1. Introduction
The Reactor Coolant Pump (RCP) Drive is a critical single-point-of-failure electrical equipment in the third-generation AP1000 nuclear power plant. It provides functions such as frequency and voltage conversion for the RCP and limits the RCP starting current, serving as the sole power supply for the RCP. The unexpected failure of any RCP Drive will trigger a reactor low-flow trip signal within 1.1 seconds, leading to an unplanned reactor shutdown.
Figure 1: Four RCP Drives per Unit
Figure 2: Original RCP 60Hz Power Supply Single Line Diagram
Shandong Nuclear Power's Units 1 and 2 are among the world's first third-generation AP1000 nuclear power units and also serve as China's key project for the indigenous development of third-generation nuclear power technology. Since the commercial operation of the first unit in 2018, multiple unplanned shutdowns have occurred due to failures of the imported RCP Drives. These incidents posed safety hazards to the nuclear units and resulted in direct economic losses of approximately 12 million RMB per unit per day, accumulating to over 100 million RMB in total. Therefore, solving the "bottleneck" challenge of RCP Drives to further enhance equipment reliability and ensure nuclear power operation safety is extremely urgent.
2. System Solution Overview
In 2022, Shandong Nuclear Power Company Ltd. (SDNP), Shanghai Nuclear Engineering Research & Design Institute (SNERDI), and Shanghai Nancal Electric Co., Ltd. initiated a collaboration and successfully developed an RCP Drive for the third-generation AP1000 nuclear power plant in 2023.
The original AP1000 RCP power supply system design is shown below:
Figure 3: Original System: One-Drive-One, No Hot Standby
Based on the RCP power configuration of the AP1000 reactor type, the original RCP power supply system (four operating drives) was upgraded by adding one drive, EV-71, as a common hot standby power source. The single line diagram of the upgraded system is as follows:
Figure 4: "4-operation and 1-standby" System Topology
In the event of a failure of any RCP Drive, the common hot backup drive can be quickly switched in (backup drive activation). A repaired drive can be returned to operation using failback technology (backup drive restores to hot standby status). Furthermore, in extreme scenarios where the failback fails, the backup drive can be reactivated. This significantly enhances the operational reliability of the RCP Drives and the flexibility of standby RCP Drive switching, further improving the operational reliability and economic efficiency of AP1000 nuclear power units.
3. Basic Configuration of the RCP Drive
4. Whole Equipment of the RCP Drive
Figure 5: Whole Equipment of the RCP Drive
4.1 Low Voltage Distribution and Medium Voltage Pre-charging Cabinet
The low voltage distribution cabinet integrates multiple 380V fan power supplies and 220V control power supplies. After automatic switching, these supplies are distributed to various functional sections of the RCP Drive.
The pre-charging circuit adopts a medium voltage pre-charging scheme, consisting of a pre-charging resistor and a pre-charging bypass circuit breaker connected in parallel. Before applying medium voltage to the drive, the DC capacitors of the power cells are pre-charged through the pre-charging resistor. This effectively reduces the inrush current at the moment of medium voltage switch closure, minimizing impact on the medium voltage grid.
4.2 Transformer Cabinet and Air-Water Cooling
The transformer cabinet houses a dry-type phase-shifting rectifier transformer with a rated capacity of 12,000kVA. The primary side receives a 10.5kV input, and the secondary side provides nine sets of 1,650V outputs, supplying power to the nine power cells.
To fundamentally solve potential water leakage issues, an air-water cooling method is employed. Heat generated by the dry-type transformer is dissipated via external cooling water. The cabinet-top cooling fans feature a redundant design and can be replaced online.
4.3 Power Cell Cabinet
The power cell cabinet is the core component of the drive. Each drive phase consists of three series-connected power cells, totaling nine power cells for three phases. Each power cell is a three-phase input, single-phase output AC-DC-AC voltage source inverter. The three power cells per phase produce a 7-level voltage output. Under extreme conditions, the drive can still provide full-load output even with two power cells bypassed.
4.4 Control Cabinet
The control cabinet contains the main control board, PLC, Human-Machine Interface (HMI), etc. The main control board, fiber optics, and input/output current and voltage sampling all employ a redundant design, with a typical switching time of <1 ms.
4.5 Water-Cooling Cabinet
The coolant with constant pressure and flow continuously circulates through the heat exchanger, transferring heat away from the power cell circuit of the RCP Drive. Redundant design is applied to the cooling pumps, ion exchange vessel, plate heat exchanger, nitrogen cylinder, and key instruments.
5. Milestones in RCP Drive Localization
To enhance the reliability of the RCP power supply for AP1000 nuclear plant, Nancal Electric has assisted Shandong Nuclear Power in implementing a step-by-step "Three-Phase" strategy for RCP Drive localization, providing a replicable and scalable "Shandong Nuclear Solution" for similar plant in China.
From October to November 2023, Units 1 and 2 completed the "4-operation and 1-standby" retrofit, adding one standby localized RCP Drive per unit.
In September 2024, the first localized replacement of an RCP Drive was completed on Unit 2. Practical application fully validated the feasibility and correctness of this technical route.
In October 2025, the replacement of the remaining three RCP Drives on Unit 2 with localized versions was completed. All full-load tests were successful on the first attempt, making it China's first AP1000 plant operating under full load relying entirely on localized RCP Drives.
The replacement of the remaining four RCP Drives on Unit 1 is planned for the first half of 2026.
6. Key Technical Indicators for Main/Backup drive Switching
6.1 Equipment Coding
6.2 Scenario One: Switching from Failed Main Drive to Backup drive
Main drives EV-31/41/51/61 operate the RCPs at the rated operating point, while the backup drive EV-71 is in hot standby status. Simulating a main drive failure, the output switch of the failed main drive is opened. After the output switch is fully opened, the output switch of the backup drive EV-71 is closed. Upon closure, EV-71 first performs phase-locked tracking based on the sampled residual magnetic voltage of the motor. Once the phase-locked loop tracking is effective, it can directly output torque for operation. The total time from failure to the cessation of speed drop does not exceed 400 ms.
Figure 6: Switching from Failed Main Drive to Backup drive
6.3 Scenario Two: failback from Backup drive to Repaired Main Drive
After a failed main drive is repaired, an online failback from the backup drive to the main drive can be performed without requiring a unit shutdown. The failback time is ≤30 ms.
Figure 7: failback from Backup drive to Main Drive
7. Awards and Recognitions
2024: Added to the National Energy Administration's List of First Major Technical Equipment in the Energy Sector.
2025: Awarded the Second Prize for Science and Technology Progress by the China Nuclear Energy Association (CNEA).
8. Conclusion
The AP1000 nuclear power plant RCP Drive power supply system adopts the "4-operation and 1-standby" solution. In the event of a failure of any RCP Drive, the hot backup drive can automatically take over within 360 ms to 700 ms, significantly improving the reliability of the RCP power supply. A repaired RCP Drive can be returned to operation within 30 ms using failback technology, restoring the backup drive to hot standby status.
The main control system of the RCP Drive employs a redundant design. The typical switching time between the main and backup control systems is less than 1 ms, and components can be replaced online. This technology is internationally leading, ensuring long-term, continuous, and stable operation of the unit and greatly enhancing the intrinsic reliability of the RCP Drive.
The engineering practice of the localization retrofit at Shandong Nuclear Power Phase I has fully validated the feasibility and correctness of this technical route. It can be promoted and applied in the RCP power supply systems of AP1000/CAP1000 units.