THREE PHASE PAD MOUNTED TRANSFORMERS BASIC AND TUTORIALS

THREE PHASE PAD MOUNTED TRANSFORMERS BASIC INFORMATION
Example Of Three Phase Pad Mounted Transformers


Three-Phase Pad-Mounted Transformers
Three-phase pad-mounted transformers are typically applied to serve commercial and industrial threephase loads from underground distribution systems. Traditionally, there have been two national standards that detailed requirements for pad-mounted transformers — one for live front (ANSI C57.12.22) and one for dead front (IEEE C57.12.26). The two standards have now been combined into one for all pad mounts, designated IEEE C57.12.34.


Live Front
Live-front transformers are specified as radial units and thus do not come with any fuse protection. See Figure 2.2.29.

The primary compartment is on the left, and the secondary compartment is on the right, with a rigid barrier separating them. The secondary door must be opened before the primary door can be opened.

Stress-cone-terminated primary cables rise vertically and connect to the terminals on the end of the high-voltage bushings. Secondary cables rise vertically and are terminated on spades connected to the secondary bushings.

Units with a secondary of 208Y/120 V are available up to 1000 kVA. Units with a secondary of 480Y/277 V are available up to 2500 kVA. Although not detailed in a national standard, there are many similar types available.

A loop-style live front (Figure 2.2.30) can be constructed by adding fuses mounted below the primary bushings. Two primary cables are then both connected to the bottom of the fuse. The loop is then made at the terminal of the high-voltage bushing, external to the transformer but within its primary compartment.

Dead Front
Both radial- and loop-feed dead-front pad-mounted transformers are detailed in the standard. Radialstyle units have three primary bushings arranged horizontally, as seen in Figure 2.2.31. Loop-style units have six primary bushings arranged in a V pattern, as seen in Figure 2.2.32 and Figure 2.2.33.

In both, the primary compartment is on the left, and the secondary compartment is on the right, with a rigid barrier between them. The secondary door must be opened before the primary door can be opened.

The primary cables are terminated with separable insulated high-voltage connectors, commonly referred to as 200-A elbows, specified in IEEE Standard 386. These plug onto the primary bushings, which can be either bushing wells with an insert, or they can be integral bushings.

Bushing wells with inserts are preferred, as they allow both the insert and elbow to be easily replaced. Units with a secondary of 208Y/ 120 V are available up to 1000 kVA. Units with a secondary of 480Y/277 V are available up to 2500 kVA.

POWER TRANSFORMERS CORE IMPROVEMENT BASIC AND TUTORIALS

POWER TRANSFORMERS CORE IMPROVEMENT BASIC INFORMATION
What Are The Transformer Core Improvements?


The major improvement in core materials was the introduction of silicon steel in 1932. Over the years, the performance of electrical steels has been improved by grain orientation (1933) and continued improvement in the steel chemistry and insulating properties of surface coatings.

The thinner and more effective the insulating coatings are, the more efficient a particular core material will be. The thinner the laminations of electrical steel, the lower the losses in the core due to circulating currents. Mass production of distribution transformers has made it feasible to replace stacked cores with wound cores.

C-cores were first used in distribution transformers around 1940. A C-core is made from a continuous strip of steel, wrapped and formed into a rectangular shape, then annealed and bonded together.

The core is then sawn in half to form two C-shaped sections that are machine-faced and reassembled around the coil.

In the mid 1950s, various manufacturers developed wound cores that were die-formed into a rectangular shape and then annealed to relieve their mechanical stresses. The cores of most distribution transformers made today are made with wound cores.

Typically, the individual layers are cut, with each turn slightly lapping over itself. This allows the core to be disassembled and put back together around the coil structures while allowing a minimum of energy loss in the completed core.

Electrical steel manufacturers now produce stock for wound cores that is from 0.35 to 0.18 mm thick in various grades.

In the early 1980s, rapid increases in the cost of energy prompted the introduction of amorphous core steel. Amorphous metal is cooled down from the liquid state so rapidly that there is no time to organize into a crystalline structure.

Thus it forms the metal equivalent of glass and is often referred to as metal glass or “met-glass.” Amorphous core steel is usually 0.025 mm thick and offers another choice in the marketplace for transformer users that have very high energy costs.