Data Center Diesel Generator System Application & Engineering Analysis

(Comprehensive Guide to Power Rating Selection, System Architecture, and Short-Circuit Calculation)

Introduction

With the rapid development of China’s “New Infrastructure” initiatives, data center construction has experienced explosive growth. As the fundamental backbone of big data and cloud computing, data centers demand extremely high power reliability and system continuity.

Diesel generator sets (gensets) serve as the long-duration standby power source in data centers. When the utility grid fails, gensets must start quickly, synchronize seamlessly, and carry the data center load from 0% to 100% in a short time. This places stringent requirements on:

• Genset stability

• Synchronization and load-sharing control

• Dynamic performance

• System architecture reliability

Although high-voltage (10kV) and low-voltage (380V) genset systems are widely used, many users still struggle with power selection, equipment configuration, and system calculations. This article provides a complete engineering guide covering:

1. Power rating types and selection methods

2. Common LV and HV system architectures in data centers

3. Short-circuit current calculation and equipment selection

4. Control systems and reliability considerations

1. Power Rating Selection for Diesel Generator Sets

According to GB/T 2820.1-2009, diesel generator power ratings are divided into four categories:

1.1 Confusion in Real Applications

Many users misunderstand the differences between PRP, COP, and ESP. Engines used in data centers often follow ISO ICXN standards, allowing 10% overload, which affects how PRP is interpreted in the communication industry (YD/T 502-2020).

1.2 Comparison with YD/T 502-2020 (Telecom Standard)

YD/T 502-2020 selects COP, PRP, and LTP, but defines PRP differently:

• PRP requires gensets to run continuously for 12 hours at rated power,

• And to run 110% overload for 1 hour in every 12-hour cycle.

This overload capability differs from GB/T 2820.1 and is based on the engine’s ICXN overload allowance.

1.3 Key Factors in Power Selection

When determining genset power, engineers must consider:

(1) Ventilation Conditions

Poor ventilation may reduce genset output by up to 30%.
This is common in soundproof rooms or incorrect acoustic design.

(2) Load Sharing Imbalance in Parallel Systems

Unequal load distribution reduces total available capacity.

(3) Non-linear Loads (UPS, servers, rectifiers)

Data centers have high harmonic content. Generators must have sufficient:

• Steady-state voltage regulation

• Voltage dip tolerance

• Harmonic withstand capacity

2. Common Diesel Generator System Architectures in Data Centers

The debate between 380V low-voltage (LV) and 10kV high-voltage (HV) genset systems has existed for years. In practice:

There is no “best solution”—only the most suitable solution based on capacity, investment, reliability, and O&M conditions.

2.1 Low-Voltage (380V) Generator Systems

Advantages

• High reliability → switching point close to load

• Short cable distance → minimal power loss

• Lower maintenance requirements

• Good for small–medium data centers (below ~6MW)

Limitations

• Current is very high → requires large cables

• Cable routing & construction difficulty increases

• Hard to parallel many gensets

• Not suitable for large scale loads

Typical LV Architecture (N Configuration)

Used in small data centers with simple structure.
(N+1 configuration may require shared generators or small parallel systems.)

2.2 High-Voltage (10kV) Generator Systems

Widely used in large data centers since ~2010.

Advantages

• Output current is 1/25 of LV systems

• Cable quantity significantly reduced

• Long transmission distance

• Flexible room layout (generators can be far from server halls)

• Easy to create a large capacity pool (many generators in parallel)

Limitations

• Higher maintenance & technical skill requirements

• Centralized switching point (10kV bus) reduces reliability

• Switching point further from critical load equipment (UPS)

Typical HV Architecture

Parallel generators feed into a 10kV bus with centralized ATS/AMF switching.

Reliability Issue

If the 10kV bus is under maintenance or testing:

• One side of the transformer may lose power

• Even with bus tie breakers, risks remain

High-Voltage Distributed Switching (Recommended for High Reliability DCs)

Moving the switching point down to each transformer’s incoming feeder increases reliability, though cost is higher.

Even so, it is still not as close to the load as LV switching at UPS input.

3. Short-Circuit Current Calculation and Equipment Selection

Many engineers mistakenly select HV generator breakers and cables the same way as the utility side, resulting in oversized and costly installations.

3.1 Example: 2000kW, 10kV Generator

Given:

• Rated capacity: 2000 kW

• Voltage: 10.5 kV

• xdʺ = 0.108 (typical)

• Base capacity Sj = 100 MVA

3.2 Short-Circuit Current Calculation

Using per-unit method:

• Xdʺ = 0.108 × (100/2) = 5.4

• Base current:
  Ij = 100 / (1.732×10.5) = 5.5 kA

• Generator terminal short-circuit current:
  Ikʺ = 5.5 / 5.4 = 1.018 kA

• Peak current ≈ 2.7 × Ikʺ = 2.74 kA

3.3 Breaker Selection

Because short-circuit current is low:

➡ 630A breaker is typically sufficient.

When 10 generators are paralleled:

• Short-circuit current at a single feeder:
  Ikʺ = 9 × 1.018 = 9.162 kA
  ➡ Breaker breaking capacity 20 kA is enough.

3.4 Cable Selection

Using thermal stability formula:

• Single machine:
  Required Smin = 7.43 mm²

• 10-machine parallel short-circuit:
  Required Smin = 66.88 mm²

➡ 70 mm² copper cable is sufficient
➡ If derating factors applied → 95 mm²

Engineering practice commonly uses 150 mm², resulting in unnecessary cost.

4. Control System & Reliability Requirements

A stable generator system relies not only on hardware but also on its “nervous system”:

• Parallel control system

• Load management (kW, kVar sharing)

• Fuel system control

• Protection relays

• Redundant power to all control devices (UPS-fed)

High reliability data centers particularly require uninterrupted control power for the entire generator control system.

5. Conclusion

This article provides a full engineering analysis for diesel generator system applications in modern data centers:

• Clarifies key power rating definitions and selection methods

• Explains the strengths and weaknesses of HV and LV generator architectures

• Offers real engineering examples for short-circuit calculation and breaker/cable selection

• Highlights the importance of generator control systems in overall reliability

By following proper design principles, engineers can:

• Improve supply reliability

• Avoid oversized equipment and unnecessary investment

• Ensure diesel gensets fully meet data center requirements