A Comprehensive Guide to Sizing a Neutral Earthing Transformer for a 110/30 kV Substation

Proper sizing of a neutral earthing transformer (NET) is crucial for ensuring reliable and safe operation of a 110/30 kV substation. This guide will walk through the detailed procedure, highlighting key steps and considerations that need to be taken into account.

Determine System Parameters

The first step in sizing a NET involves understanding the system parameters. For a 110 kV system, the line-to-line voltage is 110 kV. To calculate the line-to-neutral voltage, use the formula:

VLN VLL/√3

For a 110 kV system, the line-to-neutral voltage (VLN) can be calculated as:

V_{LN} 110 / √3 ≈ 63.5 kV

Understanding the maximum expected ground fault current is the next critical step. This is typically derived from system studies including fault analysis.

Select Earthing Method

Choosing the right earthing method is essential for accurate sizing. Options include:

Solid Resistor: Calculates the grounding resistor to limit the fault current based on the ground fault current expected. Reactance: Determines the reactance required to limit the fault current.

Calculate Transformer Rating

The kVA rating of the NET is typically based on the expected fault current. A common rule of thumb is to size the transformer to handle 25 to 50% of the maximum fault current for a duration of 1 second. The formula to calculate the transformer size (SNET) is:

S_{NET} (I_{fault} × V_{LN}) / 1000

Check for Voltage Regulation

Ensure the transformer maintains acceptable voltage levels during fault conditions. This requires checking the transformer's impedance and its effect on system voltage during a fault.

Consider Insulation Requirements

The insulation level of the transformer must be suitable for the system voltage. For a 110 kV system, the insulation class should be at least 170 kV Bipolar Insulation Level (BIL).

Evaluate Thermal and Mechanical Considerations

The transformer must handle thermal and mechanical stresses during fault conditions. This includes evaluating temperature rise and cooling requirements.

Select Connection Type

Choose the appropriate connection type (delta or wye) based on system configuration and grounding needs.

Compliance with Standards

Ensure the design complies with relevant standards and regulations, such as IEC (International Electrotechnical Commission) and IEEE (Institute of Electrical and Electronics Engineers) guidelines for grounding and earthing systems.

Consult with Manufacturer

Engage with transformer manufacturers to finalize specifications, options, and to consider any additional features such as protection devices.

Conclusion: The accurate sizing of a neutral earthing transformer is a critical task that requires careful consideration of system parameters, fault currents, and compliance with safety standards. Collaboration with experienced engineers and the use of software tools for accurate fault analysis and transformer sizing is highly recommended.

Keywords: neutral earthing transformer, substation design, 110/30 kV substation