• Electrical India
  • Mar 27, 2017

Dependence of Inrush Current in Single-Phase Transformers on Winding Geometry and Placement – An Experimental Investigation

Inrush current drawn by a transformer is expected to result in inadvertent operation of over-current protection system provided in electronic power supplies, and, therefore, is considered to be a special concern to power supply designers. In this paper, two low cost practical methods for reducing inrush current and a power supply for measuring the same are proposed. These methods are based on varying the location of winding and core window dimensions. Four line frequency transformers have been fabricated and evaluated with a view to demonstrating the effectiveness of these methods. A low cost controlled power switch has been developed to energize the transformers at selective angle of input sinusoidal waveform.  Experimental results suggest that the inrush current can be reduced by suitably locating the primary winding and designing the core window of the transformer...

- P. Renuka Nath, Mangesh B Borage,

S R Tiwari, A.C.Thakurta


 Transformers are vital and expensive components for electrical energy transfer in electronic power supplies the working of which is impinged upon by stability and security of transformer protection. Besides, outages of these transformers can interrupt the power supplies for considerable duration. Apropos, inrush current is one of the most difficult problems besetting the working of the power supplies. This current, the driving force of which is considered to be the voltage applied to the primary winding of the transformer, is caused by core saturation.

  Besides, the large transient current can cause considerable electromagnetic stress and reduce the life of transformer and the magnetic stress resulted by the inrush current may cause damage to mechanical structure. Furthermore, deleterious effects of uncontrolled inrush current include inadvertent operation of the over-current protection system and unwanted blowing of fuses provided in power supplies. Therefore, the inrush current must be reduced to overcome these.

  Majority of the methods used for protecting transformers against inrush current attempt to reduce this current to a safe value using external means instead of trying to reduce the inrush current of the transformer itself. One of the methods for limiting the inrush current of transformer is using current limiting impedance which could be a resistor, an inductance, or their series combination. But its drawback is power losses that result due to flow of steady current through them. Here, with a view to suppressing these losses, the current limiting impedance is bypassed by short circuiting. Increasing source resistance will decrease the amplitude of inrush current. Besides, it causes faster decay in the amplitude of inrush current. The amount of reduction of inrush current by electronically controlled switching angle technique may not be considerable due to remanent flux density of the core. Furthermore, this method is impractical where a number of transformers are used in a power supply and it requires additional control circuitry. The structure of winding impacts the magnitude of both leakage inductance and inrush equivalent inductance. Transformer can be designed to reduce the inrush current by adopting asymmetrical winding.

  This paper reports procedure and results of an experiment carried out to investigate the effect of winding geometry and its placement on the inrush current of a single-phase transformer. In the method proposed in this paper, when the primary winding is placed next to core or next to secondary winding, the insulating material requirement is expected to be of same. Furthermore, as in a normal transformer with shielding, shielding requirement if any can be met using single insulated copper foil between primary and secondary winding of this type of construction.

  The rest of the paper is organized as follows: Overview of transformer inrush current, its effects and mitigation methods are presented in section II. Section III describes the prototype transformer parameters and peculiarity in their design and construction. A simple power switch developed to energize the prototype transformers is presented in Section IV. Section V discusses various experimental results.

TRANSFORMER INRUSH CURRENT

  Inrush current is a form of over-current that results during energization of a transformer. Transformers are normally designed to operate below the knee of the saturation curve. However, when those are switched-on on no-load, flux builds upto a high value, thereby falls in the region of saturation and this subsequently leads to increase in current. 

  This inrush current can be up to 10 times higher than normal rated current of transformer though the steady-state magnetizing current of a transformer may be only 1-2 percent of the rated current. This transient effect which may persist for a few seconds before the steady-state condition is reached can cause tripping of some of the protective relays. Inrush currents which are highly asymmetric and have many detrimental effects are of special concern to power supplies designers. Types of transformer inrush currents include energization inrush, sympathetic inrush and recovery inrush. The most common form of inrush, energization inrush current, results when a transformer is switched on from a de-energised state. The rating and design of the transformer impact the magnitude of the inrush current significantly. The factors, on which the magnitude and duration of transformer inrush current depends, encompass saturation flux density of the core, residual flux in the core, resistance of the primary winding, source impedance, the point on the voltage wave at which the transformer is switched on, the saturation level reached by the transformers which are already connected to the system, etc. Although peaks of inrush current are higher for smaller transformers, its duration is longer for larger transformers. Modern transformers which are designed to operate at a higher flux density may have higher inrush currents.

PROTOTYPE TRANSFORMER PARAMETERS

  Since the inrush current might be seriously disturbing phenomenon, it behoves power supplies designers to pay heed to the same and limit its level. In this connection, differently from traditional methods, a few types of practical methods have been dwelled upon to enfeeble the inrush phenomenon and abate inrush currents.

  A case of single-phase line frequency transformer has been considered for the experimental study. Terminal specifications and design parameters of these transformers are summarized in table 1.

Table-1: Specifications and design parameters for the prototype transformers

  With an aim to study the effect of winding geometry and its relative placement from the core, four transformers have been made and named as T1, T2, T3 and T4.

  Transformers T1 and T2 are designed and fabricated to be identical in all respects except for the relative placement of the windings – secondary winding is wound next to the core limb and primary is wound over the secondary winding in transformer T1 whereas primary winding is wound next to the core limb and secondary is wound over the primary winding in transformer T2. This is illustrated in Fig. 1.

Fig. 1: Schematic showing relative positions of winding in transformer T1 (right) and transformer T2 (left).

  Transformers T3 and T4 have the same winding positions (primary next to the core limb and secondary wound over primary) but these transformers have different window heights. This is illustrated in Fig. 2. 

Fig. 2: Schematic showing relative window heights in transformer T2 (left), transformer T3 (middle) and transformer T4 (right).

  Considering VA rating, rated current and availability of the material, etc, CRGO Electrical steel EI laminations are selected for core. Various core dimensions are marked in Fig. 3. Transformers T1 and T2 have been made using standard EI laminations of size STN 43 and the remaining two transformers are made using cores with different window heights. Summary of these variations in the prototype transformers is given in table 2. Thus in all the transformers, the core material, core area, number of turns, type of conductor, conductor material, conductor size and the type of insulating material used are kept identical.

Fig. 3: Schematic showing various dimensions of EI laminations referred in table 2.

Table-2: Dimensions and other details of prototype transformers

CONTROLLED POWER SWITCH

  A low cost controlled power switch was developed to enable testing of prototype transformers for inrush current when they are energized at different instances on the waveform of the input supply voltage. Figure 4 shows the block diagram of the controlled power switch. It is implemented using a triac Q1 that receives ac single phase power through fuse F1 and isolator switch S1. Indicator lamp L1 shows the availability of the ac mains to the unit. The output side of triac feeds the transformer under test. Triac Q1 is put on by applying gate pulse from the control circuit enclosed in shaded box of Fig. 4. The control circuit receives a synchronizing signal from a step-down synchronizing transformer Ts. A phase shifter selectively shifts the phase of the synchronizing sinusoidal signal from 0o to 75o in steps of 15o. The phase shifted signal is then passed through zero crossing detector (ZCD) to generate firing pulse for the traic. The pulse is then modulated at 5 kHz and passed through a pulse amplifier circuit, wherein it is isolated using a ferrite transformer, and fed to the gate of triac Q1.

Fig. 4: Schematic diagram of the controlled power switch.

EXPERIMENTAL RESULTS

  The measurement setup and the prototype transformers are shown in Fig. 5(a) and 5(b) respectively. Measured waveforms of inrush current and primary voltage for four prototypes, measured at switching angle of 15 and 75 degrees, are shown in Fig. 6 to 9.

Fig. 5: Photograph showing measurement setup (a) and prototype transformers (b).

Fig. 6: Waveforms of primary voltage (ch-1, 200 V/div) and inrush current (ch-2, 10 A/div) in transformer T1 at (a) 15o and (b) 75o switching angle. X-scale: 25 ms/div.

Fig. 7: Waveforms of primary voltage (ch-1, 200 V/div) and inrush current (ch-2, 10 A/div) in transformer T2 at (a) 15o and (b) 75o switching angle. X-scale: 25 ms/div.

Fig. 8: Waveforms of primary voltage (ch-1, 200 V/div) and inrush current (ch-2, 25 A/div) in transformer T3 at (a) 15o and (b) 75o switching angle. X-scale: 25 ms/div.

Fig. 9: Waveforms of primary voltage (ch-1, 200 V/div) and inrush current (ch-2, 25 A/div) in transformer T4 at (a) 15o and (b) 75o switching angle. X-scale: 25 ms/div.

  While the waveforms of Fig. 6-9 show the measured waveforms for 15o and 75o switching angles, the measurements were carried out for intermediate switching angles also. Fig. 10 summarizes the results in the graphical form.

Fig. 10: Summary of measured inrush current in prototype transformers: (a) for various switching angles and (a) for 0o and 75o switching angles.

CONCLUSIONS

  One of the important factors on which the magnitude of this inrush current depends is the instant on the source voltage wave the transformer is switched on. The current drawn will be very high when the transformer is switched-on when the voltage wave is passing through its zero value. This is also vivid from the values of the inrush current measured on all fabricated transformers at different switching angles. The results obtained in this study suggest that the inrush current can be reduced by keeping the primary winding over the secondary winding as this leads to comparatively large area enclosed by its turn and increases the ratio of mean area covered by a single turn of the primary to net iron area. Apropos, this inrush current can also be reduced by reducing the core window height.


P Renuka Nath
Scientific Officer
Power Supplies and Industrial Accelerator Division
Raja Ramanna Centre for Advanced Technology
Department of Atomic Energy
Government of India

Dr. Mangesh Borage
Scientific Officer
Power Supplies and Industrial Accelerator Division
Raja Ramanna Centre for Advanced Technology
Department of Atomic Energy
Government of India

Shri S.R Tiwari
Scientific Officer/H
Head, Power Converter Development Section
Power Supplies and Industrial Accelerator Division
Raja Ramanna Centre for Advanced Technology
Department of Atomic Energy
Government of India

Shri Amalesh C. Thakurta
Head, Power Supplies and Industrial Accelerator Division
Raja Ramanna Centre for Advanced Technology
Department of Atomic Energy
Government of India

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