Haefely’s WA 2293 is an advanced automatic winding analyzer designed for comprehensive testing of power and distribution transformers. One of the available measurement applications that can be included with the WA 2293 allows operators to perform transformer turns ratio (TTR) measurements. Traditionally, TTR is conducted using a 100 V or 250 V AC voltage applied to the primary windings of a transformer while the secondary winding is measured.
The question is, why 100 V or 250 V? There are two popular beliefs. One, higher voltages in the primary induce higher voltages in the secondary. Two, higher test voltages can lead to higher core saturation. Both contribute to improved accuracy in the measurements. So why does the WA 2293 use only 67 V for TTR measurements?
The WA 2293 is specifically built for transformers. This means its internal algorithms and measurement techniques are designed to account for real-world transformer characteristics that can affect TTR at lower voltages, such as core saturation. While higher voltages generally aid core saturation, the WA 2293 employs sophisticated techniques to ensure accurate measurements even when the core is not fully saturated, so it effectively uses the 67 V to properly magnetize the core for the test.
The specially developed algorithm included in compensated mode reduces the influence of leakage flux while using low voltage, giving results much closer to the nominal ratio. This is crucial for accuracy at lower test voltages, as the effects of leakage flux and winding resistance can become more pronounced.
Also, the instrument can measure and analyze the excitation current, which provides valuable information about the transformer’s core condition and helps to compensate for non-ideal behavior.
As a result, the WA 2293 can achieve the following accuracies:
☑️1.0 – 100 ratio range: 0.05% accuracy
☑️100 – 2,000 ratio range: 0.1% accuracy
This precision is critical for meeting industry standards (e.g., ± 0.5% deviation for TTR) even with a 67 V test voltage.
The WA 2293 is fully automatic. It applies the test voltage, measures the induced voltage, and calculates the turns ratio and deviation. This automation minimizes human error and ensures consistency.
The “compensated mode” with advanced algorithms is key. This implies the instrument is not just performing a simple voltage ratio measurement, but is actively compensating for various non-ideal transformer characteristics to provide a true and accurate turns ratio.
While higher voltages might seem intuitively better, testing at lower voltages like 67 V for TTR has practical advantages, especially for a portable or integrated test set:
✅Safety: Lower voltages are inherently safer for personnel and equipment.
✅Reduced Risk of Damage: Minimizes the risk of overstressing insulation or damaging the transformer, particularly if there are pre-existing faults.
✅Portability and Power Requirements: Generating and controlling lower voltages is less demanding on the instrument’s power supply and makes the device more compact and portable.
✅Industry Standards: Many standards for routine maintenance testing allow for lower test voltages for TTR, provided the accuracy is maintained.
Essentially, the WA 2293 can test transformer turns ratio effectively at 67 V because it’s a specialized, high-precision instrument that uses advanced internal processing and compensation algorithms to overcome the potential limitations associated with lower test voltages, ensuring accurate and reliable results.
The same specialized measurement technology is also integrated in the WA 2293’s multiple other test applications. The WA 2293 can also test winding resistance with automatic tap changer controls and resistance during cooling curve, TTR for phase-shifted transformers, magnetic balance, short circuit impedance, automatically detect transformer vector group, and demagnetization of the core with a simple “one-time connection” system that tests all phases on both windings.
This comprehensive approach means the instrument has an all-inclusive understanding of the transformer’s electrical properties, which can indirectly aid in accurate TTR measurements by accounting for other factors.
