High-voltage (HV) transformers are some of the most important and expensive pieces of
equipment required for operating a power system. The purchase, preparation, assembly,
operation and maintenance of the HV transformers represent a large expense to the power
system.
Power Transformer Overview
When transformers are received from the factory or reallocated from another location it is
necessary to verify the following points:
- That each transformer is dry and has sustain no damage during shipping,
- That the internal connections have not been loosened,
- That the transformer’s ratio, polarity, and impedance agree with its nameplate,
- That its major insulation structure is intact, and
- The wiring insulation has not been bridged.
Apart from this there are certain factors affecting the amount of testing necessary to certify that a transformer is ready to be energized and placed in service:
- Physical size, voltage class, and kVA rating are the major factors that dictate the amount of preparation required to put transformers in service,
- Size and kVA rating also dictate the kind and number of auxiliary devices a transformer will require.
Multiple number of checks and tests are performed as a transformer is being assembled
at a substation. The test engineer may not directly perform all of the tests and
inspections but must be sure that they are satisfactorily completed, so that the final decision
over transformer bank readiness for energization can be made. Some tests and procedures are performed by the specialists during the assembly phase. Special tests, other than those listed below, may also be required. Some of the tests require special equipment and expertise that construction electricians do not have and are not expected to provide. Some tests are performed by an assembly crew, while other tests are done by the personnel making the final electrical tests on the transformers.
The following are the details for performing the entire array of tests needed to prepare transformers for service:
- Nameplate Data
- Power Meggering
- Auxiliary Components and Wire Checks
- Lightning Arrestors
- Hand Meggering
- Temperature Devices
- CT Tests
- Winding Temperature and Thermal Image
- Bushing Power Factoring
- Remote Temperature Indication
- Transformer Power Factoring
- Auxiliary Power
- Voltage Ratio
- Automatic Transfer Switch
- Polarity
- Cooling System
- Transformer-Turns Ratio
- Bushing Potential Device
- Tap Changers
- Auxiliary-Equipment Protection and Alarms
- Short-Circuit Impedance
- Overall Loading
- Zero Sequence
- Trip Checks
Before proceeding with transformer measurements following rules must be followed for all the tests. Below is the approximate sequence for transformer testing:
- Inspect transformer and parts for shipping damage and moisture
- Check nameplate and prints for proper voltages and external phasing connection to the line or bus
- Check calibration of all thermal gauges and hot-spot heater, bridge RTDs and associated alarm contacts
- Check and Megger all wiring point to point: Fans, pumps, alarms, heaters, tap changers, and all other devices on the transformer and interconnecting cables
- All banks above 150 MVA should be vacuum dried. Do not apply test voltages to the winding during the vacuum drying process. Make certain the terminals are shorted and grounded during oil circulation because of the large amount of static charge that can build up on the winding.
- After the tank has been filled with oil, confirm that an oil sample was sent to the Chemical Lab and that its results are entered in the bank test reports. Note the oil level and temperature at completion of filling.
- Power operate to verify proper rotation of pumps and fans and correct operation of the under load (UL) tap changer, when provided. Also, check heater, alarms and all other devices for proper operation.
- Winding tests
- Ratio and Polarity (Voltage Method or TTR). The preference is that all large power Transformers (>1 MVA) be tested with TTR test set.
- Impedance
- DC winding resistance
- Megger and Power Factor windings, bushing and arrestors. (Wait until 24 hours after completion of oil filling for Power Factor testing).
- Load CT circuits overall and flash for polarity
- Before energization, trip-check bank protection schemes and make sure the gas collection relay is free of gas.
- When energizing a bank or picking up load, monitor bank currents and voltages, including UL tap-changer operation.
- Check proper phasing and voltage of the bank to the system before load is picked up. When possible, large transformers (>1 MVA) should remain energized for eight hours before carrying load.
- Make in-service checks on meters and relays.
- Release to Operations and report energization information to the TNE office.
- Turn in revised prints and test reports, which should include the following:
- All test data
- Moisture and oil data
- Problems incurred
- In-service data
- Time energized and release to operation
- Any unusual problem that information will aid in future equipment testing
Detailed procedure to be continued...
