1. Introduction Since 2003, China has faced severe resource pressures, with coal, electricity, and oil all in short supply. Nearly 80% of provinces have implemented power rationing, and energy bottlenecks are becoming the most significant factor restricting the sustainable growth of the national economy. Pumps and fans are widely used power-consuming equipment across the country, particularly in thermal power plants. Statistics show that the average power consumption rate of thermal power plants nationwide is approximately 7%–8%, with pumps and fans accounting for about 75% of this consumption. Currently, the efficiency of most pumps and fans used in China is 5%–10% lower than that of similar products in industrialized countries; and the actual operating efficiency of pumps and fans is generally 10%–30% lower than in industrialized countries. Therefore, actively promoting energy conservation and consumption reduction in pumps and fans is of practical and far-reaching significance for promoting the sustainable development of the national economy, narrowing the gap with industrialized countries, improving the economic efficiency of power enterprises, and enhancing market competitiveness. 2. Equipment Overview and Existing Problems The Huaneng Weihai Power Plant Phase II 2×300 MW turbine unit is a domestically produced, imported N300-16.7/538/538 intermediate reheat condensing steam turbine unit. The condensate pumps are 9LDTNA-4 type manufactured by Shenyang Pump Factory, and the motors are YLS560 type, with a rated power of 1000 kW, voltage of 6000 V, current of 114 A, and speed of 1486 r/min. During normal operation, one of the two condensate pumps is in operation, and the other is on standby. When the operating pump fails, the standby pump automatically starts and is put into operation. After being pressurized by the condensate pumps, the condensate flows through each low-pressure heater and the main and auxiliary condensate regulating valves, finally reaching the deaerator. The opening degree of the main and auxiliary condensate regulating valves is regulated and controlled by the deaerator water level control loop (single-impulse or two- or three-impulse) consisting of the actual deaerator water level and setpoint, feedwater flow rate, and condensate flow rate. The opening degrees of the main and auxiliary condensate regulating valves during normal unit operation are shown in Table 1. Table 1 shows that the condensate pump is in a state of extreme throttling for a long time, resulting in a large waste of energy and extremely low efficiency. 3. Energy-Saving Principle of Variable Frequency Speed ​​Control AC variable frequency speed control technology is an emerging technology. With its excellent speed regulation and braking performance, significant energy-saving effect, and wide applicability, it is widely recognized as one of the most promising AC speed control methods, representing the mainstream direction of electric drive development. Currently, AC variable frequency speed control technology is developing rapidly, evolving from the original conventional control method PAM (Pulse Amplitude Modulation), which changes the amplitude of the voltage and current sources for output control, to the current PWM (Pulse Width Modulation), which modulates the voltage output waveform by changing the pulse width and SPWM speed control method (sine wave pulse width modulation). Currently, the most ideal modulation method is space vector control, which approximately simulates the speed regulation performance of a DC motor and is comparable to the speed regulation behavior of a DC motor, making it a relatively ideal speed regulation method. In the formula: n—the speed of the asynchronous motor, r/min; p—the number of pole pairs of the motor; s—the slip of the motor; f[sub]1[/sub]—the power supply frequency, Hz. From Equation 1, it can be seen that if the number of pole pairs P is constant and the slip S does not change significantly, the speed n is basically proportional to the power supply frequency f[sub]1[/sub], that is, changing the power supply frequency f[sub]1[/sub] can change the speed n (in reality, simply changing the power supply frequency will not achieve satisfactory speed regulation performance. The voltage must be adjusted simultaneously with the frequency; the accurate term should be variable frequency and variable voltage speed regulation, or simply VVVF speed regulation). According to the proportionality law, the relationship between the pump power P and the speed n is: where N—power, kW; n—speed, r/min. From Equation 2, it can be seen that the power is proportional to the cube of the speed; reducing the speed can significantly reduce the power. For example, if the pump speed is 80% of the rated speed, the power is 51.2% of the rated power, which shows a power decrease of 48.8%. Therefore, variable frequency speed regulation is a highly efficient variable frequency regulation, far superior to energy-saving regulation. [b]4 Project Scheme Introduction and Problems and Handling During Commissioning 4.1 Scheme Introduction[/b] After analyzing and comparing the performance (A-B, Siemens, and Robicon) of frequency converters from different manufacturers used in our plant, Huangdao Power Plant, and Longkou Bainian Power, we found that the AB frequency converter system is simple, highly reliable, and has good after-sales service, fully meeting our plant's environmental and technical requirements. Therefore, we decided to use Rockwell Automation products in the condensate pump frequency conversion retrofit of Unit #3 in 2005. Based on our plant's geographical environment and equipment characteristics, we decided to use AB PowerFlex7000 medium-voltage air-cooled frequency converters, three-knife switch bypass cabinets, control power distribution boxes, and one SANTAK 6kVA UPS power supply manufactured by Rockwell Automation, Canada. Variable frequency drive model AB Power1～ex7000 Type B air-cooled (csI-pVqM) Power 1 000kW Current 120A Input 6000VAC 50Hz Output 0～6 000VAC 0～70Hz (1) Structure, protection configuration and electrical secondary wiring diagram of variable frequency speed control system The variable frequency drive consists of two parts: high voltage main circuit and low voltage control circuit. High voltage main circuit: The three-phase 6000V input power supply is converted from AC to DC by the input rectifier (PWM rectifier) ​​composed of symmetrical gate commutated thyristors SGCT. After passing through the filter DC reactor, it is connected to the inverter composed of SGCT. In the pulse width modulation mode, the DC is converted to AC. Through the motor filter capacitor, the almost perfect sinusoidal current and voltage waveform is sent to the motor. According to the deaerator water level adjustment command, the voltage and frequency are adjusted to achieve the purpose of energy saving. The high voltage main circuit is shown in Figure 1. Low-voltage control circuit: The AC control power supply is provided by two power sources, which can be automatically switched. The outputs are the inverter switch cabinet control power supply, the inverter fan power supply, and the single-phase UPS power supply (providing uninterruptible power to the inverter control section). Deaerator water level control circuit: The deaerator water level is one of the important parameters for unit operation monitoring. Before the condensate pump frequency conversion modification, when the feedwater flow rate is less than 25%, the T1 switch is selected on the N side, and the deaerator water level uses single-impulse regulation. The output of regulator PID1 directly controls the opening of the regulating valve. When the feedwater flow rate is greater than 25%, the T1 switch is selected on the Y side. The deaerator water level uses three-impulse regulation. The output of regulator PID2, plus the feedwater flow signal, is used as the setpoint for regulator PID3, and compared with the condensate flow rate. After proportional-integral calculation, the output controls the opening of the regulating valve. After the condensate pump is retrofitted with a frequency converter, only a three-impulse signal is output to the frequency converter. The internal control unit of the frequency converter then outputs adjustment voltage and frequency to control the condensate pump motor speed, thereby meeting the deaerator water level regulation requirements. The deaerator water level control circuit before and after the condensate pump frequency conversion retrofit is shown in Figure 2. Frequency converter primary wiring: The power supply is connected to the 6 kV VIIIA section bus, switch number 6519, and transmitted to the frequency converter via the Gongliu/frequency converter switch cabinet, which can realize "power frequency" or "frequency conversion" operation. The electrical primary wiring diagram is shown in Figure 3. Protection configuration: 1) Electrical protection: Switch 6519 is equipped with instantaneous trip, negative sequence, undervoltage, and grounding protection, all of which trip. Among them, the instantaneous trip, negative sequence, and grounding protection trip the frequency converter after tripping. The frequency converter itself is equipped with undervoltage protection. The 6519 switch is equipped with an output pressure plate for inverter fault tripping. Before the inverter is put into operation, the "Inverter Interruption 6519 Switch" output pressure plate is engaged; before switching from "inverter" to "power frequency" operation, the "Inverter Interruption 6519 Switch" output pressure plate is disengaged. The 6519 switch protection settings use universal settings for both power frequency and inverter operation; no modification to the protection settings is required when the inverter is engaged or disengaged. 2) Thermal control interlock: The 3A# condensate pump requires the condensate to deaerator regulating valve to be fully open during inverter operation. Therefore, the condensate header pressure is as low as 0.846 MPa (currently, the total operating opening of the regulating valve is 47%, at which point valve B is fully open and valve A is fully closed). To ensure that the #3B condensate pump does not operate under low water pressure (electrical value is 1.8 MPa during power frequency operation), the setting is modified to below 0.6 MPa, triggering the standby pump. When the header pressure reaches 0.7 MPa, the thermal control indicator light will alarm. The inverter tripped, and the condensate to deaerator regulating valve was switched to 25% f. Because the unit is currently running, the DPU data cannot be modified. Therefore, it will be modified after the unit is shut down. After modification, the regulating valve will be fully open. Other thermal control protections remain unchanged. (2) The AB PowerFlex7000 inverter used in our plant is a third-generation medium-voltage current inverter from Rockwell Automation. It has a simple structure, small size, advanced technology, reliable system, convenient use and high cost performance. The inverter device is shown in Figure 4. 1) It adopts advanced CS-PWM and speed feedback full vector control technology. There are no transformers at the input and output ends; the bus inductor has a current limiting function and can reliably protect against short-circuit current. The power unit has no semiconductor fuse. The frequency conversion regulation accuracy is high, the range is wide, and the efficiency is as high as 95% or more. 2) It has excellent performance and high reliability. It has a strong adaptability to grid voltage fluctuations. It can output at full load with ±10% and can withstand a 30% drop in grid voltage and continue to operate. It can operate at full load for 5 cycles without tripping during momentary power loss, and even longer under light load. It can achieve four-quadrant operation (forward and reverse rotation and regenerative braking) and rapid start-up, with energy fed back to the grid. Without the need for additional filters, the capacitor at the motor end eliminates the main harmonic components of the motor during high-frequency operation, thus providing the motor with near-sinusoidal current and voltage, conforming to the IEEE-519-1992 harmonic standard. This reduces motor noise and the requirements for heat dissipation and insulation strength; the motor does not need to be derated or modified. There are no length requirements for the output cable. 3) Convenient to use: The inverter can perform no-load testing without a motor at the output, and can also perform no-load testing with low voltage when there is no 6kV high voltage. It adopts an innovative modular design for power units and fiber optic transmission. Units can be removed, moved, and changed from the rack; all units are interchangeable, and unit replacement can be completed within 15 minutes without special tools. The filter can be washed multiple times without deformation or failure, and can be easily disassembled and replaced during operation. 4) Comprehensive protection against overvoltage, undervoltage, phase loss, inverter and motor overload, inverter overheating, output grounding and short circuit, high-voltage cabinet door protection interlock, etc., and can trip the input side 6 kV switch, and the switch can also trip the inverter. 5) It has a universal control interface and a user-friendly operator interface. It can be used with the unit's DCS system for automatic or manual remote operation, as well as local start-up, shutdown and adjustment. The local control device is shown in Figure 5. The large-screen text and graphic display operator terminal menu guide screen completes various inverter operations such as setting, monitoring, diagnosis, alarm, etc., and displays commonly used variables such as voltage, power, speed and cumulative time. The operator interface is shown in Figure 6. 4.2 Problems encountered during commissioning and their handling 1) In the wind pressure storage setting, change the parameter of the inverter's "standby fan function" from "NO" to "YES", that is, start the standby fan function (this inverter does not have a No. 2 fan, it is a virtual fan here, and only the standby fan contact switching function on the XIO board is used to achieve mutual backup of No. 1 and No. 2 fans). Then, short-circuit terminals J2-7 and J2-9, and terminals J2-6 and J2-8 on the XIO board of the inverter. After short-circuiting, the control of fan #1 and fan #2 is actually in parallel, simultaneously controlling the operation of fan #1. The modified effect: When the 380V fan power supply is switched, the power supply to fan #1 temporarily disappears, and the air pressure drops. At this time, because the inverter has activated the standby fan automatic transfer function, the contact of fan #2 on the XIO board automatically closes, ensuring the fan contactor is closed and the fan continues to run. At this time, the inverter only issues an alarm, not a fault signal, and continues to run. 2) The touchscreen parameter status display shows that the rated speed is only 50% of the actual speed. This problem can be eliminated after the display software is upgraded. 3) During the debugging process, the "bypass output switch open" alarm continuously occurs when the inverter is working, even though the inverter's bypass switch is actually working normally. This is caused by a software logic error, and this alarm has now been disabled. This can be eliminated after a software upgrade. 5. Energy Saving Effect Analysis Currently, only the 3A# condensate pump in the condensate system of Unit 3 has been converted to variable frequency operation. Once conditions are ripe, a "one-to-two" configuration or an additional variable frequency drive will be adopted, depending on the energy saving situation. Figure 7 shows the real-time power and rated power of the 3A# condensate pump under the same load in both power frequency and variable frequency operation. Figure 8 shows the load curves of Unit 3 on Friday, Saturday, and Sunday (April 15-17, 2005). Assuming an annual operating time of 7200 hours (300 days), and using the load curve in Figure 8 as a typical daily load curve, based on the real-time power of the 3A# condensate pump under the same load in Figure 7, it can be calculated that the annual energy saving through variable frequency speed control is over 1,881,680 kWh. Based on an electricity price of 0.397 yuan/kW•h, the total annual electricity savings are 188.1680 x 0.397 = 747,000 yuan. The investment payback period T = investment amount / annual electricity savings = 280 / 74.7 = 3.75 years. The investment amount includes AB frequency converters, disconnect switch bypass cabinets, control power distribution boxes, UPS power supplies, labor costs, and construction costs. Since 2005, the power grid load in eastern Shandong has been relatively tight. From April 15th to 17th, 2005, the load factor was 82.03% (the load factor will decrease after the 500 kV Qingzhou substation is put into operation in the second half of the year), far exceeding the load factor for the entire year of 2004. In 2004, Unit #3 generated 171,158.04 million kWh, with 7,838.62 hours of usable power, resulting in an annual load factor of 72.78%. The average load was calculated to be 218.35 MW. Using interpolation, the power saving of the condensate pump under variable frequency operation at this load was calculated to be 391.9 kW, with an energy saving rate of 391.9/100 = 39.19%. Figure 9 shows the energy saving rate of the condensate pump variable frequency drive under different loads. Based on the 2004 load factor, the payback period TT = investment amount / annual electricity cost saving = 280 / (391.9 × 7,838.62 × 0.397 / 10000) = 2.29 years. Applying AB variable frequency drives can achieve considerable economic benefits in the condensate pump variable frequency retrofit; the lower the load factor, the higher the economic benefits. In both the construction of new generating units and the renovation of old units, AB PowerFlex 7000 frequency converters will have broad application prospects in terms of energy saving and consumption reduction.