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Acid Test for On-Load Tap-Changers Sturdy transient recorders stand the long-term test and provide fast and secure recording of measured data Joachim Hachmeister (for LTT Labortechnik Tasler GmbH)

On-load tap-changers are a central element for secure power supply. They are installed in power transformers and regulate their voltage to ensure consistent power supply without outages, in spite of variable load conditions in the mains supply. In a test centre, the only one of its kind worldwide, Maschinenfabrik Reinhausen is subjecting tap-changers to a series of endurance tests to ensure that they will do their job, even in extreme situations.

Photo: MR Maschinenfabrik Reinhausen GmbH

To do its task properly, a tap-changer must have a long service life and be able to perform even under extreme conditions. It must survive both a harsh Siberian winter and the heat of a summer in Abu Dhabi, a flash of lightning or short circuits due to the failure of other devices in the power supply system. To ensure precisely this, Maschinenfabrik Reinhausen (in short: MR) opened a test centre in Regensburg in 2008 that is unrivalled anywhere in the world. Here, on an area of 4800 square meters, the capability of each tap-changer type is tested before first delivery.

Tap-changers under pressure to perform The test pieces have to endure quite a lot. They are exposed to extreme cold or heat and must switch as intended even when the temperature increases from minus 70 degrees Celsius to 130 degrees heat. The temperature tests are performed in oil-filled cylinders of appropriate dimensions that are electrically heated or cooled by heat exchangers. Apart from this, two large temperature chambers are available

Inserting a tap-changer in a test cylinder for endurance testing.

for cooling purposes.

One room further on, the tap-changers are working non-stop. Suspended from supports, the equipment clatters and rattles with switching operations in 2.5 second intervals. Day and night, seven days a week, for up to three months. The test is supposed to establish whether the mechanics of the tap-changers functions faultlessly over the planned lifespan. Depending on the type, a tap-changer should be able to switch between 40,000 and 100,000 times before it is routinely checked. Vacuum switches have to complete up to 600,000 operations. MR tests up to 1.5 millions of such switching operations – more than three times as many as stipulated by the international standard IEC 60214-1.

Additional to mechanical endurance, the switching sequence is checked and pressure, vacuum and leakage tests are performed. Using sensors, high-speed cameras and thermal imaging, the mechatronics specialists and electrical engineers test the level of abrasion in the tap-changers and the switching process behaviour under heat. In addition to on-load tap-changers, the testing also includes off-circuit tap-changers and drive mechanisms.

From torque to acceleration The measurement system for the mechanical tests comprises a computer with integrated I/O cards to control the drive of the motor that moves the test pieces, a general-purpose measuring amplifier to condition the various sensor signals, a resistance measuring box to capture the contact functions, and a transient recorder as a recording device. A software application developed at MR is running on the measurement computer and controls different test variants for the motor drive plus other automatic test procedures including the regulation of temperature and pressure. Concurrently, most diverse parameters are recorded and analysed. In addition to the switching sequence of the contact functions, this includes displacement and velocity, torque, angle of rotation and speed, temperature, pressure, forces, acceleration and mechanical strain measured with strain gauge technology – typical up to 32 channels (16 analogue, 16 digital) or even more, depending on the test procedure. Precise acquisition of fast events can require channel sampling rates of up to 50 kHz, depending on the test piece.

The control room of the test centre at Maschinenfabrik Reinhausen (MR)

Fast and secure data acquisition

For data acquisition, MR counts on LTT Labortechnik Tasler, a specialist in ultra-fast measurement technology based in Würzburg. Transient Recorder Type LTT-184 – well rather a “sprinter” among measurement systems – is particularly suitable for tasks with high channel counts and large bandwidths measured over longer periods of time. The maximum sampling rate per channel is between 2.5 MHz at 16 bit and up to 20 MHz at 12 bit, depending on the desired resolution. The separate clock input allows to measure synchronously in time with external events. Via USB or Ethernet the measured data are transferred extremely fast and online to the PC which is essential for long-term monitoring. Fast RAM buffers ensure that no data is lost, even in case of interruptions.

Sturdy transient recorders for long-term use under harsh conditions

Each recorder is equipped with 8 or 16 differential analogue inputs and additionally 16 digital inputs. Synchronous acquisition with a much higher number of parallel channels is achieved by simple cascading of multiple recorders. Separate A/D converters and amplifiers for each input channel allow simultaneous sampling in all differential channels with individually adjustable input ranges and adaptive anti-aliasing filters, together with a very high and stable long-term accuracy of typical 0.05%. Two SW packages, LTTview and LTTPro, allow easy and comfortable use under laboratory conditions. An included DLL enables integration with special proprietary test bench applications. This feature is widely used at MR to automate test procedures and analyses, resulting in considerable time savings for the test engineers.

High EMC contamination does not affect the measurements Another very specific challenge is the measuring environment: It is so heavily EMC contaminated that for example persons with pacemakers are not allowed to enter the MR test lab. In spite of these conditions, the measurement equipment must provide reliable and accurate results non-stop over months. Errors in measurement or – even worse – failures due

to external stray pick-up would be fatal. The sturdy LTT recorders are designed for long-term operation in harsh environments and perform the task competently. Contact: LTT Labortechnik Tasler GmbH Science Park Friedrich-Bergius-Ring 15 97076 Würzburg Phone 0931 / 359 61 - 0 Fax 0931 / 359 61 - 50 [email protected] www.tasler.de

Pictures from Maschinenfabrik Reinhausen (MR): 1.) 16063_rk_036.jpg: Mounting sensors on the test piece at MR 2.) 16063_rk_036.jpg: Inserting an on-load tap-changer in a test cylinder for endurance testing at MR 3.) P4248022 .jpg: The control room of the test centre at Maschinenfabrik Reinhausen

Picture from LTT: LTT184, front view