PAD MOUNTED TRANSFORMERS SINGLE PHASE BASIC AND TUTORIALS

SINGLE PHASE PAD MOUNTED TRANSFORMERS BASIC INFORMATION
What Are Single Phase Pad Mounted Transformers?


Single-phase pad-mounted transformers are usually applied to serve residential subdivisions. Most single phase transformers are manufactured as clamshell, dead-front, loop-type with an internal 200-A primary bus designed to allow the primary to loop through and continue on to feed the next transformer.

These are detailed in the IEEE Standard C57.12.25 (ANSI, 1990). The standard assumes that the residential subdivision is served by a one-wire primary extension. It details two terminal arrangements for loopfeed systems: Type 1 (Figure 2.2.26) and Type 2 (Figure 2.2.27).

Type 1

Type 2

         
Both have two primary bushings and three secondary bushings. The primary is always on the left facing the transformer bushings with the cabinet hood open, and the secondary is on the right. There is no barrier or division between the primary and secondary.

In the Type 1 units, both primary and secondary cables rise directly up from the pad. In Type 2 units, the primary rises from the right and crosses the secondary cables that rise from the left. Type 2 units can be shorter than the Type 1 units, since the crossed cable configuration gives enough free cable length to operate the elbow without requiring the bushing to be placed as high.

Although not detailed in the national standard, there are units built with four and with six primary bushings. The four-bushing unit is used for single-phase lines, with the transformers connected phase-to-phase. The six-primary-bushing units are used to supply single-phase loads from three-phase taps.

Terminating all of the phases in the transformer allows all of the phases to be sectionalized at the same location. The internal single-phase transformer can be connected either phase-to-phase or phase-to-ground.

The six-bushing units also allow the construction of duplex pad-mounted units that can be used to supply small three-phase loads along with the normal single-phase residential load. In those cases, the service voltage is four-wire, three-phase, 120/240 V.

Cabinets for single-phase transformers are typically built in the clamshell configuration with one large door that swings up. Older units were manufactured with two doors, similar to the three-phase cabinets.

New installations are almost universally dead front; however, live-front units are still purchased for replacements. These units are also built with clamshell cabinets but have an internal box shaped insulating barrier constructed around the primary connections.

POWER TRANSFORMER IMPACT RECORDER BASICS AND TUTORIALS

POWER TRANSFORMERS IMPACT RECORDER - IMPORTANCE AND USAGE
What Is The Importance of Power Transformer Impact Recorders During Transport?


There are two types of impact recorders:

· Impact Indicator – is used in all power transformers that are equal or higher than 30MVA and/or of a voltage class equal or higher than 230kV for the domestic market. When the transformer is to be exported, such impact indicator is used in all power transformers that are equal or higher than 5MVA. Four (4) pieces are used, which are fixed to the tank sidewalls.

· Three-dimensional Impact Recorder – is used only when required by the customer and/or by an insurance Company. This reco rder is fixed to the main transformer lid, by means of fastening screws, the nearest possible to its geometrical center. The recorder is turned on before the transformer is loaded by Manufacturer and should remain turned on until the unit is unloaded.

They are intended to indicate whether chocks and vibrations have occurred in th e traver se, longitudinal and vertical directions during the transport, loading and unloading processes. The magnitude of vibration and chocks is recorded in terms of “g” (Multiples of gr avity acceleration), accord ing to TABLE 1.

TABLE 1 – ACCEPTABLE ACCELERATION VALUES DURING TRANSPORT

The ranges of the impact indicator are analyzed after the transformer is unloaded and should be sent back to Manufacturer transformadores. If they have proven to be out of the limits, the Technical Assistance Department will take all necessary steps.

However, if the recommended acceleration limits have been exceeded, it doesn’t mean that damages have occurred to the transformer. If such excess occurs, our technical department should carry out a deeper analysis of the occurrence, defining whether or not an internal assessment or eventually the adoption of another specific action is necessary.

It is necessary to take in account that not only the maximum value is imp ortant, but how many times such value has been reached during the transport.

The following procedures should be followed to turn off and remove the impact recorder:

· Remove the weatherproof protection;
· Check the box sealing for integrity. If damages are found, please report them immediately to Manufacturer
· Remove the box lid. In case of rain, do not allow the penetration of any kind of humidity to the inside of the recorder;
· Turn off the recorder;
· Mark the point where the transport process has been ended;
· Turn the recorder on;
· Close t he box lid;
· Unload the Transforme r;
· Remove the box lid;
· Turn off the recorder;
· Mark the point where the unloading process has been ended;
· Remove the impact recorder’s base from the unit;
· Replace the lid of the recorder box;
· Close the box with the supplied seal and send the impact recorder containing the roll of records to the sales department of Manufacturer, preferably using the same transporting company for return.

IMPORTANT: In case the transport and unloading work is carried out by the same Company, it is not necessary to turn off the recorder and switch its position between one and another operation.