Optimized MV Generator Power Plant Architectures for Large Data Centers

A Technical White Paper from Schneider Electric

For large data centers exceeding 5 MW, the design of the electrical distribution system and backup power supply represents a significant capital investment. One of the most impactful decisions is how to configure the generator power plant—the heart of the facility’s emergency power system. A new white paper from Schneider Electric’s Data Center Science Center, White Paper 262, provides a detailed analysis of optimized medium voltage (MV) generator architectures, comparing their cost and reliability. This article provides an overview of the white paper’s content and directs you to download the full document for free.

Why Generator Architecture Matters

In large data centers, the generator power plant typically represents about 30% of the total electrical distribution investment. Traditionally, generators are provided in a 2N configuration to yield a high level of availability. However, this comes at a high cost, both in terms of capital expenditure (CAPEX) and operating expenses (OPEX).

The key optimization proposed in this white paper is moving from a traditional 2N or N+1 low voltage (LV) generator architecture to a centralized MV generator power plant connected to the MV distribution network. This approach offers several advantages:

Reduced CAPEX: Fewer generators and less associated switchgear
Simplified Design: Centralized control and monitoring
Reduced Footprint: Less space required for generator sets
Improved Scalability: Easier to expand capacity as the data center grows
Lower Carbon Footprint: Fewer generators to maintain and run

The paper explores whether 2N generator plants are truly required, given that large data centers are often connected to highly reliable MV or HV transmission grids. The authors argue that N+1 (or N+2) generator architectures are becoming the reference standard design.

Understanding Tier Classifications

The architectures described in the paper are based on the Uptime Institute’s Tier classification system. This system defines the expected redundancy and fault tolerance of the data center infrastructure:

Tier I — Basic capacity, no redundancy: Impact on generator power plant: a single failure can lead to generator unavailability.
Tier II — Redundant capacity components: Impact on generator power plant: redundancy on emergency generators and electrical distribution is needed.
Tier III — Concurrently maintainable (Tier II + maintenance without impact): Impact on generator power plant: power plant may experience a blackout due to a failure, but the operator can manually reconfigure.
Tier IV — Fault tolerant (no single point of failure): Impact on generator power plant: power plant is fault tolerant, with automatic response to any failure.

Generator Power Plant Architectures Evaluated

The paper evaluates several specific architectures for generator power plant MV distribution. Each is designed to meet a specific Tier level with cost and reliability trade-offs.

Tier II Architecture

Simple N+1 Design: A single common MV switchboard with a basic protection system (overcurrent and directional protection). All generators connect to this switchboard, which supplies both power distribution paths.

Cost Base: The least expensive option
Reliability: Acceptable for many large data centers
Limitation: A major fault or scheduled maintenance on the MV switchboard can make the entire generator plant unavailable

Tier III Architectures

Double-Fed Architecture:

Each generator connects to two redundant MV switchboards (Gen-A and Gen-B) via two feeders
Only one feeder is closed at a time to maintain a simple protection system
Gen-A and Gen-B switchboards are interconnected by a tie

Open-Loop Architecture:

A further simplified design that removes the Gen-A and Gen-B MV switchboards
Each generator switchboard can be isolated for maintenance
In the event of a fault, the power plant experiences a blackout until manual reconfiguration

Key Finding: Tier III architectures cost 2 to 4 times more than Tier II but do not necessarily improve reliability.

Tier IV Architectures

Double-Fed with Automatic Re-Configuration:

Each generator connects to Main and Backup MV switchboards via two feeders
An automatic control system can switch feeders to maintain supply
Requires redundant controllers to avoid a single point of failure (SPOF)
More complex and costly than Tier III

Closed-Loop Architecture (Recommended):

A fault-tolerant, cost-optimized design
Uses busbar differential protection (87B) and line differential protection (87L) to isolate faults automatically
No complex control system is needed; service continuity is ensured by the protection system itself
Costs less than the Double-Fed Tier IV design

Cost Comparison Results

The study compared costs (including MV switchgear, cables, and installation) for different architectures, considering three short-circuit current levels (<20 kA, <25 kA, and <31.5 kA).

Tier II: Relative cost: 1.0 (Base) with 4x generators; 1.0 (Base) with 9x generators.
Tier III – Double-Fed: Relative cost: 2.5–6.3 with 4x generators; 2.8–7.1 with 9x generators.
Tier III – Open Loop: Relative cost: 1.8–4.5 with 4x generators; 1.7–5.2 with 9x generators.
Tier IV – Double-Fed: Relative cost: 3.4–7.4 with 4x generators; 3.4–7.9 with 9x generators.
Tier IV – Closed Loop: Relative cost: 2.0–4.3 with 4x generators; 2.0–6.0 with 9x generators.

Key Observations:

Moving from Tier II to Tier III increases MV distribution costs by 2 to 4 times
The cost gap between Tier III and Tier IV is smaller: 10% to 30%
The closed-loop topology saves about 20% on average compared to the double-fed design
Short-circuit current rating has a major cost impact: +27% average for 20 kA to 25 kA, and +61% for 25 kA to 31.5 kA

Reliability and Availability Findings

The study performed a detailed reliability analysis (using FMEA and fault tree analysis) on a representative large data center infrastructure with 9 generators and 4 distribution blocks.

For an N+1 Generator Plant

All Architectures: Mean failure frequency: ~0.0055 failures/year. Availability: ~99.99965% (five nines).

Key Insight: For an N+1 design, the generator power plant MV distribution architecture does not significantly affect the overall reliability or availability. The dominant failure mode is "loss of the HV/MV substation combined with loss of 2 generators." The MV architecture itself is not a major contributor to downtime.

For an N+2 Generator Plant

Tier II: Failure probability (over 10 years): 1/200. Availability: 99.9999%.
Tier III: Failure probability (over 10 years): 1/100. Availability: 99.9999%.
Tier IV (Closed Loop): Failure probability (over 10 years): 1/550. Availability: 99.9999%.

Key Insights:

Adding a second redundant generator (N+2) significantly improves reliability
Tier IV (Closed Loop) provides the highest reliability—about 2.7 times better than Tier III
Tier III architectures proved less reliable than Tier II in this study because they add more equipment (more failure points) without sufficient automatic fault tolerance

Recommendations

The paper concludes with clear recommendations based on cost and reliability analysis:

For Most Large Data Centers: A fully redundant architecture with a Tier II generator power plant and N+1 generators can be sufficient. It provides five nines of availability and a failure probability of 1/20 over 10 years at the lowest cost.
For Higher Performance Needs: Move directly to a Tier IV generator power plant (preferably the closed-loop architecture). Tier III designs do not improve reliability over Tier II for the additional cost.
For Ultra-High Reliability: Use N+2 generator redundancy with a Tier IV (Closed Loop) design. This achieves six nines availability and a failure probability of 1/550 over 10 years.
Consider the Loop Topology: The open or closed-loop configurations offer cost and footprint savings without sacrificing performance.
Study Generator Reliability First: Since the generator sets themselves (including controllers) are the primary contributors to failure sequences, the first design step should be analyzing generator reliability and redundancy needs before addressing the MV distribution architecture.

Download the Full White Paper

This overview is based on White Paper 262: "Optimized MV Generator Power Plant Architectures for Large Data Centers." The full document provides much greater depth, including:

Detailed architecture diagrams
Complete cost comparison tables and assumptions
Comprehensive reliability and availability study methodology
Failure mode analysis data
Appendices with reliability and maintenance data

The white paper is available to download for free directly from Schneider Electric:

Download White Paper 262 (PDF)

Note: This article provides an overview of a third-party white paper. While every effort has been made to summarize the key findings accurately, always refer to the official Schneider Electric document for the most up-to-date and detailed technical information. Final design responsibility rests with the licensed professional engineer in charge of the project.