Abstract: The correct selection of frequency converters is crucial for the normal operation of the electrical control system of mechanical equipment. When selecting a frequency converter, the first step is to start from the actual situation of the enterprise and rationally select the voltage level of the frequency converter. Within a certain power range (below 1000Kw), low-voltage frequency converters should be selected as much as possible. At the same time, the type of mechanical equipment, load torque characteristics, speed range, static speed accuracy, starting torque, and operating environment requirements should be considered to determine the type of frequency converter with the appropriate control method and protection level. Keywords: Variable Frequency Drive (VFD) accuracy selection. VFDs are mainly used for regulating the speed of AC motors and are an ideal speed control solution. With the rapid development of China's economy, the market demand for transmission products is gradually increasing. VFD speed control, with its advantages of wide speed range, high speed accuracy, and good dynamic response, plays an increasingly important role in many speed control applications. In addition to its excellent speed control performance, it also has significant energy-saving effects. VFDs are particularly useful in the application of fans and pumps. These types of loads are numerous and widely distributed across various sectors of the national economy, consuming huge amounts of electricity, while their speed range is limited (generally 50%-100%), making it a promising application area. Taking power plants as an example, the transmission systems of many boiler auxiliary machines generally have large capacities, resulting in serious energy waste when operating at constant speed. Using high-power VFDs can complement or modify boiler fans and pumps in power plants, such as boiler blowers, boiler induced draft fans, boiler water pumps, condensate pumps, slag flushing pumps, and slurry pumps, resulting in significant energy savings. At the same time, it can achieve a high degree of automation control, making the boiler operation safer and more reliable. For example, the variable frequency speed regulation of the induced draft fan makes the negative pressure in the furnace more stable; the variable frequency speed regulation of the primary air fan makes the air delivery more stable and the coal combustion more complete; the variable frequency speed regulation of the slurry pump can protect the slurry pump motor and prevent the pump from clogging, overload and burning out the motor. I. Current Status of High-Power Motors in Domestic Power Plants Most of the auxiliary boiler fans and pumps in domestic power plants have a power output between 200kW and 2000kW. According to domestic practice, motors above 200kW are typically 6kV motors, resulting in the main auxiliary equipment in power plants using 6kV motors. From the perspective of reducing line losses, the power sector hopes to increase the supply voltage and vigorously promotes 10kV. Users, from the perspective of simplifying configuration, naturally prefer motors and frequency converters above 200kW to use 6kV or 10kV. The advantages of this configuration are that it saves on the purchase cost and installation workload of a single motor; a bypass switch for the frequency converter can be directly installed, allowing switching to the 6kV or 10kV plant busbar directly in case of frequency converter failure, without interrupting operation. However, choosing 6kV or 10kV "direct" frequency converters is unreasonable from both a technical and economic perspective. Currently, all "direct" high-voltage frequency converters are not true direct frequency converters; they all have transformers on their input side or rely on series connection of electronic components. Therefore, it is unnecessary for the motor and frequency converter to be consistent with the grid voltage. From an economic perspective, a 560kW high-voltage frequency converter costs more than double the price of a low-voltage 660V or 690V frequency converter, exceeding 1.2 million yuan. This indicates that within a certain power range, even considering the costs of replacing the motor and adding a rectifier transformer, choosing a low-voltage solution is much more economical than a high-voltage solution, with lower subsequent equipment maintenance costs and less demanding technical requirements for repairs. II. Comparison of High and Low Voltage Frequency Converter Structures In recent years, various high-voltage frequency converters have emerged. However, to date, high-voltage frequency converters do not have the nearly unified topology of low-voltage frequency converters. A low-voltage frequency converter typically consists of a diode rectifier circuit, an intermediate DC circuit, and an inverter bridge. The inverter bridge is usually composed of 1.2kV (380V frequency converter), 1.7kV (690V frequency converter), and IGBTs, with each bridge arm generally using only one power device. The structure of high-voltage frequency converters is relatively complex. Through years of continuous research, a high-voltage frequency converter manufactured using multiplexing technology, represented by Beijing Leadway, has emerged, employing a method of achieving high-power conversion using small-power devices. The so-called multi-phase technology involves each phase consisting of several low-voltage PWM power units connected in series or parallel at their output terminals through some means (such as transformers). Each power unit is powered by a multi-winding isolation transformer. The low-voltage units are superimposed in series to achieve high-voltage output, or connected in parallel to achieve large-capacity output. Another type is the high-voltage frequency converter with a two-level conversion circuit, represented by Chengdu Jialing, which uses multiple transistors directly connected in series. This type mainly uses power devices in series and parallel to meet the requirements of high-voltage frequency conversion in the system. Various devices connected in series and parallel are used as individual devices. The high-voltage frequency converter product is realized by utilizing the relatively mature circuit topology control strategies and methods of low-voltage frequency converters. The difficulty of doing this is that the series-connected switching transistors need to achieve dynamic and static voltage equalization, which places high demands on the overall drive control circuit. It also requires solving many technical problems such as du/dt, common-mode voltage resistance, and sine wave filtering. Another type is the diode-clamped three-level converter high-voltage frequency converter, represented by the Department of Electrical Engineering at Tsinghua University. Its purpose is to overcome the voltage equalization problem required when power devices are directly connected in series. It introduces the concept of clamping diodes on the basis of multiple transistors being directly connected in series, avoiding the dynamic voltage equalization problem caused by direct device series connection. Besides the three mainstream topologies of high-voltage frequency converters mentioned above, in recent years, various forms of high-voltage frequency converters have emerged, such as flying capacitor clamped three-level converter circuits and capacitor bootstrap multilevel converter circuits. Regardless of the form, each is accompanied by its own corresponding control methods and theories. Any control method is more complex than that of ordinary two-level frequency converters of low voltage, and involves a series of problems to be solved, such as power device selection, stray parameter handling, insulation, electromagnetic interference, heat dissipation and cooling, field analysis of devices, energy control, the influence of du/dt and di/dt, and transient process control. These are problems that low-voltage frequency converters have never encountered or rarely encounter, or problems that are easily solved by low-voltage frequency converters. Therefore, using low-voltage frequency converters is more reliable and simpler for the system. III. Inverter Selection The correct selection of an inverter is crucial for the normal operation of the electrical control system of mechanical equipment. When selecting an inverter, the first step is to consider the actual situation of the enterprise and rationally choose the inverter's voltage level. Within a certain power range (below 1000kW), choose a low-voltage inverter whenever possible. Simultaneously, determine the control method and protection level of the inverter based on the type of mechanical equipment, load torque characteristics, speed range, static speed accuracy, starting torque, and the requirements of the operating environment. 3.1 Rational Selection of Inverter Voltage Level Currently, the reasonable voltage level of an inverter is determined by the voltage withstand level of the power devices, while the possible power range of the inverter at a given voltage level is determined by the current carrying capacity of the power devices. Due to the limitations of the voltage of power electronic devices and the allowable du/dt of the motor, 10kV inverters must be multi-level with multiple devices connected in series. This results in complex wiring, high price, poor reliability, and high maintenance costs. For a 10kV frequency converter, if 1700V IGBT devices are used, 10 series are needed to supply 120 devices across three phases; if 3300V devices are used, 5 series are needed to supply 60 devices, a huge number that will inevitably affect reliability. On the other hand, high-voltage frequency converters generally use devices with low current, and their current capacity is not fully utilized. For example, a 710kW 10kV motor has a current of only about 50A, while current 1700V IGBTs have a current of 2400A, and 3300V devices have a current of 1600A. Using a large number of low-current devices in series when high-current devices are available is extremely unreasonable. Even if the motor power reaches 2000kW, the current is only about 150A, still very small. In practical applications, to achieve level isolation, improve the input current waveform, and reduce harmonics, most high-voltage frequency converters have an input transformer on the input side. Since a transformer is present, the voltage of the frequency converter and the motor does not need to be the same as the grid voltage; it is not necessary to use 6kV or 10kV. Therefore, the issue of appropriate voltage levels for frequency converters and motors arises. In the past, the 200kW low-voltage cutoff for motors primarily considered direct starting, with a starting current 5-7 times the rated current. If a 10kV/380V power transformer with a capacity of 2000kVA and a short-circuit impedance of around 6% is used, the instantaneous tripping value of the switch becomes very large. To improve power system stability and power quality, it is desirable to further reduce the transformer impedance; however, lower impedance leads to a larger system short-circuit current, making the selection of electrical equipment more difficult, and requiring further increases in the breaking capacity of switches in the circuit, inevitably increasing selection costs and overall construction costs. If the 380V bus voltage drop during motor starting is limited to around 50%, the transformer capacity must be further increased, resulting in an excessively large short-circuit current that the low-voltage switch cannot withstand. After adopting frequency converter speed regulation, the starting current is limited to the rated current value. The capacity of low-voltage frequency converters can now be very large. Companies like ABB have mastered the parallel output technology of inverter units, allowing low-voltage frequency converters with capacities up to 2900kW, making it possible to widely adopt low-voltage high-power frequency converters. In practice, we can replace the original 10kV/6kV or 6kV/6kV incoming line transformers, which only served an isolation function, with 6kV/690V or 6kV/380V incoming line transformers, thus obtaining 690V and 380V power supply voltages, making it possible to widely adopt low-voltage high-power frequency converters. Currently, the capacity of 660V or 690V low-voltage motors in China has reached 1000-1200kW, making selection more convenient. 3.2 Load Torque Characteristics of Mechanical Equipment In practice, production machinery is often divided into three main types based on different load torque characteristics: constant torque loads, constant power loads, and fan and pump loads. 3.2.1 Constant Torque Load In this type of load, the load torque TL is independent of the speed n. At any speed, TL remains constant or essentially constant, while the load power increases linearly with increasing load speed. Friction loads such as conveyor belts, mixers, extruders, and feed mechanisms of mechanical equipment, as well as gravity loads such as cranes, hoists, and elevators, all belong to constant torque loads. When a frequency converter drives a constant torque load, the output torque at low speeds must be sufficiently large, and it must have sufficient overload capacity. If long-term stable operation at low speeds is required, the heat dissipation capacity of the standard squirrel-cage induction motor should be considered to avoid excessive motor temperature rise. 3.2.2 Constant Power Load The characteristic of this type of load is that it needs to remain relatively constant. Spindles of metal cutting machine tools and rolling mills, paper machines, and winding and unwinding machines in film production lines all belong to constant power loads. The constant power nature of the load should be relative to a certain speed variation range. When the speed is very low, due to mechanical strength limitations, TL cannot increase indefinitely, and it transforms into a constant torque nature at low speeds. The constant power and constant torque regions of a load significantly influence the selection of the transmission scheme. When the motor is under constant flux speed regulation, the maximum allowable output torque remains constant, which is constant torque speed regulation. However, after exceeding the field weakening point, it enters the field weakening speed regulation region, where the inverter output voltage no longer increases, the speed increases, and the torque decreases. The maximum allowable output torque is inversely proportional to the speed, which is constant power speed regulation. 3.2.3 Fans and Pumps The torque of these loads is proportional to the square of the speed, and the power is proportional to the cube of the speed. Various fans, pumps, and oil pumps are typical fluid loads. Fluid loads can significantly save energy by adjusting airflow and flow rate through inverter speed regulation. Because the power demand of fluid loads increases rapidly at high speeds, proportional to the cube of the load speed, these loads should not be operated above the power frequency. This type of load accounts for a large proportion of equipment in power plants. Understanding the load type of the equipment is not enough for selecting an inverter; the correct voltage level must also be selected. Knowing the type of load driven by the motor and selecting the correct power supply voltage, the primary basis for selecting a frequency converter should be the motor's rated current; the motor's rated power should only be used as a reference. Furthermore, the high-order harmonics in the frequency converter's output should be fully considered, as they can negatively impact the motor's power factor and efficiency. Therefore, compared to powering the motor from the mains frequency grid, using a frequency converter will increase the motor current by about 10% and the temperature rise by about 20%. This should be taken into account when selecting a frequency converter, allowing for a margin to prevent excessive temperature rise and extend the frequency converter's lifespan. Additionally, when the installation distance between the frequency converter and the motor is long, measures should be taken to suppress the influence of the long cable's ground coupling capacitance to avoid insufficient frequency converter output. Therefore, a higher-range frequency converter should be selected, or an output reactor should be installed at the frequency converter's output terminal. Only by correctly selecting the frequency converter can the transformation and production proceed smoothly. The following specific example illustrates the successful application of low-voltage, high-power converters in new power plant projects. IV. Practice of Low-Voltage High-Power Frequency Converters Economic development prioritizes thermal power. Against this backdrop, Qinhuangdao Tonghe Thermal Power Co., Ltd. undertook its second-phase expansion project in 2004. The project mainly involved the construction of a 1×24MW double-extraction condensing steam turbine generator unit with a 170t/h circulating fluidized bed boiler, the main plant building, and foundation engineering. The boiler's auxiliary equipment consisted of four high-power motors: induced draft fan motor (rated power: 1120KW); primary blower motor (rated power: 710KW); secondary blower motor (rated power: 450KW); and circulating pump motor (rated power: 680KW). Using high-voltage 6kV frequency converters for all of these would have significantly increased the initial investment cost of the expansion project. Through our strong recommendation to use low-voltage high-power frequency converters, and after on-site investigation by the manufacturer, ABB low-voltage frequency converters were ultimately adopted. The specific frequency converter model and application are as follows: ACS800-07-0580-7; specific current value: 488A. Used to drive a 450KW/690V secondary air fan variable frequency motor; motor current: 441A. ACS800-07-0870-7; specific current value: 729A. Used to drive a 680KW/690V circulating pump variable frequency motor; motor current: 705A. ACS800-07-1060-7; specific current value: 885A. Used to drive a 710KW/690V primary air fan variable frequency motor; motor current: 736A. ACS800-07-1500-7; specific current value: 1208A. IV. Detailed description of the accessories of the set of equipment is as follows: 1) All four sets of equipment are equipped with an incoming line reactor as an AC-DC-AC frequency converter. The incoming line reactor can not only greatly reduce the harmonics of the incoming power grid, but also reduce the current impact on the equipment, ensuring the reliability of the equipment. ABB integrates the incoming line reactor as a standard feature within the rectifier module. Its main function is to suppress or eliminate interference from high-order harmonics in the inverter's input current via the power grid, thus preventing interference to electronic circuits and equipment. 2) All four units are equipped with fuse combination switches. Their main function is to protect the electronic components of the rectifier's incoming line unit. To provide operators with a clear point of disconnection, an isolating switch must be installed on the inverter's incoming line side as a visible operational disconnect point. This ensures a clear point of disconnection for maintenance personnel during operation or shutdown. It is strictly forbidden to disconnect this switch under load. 3) All four units are equipped with contactors and emergency stop switches. Their main function is to ensure reliable equipment protection. In the event of equipment failure, pressing the emergency stop switch activates the incoming line contactor as an automatic tripping device, cutting off the main circuit and ensuring equipment and personnel safety. This makes the system safer, more convenient, and easier for remote operation. 4) All four units are equipped with Du/dt filters. Their main function is to address the significant winding insulation surge voltage and asymmetrical bearing current that can be introduced into the motor by the frequency converter, significantly impacting its lifespan. Unless a special motor is used, frequency converters for 690VAC motors typically require Du/dt filters to ensure reliable operation. Additionally, filters reduce motor noise, lower electromagnetic radiation, significantly attenuate high-frequency components, ensure a clean electromagnetic environment, and extend motor lifespan. 5) All four units are equipped with 12-pulse rectifiers. Each unit has two 6-pulse rectifiers, forming a 12-pulse rectifier – effectively eliminating harmonics below the 11th order in the current, reducing harmonic interference to the power grid, and purifying the power grid environment. 4.1 Working Principle and System Structure The auxiliary equipment for the 170-ton boiler at Qinhuangdao Tonghe Power Plant uses the ABB ACS800 high-power frequency converter, employing Direct Torque Control (DTC). This DTC system abandons the decoupling concept of vector control, instead comparing the flux amplitude and torque values ​​with the given flux and torque values ​​in real-time. The required voltage vector value is then directly output by the flux and torque regulators. The DTC control system consists of a speed control loop and a torque control loop. ABB combines DTC technology with fuzzy control theory to produce high-performance, low-cost frequency converter speed control products, with performance significantly superior to vector control frequency converters. In DTC, stator flux and torque are used as the primary control variables. A high-speed digital signal processor combined with an advanced motor software model updates the motor state 40,000 times per second. Because the motor state and the comparison between the actual and given values ​​are constantly updated, each switching state of the inverter is determined individually. This means the drive can generate optimal switching combinations and respond quickly to dynamic changes such as load disturbances and momentary power outages. The system responds according to the set conditions. Due to the presence of an intermediate DC link, voltage drops are effectively suppressed, ensuring the inverter can determine whether to continue operating or perform fault protection. DTC eliminates the need for separate PWM modulators for voltage and frequency control. Therefore, there is no fixed chopping frequency, and in actual operation, it avoids the high-frequency noise emitted by other inverters driving motors, while also reducing the inverter's power consumption. Rich and flexible input/output port definition functions allow for user-defined requirements in most applications without the need for any additional circuitry. 4.1.1 Function of the Three Fans The circulating fluidized bed boiler adopts a balanced ventilation system. Each boiler is equipped with one primary air fan and one secondary air fan. The air required for the combustion chamber is drawn in by the primary and secondary air fans and then sent to the air preheaters for heating. The heated primary air is then sent to the fluidized bed and the feeder, while the secondary air enters the furnace. Fuel mixes and burns with the air in the combustion chamber. The primary air is evenly distributed into the furnace from the bottom through the grate, serving as the combustion chamber's fuel. The secondary air is preheated through the furnace jacket before entering the secondary combustion chamber for secondary combustion. The primary and secondary air are controlled by fixed butterfly valve positions. Based on the combustion conditions in the furnace, the fan motor speeds are changed via frequency converters for the primary and secondary air fans to adjust the air consumption. The induced draft fan is used to maintain negative pressure combustion in the furnace. 4.1.2 Function of the Induced Draft Fan The induced draft fan works in conjunction with the boiler control system to save fuel, improve boiler combustion efficiency, and reduce environmental pollution. Utilizing modern automatic control technology and a small-scale distributed two-level control system via a higher-level DCS, it monitors multiple parameters such as furnace pressure, flue gas oxygen content, and exhaust gas temperature, based on the combustion status of the medium, to reliably control the frequency converter and adjust the induced draft fan's operating speed in real time. This automatically monitors the furnace pressure to maintain negative pressure combustion. It is required to maintain a constant main steam pressure under different load conditions, balancing the boiler's steam output with the turbine's steam consumption and heat supply by adjusting the amount of fuel and air entering the furnace. Too low a furnace negative pressure, or even a positive pressure, can easily lead to localized flameouts, which is detrimental to safe production and environmental hygiene. Conversely, too high a negative pressure will allow a large amount of cold air to leak into the furnace, increasing the load on the induced draft fan and the heat loss carried away by the exhaust gas, which is detrimental to economical combustion. The furnace negative pressure must be controlled by the frequency of the induced draft fan frequency converter, and the air volume is introduced as a feedforward signal to participate in the entire control process. The automatic control process is completed by the host computer. 4.1.3 The Role of the Primary Air Blower The primary air blower works in conjunction with the boiler control system to save fuel, improve boiler combustion efficiency, and reduce environmental pollution. Through the host computer's DCS system, the operating speed of the primary air blower is adjusted in real time, thereby automatically monitoring the oxygen ratio in the furnace and maintaining normal combustion. Maintaining an appropriate air-fuel ratio is the optimal operating condition for the combustion process and a key measure to improve boiler efficiency and economy. Maintaining economical combustion is crucial for maintaining a suitable air-fuel ratio. When the volumetric flow rate of coal changes, the primary air volume should be adjusted according to a certain ratio. Changing the air volume is achieved using the primary air blower frequency converter. First, the opening of the dampers in each air chamber under the grate is adjusted, and then the frequency of the primary air blower frequency converter is adjusted as a means of controlling the air volume. The automatic control process is completed by the host computer. Using ABB frequency converters allows for soft-start control of the blower, reducing the impact on the power grid during motor startup, reducing blower start-up wear and equipment impact energy consumption, and extending the electrical and mechanical lifespan of the equipment. 4.1.4 Function of Secondary Air Fans Secondary air fans are used in conjunction with the boiler control system to primarily save fuel, improve boiler combustion efficiency, and reduce environmental pollution. Through the DCS system, the operating speed of the secondary air fans can be adjusted in real time to maintain an appropriate air-fuel ratio, ensuring optimal operating conditions for the combustion process. This is a key measure to improve boiler efficiency and economy. Maintaining economical combustion hinges on maintaining a suitable air-fuel ratio. When the volumetric flow rate of coal changes, the air supply should be adjusted according to a certain ratio. This is difficult to achieve by simply changing the primary air fan's air volume; the cooperation of the secondary air fan is necessary to ensure complete combustion of coal in the secondary combustion chamber. The secondary air fan is affected by the primary air fan, so careful attention must be paid during its use, and the minimum motor speed should be limited to fully utilize the secondary air volume to achieve optimal combustion and allow the boiler to operate at full load. Using frequency converters for the aforementioned fans not only meets production process requirements but also enables soft-start control of the fans. This reduces the impact on the power grid during motor startup, reduces fan start-up wear and equipment impact energy consumption, and extends the electrical and mechanical lifespan of the equipment. In addition, the inverter output carries certain high-order harmonics, which may accelerate the aging of the motor insulation. This should be noted during use. V. Incoming Line Unit Incoming Power Supply: 690V. All four large inverters are equipped with incoming line switch cabinets, contactors, and emergency stop switches. Optional incoming line contactor components include an emergency stop button on the cabinet door. An external emergency stop button can be led to the terminal block inside the cabinet. Pressing the emergency stop switch locks the inverter semiconductors, disconnects the main contactor, and allows the motor to stop freely. The device is equipped with a device to prevent accidental starting. A 12-pulse rectifier is selected, consisting of two 6-pulse rectifier bridges connected in parallel. Using a 12-pulse rectifier reduces total harmonic distortion and eliminates the fifth and seventh harmonics. Because EMC filters cannot be used with 12-pulse rectifiers (the transformer secondary is floating!), it is recommended to install a shielded power transformer to reduce transmission radiation. The rectifier unit includes two diode rectifier units (DSUs), model: ACS800-507-0680-7. VI. Rectifier Transformer Data To obtain a 12-pulse phase shift, a multi-winding transformer is required. Another purpose of the transformer is to provide sufficient impedance to limit grid-side harmonics within the limits required by IEEE 519. We use a dry-type transformer manufactured in Shunde, Guangdong. The isolation transformer has two secondary windings, one Y-connected and the other Δ-connected, thus creating a 30° phase difference between the two secondary windings, providing 12 pulses to meet the requirements of a 12-pulse input rectifier bridge. Overload ratio: 1.5 times (every 10 minutes to 1 minute) No-load ratio: 6kV ± 2x2.5% Tap/0.72kV / 0.72kV Rated frequency (Fn): 50Hz ± 2% Connection group: Dyn 11 d0 Short-circuit impedance voltage drop: 8% for each of the two secondary windings Short-circuit impedance voltage drop deviation: £3% deviation between the two secondary windings Voltage deviation: £0.3%*UN voltage deviation of the two secondary windings under rated load Ambient temperature: Maximum 40 degrees Celsius Other: A shielding layer is added between the high and low voltage windings, with terminals leading out from the shielding layer. Terminals are led out from the center point of the secondary Y winding. The withstand voltage, insulation, and safety of each winding, shielding layer, and terminal conform to the relevant national standards. Equipped with an aluminum alloy protective cover, and an air-cooled temperature control device with a PT100 temperature sensing element (low-voltage winding + iron core). VII. Inverter Commissioning 7.1 Inverter Function Settings The inverter parameters are set to meet the normal operation requirements of the power plant boiler auxiliary equipment. Parameters can be set individually via the included CDP312 control panel, or a pre-programmed application macro can be selected for quick and easy startup of the ACS800. Standard application macros include factory macros, manual/automatic macros, PID application macros, torque control macros, and sequential control application macros. 7.2 Control Methods The inverter has the following three control methods: Local control: Completed via the included CDP312 control panel. Remote control: Accepts switch or analog control signals from the field or control room via the inverter terminals. Bus Control: Communication signals from a PLC or host computer are received via a bus adapter connected to the inverter's expansion port. The ACS800 has multiple serial communication interfaces, allowing communication with the CDP312 control panel and a PC. It can also communicate with other host control systems by matching different area bus adapters, thus achieving operation, debugging, diagnosis, and control. This project uses remote control via a host computer. Parameter settings are completed by the CDP312 control panel. 7.3 Acceleration and Deceleration Time Setting The fan has a large moment of inertia, so acceleration and deceleration times are generally set relatively long to prevent the inverter from reporting overcurrent or overload faults during startup. Too short a deceleration time can easily cause an intermediate DC overvoltage fault; therefore, both time periods are generally greater than 90 seconds. 7.4 Trial Run After the inverter speed control system parameters are set, motor parameter identification is performed first. After completion, the system can be trial run. First, perform low-frequency operation on the control panel to observe whether the motor's rotation direction is correct, the speed is stable, the temperature rise is normal, and the acceleration and deceleration are smooth. Then, continue test runs at frequencies of 20, 30, 40, and 50 Hz. If the test run is normal, the frequency converter can be put into trial production. VIII. Conclusion This article, through case studies, illustrates that using high-power frequency converters for power plant auxiliary equipment is not only feasible but also economical. The control and wiring are very simple, and coupled with the frequency converter's comprehensive fault diagnosis and display functions, the reliability and maintainability of the entire speed control system are greatly improved. The automation level of the power plant is further enhanced, which has been affirmed by users and has a very broad market prospect.