Low Voltage Ride Through for VFDs in Continuous Production

In continuous production industries like polymer manufacturing, even a brief voltage sag can cause costly downtime. A voltage sag, often called a “momentary dip,” is a short-term reduction in voltage that can last from a few milliseconds to several seconds. For facilities relying on variable frequency drives (VFDs), these events are particularly disruptive. When the DC bus voltage inside a VFD drops below a critical threshold, the drive trips to protect itself, stopping the motor and interrupting the process. This article explores how low voltage ride through (LVRT) technology keeps VFDs running during voltage sags, ensuring uninterrupted production and product quality.

The Real Cost of Voltage Sags in Continuous Processes

In a polymer plant, processes like extrusion, spinning, and pelletizing are tightly coupled. A single VFD trip can halt an entire production line. For example, a pelletizer driven by a 37 kW motor might stop due to a sag lasting only 100 ms. The resulting downtime can lead to material waste, equipment cleanup, and lost output worth thousands of dollars per hour. Beyond financial losses, sudden stops can create safety hazards if material solidifies in equipment or if emergency systems are affected.

Traditional solutions like UPS systems are often impractical for large motor loads due to cost and size. Instead, LVRT devices target the VFD’s DC bus directly, providing a more efficient and compact solution.

How a VFD Works and Why It Trips

A VFD converts fixed-frequency AC power to variable-frequency AC to control motor speed. The process involves three stages:

1. Rectifier: Converts incoming AC (e.g., 380V three-phase) to DC. A typical DC bus voltage is around 520V.
2. DC Link: Filters and smooths the DC voltage using capacitors.
3. Inverter: Converts DC back to AC at the desired frequency and voltage.

When the grid voltage sags, the rectifier output drops, and the DC bus voltage falls. If it dips below the VFD’s undervoltage trip level (often around 70-80% of nominal), the drive shuts down to prevent malfunction. The key to ride-through is maintaining that DC bus voltage during the sag.

LVRT System Design: The Boost Module Approach

An LVRT system acts as a backup DC power supply for the VFD’s DC bus. It monitors the bus voltage and instantly injects power when a sag is detected. The core component is the voltage sag boost module, which includes:

• Rectifier and Boost Converter: Takes AC input, rectifies it, and boosts it to a regulated DC output (e.g., 490V ±5%).
• Energy Storage: Capacitors or batteries provide the ride-through energy. For short sags (up to a few seconds), capacitors are sufficient.
• Isolation and Control: Intelligent differential isolation modules prevent circulating currents between multiple boost modules and protect against reverse voltage from the load.

The system operates in hot standby. When the grid voltage is normal, the boost module is isolated. When a sag is detected (e.g., voltage drops below 90% of nominal, adjustable), the module switches in within microseconds, supporting the DC bus. Once the grid recovers, the module disconnects.

Real-World Application: Protecting Pelletizers and Extruders

Consider a polymer plant with multiple VFD-driven loads. The table below shows a typical set of equipment needing protection:

For these loads, an LVRT system can be configured in a 1:1 or 1:N arrangement. A single cabinet (e.g., 2260x800x600 mm for a 50 kW drive) houses the boost modules, DC distribution, and control. The system connects to the VFD’s DC bus via fuses and contactors, remaining isolated during normal operation.

Key Components and Their Functions

• System Controller with HMI: Displays AC input, DC output current, operating status, event logs with timestamps, and ride-through statistics. It also interfaces with plant safety systems (e.g., MFT) for coordinated shutdown.
• Voltage Sag Boost Module: The heart of the system. It uses a boost converter to maintain a stable DC output (e.g., 490V) even when input AC drops to 90% or lower. The threshold is adjustable, typically set to 80% of nominal voltage.
• Intelligent Differential Isolation Module: Prevents circulating currents when multiple boost modules feed a common DC bus, and blocks reverse voltage from the load.
• Hall Effect Sensors: Monitor DC output currents for each channel, enabling fault isolation and improving system reliability.
• DC Distribution: Includes DC circuit breakers, fuses, and contactors for each VFD connection. Large systems may have multiple output channels.

Installation and Integration Considerations

LVRT systems are designed for easy retrofitting. They are installed in parallel with existing VFDs, requiring no changes to the AC supply wiring or VFD parameter settings. The cabinet is placed near the VFD panel. A critical requirement is a stable control power source. If the plant has a UPS, it can power the LVRT controller directly. Otherwise, a small dedicated UPS or DC backup system is needed to keep the control circuit alive during a sag, preventing the LVRT from shutting down due to loss of its own control power.

The system includes safety features like an emergency stop button on the cabinet door and automatic disconnect on MFT (Master Fuel Trip) signals. It is designed so that a fault in one VFD or its LVRT channel does not affect others.

Operational Modes and Settings

The LVRT system operates in distinct modes:

• Normal Mode: Grid voltage within ±10% of nominal. Boost module is in hot standby, isolated from the DC bus.
• Ride-Through Mode: Voltage drops below the sag threshold (e.g., 90% of nominal, adjustable). The boost module instantly activates, regulating the DC bus to 490V ±5%. This supports the VFD for a preset duration (typically up to several seconds, depending on energy storage).
• Recovery Mode: Grid voltage returns above the recovery threshold (e.g., 90% of nominal). The boost module disconnects, and the VFD resumes normal rectifier operation.
• Fault/Emergency: On MFT signal or emergency stop, the system immediately disconnects and returns to standby.

Thresholds are adjustable via the HMI, allowing customization to site-specific power quality conditions. The system logs all events, helping engineers analyze sag patterns and system performance.

Benefits Beyond Downtime Prevention

Implementing LVRT technology offers several advantages:

• Process Continuity: Eliminates production stops due to voltage sags, preserving output and quality.
• Equipment Protection: Reduces mechanical stress from sudden stops and restarts, extending motor and drive life.
• Safety: Prevents hazardous conditions caused by unexpected process interruptions.
• Cost Savings: Avoids scrap, cleanup, and restart costs. Typical payback periods are often less than a year in sag-prone areas.
• Scalability: Systems can be sized for individual drives or groups, making them suitable for both small and large installations.

Selecting the Right LVRT Solution

When choosing an LVRT system, consider the following:

• Load Profile: Total kW of VFDs to be protected, and whether they can be grouped on a common DC bus.
• Sag Characteristics: Typical depth and duration of voltage sags at the site. This determines the required energy storage capacity.
• Space Constraints: Cabinet dimensions and installation location.
• Control Power: Availability of a reliable UPS for the LVRT controller.
• Integration: Compatibility with existing plant safety systems and communication protocols.

Modern LVRT systems are modular and can be expanded as needs grow. They are a proven, cost-effective way to enhance power quality resilience in critical manufacturing processes.

Conclusion

Voltage sags are an unavoidable reality in industrial power systems, but their impact on VFD-driven processes can be mitigated. Low voltage ride through devices provide a targeted, efficient solution that keeps motors running and production lines moving. By maintaining DC bus voltage during disturbances, these systems ensure the reliability and profitability of continuous manufacturing operations. For plants looking to eliminate sag-related downtime, LVRT technology is a smart investment.