Successful application of high voltage inverter in induced draft fan of thermal power plant

1 Introduction

The electricity consumption of electric motors in my country accounts for about 60% to 70% of the national power generation, and the annual power consumption of fans and pumps accounts for about 1/3 of the national power consumption. The main reason for this situation is that the traditional adjustment method of fans, pumps and other equipment is to adjust the air supply and water supply by adjusting the damper and valve opening at the inlet or outlet, and the output power is consumed in large quantities in the interception process of the damper and valve. Since fans and pumps are mostly square torque loads, and the shaft power and speed are in a cubic relationship, when the speed of fans and pumps decreases, the power consumption also decreases greatly. Therefore, the energy-saving potential is very large. The most effective energy-saving measure is to use variable frequency speed regulators to adjust the flow and air volume. The power saving rate of frequency converters is generally 20% to 50%. In addition, in the design, the capacity of the user's water pump motor design is much higher than the actual need, and there is a phenomenon of "big horse pulling a small cart", which is inefficient and causes a lot of waste of electricity. Therefore, the promotion of AC variable frequency speed regulation devices is very beneficial.

2 Analysis of energy consumption of induced draft fan before transformation

A thermal power plant has two 410t/h circulating fluidized bed (CFB) boilers with a total installed capacity of 100MW steam turbine generator sets, which mainly provide thermal power supply to a large chemical plant. The standard coal consumption for power supply is 360g/kW·h, which is higher than the industry average. Each of the two CFB boilers is equipped with two high-pressure induced draft fans, model ykk630-6w-1250kw. Before the frequency conversion transformation, the rated 1250kw induced draft fan motor has a normal operating load of about 830kw. Its output power is adjusted by the damper opening. Under normal conditions, the damper opening is basically maintained at about 40%. A considerable part of the electricity is consumed on the damper baffle, which is a serious waste of energy and has great energy-saving potential.

3 Analysis of energy-saving transformation principles

3.1 Introduction to General High Voltage Inverter

Among the various speed regulation methods for AC asynchronous motors, variable frequency speed regulation has the best performance, a large speed regulation range, good static stability, and high operating efficiency. The working principle of a general frequency converter is shown in Figure 1.

In Figure 1, the function of the rectifier is to rectify three-phase (or single-phase) AC into DC. The function of the inverter is to regularly control the on and off of the main switch device in the inverter, and to obtain a three-phase AC output of any frequency. There will always be reactive power conversion between the intermediate DC link and the motor. This reactive power must be buffered by the energy storage element (capacitor or reactor) in the intermediate DC link. Control circuit: It is usually composed of an operation circuit, a detection circuit, a control signal input, an output circuit, and a drive circuit. Its main task is to complete the switch control of the inverter, the voltage control of the rectifier, and various protection functions.

3.2 Siemens Robincon Perfect Harmony Inverter Principle

This equipment transformation selected Siemens Robicon perfect harmonic inverter, model ph-6-6-1250, and the inverter circuit diagram is shown in Figure 2. Each secondary of the input isolation transformer t1 only supplies one power unit, and each power unit receives modulation information through optical fiber to generate the output power frequency required by the load. Each power unit can be divided into a rectifier part, a DC link, and an inverter part.

The schematic diagram of a single power unit is shown in Figure 3, and the working principle of the IGBT is shown in Figure 4. When IGBT Q1 and Q4 are closed at the same time, the voltage on the motor is high at point A and low at point B; when IGBT Q2 and Q3 are closed at the same time, the voltage on the motor is low at point A and high at point B. In this way, an alternating voltage, that is, alternating current, is formed at both ends of the motor by continuously opening and closing. Siemens perfect harmonic converter obtains a medium voltage (6kV) waveform that is similar to a sine wave by superimposing the outputs of multiple low-voltage power units (690V). Figure 5 shows the approximate sine wave waveform output after the superposition of three power units.

3.3 Calculation of the power saving principle of variable frequency speed regulation of induced draft fans

Taking the induced draft fan of No. 2 furnace as an example, the operating conditions and basic parameters of the two induced draft fans (2a, 2b) of No. 2 CFB furnace before the transformation are analyzed first, as shown in Table 1.

(1) 2a induced draft fan power before transformation

p1 =u×i×1.732×cosφ

=6.3×90×1.732×0.85

=835kw

The operating power factor cosφ is taken as 0.85.

(2) Estimated power of 2a induced draft fan after conversion

Calculated based on the local average external electricity price of 0.72 yuan/kw·h (tax included) and 330 days of operation throughout the year:

2a induced draft fan saves about (835-436)×24×330×0.72=2.275 million yuan in electricity per year

The electricity consumption data of the 2a induced draft fan before and after the transformation are compared, as shown in Table 2.

4 Engineering Application

4.1 Determination of frequency conversion control scheme for induced draft fans

Based on the analysis in the previous section, the thermal power plant carried out frequency conversion transformation on one induced draft fan of each of the two furnaces. At the same time, in order to meet the requirements of continuous heating and power supply of the chemical plant, the high-voltage inverter of the induced draft fan should have the function of online switching between working frequency and variable frequency, that is, the inverter failure can automatically switch to working frequency operation, and after the variable frequency is repaired, it can be manually switched back to variable frequency operation without affecting the continuous normal operation of the boiler.

The primary system diagram after the induced draft fan frequency conversion transformation is shown in Figure 6. dl is the 732# cabinet circuit breaker of the plant substation; j1, j2, and j3 are circuit breakers used in conjunction with the frequency converter to achieve mutual switching between industrial frequency and variable frequency. The frequency conversion control scheme is as follows:

(1) When j1 and j2 are closed and j3 is open, it is the variable frequency state; when j1 and j2 are open and j3 is closed, it is the industrial frequency state.

(2) The inverter output frequency can be controlled by DCS. When automatic is selected, the output is automatically adjusted according to the furnace negative pressure setting value; when manual is selected, the output value is manually input by the operator, and the input value is 0~100%, corresponding to the inverter output 0~50Hz.

(3) Manual frequency conversion to industrial frequency: The DCS sends a signal to trip J1 and J2, and then close J3. After completion, it is necessary to manually reset the frequency conversion to industrial frequency on the DCS screen, and the frequency conversion to industrial frequency is completed.

(4) Convert to industrial frequency when the inverter fails: When the inverter fails, the DCS system immediately issues an alarm and closes the inlet damper to a certain opening (40%). At the same time, when the inverter fails, it sends a signal to trip j1 and j2 and energize the time relay sj, delaying the closing of j3, and completing the conversion from inverter to industrial frequency. At this time, the negative pressure in the furnace will fluctuate to a certain extent, and the operator can intervene manually to ensure the stability of the boiler pressure.

(5) Power frequency conversion: DCS sends a signal to open J3 and close J1. After a delay of 2s, J2 closes (avoiding the influence of the motor's back electromotive force), and the power frequency conversion is completed.

(6) The operation flow of the induced draft fan variable frequency control is shown in Figure 7.

4.2 Debugging

4.2.1 First commissioning (motor no-load)

(1) Frequency conversion to industrial frequency test: A frequency conversion to industrial frequency command is manually sent from a remote DCS, the J1 and J2 circuit breakers are tripped, the J3 circuit breaker is closed, and the motor switches from frequency conversion to industrial frequency operation. The switch is successful and the motor runs normally.

(2) Power frequency switching to frequency conversion test (first time): DCS sends a power frequency switching to frequency conversion command, J3 trips, J1 and J2 close, after J1 and J2 close, the inverter's overcurrent (ioc) alarm is activated, J1 and J2 are tripped again, and then J3 is closed to switch back to power frequency operation. Power frequency switching to frequency conversion is unsuccessful.

(3) We considered that the back EMF of the motor might be out of sync with the power supply on the upper side of J1, causing the inverter IOC to operate. Therefore, we connected the J2 closing auxiliary contact in series with J1 so that J1 could be closed only after J2 was closed.

(4) Power frequency switching to variable frequency test (second time): After the motor is started, j3 is manually tripped and j2 is closed. At this time, j1 is not closed. Then the inverter outputs an "ioc" alarm and automatically switches back to the power frequency due to a frequency conversion failure. Therefore, it can be proved that "ioc" comes from before j1 is closed and the inverter is started.

(5) Through the previous step, it can be confirmed that the IOC alarm is caused by the reverse electromotive force of the motor. In order to avoid the influence of the motor's reverse electromotive force, we modified the control circuit and connected a time relay SJ in series. That is, after J3 tripped, J1 and J2 were delayed. The initial delay time was set to 4s.

(6) Power frequency to variable frequency test (third time): The DCS sends a command to switch from power frequency to variable frequency, and the switch is successful. Since the motor is a rotating load at this time, the inverter captures and restarts the running motor, which takes a long time. After about 50 seconds, the motor reaches the normal rated speed of 1000r/min, which cannot meet the furnace pressure requirement (±2.5kpa).

(7) Power frequency to variable frequency test (fourth time): Adjust the SJ delay time to 5s. The fourth time DCS sends the power frequency to variable frequency command. The switch is successful, but the inverter capture and restart time is longer. After about 100s, the motor reaches the normal rated speed of 1000r/min, which cannot meet the furnace pressure requirements.

4.2.2 Second commissioning (motor no-load)

After studying the manual of the inverter, the inverter has the characteristics of dealing with rotating loads, allowing the inverter to measure the speed of the motor that is already in operation. The inverter can provide the motor with an output voltage with the same frequency as the rotating motor, so that the impact on the motor when the inverter is powered is minimized. The rotating load characteristics are divided into 2 stages. In the first stage, the rotating load operation is automatic and the user does not need to make any adjustments. The inverter monitors the motor flux and can start the motor immediately. This stage lasts until the motor flux can be detected. In a typical case, if the interval between the inverter prohibition and restart is 3

The second stage includes a scanning feature, during which fixed currents of different frequencies are added to the motor. The inverter monitors the motor flux. When the motor flux reaches the flux threshold, it is assumed that the frequency added by the inverter is equal to the rotation speed of the motor. At this stage, the parameters need to be adjusted to make the "scanning function" work properly. In other words, if the inverter restarts within 3 to 4 motor time constants, it can start immediately. Therefore, we believe that the long SJ delay causes the inverter to capture the slow restart time.

Modify the control parameters, open j3 and then close j1 and j2 after a delay of 2s. The inverter self-test time is about 3s.

Power frequency switching to variable frequency test (fifth time): The power frequency switching to variable frequency start-up was successful, the inverter started immediately and quickly increased the speed to 600r/min. The frequency conversion output accelerated from 0hz to 45hz in 20.9s, which can basically meet the needs of the furnace pressure.

4.2.3 Third commissioning (motor with load)

Under the condition that the primary and secondary fans, two induced draft fans, and high-pressure return fan of the 2# boiler are all turned on, no coal is added, and the boiler load is 100t/h, the power frequency switching frequency conversion control circuit is modified and debugged: j1 is closed immediately after j3 is tripped, and j2 is closed after a delay of 2s, shortening the inverter starting time, which is close to the actual working condition test of the boiler. The debugging situation is as follows:

(1) Frequency conversion to power frequency test: The DCS sends a command for manual switching. After the command is sent, J1 and J2 trip and J3 closes, and the frequency conversion is successfully switched to power frequency.

(2) Power frequency to variable frequency test: Before the test, the process damper opening is 40%, the motor current is 128A, and the variable frequency output is set to 100%. The DCS issues a power frequency to variable frequency command. After the command is issued, the inverter starts within 10 seconds and increases the speed to the set speed. During the switching process, the furnace negative pressure fluctuates between -0.6kPa and 0.5kPa. The pressure setting value for the furnace interlock shutdown is within ±2.5kPa. The boiler operates normally and the switching is successful.

(3) Simulate inverter failure and switch to working frequency: before the test, the process air door opening is 100%, the furnace pressure is -0.1kpa, the 2b# induced draft fan air door is automatic, the inverter emergency stop button is pressed manually, j1 and j2 trip, j3 delays 12s to close, the furnace pressure fluctuates between 0.4kpa and 0.88kpa, the 2b# induced draft fan damper opens from 0 to 12%, and then closes back to 9%, and the switch is successful.

(4) Switching from industrial frequency to variable frequency: The parameters before switching were damper opening 40%, motor current 128A, frequency setting 100%, and it took 34 seconds from DCS issuing the command to switching completion and restoration of stability. The furnace pressure fluctuated between 0 and 1.3 kPa, and the switching was successful.

(5) Frequency conversion to power frequency: The parameters before switching are damper opening 100% and furnace pressure 0 kPa. After the switching command is issued, j1 and j2 trip, and j3 closes after a delay of 13.8 seconds. The furnace pressure fluctuates between 0 and 1.06 kPa, and the switching is successful.

(6) Switch from industrial frequency to variable frequency: Before switching, the induced draft fan damper opening is 40%, the motor current is 128a, the inverter setting is 100%, the 2b# induced draft fan damper is manually switched, and the furnace pressure is 0kpa. After the switching command is issued, j3 trips, j1 closes, and after a delay of 2s, j2 closes. After 32s, the inverter completes the startup until the motor speed is fully restored. During the switching process, the maximum furnace pressure is 0.8kpa, and the switching is successful.

(7) The above three times of switching from variable frequency to industrial frequency and from industrial frequency to variable frequency have all been successfully debugged, which can ensure the continuous operation of the induced draft fan during the switching process and the boiler pressure fluctuation is within the allowable range. However, it should be pointed out that the above are test data under the condition of no coal added to the boiler and a load of 100t/h. If the boiler is operated at full load of 410t/h, the online switching between industrial and variable frequency needs to be verified in practice.

5. Performance analysis and economic evaluation after transformation

In February 2009, after the 2a induced draft fan inverter was put into use, after three months of continuous operation, the situation has been very stable, and the power saving effect is very obvious. The analysis is as follows:

(1) The motor power of 2a# induced draft fan is reduced from 830 kW to about 400 kW, and the motor load of 2b# induced draft fan remains unchanged. After deducting the power consumption of 20 kW for the inverter room air conditioner, the motor can save (830-400-20)×24=9840 kW·h per day compared with the motor before the frequency conversion transformation. The annual power saving (based on 330 days of operation) is 9840×330 days=3247200 kW·h. Based on the average external power price of about RMB 0.72/kW·h (tax included), the annual power saving is RMB 2.338 million, which fully achieves the expected power saving effect.

(2) After the frequency conversion transformation, the plant's standard coal consumption for power supply decreased from 360g/kw·h to 0.8g/kw·h. On the one hand, this improved the technical and economic indicators. On the other hand, based on an annual power generation of 500 million kw·h and a coal price of 600 yuan/t, the plant can save 400 tons of coal throughout the year, saving about 240,000 yuan in fuel costs.

(3) The investment in a single frequency converter is about RMB 2.5 million, and the entire investment can be recovered in less than one year. The frequency conversion transformation of the 1a induced draft fan of another CFB boiler will be carried out during the CFB boiler maintenance in the second half of 2009.

6 Conclusion

Through the above analysis, we can draw the following conclusions:

(1) After the fan was transformed by frequency conversion, the power saving effect was very obvious. Although the initial investment was larger, the one-year investment recovery period was enough to make up for the large initial investment. After the transformation of one induced draft fan in the thermal power plant, the damper opening of another induced draft fan of the CFB boiler was only about 40%. In addition, the damper opening of the primary fan was 58%, and the damper opening of the secondary fan was 43%. The energy consumption was still serious, and the power saving potential was great. It is recommended to invest in the transformation to frequency conversion operation as soon as possible.

(2) This frequency conversion modification of the induced draft fan meets the thermal power plant's requirement for continuous heating and power supply to the chemical plant, and realizes the online switching function between industrial and variable frequency, which can fully ensure the continuous and normal operation of the power plant boiler and solve the problem that the power plant frequency converter cannot switch back from industrial frequency to variable frequency online. It is particularly suitable for thermal power plants with long-term continuous operation.