ieee trans - reduce inrush current in delta-wye transformer

2nd IEEE International Conference on Power and Energy (PECon 08), December 1-3, 2008, Johor Baharu, Malaysia

Reduction of Magnetizing Inrush Current in a Delta Connected Transformer

K. P. Basu * and Ali Asghar ** * Faculty of Engineering, Multimedia University, Cyberjaya, Malaysia. Email: [email protected] ** Faculty of Engineering, Multimedia University, Cyberjaya, Malaysia. Email: [email protected]

Abstract Magnetizing inrush current in power transformer creates severe power quality problems. Energization of the transformer from the delta side of a delta-star transformer does not allow the control of neutral resistor at the time of switching the supply for reducing the inrush current. Controlled switching at peak value of the voltage may reduce inrush current in one winding of the transformer, but the line currents are not reduced. Delayed and controlled energization of the third phase may reduce inrush winding currents in two windings and only one line current is reduced, but the inrush currents in the other two lines become very high [11].

The inrush current may be reduced either by inserting suitable value of resistance connected to each of the line wire or by combining the controlled switching at peak value of the voltage in one winding and resistance switching in the third phase. The resistances are shorted within a very short time after switching on the delta winding. Simulation studies of delta side energization of a delta-star transformer depict the results under various operating conditions. Optimum resistance value is decided to get a quick decay of inrush current, low voltage drop and losses before it is shorted.

Keywords Magnetizing inrush current; delta side energization; series resistor; controlled switching

I. INTRODUCTION Production of high inrush current in a big transformer

during switching in the supply always creates a big power quality problem. In a deregulated market with disperse generation having transformers located at different points in a system makes the problem more acute. The transient voltage dip may cause motor tripping, protective relay mal-function etc. Over the past many decades, a large number of simulation and experimental studies have been reported on the production and control of inrush currents in single and three-phase transformers [1-8]. Commonly used techniques to reduce inrush current are insertion of series resistor, point-on-wave switching etc. Energization of the transformer from the star side allows the control of neutral resistor utilizing the advantage of unbalancing in the 3-phase inrush current and sequential switching of phases to achieve the reduction of inrush current [9-10]. No extensive research on inrush current is reported for the delta side energization of the transformer barring one [11], which concludes that delayed and controlled energization of the third phase reduce inrush current only

in one phase. Therefore, the only option remains for the reduction of inrush current of a delta connected primary is to insert external resistors in series with the line. These resistors may be shorted after the inrush current decays out. The resistors may be inserted at the secondary of protective current transformers, which may increase the VA rating of the cts to high values and are not acceptable. .

This paper reports the simulation studies on the inrush current produced by the delta side energization of delta-star transformer with additional resistors connected in series with the line. Optimum value of the resistor produces quick decay of the inrush current enabling the resistors to be shorted with very short time delay. Three resistors in series with 3 phases need a circuit breaker having 6 contacts. But controlled switching of one winding with resistor switching of only one line also reduces inrush current and needs a circuit breaker having 4 contacts.

II. DELTA SIDE ENERGIZATION Fig.1 shows the delta-side energization of a delta-star

transformer. Unlike star-side energization, series resistance insertion to reduce inrush current is possible only in the line and not in the delta winding of the transformer.

a

b

c

TransfornerrsCB

G

Fig.1 Delta-side switching of transformer

The equations governing currents and voltages of the delta winding with the star side open are presented below. Leakage reactance and remnant flux in the transformer core are neglected.

Eab = Emax sin(t+) = n1(dab/dt) + r1iab + rs(ia - ib) - -(1) Ebc =Emax sin(t+-2/3) = n1(dbc/dt) + r1ibc+rs(ib-ic) (2) Eca = Emax sin(t++2/3)= n1(dca/dt) + r1ica+rs(ic-ia) (3) Where, Eab, Ebc, Eca instantaneous voltages across

windings iab, ibc, ica- instantaneous winding currents ia, ib, ic- instantaneous line currents ab, bc, ca- instantaneous fluxes linking each winding

1-4244-2405-4/08/$20.00 2008 IEEE 35


n1, r1- number of turns/phase, resistance/phase of winding

rs series resistance in each line; - switching angle The first peak of inrush current depends upon the (i)

instant of switching () or the voltage magnitude at the instant of energization, (ii) winding resistance and external resistance connected in series with the winding or with the line, and (iii) remnant flux in the core. As the no-load impedance angle of the transformer is almost 90, controlled switching at = 90 causes maximum reduction in inrush current. Balanced or unbalanced external resistance may only be inserted in the lines of a delta connected transformer. Though high value of resistance at the time of switching may help to reduce the initial peak and quick damping of dc offset current and thereby, core saturation, it causes extra loss and voltage drop during normal operation. Therefore, shorting the external series resistor is essential as soon as the inrush current dies out.

A bank of 3 single phase transformers, each having a rating of 1 MVA, 6.9 kV/69 kV, 50 Hz, connected in delta-star are used for the simulation study. The 3-phase transformer rating becomes 3 MVA, 6.9 kV/120 kV, 251A/14.5A. The primary (6.9 kV) winding number of turns per phase is n1 = 4050 and r1 = 0.25 . Simulation models [12-13] are generally used to obtain magnetic and electric circuit parameters of 3-phase, 3-limb or other type of transformers. However, magnetization curve with deep saturation data of a stalloy core [7] is used for the simulation. A MATLAB program was prepared for the simulation study. Simulation results are shown in Fig.2 and Fig.3.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-1000

-500

0

500

1000

1500

2000delta = 0 rs = 0iab ibc ica

A

T(sec) Fig.2 Winding currents =0 rs = 0 Due to backlash and wear and tear of circuit breaker

contacts, all the contacts of the CB may not close simultaneously. Closing of any 2 CB poles (say a, b) applies line voltage Eab across the winding a-b and across windings b-c and c-a in series. Due to symmetrical construction of the transformer, voltages across b-c and c-a are half of Eab until the third pole of CB is closed. Eab/2 produce winding currents ibco and icao, which can not saturate the cores b-c and c-a. Delayed closing of the 3rd pole may produce higher inrush currents in the other 2 windings compared to those of the simultaneous closing of contacts. It has been reported earlier [11] that delayed and controlled switching of the third pole of the CB may

produce negligible inrush current in one line only. Currents in the other two lines become very high.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-2000

-1500

-1000

-500

0

500

1000

1500

2000delta = 0 rs=0ia ib ic

A

T(sec) Fig.3 Line currents =0; rs = 0. Therefore, the only option remains for the reduction of

inrush current is to insert external resistors (rs) in series with the line. Because of delta connection of the windings either one resistor in each line or resistors in any 2 of the 3 lines may be inserted to reduce the current. In the later case the inrush currents become unbalanced. These resistors are to be shorted during normal operating condition. Insertion of these resistors directly in series with the high voltage line with the provision of shorting them needs the requirement of an extra 3-pole circuit breaker, which may not be cost effective. But suitable value of rs may damp out the inrush current quickly to drop below the full load current amplitude within a very short time of 0.1 second (say) and the resistance is shorted by 3 additional contacts of the same circuit breaker. Thus a single circuit breaker having 6 contacts including 3 delayed closing contacts may be used to reduce the inrush current during delta side switching of transformer (refer Fig. 4). The resistor rs should be as low as possible to reduce the voltage drop and power loss to a minimum before shorted. The main objective is to minimize the period of power quality disturbance.

Current transformers (cts) are generally connected to

each line for the purpose of protection and metering. Suitable value of resistors may be connected to the secondary of these transformers to reduce the inrush current and to make the decay of the inrush current faster, such that the power quality disturbance period only persists for a few cycles. Shorting of these resistors under normal operating condition is not difficult and does not

rs

Tr

G

CB

Fig.4 Resistance connected to line

36


require any costly equipment. But massive increase of VA ratings of these cts is observed due to high value of secondary resistors. As for example for a protective ct ratio of 250A/1A, the value of rs connected to the ct secondary becomes 15.625 k and the power rating becomes a few hundred kVA. Therefore, connection of resistor to ct secondary becomes impractical.

Fig.5 and Fig.6 show the simulation results of inrush currents with 0.25 ohm external resistor added in each line. The plots clearly indicate the reduction and quicker decay of inrush currents with added resistance. The magnitude reduction of the first peak of inrush current is not very high but the decay is very fast and the current reduces below the full load peak current within 0.1 second. The peak voltage drop at the instant of first peak is less than 450 volt and does not create any insulation problem of the external resistor.

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-1000

-500

0

500

1000

1500delta = 0 rs = 0.25 delay = 0.1seciab ibc ica

A

T(sec) Fig.5 Winding currents =0; rs = 0.25 delay = 0.1sec

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-2000

-1500

-1000

-500

0

500

1000

1500delta = 0 rs = 0.25 delay = 0.1secia ib ic

A

T(sec) Fig.6 Line currents =0 rs = 0.25 delay = 0.1sec. Another method of reducing the inrush current is to

energize any one winding of the delta side with controlled switching at = /2 and resistance switching of the third phase. The resistance rs is shorted after a short time delay of 0.1 second as discussed earlier. The controlled switching at the peak of any line voltage ( = /2) may be

carried out with the help of a point-on-wave switching device.

A circuit breaker having 4 contacts including one

delayed closing contact may be used for this purpose (refer Fig.7). Simulation results are shown in Fig.8 and Fig.9. The winding current of phase ab is reduced to minimum due to switching at voltage peak of eab. The value of rs is higher than that of 6-pole circuit breaker scheme.

Fig.8 Winding currents = /2; rs = 0.5 delay = 0.1sec

Fig.9 Line currents = /2; rs = 0.5 delay = 0.1sec

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-1500

-1000

-500

0

500

1000

1500delta = pi/2 rs = 0.5 delay = 0.1sec iab ibc ica

A

T(sec)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4-2000

-1500

-1000

-500

0

500

1000

1500delta = pi/2 rs = 0.5 delay = 0.1sec

ia ib ic

A

T(sec)

CB Fig.7 Point-on-wave switching with resistance

connected to one line

rs

Tr

GPoint-on-wave

switching device

37


III. CONCLUSIONS Simulation results of the production of magnetizing

inrush currents due to delta-side energization of a delta-star transformer have been presented. Delayed and controlled switching of the third pole of the CB may produce negligible inrush current in one line only. Currents in the other two lines become very high [11]. The inrush current may be reduced either by series resistance insertion in each line or by combining the controlled switching at peak value of the voltage in one winding and resistance switching in the third phase. The series resistances are shorted within a very short time after the delta winding energization in both the schemes. The first scheme requires a 6-pole circuit breaker and the second scheme requires a 4-pole circuit breaker with a point-on-wave switching device. Simulation studies of delta side energization of a delta-star transformer depict the results under both the schemes. Optimum resistance value is decided to get a quick decay of inrush current, low voltage drop and losses before it is shorted.

REFERENCES [1] T. Specht, Transformer inrush and rectifier transient currents,

IEEE Trans. Power App. Syst., vol. 88, no. 4, pp. 269-276, Apr. 1969

[2] K. Smith, L. Ran, and B. Leyman, Analysis of transformer inrush transients in offshore electrical systems, Proc. Inst. Elect. Eng. Gen. Trans. Distrib., vol. 146, no. 1, pp. 89-95, Jun. 1999.

[3] CIGRE working group task force 13.07, Controlled switching of HVAC circuit breakers, in 1st Part Elektra, no. 183, pp. 43-73, Apr. 1999.

[4] Westinghouse, Electrical Transmission and distribution Reference Book. Chicago: R. R. Donnelley & Sons co., 1944, pp. 411-417.

[5] B. Holmgrem, R. S. Jenkins, and J. Rinbrugent, Transformer

inrush current, in CIGRE Proc. 22nd session, vol. 1, 1968, pp. 1-13.

[6] R. Yacamini and A. Abu-Nasser, The calculation of inrush current in three-phase transformer, IEE. Proc. Elect. Power Appl., vol. 133, no.1, pp.31-40, Jan. 1986.

[7] Mohibullah and Basu K. P., Computerised Evaluation of the Magnetising Inrush Current in Transformers, Electric Power System Research, vol.2, pp. 179-182, 1979.

[8] M. A. Rahman and A. Gangopadhayay, Digital Simulation of magnetizing inrush currents in three-phase transformers, IEEE Trans. Power Del., vol. PWRD-1, no. 4, pp. 235-242, Oct. 1986

[9] Yu Cui, S. G. Abdulsalam, S. Chen and Wilsun Xu, A Sequential Phase Energization Technique for Transformer Inrush Current Reduction-Part I: Simulation and Experimental Results, IEEE Trans. On Power Delivery, Vol. 20, No.2, pp. 943-949, Apr. 2005.

[10] Wilson Xu, S. G. Abdulsalam, Yu Cui, and X. Liu, A Sequential Phase Energization Technique for Transformer Inrush Current Reduction-Part II: Theoretical Analysis and Design Guide, IEEE Trans. On Power Delivery, Vol. 20, No.2, pp. 950-957, Apr. 2005.

[11] K. P. Basu, Ali Asghar and Stella Morris, Effect of Sequential Phase Energization on the Inrush Current of a Delta Connected Transformer, International Conference on Power Electronics, Drives and Energy Systems for Industrial Growth (PEDES2006), New Delhi, India 12-15 December 2006, pp.1-4.

[12] M. Elleuch and M. Polujadoff, A contribution to the modeling of three phase transformers using reluctance, IEEE Trans. Magn., vol. 32, no. 4, pp. 1199-1204, Oct. 2000.

[13] S. G. Abdulsalam, Wilson Xu, W. L. A. Neves, and X. Liu, Estimation of Transformer saturation characteristics from inrush currents waveforms, IEEE Trans. On Power Delivery, Vol. 21, No.1, pp. 170-177, Jan. 2006.

38

ieee trans - reduce inrush current in delta-wye transformer

Documents