TWA Advanced Series

TWA ADVANCED SERIES

• Resistance measurement of all tap positions in all six windings performed in a single test
• True three-phase on-load tap changer dynamic resistance measurement
• Extremely quick measurement, single-step cable setup
• Rapid automatic demagnetization
• Fully automated test mode
• Large 10.1’’ or 7’’ graphical touchscreen display
• Temperature measurement channel

Description:

The TWA advanced series instruments are designed for six-winding resistance measurement and simultaneous three phase on-load tap changer analysis of both the primary and the secondary transformer windings. This is performed with a one singlestep cable setup, with test currents of up to 40A. Each transformer configuration has a special measurement algorithm which is optimized for the fast stabilization of test results. The TWA instruments generate a true DC ripple-free current. Both the injection of the current and the discharge of energy from the magnetic circuit are automatically regulated.

Application:

The list of the instrument application includes:

• Six-winding measurement of transformer winding resistances with a one-time cable connection.
• A special mode which enables measuring the resistances of three transformer windings in the YN configuration simultaneously.
• Dynamic resistance measurement (DVtest) of on-load tap changers.
• An evaluation of synchronization between on-load tap changer phases.
• A measurement of on-load tap changer motor current by using a dedicated channel.
• A three-phase or single-phase automatic transformer demagnetization.
• Fully automated test mode, with automatic detection of result stabilization.

Connecting the TWA to a Transformer

Using two sets of four cables, all bushings of the primary and the secondary sides are connected only once. The connection to the transformer is made using two-contact clamps that provide the four-wire Kelvin test method. The figure presents the simultaneous testing of both windings (high side and low side) on a three-phase transformer. The setup time is minimized and the test is performed very quickly. The speed is increased by saturating the magnetic core through the HV and LV windings at the same time, so the total test time is very short. The TWA test leads are interchangeable with the test leads for the TRT Three-Phase Transformer Turns Ratio Testers.

Benefits and Features Six-Winding Resistance Measurement

The TWA injects the current with a voltage value as high as 55 V. This ensures that the magnetic core is saturated quickly and duration of the test is as short as possible. All transformer windings, both primary and secondary, can be measured with a single cable setup. The TWA has internal memory capacity to store up to 500 measurements. All measurements are time and date-stamped. The instrument is equipped with thermal and overcurrent protection. The TWA has very high ability to cancel electrostatic and electromagnetic interference that exists in HV electric fields. It is achieved by a proprietary solution applied to both the hardware construction and the application software implementation. A special mode is provided for the resistance measurement in multiple de-energized tap changer (DETC) positions. Another special mode enables measuring the resistances of three transformer windings in the YN configuration simultaneously. It is also possible to test the resistances of all tap changer positions of all three phases in a single pass through the tap changer positions.

On Load Tap Changers – Simultaneous Dynamic Resistance Measurement (DRM) of All Three Phases

The TWA can be used to measure the winding resistance of the individual taps of a power transformer without discharging between the tests. The unit also checks whether the on-load tap changer (OLTC) switches without an interruption. The moment a tap position is changed from one tap to another, the device detects a sudden, very short drop of the test current. These drops called "Ripple" should be consistent, where any drop out-of-line should be investigated. Tap changer malfunctions can be detected by analyzing the measurements of transition ripple, transition time, and visualizing DRM graphs, can be observed too. The currents in all three phases are recorded simultaneously. Test currents can reach up to 40 A in total during the test.

Dynamic resistance graphs are recorded for all three phases simultaneously, so the synchronization is verified using the cursors provided in the DV-Win software. All three phase traces are plotted on the same graph. In addition, the tap changer motor current is recorded, and displayed on the same graph. The built-in tap changer control unit enables remote control of the tap changer operation from the instrument's keyboard. The test can be performed using either a standalone instrument or DV-Win PC software.