7/29/2019 A2_306

1/8

avilla@edelca.com.ve

SYMPATHETIC INTERACTION BETWEEN STEP-UP TRANSFORMERS UP13.2/400 kV OF MACAGUA HYDROELECTRIC COMPLEX FROM CVG EDELCA

A. VILLACVG Electrificacin del Caron, C.A. (CVG EDELCA)

(Venezuela)

SUMMARY

This work show the analysis of sympathetic interaction phenomena between step-up transformers up13.2/400 kV for Power House #2 of Macagua Hydroelectric Complex from CVG EDELCA. Whenthese equipments are energizing from a 400 kV field with others transformers from Power House #2connected to the system, the activation of the differential protection, produce the trip out of service ofthese transformers. The analysis was focused to determine through ATP simulations the transformersinrush currents magnitude by energizing, the harmonic spectrum and the effect from the use of pre-insertion resistors by the 400 kV circuit breakers to reduce the magnitude and time to decay. The

results show that the sympathetic interaction between step-up transformers has an influence in themagnitude, duration and harmonic spectrum of the inrush currents, furthermore the use of pre-insertion resistor affects the initial magnitude and duration of these currents, but it has a less impact onthe harmonic spectrum. Finally, to avoid the activation of the differential protection, it was necessaryto adjust the detection band of second harmonic magnitude and increase the blocking time of theprotection, while is attenuating the inrush current.

KEYWORDS

Transformer Differential Protection Inrush Current ATP TACS.

21, rue dArtois, F-75008 PARIS A2-306 CIGRE 2006http : //www.cigre.org

7/29/2019 A2_306

2/8

2

INTRODUCTION

The 400 kV System Guri Macagua II Guayana B of CVG EDELCA, has associated the higherpart of the region load (Steel industries, Aluminum, etc). At Guayana B substation arriving five (5)transmissions lines from Guri and Macagua II as show in Figure #1, to keep the electric servicecontinuity in case of contingencies that involve some lines out of services.

Figure #1. 400 kV Transmission System Guri-Macagua II Guayana B.

Figure #2. Power House #2 and substationMacagua II up 400 kV layouts.

The Macagua Hydroelectric Complex has three (3) power houses, which the power generating by eachhouse are grouped in six (6) units of 60 MW, twelve (12) units of 198 MW and two (2) units of 86 MWrespectively for a total installed power of 2908 MW. The power house #1 & 3 step up the voltage up 115kV and the power house #2 step up the voltage from 13.2 to 400 kV and is interconnect to the field orMacagua II substation through six (6) transmission lines of 2 kms length, as show in Figure #2.

On the days 15th and 24th of April 1997, the 13.2/400 kV step-up transformers of Power House #2from Macagua Hydroelectric Complex, trip out of service due the activation of differential protection,when is energize the equipment from 400 kV field (Macagua II substation) with others step-uptransformers of Power House # 2 connected to the system. The event report for the associateddifferential protection from the step-up transformer showed activation by inrush currents.

2. THE INRUSH CURRENT AND SYMPATHETIC INTERACTION BETWEENTRANSFORMERS

In the transformer energizing a transient inrush current appears that is produce by equipment coresaturation and it has for characteristic to be unidirectional, with higher magnitude and it decay after

some period of time until the value of magnetizing current due the normal operation conditions.

Usually, the transformer inrush currents are calculating as the transformers are connecting to a systemwithout other transformers on service, but in the practice these equipment are energizing in series orparallel with others transformers that are on services and this condition can cause a transientinteraction or a sympathetic interaction between the energizing transformer and the other transformerson service, which change the magnitude and duration of the inrush currents. A similar situation canarise in system with higher series resistances, like some with longer transmission lines [1].

The inrush current can be affected by different factors like as followings:

- The voltage wave point where is energizing the transformers.

- The total system impedance through is flowing the inrush current.

7/29/2019 A2_306

3/8

3

- The saturation or maximum magnetic flux densities of ferromagnetic material from transformercore.

- The residual flux into the transformer core and its polarities respect to the first half cycle ofalternative flux in steady state.

- The saturation level reach by other transformers connected to the system.

As mentioned before, whatever the condition that impose an instantaneous change on the inductionfluxes of power transformers can produce an abnormal flux of higher magnetizing currents and thiscan cause a tendency of operation of the differential protection, that to be avoided request toinsensibility the same during the period of inrush current duration, but keeping the protection, thecapability of distinguish between the short circuit wave current and the inrush current.

The inrush currents are characterized by having higher harmonic components that are not present inshort circuit current and whose amplitude in fundamental percent is shown in Table #I [2].

TABLE #IHarmonic amplitude in percentage of inrush current fundamental

HarmonicComponent

Amplitude(% of Fundamental)

2 63.03 26.84 5.15 4.16 3.77 2.4

3. OBJ ETIVE

Determine through ATP simulations, the inrush currents magnitude by energizing of step-uptransformers up 13.2/400 kV of Power House #2 from Macagua Hydroelectric Complex of CVGEDELCA, the harmonic spectrum and the effect of using pre-insertion resistors by the 400 kV circuitbreakers to reduce the magnitude and time to decay.

4. THE MODEL USED IN SIMULATIONS

As next, is a description of the use models to represent the different equipments associated to a 400 kVMacagua II substation, a generator circuit breakers and the step-up transformers of power house #2[3]. In Figure #3 is shown the complete layout of these models when a step-up transformer isenergized.

Figure #3. Complete layout of the models used insimulations.

Figure #4. The step-up transformers core voltageversus current characteristic.

7/29/2019 A2_306

4/8

4

4.1 Thevenin Equivalent of 400 kV Macagua II

The associated system to Macagua II substation was considered through a Thevenin equivalent sawfrom a 400 kV busbar, which comprise an ideal voltage source with constant frequency (60 Hz)connected behind a mutually couple resistive and inductive circuit.

4.2 The 400 kV circuit breakers

These equipments are represented like one ideal phase circuit breakers and when it has pre-insertionresistors, a time of half cycle (8.33 ms) was used like insertion time.

4.3 The 13.2/400 kV step-up transformers

These equipments are three phases shell type, in star and delta connection for high and low voltagessides. These units have a 400 kV high voltage winding and two 13.2 kV low voltage winding with a500/250/250 MVA nominal power, FOA cooling system and short circuit impedances for a 60 Hzoperating frequency of 13.8% for HV/LVY and HV/LVX and 23.36% between LVY/LVX. The tap

changer is located in series with the high voltage winding; it has five (5) different positions to regulatewithout load in a range of 5% from 400/3 kV.

These step-up transformer were represented by a mutually coupled resistive inductive (RL) circuit,whose parameters for self (Zs) and mutual (Zm) impedances were calculated from the measuringimpedances of short circuit equipment test and the core voltage versus current characteristic as showin Figure #4, was obtained from the not load transformer test. The cables connection to the generators,was considered through a capacitance by phase of 0.25F.

4.4 The differential protection

These protection equipments have a different logical circuit of control that evaluated the input and

output currents in the protected zone and its activation are produced when a difference between thesecurrents exist or by its higher level of harmonics content [4]. The principle of operation consist, incalculus of the current difference (Id) between the measuring of primary and secondary circuits,furthermore, the average current (Ip) to determine the current difference percent of the average current(%Id/Ip), in the case that overcome the adjust value (%K), is generated the trip order for the circuitbreakers. Additionally, the second harmonic component on the current difference (Id) is calculated toavoid that during the transformer energizing, an erratic activation is produced, due the inrush currentsunbalance [5]. The adjustment using by the differential protection to avoid the above mentioning arethe followings:

1) The second harmonic magnitude has to be less to an adjustable value between at 8 to 20% of thefundamental inrush current magnitude.

2) The time during the second harmonic component are present and has to be less to an adjustable timebetween at 1 to 99 s to allow the inrush current attenuation.

The operation scheme used for this protection to calculate the second harmonic content was simulatedthrough the program for Transient Analysis of Control Systems (TACS) and its equivalent blockdiagram as show in Figure #5.

Figure #5. Block diagram used to calculate the second harmonic content by differential protection.

7/29/2019 A2_306

5/8

5

5. SIMULATION OF 13.2/400 KV STEP-UP TRANSFORMER ENERGIZING

This consists in simulate the step-up transformer energizing by a high voltage terminal (400 kV) in thefollowings conditions or cases:

1) Without others step-up transformers connected to a 400 kV busbar.2) With 1 to 5 step-up transformers not loaded connected to a 400 kV busbar.3) With 1 step-up transformer connected to a 400 kV busbar and two generators on service, given 100

MVA by each one with 95% lagging power factor.4) Without other step-up transformer connected to a 400 kV busbar and line circuit breakers with pre-

insertion resistors values of 0.1 to 3 k.5) With 2 step-up transformers not loaded connected to a 400 kV busbar and line circuit breakers with

pre-insertion resistors values of 0.1 to 3 k.

In all above cases, the three phases circuit breakers were closed simultaneously and to reach themaximum inrush current at the step-up transformer phase A, the close was done when the phase Avoltage goes through zero.

5.1 Result analysis

The energizing of a step-up transformer without another step-up transformer at 400 kV busbar (case1), give an inrush current of 1.97 p.u. or 2,011 Amperes peak at the high voltage side, as show inFigure #6 and whose time to decay (t =L/R), depend of the magnetizing inductance and the total resistance(systems plus step-up transformer) relationship as indicate in zone 1 and 2 respectively. The decrease of coresaturation, increase the core inductance and produce an increase of the inrush current decay time, as show inzone 2. In Figure #6a, can be seen the distortion D in the phases inrush current wave, due the step-uptransformer core circulating currents.

Figure #6. Step-up transformer phases inrushcurrent.

Figure #6a. Step-up transformer phases inrushcurrent enlargement.

In Figure #7 is shown the second harmonic magnitude contained by phases from the step-uptransformer inrush current and the average value (to facilitate the next analysis), expressed inpercentage of fundamental. In this Figure, can be appreciate, that the second harmonic in phase Cand Average, decay more slowing and decrease until 20% of its magnitude in approximately a time of

4s.

7/29/2019 A2_306

6/8

6

Figure #7. Second harmonic magnitude contained by phases from step-up transformer inrush currentand the average value, expressed in percentage of fundamental.

In Figure #8 & 8a shown the inrush currents by the step-up transformer and the system, where it can see thatthe step-up transformer # 2 saturate as result of step-up transformer # 1 energizing and produced asympathetic current with opposite polarity to the step-up transformer #1 current. The magnitudes of thesecurrents will be the same after a time, and the resultant dc component, that is flowing in the ring made by thestep-up transformers, causing that the cores remain saturates and increasing the inrush current decay time; itdepend of the inductances and resistances closed in the ring and whose decay time is larger that the given bythe system and the step-up transformers.

Figure #8. Inrush currents see by step-up transformerand the system.

Figure #8a. Enlargement of inrush currents see bystep-up transformer and the system.

In Figure #9 is shown the decay time increase from the second harmonic current for the study cases with 1 to5 not loaded step-up transformers connected to the same 400 kV busbar and whose time are between a 9 to 22

s to decay until a 20% magnitude. In case that the step-up transformers connected to the busbar has beingfeeder with the generators, the decay time is reduced from 9 to 7 s as can be appreciate during one step-up

7/29/2019 A2_306

7/8

7

transformer energizing with another step-up transformer connected to the same busbar and 2 generators onservice (case 3).

FIGURE #9. The step-up transformer second harmonic inrush current content with another step-uptransformer in busbar, cases without load and with generators in service.

In Figure #10 & 11 show the decrease of the first inrush current peak by using different pre-insertionresistor values (0.1 to 3 k) and where it can see that the higher reduction is get with 1 k resistor bythe cases without and with step-up transformers connected to a 400 kV busbar (cases 4 & 5). Thisreduction was higher than the 50% of inrush current magnitude getting by cases of circuit breakerswithout pre-insertion resistors.

Figure #10. First peak inrush current magnitude vs.

pre-insertion resistor values by circuit breaker forcases without step-up transformer in a 400 kV busbar.

Figure #11. First peak inrush current magnitude vs.

pre-insertion resistor values by circuit breaker forcases with 2 step-up transformers in a 400 kV busbar.

7/29/2019 A2_306

8/8

8

In Figure #12 show the second harmonic decay time for cases without and with step-up transformersconnected to the same 400 kV busbar and circuit breakers without and with pre-insertion resistor (1k), where it can be see that the decaying time is keeping constant (cases 4 & 5). The above indicatedthat the delayed action in the activation of differential protection avoided an erratic performance byinrush currents.

Figure #12. Step-up transformer second harmonic inrush current content, cases without and with 1 k pre-insertion resistor and with and without another step-up transformers in a 400 kV busbar.

6. CONCLUSIONS

These can be summarized as next:

- The sympathetic interaction among the step-up transformer influences the magnitude, duration andharmonic content of inrush currents.

- The use of pre-insertion resistors affects the initial magnitude and duration of inrush currents buthas a less impact over the harmonic spectrum.

- Finally, to avoid the differential protection activation it was necessary to adjust the detection band

of the second harmonic magnitude and increase the protection blocking time while the inrushcurrents is attenuating.

BIBLIOGRAPHY

[1] H.S. Bronzeado, P.B. Brogan, R. Yacamini, Harmonic Analysis of Transient Currents duringSympathetic Interaction (IEEE Trans. On Power Systems, Vol.11, No.4, November 1996)

[2] C. Russell Mason, El Arte y la Ciencia de la Proteccin por Relevadores (Compaa EditorialContinental, S.A. Mxico, 1978)

[3] Working Group 33.02 (Internal Overvoltages), Guidelines for Representation of NetworkElements When Calculating Transients (CIGRE, 1990)

[4] J. Raull, Diseo de Subestaciones Elctricas (McGraw-Hill de Mxico S.A., 1987)

[5] Three Phase Differential Protection for Transformers (GEC Alsthom, T&D, 1993)[6] Alternative Transients Program (Rule Book, LEUVEN EMTP Center, 1992)