75kva 3 phase transformer

The following is a detailed description of the **75kVA three-phase transformer**, combined with American standards 

and practical application scenarios:

 I. Basic Parameters and Types

**Capacity**: 75 kilovolt-amperes (kVA)

**Number of Phases**: Three-phase (3-phase)

**Common Voltage Levels** (commonly used in the United States):

- **Primary Voltage**: 480V, 2.4kV, 4.16kV, 13.8kV (distribution side)

- **Secondary Voltage**: 208Y/120V, 240V, 480V (load side)

**Connection Modes**:

- **Delta-Wye** (Δ-Y, step-down transformer, such as 13.8kVΔ → 208Y/120V)

- **Wye-Delta** (Y-Δ, step-up transformer)

- **Delta-Delta** (Δ-Δ, applied in industrial motors)

# II. Application Scenarios

 III. Technical Parameters and Key Points for Model Selection

# 1. Key Parameters

 2. Model Selection Steps

1. **Load Calculation**:

   - Total Load Power (kW) = kVA × Power Factor (generally taken as 0.8)

   - **Example**: A 75kVA transformer can carry a 60kW load (with a power factor of 0.8). After reserving a 20%

 redundancy, it can carry 72kW.

2. **Voltage Matching**:

   - Confirm that the primary/secondary voltage is compatible with the power grid and the load (for example,

 in American industry, it is common to have 480VΔ input → 208Y/120V output).

3. **Cooling Method**:

   - **Natural Cooling (AN)**: Suitable for an ambient temperature ≤ 40℃ without forced ventilation.

   - **Air Cooling (AF)**: Fans can be optionally equipped to increase the overload capacity by 15-25% (such as

 the Eaton 75kVA AF type).

 IV. Installation and Wiring Specifications (NEC Standard)

 1. Physical Installation

- **Space Requirements**:

  - Clearance at the top ≥ 1.8 meters (NEC 110.26)

  - Distance from the building ≥ 3 meters (outdoor) or ≥ 1 meter (indoor, NEMA 1 enclosure)

- **Grounding**:

  - **Neutral Point Grounding**: The neutral point of the secondary winding needs to be connected to **Ufer

 Grounding** (resistance < 5Ω, NEC 250.52)

  - **Enclosure Grounding**: Use a #6 AWG copper cable to connect to the grounding system (NEC 250.122)

 2. Wiring Method

**Typical Wiring of Delta-Wye Transformer**:

- **Primary (Δ)**: 3 phase wires (no neutral wire)

- **Secondary (Y)**: 3 phase wires + 1 neutral wire (208Y/120V)

- **Identification**:

  - Primary: H1, H2, H3

  - Secondary: X1, X2, X3 (phase wires), X0 (neutral wire)

 V. Configuration of Protection Devices

 1. Overcurrent Protection

 2. Other Protections

- **Temperature Sensor**: Alarm at 105℃, trip at 120℃ (IEEE C57.91)

- **Pressure Relief Valve**: Triggered when the internal pressure > 10psi (UL 1561)

 VI. Recommended Brands and Models (U.S. Market)

 VII. Maintenance Suggestions

1. **Annual Inspection**:

   - Measure the DC resistance of the winding (deviation < 2%)

   - Test the dielectric strength of the insulating oil (≥ 30kV, ASTM D877)

2. **Load Monitoring**:

   - Ensure that the unbalance of the three-phase current < 10% (NEC 210.10)

   - Use the Fluke 345 Three-phase Power Analyzer to detect the total harmonic distortion (THD < 5%)

 VIII. Fault Case: 75kVA Transformer Burned Out

**Scenario**: The transformer in a food processing factory in California smoked and burned out.

**Reasons**:

- The loose neutral wire on the secondary side led to the drift of the neutral point, and the single-phase

 voltage rose to 250V.

- The **Neutral Wire Current Monitoring Device** was not installed (recommended by IEEE C57.140).

**Rectification Measures**:

1. Replace it with a **Delta-Delta Transformer** (no neutral wire, suitable for motor loads).

2. Install a **Residual Current Device (RCD)**, with a threshold of 30mA.

**Conclusion**: For a 75kVA three-phase transformer, the connection mode should be selected according to the 

load type (motor/lighting/non-linear load), and the NEC grounding and protection specifications should be strictly 

followed. It is recommended to choose the fully sealed models of Cooper or Eaton to adapt to the outdoor

 environment in the United States.