The Application of Frequency Converter in the Control System of the Dust Pump in Banshan Power Plant
Abstract: This article outlines the implementation of a frequency control system for a mortar pump in a power plant, focusing on the role of the drive unit within the system. The system enables variable speed operation of the mortar pump while still allowing for power frequency operation, improving production efficiency, reducing equipment wear, and enabling remote control capabilities.
1. Introduction
Hangzhou Banshan Power Plant Co., Ltd. operates four older coal-fired units in its old factory area, each equipped with two gray-water pumps designed to operate at 760m³/h. The gray water generated during unit operations is transported to dehydration silos and clarifiers, completing a closed-loop slag water circulation. After large slag particles are dehydrated, the remaining slurry is clarified and returned to the boiler via a backwater pump for reuse in slag flushing. In addition, due to environmental regulations, a sewage treatment system was introduced to prevent gray water overflow and ensure the normal functioning of the wastewater system. As a result, cooling water previously directed into the boiler, induced draft fan, and air preheater was redirected for reuse.
Due to the high volume of recovered cooling water, the water level in the forehearth of the mortar pump often drops too low, causing the pump to run dry. To prevent this, the outlet control valve of the mortar pump was adjusted, but the small opening (typically around 15%) led to excessive wear on the valve and piping, requiring replacement every two to three months. Additionally, throttling losses caused the motor to operate at higher speeds, resulting in energy waste. Furthermore, if industrial water was used to replenish the system, it not only wasted resources but also risked gray water spillage, disrupting the sewage treatment process. To address these issues, an inverter-based control system was implemented to improve efficiency and reduce maintenance costs.
2. System Components and Inverter Characteristics
The mortar pump control system, as shown in Figure 1, allows for parallel operation of two pumps, with only #1 pump controlled by frequency. An ultrasonic water level sensor automatically adjusts the water level before the pump. A bypass control function ensures that in case of inverter failure, the pump can still operate at power frequency, ensuring system safety. The #1 mortar pump motor model is Y355M3-6, with a rated power of 200kW, voltage of 380VAC, current of 374A, and speed of 990rpm.
The inverter used is the ABB ACS807, which features a complete ventilation and cooling system and dust protection. Key technical specifications include: Model: ACS800-07-0260-3 + P901; Rated output power: 200kW; Rated output current: 445A; Recommended maximum motor cable length: 300m; Environmental conditions: temperature 0–50°C, humidity 5–95% (no condensation), no conductive dust allowed.
According to the inverter's over-temperature derating characteristics, at 40°C ambient temperature, the rated current remains at 445A. For each additional degree above 40°C, the rated current decreases by 1.5%. At 50°C, the rated current is reduced by 15%, down to 378A, which meets the motor's requirement of 374A.
3. Inverter Control and Bypass Mode Switching
In normal operation, the #1 mortar pump is controlled by the inverter for frequency regulation. To ensure system safety in case of inverter failure, the system retains the ability to operate at power frequency through a bypass control mechanism. This requires switching between inverter control and bypass control modes using two knife switches. When both switches are set to the lower position, the motor receives power directly from the motor control center, bypassing the inverter for power frequency operation. When both are set to the upper position, the inverter controls the motor, enabling variable frequency operation. If the switches are in different positions, the system is disconnected, preventing any control action.
4. Operation and Control Functions
The mortar pump control system includes two main parts: the motor control center (original control cabinet) and the inverter. The motor control center handles power supply and tripping functions for power frequency operation. The inverter part includes start/stop and speed control. Local and remote operation modes are available, with local settings configurable. The inverter control panel provides signals such as power switch, trip, start, and stop, enabling direct control of the pump.
For variable frequency operation, the power switch and trip signals from the motor control center are used to send power to the inverter. Under normal conditions, the pump is kept in hot standby. To start the pump, either a local or remote command is issued. If a fault occurs, the system can switch back to power frequency operation via the control cabinet. In manual mode, speed can be adjusted locally using analog signals (4–20mA) or through the inverter’s operation panel. If there is a fault in the analog channel, local buttons allow for step-by-step adjustment, ensuring safe operation. The inverter panel provides the final control option when other methods fail.
5. Summary
The implementation of a frequency converter in the mortar pump system has significantly improved operational efficiency and reduced maintenance costs. By replacing the traditional throttling method with variable speed control, several benefits have been achieved:
◠Reduced wear on valves and pipes, extending their lifespan and saving significant equipment costs (a single valve can cost nearly 30,000 yuan). ◠Enhanced automation by maintaining stable water levels in the forehearth, reducing cavitation risks and lowering maintenance requirements. ◠Extended pump life by minimizing frequent starts and stops. ◠Lowered energy consumption, with the average current dropping from 320A to 175A after installation, representing a 45% reduction. ◠Enabled remote control, laying the groundwork for future integration with the plant’s DCS system.
This innovation demonstrates how the use of frequency converters not only improves system performance but also enhances operational efficiency and reduces resource consumption. It provides a convenient and effective way to switch between frequency and power frequency modes, while enabling remote control and contributing to overall cost savings and productivity improvements.
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