Abstract: Variable frequency drive (VFD) technology plays an increasingly important role in the thermal power industry. In-depth research is essential to achieve further technological advancements and ensure its effective utilization in this sector. Through years of practical exploration and comprehensive research, this paper presents three solutions for addressing key practical problems: in-depth analysis of flux optimization functions; research on current transformer connection methods; and the application of VFDs to handle multiple thermal power equipment in a single-use/standby configuration. The optimal solution was derived from three different solutions. Furthermore, innovative solutions were developed using six methods and five controllers for the widely used constant pressure water supply and pressure stabilization systems in the thermal power industry.
Abstract: Frequencyconversiontechnologyinthermalpowerindustrybecomesincreasinglyplayadecisiverole,Atthesametimemustbestudiedinordertoprogressoftechnologyachievements,Inordertomakethefrequencyconversiontechnologyinthermalpowerindustrywellproductivity,Throughyearsofpracticeandresearch,Analyzesthoroughlytothemagneticfluxoptimizationfunction,Tosolvethepracticalapplicationinthefinalproblem ;Currenttransformerconnectionmethodalsosolvestheaccurateoperationparametersoftheproblem;Thermalequipmentwithapreparationoperationusinginvertermulti-splitsolution,Created3kindsofsolution,Obtains thesynergy,Inthethermalpowerindustry,Widespreadapplicationconstantpressurewatersupplysystemandconstantpressuresystem,Usingthe6methodsof5kindsofcontrollerofnewtechnology,Obtainstheinnovationplan.
Keywords: frequency converter; thermal; magnetic flux optimization; current transformer; multi-split frequency converter; constant pressure water supply; controller
Keyword: Frequencyconversion; Thermal; Fluxoptimization; Currenttransformer; Invertermulti-split; Constantpressurewatersupply; Controller
Introduction: Although variable frequency technology is widely used, the thermal power industry has unique characteristics that demand in-depth understanding of the technology. Due to the scarcity of relevant literature, I have researched several new technologies. The goal of this paper is to achieve optimal application of the innovative variable frequency technology solutions studied below within the thermal power industry. This will have profound implications for the development and efficiency of the entire thermal power industry.
Having worked in electrical design and on-site construction in the heating industry for many years, I have discovered many technical problems in my work. Due to the special nature of the heating industry and the depth of the problems, these issues have created gaps in the industry. After years of research, I have achieved many results.
In a heat exchange station of a residential building, a 45KW circulating pump motor was used. The inverter used a standard macro, and all parameters were set to the default standard macro settings. The inverter used a motor with the same power value as the default power of the parameters. The external wiring of the inverter consisted of only a potentiometer for adjusting the frequency and a starting contact (an auxiliary contact of an intermediate relay, whose coil was in the main circuit, forming a simple start-stop circuit). That is, the circuit connected to the inverter was simple and normal, as shown in Figure 1. However, after a few minutes of startup, the inverter displayed f0001 (overcurrent) on the basic control panel and immediately stopped. This problem is rarely encountered. The relevant information was not found. After testing, the motor and wiring were normal, and the main circuit and secondary circuit were normal. The problem was concentrated in the inverter. When checking the internal parameters, it was found that parameter 2602 was set to 1 to enable the flux optimization function. Increasing the flux would saturate the iron core, resulting in excessive excitation current, which could damage the motor due to overheating of the windings [1]. The flux optimization function can adjust the flux amplitude according to the load size, thereby saving energy…; however, the circulating pump load belongs to the fan pump type, and its speed gradually increases. Continuously adjusting the flux amplitude will inevitably affect the current, leading to abnormal operation. For the ABBACS510 frequency converter, parameter 2602 should be set to 0 to disable the flux optimization function. However, this parameter 2602 was not present in the previous generation ABBACS400 frequency converter, indicating that it is a newly added and still immature parameter. Therefore, its function should be disabled, while other equipment remains unchanged. After disabling it, the operation is normal.
One important issue is that the current transformer must be connected above the frequency converter because instruments are generally required to operate at 50Hz. However, the frequency converter displays the output current, while the current transformer, when connected above the frequency converter, displays the input current. This discrepancy in engineering practice can lead to misjudgments and operational errors, and must be taken seriously.
Chart 1 Frequency converter control schematic
In the heating industry, there is a common operating configuration for heating equipment, with one unit in use and one on standby. Using two frequency converters, each dedicated to a specific motor, would be wasteful. A solution is to use a single frequency converter to power multiple units. This allows for both full-load operation and individual unit operation. Note that this study focuses on individual unit operation of a single frequency converter supporting two units, not simultaneous operation. The above is a common circuit using a double-throw switch to switch the motor. However, my research revealed a drawback: switching motors can easily lead to the switch being opened and closed under load. Therefore, I researched and implemented the following improved design method. Improvement method one is shown in Figure 2.
Chart 2 Improved Method 1
Method 1, as shown in Figure 2, involves connecting a voltage relay to the inverter to detect the voltage. During startup, there is no voltage or current, allowing normal startup. However, after startup, current appears, triggering the KV relay. Since the voltage relay only activates after reaching a certain value, this causes the inverter to stop. This method is unsuitable.
Improved method two, as shown in Figure 3, involves the following steps: During startup, the time relay is not activated, allowing the inverter to start normally. After startup, current is present, causing the current relay KC to trip. Since the time relay only trips after a certain time, the intermediate relay KA does not trip, preventing the inverter from stopping. When the double-throw disconnect switch is opened under load, the current relay de-energizes, causing the intermediate relay KA to trip, thus stopping the inverter and preventing the disconnect switch from being closed under load. However, this does not prevent the disconnect switch from being opened under load.
The third improvement method is to replace the knife switch with two contactors and interlock them in the control circuit. This avoids closing the knife switch under load and also avoids opening the knife switch under load, and simplifies the wiring.
Chart 3 Improvement Method Two (Chart2 CorrectivemethodTWO)
In the heating industry, constant pressure water supply systems and pressure stabilization systems are widely used to maintain a constant water pressure in the pipe network. These systems have a fixed setpoint and are considered constant-value control systems. For example, constant pressure water supply exists in the makeup water system or secondary network pressure stabilization system of a city-wide heating network circulation system. In the design process, there are issues with the energy consumption of variable frequency (VFD) water supply and complex wiring. The energy consumption problem is that when the VFD water supply reaches the set pressure, the VFD still has a low-frequency output. In reality, there should be no output power in this state, but due to PID control issues and pipe network fluctuations, power output for pressure regulation is inevitable. This results in energy consumption, which is a huge waste for a city-wide heating network system. Research has found that the sleep function of some VFDs is very practical and suitable for application in frequency ranges, such as the 0-5Hz range when the low-frequency output is in the set pressure range. The sleep function can be applied to reduce the frequency output to 0, thus solving the energy consumption problem.
Complex wiring is currently the most widespread problem in the industry, stemming from the equipment itself. Constant pressure water supply systems are designed using either traditional wiring methods or controller wiring methods. Traditional wiring methods involve building circuits with relays and contactors, using frequency converters to regulate pressure, and external or internal PID controllers to output control signals. Controller wiring methods use controllers to build circuits, simplifying the actual wiring by replacing hard circuits with soft circuits, which has yielded excellent practical results. Firstly, a general-purpose controller is used. This controller is typically placed near the electrical equipment in the field, enabling control of all equipment and uploading operational fault parameters to the city's dispatch and command center. An application program for the constant pressure water supply system is written into the general-purpose controller, enabling regulation functions. Because general-purpose controllers are deployed on-site for operation, the wiring becomes complex due to the addition of other equipment such as control and signal lines. Furthermore, the general-purpose controller chip must possess extremely high processing capabilities, including data processing, control processing, fault handling, and communication processing, which can lead to instability. Therefore, programmable logic controllers (PLCs) were experimentally implemented, simplifying the actual wiring. However, programming remains a challenge. While some professional engineers can handle this, it is a highly specialized technical task for most practitioners, forcing many organizations to hire specialized companies at high salaries. Additionally, debugging and maintenance are extremely inconvenient. In engineering practice, water supply controllers have emerged. These controllers utilize the relatively consistent functions and fixed procedures of constant pressure water supply systems, thus becoming independent controllers. Moreover, the program is embedded within the controller, eliminating the need for programming and simplifying complex programming tasks—a seemingly excellent solution. However, these controllers also suffer from relatively complex wiring. Research has shown that utilizing a control unit integrated into a frequency converter completely solves the above problems and is the best solution among several options. There are two types of such control units: PFC and SPFC.
PFC alternating control mode can be further divided into 1+6 (1 regulating device) + (6 network-connected devices) mode; 1+3+3 (1 regulating device) + (3 network-connected devices) + (3 standby devices) mode; and automatic switching mode: 1-6 devices. In the first mode, the main circuit wiring involves one motor connected to the frequency converter, and a maximum of six other devices connected to the mains frequency. Initially, the device connected to the first frequency converter is controlled by the frequency converter's own PFC controller to regulate the constant pressure water supply. If the requirement is not met, the PFC controller regulates and starts the second to sixth devices until the requirement is met. When the constant pressure water supply is regulated, the PFC controller stops the devices in reverse order, but the first device always stops last. The interlock function (if used) can identify inactive (unused) devices, allowing the PFC controller to skip these devices and call the next usable device. The automatic switching function (if used and corresponding switches and contactors are used) allows the load time to be evenly distributed among the devices. The second method involves connecting one motor to the frequency converter in the main circuit, with three other devices connected to the mains frequency and three as backups. The third method connects each motor to both the frequency converter and the mains frequency, but only one frequency converter is used, allowing a maximum of six devices to be connected. The automatic switching function periodically adjusts the position of each motor – for example, a speed-regulating motor becomes the last auxiliary motor to be called, while the first auxiliary motor becomes the speed-regulating motor. The significant advantages of this PFC controller are its simple wiring, convenient control, flexible wiring methods, and complete automation, making it ideal. However, it suffers from the drawback of direct start-up.
SPFC cyclic soft-start application (up to 6 motors) overcomes the shortcomings of PFC alternating control mode, achieving optimal design. The main circuit wiring connects each motor to both a frequency converter and a mains frequency circuit, but only one frequency converter is used, allowing a maximum of 6 motors to be connected. The main circuit wiring is the same as the third method of PFC alternating control mode. However, it uses the frequency converter to start the auxiliary equipment. First, the frequency converter starts the first device, and the frequency converter's own PFC controller regulates the constant pressure water supply. If the requirement is not met, the PFC controller controls the frequency converter to start the second device, and the first device starts at the mains frequency, and so on, until the sixth device is started, until the requirement is met. When the constant pressure water supply regulation is satisfied, the PFC controller stops the devices in reverse order, i.e., first-start, first-stop, but the device driven by the frequency converter always stops last. The difference between SPFC cyclic soft-start application and the third method of PFC alternating control mode is that the latter has the drawback of direct start.
Through the above detailed analysis and research, we have obtained the above technologies and their respective characteristics. Each of these technologies has its own advantages and will play a pivotal role in industry applications. We hope that these research results over the years will contribute to various industries in society and maximize their effectiveness.
