Friday 27 April 2012

High-voltage impulse testing

The DL850 ScopeCorder, from Yokogawa, is at the heart of a test set-up created by SP -Technical Research Institute of Sweden - to offer an impulse-voltage calibration and testing service based on the international IEC60060 standard for high-voltage testing.
High-voltage equipment in the power grid runs a high risk of being subject to transient overvoltages caused by lightning strikes. In order 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 1.2 µs and a time to half-value of 50 µs.

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.

There are three major parameters to measure 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 20 m tall - and this, together with the requirement for a flat frequency response from DC into the megahertz range, presents significant challenges to both the design and calibration of the measurement equipment.

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, and so SP has developed a test set-up, with the DL850 as an integral part, to bring the calibration laboratory to the customer.

By bringing a high-accuracy reference measuring system for voltage levels up to 500 kV to the customer, 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 200 V 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 aging, and the accuracy was no longer good enough. The minimum requirements for the digitiser according to IEC61083-1 are a sampling rate of 60 MS/s and 9 bits of resolution. These values may seem modest for most applications, where the usual choice is between high sampling rate or high resolution. The combination of medium/high sampling rate and medium/high resolution found in this application is rare.

When the SP team were searching for a new replacement, one of the candidates was the Yokogawa DL850 ScopeCorder, and SP eventually selected an instrument configured for optimum lightning impulse accuracy. A key benefit of this instrument was the ability to control all pertinent operating parameters using a computer.

The DL850 ScopeCorder was calibrated using a calculable impulse voltage calibrator developed at Helsinki University by Dr. Jari Hällström, 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. This is well below the requirement for the application, where the specification is for the reference measuring system to measure with an expanded uncertainty of <=1% for peak voltage and <=5% for time parameters.

SP have developed their own software for measuring impulse voltages, which is used for both controlling the DL850 ScopeCorder and analysing the results. The analysis part is freely accessible here.

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 even more important to use a test instrument that can efficiently access the raw measured data. The fact that the Yokogawa DL850 communicates using the standard VISA protocol was therefore a further strong point in its favour.

The Yokogawa DL850 ScopeCorder is a versatile instrument that combines the benefits of a high-speed oscilloscope and those of a traditional data acquisition recorder in a single, portable instrument.

The instrument features high-speed acquisition - up to 100 megasamples per second - over up to 128 channels, the ability to carry out real-time recording, a powerful user interface, and a comprehensive range of PC interfacing capabilities for wider functionality and ease of use. A key feature for electrical measurements is the isolated input module which, when combined with the 100 MS/s acquisition speed and the 12-bit resolution, makes the DL850 ideally suited to measurements such as high-voltage impulse testing.

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