The article describes the complete procedure carried out in order to determine the cause of high levels of gases detected in chromatographic determinations performed in Current Transformers (CT) oil.
These analyses of gases are normally included in routine maintenance work. The obtained results could evidence some possible damage in the machine insulation.
According to the electrical stresses that Current Transformers are typically subjected to, it was presumed that the cause of the possible damaged could be the high-frequency transients that take place during air disconnect switch operations.
Therefore, with the aim of determining the presence of such electrical stress, field measurements were performed. A specially designed measuring system was utilized to accomplish this goal.
Taking into account the difficulties normally encountered when measuring currents flowing through Current Transformers to ground, especially in terms of accuracy, it was decided to perform voltage measurements. Then, by determining the real frequency response of the CT in the range of interest, it was possible to achieve such currents by computer simulations.
In order to achieve accurate results in the simulations, it was necessary to develop an appropriate electrical model of this type of transformers in the analyzed frequency range. Once the model was carefully adjusted, computer simulations were performed. As a result, both shape and magnitude of such currents were also achieved.
Finally, one of the Current Transformers with high levels of gases was taken out of service. Then, it was completely disassembled so as to confirm the suspected damage in the insulation. The results yielded in the inspection of the insulation are commented at the end of this work.Instituto de Investigaciones Tecnológicas para Redes y Equipos Eléctricos (IITREE

Barbera, Gustavo Ariel

Esteban, Daniel A.

Funes, A.

Issouribehere, Pedro

Ledesma, Alicia I.

Pozo, Miguel Dionisio del

English

El Servicio de Difusión de la Creación Intelectual

mdelpozo@iitree-unlp.org.ar 
 
 
Field measurements and modelling of high frequency transients during 
disconnect switch operations in EHV Substations. Assessment of their effects 
on Current Transformers 
 
 
M.D. DEL POZO     D.A. ESTEBAN     P.E. ISSOURIBEHERE     G.A.BARBERA 
IITREE FI-UNLP 
Argentina 
 
         A. FUNES                           A. LEDESEMA 
YACYLEC 
Argentina 
 
SUMMARY 
 
The article describes the complete procedure carried out in order to determine the cause of high levels 
of gases detected in chromatographic determinations performed in Current Transformers (CT) oil. 
These analyses of gases are normally included in routine maintenance work. The obtained results 
could evidence some possible damage in the machine insulation. 
According to the electrical stresses that Current Transformers are typically subjected to, it was 
presumed that the cause of the possible damaged could be the high-frequency transients that take place 
during air disconnect switch operations. 
Therefore, with the aim of determining the presence of such electrical stress, field measurements were 
performed. A specially designed measuring system was utilized to accomplish this goal. 
Taking into account the difficulties normally encountered when measuring currents flowing through 
Current Transformers to ground, especially in terms of accuracy, it was decided to perform voltage 
measurements. Then, by determining the real frequency response of the CT in the range of interest, it 
was possible to achieve such currents by computer simulations. 
In order to achieve accurate results in the simulations, it was necessary to develop an appropriate 
electrical model of this type of transformers in the analyzed frequency range. Once the model was 
carefully adjusted, computer simulations were performed. As a result, both shape and magnitude of 
such currents were also achieved. 
Finally, one of the Current Transformers with high levels of gases was taken out of service. Then, it 
was completely disassembled so as to confirm the suspected damage in the insulation. The results 
yielded in the inspection of the insulation are commented at the end of this work. 
 
KEYWORDS 
 
Electrical stress – Current Transformer – High-frequency transient – HV equipment – Insulation – 
Measurement – Model – Simulation. 
 
 
21, rue d’Artois, F-75008 PARIS         A3_207_2010       CIGRE 2010 
http : //www.cigre.org   
 
  2 
 
1. INTRODUCTION 
A Transmission Company of Argentina detected different anomalies in several EHV Current 
Transformers. The main anomaly consisted in the observation of high oil level probably due to the 
presence of different gases. 
In order to determine the presence of gases a sample of oil was taken and then chromatographically 
analysed. By the tests performed to the sample of oil it was detected a severe increment in the amount 
of several gases: hydrogen, methane and carbon dioxide, among them. In Fig. 1 the results of the 
chromatographic analyses are shown. It has also been included the Total Dissolved Combustible 
Gases (TDCG). 
Hydrogen Methane 
  
Carbon Dioxide Total Dissolved Combustible Gases 
Figure 1. Results of gases analyses performed in oil. 
The charts indicate the levels of gases in parts per million [ppm] as a function of time. In all the cases 
the amount of gases increased dramatically between the analyses carried out in March of 2000 and the 
next ones performed after the CT was taken out of service, in January of 2002. After that, personnel of 
the Company continued taking samples of oil and performing the same corresponding analyses. For 
this reason, the trends decrease once the peak has been reached. 
Naturally, such an increment in the amount of gases in the oil evidenced a possible damaged in the CT 
insulation. Because of this reason, several CTs were taken out of service. 
Since it was necessary to confirm whether the insulation was damaged, one of the CTs taken out of 
service was carefully disassembled in its totality. 
Additionally, it was also essential to determine the reasons for this premature damage. It was 
immediately thought that a possible cause of this could be the electrical stress that CTs suffer during 
air disconnect switch operations. It consists of high-frequency transients that arise in CTs while the arc 
is present in the disconnect switch. They take place in both opening and closing operations. 
Photograph in Fig. 2 shows the very moment in which the arc is present. 
With the purpose of determining the existence of such high-frequency transients, field measurements 
in several Transformer Stations were carried out. 
  3 
 
 
Figure 2. Arc present in the air disconnect switch during an operation. 
2. HIGH-FREQUENCY TRANSIENT MEASUREMENTS 
2.1. Measuring System Description 
A specially designed measuring system was utilized to carry out field transient measurements. This 
system basically consists of an electric field (E) sensor, an E/V transducer, and finally a transmitter 
and a receiver connected by a fiber optic link. The signal arriving at the receiver can be easily 
observed with either an oscilloscope or a spectrum analyzer. Its bandwidth is from 5 Hz to 10 MHz. In 
Fig. 3, some photographs of the system are shown. 
 
Figure 3. Photographs of the measuring system. 
The main advantage of this measuring system lies in the fact that measurements can be performed 
without affecting the normal operation of the substation since it has no physical contact with the 
installation. Added to this, due to the fiber optic link, measurements are not affected by the 
electromagnetic noise present in the substation. 
By the use of this measuring system it is possible to obtain waveforms of voltage transients. 
 
Receiver 
Fiber 
optic link 
Sensor and 
Transmitter 
  4 
 
2.2. Waveforms captured in field measurements 
As it was mentioned in the Introduction, in order to determine the presence of high-frequency 
transients, field measurements in several EHV Transformer Stations were made. Thus, some of the 
oscillograms captured by the measuring system in a particular EHV Transformer Station will be 
shown as follows. 
The voltage records were taken upstream one of the Current Transformers existing in two fields of the 
Station – called Field 1 and Field 2 in the present paper. The sensors were located below the 
connection bar of the transformers. In this way, the line-to-ground voltage was recorded. 
In Fig. 4 two oscillograms belonging to a closing operation in Field 1 are depicted. The first one was 
captured at the beginning of the arc, whereas the second was obtained 790 ms after the arc began. 
  
Figure 4. Oscillograms during closing operation. At the beginning and close to the end of the arc. 
As it can be observed in both graphs the perturbation due to the arc in the disconnect switch does exist 
and appears on the CT. At the beginning of the arc voltage steps of great amplitude, which only occur 
twice, one positive and another negative, during each period of the power-frequency are present. On 
the other hand, close to the end of the arc, as its length is reduced, the steps are of smaller amplitude 
than at the beginning, but their amount is greater during each period of the power-frequency. 
In Fig. 5 two oscillograms obtained during an opening operation are shown. The comments are 
analogous to those made in the previous case, but now the voltage steps of great amplitude take place 
at the end of the arc, while those of smaller amplitude appear at the beginning. 
  
Figure 5. Oscillograms during opening operation. At the beginning and at the end of the arc. 
These four records were done using a time scale of 20 ms per division. Nevertheless, new oscillograms 
with different time scale were captured. First, with the aim of knowing the front time of the voltage 
steps, faster records (1 s per division) were taken. Then, slower records (0.4 ms per division) were 
also obtained so as to know the time length of the arc during both operations. 
  5 
 
In order to summarize the totality of the results obtained in the Transformer Station Table I shows the 
maximum voltage step found throughout all the operations carried out in both fields. 
Table I. Maximum Voltage Step in Each Operation 
Maximum Voltage Step ΔUmax [kV] FIELD 
Closing Operation Opening Operation 
1 590 ; 530 ; 50 0; 470 ; 440 ; 420 ; 410 560 ; 530; 480 ; 400 
2 520 ; 450 620 ; 600 
In addition, Table II shows the range within front times lay for both kinds of operations. 
Table II. Front Times Recorded 
Front Times [ns] FIELD 
Closing Operation Opening Operation 
1 From 500 to 600 From 550 to 560 
2 780 810 
Finally, in Table III the durations of the arcs are given considering both the average value and their 
standard deviation. 
Table III. Time Lengths of Arcs 
Time duration [ms] 
FIELD Average Value 
Closing / Opening 
Standard Deviation 
Closing / Opening 
1 1235 / 1780 26 / 131 
2 345 / 720 25 / 40 
3. COMPUTER SIMULATIONS 
3.1. Purpose of simulations 
All the records performed involved only voltage measurements. However, in order to assess the actual 
solicitations that Current transformers are exposed to it is worth knowing the current flowing through 
transformers to ground as well. 
Since such currents were not measured – the sensor included in the measuring system performs an E/V 
conversion – they were obtained by computer simulations. 
In order to achieve successful results in computer simulations it is necessary to obtain an accurate 
model of the Current Transformer. For this reason, a detailed study of the CT frequency response in 
the range of interest was carried out. This included the measurement of its actual response in situ. 
Once the electrical model was properly adjusted, this was excited by a voltage generator with the 
shape of the oscillograms recorded in the Transformer Stations so as to determine the required current. 
3.2. Frequency Response Determination 
The frequency response of a Current Transformer of the same brand and model as those installed in 
Field 1 was obtained by field measurements. By the use of a spectrum analyzer which includes a built-
in tracking generator it was obtained the impedance of both CTs as a function of frequency between 
10 kHz and 5 MHz. 
In Fig. 6 both voltage and current measured spectrums are depicted. These responses also include 
contribution of the measuring circuit (coaxial cables, etc.). 
As it can be seen, the resonant frequency lies in 1.127 MHz. In addition, the total equivalent series 
resistance is 7.46 Ω. 
  6 
 
Att  40 dB
22.Feb 07 10:59
*
TG  0 dBm
RBW 3 kHz
VBW 10 kHz
SWT 10 s*Ref  50 dBmV
1 SA
VIEW
2 SA
VIEW
Center 2.505 MHz Span 4.99 MHz499 kHz/
0
5
10
15
20
25
30
35
40
45
50
1
Marker 1 [T1 ]
          32.76 dBmV
     1.127760000 MHz Att  30 dB
*
TG  0 dBm
RBW 3 kHz
VBW 10 kHz
SWT 10 sRef  45 dBmV
Center 2.505 MHz Span 4.99 MHz499 kHz/
1 SA
VIEW
2 SA
CLRWR
*
22.Feb 07 10:58
-5
0
5
10
15
20
25
30
35
40
45 1
Marker 1 [T1 ]
          43.40 dBmV
     1.097820000 MHz
 
Figure 6. Spectrums of voltage and current of a CT between 10 kHz and 5 MHz. 
The next challenge was to achieve the reproduction of both responses in the model proposed in the 
simulation program. This model has to include not only the components related to the Current 
Transformer itself but also those belonging to the measuring circuit. 
3.3. Computer Modelling of Current Transformers 
The CT can be modelled as a series impedance including a capacitance (Cts), an inductance (Ls) and a 
resistance (Rs). At this point, the values of these three components are unknown. 
Added to this, since CTs are over the ground level by means of a support, it is also necessary to 
include the inductance representing the influence of this support (Lbase). Its magnitude can be 
estimated theoretically from the geometry. 
As regards the magnitude of the components belonging to the measuring circuit, since the signal 
connection used during the measurements is of known geometry, it is also possible to estimate their 
value theoretically. 
Then, the complete circuit was drawn in the simulation program. The values of Cts, Ls and Rs were 
determined by trial and error, i.e. they were adjusted many times until the simulated responses 
matched the measured ones. As a result, it was achieved that Cts=1 nF, Ls=4,4 H and Rs=6,5 . 
In Fig. 7 the responses obtained by simulations are depicted (voltage in red and current in blue). Thus, 
it has been proved that the model achieved for the CT and the measuring circuit works successfully. 
 
Figure 7. Responses achieved by simulation, tuning the values of the components. 
  7 
 
3.4. Currents flowing through the CT 
Once the complete model was properly adjusted, it was possible to run simulations so as to obtain both 
shape and magnitude of the current flowing through the Current Transformer to ground. 
Therefore, the circuit was analyzed by applying a step voltage generator representative of the recorded 
transients, especially in terms of the measured front times. As from the simulations that are described 
as follows, the current transient produced by a voltage step is quickly attenuated (duration shorter than 
10 μs), it is valid to analyze the successive steps as if they were independent events since they appear 
with a difference of 2 ms to 20 ms between them. 
Provided that each of the voltage steps corresponding to the real complete transient has different 
amplitudes, the simulations were carried out with reference amplitude of 100 kV. The reason of that 
was to enable later the conversion of the obtained results to proportional current values, according to 
the step amplitude of real recorded voltages. 
The result of one of the simulations carried out appears in Fig. 8. As expected, the current resulted in a 
damped oscillation, whose oscillation frequency is 1.9 MHz. It is worth remarking that its amplitude is 
reduced to less than 10 % of its maximum value at 6 μs after the transient begins. In addition, the 
current peak reached a value of 280 A for each 100 kV of the step voltage amplitude. 
           Time
0s 1us 2us 3us 4us 5us 6us 7us 8us 9us 10us
I(Cts1)
-300A
-200A
-100A
0A
100A
200A
 
Figure 8. Current through CT to ground. 
By using the conversion factor of 2.80 A/kV and the actual magnitudes of the different voltage steps 
recorded it was possible to determine the maximum and the average values of the peak currents that 
would be expected. These are 1206 A and 1060 A, respectively. 
It is pointed out that such impulsive currents are applied to the real transformer as a train of impulses 
during each operation of the air disconnect switch. 
4. CURRENT TRANSFORMER DISASSEMBLY 
With the aim of detecting some damage in the insulation of the analyzed Current Transformers due to 
the solicitation above described, one of the machines taken out of service was completely 
disassembled. 
The primary insulation of this type of Current Transformers basically consists of multiple layers of 
paper, which is impregnated with oil. They also include a number of equipotential layers in order to 
accomplish an adequate potential distribution inside the insulation. 
The methodology employed to disassemble the CT was, as much as possible, just the opposite to that 
carried out during the manufacture of it. 
  8 
 
Therefore, first the oil was completely removed. Then, the expansion bellows were also taken out and 
no anomaly was detected. The next step was to remove all the elements related to the primary of the 
CT. All the components were found in good state, as well. 
Finally, it was the turn of the active part of the insulation. The procedure began by the high-voltage 
electrode, then the first equipotential layer (S1) and the three inner layers, reaching finally the last one 
(S5) corresponding to the earth electrode. 
As soon as the disassembly of this part began it was possible to observe some damage in S1 layer. The 
metallic had been noticeably affected by internal discharges. Once S1 was removed, some severe 
damage was also observed in the oil-impregnated paper located between S1 and S2. 
In Fig. 9 photographs of the metallic layer and the paper are presented. Both pictures show clearly the 
damage caused by the electrical stress to which the isolation was subjected to. 
Naturally, the level of the stress was high enough to “carbonize” the paper. Normally, discharges 
occur at the edge of the layers, but they can propagate rapidly in oil-impregnated paper. 
After the destructive effects of the discharges were observed, the disassembly continued according to 
the originally planned schedule. As a result, in the S2 layer as well as in the paper situated between S2 
and S3 some damage was also detected, but not in the same level as in the previous case. Before 
reaching the layer S3 the carbonization of the paper disappeared completely and no important anomaly 
was found until the end of the work. 
Obviously, this damage in the insulation of the CT led to the production of the gases detected in the 
chromatographic determinations performed at the beginning of the study. 
Figure 9. Metallic layer and paper affected by internal discharges. 
This “puncture” on the insulation of the CT is thought to have been caused by the high-frequency 
transients that were dealt with in the previous sections. Discharges are presumably initiated at the edge 
of a layer with the first operations, but in the long term they can result in considerable propagation, 
extending through several layers of paper. If these anomalies are not detected at the correct time, they 
could lead the Current Transformer to complete breakdown. 
In the schematic drawing of Fig. 10 it is represented the area where the main damage of the insulation 
was located. 
 
  9 
 
 
Figure 10. Schema showing the area affected by the perforation. 
5. FINAL COMMENTS AND CONCLUSIONS 
 By both field measurements and simulations it was possible to determine the presence of high-
frequency perturbations caused by the normal operations of air disconnect switches. 
 The amount of gases detected in chromatographic determinations indicated some possible 
damage in the insulation. 
 The suspected damage was confirmed once one of the CTs was disassembly. It proves that 
analyses of gases are a reliable indicator of the insulation state. 
 It is important to remark that the knowledge acquired from this research will be useful in the 
design of new Current Transformers under real electrical stresses. 
 Personnel of Transmission Companies also should address the issue dealt with in this paper so 
as to prevent high-voltage equipment from being damaged. 
 This work raises the question whether these sorts of electrical stresses are included in the type-
tests described in the Standards concerning high-voltage testing. And, if so, are they properly 
represented? 
BIBLIOGRAPHY 
[1] IEC 60044-1. Instrument transformers - Part 1: Current transformers, 2003. 
[2] IEC 62271-102. High-voltage switchgear and controlgear - Part 102: Alternating current 
disconnectors and earthing switches, 2003. 
[3] “Application of Fibre Optics to the Measurements of Corona Losses in UHV Experimental 
Lines”. Third Internacional Symposium on High Voltaje Engineering. Milan, August 1979. L. 
Thione, G. Costa, E. Elli (CESI – Milano, ITALY). A Manzini, A. Centemeri (ENEL CREL 
Milano, ITALY). 
[4] L. L. Alston. “Alston High-Voltage Technology”. Oxford University Press, 1968. 
[5] M. D. del Pozo, D. Esteban, P. Issouribehere, A. Funes, A. Ledesma. “Mediciones de 
transitorios durante las maniobras de seccionadores en Estaciones Transformadoras de 500 kV”. 
XIII ERIAC. Puerto Iguazú, Argentina, Mayo de 2009. 
Cores 
Detected 
Perforation 
Equipotential 
Layers (S1 to S5) 
Layers 
of paper 
Metal Box 
(ground) 


Field measurements and modelling of high frequency transients during disconnect switch operations in EHV Substations : Assessment of their effects on Current Transformers

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Field measurements and modelling of high frequency transients during disconnect switch operations in EHV Substations : Assessment of their effects on Current Transformers

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