Consulting Case studies

📘Method of a study

1. Terms of reference: need (energy, power, profile, quality, reliability), site constraints, budget, horizon.
2. Data: weather (Site weather data module), consumption records, characteristics of the existing network, prices.
3. Variants: at least two technically correct solutions (centralised / decentralised, 24 / 48 V, PV / hybrid…).
4. Calculations: sizing, wiring, load flow and short circuit if a network is involved, economics.
5. Sensitivity: on the uncertain assumptions (prices, rates, demand growth, resource).
6. Reasoned recommendation and traceable design note (assumptions, methods, results, standards).

🗂️Case 1 — Electrification of a village (centralised system or individual kits?)

Data

Site: Dakar (locations database). LV network of the centralised system: 30 m per customer, 6 000 kF/km, maintenance 60 kF/km/yr. Current imposed equipment (400 Wp module, 12 V 200 Ah AGM battery, 60 A MPPT controller, 1.5 kVA inverter; prices of level 3). Centralised: 48 V bus; individual kits: 12 V bus. Discount rate 8 %, inflation 3 %, 20 years. File: exemples/pv_village.nxpv.

Reference results

Analysis

• The centralised system costs half as much per kWh despite the LV network: a single inverter and a single battery bank pool the peaks (diversity), and the mill, very poorly served by a 12 V kit, benefits from the common bank.
• The wiring of the 48 V centralised system reveals its limit: the 28-string array delivers 380 A at 93 V; a single 30 m link would require 2 × 300 mm². In practice the array is split between several controllers placed near the junction boxes, or a higher-voltage bus is chosen: a point to address in the design note.
• Individual kits remain relevant for very scattered dwellings (network > 100 m per customer): redo the study with 0.1 km per customer.
• Extension: replace part of the batteries by a genset (economic dispatch, level 5) and compare.

🔌Case 2 — 500 kWp PV plant connected to the MV network

Data

64 strings of 20 modules of 400 Wp (512 kWp; 620 V per string), 8 junction boxes of 8 strings, 8 inverters (500 kVA in total), 0.4/30 kV transformer 30 m from the inverters. Cost excluding wiring 400 kF/kWp; operation 8 kF/kWp/yr; inverters replaced at 12 years (20 MF); yield 1 581 kWh/kWp/yr (example 3.5), degradation 0.5 %/yr; 25 years, discount rate 8 %, inflation 3 %.

Wiring (calculated by NRJXpert, 40 °C, grouping 0.8)

In aluminium, the AC link becomes 4 × (4 × 240) mm² Al for 6.9 MF: a saving of 4.5 MF. The box → inverter link is sized by heating (the voltage drop alone would require 25 mm²).

Economics

Investment 226.6 MF; production 809 MWh in the first year; LCOE 35.7 F/kWh: this is the minimum tariff. At 35 F/kWh, a subsidy of 2.7 % is enough.

To be completed by the student: study of the connection to the MV feeder (load flow: voltage rise at the connection point, hosting capacity, level 5) and contribution to the short-circuit current (I_skPF of the inverters).

💻Case 3 — Delivery substation and industrial installation

Data

30 kV network with a short-circuit power of 250 MVA; 1 000 kVA, 30/0.4 kV, Dyn11 transformer, u_sc = 6 %, P_sc = 10.5 kW; 600 kW factory at cos φ = 0.8; one 30 kW motor (efficiency 0.9, cos φ 0.85) 80 m from the main LV switchboard.

Calculations

1. Impedances referred to 400 V: upstream Z_a = 1.1 × 400²/250·10⁶ = 0.704 mΩ (X = 0.995Z, R = 0.1X); transformer Z_t = 9.60 mΩ, R_t = 1.68 mΩ, X_t = 9.45 mΩ.
2. Short circuit at the main switchboard: R = 1.75 mΩ, X = 10.15 mΩ; I"k3 = 1.05 × 400/(√3 × 10.30 mΩ) = 23.5 kA; κ = 1.60; ip = 53.4 kA. Main breaker 1 600 A (transformer I_n 1 443 A), breaking capacity ≥ 25 kA, making capacity ≥ 53 kA.
3. Motor feeder: I = 30 000/(0.9 × 0.85 × √3 × 400) = 56.6 A; cable chosen by NRJXpert 4 × 16 mm² Cu (ΔU 2.55 %, 63 A protection). Short circuit at the end: I"k3 = 2.10 kA; minimum phase-to-phase (c = 0.95) ≈ 1.65 kA. A 63 A D-curve breaker (magnetic tripping up to 14 I_n = 882 A) protects the line well.
4. Compensation: to go from cos φ = 0.8 to 0.93: Q_c = 600 × (0.750 − 0.395) = 213 kvar; S goes from 750 to 645 kVA, the current from 1 083 to 931 A: the transformer regains 14 % of margin.

🌬️Case 4 — Wind pumping on the northern coast (Saint-Louis)

Data

Wind at 10 m: Weibull c = 6.0 m/s, k = 2.4; roughness 0.03 m; ρ = 1.18 kg/m³. 10 kW wind turbine (curve of example 3.7), hub at 24 m, losses 6 %. Need: pumping of 60 kWh/d on average for market-garden irrigation.

Results

Wind at hub height: c = 6.90 m/s, mean 6.12 m/s, power density 220 W/m². Mean power 3.11 kW; net production 25.6 MWh/yr, i.e. 70 kWh/d on average; capacity factor 29 %. One machine covers the need on an annual balance (0.85 machine). But the wind weakens in the rainy season: check month by month (tab 4 of the wind window) and store water rather than electricity (tank of 2 to 3 days of consumption, far cheaper than a battery bank).

To be completed: wiring of the machine (tab 5, 24 m tower + 30 m), economic study compared with a diesel motor-pump, and PV + wind variant (seasonal complementarity).

💧Case 5 — Solar water pumping for a village (borehole and water tower)

Data

Village of 500 inhabitants (50 l/inh/d, i.e. 25 m³/d), borehole with a TDH of 15 m, irradiation 5 kWh/m²/d, submersible centrifugal pump (η_p = 0.5, η_m = 0.85), array losses 10 %, pumping over 9 h. File: pompage_village.nxpomp (Help › Open a tutorial example…).

Results

Pumping energy 2 404 Wh/d; peak power 534 Wp (energy method) or 583 Wp (power method); 420 W motor-pump. The hourly simulation shows that 6 modules (600 Wp) cover only 86 % of the demand: the selected array is 9 modules of 100 Wp (100 % coverage), with a 50 m³ tank. With the prices of the supplied database (November 2026) and the default economic parameters, the cost per m³ is about 126 F against about 270 F with a diesel motor-pump providing the same service.

To be completed: choice of the pump in the database (“Pumps” tab), sensitivity to the TDH and to the design margin (10 % by default), battery variant (628 Wp by the formula, but a higher cost per m³), and maintenance plan (pump replacement every 8 to 10 years).

🧩Design note template

1. Purpose of the study, client, reference and revision index.
2. Documents and standards: NF C 15-100, UTE C15-712-1, IEC 60909-0, IEC 61400-12-1, IEEE 1547; terms of reference.
3. Input data: site, weather (source and period), needs (table of appliances), network, prices (date of the quotations).
4. Assumptions: efficiencies, safety factors, temperatures, lifetimes, discount and inflation rates.
5. Methods: formulas or software (NRJXpert, version and project files attached).
6. Results: sizing, wiring (table of links), short-circuit currents and protection, voltage profile, costs.
7. Checks: hand calculation of at least one result of each type; differences explained.
8. Sensitivity and risks.
9. Conclusion and recommendation; reservations.

👤Author

November 2026