Tap changer in endurance test

At the test center of Maschinenfabrik Reinhausen, on-load tap-changers are subjected to a series of endurance tests to ensure that they remain functional even in extreme situations.

Tap changers are a central element for safe power supply. They are installed in power transformers and regulate their voltage so that the power supply remains constant despite changing load conditions in the network and failures do not occur. In a unique test center at Maschinenfabrik Reinhausen, such switches are subjected to a series of endurance tests to ensure that they remain functional even in extreme situations.

To do its job well, a tap-changer must have a long service life and do its job even under extreme conditions. It has to withstand the harshness of a Siberian winter as well as the heat of a summer in Abu Dhabi, a lightning strike as well as short circuits caused by the failure of other devices in the power grid. In order to ensure precisely this, Maschinenfabrik Reinhausen (MR for short) opened a test center in Regensburg in 2008 that is unique in the world. Here, every on-load tap-changer type is tested for its suitability on an area of 4800 square meters before it is delivered for the first time.

Stufenschalter unter Leistungsdruck

The test specimens have to put up with a lot. They are exposed to extreme cold or heat and must switch as intended even when the temperature is increased from minus 70 degrees Celsius to 130 degrees. The temperature tests are carried out in appropriately sized oil vessels that are electrically heated or cooled by means of heat exchangers. In addition, two large temperature chambers are available as cooling cells.

One room further on, the tap changers work in piecework. Suspended in linkages, the devices clatter and rattle at 2.5-second intervals with every switch. Day and night, seven days a week, for up to three months. The test is intended to clarify whether the mechanics of the switches will function properly during their intended service life. Depending on the type, a tap-changer should be able to switch between 40,000 and 100,000 times before it is routinely inspected. Vacuum switches must even complete up to 600,000 switching operations. MR tests up to 1.5 million such switching operations - three times as many as required by the relevant IEC 60214-1 standard.

In addition to the mechanical endurance test, tests are also carried out on the switching sequence, pressure, vacuum and leakage tests. With the help of sensors, high-speed cameras and thermal images, the mechatronics and electrical engineers check exactly how much abrasion there is in the on-load tap-changer or how the switching process behaves under heat. In addition to on-load tap-changers, the associated diverters and motor drives are also put to the test.

From torque to acceleration

The measuring system for the mechanical tests consists of a computer with integrated I/O cards for controlling the motor drive for moving the test specimens, a universal measuring amplifier for processing the various sensor signals, a resistance measuring box for recording the contact functions, and a transient recorder as a recording device. Software developed at MR runs on the measuring computer. This controls the motor drive in various test variants and also other automatic test sequences, such as the control of temperature and pressure.

At the same time, a wide variety of measured variables are recorded and evaluated. In addition to the switching sequence of the contact functions, these also include displacement and speed, torques, angle of rotation and speed, temperature, pressure, forces and accelerations, as well as mechanical stresses using strain gauge measurement technology - typically up to 32 channels in total (16 analog, 16 digital), and in some cases more, depending on the test sequence. To ensure that even fast events are recorded precisely, the required channel sampling rates can be up to 50 kHz, depending on the test object.

Fast and secure measurement data acquisition

For measurement data acquisition, MR relies on Labortechnik Tasler, the Würzburg-based specialist for fast measurement technology. The transient recorders are also particularly suitable for applications in which many channels with high bandwidth and over longer periods of time must be acquired. Depending on the desired resolution, the maximum possible sampling rate per channel is between 2.5 MHz at 16 bits and up to 20 MHz at 12 bits. A separate clock input also offers the possibility of time-synchronous measurement to external events. Via USB or Ethernet, the measurement data can be transferred extremely fast and online to the PC, which is indispensable for long-term monitoring. Fast RAM buffers ensure that no data is lost even in the event of interruptions.

Robust transient recorders for rough continuous operation
A single recorder has 8 or 16 differential analog inputs and an additional 16 digital inputs. However, by simply cascading multiple units, synchronized acquisition is possible with many more parallel channels. Separate A/D converters and amplifiers for each input provide simultaneous sampling of all differential channels with individually adjustable input ranges and adaptive anti-aliasing filters. And all this with highest and stable long-term accuracy of typically 0.05% absolute. The LTTPro software makes the LTT24 very easy and comfortable to use in the lab. In addition, an included DLL allows the integration into special own test bench applications. This is a feature that is also used intensively at MR, because it allows test sequences and evaluations to be automated to the greatest possible extent, which saves measurement engineers a great deal of time.

Even a high EMC load does not influence the measurement
The measurement environment poses another very special challenge: It is so extremely EMC contaminated that, for example, people with pacemakers are not allowed to enter the MR test lab at all. Despite these special conditions, however, the measuring devices must reliably deliver flawless results for months on end. Measurement errors or even dropouts due to external interference would be fatal. But the very robust LTT24s, designed for rough continuous operation, also master this task with aplomb.