Technical Details- AVR
1. Overview
An Automatic Voltage Regulator (AVR) is a device used with power transformers to maintain a constant output voltage regardless of load or input voltage fluctuations. It plays a crucial role in voltage regulation, grid stability, and power quality.
2. Applications
- Power distribution transformers
- Industrial power systems
- Generator step-up (GSU) transformers
- Grid substations
- Renewable energy integration (solar/wind transformers)
3. Technical Specifications
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Parameter
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Typical Values / Ranges
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|---|---|
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Input Voltage
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6.6 kV, 11 kV, 22 kV, 33 kV, or higher
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Output Voltage Regulation Range
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±10% to ±20% of rated voltage
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Response Time
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10–30 milliseconds (fast acting)
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Accuracy
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±1% or better
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Control Mode
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Manual / Automatic (with feedback loop)
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Power Rating
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Matches transformer kVA/MVA rating
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Type
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Tap changer-based or electronic-based
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Cooling
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Same as transformer (ONAN, ONAF, etc.)
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4. Working Principle
AVRs maintain the transformer output voltage within a pre-set range by adjusting the transformer tap positions using:
a. OLTC (On-Load Tap Changer) AVR
- Installed on high-voltage winding side.
- Adjusts winding turns ratio without interrupting the load.
- Controlled by AVR relay based on sensing the output voltage.
b. Electronic AVRs (for small units or generators)
- Use power electronics (SCRs, IGBTs) for voltage regulation.
- Suitable for low-voltage or medium-voltage applications.
5. Key Components
- Voltage sensing circuit (typically PT input)
- Control relay/unit (AVR relay with setpoint adjustment)
- Tap changer mechanism (motorized)
- Current limiter and protection circuitry
- Feedback loop for maintaining accuracy
6. Protections & Alarms
- Overvoltage/Undervoltage protection
- Tap position limiters
- Blocked tap changer alarms
- AVR relay failure
- Manual/Auto changeover switch
- Voltage imbalance protection
7. Design Standards
- IEC 60214 – Tap-changers
- IEC 60076-1 – Power Transformers
- IS 8478 – AVR for synchronous machines
- IEEE Std C57.131 – Guide for Tap Changer Operation
- IS 1180 Part 1 – Distribution transformers (India)
8. Advantages
- Stable output voltage under varying load conditions
- Enhanced equipment life due to voltage consistency
- Improved system reliability and efficiency
- Uninterrupted tap changing under load (OLTC-based)
- Protection against voltage fluctuations
9. Limitations
- Mechanical wear and tear in OLTC mechanisms
- Requires maintenance of tap changer contacts
- Limited speed of regulation compared to fully electronic systems
- Initial cost and complexity for large installations
10. Use Case Example:
For an 11 kV/415 V, 2 MVA transformer, an AVR with OLTC can regulate output voltage at 415 V ±5% by automatically adjusting the tap changer according to the load voltage.
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