Components & Periphery
Frequency Response of
Instrument Transformers in the kHz range
Kerstin Kunde, Holger Däumling, Ralf Huth, Hans-Werner Schlierf, Joachim Schmid
As the number of non-sinusoidal sources and loads in the network
continues to grow, the importance of information concerning the quality
of the current and the voltage becomes ever more important. Each
deviation in frequency can be regarded as harmonic contamination of
the network that can cause issues for the network operator and the
users. It is questionable, however, whether the instrument transformers
so far used for measuring the Power Quality can cope with the
accuracy requirements for the analysis of network quality.
Instrument transformers of various
different construction and from various
different manufacturers vary greatly in
their frequency response behaviour.
The causes of this are varied and extend from manufacturing tolerances in a
series up to various different operating
conditions. For the frequency response
behaviour, in addition to the structure,
Öl VT
G3
%
Transmission error
Because of the harmonic contamination in the network, users require that
the network operators demonstrate
and ensure the quality of the prepared
energy supply is in accordance with
DIN EN 61000-4-30 (VDE 0847-4-30) [1]
and DIN EN 50160 [2]. From the point of
view of the network operator it is important to determine the cause of the
harmonic contamination of the network,
in order to be able to carry out counteracting measures. The execution of the
required measurement of current and
voltage should, where possible, be carried out using the existing measuring
systems from the inductive or capacitive instrument transformers.
Measuring principles and
influence for harmonic
vibration measurement
SF6 Kombi
G3
GIS VT
G3
GIS VT
36 kV
MS VT
36 kV
“
“
GIS VT
G3
1 Frequency
MS VT
12 kV
1 Hz
Fig. 1. Transmission errors with various different inductive instrument transformer types
Dr. Kerstin Kunde is responsible for product life-cycle management
in the Business Segment Instrument Transformers at Siemens AG in
Erlangen.
Email: kerstin.kunde@siemens.com
Dr.-Ing. Holger Däumling is Managing Director of
Ritz Instrument Transformers GmbH in Hamburg.
Email: info@ritz-international.com
Dipl.-Ing. Ralf Huth is Asset Manager for Substations at Tennet TSO
GmbH in Bayreuth.
Email: ralf-huth@tennet.eu
Dipl.- Ing. Hans-Werner Schlierf was, until recently, Manager of the
Service Teams Primary Technology at Amprion GmbH in
Lampertheim.
Email: info@amprion.net
Dr.-Ing. Joachim Schmid has global responsibility for R&D in the
Instrument Transformer Sector at
Siemens Schweiz AG in Zürich.
Email: joachim.schmid@siemens.com
1
Because of design restrictions,
instrument transformers are used for
transmission as close as possible to
the illustration of the primary technology measuring parameters, such as
current or voltage. This requirement of
the response behaviour applies to the
range of measurement frequency and is
ensured by the instrument transformer
manufacturers. In order to be able to
assess the network quality and compliance with existing standards as well as
at the fundamental frequency we need
to accurately measure harmonics up to
50 times the rated frequency in terms of
both magnitude and phase angle.
the most significant responsible factors are the voltage level and the connected load impedance. Measurements
have shown that the resonance points
move to lower frequencies the higher
the voltage level. With the connected
load impedances (protection, measuring system), the frequency behaviour
is generally unknown. However, it has
a considerable influence on frequency
response.
Inductive voltage transformers convert the high voltage to a low voltage
signal using the transformer principle.
In this case the secondary voltage behaves in the linear area of the “opened”
transformer in a reciprocal manner to
the response ratio.
Heft 6/2012 ●
Components & Periphery
500
`
400
`
300
`
200
`
100
“`
0
0
500
1 000
1 500
Frequency
2 000
Hz
Phase
Rel. Amplitude
%
“`
2 500
Fig. 2. Amplitude and phase errors of an inductive voltage transformer at various different
frequencies
900
kV
System voltage
700
600
Depending on the selected insulation
(oil-paper, gas or cast resin) various
different geometric structures of the
primary and secondary windings are
produced, and these lead to various different response behaviours depending
on the concentrated parameters capacitance, inductance and resistance.
On the other hand, capacitive voltage transformers produce the secondary voltage as a transmission ratio
between the primary and secondary
capacitance. They consist of a stack
of condensers wired in series and an
inductive unit required for the power
provision on the low voltage side. Capacitive voltage transformers are primarily structured with a mixed dielectric in
the condenser stack (C1/C2) in order to
achieve the class accuracy even in various different temperature ranges.
Conversion with inductive current
transformers is carried out using the
transformer principle, as is the case
with the voltage transformers. However, the windings are almost shortcircuited.
500
400
300
200
100
0
0
1 000
2 000
3 000
Frequency
4 000
5 000
Hz
6 000
Fig. 3. Frequency of the first resonant point for various different voltage levels
250
`
`
150
“`
100
“`
50
“`
0
0
100
200
300
400 500 600
Frequency ƒ
700
800
900
Phase
Rel. Amplitude
%
200
“`
Hz 1 100
Fig. 4. Amplitude and phase errors of a capacitive voltage transformer
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Components & Periphery
On inductive current transformers
the transmission property is less severely determined by the capacitive
layer influences, so that a higher linear
transmission frequency spectrum is to
be expected. This process is supported
by light loading. The size of the errors
that can arise in the various different
converter types in the frequency range
up to 50 kHz is shown in Fig. 1. For example, Fig. 2 shows the error in an oilpaper insulated, inductive 420 kV voltage converter for the frequency range
up to 2.5 kHz according to magnitude
and phase.
For the same types of design and insulation principles we can determine
that the occurrence of the first resonance point falls depending on the voltage level. That is shown in Fig. 3 using
various different voltage transformers.
Capacitive voltage transformers are
set to the nominal frequency of 50 Hz or
60 Hz. The accuracy is guaranteed only
Frequency response behaviour
of instrument transformers
The frequency response of inductive
and capacitive voltage transformers is
determined by the geometrical structure of each individual product. For this
reason, there may be differences in the
resonant frequencies between oil, gas
and cast resin insulated voltage transformers. The influencing factors are:
• The winding resistance in the high
voltage winding,
• The leakage inductance in the high
voltage winding,
• The leakage inductance between the
high and low voltage windings,
• The layer capacity,
• Capacitance between the high
voltage coil and the earth-side end of
the insulation,
• The inductance of the iron core
• Resistive losses in the iron core.
for a narrow band (Fig. 4). The lowest
resonant frequency is a few hundred Hz.
The frequency range to which the various different technologies are suited is
clearly shown in Fig. 5. The class error
is taken into account here.
Inductive current transformers transmit the signals over several kHz without
major errors. Fig. 6 shows the measurements at various different frequencies.
The error can be ignored up to 5 kHz.
However, the existing measuring equipment does not allow differentiation between amplitude and phase errors.
Also, investigations were undertaken
to show the difference between primary
and secondary signals with frequencies
overlaid in different ways. An amplitude
or phase measurement is not undertaken in this investigation. The result of
this analysis also confirms the suitability of inductive current transformers for
the measurement of higher frequency
harmonics.
optical VT/RCVT
C-divider
R-divider
10 MHz
1 MHz
LV
100 kHz
MV
10 kHz
15 Hz
HV
1 kHz
capacitive
voltage
transformers
inductive
voltage
transformers
50 Hz
100 Hz
electronic
voltage
transformers
Frequency
Fig. 5. Frequency response behaviour of various different voltage transformer technologies
in accordance with IEC/TR 61869-103 [3]
%
0
500 Hz
Error
1 000 Hz
3 000 Hz
“
1 800 Hz
5 000 Hz
“
0
10
20
30
40
50
60 70
Current
80
90
200 Hz
45 Hz
100 110 120
A
140
Fig. 6. Amplitude errors in an inductive current transformer at various different frequencies
3
Heft 6/2012 ●
Components & Periphery
Protection requirements
Result
Literature
Because the majority of protection
principles are based on fundamental
frequency values, protection systems
evaluate these signals in the permissible frequency working range. Digital protection devices are capable, in
this case, of precisely filtering out unwanted frequency components.
Although harmonics play a subordinate role in the protection, there are
also protection principles that use
them. The current differential protection assesses the 2nd to 5th harmonics for stabilization purposes in the
current. This is not critical because
current transformers transmit these
frequencies without any problems.
Earth short wiper principles are based
on the evaluation of higher frequency
current and voltage signals (< 5 kHz) in
the first periods after the occurrence
of the error. The principle is used exclusively in the distribution network, in
other words, at medium voltage. At this
voltage level, the frequency response
behaviour of the voltage transformers
is considerably better. Principles for
protection of capacitor banks are also
based on the assessment of higher frequency signals. The protection criteria
that are used use current measuring
principles.
Both inductive and capacitive voltage transformers, at the current state
of technology, are not suited for the
measurement of harmonics without
additional measures, because of the
occurrence of resonant frequencies,
particularly at high voltage. Both types
of transformers are dimensioned for
the measurement and protection at
nominal frequencies. Resonances between the winding inductance and the
stray capacitance (between the layers)
can cause large amplitude and phase
errors.
If the measuring task is the measurement of higher frequency harmonics,
then we need to use RC dividers for
high voltage and C- or R-dividers for
medium voltage. These are suitable
both for the measurement of harmonics and for the measurement of DC
voltages (albeit with negligible power
output). Inductive current transformers
transmit harmonics up to several kHz
in correct phase and with negligible
errors. If you need more detailed information concerning the resonant
frequencies of a conventional measuring converter this should initially be
requested from the instrument transformer manufacturers.
[1] DIN EN 61000-4-30 (VDE 0847-4-30):
2009-09 Electromagnetic Compatibility (EMC) – Part 4-30: Test and
measuring procedure - procedure
for measuring the voltage quality.
VDE VERLAG
[2] DIN EN 50160:2011-02 Voltage
characteristics in public electricity
supply networks. Berlin: Beuth
[3] IEC/TR 61869-103:2012-05 Instrument transformers - The use of
instrument transformers for power
quality measurement. Geneva/
Switzerland: Bureau Central de la
Comission Electrotechnique Internationale
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