Calculation of 35KV bus voltage

A 35 kV bus is typically rated at 35 kV line-to-line, with current and busbar sizing determined by transformer ratings, load, and short-circuit requirements.

Rated Voltage and Current

For a 35 kV bus, the line-to-line voltage is 35 kV. To calculate the rated current for a connected transformer or load, use the formula:

where $S$ is the apparent power in VA (or MVA) and $V$ is the line-to-line voltage. For example, a 75 MVA transformer at 34.5 kV has a rated current:

If multiple parallel cables are used per phase, the current per cable is divided accordingly. For 6 cables per phase: $1.255\text{ kA}/6\approx 209\text{ A per cable}$ ( ).

Busbar Sizing

Busbars must be sized to carry the rated current continuously and withstand short-circuit currents. Aluminum or copper busbars are selected based on:

• Current carrying capacity for the expected load
• Short-time withstand for fault conditions
• Temperature rise limits (e.g., 50°C above ambient) ( ) For example, a 4" EH IPS aluminum tube may carry up to 3590 A with proper spacing and support, suitable for a 35 kV bus ( ).

Insulation and Clearance

35 kV switchgear requires proper air insulation, creepage distances, and BIL (Basic Insulation Level), typically around 170 kV, to ensure safe operation under transient overvoltages ( ). Clearance between phases and to ground must comply with IEC or IEEE standards, considering pollution degree and altitude.

Short-Circuit Considerations

The bus must withstand short-circuit currents, often in the range of 25–40 kA for 1 second. Busbars and switchgear are rated accordingly, and protective relays (50/51, 50N/51N, 27/59, etc.) are coordinated to isolate faults safely ( ).

Cable Bus or Bus Duct

When connecting transformers or feeders, cable bus or bus duct arrangements are used. Proper spacing, parallel cable sizing, and grounding are essential to minimize unbalance and ensure cooling. For example, a 35 kV single-core cable in trefoil arrangement with spacing 2.15 times the cable diameter improves ampacity and reduces heating ( ).

Summary

To calculate and design a 35 kV bus:

1. Determine rated voltage (35 kV line-to-line) and load current using transformer or feeder ratings.
2. Select busbar material and cross-section to handle continuous current and short-circuit stresses.
3. Ensure insulation, clearances, and BIL meet standards.
4. Coordinate protection relays and short-circuit withstand.
5. Design cable bus or bus duct for proper current distribution and cooling. This approach ensures safe, reliable operation of a 35 kV bus in substations or industrial distribution systems.