Abstract: This paper explores the experience of improving the mechanical efficiency and saving energy in the feedwater system of a 600MW unit by replacing the hydraulic coupling with a domestically produced 10MW-class high-voltage ultra-high-power frequency converter for mechanical speed regulation. Practice has proven that good energy-saving benefits can still be achieved by using a high-voltage frequency converter on the basis of existing hydraulic coupling speed regulation.
Keywords: water pump, high-pressure variable frequency, application
I. Current Status of the Project
1. Overview
A power generation company in Hebei Province currently has two 600MW thermal power generating units, operating in a unit-based configuration. Each boiler feedwater system is equipped with three KSB-manufactured electric variable-speed feedwater pumps, operating at 50% of the unit's full-load capacity, in a two-in-operation-one-standby configuration. The feedwater pump system consists of a booster pump, electric motor, hydraulic coupling, and the pump body. The process flow involves three low-pressure feedwater pipes from the deaerator tank being pressurized by the booster pump and feedwater pumps, then converging into a mains pipe leading to the high-pressure heater, boiler economizer, and other heating equipment, before entering the steam-water separator to maintain a stable liquid level. The system's process flow is shown in Figure 1.
Figure 1. Process flow diagram of water supply system
To ensure safe boiler operation, the unit currently controls the feedwater flow rate and stabilizes the steam-water separator level by adjusting the output speed of the feedwater pump hydraulic coupler. The feedwater pump hydraulic coupler is equipped with a speed-increasing gear, allowing the turbine speed to exceed the prime mover speed. Speed regulation then extends within a range from this higher value to a lower speed. At low loads of 350MW and below, a single feedwater pump operates; at high loads above 350MW, two feedwater pumps operate in parallel, with the hydraulic coupler speed governor output speed adjusted between 69% and 91%, and the system has no feedwater regulating valve.
2. Problems with the hydraulic coupling speed control system
2.1 The water pump uses a liquid coupling drive for speed regulation, which results in large transmission losses, low system efficiency, and a significant waste of energy.
2.2 The liquid coupler speed controller is a flexible connection drive. When the scoop tube opening is used for adjustment, the system response speed is slow, the adjustment dead zone is large, and the linearity is poor.
2.3 The hydraulic speed governor uses high-pressure transmission oil, which generates a large amount of heat loss during the mechanical energy transmission process.
2.4 During the direct start-up of a 10MW high-pressure feedwater pump, the peak current of 5~8In has a significant impact on the power grid.
One important approach to solving the aforementioned problems is to replace the mechanical speed control method of the hydraulic coupling with the currently efficient, energy-saving, and widely used high-voltage frequency converter electronic speed control. By replacing the current hydraulic coupling speed control of the feedwater pumps with high-voltage frequency converters, the plant's power consumption can be reduced while meeting the process regulation requirements of the feedwater system. This not only improves and enhances the system's regulation performance but also increases system operating efficiency and reduces feedwater pump power consumption, providing a good way to reduce the power plant's power consumption rate.
II. Selection of Technical Solutions
Currently, the power system of the boiler feedwater pump unit in a 600MW unit is characterized by high power, lack of other third-party speed regulation methods, inability to start directly under load, and high requirements for technical safety and reliability. If variable frequency speed control technology is adopted for energy-saving retrofitting, the advantages of frequency converter speed regulation are high speed regulation efficiency, low starting energy consumption, wide speed range, stepless speed regulation, fast dynamic response speed, small dead zone, simple operation, and ease of implementation of PID control strategies for boiler drum water level. A simple one-to-one direct-drive structure is recommended for the frequency conversion retrofit system.
The water pump equipment originally used a hydraulic coupling to achieve functions such as pump start-up and speed regulation; now, after switching to high-pressure variable frequency speed control, the following two solutions can be selected based on the system structure:
Option 1: Keep the hydraulic coupling unchanged, with the scoop tube opened to 100% output, achieving both transmission and speed increase. The frequency converter controls the motor speed via electrical characteristics to regulate the water pump flow. The drawback of this method is that the hydraulic coupling is not removed, meaning maintenance of the coupling is still required; furthermore, due to the inherent efficiency limitations of the hydraulic coupling, there will still be a reduction in energy efficiency.
Option 2: Remove the hydraulic coupling and replace it with a speed-increasing gearbox to achieve a rigid transmission connection; this solves the efficiency loss problem in the transmission of mechanical torque in the system. However, this option requires the manufacture and replacement of mechanical equipment, resulting in a long engineering modification cycle and significant equipment investment and downtime losses. Therefore, it presents certain implementation challenges in practice.
In view of the above, and considering the actual situation of a power plant in Hebei Province, it is proposed to adopt the first option for renovation and demonstrate its feasibility for implementation.
III. Technical Solution
1. Primary power system scheme
The main power system scheme uses two frequency converters to power two feedwater pumps in a one-to-one configuration. The original standby mode for feedwater pump #3 remains unchanged and is still in standby mode. The specific system structure and principle are shown in Figure 2.
Figure 2. Schematic diagram of the primary power system
In this designation, QF represents a high-voltage switch, TF represents a high-voltage frequency converter, QS represents a high-voltage disconnect switch, and M represents a feedwater pump motor. QF10, QF20, QF30, QF31, and M are existing equipment on site. During normal operation, QF11, QF12, QF21, and QF22 are in the closed state, connecting the frequency converter output to the motor. When the feedwater pump or motor requires maintenance, the frequency converter is stopped, and the high-voltage disconnect switch cabinet trolley is pulled out to the open position to ensure safe operation and maintenance. The frequency converter provides complete motor protection functions for the output-side motor, including overvoltage, undervoltage, overcurrent, overload, instantaneous trip, phase loss, and grounding protection, eliminating the need for a neutral cabinet and differential protection devices in frequency converter applications.
When the frequency converter is under maintenance, the water pump can be switched to mains frequency operation. The switch status is as follows: QF11, QF12, QF21, and QF22 are in the open position, and QF13 and QF23 are in the closed position.
2. Secondary system control
After the system was upgraded with frequency converters, the differential protection circuit for the motors in the original electrical system was eliminated. The motor overload, overcurrent, overvoltage, undervoltage, phase loss, instantaneous trip, and grounding protection functions are now implemented by the frequency converter. The speed regulation command and speed feedback signal of the hydraulic coupler are connected to the frequency converter side for speed regulation. Other control and monitoring signals related to the hydraulic coupler are eliminated, while the original DCS water supply system control strategy remains unchanged.
To ensure system security and reliability, the system employs multiple protection measures based on hierarchy, segmentation, and pattern recognition to ensure effective protection without failure to activate, erroneous activation, or misoperation, and to guarantee appropriate and effective protection. The system's protection primarily includes:
1) The high-voltage input switch QF1 of the frequency converter is equipped with a transformer integrated protection device to protect the frequency converter;
2) The inverter input side is equipped with overcurrent, overload, grounding, phase loss, overvoltage, undervoltage, and transformer overheat protection;
3) The inverter output side is equipped with overcurrent, instantaneous trip, overload, phase loss, overvoltage, undervoltage, and unit overheat protection.
This technical solution provides the HARSVERT series of harmonic-free high-voltage frequency converters. This series of converters uses several low-voltage PWM power units connected in series to achieve direct high-voltage output. The converter features low harmonic pollution to the power grid, high input power factor, good output waveform quality, and eliminates problems caused by harmonics such as additional motor heating, torque ripple, noise, dv/dt, and common-mode voltage. It also eliminates the need for an output filter, making it suitable for ordinary asynchronous motors.
IV. Key Points of 10MW-class Ultra-high Power High Voltage Variable Frequency Technology
1. Selection of key components
The main inverter section inside the high-voltage frequency converter uses high-performance IGBTs manufactured with high-quality German fourth-generation IGBT chips and PRIMEPACK packaging technology. Its technological advantages are mainly reflected in:
1) The fourth-generation IGBT improves the operating characteristics of the IGBT, making its operation smoother than that of the third-generation IGBT;
2) The fourth-generation IGBT can accommodate smaller drive resistors without generating severe voltage spikes or glitches, achieving lower switching losses than the third-generation IGBT.
3) The fourth-generation IGBT has enhanced chip temperature characteristics and can operate at 150℃ with a maximum tolerable temperature of 175℃, while the third-generation IGBT can only operate at 125℃ with a maximum tolerable temperature of only 150℃.
4) The fourth-generation IGBT has the same short-circuit withstand capability as the third-generation IGBT, which can ensure safe and reliable operation;
5) Compared with the second and third generation IGBTs, the fourth generation IGBTs exhibit superior performance in power cycle life, as detailed in the table below:
6) The fourth-generation IGBT retains the positive temperature characteristics of the third-generation IGBT, making it easy to connect in parallel.
2. Device current sharing problem
Because the current-carrying capacity of a single IGBT chip is limited, high-power products typically use IGBTs connected in parallel to increase the output current capability. IGBTs themselves have a positive temperature coefficient and self-current sharing capability, making them suitable for parallel connection. To ensure the reliability of the equipment, the design margin factor is increased when calculating the capacity of components, approximately doubling the margin.
Dynamic and static current sharing techniques are employed to reduce the impact of IGBT saturation voltage drop Vce(sat) and anti-parallel diode forward voltage drop Vf on static current sharing effect; and to reduce the impact of IGBT transconductance gfs, gate-emitter threshold voltage Vge_th, and anti-parallel diode reverse recovery characteristics on dynamic current sharing effect.
3. Component heat dissipation issues
In ultra-high power frequency inverters, the heat dissipation power density is much higher than that of conventional frequency inverters, and conventional heat dissipation structures cannot meet the requirements of high-density heat dissipation. Therefore, we adopt a special heat dissipation structure and layout design to improve the heat dissipation power density and optimize the thermal field distribution to avoid IGBT junction temperature from becoming too high and causing device damage.
4. High-current electromagnetic noise suppression problem
The voltage spikes generated on the parasitic inductance of the busbar during IGBT switching are a major cause of IGBT damage. This voltage is proportional to the operating current and parasitic inductance, and inversely proportional to the IGBT's operating time. Since the IGBT's operating time varies very little under different currents, the voltage spikes will increase proportionally as the device current increases. The parallel connection of IGBTs in the main circuit creates differences in line inductive reactance, which severely affects the dynamic operating characteristics of the IGBTs. Adopting a symmetrical main circuit structure effectively suppresses high-current noise.
V. Energy Saving Benefit Analysis
After replacing the hydraulic coupling speed control method with high-pressure variable frequency speed control in the 600MW boiler feedwater system, the efficiency of the hydraulic coupling stabilized at 97%, and the hydraulic coupling loss was reduced to a minimum. The application of variable frequency speed control improved the efficiency of the feedwater pump drive system. The relationship between system transmission efficiency and power transfer is shown in Figure 4 below.
Figure 4. Schematic diagram of efficiency and power transmission after frequency conversion modification
The power consumption calculation data of the water supply system under different load conditions after frequency conversion modification are shown in Table 1 below.
The above data analysis shows that, under the application conditions of ultra-high power equipment in the boiler feedwater system of a 600MW unit, replacing hydraulic coupling speed regulation with high-voltage frequency converter speed regulation can still achieve good energy-saving effects and significant energy-saving benefits. This is of practical significance for further reducing the unit's plant power consumption rate.
Reference: Yi Peng. High Voltage High Power Frequency Converter Technology Principles and Applications. Beijing: Posts & Telecom Press, 2008.
