[Webinar Recap] Utilizing Reactive Power Compensation for Mitigating Voltage Sags Due to Motor Starting

Voltage sags during motor starting can create serious challenges for industrial and utility systems. From equipment disruptions to insufficient starting torque, voltage dips can lead to operational inefficiencies, increased wear on electrical components and even unexpected shutdowns.

Powerside Senior Technical Fellow Chris Duffey’s webinar takes on the challenge of voltage sags during motor starting—why they happen, how to predict them, and most importantly, how to mitigate them effectively. He explores the importance of properly adopting the right simulation methods for accurately modeling these events, and demonstrates how expert application of reactive power (VAR) compensation can be a game-changer in reducing their impact.

Keep reading for webinar highlights, and watch the full recording on-demand for additional detail.  

Why Do Voltage Sags Happen?

When large motors start, they draw a significant amount of current, often leading to voltage sags. Several factors contribute to the severity of these sags, including:

• Poor power factor at motor startup (as low as 0.10 to 0.25)
• Utility system impedance
• The inability of reduced voltage starters to sufficiently lower inrush current

Unfortunately, key details like the starting power factor are often missing from standard motor data, making it difficult to fully assess motor impact.

Using Simulations to Understand Motor Starting Impacts

To accurately predict voltage sag events and optimize mitigation strategies, various simulation techniques can be used, ranging from basic power flow analysis to advanced finite element modeling:

• Power Flow Analysis: A simple method but lacks time-domain insights.
• Hybrid Simulations: Offer a more detailed view by incorporating motor load behavior.
• Dynamic & Transient Simulations: Essential for capturing grid-wide and real-time motor performance.
• Finite Element Simulation: So highly detailed that it becomes impractical for voltage sag analysis.

Among these methods, transient simulations using PSCAD stand out as a valuable tool. PSCAD can reveal switching transients, soft-start distortion effects and real-time equipment responses, making it particularly useful for analyzing and mitigating voltage sag issues.

Strategies for Voltage Sag Mitigation

While simulations are crucial for understanding the problem, they do not provide direct solutions. Once the issue is well-defined, the next step is identifying effective mitigation strategies to reduce voltage sags and ensure stable motor performance.

Reducing system impedance or upgrading transformers can help, but these approaches can be costly and impractical. Alternative mitigation methods include:

• Reduced Voltage Starters & Soft Starts: These reduce voltage sags but extend motor starting time and fail to address poor power factor.
• Wound Rotor & Wye-Delta Motors: Help lower current draw but are dependent upon having these specific motor types already installed.
• STATCOMs & Adjustable Speed Drives: Do mitigate sag but introduce harmonic distortion and are prohibitively expensive in most cases.

Since many of these methods come with limitations, another approach is needed—one that effectively improves voltage stability while optimizing motor performance. This is where switched VAR compensation comes in.

Enhance Motor Starting with Switched VAR Compensation

Switched VAR compensation is a highly effective approach that reduces net starting current and improves voltage stability by injecting reactive power. This compensation method helps motors start more efficiently with less strain on the electrical system—meaning that power factor is also improved.

Powerside’s PowerMVar and SynchroMVar medium voltage metal-enclosed capacitor and filter banks utilize switched VAR compensation to do this and more, including:

• Reduce net starting current magnitude
• Shorten motor starting time
• Minimize voltage sag depth
• Provide harmonic filtering to mitigate soft-start distortions
• Ensure transient-free switching with SynchroMVar’s point-on-wave technology

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