Recorder meets testing requirements

17 July 2012

A test set-up created by SP –Technical Research Institute of Sweden has been designed to offer an impulse-voltage calibration and testing service based on the international IEC60060 standard for high-voltage testing. A Yokogawa DL850 ScopeCorder is at the heart of this.

High-voltage equipment in the power grid runs a risk of being subject to transient overvoltages caused by lightning strikes. To test the ability of electrical equipment to survive these harsh conditions standards, such as IEC60060, prescribe testing with a standardised simulated lightning overvoltage. This voltage is defined in IEC60060 as an ideal double-exponential waveshape with a front time of 1.2µs and a time to half-value of 50µs.

In actual testing, the impulse generator will often create a waveshape distorted with front oscillations at a higher frequency. Research has shown that these oscillations have a small impact on the electrical withstand of the test object, and IEC60060 specifies a standardised filter that conforms to the experimental results. The filter is defined as a digital filter to be applied on digitised measured data.

Three major parameters need to be measured on a lightning impulse – peak voltage, front time and time to half-value. These need to be measured at very high voltages of 1000 kV and higher. The test object and the high-voltage divider are of considerable size – up to 10 or 20mm in height – and this, together with the requirement for a flat frequency response from DC into the megahertz range, presents challenges to both the design and calibration of the measurement equipment.

IEC60060 requirements
IEC60060 requires that the tests are performed with measuring equipment that has a traceable calibration. With equipment for these voltage levels, it is not generally feasible to transport it to a calibration laboratory, so SP has developed a test set-up, with the DL850 as an integral part, to bring the calibration laboratory to the customer. The modular, waveform recording instrument is able to measure voltage, current, strain, acceleration, and other phenomena-- simultaneously.

By providing a high-accuracy reference measuring system for voltage levels up to 500 kV, SP provides traceability to internationally recognised standards of measurement. Above 500 kV, the linearity of the system is proven by other methods that are recognised in the standard.

The first stage of the test system is a resistive voltage divider. The scale factor is determined by measuring DC resistance, and is then extended to the megahertz range by measuring the step response using a special 200V step generator and applying convolution methods. The cables, attenuators and termination resistor are also part of the measuring system, and the resistance values of these are used when determining the scale factor. The frequency response of this set-up is flat to at least 500 MHz, providing excellent performance.

The digitiser previously used at SP was no longer able to provide the accuracy needed. The minimum requirements for the digitiser according to IEC61083-1 are a sampling rate of 60 MS/s and 9 bits of resolution. It is the combination of medium/high sampling rate and medium/high resolution found in this application that is rare.

The instrument was calibrated using a calculable impulse voltage calibrator developed at Helsinki University, resulting in errors for peak voltage below 0.4% and for time parameters below 1.5% for a 0.84/50 µs standard lightning impulse. SP has developed its own software for measuring impulse voltages, which is used for both controlling the ScopeCorder and analysing the results.

SP takes part in standardisation work and this software has formed a key part of the ‘round robin’ tests used to establish the reference values in the upcoming revision of IEC61083-2: requirements for software for impulse tests. It is important for SP to be able control all aspects of the software in order to be able to vouchsafe for the quality, making it ‘ more important to use a test instrument that can efficiently access the raw measured data. The fact that the DL850 communicates using the standard VISA protocol went in its favour.

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