Traceable measurement of transmitted overvoltages in instrument transformers

Alf-Peter Elg (Corresponding Author), Fernando Garnacho, Tomás Garcia, Jorge Rovira, Jari Hällström, Tatu Nieminen

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Abstract

Transmitted over-voltages on instrument transformers are becoming a concern for the grid operators due to the risk of damaging the instruments and control systems connected to the low voltage side. Present methods allow for low voltage tests; however, traceability is lacking for these waveshapes and assessments of measurement uncertainties.

The test referred in the clause 7.4.4 of IEC Standard 61869-1 is used to check the overvoltage that appears on the secondary terminals of instruments transformer when a specified overvoltage is applied to the primary terminals.

The general test procedure is indicated in IEC 61869-1, but no specific requirements are given, such as the test voltage level to be applied, or how the peak value should be measured, or which are the requirements to be satisfied by the measuring instruments.

These specific requirements can significantly influence the test result.

The aim of this paper is to present a method for traceable measurement of transmitted over-voltages in voltage instrument transformers. The present IEC 61869-1 standard defines the limits of permissible transmitted over-voltages by using low voltage impulses.

However, the Standard does not state the measurement uncertainties for the measurement.

The testing setup for the determination of transmitted over-voltages consists of a high voltage impulse generator generating a wave shape of 0.5/50 μs up to 250 kV. A reference measuring system simultaneously records the applied wave shape and the transmitted overvoltage. A wideband attenuator is connected on the secondary terminals of the tested transformer.

We present a traceability chain for 0.5/50 μs impulse shape. Measurements results are presented for four different types of instrument transformers commonly found in high voltage grids; a 145 kV inductive voltage transformer, a 400 kV and a 132 kV capacitive inductive voltage transformer (CVT) and a 145 kV capacitive voltage divider.

Measurements show that LV tests may both overestimate and underestimate the transmitted overvoltages in voltage transformers. In one case the difference was as large as a factor of two compared to measurements at full voltage. No requirements on neither the method nor the measurement system for the LV test, e.g. no requirements on the transient recorder, combined with voltage non-linearity of the phenomenon and undefined burden in the voltage transformer lead to unacceptable large errors and measurement uncertainties.

Generation of impulses with 0.5 μs front time is difficult without excessive overshoot.

Conditions in the power grid have long since been standardised to LI withstand tests with a shortest wave front of 0.84/50 μs. Our recommendation is to use a standard LI wave shape of 0.84/50 μs and do full voltage tests with the same well specified requirements.
Original languageEnglish
Pages (from-to)58-63
Number of pages6
JournalCIGRE Science and Engineering
Issue number16
Publication statusPublished - Dec 2019
MoE publication typeA1 Journal article-refereed

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Electric instrument transformers
Electric potential
Voltage dividers

Cite this

Elg, A-P., Garnacho, F., Garcia, T., Rovira, J., Hällström, J., & Nieminen, T. (2019). Traceable measurement of transmitted overvoltages in instrument transformers. CIGRE Science and Engineering, (16), 58-63.
Elg, Alf-Peter ; Garnacho, Fernando ; Garcia, Tomás ; Rovira, Jorge ; Hällström, Jari ; Nieminen, Tatu. / Traceable measurement of transmitted overvoltages in instrument transformers. In: CIGRE Science and Engineering. 2019 ; No. 16. pp. 58-63.
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Elg, A-P, Garnacho, F, Garcia, T, Rovira, J, Hällström, J & Nieminen, T 2019, 'Traceable measurement of transmitted overvoltages in instrument transformers', CIGRE Science and Engineering, no. 16, pp. 58-63.

Traceable measurement of transmitted overvoltages in instrument transformers. / Elg, Alf-Peter (Corresponding Author); Garnacho, Fernando; Garcia, Tomás; Rovira, Jorge; Hällström, Jari; Nieminen, Tatu.

In: CIGRE Science and Engineering, No. 16, 12.2019, p. 58-63.

Research output: Contribution to journalArticleScientificpeer-review

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T1 - Traceable measurement of transmitted overvoltages in instrument transformers

AU - Elg, Alf-Peter

AU - Garnacho, Fernando

AU - Garcia, Tomás

AU - Rovira, Jorge

AU - Hällström, Jari

AU - Nieminen, Tatu

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N2 - Transmitted over-voltages on instrument transformers are becoming a concern for the grid operators due to the risk of damaging the instruments and control systems connected to the low voltage side. Present methods allow for low voltage tests; however, traceability is lacking for these waveshapes and assessments of measurement uncertainties.The test referred in the clause 7.4.4 of IEC Standard 61869-1 is used to check the overvoltage that appears on the secondary terminals of instruments transformer when a specified overvoltage is applied to the primary terminals.The general test procedure is indicated in IEC 61869-1, but no specific requirements are given, such as the test voltage level to be applied, or how the peak value should be measured, or which are the requirements to be satisfied by the measuring instruments.These specific requirements can significantly influence the test result.The aim of this paper is to present a method for traceable measurement of transmitted over-voltages in voltage instrument transformers. The present IEC 61869-1 standard defines the limits of permissible transmitted over-voltages by using low voltage impulses.However, the Standard does not state the measurement uncertainties for the measurement.The testing setup for the determination of transmitted over-voltages consists of a high voltage impulse generator generating a wave shape of 0.5/50 μs up to 250 kV. A reference measuring system simultaneously records the applied wave shape and the transmitted overvoltage. A wideband attenuator is connected on the secondary terminals of the tested transformer.We present a traceability chain for 0.5/50 μs impulse shape. Measurements results are presented for four different types of instrument transformers commonly found in high voltage grids; a 145 kV inductive voltage transformer, a 400 kV and a 132 kV capacitive inductive voltage transformer (CVT) and a 145 kV capacitive voltage divider.Measurements show that LV tests may both overestimate and underestimate the transmitted overvoltages in voltage transformers. In one case the difference was as large as a factor of two compared to measurements at full voltage. No requirements on neither the method nor the measurement system for the LV test, e.g. no requirements on the transient recorder, combined with voltage non-linearity of the phenomenon and undefined burden in the voltage transformer lead to unacceptable large errors and measurement uncertainties.Generation of impulses with 0.5 μs front time is difficult without excessive overshoot.Conditions in the power grid have long since been standardised to LI withstand tests with a shortest wave front of 0.84/50 μs. Our recommendation is to use a standard LI wave shape of 0.84/50 μs and do full voltage tests with the same well specified requirements.

AB - Transmitted over-voltages on instrument transformers are becoming a concern for the grid operators due to the risk of damaging the instruments and control systems connected to the low voltage side. Present methods allow for low voltage tests; however, traceability is lacking for these waveshapes and assessments of measurement uncertainties.The test referred in the clause 7.4.4 of IEC Standard 61869-1 is used to check the overvoltage that appears on the secondary terminals of instruments transformer when a specified overvoltage is applied to the primary terminals.The general test procedure is indicated in IEC 61869-1, but no specific requirements are given, such as the test voltage level to be applied, or how the peak value should be measured, or which are the requirements to be satisfied by the measuring instruments.These specific requirements can significantly influence the test result.The aim of this paper is to present a method for traceable measurement of transmitted over-voltages in voltage instrument transformers. The present IEC 61869-1 standard defines the limits of permissible transmitted over-voltages by using low voltage impulses.However, the Standard does not state the measurement uncertainties for the measurement.The testing setup for the determination of transmitted over-voltages consists of a high voltage impulse generator generating a wave shape of 0.5/50 μs up to 250 kV. A reference measuring system simultaneously records the applied wave shape and the transmitted overvoltage. A wideband attenuator is connected on the secondary terminals of the tested transformer.We present a traceability chain for 0.5/50 μs impulse shape. Measurements results are presented for four different types of instrument transformers commonly found in high voltage grids; a 145 kV inductive voltage transformer, a 400 kV and a 132 kV capacitive inductive voltage transformer (CVT) and a 145 kV capacitive voltage divider.Measurements show that LV tests may both overestimate and underestimate the transmitted overvoltages in voltage transformers. In one case the difference was as large as a factor of two compared to measurements at full voltage. No requirements on neither the method nor the measurement system for the LV test, e.g. no requirements on the transient recorder, combined with voltage non-linearity of the phenomenon and undefined burden in the voltage transformer lead to unacceptable large errors and measurement uncertainties.Generation of impulses with 0.5 μs front time is difficult without excessive overshoot.Conditions in the power grid have long since been standardised to LI withstand tests with a shortest wave front of 0.84/50 μs. Our recommendation is to use a standard LI wave shape of 0.84/50 μs and do full voltage tests with the same well specified requirements.

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