ถอดรหัสกราฟแรงดันตก Sag/Dip 📊

Understanding Voltage Sag and Dip Graphs: A Comprehensive Guide to Analysis, Planning, and Decision-Making

At Truewatts, we have had the opportunity to assist numerous clients in analyzing data from Power Quality Meters (PQ Meters), particularly regarding voltage sag events. These events often come with common questions such as:

👉 “What do the values in the graph mean?”

👉 “How can this data be utilized further?”

This article will guide you in effectively understanding Voltage Sag/Dip graphs. We will provide insights into analysis, planning, and decision-making from a technical perspective.

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🧩 Are Voltage Sag and Voltage Dip Different?

Both terms refer to “temporary voltage drops” in electrical systems. However, they differ based on the standards used for reference:

• ✅ Voltage Sag: Commonly used in North America, such as IEEE and EPRI standards.

• ✅ Voltage Dip: Used in Europe and international standards like IEC.

  
  

According to the IEEE 1159 definition:

"A decrease to between 0.1 and 0.9 per unit in RMS voltage or current at the power frequency, for durations of 0.5 cycle to 1 minute."

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📉 Example Graphs from PQ Meters

The images below are taken from measurement points at the MDB using a Power Quality Meter.

These graphs illustrate the behavior of voltage drops over time (Voltage Sag/Dip).

🔍 Data from the Graph

• Voltage dropped to 70.62% of nominal.

• Duration: 0.689 seconds.

• Clear identification of the date and time of the event.

  
  

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🧠 Why Are Magnitude and Duration Important?

To accurately analyze the impact of Voltage Sag, two values must be considered together:

🔹 Magnitude – This refers to the depth of the voltage drop compared to the nominal voltage, expressed as a percentage. For example, 70% or 50%.

• If below the device's tolerance threshold, there is a risk of tripping or resetting.

• For instance, PLCs typically tolerate 80–90%, while inverters can handle 70–80%.

  
  

🔹 Duration – This is the length of time the voltage sag occurs, such as 0.1 seconds or 0.7 seconds.

• If the sag lasts only 20–40 ms, some devices may still function.

• However, if it exceeds 100–200 ms, the risk of tripping increases significantly.

  
  

💡 If the voltage sag drops to 70% for 0.689 seconds, it is considered "deep and long." Most devices cannot withstand this.

📌 This value can be compared against standards such as:

• SEMI F47

• IEC 61000-4-11

• To plan for appropriate protective equipment.

  
  

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🎯 What Can You Do with This Information?

1️⃣ Analyze the Causes of Machine Downtime

• If, at the same time as the event, there are PLC resets, inverter trips, or production halts, it indicates that Voltage Sag is the primary cause.

• This helps reduce wasted time searching for other causes.

  
  

2️⃣ Assist in Investment Decisions for Protective Equipment

• For example:

- Voltage Compensation Systems

- UPS for Control Circuits

✅ In this case, the sag lasted 0.7 seconds, exceeding the tolerance of most devices (e.g., PLCs can only handle 20–100 ms). Therefore, consider installing compensation systems at critical points.

3️⃣ Serve as Technical Evidence

• If there are damages or production halts, this data can be included in reports to:

- Utility companies

- OEM equipment manufacturers

→ For verification or claims based on sag immunity specifications.

4️⃣ Compare with Sag Tolerance Standards

• Use the data:

- Voltage at 70.62%

- Duration at 689 ms

→ Compare with curves such as:

- CBEMA / ITIC

- SEMI F47

- IEC 61000-4-11

If the equipment should withstand it but trips, the design of the control circuit may need to be reviewed.

5️⃣ Plan Electrical System Improvements

• Install permanent PQ Meters or consider relocating measurement points.

• Separate critical and non-critical loads.

• Assess the feasibility of installing ATS or Fast Transfer systems.

  
  

📬 If you want to maximize the benefits from power quality event data or are considering equipment to mitigate voltage sag issues, we are here to assist: Info@truewatts.co.th