Application of high-voltage frequency converters in feed water pumps at Longmei Thermal Power Company

Application of high-voltage frequency converters in feed water pumps at Longmei Thermal Power Company

The Application of High Voltage Inverter in Water Pump of Longmei Thermal Power Company  

Liu Hongwei, Longkou Mining Group Thermal Power Company

Xu Changhai and Guo Zongzhen, Shandong Xinfengguang Electronic Technology Development Co., Ltd.

Liu Hongwei, Xu Changhai, Guo Zongzhen

Abstract   Abstract: This article introduces the application of Fengguang brand high-voltage frequency converters in the boiler feedwater pump frequency conversion system of Longmei Thermal Power Company, elaborates on the implementation mechanism, function and characteristics of the frequency conversion speed control system, and analyzes the energy-saving effect. From a new perspective, it interprets the efficient integration of the high-voltage frequency conversion speed control system with the on-site DCS system, enhancing the automation performance of the boiler feedwater system.

Keywords: Variable frequency speed control system, boiler feedwater system, high-voltage frequency converter, DCS

Abstract: The article introduces the application instances of FengGuang high voltage inverter in Longmei Thermal power company boiler water supply systems, with a description of the frequency control system implementation mechanism, function and features, and the energy savings is analyzed. From a new perspective, interpretation of the high-voltage frequency control system and DCS systems in efficient docking site, and enhance the automation of the boiler Boiler water supply system performance quoted.

Key words: Frequency control system Boiler water supply system High voltage inverter   DCS

1 Introduction
Longkou Mining Group Thermal Power Company is located in Longkou City, on the northern outskirts of the city, approximately 4 kilometers from downtown Longkou. After upgrading its boiler, the company introduced a JG140-9.8/540-M type circulating fluidized bed boiler, equipped with two dedicated feedwater pumps and motors for this boiler's feedwater system, and successfully put it into operation on the first attempt. The project employed advanced technologies such as air cooling, flue gas desulfurization, plasma ignition, greywater recycling, and electrostatic high-efficiency dust removal. The construction of this project not only solved the problem of full-load power generation for the original generator units but also met the heating needs of users in several nearby areas.

Figure 1. Boiler No. 6 of Longkuang Thermal Power Company

2. Variable Frequency Drive Retrofit Project Operating Conditions
Our company's No. 6 boiler is equipped with two feedwater pumps, as shown in Figure 1. Previously, water flow was controlled by valve opening. During unit operation, adjusting the feedwater pump outlet valve is complex. Because this method only changes the flow resistance of the channel, without altering the output power of the drive source, the throttling loss is considerable, wasting a significant amount of electrical energy. Its main drawbacks are as follows:
(1) The pressure difference across the regulating valve increases, the working safety characteristics deteriorate, the pressure loss is serious, and the energy consumption increases;
(2) The water pump motor runs at a constant speed, the valve adjustment throttling loss is large, the outlet pressure is high, the system efficiency is low, and energy is wasted;
(3) Excessive pipeline pressure threatens the sealing performance of system equipment;
(4) Long-term valve opening accelerates valve wear and leads to a deterioration in valve control characteristics;
(5) The equipment has a large starting inrush current, which requires an increase in the capacity of the power distribution equipment and thus an increase in investment.
To address the aforementioned issues, after extensive technical evaluation, Longkou Mining Group Thermal Power Company decided to replace the traditional valve-based water supply regulation method with high-voltage frequency converters. Stable motor operation is crucial for normal production and places particularly stringent requirements on the equipment; therefore, the selection of high-voltage frequency converters was extremely cautious. To overcome these drawbacks, the thermal power company selected high-voltage frequency converters manufactured by Shandong Xinfengguang Electronic Technology Development Co., Ltd. to simultaneously upgrade two water supply pumps. One high-voltage frequency converter was selected, employing a one-to-two configuration, with one water supply pump in operation and the other as a backup. The technical parameters of the two water supply pump motors and frequency converters are shown in Tables 1 and 2:

 model

 YKK5005-2

 Rated voltage

 10000V

 Rated current

 69.6A

 Rated power

 1000kW

 Rated frequency

 50Hz

 Rated speed

 2985r/min

 Power factor

 0.88

 Connection method

 Y 

 Manufacturer

 Xi'an Xima Electric Co., Ltd. 

3. Introduction to the energy-saving principle of water supply pumps
3.1 Mechanical characteristics of water supply pump operation

The basic principle of adjusting the pump speed is to change the pump's head characteristic curve to alter the operating point. Since the control theories of pumps and fans are similar, both based on fluid mechanics, energy-saving methods applicable to fans are also generally applicable to pumps. However, liquids are significantly denser than gases, resulting in much higher pressure; the liquid level difference in the pipeline resistance is also substantial. Therefore, the basic energy-saving methods have three main elements:
(1) Reduce unnecessary traffic
(2) Reduce pipeline resistance
(3) Using efficient methods to control flow rate. There are many specific methods to change flow rate, but they are all based on the premise of reducing unnecessary flow rate and head. Methods to control flow rate: intermittent operation, parallel number control, series number control, wing angle control, speed control, etc. Variable frequency speed control has a higher initial investment, but the power consumption during operation can be greatly reduced. It is suitable for occasions with large flow rate changes and large head changes.

3.2 Energy-saving principle of water supply pump

According to formula (3), it can be calculated that when the frequency is reduced from 50Hz to 40Hz, nearly half of the energy consumption can be saved.
A more intuitive diagram of the pump (or fan) operating curve is shown in Figure 2: The normal operating point of the pump (or fan) is A. When the water volume (or air volume) needs to be adjusted from Q1 to Q2, valve regulation is used. The pipeline characteristic curve changes from R1 (valve fully open) to R2 (valve partially closed), and its operating point is adjusted to point B. Its power is the area enclosed by OQ2BH2', and its power change is very small, but its efficiency decreases accordingly. When variable frequency speed control is used, the motor speed can be increased or decreased as needed, changing the equipment's performance curve. In the figure, from n1 (rated speed) to n2 (speed decrease), its operating point is adjusted to point C, so that its parameters meet the process requirements. Its power is the area enclosed by OQ2CH2, and its efficiency curve also shifts accordingly, still operating in the high-efficiency zone. The shaded area in the figure represents the actual energy savings.

Figure 2. Working curve of water pump (or fan)

If pump speed regulation is performed in a system with unchanged pipe network characteristics and no water pressure requirement, the energy-saving benefits are much more significant than those of constant pressure water supply.
4. Introduction to Variable Frequency Drive Retrofit Scheme
The variable frequency speed control system is configured with the existing DCS in our factory. The DCS controls the start-up, shutdown, and speed regulation of the frequency converter, and can display the operating data and current status of the frequency converter on the DCS, and monitor the system operation in real time.
To ensure the reliability of the boiler feedwater system, the frequency converter has a manual bypass device for the mains frequency. When the frequency converter fails and stops running, the motor can be manually switched to run at the mains frequency. This ensures the boiler's water supply requirements and improves the safety and stability of the entire system.
In terms of operation, there are two control methods: remote control and local control. These two control methods can improve the safety performance of the system. The DCS performs closed-loop control. According to the unit's load, the DCS detects the main pipe pressure according to the set program, calculates and gives the frequency converter a suitable frequency value, thereby realizing automatic control of the boiler feedwater pump motor speed and ensuring the stability of the main pipe pressure.
The variable frequency drive (VFD) system for water pumps has the following characteristics: It can operate under variable frequency speed control or directly at the mains frequency, while also incorporating a reliable interlocking circuit; when the 220V AC control power supply to the VFD fails, since the control power supply and mains power supply have no phase or synchronization requirements, the VFD can continue operating using a UPS without shutting down; when the speed setpoint signal in the on-site DCS goes offline, the VFD provides an alarm while continuing to operate at the original speed, maintaining the unit's operating conditions unchanged; the VFD is equipped with a unit bypass function, allowing it to bypass the faulty unit and continue operating at a reduced rating during partial faults, minimizing losses caused by sudden shutdowns; the original motor can be retained for continued use without altering any foundation of the existing motor equipment.
5. High-pressure condensate pump frequency conversion solution
5.1 Power System Scheme
Our plant's water supply system frequency conversion upgrade adopts a one-to-two manual bypass scheme. Its primary circuit is shown in Figure 3, which involves equipping one high-voltage frequency converter. By switching the high-voltage isolating switch, the high-voltage frequency converter can be switched to the desired water supply pump. The high-voltage frequency converter can drive the #7 water supply pump motor for frequency conversion operation, or it can drive the #9 water supply pump motor for frequency conversion operation. Both water supply pump motors have a power frequency bypass function.

Figure 3 Wiring diagram of high voltage frequency converter primary system

k1 and k2 are high-voltage disconnect switches in the inverter bypass switch cabinet;
QF4 and QF5 are high-voltage circuit breakers for frequency converter bypass switch cabinets.
QF1 and QF3 are the high-voltage circuit breakers for the power frequency power supply of the No. 9 and No. 7 water pumps on site, respectively.
QF2 is a high-voltage circuit breaker for field frequency converter power supply;
The frequency converter is a Fengguang JD-BP38 series high-voltage frequency converter;
Pumps #7 and #9 are controlled by frequency converters.
The frequency converter controls two motors in a one-to-two control configuration, with the bypass switch cabinet used for switching between industrial and frequency conversion operation. K1 and K2 are two high-voltage disconnect switches; when the frequency converter is running, K1 and K2 must be closed simultaneously. When QF4 is closed, QF5 is open, and QF1 is open, the No. 9 feedwater pump operates at frequency conversion; when QF4 is open and QF1 is closed, the No. 9 feedwater pump operates at industrial frequency; when QF5 is closed, QF4 is open, and QF3 is open, the No. 7 feedwater pump operates at frequency conversion; when QF5 is open and QF3 is closed, the No. 7 feedwater pump operates at industrial frequency. QF4 is electrically interlocked with QF1 and QF5, and QF5 is electrically interlocked with QF3 and QF4. Remote control of the feedwater pumps can be achieved by setting the "remote control" switch of the control cabinet to the "remote control" position and setting the "local/remote" switch of circuit breaker QF2 to the "remote" position.
5.2 Control System Scheme
Our plant's water supply system primarily employs a differential pressure water supply scheme. The signals are taken from the pressure sensors at the boiler drum and water pump outlets, and sent to a differential pressure transmitter. This transmitter compares the differential pressure signal with the water supply regulation signal output from the DCS, forming a closed-loop control system. This signal is then sent from the DCS to the frequency converter as its setpoint. When the system load changes, the boiler drum pressure continuously changes, and the frequency converter's output frequency automatically adjusts upon receiving the regulation signal. The water pump's outlet speed also changes accordingly, ensuring that the pump's outlet pressure always follows the changes in the boiler drum pressure. The frequency converter mainly regulates the water supply through the motor, i.e., the pump's speed, to maintain the boiler drum water level. This system offers fast response, good stability, and strong anti-interference capabilities. Furthermore, in applications where flow rate is regulated by speed, the pump's power consumption is significantly reduced by a factor of three, resulting in substantial energy savings.
The DCS monitoring screen of this system is shown in Figure 4:

Figure 4. DCS monitoring diagram of feedwater pump for boiler #6

When starting the feedwater pump via frequency conversion, first operate the corresponding QF2 high-voltage circuit breaker to charge the frequency converter, then press the frequency converter start button to start the frequency converter. To prevent the feedwater pump from experiencing "steam turbidity" during frequency conversion startup, the feedwater pump starts in minimum flow control mode, opening the feedwater pump recirculation valve to meet the minimum flow of the feedwater pump. During startup, when the outlet flow exceeds the minimum flow, the outlet valve will open after a delay. After the outlet valve is fully open, the recirculation valve will close. As the flow increases, the main regulating valve entering the main pipe will also open to its maximum according to the flow change. During operation, the flow in the pipeline is mainly regulated by the rotational speed to meet the needs of water level changes in the steam drum.
Since the frequency converter cannot be started in pairs, only one of the No. 7 and No. 9 water pumps can be operated by frequency converter, while the other is operated by fixed frequency. The method of interlocking the two pumps is the same as before the modification.
5.3 Starting conditions for water supply pumps (1) The inlet and outlet pipes of the water supply pump must be filled with water;
(2) The inlet valve of the water supply pump is fully open;
(3) The outlet valve of the water pump is fully closed;
(4) The water pump recirculation valve is opened;
(5) The cooling system is normal;
(6) The mechanical seal water is normal;
(7) The oil level is normal;
(8) Electrical interlocking: incoming circuit breaker QF2 closes, frequency converter contactors KM1 and KM2 close;
(9) The inverter is powered on, the system self-test is normal, and the "ready" signal is received from the inverter.
6. Operational Status and Benefits

Figure 5. Field application of high-voltage frequency converter

On March 2, 2010, this frequency converter was successfully commissioned on the first attempt. Since its commissioning, the frequency converter has been operating generally well. Figure 5 shows the field application of the high-voltage frequency converter.

6.1 Direct Benefits

The motor's variable frequency drive (VFD) operating parameters are recorded in Table 3. According to the operating conditions, the motor operates between 40 and 45 Hz, and for extended periods at around 45 Hz. At this frequency, the VFD's input power is approximately 785 kW, resulting in significant energy savings.
Previously, before our company used frequency converters, the water pumps operating at the mains frequency had a motor voltage of 10.3kV, a current of 60A, and a power of 942kW. After the upgrade, the water pumps generally operate at 45.6Hz, with a frequency converter output voltage of 9.7kV, an output current of 57A, and a power of 842kW, saving 2400 kWh per day.
Since the implementation of frequency converters, the way operators frequently operate pump outlet valves has changed, reducing the impact of water flow on the valve's internal structure and extending the valve replacement cycle. Previously, valves needed to be replaced every 1-2 years, but since the system was put into operation in 2009, they have not needed to be replaced at all. This has not only saved on material costs but also reduced the amount of equipment maintenance required.
6.2 Indirect benefits (1) After frequency conversion control, the motor can be soft-started, the starting current is less than the rated current value, and the start-up is smoother.
(2) The speed of the motor and the load decreases, the system efficiency is improved, and energy saving is achieved. This greatly reduces the amount of equipment maintenance and saves human and material resources.
(3) As the motor and load are adjusted by speed regulation, the working characteristics change, the equipment working conditions are improved, and the service life of the equipment is extended.
(4) Using frequency conversion regulation, the parameters can be kept constant in real time, which improves the safety and stability of the system operation.
(5) Due to the adoption of automatic control, the automation level of equipment operation control and system operation management has been further improved, thereby truly realizing automatic adjustment and greatly enhancing the safety and reliability of operation.
6.3 Operational Status
The JD-BP38-1000F high-voltage frequency converter monitoring screen categorizes fault types into several types: main control board fault, protection board fault, external fault, shutdown fault, three-second power-off fault, and frequency setting fault. Two issues were encountered during operation.
(1) Due to the constraints and influences of the company's operating environment, dust often clogs the filter screen, causing the internal dry transformer temperature to rise and trigger an alarm. However, after timely descaling of the filter screen, the normal ventilation of the frequency converter is ensured, and the equipment can operate normally. Therefore, the filter screen should be cleaned regularly.
(2) In November 2010, a C-phase unit failure was detected on the inverter monitoring screen. Upon inspection, it was found that a rectifier unit inside the cabinet was damaged. After replacement, the equipment was put into operation and all equipment operated normally.
Apart from the two faults mentioned above, the Fengguang brand high-voltage frequency converters are operating well in our company.
7. Conclusion
The JD-BP38-1000F high-voltage frequency converter was adopted in the boiler feedwater system of Longkou Mining Group's thermal power plant. This converter is not only convenient and easy to operate with minimal maintenance, but also offers significant energy savings. The application of the high-voltage frequency converter in the boiler feedwater system allows for motor adjustment based on the boiler drum water level, greatly improving boiler stability and resulting in excellent coal savings and improved operational economy. Based on the experience gained from its application in the feedwater pumps, company leaders are striving to promote its widespread adoption in other equipment throughout the plant.

References
Shandong Xinfengguang Electronics User Manual [Z] Shandong Xinfengguang Electronics Technology Development Co., Ltd.

Personal Profile:
Liu Hongwei is a senior engineer currently employed at the Thermal Power Company of Longkou Mining Group.

Address: Wenshang Economic Development Zone, Shandong Province Tel: 0537-7237196
Postal code: 272500
Contact person for this article: Guo Peibin Tel: 0537-7237007