From Electrical Engineer's Portable Handbook, Second Edition

3.6 TRANSFORMER ELECTRICAL CHARACTERISTICS

Introduction

Transformers are a critical part of electrical distribution systems because they are most often used to change voltage levels. This affects voltage, current (both load and fault current levels), and system capacity. They can also be used to isolate, suppress harmonics, derive neutrals through a zig-zag grounding arrangement, and reregulate voltage. Their electrical characteristics are as follows (see Tables 3.31-3.34 and Figure 3.31).

Table 3.31: Transformer Full-Load Current, Three-Phase, Self-Cooled Ratings

Table 3.34: Transformer Primary (480-Volt, Three-Phase, Delta) and Secondary (208Y/120-Volt, Three-Phase, Four-Wire) Overcurrent Protection, Conductors and Grounding


Figure 3.31: Electrical connection diagrams.

Auto Zig-Zag Grounding Transformers

Three single-phase transformers can be connected in an autotransformer arrangement for developing a neutral from a three-phase, threewire supply (phase-shifting). For proper overcurrent protection, refer to NEC Article 450.4. Figure 3.32 shows the one line and wiring diagrams for this arrangement.


Figure 3.32: Auto zig-zag grounding transformers for deriving a neutral-schematic and wiring diagram.

Table 3.35 shows the nameplate kVA for each transformer, number of transformers required, three-phase kVA rating, and maximum continuous amp load per phase (@ 277 V) for a primary input of 480 V, threephase, three-wire, to a secondary output of 480Y/277 V, three-phase, four-wire.

Table 3.35: Auto Zig-Zag Grounding Transformer Ratings

Connection diagram (using 3 pieces of 1 phase. 60 hertz transformers connected zig-zag auto for developing a neutral (4th wire) from a 3 phase 3 wire supply, Applicable for the above connection...


Products & Services
Single Phase Transformers
Single-phase transformers output single-phase AC power, typically while stepping-up or stepping-down the voltage between two circuits.
Three Phase Transformers
Three phase transformers are designed to supply electric power to three-phase systems.
Electrical Receptacles
Electrical receptacles, outlets, and wall sockets accept plugs and provide current to run electrical devices.
Power Transformers
Power transformers convert power-level voltages from one level or phase configuration to another.  They can include features for electrical isolation, power distribution, and control and instrumentation applications.

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