liu, "Estimation of Transformer
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saturation characteristics from inrush currents waveforms," IEEE Trans. On Power
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Delivery, Vol. 21, No.1, pp. 170-177, Jan. 2006.
8
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V.Molcrette, J.-L. Kotny , J.-P. Swan and J.-F. Brudny "Reduction of inrush current in single-
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phase transformer using virtual air gap technique", IEEE Trans. Magn., vol. 34, no.
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4, pp.1192 -1194 1998.
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Yu-Hsing Chen and Po-Tai Cheng, "An inrush current mitigation technique for the line-
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interactive uninterruptible power supply systems," IEEE Trans. on Industrial
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Applications, vol. 46, no.4, July/August 2010.
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R. A. Turner and K. S. Smith "Transformer inrush currents", IEEE Ind. Appl. Mag., vol.
16, no. 5, pp.14 -19 2010.
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N. Chiesa and H. K. Hoidalen "Novel approach for reducing transformer inrush currents:
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Laboratory measurements, analytical interpretation and simulation studies", IEEE
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[1]
Figure 1: The inrush current in the transformer.
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v (t)  (t)
t
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Figure 2: The phaser voltage and magnetic flux of the transformer.
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 ,Wb
 ,Wb
f1 =50Hz
f2 =60Hz
f3 =70Hz
f1
f2
mmf
A.turns
f3
im
(a)
(b)
f2
f1
f3
t
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(c)
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Figure 3 (a) Magnetic flux on transformer cores. (b) magnetization curve.
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(c) magnetization Current.
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Voltage
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Time
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Figure 4: The voltage resulted from the adjustable frequency value with
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the Down-Chirp method.
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Table 1. Parameters of the 1200VA 50Hz transformer.
Parameter
Voltage(V)
Normal
Current (A)
R(ohm)
L(H)
LV winding
115
0.35
8.27
0.148
HV winding
230
0.17
14.9
0.41
14
4
Current (A)
3
2
1
0
-1
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
Time (s)
(a)
3.76A
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(b)
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Figure 5: Inrush Current: (a) results of the MATLAB simulation
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(b) results from the actual measurement.
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298
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301
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Figure 6: Circuit simulation of reducing inrush current by Down-Chirp supply voltage.
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Initial frequency = 60 Hz
4
3.5
3
2.5
Current (A)
2
1.5
1
0.5
0
-0.5
-1
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.3
0.35
0.4
0.45
0.5
0.3
0.35
0.4
0.45
0.5
0.3
0.35
0.4
0.45
0.5
Time (s)
Initial frequency = 70 Hz
4
3.5
3
2.5
Current (A)
2
1.5
1
0.5
0
-0.5
-1
0
0.05
0.1
0.15
0.2
0.25
Time (s)
Initial frequency = 80 Hz
4
3.5
3
Current (A)
2.5
2
1.5
1
0.5
0
-0.5
-1
0
0.05
0.1
0.15
0.2
0.25
Time (s)
Initial frequency= 90 Hz
4
3.5
3
Current (A)
2.5
2
1.5
1
0.5
0
-0.5
-1
310
0
0.05
0.1
0.15
0.2
0.25
Time (s)
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Figure 7: The result of inrush current reduction in the power transformer using voltage
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Down-Chirp technique.
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