How to size a 750 kVA standby diesel generator for a Saudi cold storage facility

## TL;DR

Sizing a 750 kVA generator for a Saudi cold storage facility is a mission-critical task where thermal inertia and motor starting torque take precedence. Unlike commercial buildings, cold storage facilities rely on heavy refrigeration compressors that present high-inertia loads and significant inrush currents. A 750 kVA unit must be sized to handle the 'worst-case' scenario: a total power failure during peak summer (50°C ambient) when all compressors attempt to restart simultaneously. Procurement managers must ensure the alternator is sized for high motor-starting kVA (SkVA) to prevent voltage dips that could trip sensitive control electronics. Compliance with SASO and SEC is mandatory, alongside ISO 8528 G3 standards for voltage and frequency stability. In this environment, a 750 kVA rating often serves a continuous cooling load of no more than 500-550 kVA to allow for the cyclic nature of refrigeration compressors and the inevitable derating due to the harsh Saudi climate.

## Calculating the standby diesel generator duty point

The duty point for a cold storage facility is defined by the peak refrigeration demand plus ancillary loads like lighting and dock levellers. The sizing equation follows: kVA = (Running kW / Power Factor) × 1.25 safety margin, plus the surge allowance for the largest compressor. In cold storage, compressors often utilize Part-Winding or Star-Delta starters. For a 750 kVA generator, the 'Step Loading' capability is vital. If the facility has four 100 kW compressors, they must be programmed via a PLC to start sequentially (staggered) rather than all at once. If two compressors start together, the transient voltage dip could exceed the 15% limit allowed for industrial electronics. The 750 kVA rating must be viewed through the lens of 'Maximum Permissible Voltage Dip'. You are not just sizing for running Amps; you are sizing for the alternator's reactance to withstand the massive inductive kick during compressor engagement.

## Standards and Saudi Arabia codes that apply

All generators in Saudi cold storage facilities must comply with SASO standards for energy efficiency and safety. Because these facilities often store perishable food or pharmaceuticals, the generator system often falls under stricter 'Emergency Power' classifications similar to NFPA 110 Level 1. The SEC (Saudi Electricity Company) requires specific protection relays at the point of common coupling. Furthermore, the alternator must meet IEC 60034 standards, specifically regarding temperature rise. In a 50°C Saudi environment, a standard Class H insulation alternator should be limited to a Class F temperature rise to ensure longevity. ISO 8528-5 G3 is the critical performance standard here; it dictates that the generator must recover its frequency and voltage within tight tolerances after a 50% load step, which is common when large refrigeration racks kick in.

## Common procurement traps for cold storage facility

The biggest trap is 'Under-sizing the Alternator'. Many buyers look at the engine's kW but forget that the alternator's SkVA (Starting kVA) capacity is what starts the compressors. A 750 kVA engine might be paired with an undersized alternator that can't handle the heat or the surge. Another pitfall is ignoring 'Altitude and Ambient Derating'. A facility in Abha (high altitude) or Riyadh (high heat) will see a 750 kVA unit perform like a 630 kVA unit. Furthermore, 'Fuel Quality and Filtration' is often overlooked. Diesel in remote Saudi industrial zones can contain contaminants or moisture; specifying a heavy-duty fuel-water separator is essential to prevent injector failure. Lastly, ensure the radiator is a '50°C Ambient' version; standard 40°C radiators will lead to engine shutdowns within minutes of full-load operation in the Saudi summer.

## Worked example for a 750 kVA cold storage facility

Consider a facility with the following profile:

1. **Base Load (Lighting, Fans, Control):** 150 kW.

2. **Cooling Load:** Three compressors at 110 kW each.

3. **Total Running kW:** 150 + (3 × 110) = 480 kW.

4. **kVA at 0.8 pf:** 480 / 0.8 = 600 kVA.

5. **Applying the 1.25 Margin:** 600 × 1.25 = 750 kVA.

6. **Starting Analysis:** The largest load is a 110 kW compressor. With a Star-Delta starter, the surge is approx. 3.5x running current.

- Motor running kVA = 110 / 0.8 = 137.5 kVA.

- Surge = 137.5 × 3.5 = 481 kVA.

7. **Check:** When the generator is running at base load (150 kW/187 kVA) and the last compressor starts, the total instantaneous load is 187 + (2 × 137.5) + 481 = 943 kVA.

8. **Recommendation:** A standard 750 kVA unit might struggle with a 943 kVA peak surge. The procurement manager should specify an 'Oversized Alternator' (e.g., an 800 kVA frame on a 750 kVA engine) to handle the 943 SkVA requirement without exceeding a 20% voltage dip.

### Can I use a 750 kVA generator for a facility with 600 kW of total load?

No. In a Saudi summer, a 750 kVA generator (which is 600 kW at 0.8 pf) will be at its absolute limit. After 10-15% ambient derating, you only have 510-540 kW of actual capacity. You should upsize to at least 1000 kVA for a 600 kW load.

### Why does my generator shut down when the large compressor starts?

This is likely due to 'Voltage Dip'. If the compressor's starting kVA exceeds the alternator's surge capacity, the voltage drops, causing the control panel or the motor contactors to trip. You may need a soft starter or a larger alternator frame.

### What is the recommended maintenance interval for cold storage generators in KSA?

Due to the dusty environment and high heat, air filters and oil should be checked monthly. A full service every 250 operating hours or every 6 months (whichever comes first) is standard for ensuring reliability in critical cold-chain applications.