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. The system enables variable speed operation while retaining the ability to run at power frequency, thereby enhancing production efficiency, reducing equipment wear, and supporting remote control capabilities.

1. Introduction

Hangzhou Banshan Power Plant Co., Ltd. operates four older coal-fired units equipped with two gray water pumps designed to operate at 760m³/h. The gray water generated from boiler operations is transported to dehydration silos and clarifiers, creating a closed-loop slag water system. After large slag particles are dehydrated and reused, the clarified water is pumped back into the boiler for further use. Additionally, due to environmental regulations, a wastewater treatment system was introduced to prevent gray water overflow and ensure smooth operation. As a result, cooling water previously directed to the boiler, induced draft fan, and air preheater was redirected for reuse.

With the increased recovery of cooling water, the water level in the forehearth of the mortar pump often drops too low, leading to pump cavitation. To prevent this, the outlet control valve was adjusted to regulate the flow. However, this resulted in frequent valve replacements (every 2–3 months) due to excessive wear. Moreover, throttling losses forced the motor to run at higher speeds, increasing energy consumption. Additionally, using industrial water to replenish the system wasted resources and risked gray water spills that could disrupt the wastewater treatment process. To address these issues, an inverter-based control system was implemented to optimize performance and reduce costs.

2. System Components and Inverter Characteristics

The mortar pump control system, as shown in Figure 1, features parallel operation of two pumps, with only #1 pump controlled via frequency. An ultrasonic level sensor automatically adjusts the water level before the pump. A bypass control function ensures power frequency operation if the inverter fails, maintaining system safety. The #1 mortar pump motor model is Y355M3-6, rated at 200kW, 380VAC, 374A, and 990rpm.

The inverter used is the ABB ACS807, which includes a complete ventilation and dust-proof design. Key specifications include: Model ACS800-07-0260-3 + P901, rated output power 200kW, rated current 445A, and maximum cable length of 300m. It operates in temperatures between 0–50°C and humidity levels of 5–95% (non-condensing). At 40°C, the rated current is 445A, but it decreases by 1.5% per degree above 40°C, reaching 378A at 50°C, which is sufficient for the motor’s 374A requirement.

3. Inverter Control and Bypass Switching

Under normal conditions, the #1 mortar pump is controlled by the inverter for frequency regulation. In case of inverter failure, a bypass allows the pump to operate at power frequency. The system uses two knife switches to toggle between inverter and power frequency modes. When both switches are in the lower position, the motor receives direct power, bypassing the inverter. If both are in the upper position, the inverter controls the motor. If they are in different positions, the system is disconnected, preventing any operation.

4. Operation and Control Functions

The control system includes the motor control center and the inverter. The motor control center handles power switching for power frequency operation, while the inverter manages start/stop and speed control. Local and remote operation options allow flexible control. In variable frequency mode, the inverter receives power from the motor control center and adjusts the pump speed based on the water level. Automatic control uses a PID regulator to maintain the desired level, while manual control offers local analog or switch adjustments. Remote signals can also be used to set speed commands.

5. Summary

The integration of a frequency converter into the mortar pump system has significantly improved operational efficiency and reduced maintenance costs. Key benefits include:

● Reduced wear on valves and piping, extending their lifespan and saving on replacement costs (a single valve costs nearly ¥30,000).

● Enhanced automation through automatic water level control, minimizing cavitation risks and improving system reliability.

● Extended pump life by reducing frequent starts and stops.

● Lower energy consumption, with the average current dropping from 320A to 175A after inverter installation, a reduction of 45%.

● Enabled remote control, paving the way for future integration with the plant’s DCS system.

This innovation not only improves system performance but also enhances operational efficiency and reduces labor requirements, making it a valuable upgrade for the power plant.

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