MEGGER

Transformer Resistance Testing

Transformer insulation resistance deterioration is one of the most common causes of transformer failure: a failing transformer is a costly replacement in an electrical system with the potential for a long downtime. If you fail to maintain your transformer with regular insulation resistance testing (as can be carried out by the Megger Transformer Ohmmeter (MTO) series of transformer test equipment) then it’s likely to fail before reaching its maximum operating life.

By measuring a transformer's winding resistance from one HV transformer bushing to another, transformer resistance testing can reveal a great deal of information about the transformer. In addition to the obvious faulted transformer winding (i.e., an open winding or shorted turn), subtler problems can be detected. The DC current, in addition to flowing through the winding, also flows through the off-load ratio adjusting switch (DETC), the on-load ratio adjusting switch (on load tap changer or OLTC), as well as numerous welded and mechanical connections. Hence, the integrity of all these components can be verified with the use of transformer resistance test instruments. Applying DC test current through OLTC switches under transition (tap changes) validates proper make-before-break operation. From experience, it is known that the on load time changer has the greatest risk of misoperation as it operates within a transformer.

Transformer problems or faults occur due to poor design, assembly, handling, environmental damage, overloading or subpar maintenance. Measuring the resistance of the transformer windings ensures that the connections are correct and the resistance measurements indicate that there are no severe mismatches or opens. Most power transformers have taps built into them. These taps allow the ratio to be increased or decreased by fractions of a percent. Ratio changes involve a mechanical movement of a contact from one position to another, and a transformer tap changer test should also be undertaken during the transformer winding resistance test to validate proper operation.

LTC135
LOAD TAP CHANGER TESTING POWER SUPPLY

  • Winding resistance measurement
  • Dynamic resistance measurement (DRM)
  • Switch sequence timing
  • Transition resistor measurement
LTC135 is an accessory for circuit breaker analysers to test load tap changers where it delivers test currents for the measurements.

The load tap changer is the only moving part connected to the transformer windings. Taking a transformer off the system to investigate an internal problem with a tap changer is an expensive exercise; therefore it is in every utility’s interest to carry out condition assessments of their tap changers to help detect developing faults at an early stage.

LTC135 is designed to be used together with the following Megger instruments:

EGIL TM1600/MA61 TM1700 series TM1800

Together with the circuit-breaker analyzer (CBA) it becomes a a test setup for performing dynamic measurements on load tap changers.

The unit can also be used for winding resistance measurements and other applications where a constant DC current test signal is requested. It is a power supply designed for DC current testing of inductive loads with provisions for discharging the transformer winding when the test is finalised.

MTO106
TRANSFORMER OHMMETER

  • Entry level transformer ohmmeter
  • 2 channels with max 6 A current output
  • ±0.25% measurement accuracy
  • 48 V test voltage for fast saturation of transformers
  • Multimeter type operation, intuitive and fast
  • Safety through automatic, passive discharge circuitry
  • True portability – compact size and low weight
The MTO106 transformer ohmmeter is an easy-to-use, line-operated instrument specifically designed for safe and accurate field measurement of winding resistance in smaller transmission and distribution transformers. It has dual channels with a measurement range and accuracy suited to provide accurate information about the vast majority of power transformers, reactors and instrument transformers. The test current can be manually set in five different ranges to fit transformers of various sizes.

The instrument is exceedingly simple to use, which minimises the need to train users. It has a single selector switch for test current and a single button to activate the test cycle. The instrument is also supplied with test leads fitted with Kelvin clamps that allow for a single, one-time connection to the unit under test. The test leads are 10 m (33 ft) in length to allow for easy testing of pole mounted transformers.

The compact and lightweight instrument is housed in a rugged plastic case for true portability. When closed, the case is rated IP67, which means your instrument will always arrive safely to the testing location even in the harshest conditions.

Users are protected by an automatic discharge function that de-energises the transformer winding at the end of every test. The discharge function is passive and thus also functions if there is an inadvertent loss of power or if the test or mains leads are accidentally pulled.

MTO210
TRANSFORMER OHMMETER

  • Direct 2-channel digital reading (1 µΩ to 2000 Ω)
  • DC test current up to 10 A maximum
  • 0.25% measurement accuracy
  • Integrated demagnetization feature
  • Tests operation of on-load tap-changers
  • Very fast auto safety discharge circuit
The MTO210 transformer ohmmeter is a line-operated, field-portable instrument designed specifically to measure the dc resistance of all types of magnetic windings safely and accurately. It can test transformers and rotating machine windings and perform low-current resistance measurements on connections, contacts and control circuits.

The Transformer Ohmmeter is extremely useful when testing the windings and contact resistance of tap-changers with “make-before-break” contacts and voltage regulators. This action will check for pitted or misaligned contacts as the instrument will give an indication if either condition occurs.

MTO250
50 A TRANSFORMER OHMMETER

  • Capable of testing transformers up to 1000 MVA
  • DC test current up to 50 A maximum
  • Integrated demagnetisation feature
  • Computer control and internal data storage
  • Validates proper operation of on-load tap-changers
  • Protected against accidental current lead disconnection (through potential leads)
The MTO250 Transformer ohmmeter is a line-operated, field-portable instrument designed specifically to measure the resistance of all types of magnetic windings safely and accurately. It tests transformers, shunt reactors, rotating machine windings and performs low resistance measurements on connections, contacts and control circuits.

The dual set of potential inputs enables the resistance measurement of either two primary, two secondary windings, or of a primary plus a secondary winding simultaneously. This dual reading, dual injection characteristic, is a highly efficient method of completing tests in a timely manner.

MTO300 and MTO330
AUTOMATED SIX-WINDING TRANSFORMER OHMMETER

  • Automated 8-terminal/6-winding measurement capability
  • Simultaneous winding magnetization
  • Tests operation of on-load tap-changers
  • Built-in auto-demagnetization feature
  • Automatic discharge
  • Power DB software (MTO330)
The MTO 300 series delivers full eight-terminal/six-winding resistance measurement capability. It is designed to save time for the user by testing all normal 6 windings without having to disconnect and reconnect leads during testing.

The MTO3xx series also shares the Megger TTR300 series lead set. Users who test both turns ratio and winding resistance can save time and money since only one lead set is needed to connect to the transformer.

MWA300 and MWA330A
3-PHASE RATIO AND WINDING RESISTANCE ANALYSER

  • Combines TTR and resistance testing in one box - faster, more efficient testing
  • 55% smaller, 40% lighter than individual instruments
  • Only one set of leads required - lighter, smaller and less expensive
  • One test form - easier, faster to complete
  • Built-in demagnetisation
The Megger MWA300 and 330A Transformer Winding Analysers are here to offer you a faster, more efficient testing device through combining TTR and resistance testing in a single unit.

This advanced 3-phase transformer test system provides complete ratio, phase and winding resistance measurements with only one 3-phase lead-set connection. What’s more, it offers portability, a reduced set-up time and increased job safety.

Once connected, the MWA300 performs DC resistance measurements on all high- and low-side windings without reconnection. It has an 8-terminal/6-winding resistance measurement capability and doesn’t need any inter-connected boxes, allowing you to test all 6 windings without having to disconnect and reconnect leads.

The MWA300 series saves you even more time by using a single software platform, PowerDB, which means you only need one set-up and one easy-to-use test form.

The MWA300 series can be used effectively test power transformers, distribution transformers, CTs and VTs, and motors and generators.

The following tests are easily performed with a single instrument, the MWA300 series, and a single 3-phase lead set connection:
  • 3-phase turns ratio
  • 3-phase winding resistance
  • OLTC continuity (make-before-break)
  • 3 phase core demagnetization
  • Magnetic balance / flux distribution
  • Excitation current
  • Polarity and phase angle deviation
  • Auto vector detection
  • Heat-run test
The MWA330A comes equipped with a built-in computer that has a 305 mm (12 in) colour display and touch-screen interface. The computer is designed to control the unit and it can store up to 100, 000 data sets.
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