Journal of ELECTRICAL ENGINEERING, VOL. 61, NO.

1, 2010, 11–19

CONTROLLED SWITCHING OF THE T402 TRANSFORMER

IN THE KRIŽOVANY 400kV SUBSTATION
∗ ∗
František Janı́ček — Martin Mucha
∗∗ ∗∗
Karol Česnek — Jozef Kováčik

The paper deals with simulating the circumstances of switching surge and trigger current at switching-in of the T402
autotransformer recently installed in Križovany, with the tests conducted by means of ATP (Alternative Transient Program)
simulation software for simulating transient electromagnetic phenomena. The target has been to determine temporal curves
for the most critical surge instances against earth and between the phases and the curves for trigger current with and without
surge limiters connected, and to determine by means of simulations the size and statistical frequency of switching surges and
based on a controlled switching simulation, to describe the elimination of transient phenomena during the process and in
reference to the simulations and to operating manual for the PSD02 Siemens control unit, suggest the required configuration
settings for the unit.
K e y w o r d s: switching surge, trigger current, substation, transformer, T402 transformer, EMTP - ATP, Križovany,
surge current

1 INTRODUCTION idle transformer. For examining the least favourable in-

stances in terms of surge voltages, maximum voltages of
Issues relating to non-controlled switching of large 420 kV and 242 kV respectively have been set for the as-
transformers in very high voltage networks have been paid semblies, with secondary and tertiary transformer wind-
particular attention in available sources. Surge current ings idle. The TR402 and TR403 400 kV fields have been
in switching large transformers along with the respective modelled by means of LCC component with view to lay-
switching surges are an additional strain upon very high out and dimensions of respective bus bars and connec-
voltage assemblies but also may result in network stabil- tions. With view to feedback effects of surge waves, the
ity issues. 400kV bus bar has not been connected to the network
This paper deals with simulating switching surges in directly but through dedicated connections modelled as
the newly installed T402 transformer in the Križovany line elements with evenly distributed parameters. The
substation conducted by means of ATP software for sim- line parameters have been defined in reference to mea-
ulating transient electromagnetic phenomena. The results sured values. The lines have been terminated with net-
aim at determining statistical frequency of surge voltages work connections modelled by means of ideal voltage
when idle transformer switches in and determining tem- source with surge impendance in a consecutive compo-
poral curves for the most critical surge voltages against nent assembly calculated in reference to 1-phase and 3-
earth and between phases and temporal curves for clos- phase shortcut surges to reach the Križovany substations
ing current with and without connected surge limiters. while deducting line impendance values. The transform-
Moreover, curves are examined for switching off the trans- ers have been modelled by means of BCT component as
former and a controlled switching automation example 3-phase autotransformers TR402 and TR403 with 5-core
with the respective curves indicated that testify to elim- magnetic circuit and as three one-phase TR401 compo-
inating transient phenomena in the process. In the last nents. Based on label indications and measured current
part in reference to the simulations conducted and to the and output values at the source and for idle system, an
Siemens PSD02 unit user manual, ideal setup parameters alternative transformer scheme has been calculated. The
are suggested for the unit. transformer model also includes residual capacities of the
different windings against earth and between each other.
The model assembly further includes PTP residual ca-
2 MODEL DESCRIPTION pacities. Diverters are modelled by means of non-linear
MOV 3 resistor component in which the defined VA char-
Figure 1 shows the circuit layout of the model em- acteristic of the ZnO limiter is modified for applicable
ployed for simulating switching surges in switching on UT .

∗
Faculty of Electrical Engineering and Information Technology of the Slovak University of Technology in Bratislava, Ilkovičova 3,
812 19 Bratislava, frantisek.janicek@stuba.sk, martin.mucha@stuba.sk;
∗∗
VÚJE, a. s., Okružná 5, 918 64 Trnava, cesnek@vuje.sk, kovacik@vuje.sk,

c 2010 FEI STU

ISSN 1335-3632
12 F. Janı́ček — M. Mucha — K. Česnek — J. Kováčik: CONTROLLED SWITCHING OF THE T402 TRANSFORMER IN . . .

a mean deviation of σ = 2 ms. Based on the simulation

results, percental frequency has been determined of surge
voltage for each selected closing point in the cycle. By
means of the approach, it has been ensured that the re-
sults comprise apart from the dependency of emergent
surge voltage upon momentary voltage also surge voltage
dependency upon the closing sequence of the different
contacts while considering the closing delay between the
different contacts. Along with the high number of closing
instances, this ensures statistically relevant results that
closely reflect real-life circumstances. In Table 1, average
number is indicated for emergent surge voltages over the
entire period and the dependency visualized in Figure 2.
The significance of the different points is as follows: for in-
stance the point that corresponds to the value of kf = 1.9
indicates that 20 % of surge instances when switching on
idle TR402 transformer was greater than or equal to 1.9
times the normal value. For better visualization, the surge
values are quantified as phase-to-neutral surge coefficient
uf p max
defined as kf = √2(U/ √ , where uf p max is the peak
3)
value of phase-to-neutral surge and U the effective value
of stabilized phase-to-phase voltage for the network prior
to the point of surge emergence.

Fig. 2. Percental frequency distribution of emergent switching

Fig. 1. Circuit layout for the mathematical model surges in non-controlled switching of idle TR402 Križovany without
surge limiter connected over the entire period

3 SWITCHING ON OF IDLE TRANSFORMER In Fig. 2, the frequency curve is shown for emergent
WITHOUT SURGE LIMITER switching surges under non-controlled switching of idle
TR402 over the entire period.
The following Chapter provides the results for theoret- The peak phase-to-phase closing surge reached a value
ical surge voltages in switching on idle transformer with- 2.5 times higher than nominal voltage — 815.9 kV and
out attached surge limiter. In order to determine statisti- 1150 kV respectively between phases, ie, two times the
cal frequency distribution for switching surges in switch-
value but less than nominal proof voltage for switching
ing on idle T402 transformer, simulations by means of
impulse against earth — 1050 kV and 1500 kV respec-
ATP software have been conducted employing a special
tively between phases.
STATISTICAL SWITCH. Over the period of a network
voltage cycle (T = 0.02 s) for 20 evenly distributed points Temporal curves for maximum emergent surge volt-
in the cycle in ∆t = 1 ms intervals, 600 closing instances ages are shown in Figs. 3 and 4. Peak value for closing
have been simulated for an identical mean closing time current in the initial period was 556.77 A, switch recovery
value; in total in each period for 20 selected points in the voltage was 538 kV as shown in Figs. 5 and 6. The closing
cycle, 12000 transformer switching on instances have been current peak value caused by charging residual capacities
conducted with three switch contacts featuring Gaussian did not occur this time as the most unfavourable circum-
statistical distribution curve for each contact closing with stances in inductivity switching are the most favourable
 Journal of ELECTRICAL ENGINEERING 61, NO. 1, 2010 13

Fig. 3. Maximum phase-to-neutral surge voltage for primary con- Fig. 4. Maximum interphase surge voltage for primary connectors
nectors for TR402 Križovany for switching o idle TR402 without for TR402 Križovany for switching on idle TR402 without surge
surge limiter limiter

Table 1. Mean Percentual Frequency Chart for Emergent Surge

Voltages under Non-Controlled Switching of Idle TR402 Trans-
former over the Entire Period

Phase- Emergent Phase- Emergent

to-phase surge to-phase surge
surge frequency surge frequency
coeff. (%) coeff. (%)
1 100 1.8 26.85
1.1 81.95 1.85 23.65
1.15 77.25 1.9 20.53
1.2 72.9 1.95 17.88
1.25 67.28 2 14.8
1.3 59 2.05 11.3
Fig. 5. Recovery voltage curve for the QM TR402 Križovany switch 1.35 55.83 2.1 8.6
for switching on idle TR402 without surge limiter 1.4 52.65 2.15 6.5
1.45 49.78 2.2 4.375
1.5 46.68 2.25 3
1.55 43.43 2.3 1.8
in capacitor switching which is why the transient phe-
1.6 39.975 2.35 0.9
nomenon practically did not occur in this instance. The 1.65 36.38 2.4 0.275
high closing current in Fig. 6 at the point of transformer 1.7 32.98 2.45 0.1
switching in is caused by the transformer connection to 1.75 29.53 2.5 0
network at the most unfavourable point in the cycle when
there is a zero voltage value and emergent offset magnetic
current in the core that combines within the semiperiod
with nominal magnetic current, hence the total magnetic Table 2. Average percental frequency of emergent switching surges
in switching on idle TR402 over the entire period with 336/267 surge
current in core is twice the value of nominal magnetic cur-
limiter
rent which is why the core material becomes saturated
with magnetizing current reaching extreme values. The
Phase- Emergent Phase- Emergent
Fourier analysis results imply significant phenomena in
to-phase surge to-phase surge
the closing current initial period with significant one-way surge frequency surge frequency
component and basic harmonic and significant 2nd , 3rd coeff. (%) coeff. (%)
and 4th harmonic. In the FFT area of the transient phe- 1 100 1.4 53.18
nomenon becoming stabilized for the most unfavourable 1.05 88.375 1.45 50.05
point in the cycle of transformer switching in, there is 1.1 82.38 1.5 47.08
a comparable amount of higher harmonic as compared 1.15 76.95 1.55 44.13
to basic harmonic due to transformer operating in the 1.2 71.93 1.6 39.88
1.25 66.48 1.65 32.58
magnetizing curve saturation range for the transformer
1.3 58.65 1.7 17.88
core material when idle but these are relatively small and 1.35 56.03 1.75 0
comparable to current values for idle transformer.
14 F. Janı́ček — M. Mucha — K. Česnek — J. Kováčik: CONTROLLED SWITCHING OF THE T402 TRANSFORMER IN . . .

Fig. 6. Closing current curve for non-controlled switching of idle TR402 Križovany without surge limiter: (a) — overall curve, (b) —
detail for the initial phase, (c) — FFT for the initial period, (d) — detail after 10 seconds, (e) — FFT for the last period

in the initial section. Surge voltages were limited to 1.75

times the nominal value of phase-to-neutral voltage.
Peak phase-to-neutral switching surge reached 1.75
times the nominal value — 599.5 kV and 872 kV respec-
tively between phases, ie1.54 times, yet this is less than
nominal proof voltage for switching impulse against earth
— 1050 kV and 1500 kV respectively between phases.
Temporal curves for peak emergent surge voltages are
shown in Fig. 8 and 9. Peak value for closing current
reached 556.75 A in the initial period, the switch recovery
voltage was 538 kV as shown in Figs. 10 and 11.

Fig. 7. Percental frequency curve for emergent surge voltages for

non-controlled switching of idle TR402 Križovany over the entire
period with 336/267 surge limiter
5 CONTROLLED SWITCHING ON OF IDLE
TRANSFORMER WITHOUT SURGE LIMITER

In order to prevent transient phenomena and the re-

4 SWITCHING ON OF IDLE TRANSFORMER
WITH 336/267 SURGE LIMITER lated switching surges in controlled inductivity switch-
ing, voltage needs to be applied through switch at the
peak point as inductivity current is delayed by one fourth
In this section, the results are listed for theoretical of the period, hence equals zero at the point. For three-
surge voltages in switching on idle transformer with con- phase transformers with earthed neutral, this would mean
nected surge limiter corresponding to real-life circum- to connect the different phases in a sequence of their
stances. All the reference assumptions apply as described maximum voltage, ie, each delayed compared to previous
 Journal of ELECTRICAL ENGINEERING 61, NO. 1, 2010 15

Fig. 8. Peak phase-to-neutral surge voltage curve for primary con- Fig. 9. Peak interphase surge voltage curve for primary connectors
nectors of TR 402 Križovany for switching on idle TR402 Križovany of TR402 Križovany for switching on idle TR402 with connected
with connected 336/367 surge limiter 336/367 surge limiter

Fig. 10. Recovery voltage curve for the QM TR402 Križovany Fig. 11. Closing current curve for non-controlled switching on of
switch in switching on idle TR402 with connected 336/367 surge idle transformer TR402 Križovany with connected 336/367 surge
limiter limiter

Fig. 12. Phase-to-neutral voltage curve for primary connectors of Fig. 13. Phase-to-phase voltage curve for primary connectors of
TR402 Križovany in controlled switching TR402 Križovany in controlled switching

phase by one third of the period. However, due to mag- increases to nominal value. The current is separated by
netic coupling between the different phases, the closing se- the remaining part of the core in which magnetic current
quence would not prevent switching surge from emerging. is not generated by the remaining windings. If the remain-
For switching idle three-phase transformers with earthed ing two phases are applied simultaneously one fourth of a
neutral, one phase is applied at the peak of its supply period of 5ms (peak point of phase-to-phase voltage be-
voltage (with acceptable tolerance of 2 ms from the peak tween the two phases) after the first phase as their values
credit to fair gradient of the sine curve in the area). For are equal in nominal terms yet of opposite polarity (mag-
sudden changes, voltage affects inductivity immediately, netic current generated by the first winding voltage being
ie, one phase remains at nominal voltage and magnetic zero at the point, zero voltage exposure time), magnetic
current in the respective part of the transformer core also current for the remaining two windings remain separated
16 F. Janı́ček — M. Mucha — K. Česnek — J. Kováčik: CONTROLLED SWITCHING OF THE T402 TRANSFORMER IN . . .

current), there are no transient phenomena and hence no

surge voltages occur. In setting up switching time in prac-
tical applications, the mechanical closing time needs to
be considered for each pole in the switch and arc preig-
nition time in the switch at maximum phase-to-neutral
voltage for the first phase and at half the value of peak
phase-to-phase voltage for the remaining three phases.
The curves for phase-to-neutral and phase-to-phase volt-
ages for primary connectors of T402 Križovany and the
curve for closing current in controlled switching are shown
in Figs. 12, 13 and 14. In controlled switching, closing cur-
rent equals idle transformer current. The peak value for
240 A closing current is the charging current of residual
capacities; in a model that does not include the capacities,
there would be no peak value in closing current for the
transformer. In controlled switching, the transient phe-
nomenon is the most pronounced. In this particular case,
the peak value is not significant but referential only as
there have been no precisely measured values for a par-
ticular T402 but for a T403, and it has been modelled
with a joint capacitor at the transformer connections as
this is not practically implementable otherwise in ATP,
and this is an approximation of reality with even capacity
distribution with loading current featuring different char-
acteristics. In Fig. 14b, Fourier analysis is shown of clos-
ing current that highlights the significant substance of the
first harmonic and of one-way component. A possible 2ms
tolerance example in phase switching is demonstrated in
Figs. 15–18. Figure 19 shows a case without considering
residual capacities, making the induced voltage in the re-
maining phases more evident at half of its value. Surge
current is minimal in controlled switching as shown in
Fig. 14 and in switching on with 2 ms tolerance, there
were peak values of 261 A and 177 A respectively; how-
ever, this is an extreme case with a similar occurrence
Fig. 14. Closing current curve for three phases for TR 402 likelihood to that of maximum switching surge in non-
Križovany: (a) – in controlled switching, (b) – detail, (c) — FFT controlled switching. In order to highlight the elimina-
analysis
tion of switching surges in controlled switching, no surge
limiter has been included in any of the simulations.

6 SWITCHING OFF IDLE TR 402

In switching off idle transformer, there have been no

significant surge voltages, as shown in Figs. 20 and 21.
Phase-to-neutral surges did not exceed 1.25 times the
nominal voltage even though 99 % of the surges exceeded
1.2 Um .

Fig. 15. Phase-to-neutral voltage curve for primary connectors of 7 DETERMINING CONTROLLED SWITCHING
TR402 Križovany in controlled switching with early phase switching PARAMETERS FOR SIEMENS PSD02 UNIT
by 2 ms against peak values

The simulation results imply that it is sufficient to

in their respective parts of core and negate each other (un- control the processes of switching on the transformer as
der the scenario, there is no sudden change in magnetic there are no significant surge voltages in switching off
 Journal of ELECTRICAL ENGINEERING 61, NO. 1, 2010 17

Fig. 16. Closing current curve for three phases for TR 402 Fig. 18. Closing current curve for three phases for TR 402
Križovany in controlled switching (a) — with early phase switching Križovany in controlled switching with delayed phase switching
by 2ms against peak value, (b) — detail (a) by 2 ms against peak value, (b) — detail

Fig. 17. Phase-to-neutral voltage curve for primary connectors Fig. 19. Phase-to-neutral voltage curve for primary connectors
of TR402 Križovany in controlled switching with delayed phase of TR402 Križovany in controlled switching without considering
switching by 2 ms against peak value residual capacities of windings

in standard operation. In the following sections, ideal • In-switch arc pre-ignition time at half the maxi-
parameter calculation for the PSD02 unit is described. mum
√
value of momentary phase-to-phase voltage value
2 2
Data provided by the manufacturer of the switch em- 2 U n tprearc ;
Where Un is network nominal phase-to-phase voltage.
ployed (best measured and for each pole):
In compiling the study, it has not been possible to ob-
• Mechanical operating time for the switch — tbreaker ± tain the data from the switch manufacturer which is why
0.1 ms; the following text may serve as a theoretical guide for cal-
culating controlled switching parameters. After obtaining
• In-switch arc pre-ignition time at maximum momen-
q the required times, the formulas may be easily populated
2 1
tary phase-to-neutral voltage value 3 Un − tprearc ; with the values.
18 F. Janı́ček — M. Mucha — K. Česnek — J. Kováčik: CONTROLLED SWITCHING OF THE T402 TRANSFORMER IN . . .

Fig. 20. Phase-to-neutral voltage curve for primary connectors of Fig. 21. Phase-to-phase voltage for primary connectors of TR402
TR402 Križovany in switching off idle transformer Križovany in switching off idle transformer

Set parameter (phase shift/pole) in degrees separately voltage and 2.65 times the value of phase-to-phase volt-
for each phase to age, with installed surge limiter a maximum of 1.75 times
the nominal value of phase-to-neutral voltage and 1.54
A = 90◦ , B = 180◦, C = 180◦ . times phase-to phase voltage and closing current may
reach several times the value of idle current. The val-
Set parameter (adjusting time – closing, or arcing time ues are lower than nominal proof voltage against earth
– opening ) in ms separately for each phase at negative at closing impulse — 1050 kV and 1500 kV respectively
value between phases but with automated switching control,
transient phenomena are eliminated, thus protecting the
A = −t1prearc , B = −t2prearc , C = −t2prearc . transformer as a preventive measure but also the trans-
former switch and other equipment exposed to the effects.
Set parameter (closing time/opening time) – if not set Based on the simulations, parameters are specified at
initially – to the end automated controls on the Siemens PSD02 unit
tbreaker . need to be set for in controlled switching of transform-
ers with earthed neutral on the switched side. It has not
Should it prove impossible to obtain the required been possible to obtain exact arc preignition times for
switch data from the switch manufacturer, we recom- specific switches and voltages and mechanical switch clos-
mend to initiate the ing times, hence only variables are indicated for the val-
q closing test with a preignition time √
t1prearc = 3 ms at 2
and t2prearc = 2.5 ms at 2 ues to which the particular parameters need to be set in
3 Un 2 Un
follow-up to obtaining the data from the manufacturer.
and mechanical operating time for the switch of tbreaker = Should it prove impossible to obtain the data, we pro-
40 ms. vide reference values that shall be used for setting up
The results of the oscillographic record of closing cur- test assemblies and further elaborated in more detail in
rent at closing test (but also if initial setting is left reference to obtained results as is the standard procedure
on), we recommend to check the closing current oscil- in some cases for setting up controlled switching.
logram. Ideal closing times can be derived from cal-
culated times: closing time, make time, command delay Acknowledgements
and closing time difference for the last closing instance
recorded by the equipment, by changing values of the pa- The paper has been compiled under grant provided by
rameters, adjusting time – closing, or arcing time – open- scientific research grant agency with the Ministry of Edu-
ing — based on recorded curves towards reaching mini- cation of the Slovak Republic and the Slovak Academy of
mum closing current. Sciences 1/3092/06 and of the Slovak Agency for Research
and Development under Ref. No. APVV - 20-023505.

CONCLUSION
References
In the article, simulation results are indicated of simu-
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[3] MARTON, K. : High Voltage Technology, Vydavatestvo ALFA, in 1979 and took the position of assistant lecturer with the
Bratislava, 1983. (in Slovak) Department of Electrical Power Engineering of EF SVT. In
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Energy Division Raychen 1996.
Research and Development Agency.
[12] STN EN 99-5 Surge voltage protector, recommendations for the
selection and use.
Martin Mucha graduated from the Faculty of Electrical
Engineering and information technology at the Slovak Uni-
[13] BIEKFORD, J. P. et al : Computation of Power System Tran-
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CHEN, Energy Division, Bratislava, 2004. (in Slovak)
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Technická literatura, Praha, 2005. (in Czech) surements in electric distribution with a nationwide scope.
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proposals for EU standards in theory and practice, EN IEC Technology ZSE Bratislava. Since 1990, he worked as Deputy
62305 Lightning protection. (in Czech) Director General SEP Bratislava, where later in the trans-
[20] GERT, R. : Operating surges in power systems, SNTL, Praha, formation to the SE and Bratislava tenure as the director
1964 pages142. (in Czech) and Chairman of the directorate. worked as project manager
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John Wiley & Sons, New York, 1991 pages751. cal Engineering (EF) at the Slovak University of Technology
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voltage protection.
Maintenance of Power Transformers in ZSE Bratislava. In that
[25] STN EN 60071-1 Insulation coordination - definitions, princi-
position he was involved on Operational Diagnostics of power
ples, rules.
devices within power substations. Regarding of technical fo-
[26] STN EN 60071-2 Insulation coordination - users guide.
cus of that Department one was dealing within whole terri-
[27] ABB Surge Arresters – Buyer’s Guide Edition 5, 2004-10. Cat-
tory of Slovakia. It was done in connection with SE Bratislava
alog Publ: 1HSM 9543 12-00en.
and SEPS Bratislava. Nowadays these technical activities are
[28] TRIDELTA Metal Oxide Surge Arrester for High Voltage Sys-
tems.
doing in connection with company VUJE, a.s. Trnava. He au-
thored or co-authored 15 articles in periodicals, and 10 lectures
Received 8 September 2009 at international scientific conferences. In area of Diagnostics
of Power Transformers the technical works were done within
František Janı́ček graduated from the Faculty of Electri- Power Grid of Slovakia as well with partners from abroad in
cal Engineering (EF) at the Slovak University of Technology Europe.