Technical Insights | Understand the Differences and Setting Rules for Voltage Overlimit, Voltage Deviation, and Undervoltage in One Article!
Release Date:
2025-09-24
Technical Insights | Understand the Differences Between Voltage Overlimit, Voltage Deviation, and Undervoltage—and Their Setting Rules—in One Article!
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In the daily operation and maintenance of electrical systems, “voltage anomaly” alarms sound frequently. Yet have you ever wondered: what exactly are the differences among concepts that sound so similar—such as voltage overlimit, poor voltage quality, undervoltage protection, and low-voltage protection?
On the surface, it all looks like “voltage anomalies,” but the underlying mechanisms and objectives are worlds apart! If “worse” is mistakenly set as “undervoltage,” it may cause the protective device to Maloperation or refusal to operate , leading to serious accidents. Today, we will thoroughly clarify these key concepts.
I. Why Is It Easy to Get Confused? First, Clarify the Core Objective!
Voltage-based protections are easily confused because they all monitor the same physical quantity—voltage—yet each one Triggering principle, action logic, and ultimate purpose Completely different. Simply put:
Some only “report the alarm” and ignore tripping. (e.g., voltage out-of-limit).
Some are specifically “locked out” to prevent accidental tripping. (e.g., lockout when voltage deviation exceeds the limit).
Only some are the protections truly responsible for “tripping.” (such as undervoltage and low-voltage protection).
Next, let’s quickly grasp the core differences through a table.
II. Overview of Core Differences
| Protected Name | Trigger Condition | Objective of Action | Common Application Scenarios | Action Result |
|---|---|---|---|---|
Voltage out of limits | Voltage is too high or too low, exceeding the normal operating range. | Monitoring and Early Warning | Substation and Distribution Room Monitoring System | Issue an alarm signal , no tripping |
Lockout when voltage is poor | Three-phase voltage unbalance exceeds the set value. | Prevent protective maloperation | Interlocking Logic in Various Protective Devices | Locking Related protection tripping functions |
Undervoltage protection | System Voltage Continuous Below the rated value | Protect electrical equipment | Low-voltage motors, low-voltage lines | Trip after time delay , to prevent equipment damage |
Low-voltage protection | Voltage Sudden severe drop Below the limit value | Ensure system stability | Generator, critical busbars | Fast tripping or switch the power supply |
III. Detailed Explanation of Four Types of Voltage Protection
1. Voltage Out-of-Limits: The System’s “Sentinel”
Positioning : The first line of defense for operational monitoring, serving as an “early warning” function.
Function : When the voltage deviates from the normal range (e.g., ±10% of Un), an alarm is triggered for the operating personnel to indicate an abnormal trend in the system, facilitating Early intervention , to prevent the situation from worsening.
Setting Rules : Typically set at ±10% of the rated voltage. For example, in a 220 V system, the over- and under-voltage alarm thresholds can be set at 198 V (lower limit) and 242 V (upper limit). Even if the voltage drops as low as 170 V, only an alarm will be triggered, while It will not trip directly. 。
2. Voltage Degradation Lockout: The “Safety Valve” of Protection
Positioning : an important Auxiliary Protection Logic , the core function is “locking” rather than “action.”
Function : When a PT (potential transformer) open-circuit occurs, its fuse blows, or a severe asymmetric short circuit takes place, it can lead to severe three-phase voltage imbalance. Under such conditions, certain protective functions (such as low-voltage protection) may misoperate. The voltage-quality-based blocking function is capable of detecting this abnormality and Immediately lock out protections that may operate erroneously. , thereby enhancing the reliability of protection.
Setting Reference : Typically, the setting is 15% to 20% of the rated voltage. For example, if the system’s rated voltage is 220 V, phase A and phase C are at 230 V, while phase B is at 180 V—a difference of 50 V (approximately 23%)—the protective device will be locked out.
3. Undervoltage Protection: The “Guardian” of Your Equipment
Positioning : For electrical equipment Time-delay backup protection 。
Function : Preventing motors and other equipment from Long-term low voltage Operating under such conditions while faulty can lead to overheating, reduced efficiency, and even burnout. It will only trip when both “low voltage” and “long duration” are present simultaneously.
Tuning Method ：
Operating voltage : Typically set at 70% to 80% of the rated voltage (e.g., for a 380 V system, set around 300 V).
Action delay : Typically ranges from 0.5 to 5 seconds, requiring sufficient time to avoid transient voltage fluctuations in the power grid and the operating time of upstream circuit breakers.
4. Low-Voltage Protection: The System’s “First Responder”
Positioning : Responding to Severe Power Grid Faults Fast Primary Protection 。
Function : When the system experiences a fault such as a short circuit, causing the voltage Instant collapse At such times, promptly shed non-essential loads or disconnect generators to prevent system collapse and ensure power supply to critical loads.
Key Differences ：
Undervoltage protection It is aimed at “slow, sustained” voltage drops.
Low-voltage protection It is designed to address “sudden, severe” voltage sags, with lower setpoints and faster operation.
To summarize:
Don’t panic when you see “over limit” —It serves as an “alarm” to alert operations and maintenance personnel.
Be on guard when you encounter “the worse” —It serves as a “fuse” to prevent unauthorized operation.
“Undervoltage” tripping to protect equipment —It is the “time-delay fuse” for electrical equipment.
“Low-pressure” action to safeguard the system —It is the “emergency brake” for the entire power system.
Accurately understanding and properly setting these protective relays is the cornerstone of ensuring the safe and stable operation of power systems. It is our hope that this article will provide you with a clear understanding.
Hubei RuiNeng Electric specializes in power system protection and control technologies, ensuring the safe and stable operation of your power grid!
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