Transformer protection fuses are designed to remove transformer faults from the system. A two-part protection system is typically employed, comprising the Bay-O-Net (BON) fuse for overcurrent protection and the ELSP backup fuse for severe short-circuit fault current limiting. Below is a detailed analysis of their selection and coordination.

 

1. Structure and Role

Bay-O-Net Fuse

• Provides overcurrent protection.
• Fuse elements can be independently replaced without requiring core lifting or oil draining.

 

ELSP Backup Fuse

• Offers fast current limiting protectionduring severe short circuits.
• Requires core lifting and oil draining to replace the fuse element.

 

2. Transformer Operating Current Tolerance

Inrush Current

According to IEEE C37.48.1, inrush current values based on operational data and transformer characteristics are:

• 12x transformer full-load current for 0.1s.
• 25x transformer full-load current for 0.01s.

 

Cold Load Pickup and Short-Term Surge Loads

These conditions, relevant to conventional transformers, are less applicable to current source generation systems like photovoltaics.

 

Photovoltaic Low Voltage Ride-Through

In photovoltaics, the inverter’s output current characteristics under low voltage ride-through are as follows:

• 30–40 μs:Instantaneous overcurrent reaches 5x peak value.
• 30 ms:Current stabilizes to 1–1.2x nominal value.
• 60 ms:Reactive current reaches 90% of target value.

 

The inrush current values far exceed this range, so they do not impact the fuse selection process.

 

3. Coordination of BON and ELSP Fuses

Protection Characteristic Requirements

The protective characteristics of the fuses must lie to the left of the transformer damage curve to prevent damage. Key points on the transformer damage curve are:

• 1800s: 2x full-load current.
• 300s: 3x full-load current.
• 60s: 4.75x full-load current.
• 30s: 6.3x full-load current.
• 10s: 11.3x full-load current.
• 2s: 25x full-load current.
• 78s: 40x full-load current.

 

BON Fuse Interruption Capacity

Depends on transformer oil type (mineral oil or FR3):

 

• 1200A @ 34.5kV, 60Hz.
• 1000A @ 38kV, 60Hz.

 

This interruption capability exceeds the transfer current interruption limit of SF6 load switches (800A @ 36kV, 50Hz). However, the BON fuse’s maximum interruption capacity imposes a 5000 kVA limit on the transformer capacity it can protect.

 

4. Transformer Capacity and Inrush Current

IEEE C37.48.1 provides the following empirical data:

• 4000 kVA Transformer: Continuous current 67A, inrush current 804A @ 0.1s.
• 5000 kVA Transformer: Continuous current 84A, inrush current 1008A @ 0.1s.

 

As transformer capacity increases, the inrush current curve shifts to the right (Figure 2). To ensure reliable avoidance of inrush currents:

• The BON fuse’s melt and clear curvesmust shift rightward.
• This causes the overcurrent point on the BON fuse curve to approach its maximum interruption capacity.

 

5. Coordination Under Secondary Short Circuits

For a 5000 kVA, 34.5/0.8–0.8 kV transformer with a short-circuit impedance of 6%:

• Secondary short-circuit current reflects as 1395Aon the primary side, exceeding the BON fuse’s interruption capacity of 1200A @ 34.5kV, 60Hz.

 

Solution

Increase the short-circuit impedance to 7.5% or higher, which reduces the primary side current to 1116A or below, within the BON fuse’s capacity.

 

6. Practical Recommendations

From an equipment manufacturer’s perspective, system safety should take precedence over operational continuity:

• BON and ELSP fuse coordination must ensure protection without compromising reliability.
• The configuration must allow the system to handle transient and fault conditions effectively.

 

Conclusion

The two-part protection system with BON and ELSP fuses provides robust protection for transformers. By carefully selecting fuse types and coordinating their characteristics, both normal operation and fault conditions can be managed