Renewable Integration at Remote Microgrid
Engineering Case Study
Case Study 2: Renewable Integration at Remote Microgrid
Scenario A hybrid microgrid serving a mining camp in the Pilbara region of Western Australia integrates a new 5 MW solar PV plant and 2 MWh lithium-ion battery storage. The existing diesel-based 6.6 kV system (base power 25 MVA) was originally designed for 12 kA symmetrical fault current. Constraints include extreme ambient temperatures (>45°C), limited transport access for equipment, and mandatory compliance with AS/NZS 3000 and IEEE 1547-2018 for inverter fault contribution.
Given Data
- Voltage: 6.6 kV
- System Base Power: 25 MVA
- Total Impedance: 0.18 pu (calculated including diesel generator subtransient reactance X"_d = 0.15 pu, MV cables Z = 0.025 pu, and inverter-based resource contribution modeled as 1.2× rated current for 0.5 s → equivalent impedance ≈ 0.005 pu; total = 0.15 + 0.025 + 0.005 = 0.18 pu)
Calculation Using the same per-unit formula:
$$ I_{SC} = \frac{S_{base}}{\sqrt{3} \times V_{LL}} \times \frac{1}{Z_{total(pu)}} $$
- $S_{base} = 25\ \text{MVA}$
- $V_{LL} = 6.6\ \text{kV}$
- $Z_{total(pu)} = 0.18$
Base current: $$ I_{base} = \frac{25,000\ \text{kVA}}{\sqrt{3} \times 6.6\ \text{kV}} = \frac{25,000}{11.43} \approx 2.187\ \text{kA} $$
Then: $$ I_{SC} = \frac{2.187}{0.18} \approx 12.15\ \text{kA} $$
The Short Circuit Current Calculator returns: 12.15 kA.
Result and Decision Although the original switchboard was rated for 12 kA, the 12.15 kA result exceeded the margin (typically ±5% tolerance per AS 61439.1). Engineers opted to retrofit existing 6.6 kV ACBs with higher-rated trip units (16 kA Icu) and added dynamic fault current limiters (FCLs) on the PV interconnection point to cap contribution during faults—ensuring coordination with diesel generator overcurrent relays and meeting IEEE 1547’s 2× rated current limit clause.
Lesson Inverter-based resources significantly reduce effective system impedance; their fault contribution—though transient—must be included in total impedance even if duration is <100 ms, otherwise thermal and magnetic stress on breakers may exceed design limits during first-cycle asymmetry.