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ARWtr2013
Advanced Research Workshop on Transformers.
28 -30 October 2013. Baiona– Spain
Behaviour of Inductive Voltage Transformers
against Very Fast Transient Overvoltages
Adolfo IBERO, Javier AGUIRRE
High Voltage Instruments Transformers Department, ARTECHE
Derio Bidea 28, 48100 Mungia, Spain
Phone: (+34) 946011200, fax: (+34) 946740958, e-mail: [email protected], [email protected]
Abstract — One of the well-known phenomena existing in GIS substations is the influence of over voltages of a very
high frequency. Those surges, called Very Fast Transient over-voltages (VFTO), are due to the operation of the SF6
circuit breakers and appear specially when there are pre-strikes or re-strikes during the switching.
In the last years, it is becoming common to hear about such switching failures also in high voltage AIS type
substations, because the older circuit breakers are being replaced with modern ones which don’t have grading
capacitors and the transient surges are faster .
This paper discusses about the influence of those very fast surges on the instrument transformers insulation and
their transient behaviour.
Keywords — Instrument Potential transformer, VFTO, Transient response.
I.
INTRODUCTION
In the literature that has been investigated, this subject is treated as the behavior of a primary winding of a
power transformer against high voltage pulses with a rise time of a few nanoseconds, approximately 50
nanoseconds.
Those kind of pulses occur in cases of a standard closing of an SF6 circuit breaker (pre-striking), also in the
case of sudden re-closings or when opened capacitive loads (re-striking) and in any of the two before cases
where there are trapped charges on the line which is going to be connected or disconnected (eg. No load line,
capacitor bank, ...).
What is well known is the behavior of the power transformer and their problems when these voltage’s "steps"
are applied.
All these faults in power transformers are due to not expected voltage distributions inside of the primary
windings, and subjected to an excessive stress in some areas. We're talking about failures because of voltage
poorly distributed across the winding’s layers.
In our case, we will analyze the instrument transformer’s behavior and we will see that the effect is not the
same, and that the origin of the over voltages is the current generated by this very fast voltage surge.
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ARWtr2013
28 -30 October 2013. Baiona– Spain
Advanced Research Workshop on Transformers.
II. SIMPLIFIED EQUIVALENT CIRCUIT UNDER STUDY
At first, we begin by describing the high-frequency equivalent circuit corresponding to a conventional power
transformer.
This phenomenon has been widely studied, and for that reason, we will summarize only the most important
points to understand the failure mode.
The high frequency pulse, which is applied to the transformer primary winding, flows through the stray selfcapacitances of the primary turns. These primary turns can be distributed in different ways, but in short, circuit
under study is presented as follows.
In this circuit, it is shown the capacitance CS between the high voltage primary coils o primary sections, and
shunt capacitance Cg to ground and magnetic core. (See Fig.1).
H.V.
Cs
Cs
Cs
Cg
Cg
Cg
Cs
Cg
Cs
Cg
Cg
Cg
Cg
Fig. 1
Voltage distribution which appears in the transformer primary winding voltage, corresponds to the capacitor
network shown on Figure 1.
The voltage of each primary section in an array of this type, is:
𝐸=𝑉∙
𝑠𝑒𝑛ℎ(𝛼∙𝑋⁄𝑙 )
𝑠𝑒𝑛ℎ 𝛼
(1)
Where:
E = Voltage of each section to earth.
V = Full applied voltage from high to low voltage side.
l = Full length of the divider net.
X = Calculation point against earth.
𝐶𝑔
𝛼 = √ ⁄𝐶𝑠
(2)
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ARWtr2013
28 -30 October 2013. Baiona– Spain
Advanced Research Workshop on Transformers.
In fig. 2 we can see the voltage distribution against earth of a primary winding with a "neutral ground" in which
can be seen the enormous impact the  value on stress distribution.
𝐶𝑔
(𝛼 = √ ⁄𝐶𝑠)
When Cg << Cs the voltage distribution is linear and Cg capacity does not affect at all on the stress distribution
and hence neither at the voltage gradient at which coil is subjected to.
Porcentaje voltaje to ground
When the Cg capacitance values are similar to those of CS values, then the voltage distribution begins to be a
non-linear distribution which causes a short length of coil may have to withstand more than half of the applied
voltage, causing a dielectric failure.
=0
=5
=1
 = 10
x/l
Fig. 2. Initial distribution down a uniform winding in response to a step function voltage surge.
For the design of ARTECHE instrument voltage transformers, primary winding insulations are designed with a
layers distribution known as "no resonance coil".
This design makes that layers are distributed so the capacitance between layers "CS" is always the same and
also the capacitance with respect to ground "Cg" of each primary winding layer is proportionately much smaller
than that capacity series.
Being Cg capacitance much smaller than the Cs capacitance, then the divider will not be affected by the Cg
capacitance of each layer with respect to earth and the distribution of stresses between layers will be almost
linear.
Maintaining linear stress distribution, dielectric work is balanced and is capable of perform well at both low
frequencies and high frequencies.
As seen, the  coefficient of those instrument transformers is practically 0 and therefore as you can see in
Figure 2, the distribution is linear and we'll forget the distorting effect of the "stray" capacitance Cg studied in
the previous voltage divider.
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ARWtr2013
28 -30 October 2013. Baiona– Spain
Advanced Research Workshop on Transformers.
On the other hand, the voltage applied between high and low voltage areas changes rapidly. This means that the
current flow through the divider entering as conduction current, passing across the dielectric as displacement
current and go out again as conduction current.
If this voltage distribution in instrument transformers is unlike the case of power transformers, and also is
properly distributed, then we cannot talk about over voltages inside the primary windings due to incorrect
distribution of internal stresses.
However, a different phenomenon appears that can generate dangerous over voltages if care is not taken into
account to improve the behavior. These over voltages are due to current flow through the screens and internal
wiring of equipment that can induce over voltages and electrical stresses between turns by coupling at high
frequency.
The occurrence of high frequency and high current surges induce over voltages inside of the primary winding
that slowly damage primary insulation.
III. APPLICATION OF THE VOLTAGE PULSE
We will show the waveforms of voltage and current waves in a raw capacitance subjected to a step impulse
applied to an inductive voltage transformer with a very short line between the instrument transformer and the
circuit breaker.
In this case, the circuit can be assumed as having lumped constant and the voltage surge was a theoretical step
impulse.
We are talking about distances of 10, 20 or 30 meters from the transformer to the circuit breaker.
The diagram under study is shown in Figure 3:
L
T1
C
Z
Fig. 3
The Equivalent circuit to calculate will be as follows. (See figure 4):
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ARWtr2013
28 -30 October 2013. Baiona– Spain
Advanced Research Workshop on Transformers.
iT (s)
LS
iC (s)
1 / CS
Z
VC (s)
Fig. 4.
The current through the instrument transformer is:
𝑉
𝑖𝑐(𝑡) = 𝐿 ·
1
𝑚
· 𝑒 − 2 ·𝑡 · 𝑠𝑒𝑛 (√𝑛 −
2
√𝑛−𝑚
𝑚2
4
· 𝑡)
(3)
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Calculating values for the following cases,
Using 30 meters length,
L = 30 µH y
C = 200 pF ( Primary winding capacitance)
Z = Z Line = 288 Ω.
We get the following result:
îc (t) = 426 Â
Oscillating frequency = 4 MHz.
Using 30 meters length again,
L = 30 µH
C = 4000 pF ( Transformer primary capacitance plus an additional capacitance of 3800 pF in parallel)
Z = Z Linea = 288 Ω.
We get the following result:
îc = 3089 Â, but ic across transformer now is only 190,5 Â
Oscillating frequency is = 0,445 MHz.
Therefore,
When a capacitance is added in parallel with the instrument transformer, the current flowing through the
transformer is reduced by half. ( 190 Â Vs 426 Â ). But mainly, the frequency is reduced by 10. From a
frequency of 4 MHz to a final frequency of 0,44 MHz.
Then the induced voltage and the damaging effect of this current are approximately 20 times lower.
Should be mentioned that these capacitors added must function as a “capacitor”, with an operating frequency of
at least 500 kHz.
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ARWtr2013
Advanced Research Workshop on Transformers.
IV. FLOW
28 -30 October 2013. Baiona– Spain
OF CURRENT AND THEIR OVER-VOLTAGES BY HIGH FREQUENCY COUPLING BETWEEN
TURNS
The voltage impulse generates in the capacitance of the instrument transformer winding, a current impulse
flowing through the internal elements of the transformer.
The high frequency current enters through the High Voltage screen as conduction current. Hence becomes
displacement current and goes through the high voltage winding insulation as if it were a capacitance. This
current is going to be collected in the low voltage screen again to become conduction current and go outside of
the low voltage primary winding output. This output is connected to the transformer ground.
High voltage and low voltage screens are metallic and don’t present any resistivity problem nor heating
problem due to these short pulses of current.
The current entering trough the high voltage screen is distributed across the length of the screen depending on
the capacitance that this screen see to the winding.
Impulse current goes out from the end of the primary winding and to avoid any over voltage on the turns, this
current is collected in a low voltage screen. Following, is shown the high frequency current path flowing across
the primary winding of these instrument transformers.
Figure 5 shows how the current flow with several paths. As this current has also high frequency, the current
tends to go through the edges of the screen and at some areas, this current circulates in parallel with the primary
windings turns generating induced over voltages and burning inter-turn enamel insulation if these currents and
over voltages are high enough and of high frequency.
High voltaje electrode
Current ic (t)
Low voltaje electrode
Current ic (t)
Output current ic (t)
Fig. 5. High frequency diagram of a primary winding inductive voltage instrument transformer
V. IMPACT IN THE INSULATION AND FIELD EXPERIENCE
The failure mode described herein is not necessarily instantaneous.
These voltage impulse surges between turns progressively deteriorate the quality of the wire insulation.
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ARWtr2013
Advanced Research Workshop on Transformers.
28 -30 October 2013. Baiona– Spain
In service, the voltage applied at the instrument transformer is reduced from that which withstand the main
insulation, and this causes the voltage between turns is very low (only about a few volts per turn) and no failure
occurs instantaneously.
The failure is evident when the enamel of the cooper wire, which gradually deteriorates with very high
frequency impulses, is completely carbonized and also the surrounding insulation paper is damaged. At this
point, insulation paper between adjacent layers does not support the service voltage to which they are subjected,
between layers which is of the order of kilovolts.
It can even be with half of the primary winding carbonized and still been in service without giving electrical
failure symptoms due to the reduced service voltages between turns. Now when the damaged area covers a big
large area and the applied voltage to that area is of several kilovolts, a complete dielectric breakdown occurs
Phase-Earth.
What it is detected when this phenomenon appears are gases (acetylene, (a little), ethylene, methane, ..)
VI. PROBLEM SOLUTION
This research has been carried out over several years, using the experience in the field and the standard
equipment installed in the network.
It was found, in collaboration with the customer, that no instrument transformer connected in the output of the
line or substation, with coupling capacitor near of the transformer failed in the numerous openings and closings
of circuit breakers.
However it was observed in some substations, that those instruments transformers which were installed even in
the same phase, with no parallel coupling capacitor had serious problems for this reason and the customer had
to remove them from service.
Later also some information concerning scrapping of equipment were collected to confirm the exact location of
faults, confirming the assumptions that had been raised previously.
Once we located the source of the problem, in the special use or operation of SF6 circuit breakers and its
influence on the equipment installed in the surrounding area, we have shown that capacitors placement in
parallel to the inductive voltage transformers effectively reduces this type of over voltage.
Another alternative that is valid is to reduce the speed of rise of SF6 shockwaves installing grading capacitors
in circuit breakers.
As discussed previously, in recent years is when it has begun to be used widely in higher insulations voltages
the circuit breakers without these grading capacitors and with SF6 insulated gas. With those circuit breakers,
the rise time of the voltage impulse is much faster what is required by standard current regulations applicable
to the instrument transformers, and over other previously used oil circuit breakers.
The High Voltage and Low Voltage screens of the transformers are necessary if you want to distribute the
voltage stress correctly. Also they serve to collect effectively the displacement current that comes from the high
voltage electrode and lead safely that current to the transformer’s earth terminal.
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ARWtr2013
Advanced Research Workshop on Transformers.
28 -30 October 2013. Baiona– Spain
This phenomenon produced by very high frequency currents that generate over voltage surges inside the
transformer insulation, comes imposed by the external circuit that are connected to the transformer.
For that reason, to avoid or mitigate the effects of these high electrical stresses induced by these currents on the
HV and LV screens, it must act on the substation circuit design and on the external elements to the instrument
transformer with the solutions mentioned above.
REFERENCES
[1]
IEC 60044-2, Instruments Transformers – IEC standard Part 2: Inductive voltage transformers.
[2]
IEEE C57.13-2008, IEEE Standard Requirements for Instrument Transformers.
[3]
Lou Van der Sluis, Transients in Power Systems, John Wiley & Sons LDT, pp. 31-134, 2001.
[4]
Greenwood, A., Electrical transients in Power Systems, John Wiley & Sons, pp. 1-91, April 1991.
[5]
ARTECHE know-how from experience and other internal technical documents.
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