Abstract: Automatic quasi-synchronizing operation is a frequent and important operation in power systems. This paper develops an experimental device based on virtual instrument technology to realize the three conditions of voltage, frequency, and phase for automatic quasi-synchronizing operation. This experimental device has the characteristics of good visibility, flexible operation, and high grid connection accuracy. Keywords: Power system; Automatic quasi-synchronizing; Virtual instrument; Experimental device 1 Introduction In power systems , due to the needs of grid operation, synchronous generators, synchronous compensators, and synchronous motors are frequently connected to or disconnected from the grid. The operation of connecting synchronous machines to the power system for parallel operation is called parallel operation. Parallel operation is divided into two types: quasi-synchronous parallel operation and self-synchronous parallel operation. During quasi-synchronous parallel operation, the inrush current can be very small, which will not cause large disturbances to the power grid and will not damage the generator set. It is the most widely used synchronization method in power plants and substations. Quasi-synchronous parallel devices are divided into two types: automatic quasi-synchronous devices and manual quasi-synchronous devices. The existing automatic quasi-synchronous devices mainly include automatic quasi-synchronous devices based on MAS series single-chip microcomputers [1], automatic quasi-synchronous devices based on DSP [2], and automatic quasi-synchronous devices based on PLC technology [3]. For students majoring in electrical engineering, it is difficult to actually perform parallel operation. Automatic quasi-synchronous experimental devices can simulate parallel operation. Based on the basic principle of automatic quasi-synchronous devices, they can simulate some actual operations. Among them, there is the ZZQ-5 type automatic quasi-synchronous experimental box, which can provide simulated system voltage and generator voltage. It can adjust the voltage and frequency of the simulated generator and system to obtain different voltage and frequency differences to meet the requirements of generator parallel operation. This device is used together with the generator synchronization simulation tester to simulate the quasi-synchronous parallel operation of generators. However, existing experimental devices suffer from poor visibility, inflexible operation, and malfunctions due to misoperation. Developing a virtual automatic quasi-synchronization experimental device using virtual instrument technology can overcome these shortcomings, offering intuitive display, high grid connection accuracy, and flexible operation. 2. Automatic Quasi-Synchronization Conditions The automatic quasi-synchronization conditions are: the voltage of the generator to be connected is the same as the grid voltage, and their frequencies are close. At the instant the phase difference between them is zero, the main contacts of the generator circuit breaker are closed, allowing the generator to smoothly connect to the grid. The quasi-synchronization conditions consist of three parts: voltage condition, frequency condition, and phase condition. The phase condition requires the generator's grid connection switch to close instantaneously when the phase difference is zero. However, the switch closure requires a certain time, called the switch closing time or lead time. Therefore, the closing pulse must be issued in advance to ensure the quasi-synchronization requirements are met. Thus, the most important task of the automatic quasi-synchronization device is to capture the timing of the closing pulse based on the current phase difference and its rate of change. The frequency and voltage conditions specified in the automatic quasi-synchronization conditions have a significant impact on this capture process. First, the rate of change of phase difference is determined by the frequency difference between the power grid and the generator. The larger the frequency difference, the faster the phase difference changes. Obviously, an excessively large frequency difference is not conducive to capturing the timing of closing. Only under a certain frequency difference can the phase difference show a periodic change, so the frequency difference cannot be zero. Second, the voltage condition determines the magnitude of the impact that may occur when the phase difference is zero. Ideally, the voltage difference is zero. Under this condition, there will be no impact when closing the grid. However, it is not only unrealistic but also unnecessary to require the generator voltage to be strictly equal to the grid voltage. As long as the voltage difference is controlled within a certain range, the impact is acceptable. In the automatic quasi-synchronous test device, as long as the voltage amplitude difference is less than the allowable value, the frequency difference is less than the allowable value, and the closing phase angle difference is less than the allowable value, parallel operation can be achieved. That is, 3 Design of Automatic Quasi-synchronous Test Device In the parallel operation of the power system, the core is the judgment of the parallel conditions. Therefore, the focus of the design of the automatic quasi-synchronous test device is also on this [4]. Its flowchart is shown in Figure 1. [align=center] Figure 1 Flowchart[/align] The LABVIEW[5-6] software is used to simulate the generator paralleling process. A program is written to complete the judgment of the three conditions before the generator paralleling, and at the same time, the generator paralleling process is simulated and displayed, so that it can be conveniently and intuitively simulated and demonstrated. The front panel of the automatic quasi-synchronous experimental device designed in this paper is shown in Figure 2. [align=center] Figure 2 Front panel[/align] The automatic quasi-synchronous experimental device mainly consists of five parts, namely the voltage effective value calculation unit, frequency measurement unit, voltage effective value comparison unit, frequency comparison unit and phase angle judgment unit. 3.1 Voltage Effective Value Calculation Unit The continuous voltage sinusoidal signal is collected into the computer through the acquisition card and stored in the form of an array to form a discrete signal. Its voltage effective value can be obtained by equation (5). (5) 3.2 Frequency Measurement Unit [align=center] Figure 3 Frequency Test Unit [/align] 3.3 Voltage RMS Comparison Unit [align=center] Figure 4 Voltage RMS Comparison Unit [/align] After the voltage RMS value and frequency can be determined, the determination of the parallel conditions is the key link of this experimental device. The voltage RMS comparison unit is shown in Figure 4. In this unit, the system voltage RMS value and the generator voltage RMS value are obtained and compared respectively. It is set that when the error range between the generator voltage RMS value and the system voltage RMS value is ±10V, the voltage is qualified and a qualified signal is issued. When the voltage is not in this range, the generator excitation is adjusted (here, it is set that the generator voltage is increased or decreased by 2V each time) to eventually reach the allowable range and prepare for closing. When the voltage is unqualified, the corresponding indicator light is lit to show the adjustment process. The principle of the frequency comparison unit is the same as that of the voltage RMS comparison unit, only the conditions are different. It is set that the closing requirements are met when the error between the generator frequency and the system frequency is about ±0.5Hz. When the generator frequency does not meet the requirements, the turbine is adjusted (here, the generator frequency is adjusted by 0.1Hz each time). During the adjustment process, the corresponding indicator lights illuminate to show the adjustment process. 3.4 Phase Angle Judgment Unit In the phase angle judgment unit, the voltage waveforms of the generator and the system are converted into square waves, as shown in Figure 5. This square wave indicates the change in phase angle. When the square wave pulse is the narrowest, it indicates that the phase difference is minimal and the closing requirements are met. At this moment, when both the effective voltage value and frequency meet the conditions, the generator sends a parallel signal, allowing the generator to be safely connected to the system grid. [align=center] Figure 5 Generator and System Square Waves[/align] 4 Conclusion This experimental device can effectively simulate the automatic parallel operation of the generator. The interface can display the changes in generator and system voltage, frequency, and phase in real time, as well as the conditions under which parallel operation can be successfully achieved. Using virtual instruments to realize the automatic quasi-synchronous experimental device, many hardware-based implementations are accomplished through software programming. This not only makes the experimental device more intuitive, with high grid connection accuracy and flexible operation, but also makes it convenient and simple to modify or add experimental functions. The innovation of this paper: This paper systematically describes the design of an automatic quasi-synchronization experimental device, and uses virtual instruments to realize its system design, which saves development time and greatly reduces the development cost of the design. References [1] Guo Jian, Zhou Bin. Design of a new type of microcomputer automatic quasi-synchronization device [J]. Electric Power Automation Equipment, 2005, 25 (8): 77-81. [2] Li Yexing, Deng Zhijie, Li Wenhui. Design and implementation of automatic quasi-synchronization device based on DSP [J]. Electrical Application, 2006, 25 (8): 27-30. [3] Guo Quanli, Zheng Junzhe, Xu Jian. Design of a new type of automatic quasi-synchronization device [J]. Journal of Shenyang Institute of Engineering (Natural Science Edition), 2004, 2 (2): 134-136. [4] Ding Shuwen. Microcomputer-based automatic device [M]. Beijing: China Electric Power Press, 2005. [5] Lei Zhenshan. Practical tutorial on LabVIEW 7 Express [M]. Beijing: China Railway Publishing House, 2004.4. [6] Yuan Fang, Jiang Wei. Design scheme of virtual instrument system [J]. Microcomputer Information, 2007, 23 (8-1): 162-163.