Effects of Magnetizing Inrush Current on Power Quality and DistributedGeneration

Manana, M.; Eguıluz, L.I.; Ortiz, A.; Dıez, G.; Renedo, C.; Perez, S.

Department of Electrical EngineeringE.T.S.I.I.T. University of Cantabria

Avda. Los Castros s/n39005 Santander, Cantabria

SpainPhone: +34942201378 Fax: +34942201385 Email: mananam@unican.es

Abstract. When a power transformer is energized there isan important transient inrush of current that it is necessary inorder to establish the magnetic field of the transformer. Somepower transformers exhibit peak current demand up to eightor ten times the nominal value. In addition, during the firstcicles high values of the homopolar components of current arealso requested by the transformer. If the power transformer isplaced in a substation that works as a common coupling pointfor distributed generation facilities some specific power qualityproblems can be found.

This paper analyzes the problem from a general point ofview considering not only the theoretical approach but also theresults obtained in a real case including two 180 MVA powertransformer and a 120 MW distributed generation.

Keywords. Power Quality, distributed generation, protec-tions, inrush current, homopolar components

1. IntroductionFrom a general point of view, the magnetizing of

power transformer can be the origin of some powerquality problems. First at all, it should be underlinedthat the transient of current that is required to establishthe magnetic field of the transformer during the mag-netization can not be considered a fault condition so itshould not cause protective relays to operate. A basicapproach to the problem can be done considering a coilsurrounding a ferromagnetic material which is suppliedfrom a sinusoidal voltage [1]. The steady-state flux canbe computed as the integral of the supplied voltage v(t),

φ(t) =1N

∫v(t)dt =

1N

∫cos(ωt)dt =

1ωN

sin(ωt)

(1)Equation 1 highlights the fact that flux lags the voltage

by 90 degrees.Considering the coil as a linear inductance, the current

will have exactly the same waveform as the flux, that is,

i(t) =1L

∫v(t)dt (2)

However, it is well known that the ratio between theflux density φ and the magnetic field intensity H in aferromagnetic material is linear while H is lower thana value that define the knee of the saturation curve.Transformers are designed to operate close the knee ofthe saturation curve of the core hysteresis loop when themachine is working under normal conditions. When atransformer is connected after a period out of service, theconditions of the analysis are different. Considering thatthe transformer is energized when the voltage is zero andthere is not residual flux or in the worst case, the residualflux has a value that is the opposite to the theoreticalmeans that the resulting flux will be up to twice its normalmaximum value. In this case the magnetizing current willbe many times the nominal value because the non-linearbehavior of the transformer core. Figure 1 shows thecurrent required to provide a given level of flux.

The duration and amplitude of the inrush current is afunction of two sets of parameters [2], [3]. The first oneconsiders parameters which belongs to the transformer.The second includes parameters from the power system.Among others, the following should be considered:

1) Nominal power of the transformer.2) Material used to build the core of the transformer.3) Residual flux just before the connection of the

power transformer.4) Short circuit power at the common coupling point.5) Distance between the bus of the substation and the

power transformer.

Points 1, 2 and 3 are specific from the transformer,while 4 and 5 are defined by the power system at whichthe power transformer is connected.

2. Power quality problems derived fromMagnetizing Inrush Current

From a power quality point of view, the magnetizinginrush current can be considered as a distorted wave withtwo kind of disturbances [1], [2]:

Time

Tim

e

Time

i(t)

v(t)

Amplitude

Exc

iting

curr

ent

φφ(t

)

resi

dual

flux

φR

Figure 1. Electrical magnitudes involved in the inrush current of thepower transformer magnetization.

• Unbalance• Harmonics

A. Unbalance

Current unbalance can not be considered a disturbance.Asymmetrical loads produce unbalanced currents. In thesame way, the magnetizing inrush current produces cu-rrent unbalance during magnetization. This condition canbe used in parallel with the second harmonic in orderto know what it is happen during the connection of thetransformer.

B. Harmonics

The current demanded by the transformer during themagnetization contains all orders of harmonics. However,only the second and third harmonics are relevant. The dccomponent can also be significant during the first cyclesdepending on the residual flux. The most significativeharmonics are the following:

• DC or offset component. A dc component canbe found almost always in the inrush current, withdifferent values for each phase of the three-phasesystem. The offset is a function of the residual flux.

• Second harmonic. The second harmonic is presentin all inrush current of all three phases. The valueof the second harmonic is a function of the degreeof saturation. Anderson [1] says that the minimum

value for the magnitude of this harmonic componentis about 20% of the excess magnetizing current.

• Third harmonic. Third harmonics in the inrushcurrent can be found with the same magnitude thatsecond harmonics. They are produced by saturation.

• Higher harmonics. Harmonics of high order arepresent with different values. Actually, they havesmall values so can be neglected.

3. Distributed generationThe protection of the generation units is more complex

than the protection of static machines like transformers.In addition, protection experts suggest the necessity ofdisconnecting the generators when harmonics or currentunbalance are upper than a threshold level. The mainreason is that harmonic components produce an incrementof the heat inside the machine. This heat can producemechanical problems derived from the resistance of thematerials used to build the generators. Ac generators aresensitive to harmonics and unbalance. Problems can besummarized as follows [3]:

Harmonics In spite of the fact that there are diffe-rent harmonic components in the inrushcurrent, special attention has to be paidto the second harmonic. This frequencycomponent is not present in fault cur-rents so can be use in order to testif the fault condition is true or only amagnetizing condition.

Unbalance This disturbance produce negative-phase-sequence components of currentwhich induce a double-frequency inthe surface of the rotor, the retainingrings, the slot wedges, and to a smallerdegree, in the field winding. Theseinduced currents can produce hightemperatures in relatively short time.

4. Magnetizing inrush supressionWhen the magnetizing inrush current is high enough

to produce power quality problems due to the trip ofthe relays that protect both the power transformer of thegenerators, there are several method that can be use inorder to avoid these problems [4], [5], [6]. The basis ideais to desensitize the relay during the transformer magneti-zation. It can be done adding a time delay or observing thecurrent harmonics. New techniques like artificial neuralnetworks can help to discriminate between magnetizationand fault conditions [7].

5. Test results

A. Real case

A real case including two 180 MVA power transformersand 120 MW distributed cogeneration has been analyzed.

Both the power transformers and the generators areconnected to a 220 kV substation. Figure 2 shows thestudied power system.

Power System220 kV

Cogeneration120 MW

Transformer 1180 MV A

Transformer 2180 MV A

Figure 2. Power System under Test.

B. Results

The power system has been put to the test in order tomeasure the magnetizing inrush current.

Figure 3 shows the rms value of the voltage at 220 kVbus during the magnetization of the power transformer.The machine is magnetized without any connected load.

0 2 4 6 8 10 12131

131.5

132

132.5

133

133.5

134

134.5

135

Power Transformer magnetization

T ime (s)

Vol

tage(k

V)

Figure 3. Voltage evolution at 220 kV bus during power transformermagnetization.

Figure 4 shows the rms value of the current demandedby the primary side of the transformer during the mag-netization process in the same conditions that figure 3.

Figure 5 shows a current snapshot of the first cyclesafter the connection of the power transformer with thesame conditions that figure 3.

Figure 6 shows the rms value of the homopolar compo-nent of the current demanded by the transformer duringthe magnetization. This component has to be supplied bythe power system and the distributed generation.

Finally, figure 7 shows the second harmonic componentof the current demanded by the transformer during themagnetization.

0 2 4 6 8 10 120

50

100

150

200

250

300

350

400

Power Transformer magnetization

T ime (s)

Curr

ent(A

)

Figure 4. Current demanded by the power transformer number 2during the magnetization (rms value).

1.32 1.34 1.36 1.38 1.4 1.42 1.44

-800

-600

-400

-200

0

200

400

600

800

1000

T ime (s)

Curr

ent(A

)

Figure 5. Current demanded by the power transformer number 2during the magnetization (snapshot).

0 2 4 6 8 10 120

10

20

30

40

50

60

70

80

90

100

Power Transformer magnetization

T ime (s)

Curr

ent(A

)

Figure 6. Homopolar current demanded by the power transformernumber 2 during the magnetization (rms value).

From the point of view of the distributed generation,figure 8 shows the rms value of the current supplied bythe distributed generation during the transformer mag-

0 2 4 6 8 10 120

20

40

60

80

100

120

140

160

180

200

Power Transformer magnetization

T ime (s)

Curr

ent(A

)

Figure 7. Second harmonic current demanded by the power trans-former number 2 during the magnetization (rms value).

netization. Figure 9 shows the homopolar componentof the current supplied by the cogeneration during themagnetizing process.

0 2 4 6 8 10 12115

120

125

130

135

140

145

Power Transformer magnetization

T ime (s)

Curr

ent(A

)

Figure 8. Current supplied by the cogeneration during the magneti-zation (rms value).

6. ConclusionsThis research work highlights the necessity of limiting

the magnetizing inrush current of power transformers, es-pecially when they share bus with distributed generation.This problem is more important when the short circuitpower at the common coupling point has reduced values.In order to avoid false trips produced by the transformermagnetization, a second condition can be add to thegeneration protection schemes.

AcknowledgmentThe authors would like to thank the support of the

Spanish Government under the CICYT research projectDPI2002-04416-C04-01.

0 2 4 6 8 10 120

2

4

6

8

10

12

Power Transformer magnetization

T ime (s)

Curr

ent(A

)

Figure 9. Homopolar current supplied by the cogeneration during themagnetization (rms value).

References[1] Anderson P.M. Power System Protection. IEEE Press, McGraw-

Hill, 1989.[2] ANSI/IEEE C37.91-1985, IEEE Guide for Protective Relay Appli-

cations to Power Transformers. ANSI/IEEE, 1985.[3] ANSI/IEEE C37.102-1987, IEEE Guide for AC Generator Protec-

tion. ANSI/IEEE, 1987.[4] Sharp R.L. and Glassburn W.E. A transformer differential relay with

second harmonic restraint. AIEE Trans., 77:913–918, December1958.

[5] Larson R.R. and Flechsig A.J. Schweitzer E.O. An efficientinrush detection algorithm for digital computer relay protection oftransformer. IEEE Trans., 97:323, March-April 1978.

[6] Sidhu T.S. and Sachdev M.S. On line identification of magnetizinginrush and internal faults in three-phase transformers. IEEE Trans.,PWRD-7(4):108–117, October 1992.

[7] Meador J.L. and Obradovic Z. Perez L.G., Flechsig A.J. Trainingand artificial neural network to discriminate between magnetizinginrush and internal faults. IEEE Trans., PWRD-9(1):434–441, 1994.