Three-Phase Winding Ohmmeters & Tap Changer & Winding Analyzers TWA Standard 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
• Built-in tap changer control unit
• Temperature measurement channel

Description

The TWA standard 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 (provided by the TWA40 model). This is performed with a one single-step 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:

• A six-winding measurement of transformer winding resistances with a one-time cable connection
• Quick YN mode, which enables measuring the resistances of three transformer windings in the YN configuration simultaneously (TWA40D)
• Dynamic resistance measurement (DVtest) of on-load tap changers, which can be performed simultaneously on all three phases, or on a single phase (TWA40D)
• An evaluation of synchronization between on-load tap changer phases (TWA40D)
• A measurement of on-load tap changer motor current by using a dedicated channel (TWA40D)
• A three-phase or single-phase automatic transformer demagnetization

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.

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

Phases

The TWA40D model 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.

DV-Win Software

The Windows-based DV-Win software enables control and observation of the test process, as well as saving and analyzing the results on a PC. It provides a test report, arranged in a selectable form as an Excel spreadsheet, PDF, Word, or ASCII format. The software provides an OLTC (tap changer) condition assessment through analysis of the graphs representing dynamic resistance values during the tap changer transitions. Additionally, the DV-Win measures and calculates the OLTC transition time, the ripple and the winding resistance for each tap changing operation. The standard interface is USB. RS232 is optional. Additional testing mode, Quick YN, is enabled by the DV-Win software for the TWA40D model. In this mode, it is possible to measure 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.

Tap Changer Motor Current Monitoring Channel

The AC and/or DC current monitoring channel enables monitoring and recording the OLTC mechanical drive motor current during the tap changer operation on the TWA40D model. The motor current waveform (or another useful signal) is printed on the same DV-Wingenerated DRM graph, and can help in detecting OLTC mechanical problems. Motor recording allows for DRM recording using the motor operation trigger, which is useful for reactance tap changers. An AC/DC current clamp is available as an optional accessory.

Automatic Transformer

Demagnetization

After a DC current test, such as a winding resistance measurement, the magnetic core of a power or instrument transformer may be magnetized. Also, when disconnecting a transformer from a service, some amount of magnetic flux trapped in the core could be present.

The remnant magnetism can cause various problems such as erroneous measurements on a transformer, or an inrush current at start-up of a power transformer, or an incorrect operation of the protective relays due to the magnetized CT cores. To eliminate this source of potential problems, demagnetization should be performed. When the discharging process has been completed, the TWA can perform fully automatic demagnetization.

Demagnetizing the magnetic core of a transformer requires alternating current applied with decreasing magnitude down to zero. The TWA provides this alternating current by internally changing the polarity of a controlled DC current. During the demagnetization process the TWA supplies current at decreasing magnitude for each step, following the proprietary developed program. Both singlephase and three-phase demagnetization can be performed.

Technical Data

Winding Resistance Measurement

• Test currents: 5 mA – 25 A DC (TWA25A) 5 mA – 40 A DC (TWA40D)

AC Current Measurement Channel (TWA40D)

• Resolution: 0,1 ms
• Amplitude resolution: 16 bit
• Output voltage: up to 55 V DC
• Measurement range: 0,1 μΩ - 10 kΩ
• Typical accuracy: ± (0,1 % rdg + 0,1 % F.S.)

Current Clamp Meter Specifications

• Nominal current: 300 ARMS or 450 A DCPK
• Measuring ranges: 30/300 A

Resolution

• 0,1 μΩ – 999,9 μΩ: 0,1 μΩ
• 1,000 mΩ – 9,999 mΩ: 1 μΩ
• Frequency range: DC to 20 kHz (-3 dB)
• Resolution: ± 50 / ± 100 mA
• Accuracy: ± 1% of the reading
• 10,00 mΩ – 99,99 mΩ: 10 μΩ
• 100,0 mΩ – 999,9 mΩ: 0,1 mΩ

Computer Interface

• USB
• 1,000 Ω – 9,999 Ω: 1 mΩ
• 10,00 Ω - 99,99 Ω: 10 mΩ
• 100,0 Ω – 999,9 Ω: 0,1 Ω
• 1 000 Ω – 9 999 Ω: 1 Ω

Data Storage

• 500 internal memory positions
• USB flash drive feature enables storage of a huge number of measurements
• Optional: RS232

Warranty

• Three years

Environmental Conditions

• Operating temperature:
• 10 ºC - + 55 ºC / 14 ºF - +131 ºF
• Storage & transportation:
• 40 ºC - + 70ºC / -40 ºF - +158 ºF

OLTC Dynamic Resistance Measurement

• Sampling rate: 0,1 ms for TWA40D, 4 ms for TWA25A
• Automatic open circuit detection and warning
• Transition current ripple measurement
• Transition time measurement using DV-Win software
• Timing measurement of different transition changes using DV-Win graph analysis tool
• Humidity 5 % - 95 % relative humidity, non-condensing

Dimensions and Weight

• Dimensions (W x H x D):
• 480 mm x 197 mm x 395 mm
• 18.9 in x 7.8 in x 15.6 in
• Weight:12,8 kg / 28.3 lbs

Mains Power Supply

• Connection according to IEC/EN60320-1; UL498, CSA 22.2
• Mains supply: 90 V - 264 V AC
• Frequency: 50/60 Hz
• Mains supply voltage fluctuations up to
• ±10 % of the nominal voltage
• Input power: 1500 VA
• Fuse 15 A / 250 V, type F, not user replaceable

Temperature Measurement

• One temperature measurement channel Thermometer Pt100

Applicable Standards

• Installation/overvoltage: category II
• Pollution: degree 2
• Low voltage directive: Directive 2014/35/EU (CE Conform) Applicable standards, for a class I instrument, pollution degree 2, installation category II : IEC EN 61010-1
• Electromagnetic compatibility: Directive 2014/30/EU (CE Conform) Applicable standard EN 61326-1
• CAN/CSA-C22.2 No. 61010-1, 2nd edition, including Amendment 1
• 50 ºC +180 ºC / -58 ºF +356 ºF
• 50 mm x 6 mm

All specifications herein are valid at ambient temperature of + 25 °C and recommended accessories. Specifications are subject to change without notice

Specifications are valid if the instrument is used with the recommended set of accessories

TWA Standard Series – Models

Ordering Information

IBEKO Power AB Stockholmsvägen 18 181 50 Lidingö, Sweden

Contact Phone: +46 70 0925 000 E-mail: sales@dv-power.com