Customer Solutions

Testing High-Voltage Surge Arresters with PXI and LabVIEW

Author(s):

Stephen Hoffarth, ABB Switzerland Ltd.

Industry:

Energy/Power

Product:

LabVIEW, PXI/CompactPCI

The Challenge:

Characterizing the performance of metal-oxide varistors used in medium and high-voltage surge arresters for overhead power distribution systems and railway power systems.

The Solution:

Developing a high-accuracy PXI-based measurement system with the PXI-5122 14-bit, 100 MS/s digitizer for high-resolution amplitude measurements and the MXI-3 fiber-optic link for electrical isolation.

ABB used two PXI-5122 digitizers and the MXI-3 remote link to improve voltage test-system measurements and safety.

Varistor-Testing Challenges

To meet the increasing reliability requirements of the worldwide energy industry, the Surge Arrester Division of ABB High Voltage Products develops and produces air and SF₆ gas insulated surge arresters which effectively protect medium and high-voltage networks from damaging overvoltages caused by lightning or circuit breaker switching. These surge arresters can have continuous operating voltages as high as 528 kV and withstand current impulses up to 100 kA with a waveshape of 4/10 ms. Because of these extreme operating conditions, testing the arresters and the metal-oxide varistors inside them presents a unique challenge.

At ABB, we produce metal-oxide varistors with diameters of 38 mm to 108 mm in voltage classes from 100 V up to 6 kV maximum continuous operating voltage. To create a medium or high-voltage surge arrester, we arrange several varistors in series and pack them into a case of insulating material. Instead of testing the fully assembled arrester, we test each varistor separately to lower the test voltage requirement to a more practicable level.

During the development and standards compliance type tests, we use an impulse current generator that injects a double exponential current impulse of up to 200 kA to the varistor. This test system generates impulse currents with wave shapes simulating lightning strikes or circuit breaker switching conditions and measures the residual voltage across and the current through the metal-oxide varistor. The residual voltage represents the peak value that appears between the terminals during the current pulse. The system also computes the momentary power and the energy injected into the metal-oxide varistor.

High Resolution for Increased Measurement Accuracy

To reduce the voltage and current to a directly measurable level, we use a voltage divider consisting of capacitors and high-ohmic resistors and a special very low-ohmic impulse current shunt designed for fast transients. Two PXI-5122 14-bit, 100 MS/s digitizers connect to the divider and shunt to measure the voltage and current transients that occur during the test. The 14-bit vertical resolution and high sample rate of the PXI-5122 digitizer are necessary because we want to measure the signals' transient characteristics. Because our previous test system was based on a 10-bit digitizer, the PXI-5122 digitizer increased the measurement accuracy 14 times, giving us the ability to better characterize the varistor's performance. We then use built-in LabVIEW analysis capabilities to determine the peak value, rise time, and time-to-half value. Additionally, we compute the momentary power input, momentary resistance, and complete converted energy using both the voltage and current data.

Fiber-Optic MXI-3 Link Provides High Data Throughput and Isolation

By switching to a PXI-based system, we now can transfer megabytes of data in the time it took to transfer only a few kilobytes with our older GPIB-based system. Because we can gather the data much faster, we now can rapidly reconfigure the test system by switching a power circuit breaker with a PCI-DIO-24 digital I/O board and relay module. With the improved data throughput speed we can perform, for instance, a high current operating duty test on the varistor within the time specified in the IEC 60099-4 standard for surge arrester test, much better than before.

Generating a 100 to 200 kA pulse with a rise time of 4 μs creates a very strong electromagnetic field that can potentially damage the host computer and harm the operator. To increase the safety of the system and minimize risk to the equipment, we installed two PXI-5122 digitizers in two separate PXI-1002 four-slot chassis. We "daisy-chained" the chassis together and then connected them to the host computer using the fiber-optic version of the MXI-3 remote link. This provides an optical coupling between the two digitizers and the host computer, thus eliminating any electrical connection between the computer and the pulse generator. Because separate digitizers in electrically isolated chassis make the voltage and current measurements, we avoid creating a large inductive loop from the cabling. If such a loop were formed in a high magnetic field environment, a large induced voltage would disturb the measurements.

By switching our test system to use LabVIEW 7 Express, the MXI-3 remote link, and the PXI-5122 digitizer, we improved our measurement results while also increasing the safety of our test system. Furthermore, due to the high throughput of PXI, we now can complete both our tests in the time required by the IEC 60099-4 standard.

For more information, contact:
Stephan Hoffarth, Head of Laboratory
ABB Switzerland Ltd - High Voltage Products
Dept. PTHA-TL
Jurastrasse 45
CH-5430 Wettingen 1
Tel: +41 58 585 57 68
E-Mail: stephan.hoffarth@ch.abb.com