Abstract: In large-scale industrial applications, systems driven by high-power motors require speed regulation for process control and energy saving. The first widely adopted mechanical speed regulation method was hydraulic coupling speed regulation. With the stabilization of power electronic products and the reduction in price, high-voltage frequency converters have begun to enter the motor speed regulation field on a large scale and have captured a major share of the market. This paper compares the advantages and disadvantages of these two speed regulation and energy-saving devices to improve the understanding of energy saving in high-power motor speed regulation. Keywords: Speed ​​regulation, energy saving, frequency converter , hydraulic coupling I. Introduction In the industrial sector, high-power motors are the heart of the entire industrial system, consuming more than 30% of the nation's total electricity, making them the system with the largest share of energy consumption. The power consumption of large and medium-sized fans and pumps driven by high-power motors accounts for more than 50% of the total power consumption of fans and pumps. Due to the various process requirements of large systems, their fans and pumps are designed according to the most severe environment. For most of the operating time, they are in a state of being overpowered and underutilized. The simple adjustment method is to use baffles or valves to adjust the air volume or flow rate to meet the requirements of load changes, which results in a serious energy loss. Using the change of motor speed to adjust the air volume or flow rate is undoubtedly of great significance for saving energy and improving equipment efficiency. However, for customers, how to choose an economical and practical speed regulation method according to their objective situation is a practical problem they face. This article provides a comprehensive analysis and comparison of the advantages and disadvantages of using high-voltage frequency converters and hydraulic couplings from both theoretical and practical perspectives. II. Working principle and performance characteristics of high-voltage frequency converters: (I) Development process of high-voltage frequency converters: High-voltage frequency converters are a type of high-voltage motor speed regulation product that has gradually developed with the development of modern power electronic devices. The development stages are roughly as follows: (1) From the perspective of power components: GTR, GTO, IGBT, IGCT. (2) According to the structural method: high-low-high, three-level, diode clamped multi-level series, capacitor clamped multi-level series, multi-level unit series superposition, direct vector control current source inverter. (3) According to the control method: thyristor capacitor forced commutation, thyristor inductor forced commutation, GTO self-turn-off, IGBT voltage control self-turn-off, IGCT current control self-turn-off. (4) According to the control system: analog control, digital industrial computer control, digital FPGA control, digital DSP control. (II) Basic composition of multi-level unit series superposition type high voltage frequency converter: (1) Main circuit composition: It consists of high voltage frequency converter, remote control operation box, machine-side operation box and bypass switch cabinet, etc. The machine-side control box and bypass switch cabinet are optional equipment. The bypass switch cabinet can be used in manual or automatic bypass mode. The single-line schematic diagram of the system is shown in the figure: [align=center] Figure 1 Single-line schematic diagram of the system[/align] (2) Composition of the high-voltage frequency converter: It is composed of eighteen identical unit modules. Each group consists of six modules, which correspond to the three phases of the high-voltage circuit. The unit power supply is provided by the phase-shifting transformer. The schematic diagram is as follows: [align=center] Figure 2 Internal structure diagram of the high-voltage frequency converter[/align] (3) Composition of the power unit: The power unit is a single-phase bridge converter, which is powered by the secondary winding of the input switching transformer. After rectification and filtering, it is controlled by 4 IGBTs using the PWM method to generate the set frequency waveform. All power units in the frequency converter have the same circuit topology and are designed in a modular manner. Its control is transmitted through optical fiber. The principle block diagram is shown below: [align=center] Figure 3 Power Unit Principle Block Diagram[/align] The control optical signal from the main controller is converted from light to electricity and sent to the control signal processor. After receiving the corresponding instruction, the control circuit processor sends the corresponding IGBT drive signal. After receiving the corresponding drive signal, the drive circuit sends the corresponding drive voltage to the IGBT control electrode to operate the IGBT to turn off and on, and outputs the corresponding waveform. The status information in the power unit will be collected and processed in the response signal circuit. After being concentrated, it is converted by the electricity-to-light converter and sent to the main controller as an optical signal. (III) Operating Principle of High Voltage Frequency Converter: Each power unit of the high voltage frequency converter is equivalent to a three-level two-phase output low voltage frequency converter. By superimposing, it becomes a high voltage three-phase AC power. Taking a 6KV frequency converter as an example, it is discussed that: For a 6KV output voltage frequency converter, there are 6 power units connected in series in each phase. The input voltage of the unit is three-phase 600V, and the output is single-phase 577V. After the units are connected in series and superimposed, the output phase voltage can be 3464V. When the inverter output frequency is 50Hz, the phase voltage is a 13-step waveform, as shown in the figure below. In the figure, UA1…UA6 are the output voltages of the six power units in phase A, which, when superimposed, form the inverter's phase A output voltage UA0. The figure shows the phase A reference voltage UAr used to generate the PWM control signal; it can be seen that UA0 closely approximates UAr. UAF is the fundamental component of the phase A output voltage. [align=center]Figure 3: Superimposed Waveform of Phase Voltage Circuit[/align] Since the inverter's neutral point is not connected to the motor's neutral point, the inverter output is actually a line voltage. The UAB output line voltage generated by the phase A and phase B output voltages can reach 6000V, which is a 25-step waveform. As shown in the figure below, the output line voltage and phase voltage are stepped waveforms. UAB not only has a sinusoidal waveform but also a significantly increased number of steps, resulting in smaller harmonic components and dV/dt. [align=center] Figure 4 Superimposed waveform of line circuit[/align] (IV) Three-phase waveform output quality of multi-level unit series superimposed frequency converter: After the high-voltage frequency converter is running, it converts the input three-phase high-voltage AC power of the power frequency into three-phase AC power that can be frequency adjusted. Its voltage and frequency are adjusted accordingly according to the V/F setting to keep the motor running at different frequencies, while the main magnetic flux in the stator core is kept at the rated level, improving the conversion efficiency of the motor. The following figure shows the three-phase input waveform of the motor collected by PT during on-site operation: [align=center] Figure 5 Input voltage waveform of motor[/align] Due to the multiple superimposed application, the harmonic content of the output voltage of the high-voltage frequency converter is very low, which has reached the harmonic content allowed by the conventional power supply voltage. At the same time, the dV/dt of the output voltage is small, which will not increase the stress of the motor winding. It can supply power to ordinary standard AC motors without the need for derating or adding output filter reactors, ensuring the versatility of high-voltage equipment. On the input side of the frequency converter, due to the uniform displacement of multiple secondary windings of the frequency converter, such as the six windings of +250, +150, +50, -50, -150, and -250 when outputting at 6KV, the corresponding current components in the primary current of the frequency converter also shift uniformly to each other, forming an equivalent 36-pulse rectifier circuit. The harmonics generated by the conversion cancel each other out and are annihilated. The power factor during operation is above 0.95, so no additional power filter or power factor compensation device is needed, and it will not resonate with the existing compensation capacitor device, and it will not interfere with electrical equipment operating on the same power grid. (V) Performance characteristics of high voltage frequency converter: (1) Application range: l Wide speed regulation range, which can be smoothly adjusted from zero speed to power frequency speed. l It can achieve soft start with small current on large motors, and the start time and start method can be adjusted according to the on-site working conditions. l Frequency adjustment is based on the voltage-frequency ratio coefficient of the motor at low frequency to output voltage and frequency. At low speed, the motor not only generates less heat, but also has a lower input voltage, which will reduce the aging speed of the motor insulation. (2) Novel technology The application of series multiplexing superposition technology realizes true high-to-high power conversion without the need for step-down and step-up conversion, reducing device losses, improving reliability, and solving the difficulties of high-voltage power conversion. The application of series multiplexing superposition technology also opens up a new way to realize pure sine waves and eliminate grid harmonic pollution. (3) High performance indicators l High power factor, reaching 0.95 or above, without the need for additional power factor compensation devices, avoiding penalties caused by reactive power. l High efficiency, reaching 96% or above, far exceeding the high-power speed control device of thyristor. l Meets the strict requirements of IEEE519-1992 standard, does not generate harmonic pollution to the grid, and does not require any filtering devices. l Does not generate harmonic pollution to the motor, effectively reducing the heat generation of the motor, and the noise is similar to that when using power frequency power supply. l The torque pulse is very low, which will not cause resonance of mechanical equipment such as motors, and also reduces the wear of transmission mechanism. l The output waveform is perfect and the distortion is less than 1%. l The electric stress intensity of the motor is similar to that when using power frequency power supply, so there is no need to equip a special motor. l The connection with the motor is not limited by the cable length. (4) High technology content l The large-scale gate array CPLD circuit is adopted to realize the high real-time performance, speed and accuracy of PWM control. l The two-fiber real-time transmission technology has obtained a national invention patent, which makes the communication between the control unit and the power unit faster and more reliable. l The specially designed H-bridge inverter circuit has obtained a national patent, which provides a guarantee for the reliability of system operation. l The perfect power unit bypass technology has obtained a national patent, which further improves the reliability of system operation. l The control part adopts high-performance DSP and FPGA chips, which greatly improves the performance of the control system, realizes constant V/F and constant torque control, and the boost characteristics can be set arbitrarily to meet the requirements of various mechanical start-up and operation. l The excellent DSP software mathematical model greatly improves the real-time performance and efficiency of system operation. III. Working Principle and Performance Characteristics of Hydraulic Couplers: (I) Structure of Hydraulic Couplers: A hydraulic coupler is a hydraulic transmission device, also known as a hydraulic coupling. The hydraulic coupler's structure mainly consists of three parts: a housing, a pump impeller, and a turbine, as shown in the figure. [align=center]Figure 6 Basic Structure of a Hydraulic Coupler[/align] The pump impeller and turbine are installed opposite each other and are collectively called the working impeller. There are radially arranged straight blades on the pump impeller and turbine, and the pump impeller and turbine do not contact each other. There is a certain gap between them (approximately 3mm-4mm); after the pump impeller and turbine are assembled into a whole, its axial section is generally circular, and its inner cavity is filled with hydraulic oil. (II) Installation Method of Hydraulic Couplers: The input shaft of the hydraulic coupler is connected to the motor and rotates with the motor, making it the active part of the hydraulic coupler. The turbine and output shaft are connected together, forming the driven part of the hydraulic coupler, and are connected to the load. Its structural diagram is as follows: [align=center] Figure 7 Hydraulic Coupling Installation Diagram[/align] During installation, the hydraulic coupling is installed between the motor and the load. Usually, due to the large load and interlocking with other equipment, the motor is moved to the rear. In the modification scheme, the motor foundation needs to be redone. (III) Working principle of hydraulic coupling: When the motor runs, it drives the housing and pump wheel of the hydraulic coupling to rotate together. The hydraulic oil in the pump wheel blades rotates together with the pump wheel. Under the action of centrifugal force, the hydraulic oil is thrown to the outer edge of the pump wheel blades and rushes to the turbine blades at the outer edge, causing the turbine to rotate under the impact force of the hydraulic oil. The hydraulic oil rushing to the turbine blades flows along the turbine blades to the inner edge, returns to the inner edge of the pump wheel, and is then thrown to the outer edge by the pump wheel again. The hydraulic oil flows from the pump wheel to the turbine and back from the turbine to the pump wheel in this way to form a circulating flow. In a hydraulic coupling, the circulating hydraulic oil flows from the inner edge to the outer edge of the pump impeller blades, where the pump impeller performs work on it, gradually increasing its speed and kinetic energy. Conversely, as it flows from the outer edge to the inner edge of the turbine blades, the hydraulic oil performs work on the turbine, gradually decreasing its speed and kinetic energy. This circulation of hydraulic oil is generated by the speed difference between the pump impeller and the turbine, creating a pressure difference at the outer edges of the blades. During operation, the kinetic energy of the electric motor is transferred to the hydraulic oil via the pump impeller, and the hydraulic oil, in turn, transfers its kinetic energy to the turbine for output during its circulation. During this circulation, the hydraulic oil experiences no additional external forces other than the force between the pump impeller and the turbine. Based on the principle that action and reaction forces are equal, the torque exerted by the hydraulic oil on the turbine should be equal to the torque exerted by the pump impeller on the hydraulic oil; this is the working principle of a hydraulic coupling. (IV) Speed ​​Regulation Method of Hydraulic Coupler: In actual operation, the hydraulic coupler works as follows: the motor drives the pump wheel to rotate, which in turn drives the hydraulic oil to rotate. The turbine is then subjected to torque. When the hydraulic oil volume is small, the torque is insufficient to overcome the starting resistance torque of the load, so the turbine will not rotate with the pump wheel. Increasing the hydraulic oil increases the torque acting on the turbine. When the torque acting on the turbine is sufficient to overcome the starting resistance of the load, the turbine starts to rotate. When the torque transmitted by the hydraulic oil is equal to the load torque, the speed stabilizes. The load torque and speed are proportional to the square. As the hydraulic oil volume increases, the output torque increases, and the turbine speed increases accordingly, thus achieving the purpose of speed regulation. The change between torque and speed during the operation of the hydraulic coupler is shown in the speed vector diagram. [align=center] Figure 8 Speed ​​Vector Diagram of Hydraulic Coupler[/align] The flow velocity VT of the spiral circulation of the oil remains constant, VL is the relative linear velocity of the pump wheel and the turbine, VE is the pump wheel outlet velocity, and VR is the combined velocity of the oil. When the turbine rotates at high speed, that is, when the output and input speeds are close to the same, the angle between the combined speed VR and the pump wheel outlet speed is large. This makes the fluid flow on the turbine very small, which will cause the output element to slip and the speed to decrease. When the oil volume is increased, both the relative speed VL and the combined speed VR become large, which makes the thrust of the fluid flow on the turbine blades become large. Until there is enough circulating oil to generate sufficient impact force on the turbine, the output speed increases. (VI) Conversion efficiency of hydraulic coupler: The speed regulation principle of hydraulic coupler shows that the change in transmission speed is actually the result of mechanical power regulation. Therefore, the decrease in the output speed of hydraulic coupler is actually a decrease in output power. During the speed regulation process, the original transmission speed of hydraulic coupler does not change. Assuming that the load torque remains unchanged and the mechanical power of the original transmission also remains unchanged, where does the difference between the input and output power go? Obviously, it is lost by the hydraulic coupler in the form of heat energy. Let the original transmission power be PM1 and the output power be PM2. The power loss is then calculated using the formula above. This illustrates that a hydraulic coupling is an energy-consuming mechanical speed regulating device. The deeper the speed regulation (the lower the speed), the greater the loss. For a square torque load, since the load torque changes according to the square of the speed, the original transmission input power decreases according to the square of the speed, resulting in relatively smaller power loss. However, the output power decreases according to the cube of the speed, and the speed regulation efficiency remains very low. Simultaneously, during operation, the coupling's oil discharge temperature is high, generally peaking at around 50% of the scoop tube position. This is because the turbine contains about half the oil, and the friction between the turbine and pump impeller generates significant heat. When the scoop tube position is low, there is less oil in the turbine, and although the heat generated by the friction between the pump impeller and the turbine is still significant, it can be cooled by the oil cooler. When the scoop tube position is high, the slip ratio is small, so the oil discharge temperature is not high. Generally, the cooling water valve of the coupling's working oil cooler is not adjusted. Therefore, the heat generated at low speeds can be carried away by the oil cooler. Consequently, as the speed increases, the working oil temperature continuously increases. However, as the rotational speed increases, the circulation volume of the working oil also increases. Therefore, the working oil has a high temperature point, and the hydraulic coupling suffers the greatest loss at the high temperature point. (VII) Performance characteristics of hydraulic coupling: (1) Application range: l Wide speed range, can be adjusted from zero. l No electrical connection, can work in dangerous places, and has low environmental requirements. (2) Mature technology: l Simple structure and easy operation. l Years of research, reasonable structure. l All domestically produced, easy to maintain. (3) Performance indicators: l Cheap price, low precision requirements l Low energy conversion efficiency. l Simple structure, low failure rate. l Requires a special cooling system during operation. l Hydraulic oil needs to be replaced regularly after aging. IV. Comparison of the advantages and disadvantages of frequency converter and hydraulic coupling: (I) Energy saving effect: 1. Frequency converter has good energy saving effect and high power factor. 2. Hydraulic coupling has low energy saving effect. At low speed, nearly 3/4 of the energy is wasted. Large capacity equipment should also add a water cooling system. (II) Installation Method: 1. Variable frequency drives (VFDs) are easy to install; the motor and load remain stationary, and the device is simply connected to the power supply. 2. Hydraulic couplings require installation between the motor and load, necessitating motor relocation for installation. (III) Safety: 1. VFDs can be operated in bypass mode if a problem occurs. 2. Hydraulic couplings require shutdown and maintenance if a problem arises. (IV) Operating Accuracy: 1. VFDs offer high operating accuracy, allowing for precise adjustment. Speed ​​is limited by the output frequency, and the speed remains constant even with load fluctuations. 2. Hydraulic couplings rely on oil volume and load for speed regulation, resulting in lower accuracy. The speed changes with load variations. (V) Maintenance Costs: 1. VFDs have low maintenance costs, with no consumables during normal operation. 2. Hydraulic couplings require hydraulic oil replacement after a certain period of operation. (VI) Operability: 1. VFDs are complex to operate, requiring specialized training for operators. 2. Hydraulic couplings are simple and convenient to operate. (VII) Economic efficiency: 1. Variable frequency speed control devices are expensive. 2. Hydraulic couplings are inexpensive. V. Practical application of high-voltage frequency converters and hydraulic couplings: In a power plant in Heilongjiang, a hydraulic coupling is used on the induced draft fan of boiler No. 10, while a high-voltage frequency converter is used for speed regulation on the induced draft fan of boiler No. 13. Boilers No. 10 and No. 13 are both 100MW units of the same model, and their induced draft fans are driven by asynchronous motors with a capacity of 630KW. (I) Data on the operation of the high-voltage frequency converter: Under the active load condition of generator set No. 13, the current, voltage and power factor of the induced draft fan driven motor were measured in the bypass and speed regulation operation conditions, and the power consumption was calculated: 1. 50MW load 2. 60MW load 3. 70MW load 4. 80MW load 5. 90MW load 6. 100MW load (II) Data on the operation of the hydraulic coupling: Under the active load condition of generator set No. 10, the current, voltage and power factor of the induced draft fan driven motor were measured in the full speed and speed regulation operation conditions, and the power consumption was calculated: 1. 50MW load 2. 60MW load 3. 70MW load 4. 80MW load 5. 90MW load 6. 100MW load (III) Power consumption of the induced draft fan during unit operation: (1) Average daily operating conditions of the unit: The power plant is a peak-shaving power plant. The daily power generation load is uniformly allocated by the dispatching department. The amount of variation in a day is extremely large. Based on the summary of the operating conditions of the power plant's units throughout the year, the average daily operating conditions of the units are as follows: 50MW: 23:00 to 3:00:00, 4 hours; 60MW: 1.5 hours before and after 50MW at night, 3 hours; 70MW: 5:00 to 8:00, 8:00 to 10:00: 5 hours; 80MW: average daytime time, 3 hours; 90MW: average daytime time, 3 hours; 100MW: average daytime time, 6 hours. (2) Power consumption of the induced draft fan motor of Unit 13 operating with high-voltage frequency converter: Daily power consumption during bypass operation: 149 * 4 + 215 * 3 + 253 * 5 + 286 * 3 + 313 * 3 + 384 * 6 = 6627 Daily power consumption during speed regulation operation: 47.33 * 4 + 76＊3 + 138＊5 + 226＊3 + 270＊3 + 367＊6 = 4787.32 Daily power saving per motor compared to bypass operation: 6627 - 4787.32 = 1839.68 (KW) (3) Power consumption of the No. 10 induced draft fan motor operating with hydraulic coupling: Daily power consumption when hydraulic coupling is running at full speed: 146.6＊4 + 271＊3 + 316＊5 + 371＊3 + 419＊3 + 480＊6 = 8227.6 Daily power consumption when hydraulic coupling is running at full speed: 95.6＊4 + 128＊3 + 194＊5 + 274＊3 + 321 * 3 + 393 * 6 = 5879.4 (4) Energy saving comparison of two speed regulation methods: The inverter bypass operation belongs to the direct drive of the induced draft fan by the motor, which is used as the reference benchmark for the constant speed operation of the induced draft fan: ● The daily power saving of a single motor is 6627 - 4787.32 = 1839.68 (KW) when the high voltage inverter speed regulation operation is compared with the direct drive operation of a single motor: 6627 – 5879.4 = 747.6 (KW) ● The daily power saving of a single motor is 1839.6 – 747.6 = 1092 (KW) when the high voltage inverter speed regulation operation is compared with the hydraulic coupling speed regulation operation of a single motor: 1092 * 365 = 398580 (KW) VI. Conclusion Through the comparison of two speed control devices, high voltage frequency converter and hydraulic coupling, high voltage frequency converter has unparalleled advantages in energy saving and precision regulation. However, the price of high voltage frequency converter is still relatively expensive, and its structure and operation are still very complicated, which hinders its promotion and application speed. However, with the rapid development of modern electronic devices and the maturity of vector drive circuits, the price and structure of high voltage frequency converter will continue to decrease and be simplified, eventually replacing hydraulic coupling. References: [1] Zhang Yonghui Comparison of high voltage frequency converter technology Variable frequency world [2] Ma Wenxing Hydraulic transmission theory and design Chemical Industry Press [3] Xie Yucheng Power transformer handbook Machinery Industry Press [4] Wang Xikui Pumps and fans China Electric Power Press [5] Bai Kuoshe Fluid mechanics·pumps and fans Machinery Industry Press [6] Liang Hao Latest national mandatory standard implementation and design selection and use technical manual for frequency converters