Comprehensive Guide to Power Transformer Sizing, Impedance (%Z) & BUET Testing in Bangladesh

1. Calculating Required Transformer Capacity

Selecting the right transformer rating is the foundational step in any industrial electrical substation design. Sizing too small results in chronic overheating and oil degradation; sizing too large leads to unnecessary capital expense and poor light-load power factor.

Calculate your total connected load in kilowatts (kW), apply diversity and demand factors (typically 0.70–0.85 for industrial plants), and divide by your expected power factor (0.85–0.90) to determine design kVA. Finally, allow a 20% growth margin for future machinery expansion.

2. Understanding Percentage Impedance (%Z)

The percent impedance directly governs short-circuit fault current levels. Standard 11kV distribution transformers in Bangladesh typically specify 4.0% to 5.0% impedance for units up to 1000kVA, and 5.5% to 6.25% for units from 1500kVA to 2500kVA.

3. Mandatory BUET Laboratory Testing

Before energization, industrial clients and utility authorities (DPDC, DESCO, BREB, WZPDCL) require full type testing from the Department of Electrical & Electronic Engineering at BUET, covering:

• Turns Ratio & Vector Group Verification
• Winding Resistance Measurement at 75°C
• No-Load Core Losses and Exciting Current
• Full-Load Copper Losses & Impedance Voltage
• Insulation Resistance & High Voltage Withstand Test
• Oil Breakdown Voltage (BDV) Dielectric Test