1. Selection of 10kV Circuit Breakers

In 10kV substations, vacuum circuit breakers are commonly adopted for both incoming lines and transformer high-voltage sides. Standard rated currents include 630A, 1250A, 1600A, 2000A, 2500A, 3150A, and 4000A, paired with corresponding breaking capacities as follows:

• 630A: 20–25kA

• 1250A & 1600A: 31.5kA

• 2500A & 3150A: 40kA

For user substations and industrial loads in Beijing, a 20–25kA breaking current suffices. For power utilities and generation facilities, circuit breakers should be rated for 31.5–40kA.

Given current power distribution transformer capacities not exceeding 2500kVA, the high-voltage current can be calculated as:

• Single transformer: Ig = 1.05 × Se / (√3 × Ue) ≈ 152A

• Dual transformers (1.3×): Ig ≈ 187A

Thus:

• If only one transformer feeder exists: use 630A breaker.

• If there are multiple feeders or standby lines: opt for 1250A or 1600A.

• With 10kV short-circuit currents in Beijing not exceeding 25kA, a 25kA breaker is typically sufficient.

2. Selection of 10kV Fuses

According to GB13539.1-2002 / IEC60269-1:1998, standard rated fuse currents range from 0.5A to 1250A.

If a load-break switch is used on the 10kV side, a combined load-break switch and fuse protection scheme is employed. Fuse rating (Irr) is calculated using:

Irr = K × Igmax
Where:

• Igmax = 1.1 × rated current

• K = 1.1–1.3 (without motor starting), 1.5–2.0 (with motor starting), typically ~1.5

Example:
For an 800kVA transformer:
Ig = 46.2A → Irr = 1.1 × 1.5 × 46.2 = 76.23A
→ Fuse rating = 80A

3. Voltage Transformer Fuse Selection (High Side)

• For metering: Rated fuse current = 0.5A

• For protection and measurement: Rated fuse current = 1A

These values ensure precise fault detection and secure operation under minimal burden conditions.

4. Selection of 10kV Surge Arresters (MOA)

Rated Voltage:

According to GB/T 50064-2014, surge protection must accommodate:

• Temporary overvoltages (TOVs)

• Switching surges

• Lightning-induced surges

Given Um = 12kV for 10kV systems:

• Low resistance grounded systems: MOA rated voltage ≥ 12kV

• Arc-suppression coil grounded: MOA rated voltage ≥ 15kV

• Continuous operating voltage (Uc):

• Low-resistance: ≥ 9.6kV

• Arc-suppressed: ≥ 12kV

Discharge Current:

• Distribution equipment: 5kA

• Motors: 2.5kA

• Transformers/neutral points: 1.5kA

Residual Voltage:

Using the formula:
Ul.p ≤ Ue.l.i / K16, with K16 = 1.25

• Arc-suppression coil grounded: Ul.p ≤ 60kV

• Low resistance grounded: Ul.p ≤ 48kV

Selection Example:
HY5WZ(S)-17/45, where:

• HY: Composite zinc oxide arrester

• 5: 5kA discharge current

• W: Gapless

• Z: Substation type

• S: Distribution type

• 17: Rated voltage (kV)

• 45: Residual voltage (kV)

Installation:

• With shared PT: installed in PT cabinet

• Without PT: placed on transformer feeder panel

5. Sectional Circuit Breaker Selection

The sectional breaker should match the incoming line breaker in rated parameters, including CTs. The instantaneous and time-delay trip settings should lie between the upstream breaker and the downstream load's maximum fault current.

In Beijing’s power grid, both 10kV and 0.4kV buses often operate in split configuration to limit short-circuit currents. Sectional switches must support 3-out-of-2 interlocking (closed-loop protection).

6. Current Transformer (CT) Selection

Primary ratings: 15A–4000A
Secondary ratings: 5A or 1A

Accuracy:

• Metering: 0.2S or 0.5S

• Protection: 10P10 or 10P15

Metering CT Selection Formula:
N = Ie / (0.7 × 5)
Where Ie = primary rated current

Example CT Ratios:

Protection CT Selection:

Ensure CT saturation current ≥ 1.3 × maximum short-circuit current.

Thermal & Dynamic Stability:

• Thermal: CT must endure 25kA for 4s (10kV)

• Dynamic: Idyn ≥ 2.55 × 25kA = 63.75kA

Zero-Sequence CTs:

• Typical ratios: 50/5, 75/5, 100/5, 20/1

• Systems with grounding via arc suppression coil: protection acts at ≥5A

• Choose low-ratio CTs for sensitivity; e.g., 100/5 gives 0.25A secondary at 5A primary

7. Busbar Cross-Sectional Area Selection

Based on DL5222-2005, for copper conductors at 25°C and 4s fault duration:

8. Cable Selection for 10kV Systems

A. Voltage Level:

Based on GB 50217-2007:

• Low-resistance grounding (≤1 min fault): ≥6.5kV

• Arc-suppressed coil grounding (<8h): ≥8.7kV

• Ungrounded (>8h): ≥11kV

B. Conductor Sizing:

1. Load Carrying Capacity:

Use:
Ig = K × Se / (√3 × 10)
Where K ≈ 1.3 for overloading margin

Adjust for temperature, soil resistance, and installation factors (K1, K2, K3).

2. Thermal Stability Check:

S ≥ 100 × √Q / C, where

• Q = Id² × t

• t = 0.245s

• C (for XLPE Cu cable) = 13279

Example:
For 25kA fault, S ≥ 93.2mm²
→ Use 150mm² XLPE cable for external feeders

For Transformer Feeder Cables:

Cables only need to meet continuous current demand—thermal performance is inherently satisfied due to lower fault levels on the HV side.

Conclusion

Designing a 10kV distribution system requires rigorous component selection rooted in electrical standards, fault tolerance, and system coordination. From vacuum breakers to surge arresters, each element must be carefully sized not only for nominal operation but also for extreme contingencies. Engineering foresight in this regard is essential to ensure reliability, safety, and long-term operational integrity.