Transformers are a part of electricity distribution and transmission networks, performing the conversion steps between the high voltage at which power is transmitted over long distances to intermediate voltages in local networks, then down to the 240 V or 120 V at which electricity is typically supplied to households and commercial users.
Achieving this process with minimal losses is a high priority for electricity distribution and transmission network operators, who only earn revenues on the power that is actually supplied to their end users. When an operator commissions a new transformer, it specifies a maximum figure for no-load losses, produced by the voltage and generated continuously when the transformer is switched on, and load losses, which are only produced when the transformer is loaded. If the transformer exceeds these specified, guaranteed values, the manufacturer can be liable for severe penalties.
The IEC 60076 and IEEE C57.12.00 international standards, which define the method used to measure losses in a transformer, specify that results have to be inclusive of any measurement error. Loss measurement results are therefore comprised of two components: the real losses attributable to core and copper losses in the transformer itself, and virtual losses attributable to uncertainty in the measurement system.
Royal SMIT Transformers B.V. supplies large, high-voltage transformers to energy generation and energy transmission companies all over the world. Part of the SGB-SMIT Group, it is based in the Dutch city of Nijmegen where it was founded more than 100 years ago.
As well as phase-shift transformers and shunt reactors it is responsible for developing, producing, selling and maintaining transformers rated from around 200 MVA up to 1,200 MVA in a voltage range up to 765 kV. Its products play a crucial role in the reliable and efficient operation of power networks that serve hundreds of millions of residential and commercial users.
Every customer requirement is unique, and nearly every transformer is custom engineered to order. Being able to measure power losses accurately to make sure products comply with specifications is an important part of the process. Each of Royal SMIT’s test bays incorporates a three-phase transformer power-loss measuring system consisting of high-voltage reference capacitors, zero-flux current transformers, and a three-phase precision power analyser supplied by Yokogawa Test and Measurement.
When Royal SMIT decided to build a new test facility, the challenge was to design a stable and reliable environment in which measurement uncertainty forms a negligible component of measured losses. Part of the solution was to use Yokogawa’s newer WT5000 model.
The WT5000 Transformer Version’s accuracy of ±0.008% at a power factor of 1 is claimed to restrict measurement error to a negligible fraction of total measured losses. It is also said to achieve the highest possible accuracy at power factors as low as 0.001.
One of the features that contributes to the WT5000’s accuracy and stability is a special aging treatment. First, the instrument is optimised by calibration at 53 Hz at power factors of 1, 0.5, 0.05, 0.01 and 0.001. Additional calibration at up to 100 kHz ensures the required performance when measuring distorted waveforms that can be encountered when measuring no-load loss currents.
Other features include a user interface with the ability to define and use event triggers and custom computations. Measurements can be viewed in numeric, waveform, bar, vector or trend formats, while multiple interface options support GPIB, USB and Ethernet communications. Backwards compatibility with communication protocols used by earlier models allows users replacing legacy instruments to continue using existing test software.
The WT5000 Transformer Version has helped Royal SMIT reduce measurement error, with 12-month accuracy improved from ±0.01% to ±0.008%. The linearity of the WT5000 Transformer Version is also superior. The newer product is said to offer one effective input range covering values from 10% to 110%, as well as sampling 50 times faster than the WT3000 and providing a resolution that is four times higher.
Calibration certificates from Yokogawa’s ISO 17025 accredited laboratory give Royal SMIT’s engineers the confidence that they can consistently produce traceable low power-factor measurements that comply with the specifications of the IEC 60076-8 and IEEE C57.12.00 standards. According to Steven Lauf, senior test engineer at Royal SMIT, the WT5000 has operated trouble free, and the three measurement channels being operated in the test system hardly need to be adjusted at all, even though the instrument can be on for days at a time.
“By installing the WT5000 Transformer Version in our test system, we can characterize the performance of every custom design more accurately than ever,” Lauf added. “This gives us the ability to offer the customer a guaranteed loss performance that is better than a competitor might be offering, and to do so with high confidence that we will deliver what we promised.”
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