The TWA Standard series measures the resistance of all six windings - primary and secondary - of three-phase transformers and autotransformers and analyses the on-load tap changer, under IEC 60076-1.
Advantages
Six-winding measurement in a single connection
Two sets of four cables connect every primary and secondary bushing at once; the true, ripple-free DC current, up to 55 V DC, quickly saturates the core and shortens the test.
DVtest: dynamic resistance of the tap changer
TWA40D records the DVtest on all three phases at once, together with the motor current, and detects the current drops that reveal switching faults.
Quick YN mode for three simultaneous phases
On YN-connected windings, TWA40D measures all three phases at once instead of one at a time, considerably shortening the test on tap changers with many positions.
Demagnetization and remote tap-changer control
Once the measurement ends, the instrument automatically demagnetizes the core and its control unit drives the OLTC remotely with a single operator.
Applications
- Commissioning and periodic maintenance of power and distribution transformers and autotransformers.
- Condition assessment of tap changers through analysis of the DVtest and motor current graphs (TWA40D).
- Synchronisation check between the phases of a three-phase tap changer (TWA40D).
- Core demagnetization after DC tests, before returning the transformer to service.
About the product
The Standard range comprises two models, TWA25A and TWA40D, which share the resistance range (0.1 μΩ-10 kΩ), the 5 mA to 25 A DC test current and automatic demagnetization. TWA40D adds simultaneous DVtest recording on all three phases, the motor current channel and Quick YN mode, with a 0.1 ms resolution against TWA25A's 4 ms.
Both models share their H and X leads with TRT, so a single connection chains six tests together without rewiring the transformer. Results are stored in a 500-record internal memory and downloaded over USB, with RS-232 optional. Accessories available: H and X cable sets in several lengths with TTA clamps, a 30/300 A current clamp for TWA40D, a transport case, cable cases and a temperature sensor.
Technical specifications
| Models | TWA25A and TWA40D |
| Measurement | Three-phase, six windings, with a single cable connection |
| Resistance range / accuracy | 0.1 μΩ - 10 kΩ; from ±0.1% of reading |
| Test current | 5 mA - 25 A DC; up to 40 A DC in simultaneous YN mode (TWA40D) |
| DVtest resolution | 4 ms (TWA25A); 0.1 ms (TWA40D) |
| Internal memory / interface | 500 records; USB, with RS-232 optional |
| Dimensions and weight | 478 × 194 × 389 mm; 12.8 kg |
Frequently asked questions
What is the difference between TWA25A and TWA40D?
TWA40D adds simultaneous DVtest recording on all three phases, the motor current channel and Quick YN mode; TWA25A measures sequentially, phase by phase.
What is Quick YN mode?
A TWA40D mode that measures the resistance of all three phases of a YN-connected winding at once instead of one at a time, cutting test time.
Can it use the same leads as TRT?
Yes: TWA shares its H and X leads with TRT, so a single connection chains both instruments' tests.
How are test results documented?
They are stored in the internal memory and downloaded over USB; the PC software displays them numerically and graphically and exports reports to Excel, Word or PDF.
