Application of Nancal Medium-Voltage Drive in Redundant Retrofit of Propylene Compressors

In recent years, driven by national goals for energy conservation, emission reduction and green production, many leading domestic chemical companies have transitioned their large compressors from steam-driven to electric-driven systems. This shift significantly reduces energy consumption, air pollution and greenhouse gas emissions, while delivering substantial economic benefits. However, users have faced challenges regarding the stability and reliability of electric-driven systems. With the rapid advancement and maturation of high-capacity medium-voltage frequency drive technology, electric drives for large-scale compressors have gained wider acceptance in the chemical industry.

In a chemical gasification plant, the propylene compressor plays a crucial role with high demands for operational reliability and stability. Equipment failure can have severe impacts on the plant and downstream production. The plant originally used an imported medium-voltage drive, but its failure rate has been increasing. A significant failure, especially one that cannot be quickly repaired, could halt gasification operations, disrupt downstream production, and lead to substantial economic losses.

After extensive research and technical discussions, the user selected Shanghai Nancal’s medium-voltage drive for hot standby, enabling the redundant automatic switching technology of NC HVVF to resolve issues caused by failures in the original imported drive and ensure uninterrupted production.

To enhance system reliability, this retrofit project incorporates a Nancal medium-voltage drive of the same capacity as the original. This forms a “1-operation and 1-standby” system, ensuring redundancy across the entire machine. When the original drive fails, Nancal’s hot standby technology automatically takes over, maintaining operation without interrupting the gasification process, minimizing speed fluctuations and ensuring load stability.

Under normal conditions, the original imported drive manages frequency conversion and speed regulation. If it fails, the hot standby drive must take over within 200 ms, quickly stops falling speed of motor rotation and recovering torque. The hot standby drive continues operation as needed. Once the original drive is repaired, the system can revert back to the original, or the standby drive can maintain control if system fails revert back, greatly enhancing reliability.

Fig.1 Single Line Diagram of Nancal frequency drive

The system utilizes several high-reliability functions to enhance performance:

• Control System Redundancy and Online Replacement

The main control board, power supply, voltage/current sensors, and fiber-optic connections are all redundant, ensuring automatic system switching within 1 ms and preventing downtime. The dual control system also supports online replacement, improving production continuity.

Fig.2 Control system redundancy design diagram

Fig.3 Physical control system redundancy design

• Fast Automatic Bypass for Power Cell Failures

If a power cell fails, the system uses an automatic bypass function to maintain continuous operation, bypassing the faulty cell and adjusting phase balance through neutral point shift technology to prevent instability. Fault information is displayed on the user interface in real time for easy troubleshooting. The specific bypass process is as follows:

① Frequency drive blocks the pulse output immediately after detecting fault of any cell.

② Closing bypass contactor of the faulty cell to physically bypass after blocking pulse.

③ After blocking pulse, the drive outputs pulses again and the motor returns to normal operation.

Fig. 4 Before bypassing power cell / After bypassing power cell (one cell fault in phase A)

According to motor parameters and load requirements, the basic configuration of Nancal drive is as follows:

Fig.5 Motor nameplate

Fig.6 Nancal standby drive on site

The main goal of this retrofit was to ensure high reliability for the gasification plant’s operation. Key concerns were the reliability of switching between the primary and standby frequency drives and the speed of the switching process. Normally, the original device drives motor and Nancal drive is in hot standby ready state after connecting to medium-voltage power supply.

• Hot Standby Drive Switching Process

① ITCC (Integrated Turbine & Compressor Control System) sends a "hot standby drive input" command to the standby drive.

② The standby drive sends a "drive hot standby" signal back to the ITCC.

③ Under typical operating conditions, the fan power supply in the original drive cabinet is disconnected to simulate the failure of the original drive. 

    (Conditions: output power 4059 kW, output voltage 8591 V, output current 302 A, motor speed 1413 rpm)

④ When the original drive fails, the output switchgear QF2 trips and sends a "original drive failure" signal to the standby drive.

⑤ The standby drive receives the original drive output switch break signal and triggers the output switchgear QF3 to close.

⑥ After closing output switchgear QF3, the standby frequency drive starts immediately and outputs torque.

⑦ The standby frequency drive sends signals indicating "standby drive is running" and "successful switching to standby drive " to the ITCC.

⑧ The ITCC performs speed regulation on the standby drive.

As shown in the switching waveform below, the overall switching process is smooth. Key timings are as follows:

• The time from the original drive fault to the signal issuance is 138.6 ms.

• The time for the switching of the original and standby drive output switchgear is 125.1 ms.

• The standby machine startup speed stabilization time is 37.8 ms.

• The total time from fault detection to switching completion is 162.9 ms (≤ 200 ms, meeting technical requirements).

Additionally, the trend in rotational speed shows a 4.6% drop during the switching process. After the switch, system flow, pressure, vibration, and other parameters were within normal operating range.

Fig. 7 Drive Failure Switching Process

Note:  

- C1 (Yellow): Original drive fault signal

- C2 (Red): Motor speed of the standby drive

- C3 (Blue): Standby drive output voltage  

- C4 (Green): Motor current

• Standby Drive with Load Shaking Function Test

① The standby drive is tested with a propylene compressor in control system redundancy mode. The main control board A drives the motor to 50 Hz, while main control board B remains in hot standby mode.

② The input switch QF11 is artificially disconnected, then closed after 1.2 seconds, followed by a low voltage ride through (LVRT) test.

③ The standby frequency drive is put into operation with pre-charging, and the LVRT function test is performed. After the grid voltage is restored, the output frequency ramps up from the present running frequency of the motor. The output speed and current are measured to be stable, and the waveforms are shown below:

Fig. 8 Standby Drive LVRT Function Test Waveforms

Note:

- C1 (Yellow): Drive input voltage  

- C2 (Red): Motor speed  

- C3 (Blue): Drive output voltage  

- C4 (Green): Motor current

Fig. 9 Standby Drive Hot Standby State (HMI Display）

In response to the reliability requirements of the propylene compressor drive system, the project incorporates an additional high-reliability, medium-voltage drive as a hot standby to achieve redundancy. This paper has described in detail the unique high-reliability features and functions of the hot standby drive. The on-site test results, including measured functions and data, have satisfied the users, who believe that the Nancal medium-voltage drive meets the operational and reliability needs of the process control system.

The seamless automatic switching control system of Nancal's medium-voltage drive optimizes the process control of the propylene compressor unit, improves operator convenience, and provides significant economic benefits to the chemical enterprise.

If the original frequency drive were also a Nancal medium-voltage drive, the fault switching time could be further reduced. Moreover, after switching to the standby drive, operations could continue without concerns about the availability of the hot standby drive. The system's design allows for automatic interchange between the main and standby drives. In cases where process requirements necessitate switching back to the original frequency drive, this can be done online without the need for production downtime.

Overall, the switch function and cntrol system redundancy design of Nancal’s high-reliability, medium-voltage frequency drive system ensure high system stability, reducing risks and potential losses associated with downtime and meeting the user's demands for operational continuity.