Abstract — This document outlines key considerations for breaker connections in single-phase completely self-protected (CSP) transformers with four low-voltage (LV) bushings, particularly when configured in a bank to supply three-phase loads. It details wiring connections for both 120-240 V and 120 V setups, addressing scenarios where using three single-phase transformers is necessary due to shipping constraints.
Keywords— Breaker Connections, CSP Transformers Grounding Procedures, Three-Phase Loads
I. GENERAL
This document outlines the key considerations for breaker connections in single-phase completely self-protected (CSP) transformers equipped with four low-voltage (LV) bushings, particularly when used in a bank configuration to create a three-phase LV system. When utilizing three single-phase transformers to power three-phase loads, precise breaker wiring is essential to ensure proper operation and safety. This guide explains the necessary connections for both 120-240 V single-phase and 120 V single-phase connections within a three-phase bank setup, the latter used in cases where it may be impossible or impractical to fabricate or ship a three-phase transformer with an extremely large MVA capacity. A bank of three single-phase transformers may then be the best solution, although three single-phase units will most likely exceed the total size, weight, and cost of one three-phase unit. Special attention should be given to grounding procedures to avoid potentially hazardous conditions, such as floating windings, which can occur if neutral grounding is compromised when the breaker opens.
II. SINGLE-PHASE CSP 120-240 V
To connect the breaker, it is necessary to create an external jumper between X3 and X2. This will allow you to configure a single-phase transformer with a secondary voltage of 120-240V. Additionally, the breaker terminals should be connected between X1 and X4. In this setup, the neutral is located at X3-X2 and is grounded externally.
III. THREE-PHASE TRANSFORMER BANK
For the three-phase transformer bank, it is necessary to install two external jumpers in the secondary winding: one between X1 and X3, and another between X2 and X4. This setup allows for field installation or customization to achieve 120V for bench mounting. Additionally, the circuit breaker connection must be switched from terminals X4 to X3. This configuration ensures a voltage of 120V between X1-X3 and X2-X4, with the neutral at X2-X4, and is grounded externally.
To ensure proper breaker operation, it is critical that neutral grounding is maintained when the breaker opens the contacts. Failure to do so can result in a semi-winding condition, causing the loss of ground reference and creating a "floating" system, which creates hazardous working conditions for the transformer’s secondary winding, particularly in a three-phase bank configuration.
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