Abstract: This paper introduces the application and effects of low and medium voltage inverters in the frequency conversion retrofitting of fans and pumps in thermal power plants. Keywords: inverter, energy saving, retrieving term of investment. Introduction Variable frequency speed control has unparalleled advantages over other speed control devices in terms of energy saving, speed control accuracy, and speed control range. It also facilitates communication with automated control systems (such as DCS systems), leading to its widespread application in various fields. This paper introduces and summarizes several successful examples of our company's technological transformation using variable frequency speed control in recent years, thereby illustrating the broad application prospects of frequency converter devices in technological transformation across various industries. 1. Variable Frequency Drive (VFD) Speed ​​Control for Boiler Fan Motors 1.1 VFD Speed ​​Control for Boiler Secondary Air Fan Motors Our company's boilers are 75 (T/H) circulating fluidized bed boilers. Each boiler is equipped with one induced draft fan, one primary air fan (forced draft fan), and one secondary air fan. The main technical parameters of each motor are as follows: Before the VFD upgrade, the operating data of each fan under normal conditions, based on the annual operating conditions in 2000, are as follows: We know that due to the large variations in fuel composition, heat load, electrical load, and seasonal factors during normal boiler operation, the amount of air required for boiler combustion also varies considerably under different conditions. However, the boiler fans are designed based on the maximum air volume required under the boiler's maximum output conditions, and a certain air volume margin must be considered in case of boiler accidents. Therefore, the power configuration of the fan motors is generally large. The statistical data in the table shows that the average opening of the boiler fan dampers is approximately 48% for the induced draft fan and only about 45% for the secondary air fan under normal conditions. Using baffles for control resulted in significant energy waste due to the resistance of the baffles, leading to high plant power consumption and impacting the unit's economic operation. At the end of 2000, we first attempted a modification on the secondary air fan of Boiler #1. Considering the large power margin in the secondary air fan motor design, we intentionally selected a 132kW frequency converter to control a 160kW motor. The frequency converter was a Mitsubishi FR-F540L-132K with a voltage rating of 380V. Installation and commissioning were completed in January 2001, and the frequency converter was put into operation. After a period of testing, the secondary air fan's power frequency current decreased from an average of 135A to an average of 70-75A, demonstrating significant energy savings. Furthermore, the frequency converter's technical performance fully meets the boiler's operational requirements (mainly air pressure, air volume, and the rate of air injection/reduction). According to electricity meter readings, the energy efficiency is around 45%, meaning the investment can be recovered within one year. Moreover, the motor's starting, operation adjustment, and control functions have all been greatly improved. Due to the significant benefits, in February 2001, our company also upgraded the frequency converters for the secondary air fans of boilers #2 and #3, and they have been operating well ever since. In the bypass design of the boiler secondary air fan motor, we adopted a double-throw switch with manual switching, which has also worked very well in actual use. It not only saves investment, but also makes the wiring simple and reliable, and the installation is quite convenient. The wiring diagram of the secondary air fan frequency converter is shown in Figure (1). [align=center]Figure 1 — Wiring Diagram of Secondary Air Fan Inverter[/align] 2. Application of Inverter Speed ​​Control in Boiler Induced Draft Fan Motor After the successful modification of the inverter regulating device on the boiler secondary air fan, we saw the huge energy-saving potential and good benefits of inverter technology modification. In December 2002, we carried out high-voltage inverter speed control system modification on the No. 3 boiler induced draft fan. After extensive technical investigation and comparison, we believe that the HARSVERT-A06/050 inverter produced by Beijing Leadway Technology Co., Ltd. has a good performance-price ratio advantage, and the product is widely used in the domestic market with comprehensive and thoughtful after-sales service. Figure 2 — Wiring Diagram of Induced Draft Fan Inverter. The HARSVERT-A06/050 inverter speed control system adopts multi-level module series, AC-DC-AC, high-to-high type circuit, the power converter adopts 30 pulse, diode three-phase full bridge, and the output adopts IGBT. The inverter bridge is connected in series. The original motor power supply is directly used as the inverter input power supply, and then the inverter output is connected to the motor. To ensure the reliability of the system, the inverter is also equipped with a power frequency bypass device. In case of inverter failure, the inverter stops operating, and the motor can be manually switched to power frequency operation. The bypass consists of three high-voltage disconnect switches QS1, QS2, and QS3 (see Figure 2, where QF is the circuit breaker in the original high-voltage switch cabinet). During frequency conversion operation, QS1 and QS2 are closed, and QS3 is open; during power frequency operation, QS3 is closed, and QS1 and QS2 are open. Variable frequency speed regulation is remotely controlled by the start, stop, and speed adjustment switches installed on the boiler control panel. It can also interface with the DCS system to realize the speed control of the inverter through the DCS. The variable frequency speed regulation device also provides alarm indication, fault indication, standby status, running status, bypass status, high-voltage closing permission, high-voltage emergency disconnection and other protection information, as well as necessary indications such as speed setpoint and actual fan speed, to facilitate operator control. The equipment arrived on April 24, 2003, and installation and commissioning were completed on April 28, a total of four days. A series of dynamic tests were conducted, including: a 50Hz full-load operation test of the frequency converter, a critical vibration test of the motor-fan system, a bus voltage fluctuation test, an interlock test between the frequency converter and the high-voltage switch, and a continuous rapid load increase/decrease test. After everything was deemed normal, a continuous 72-hour trial run was conducted, and the equipment was put into normal operation on May 1. Since then, the equipment has been operating normally without any abnormalities. Before the frequency converter modification, according to statistics, the operating current of the boiler induced draft fan was around 25A. After the modification with the frequency converter adjustment device, under the same boiler operating conditions, the current of the boiler induced draft fan after frequency conversion is currently around 9-10A, an average current reduction of 15-16A. Calculations show an average power saving of 128-136 kWh/h. Considering the unit's own power consumption and future air conditioning power consumption of 16 kWh/h, the overall power saving can reach 112-120 kWh/h, with a power saving rate of 53%-56%, resulting in significant economic benefits. Therefore, the boiler induced draft fan high-voltage frequency converter energy-saving technology renovation project is quite successful. Our company plans to further renovate other boiler induced draft fans using frequency converter regulation devices to achieve even greater economic benefits. 3. Application of Frequency Converter for Water Supply Pump Motor Regulation and Control Our company's thermal power plant consumes approximately 6,000-7,000 tons of water daily, mainly for water purification, industrial water, and domestic water. The pump house has three water supply pumps, and their motor technical parameters are as follows: On the one hand, the output of pump #3 is too small to meet the daily water consumption needs, while the output of pumps #1 and #2 is too large, making the pumps difficult to control and regulate, resulting in excessively frequent motor start-stops. On the other hand, considering the water purification effect of water purifiers, the ideal purification method is to maintain a certain water volume for continuous water supply, which results in high purification efficiency and good effect. Frequent starting and stopping of the water pump can easily lead to serious water outages. During periods of water purifier shutdown, prolonged exposure to sunlight can cause the plastic inclined tubes inside the purifier to age, affecting their lifespan. If the water purifier is shut down for too long, it needs to be backwashed again upon restarting, resulting in significant water waste. Furthermore, the distance between the water supply pump room and the water purifier is approximately 600 meters, and the pipeline is relatively shallow, leading to pipe ruptures in winter. All of these issues are caused by prolonged water outages, resulting in stagnant water in the pipes freezing at low temperatures. Therefore, we modified the motors of water supply pumps #1 and #2 by adjusting their frequency converters to achieve continuous water supply control. The selected frequency converter is a Mitsubishi FR-F540L-37 with a voltage rating of 380V. The modification of the water supply pump to use a frequency converter for water supply regulation is not only due to its energy-saving benefits for the motor, but more importantly, from the perspective of production equipment operation safety. The modified pump operates well and improves the water purification effect of the water purifier. It also achieves the goal of continuous water supply. Our design for the frequency converter control of the water supply pump follows the scheme below (wiring diagram shown in Figure 3). 3.1 To fully utilize the frequency converter, we use one frequency converter to control the speed of two motors; 3.2 Both water supply pumps can operate in both variable speed and constant speed modes. The frequency converter can only power one motor at a time, so the start and stop of each motor must be mutually interlocked, controlled by logic circuits to ensure reliable switching. A double-throw switch is used at the outlet; 3.3 When the two water supply pumps are working, one is powered by the mains frequency for constant speed operation, and the other is powered by the frequency converter for variable speed operation. The variable speed and constant speed operation of the same motor are mutually interlocked by AC contactors. That is, during variable speed operation, constant speed cannot be engaged, as shown in the diagram below, 1C1 and 1C2, and 2C1 and 2C2 are not allowed to be engaged simultaneously; 3.4 To ensure safe and reliable process control, the frequency converter and the control, protection, and measurement units of both motors are all centralized in a local control cabinet. Control and adjustment are led to the control room via shielded signal cables; Figure 3 - Wiring of the water pump motor frequency converter, the dotted box indicates the added part of the modification . 4. Some Technical Issues to Note in Frequency Converter Speed ​​Regulation Modification Power plants can fully utilize frequency converters for energy-saving technology modification, which can not only improve economic efficiency but also generate huge social benefits and promote the technological progress of enterprises. However, technically, frequency converters with appropriate characteristics should be selected according to different production equipment. For example, in the frequency converter modification of boiler fans, in addition to considering whether the speed-up and speed-down characteristics of the frequency converter meet the requirements of the combustion process, the following issues must also be considered to avoid investment losses. 4.1 The safe operation of the boiler is the fundamental guarantee of the plant's power. Although the frequency converter speed regulation device is reliable, once a problem occurs, it is necessary to ensure the safe steam supply of the boiler. Therefore, a switching system (bypass system) for power frequency to frequency converter operation must be implemented. In the production process, if manual switching can meet the equipment operation process requirements, it is recommended to avoid using automatic bypass. For general small-power motors, using a double-throw switch as a manual/automatic switching method is also a relatively ideal approach. 4.2 For motors with large inertia loads (such as boiler induced draft fans), after frequency conversion modification, attention should be paid to the possibility of torsional resonance in the fan. If resonance occurs during operation, it will severely damage the fan and the driven motor. Therefore, it is necessary to calculate or measure the critical speed for torsional vibration of the fan-motor connection shaft system and take corresponding technical measures (such as setting a frequency jump function to avoid the resonance point, flexible connections, and adding vibration-absorbing rubber to the frame). 4.3 After adopting frequency conversion speed control, if the frequency converter operates below 1/2 of the power frequency for a long time, as the motor speed decreases, the motor's heat dissipation capacity also decreases, and the heat generated by the motor also decreases. Therefore, the motor's own temperature actually decreases, and it can still operate normally without overheating. 4.4 The frequency converter cannot be powered in reverse from its output port. This must be considered in the electrical circuit design. For example, in the wiring diagram of the high-voltage frequency converter for the induced draft fan, QS2 and QS3 cannot be closed simultaneously. In the wiring diagram for the frequency converter retrofit of the water supply pump, 1C1 and 1C2, and 2C1 and 2C2 cannot be closed simultaneously. This requires not only electrical interlocking in the secondary electrical circuits but also mechanical interlocking to ensure the safe operation of the frequency converter. 4.5 Low-voltage frequency converters are relatively easy to install in retrofits due to their smaller size. However, high-voltage frequency converter systems are relatively large, generally consisting of 4-5 cabinets. For retrofit projects, a new frequency converter room usually needs to be built. Therefore, when selecting the location of the frequency converter room, it is necessary to consider both the distance from the motor equipment and the potential impact of the surrounding environment on the operation of the frequency converter. The installation and operation environment of frequency converters has high requirements. To ensure long-term stable and reliable operation, the indoor temperature of the room where the frequency converter is installed should ideally be controlled between 0 and 40℃. If the temperature exceeds the allowable value, appropriate air conditioning equipment should be considered. Simultaneously, the room should be free of large amounts of dust, corrosive or explosive gases, conductive dust, etc. 4.6 A reliable connection between the frequency converter cabinet and the factory ground must be ensured to guarantee the safety of personnel and equipment. To prevent signal interference, the control system should ideally have an independent grounding system, with a grounding resistance not exceeding 4Ω. Signal lines to the frequency converter must use shielded cables, and one end of the shield must be reliably grounded. 4.7 When selecting a frequency converter, based on the actual operating conditions of the equipment, a lower power rating can be considered. For example, our company uses a 132 kW frequency converter to control a 160 kW motor in our secondary air fans, and the operation has consistently been excellent. 5 Conclusion With the development of power electronics technology, the technical performance of frequency converters has been broadened and improved. In the thermal power industry, there are many fan and pump loads, and the full application of frequency converters for energy-saving renovation has gradually been accepted. For low-voltage frequency converters, the investment is relatively low and the benefits are high, and the investment can be recovered in about one year, so they are widely used. However, compared with high-voltage frequency converters, the price is still relatively high, and the investment recovery period generally takes about two years. However, with the rapid development of domestic high-voltage frequency converters, the performance-price ratio of frequency converters has been greatly improved, providing a broader prospect for energy-saving technology renovation using frequency converters. References: [1] One Hundred Examples of Variable Frequency Speed ​​Regulation Application / Edited by Wang Zhankui et al. — Beijing Science Press, April 1999 [2] Frequency Converter Application Manual / Edited by Wu Zhongzhi and Wu Jialin — 2nd Edition — Beijing Machinery Industry Press, July 2002