Application of 10MW high voltage inverter in 600MW unit water supply system

1. Project Status

1. Overview

A power generation company in Hebei Province of Guodian currently has two 600MW thermal power generating units, which adopt a unit operation structure. Each boiler feed water system is equipped with three electric variable speed feed water pumps produced by KSB with a full flow rate of 50% of the unit, and operates in a two-in-one standby mode. The feed water pump system consists of a pre-pump, an electric motor, a hydraulic coupling, and a feed water pump body. The process flow is to pass the three low-pressure feed water pipes coming out of the deaerator water tank through the pre-pump and the feed water pump to the high-pressure heater, boiler economizer, and other heating equipment, and then enter the steam-water separator to maintain stable liquid level operation. The process flow of the system is shown in Figure 1.

Figure 1. Water supply system process flow chart

To ensure that the boiler is operating in a safe state, the unit currently changes the feedwater flow rate by adjusting the output speed of the feedwater pump hydraulic coupling to control the stability of the liquid level in the steam-water separator. The feedwater pump hydraulic coupling is equipped with a speed-increasing gear, which makes the turbine speed higher than the speed of the prime mover, and adjusts the speed to a lower range at this higher speed value. When the unit is under low load of 350MW and below, a single feedwater pump operates; when the unit is under high load of more than 350MW, two feedwater pumps operate in parallel, and the output speed of the hydraulic coupling governor is 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 supply pump adopts hydraulic transmission speed regulation operation, which has large transmission loss and low system efficiency, resulting in a large amount of energy waste.

2.2 The hydraulic coupling speed regulator is a flexible connection drive. When the spoon tube opening is adjusted, the system response speed is slow, the adjustment dead zone is large, and the linearity is poor.

2.3 The hydraulically coupled speed regulator uses high-pressure transmission oil to operate, which generates a large amount of heat loss during the mechanical energy transfer process.

2.4 During the direct start-up of a 10MW high-pressure water supply pump, the 5~8In peak current has a significant impact on the power grid.

One of the important means to solve the above problems is to use the currently efficient, energy-saving and widely used high-voltage inverter electronic speed control method to replace the mechanical speed control method of the hydraulic coupling. Using high-voltage inverters to replace the current hydraulic coupling speed control of the water supply pump can reduce the energy consumption level of the power consumption rate of the water supply pump group while meeting the process regulation requirements of the water supply system. In this way, not only the system regulation performance is improved and enhanced, but also the system operation efficiency is improved and the power consumption of the water supply pump is reduced, which provides a good way to reduce the power consumption rate of the power plant.

2. Selection of technical solutions

At present, the power system of the boiler feed pump group of the 600MW unit has the characteristics of high power, no other third-party speed regulation means, inability to start directly with load, and high technical safety and reliability requirements. If the variable frequency speed regulation technology is used for energy-saving transformation, the advantages of the variable frequency speed regulation are high speed regulation efficiency, low starting energy consumption, wide speed regulation range, stepless speed regulation, fast dynamic response speed, small dead zone, simple operation, and easy to implement the drum water level PID regulation strategy. The variable frequency transformation system should adopt a simple one-to-one direct-connected drag structure.

Since the water supply pump equipment originally used hydraulic coupling to realize the start-up and speed regulation of the water supply pump, and now uses high-voltage variable frequency speed control, the following two solutions can be selected in combination with the system structure:

Solution 1: Keep the hydraulic coupling unchanged, open the scoop tube to 100% output, and realize the transmission and speed-increasing function. The inverter controls the motor speed through electrical characteristics to achieve flow regulation of the water pump. The disadvantage of this method is that the hydraulic coupling is not removed, and the maintenance of the hydraulic coupling still exists; at the same time, due to the efficiency problem of the hydraulic coupling itself, there is still a certain decrease in energy saving rate.

Solution 2: Remove the hydraulic coupling and replace it with a speed-increasing gearbox to achieve rigid transmission connection; solve the efficiency loss problem in the mechanical torque transmission of the system. Since the mechanical equipment needs to be remade and replaced, the engineering transformation cycle is long, and the equipment investment and downtime losses are both large. Therefore, there are certain implementation problems in actual operation.

In view of the above situation, combined with the actual situation of a power plant of State Power Corporation in Hebei Province, it is proposed to adopt the transformation method of Plan 1 for demonstration and implementation.

3. Technical Solution

1. Primary power system solution

The main power system solution is to use two inverters for two water supply pumps in a one-to-one mode. The original 3# water supply pump's power frequency standby mode remains unchanged and is still in standby mode. The specific system structure principle is shown in Figure 2.

Figure 2. Schematic diagram of primary power system

Among them, QF represents high-voltage switch, TF represents high-voltage inverter, QS represents high-voltage isolating switch, and M represents water pump motor; QF10, QF20, QF30, QF31, and M are original equipment on site. During normal operation, QF11, QF12, QF21, and QF22 are in the closed state, and the inverter output is connected to the motor. When the water pump or motor needs to be repaired, stop the inverter operation and pull the high-opening isolating switch cabinet trolley out of the disconnect position to ensure safe operation and maintenance. The inverter provides complete motor protection functions such as overvoltage, undervoltage, overcurrent, overload, quick break, phase loss, and grounding for the output side motor, which can eliminate the application of frequency conversion conditions for neutral cabinets and differential protection devices.

When the inverter is under maintenance, the water supply pump can be switched to industrial frequency operation, and the switch status is: QF11, QF12, QF21, QF22 are in the open position, and QF13, QF23 are in the closed position.

2. Secondary system control

After the system was transformed by frequency conversion, the motor differential protection circuit in the original electrical system was cancelled, and the motor overload, overcurrent, overvoltage, undervoltage, phase loss, quick disconnection, grounding and other protection functions were realized by the frequency converter. The speed adjustment command and speed feedback signal of the hydraulic coupling were connected to the frequency converter side for frequency converter speed adjustment. Other control and monitoring signals related to the hydraulic coupling were cancelled, and the original DCS water supply system control strategy remained unchanged.

To ensure the safety and reliability of the system, the system adopts multiple protection measures of grading, segmentation and pattern recognition to ensure effective protection without refusal or misoperation, and appropriate and effective protection. The system protection mainly includes:

1) The high-voltage switch QF1 at the upper input port of the inverter is equipped with a transformer comprehensive protection device to protect the inverter;

2) The inverter input side is equipped with overcurrent, overload, grounding, phase loss, overvoltage, undervoltage, and transformer overheating protection;

3) The inverter output side is equipped with over-current, quick-break, overload, phase loss, over-voltage, under-voltage, unit overheating and other protections;

This technical solution provides the HARSVERT series of perfect harmonic-free high-voltage inverters. This series of inverters uses a number of low-voltage PWM frequency conversion power units in series to achieve direct high-voltage output. The inverter has the characteristics of low harmonic pollution to the power grid, high input power factor, good output waveform quality, and no additional heating, torque pulsation, noise, dv/dt and common mode voltage caused by harmonics. It does not need to add an output filter and can be applied to ordinary asynchronous motors.

4. Key points of 10MW ultra-high power high voltage frequency conversion technology

1. Selection of key components

The main inverter part inside the high-voltage inverter adopts high-performance IGBT produced by the fourth-generation IGBT chip of German high-quality brand and PRIMEPACK packaging technology. Its technical advantages are mainly reflected in:

1) The fourth-generation IGBT improves the operating characteristics of the IGBT, making it softer than the third-generation IGBT;

2) The fourth-generation IGBT can adapt to smaller drive resistance without generating severe voltage spikes, achieving lower switching losses than the third-generation IGBT;

3) The fourth-generation IGBT has enhanced the temperature characteristics of the chip and can operate at 150°C with a maximum tolerance temperature of 175°C, while the third-generation IGBT can only operate at 125°C with a maximum tolerance temperature of only 150°C;

4) The fourth-generation IGBT has the same short-circuit tolerance as the third-generation IGBT, which can ensure safe and reliable operation;

5) Compared with the second and third generation IGBTs, the fourth generation IGBT has excellent performance in power cycle life, as shown in the following table:

6) The fourth-generation IGBT maintains the positive temperature characteristics of the third-generation IGBT and is easy to connect in parallel.

2. Device current sharing problem

Since the current capacity of a single IGBT chip is limited, high-power products usually use IGBTs in parallel to increase the output current capacity. IGBT itself has a positive temperature coefficient and self-current balancing capability, making it suitable for parallel connection. In order to ensure the reliability of the equipment, the design margin factor of the components is first increased when calculating the capacity, which is approximately twice the margin.

Dynamic current balancing and static current balancing technologies are used to reduce the influence of the saturation voltage drop Vce(sat) of the IGBT and the forward voltage drop Vf of the anti-parallel diode on the static current balancing effect; as well as the influence of the transconductance gfs and gate-emitter threshold voltage Vge_th of the IGBT and the reverse recovery characteristics of the anti-parallel diode on the dynamic current balancing effect.

3. Device heat dissipation problem

In ultra-high power inverters, the heat power density is much greater than that of conventional inverters, and conventional heat dissipation structures cannot meet the needs of high-density heat dissipation. For this reason, we use a special heat dissipation structure and layout design to increase the heat dissipation power density and optimize the thermal field distribution to avoid device damage caused by excessively high IGBT junction temperature.

4. High current electromagnetic noise suppression problem

When the IGBT switches, the spike voltage generated on the busbar parasitic inductance is a major cause of IGBT damage. This voltage is proportional to the operating current and parasitic inductance, and inversely proportional to the IGBT action time. Since the IGBT action time changes very little under different currents, when the device current increases, the spike voltage will increase proportionally. The main circuit structure of IGBTs in parallel causes differences in line inductance, which will seriously affect the dynamic working characteristics of the IGBT. By adopting a symmetrical main circuit structure, large current noise is effectively suppressed.

V. Energy-saving benefit analysis

After high-pressure variable frequency speed regulation was used to replace the hydraulic coupling speed regulation in the 600MW boiler feed water system, the efficiency of the hydraulic coupling was stabilized at 97%, and the hydraulic coupling loss was reduced to the lowest level. Through the application of variable frequency speed regulation, the efficiency of the feed water pump drive system has been improved. The relationship between the system transmission efficiency and power transmission is shown in Figure 4 below.

Figure 4. Schematic diagram of efficiency and power transmission after frequency conversion

After the water supply system is transformed by frequency conversion, the power consumption calculation data under different load conditions are shown in Table 1 below.

由上述数据分析可知，在600MW机组锅炉给水系统超大功率设备应用条件下，采取高压变频器调速替代液力耦合器调速方式，仍可取得良好的节能效果和显著的节能收益。对进一步降低机组厂用电率水平具有切实意义。

References: Yi Peng. Principles and Applications of High-voltage and High-power Inverter Technology. Beijing: Posts and Telecommunications Press, 2008.