1. Introduction
As the global energy transition accelerates and market competition intensifies, the oil&gas industry faces multiple challenges, including improving energy efficiency, reducing operational costs, and lowering carbon emissions. Ethane recovery is a critical process for enhancing the added value of natural gas and ensuring the supply of downstream chemical feedstocks. The energy consumption and operational stability of this process directly impact a project's economic and environmental benefits. Among the numerous high-energy-consumption equipment in ethane recovery units, key equipments driven by high-power medium voltage motors—such as compressors, pumps, and fans—accounts for over 60% of total energy consumption.
Medium voltage variable frequency drive (VFD) technology is one of the most advanced energy-saving technologies in the field of electrical drives today. By smoothly adjusting the motor's operating speed through changes in the power supply frequency, it enables precise control of load flow and pressure, thereby achieving significant energy savings (typically 20%–40%). Therefore, employing medium voltage VFD systems that combine grid adaptability and high reliability is crucial for minimizing shutdown losses and ensuring long-term project returns.
After extensive research and evaluation, the end-user ultimately selected the Shanghai Nancal Electric NC HVVF series high-capacity water-cooling medium voltage drive system for the project's motors. The power cell fast automatic bypass design and the Low Voltage Ride Through (LVRT) capability of Nancal Electric's medium voltage drives address internal cell failures and external grid voltage sags, respectively, effectively preventing unplanned shutdowns caused by such issues.
2. Technical Solution
The installed power of the project's natural gas export compressor unit is 27.7MW. The matched VFD model is NC HVVF 10/10-32000LO. The single line diagram is as follows:
Figure 1: SLD of the NC HVVF 10/10-32000LO VFD System
To further enhance the system reliability, Nancal Electric configured two reliability-enhancing functions for this project's medium voltage drive:
2.1 Power Cell Fast Automatic Bypass Design
When a power cell fails or a control component is damaged, the power cell bypass function ensures that the drive continues to operate stably without shutdown, meeting the project's high-reliability requirements. During actual operation, if a power cell fails, the drive employs fast automatic cell bypass technology to bypass the faulty cell. The specific bypass process is as follows:
(1) Upon detecting a cell failure, the drive immediately blocks pulse output to that cell.
(2) After pulse blocking, the bypass contactor for the faulty cell closes immediately, physically bypassing it.
(3) Once the bypass contactor is closed, the drive resumes pulse output, and the motor returns to normal operation.
Figure 2: Comparison Before and After Power Cell Bypass
The following figure shows simulation waveforms of normal output phase voltage and line voltage:
Figure 3: Simulation Waveforms of Normal Output Phase Voltage and Line Voltage
The following figure shows simulation waveforms of output phase voltage and line voltage after bypassing one cell in Phase A. It can be observed that although the phase voltage of Phase A decreases and the phase angle differences are not 120° (becoming 128° and 104°), the final output line voltages remain balanced with 120° phase angle differences.
Figure 4: Simulation Waveforms of Output Phase Voltage and Line Voltage After Bypassing One Cell in Phase A
The following figure shows an on-site switching waveforms with the power cell bypass function active:
Figure 5: On-site Switching Waveforms During Power Cell Bypass
The entire bypass process takes <200ms. The motor speed drop is minimal, and the motor voltage remains at a relatively high amplitude, allowing the drive to continuously track the motor's current speed.
The following figure shows the measured output line voltage waveform after bypassing one cell. It demonstrates that the output line voltages remain symmetrical.
Figure 6: Measured Output Line Voltage Waveform After Bypassing One Cell
2.2 Low Voltage Ride Through (LVRT) Function
When transmission lines experience grid voltage fluctuations, short-term sags, or instantaneous interruptions due to severe weather such as lightning strikes, snowstorms, or icing, the advanced ProCon (Process Continuous) technology integrated into Nancal Electric's NC HVVF series medium voltage drives can maximize continuous equipment operation.
ProCon integrates multiple technologies, including Kinetic Energy Buffering Low Voltage Ride Through (LVRT), Spinning Load Pick Up, Intelligent Torque limiting for Square-torque Loads, Enhanced PWM Modulation and etc., effectively coordinating these functional modules.
When the grid-side voltage sags by 15%–40%, the NC HVVF series medium voltage drive reduces speed and limits output torque, maintaining operation at a derated capacity. When the grid voltage drops below 60% or even during a complete grid power loss, the Kinetic Energy Buffering LVRT function converts the rotational kinetic energy of the motor and load into electrical energy to recharge the drives's power cells, maintaining cell voltage and preventing shutdown due to undervoltage.
Figure 7: On-site Photo of Nancal Electric Medium Voltage Drive
Figure 8: Measured LVRT Waveform (Riding Through Zero Voltage for 2.07s)
3. Basic Configuration
Based on motor parameters and load requirements, the basic configuration of the Nancal Electric medium voltage drive is as follows:
4. Conclusion
In summary, equipping key compressor loads in ethane recovery units with highly reliable medium voltage drives represents a strategic investment integrating advanced technology, economic benefits, and environmental friendliness.
The high-capacity water-cooling medium voltage drive supplied by Nancal Electric for this project serves as a critical link for energy saving and consumption reduction in the compressor unit's eDrive system within the ethane recovery project. Through over three years of stable and reliable operation, deeply integrated with systems such as natural gas processing and waste heat utilization, it has provided strong technical support for the oilfield's green, efficient production and deep resource utilization.
Since commissioning, this medium voltage drive has withstood multiple grid voltage sags of varying severity, significantly ensuring the stability of the continuous production process and improving production efficiency.