Frequency Response of Instrument Transformers in the kHz Range

Published by Dr. Kerstin Kunde, Dr.-Ing. Holger Däumling, Dipl.-Ing. Ralf Huth, Dipl.- Ing. Hans-Werner Schlierf and Dr.-Ing. 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.

Measuring principles and influence for harmonic vibration measurement

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, 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.

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.

Fig 1. Transmission errors with various different inductive instrument transformer types

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.

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 short-circuited.

Fig 2. Amplitude and phase errors of an inductive voltage transformer at various different frequencies

Fig 3. Frequency of the first resonant point for various different voltage levels

Fig 4. Amplitude and phase errors of a capacitive voltage transformer

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

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 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.

Fig. 5 Frequency response behaviour of various different voltage transformer technologies
in accordance with IEC/TR 61869-103 [3]

Fig. 6 Amplitude errors in an inductive current transformer at various different frequencies

Protection requirements

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.

Result

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.

Literature

[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

Publishers Information

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

Published by PQTBlog

Electrical Engineer

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