AC UPS System – Non-Redundant Configuration

Single Line Diagram (SLD) – Component & Operating Philosophy Explained

A Non-Redundant (Single Module) AC UPS System is designed to provide uninterrupted power to critical loads using one complete UPS path (no parallel backup module).

Though non-redundant, it must still ensure high reliability, protection, and monitoring.

Below is a structured technical breakdown 👇

🔹 1️⃣ Major Components & Their Significance

⚡ 1.1 AC Incomer (UPS Normal Supply)

Source: 415V, 3-Phase, TPN, 50 Hz from LV Switchgear

Components:

SFU / MCCB (Short-circuit & overload protection)
Properly sized 3.5C cables (Phase + Neutral continuity)

Significance:
Primary input supply for the UPS during normal operation.

⚡ 1.2 Input Isolation Transformer

Provides galvanic isolation between grid and UPS
Reduces harmonics and electrical noise
Eliminates ground loops
Enhances safety and reliability

Especially important in industrial and process environments.

🔌 1.3 Rectifier / Charger

Converts AC → DC
Supplies DC Bus
Charges battery in Float / Boost mode
Maintains stable DC voltage

Acts as the front-end power conditioning stage.

🔋 1.4 Battery Bank (Ni-Cd / VRLA / Lithium-Ion)

Connected through DC MCCB
Supplies DC power during mains failure
Blocking diode prevents reverse current flow
Designed based on required autonomy (e.g., 30 min / 1 hr / 2 hr)

Battery health directly impacts system reliability.

🔄 1.5 Inverter

Converts DC → Clean, regulated AC
Maintains stable voltage and frequency
Supplies critical loads continuously

This is the heart of the UPS system.

🔀 1.6 Static Switch

Fast electronic switching device between:

Inverter Output
Bypass Supply

Key Feature:
Transfer time typically in milliseconds (virtually uninterrupted supply).

Used during overload or inverter failure.

⚡ 1.7 Bypass Incomer & Bypass Transformer

Independent 415V AC bypass source
415V / 110V Bypass Transformer (3Φ / 1Φ as required)
Voltage stabilizer ensures regulated bypass voltage

Used during:

Inverter failure
UPS maintenance
Overload conditions

🔹 2️⃣ System Operating Philosophy – Step by Step

🟢 A. Power Flow Modes

1️⃣ Normal Operation

AC Incomer → Rectifier → DC Bus → Inverter → Load
Battery remains in Float Charging Mode.

2️⃣ Mains Failure

Battery → Inverter → Load

✔ Zero transfer time
✔ Continuous supply to critical loads

3️⃣ Inverter Fault / Maintenance

Static Switch transfers load to Bypass Supply automatically.

✔ Millisecond transfer
✔ Maintains supply continuity

🔹 B. Protection Philosophy 🛡️

✔ Incomer SFU / MCCB – Short-circuit & overload protection
✔ Rectifier & Inverter internal electronic protection
✔ Battery MCCB – DC fault isolation
✔ Output Circuit Breaker – Load protection
✔ Blocking Diode – Battery reverse current protection
✔ Over-temperature & DC earth fault detection

Proper coordination ensures system safety.

🔹 C. Metering & Monitoring 📊

AC Incomer & Bypass Line:

Voltmeter (0–500V)
Ammeter
Frequency meter
Phase indication (R-Y-B)

DC Side:

DC voltage monitoring
Battery charge/discharge current

UPS Output:

Voltage (V)
Current (A)
Frequency (Hz)
Load percentage

Real-time monitoring ensures operational visibility.

🔹 D. Control Functions 🎛️

✔ Rectifier mode selection (Float / Boost)
✔ Inverter ON/OFF control
✔ Static switch auto-transfer logic
✔ Battery charging management
✔ Manual / Auto bypass control

Control logic ensures seamless transitions.

🔹 E. Status Indications 💡

Typical LED/Display Indications:

✔ Rectifier ON
✔ DC Output Healthy
✔ Battery Charging / Discharging
✔ Inverter ON
✔ Load on Inverter
✔ Load on Bypass
✔ Static Switch Active
✔ Inverter Synchronised with Mains

Clear indication reduces troubleshooting time.

🔹 F. Communication & SCADA Interface 🌐

Monitoring via:

Potential-free contacts
Modbus / Ethernet
SCADA integration

Typical signals include:

✔ Rectifier status
✔ Inverter status
✔ Battery health
✔ Alarm & Trip signals
✔ UPS Healthy / Fault

Remote monitoring improves reliability.

🔹 G. Annunciation & Alarms 🚨

Common Alarm Windows:

🟥 AC Fail
🟥 Rectifier Fail
🟥 Inverter Fail
🟥 Battery Low Voltage
🟥 DC Earth Fault
🟥 Static Switch Failure
🟥 Bypass Active
🟥 Overload / Short Circuit

Proper annunciation ensures quick fault diagnosis.

🎯 Key Takeaway

A Non-Redundant UPS system may have only one power path —
but its design, protection, monitoring, and control philosophy must still be robust.

Good UPS engineering ensures:

✔ Continuous power
✔ Fast fault detection
✔ Safe isolation
✔ Reliable communication
✔ Controlled transfer logic

💬 Have you encountered challenges with single UPS systems — especially during inverter failure or bypass transitions?
Let’s discuss practical site experiences.

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Friday, February 20, 2026