Under the "Dual Carbon" strategy, China's chemical industry is accelerating energy structure adjustments and production equipment upgrades. Leading companies are transitioning from traditional steam-driven systems to electric-driven systems, particularly for large compressor units. This shift not only reduces energy consumption and carbon emissions per unit of product but also enhances intelligent production through digital transformation, achieving both environmental and economic benefits.
The breakthrough in large capacity medium voltage drives (MV VFDs) has been pivotal in enabling this transition. By providing precise speed control, MV VFDs allow compressors to operate at optimal efficiency across varying conditions, addressing the energy waste inherent in traditional power frequency drives.
Regarding the reliability of electric drive systems and equipment, the client conducted thorough research and compared multiple technical solutions currently available, including VFD soft start to grid power, redundant parallel operation of multiple motors with VFDs and full-system redundancy configurations (1+1 or 2+1 backup for VFD-driven systems).
Additionally, a comprehensive evaluation was performed comparing imported and domestic medium voltage drive brands in terms of performance, cost, reliability, and after-sales service.
Ultimately, Nancal Electric's high-capacity water-cooling medium voltage drive stood out due to its superior performance and high-reliability technological advantages. It was successfully deployed in a centrifugal compressor system at an aromatic hydrocarbon plant, demonstrating Nancal's core value in process industries.
The aromatic hydrocarbon plant in question is one of the largest single-train crystallization process facilities in China, with an annual production capacity of 1.6 million tons and a required annual operation time exceeding 8,000 hours. The compressor unit is designed without a backup machine, utilizing a domestically manufactured 3MCL1106 centrifugal compressor with a variable speed range of 60%–110% and a single-motor power of 30 MW.
Motor Specifications:
Based on the customer's technical requirements and motor parameters, the voltage-source cascaded multilevel water-cooling medium voltage drive of Nancal Electric was selected. The system's hardware primarily consists of oil-immersed phase-shifting transformer, power cells, control system and cooling system.
Key Component Features:
(1) Oil-Immersed Phase-Shifting Transformer
High-efficiency heat dissipation
Short-circuit resistant design
Multi-layer sealing technology
Delivers high reliability and extended service life in harsh industrial environments, with a low failure rate
(2) Water-Cooling System
Redundant pumps
Dual-path switching mechanism
Intelligent monitoring system
Corrosion-resistant materials
Significantly enhances system fault tolerance and reliability
(3) Overall System Reliability
Primarily dependent on power cells and control system performance.
NC Water-Cooling MV VFD Key Specifications:
Figure 1. Single Line Diagram of the Variable Frequency Drive System
2.1 Structure and Features of Water-Cooling Power Cells
The water-cooling power cell adopts an integrated and modular design, comprising the following key hardware components: rectifier circuit, DC bus circuit, inverter circuit and communication control trigger circuit.
Technical Specifications:
Utilizes IGBTs of 3300 V as primary switching devices
Power cell voltage: 1450 V
Power cell current: 2100 A
locomotive grade IGBTs
long-life film capacitors
Cooling System of Power Cell:
Rectifier diodes and IGBT modules are mounted on water-cooling plates
Simplified hydraulic design with single inlet/outlet configuration (minimizing potential leakage points)
Incorporates 60° tapered-thread connections for secure piping
Advanced Drive Circuit Technology:
Features multi-stage automatic gate resistance switching during operation (reducing switching losses and du/dt by approximately 10%)
Integrates dynamic active voltage clamping technology for effective suppression of IGBT voltage spikes
The power cell is equipped with a cell bypass function. When one cell fails, the faulty cell can be automatically bypassed, allowing the equipment to continue operating without shutdown and maintain full-load operation. In case of two or more cell failures, the system can operate at derated capacity. After bypass, there is a definitive mechanical isolation between the bypassed cell and the system, ensuring that internal faults of the bypassed cell will not affect overall equipment operation, thereby meeting the reliability requirements of this project.
2.2 Redundant Design of the Main Control System
The main control system adopts a triple-core (DSP+FPGA+ARM) architecture with clear functional division and high operational efficiency. The main control board features:
All-in-one design with high integration
Elimination of oxidation and loosening risks from board-to-board connectors
Enhanced anti-interference capability
Fully sealed dust-proof enclosure that prevents both external moisture ingress and conductive dust contamination
Additionally, the system implements comprehensive redundancy through redundant main control boards, redundant control power supplies, redundant voltage/current sampling and redundant fiber optic connections between control system and power cells. This complete redundancy configuration enables automatic switchover within 1 ms, ensuring continuous operation without shutdown and maintaining uninterrupted production processes.
Figure 2. Redundancy Design Diagram of Main Control System
2.3 Low Voltage Ride Through (LVRT) Function
In addition to reliable hardware design, to effectively address grid transient voltage fluctuations and maximize the continuity and safety of industrial production, this MV VFD is equipped with LVRT function. It can maintain long-term derated operation under grid voltage dips of up to -40% and achieve zero-voltage ride-through, significantly enhancing system operational continuity during grid fluctuations or momentary power loss.
For scenarios such as severe weather conditions, startup of large equipment within the plant, or grid section switching that cause short-term power supply fluctuations, digs, or even interruptions - when the duration of voltage dig causes the power cell voltage to fall below a specified threshold - the LVRT function activates. Through integrated technologies including kinetic energy buffering, intelligent torque limiting for square-torque loads, and dynamic speed tracking, it optimally handles power supply instability, ensuring minimal external disturbance to equipment operation and maintaining continuous operation of both main and auxiliary systems.
Figure 3. Diagram of Low Voltage Ride Through (LVRT) Function
2.4 Bumpless Synchronization Transfer
From the perspective of overall production system design, the medium voltage drive's bumpless synchronization transfer function can reduce inrush current, ensure equipment safety, maintain production continuity, and optimize energy efficiency, overcoming the limitations caused by high current impact in traditional transfer.
Bumpless transfer requires real-time tracking of grid voltage amplitude, frequency and phase, while adjusting the drive's output parameters to achieve synchronization. This places high demands on phase-locked loop accuracy and anti-interference capability.
The synchronization transfer process is as follows:
(1) Close incoming circuit breaker QF13 to initiate medium voltage pre-charging;
(2) After pre-charging completes, close QF1 to energize the drive, then close QF11 after drive self-check completes;
(3) Upon receiving start command from DCS, the drive accelerates motor to ≥90% of rated speed;
(4) Upon receiving the transfer command from the DCS, the drive drives the motor to reach rated speed and adjusts its output voltage amplitude and phase to ensure the frequency, amplitude and phase of drive output voltage match the grid voltage, maintaining these parameter deviations within thresholds for specified duration. Then it closes power frequency circuit breaker QF12, stops drive output, and opens circuit breakers QF11 and QF13.
(5) Motor continues operation at power frequency.
After the Nancal Electric water-cooling medium voltage drive was positioned on site, based on the installation and commissioning procedures, the equipment underwent component verification, on-site power cell installation, copper busbar connection, water circuit system testing, reverse power transmission testing, VF testing, no-load function testing and verification with the motor, and full-load testing and adaptation, etc. The entire process was implemented as planned. In particular, the above-mentioned testing and verification of the redundant function of the main control system, the testing and verification of the low voltage ride through function, and the bumpless synchronization transfer function, which the customer was concerned about, all met the customer's technical requirements.
Figure 4. NC MV VFD On-Site
Figure 5. LVRT Function Test (Zero-Voltage Ride Through for 3.7 s)
Figure 6. Output Voltage Waveform During the Bumpless Synchronization Transfer
Electric drives, enhanced by Nancal's high-reliability MV VFD technology, offer faster start-ups, superior disturbance resistance, and proven cost savings. Over two years of operation, the system achieved zero unplanned downtime, meeting all process and reliability requirements for aromatic hydrocarbon production.
The electric drive system features short start-stop times and strong anti-disturbance capabilities. Coupled with the high-reliability technology of the high-reliability MV VFD technology, the transformation from steam drive to electric drive has become the core path for the energy efficiency upgrade in the chemical industry, and its technical maturity and economic benefits have been widely verified. This article introduces the unique high-reliability technologies and functions of the Nancal Electric water-cooling medium voltage drive. Over two years of operation, the system achieved zero unplanned downtime, meeting all process and reliability requirements for aromatic hydrocarbon production.