Abstract This paper analyzes and demonstrates the harmonic content and magnitude of the wind turbine inverter load in the first domestically produced 60MW generator air-cooled island low-voltage power distribution system, which has practical engineering significance for the rational selection of filter equipment capacity. Keywords : Harmonic analysis on power system of 600MW air-cold sub-critical generators group in cold air island Plant Chundong Liu HngXul Xianan Wei (1. Program Management Department of China Power Engineering Corporation, Tongliao, Inner Mongolia 028000 2. Northeast Electric Power Design Institute, Changchun, Jilin 30021) Abstract: This paper aims at the analysis and argumentation of low voltage power distribution system of the first set 600MW generator group, the harmonic amount and size of air blower fan transducer load, which is significant in reasonably selecting the capacity of the filtering device. Keywords: Cold Air island, transducer, harmonic, simulation calculation, harmonic variation trend 1 Introduction Northern China has abundant coal resources and numerous thermal power generation projects. All five major power generation groups in China have thermal power plants in the region. However, due to the scarcity of water resources in northern China, traditional water-cooled systems in thermal power projects are being upgraded to adopt the internationally popular air-cooled island technology, and domestic air-cooling technology is gradually being applied. The third phase of the Tongliao Power Plant project involves the construction of a new 060MW air-cooled subcritical generator unit. Upon completion, this unit will be the first domestically produced generator unit of its capacity, and its demonstrative significance for similar projects in China is immeasurable. The primary wiring diagram of the low-voltage plant power system of the air-cooled island for this unit is shown in Figure 1. 2. Problem Statement Using frequency converters for the fans in the direct air-cooling system of the air-cooled island can bring the following benefits: A. Saving plant power; B. Reducing the radiator area; C. Making the aerodynamic field around the radiator more uniform. The biggest adverse effect of using frequency converters in the direct air-cooling system of an air-cooled island is the generation of numerous harmonics. These harmonics from multiple frequency converters overlap, creating a complex system. Harmonics can affect the normal operation of motors, cause insulation damage to power supply equipment, and lead to malfunctions or failures to operate internally or externally, especially impacting electronic products. Numerous studies have analyzed the harmful effects of harmonics in detail, so this article will not elaborate further. Generally, the magnitude of frequency converter harmonics is determined by the following main factors: ① Trigger control angle α; ② Commutation overlap angle γ; ③ Symmetry of the trigger pulse; ④ Built-in filtering technology. Without similar engineering experience, questions such as the magnitude of harmonics generated by frequency converter loads in low-voltage 380V busbar and high-voltage 10kV busbar systems, the appropriate methods for mitigation, and the suitable capacity of harmonic mitigation equipment pose challenges for both users and designers. 3. Harmonic Content Analysis and Calculation 3.1 Analysis using ETAP Powerstatlon 4.7 software. This software is a power system simulation and calculation software based on a full graphical interface of Microsoft Windows. Harmonic analysis calculations follow the IEEE 519a standard, performing harmonic power flow analysis, harmonic resonance fundamental frequency sweep analysis, and simulating voltage and current harmonic sources. It can provide calculation results such as the root mean square (RMS) of voltage and current; the sum of voltage and current peak values ​​(ASUM); the total harmonic distortion (THD); and the communication interference factor (TIF). The calculation model used for the electrical wiring in this project includes primary electrical system parameters such as generators, transformers at all levels, lines, motors, and frequency converters. In the electrical model of this project, only the frequency converter provides harmonic current as a harmonic current source. The calculation uses the EIEE conventional 6-pulse waveform provided in the ETAP software (waveform attachment omitted), without considering any filtering measures. The 5th and 7th harmonic components in the calculation results are 18.45% and 13.19% of the fundamental current, respectively. The calculated results are representative and have general significance, as shown in Table 1. The simulation calculation shows that the total harmonic current on the low-voltage bus is 1125A. Since this calculation does not consider any filtering measures, the result is definitely too high. According to the calculation data and product experience of the frequency converter manufacturer, it is entirely possible to reduce the harmonic amplitude by 30% to 40%. Considering the worst-case scenario, 30% of the harmonic current can be eliminated, so the actual harmonic current should be less than 789A; at the same time, considering that the national standard allows a harmonic current of 180A (calculation process omitted), the harmonic current that needs filtering should not exceed 609A. In this case, since the air-cooled fan does not operate at 100% motor power, in most cases, the harmonic current on the low-voltage bus can be guaranteed to be far less than the national standard requirement of 180A. 3.2 Analysis of the air-cooled fan using Rockwell software The final selected frequency converter is from Rockwell, with a total of 16 132kW frequency converters. The system data is shown in Figure 2. Based on the system electrical parameters in Figure 2, and considering the inverter's 3% incoming line reactor and built-in intermediate DC reactor, as well as a 95% load rate, the harmonic content in Table 2 was calculated through simulation. The simulated harmonic current on the low-voltage bus in Table 2 is 764.88A, which is approximately 360A (about 32%) smaller than the 1124.9A calculated using ETAP software. This difference, besides being due to the simulation principle, is likely also because the latter considered the presence of reactors connected in series on both the AC and DC sides, as well as the internal DC LC filter. 3.3 Similar Operating Condition Test Analysis To further verify the above simulation analysis, the author conducted tests on the Datong No. 2 Power Plant, which operates under similar conditions. The low-voltage system of the Zx600MW generator unit air-cooled island at Datong No. 2 Power Plant includes four busbars, each powered by a 2500kVA transformer. The main load on each busbar is 14 10kW frequency converters (Semens), with 3% series reactance on the AC side and reactors connected in series on the DC side. The test point was the secondary side of the transformer, at the 380V busbar inlet. The test data are shown in Table 3. As can be seen from Figure 3, the harmonic current distortion rate is inversely proportional to the frequency converter load rate, exhibiting a negative polarity curve characteristic, with an extreme value of 24%. The position of the extreme value is related to the frequency converter pulse count, AC/DC series reactance, and built-in filter. The harmonic voltage distortion rate is directly proportional to the frequency converter load rate, exhibiting a positive polarity curve characteristic, with an extreme value of 16%. The position of the extreme value axis is related to the current distortion rate and system impedance parameters. Further analysis of the curves in Figure 3 shows that in the Datong Power Plant air-cooled island system, when the inverter load rate is 10% (approximately 225A), the transformer load rate is approximately 62.3%, the low-voltage bus harmonic current distortion rate is approximately 27.5%, and the harmonic current value is 618.SA. Comparatively, in the Tongliao Power Plant air-cooled island system, with the same main transformer capacity and a total inverter installed power of 16 x 132 kW = 2nZ kW, when the inverter operates at 100% load, the transformer load rate is approximately 84.5% (approximately 305A), the low-voltage bus current distortion rate is approximately 24.5%, and the harmonic current value is approximately 746A. 3.4 Summary From the above three analyses, it can be concluded that for the low-voltage system of the air-cooled island in the third phase of Tongliao Power Plant, when the inverter load rate is 95%-100%, the current distortion rate of the low-voltage bus is about 25%, and the harmonic current value is about 750A-800A. According to the short-circuit capacity of 380V bus is 23MVA, considering the requirements of the national standard [3] for the harmonic current injection value, the residual injection total harmonic current value is about 180A. Therefore, the harmonic current that needs to be filtered should not be greater than 620A. Considering that the operation mode of the air-cooled fan is not entirely 10% motor power operation, in most cases, the harmonic current of the low-voltage bus can be guaranteed to be far less than the national standard requirement of 180A. Based on the comprehensive harmonic current analysis and calculation results, compared with the national standard requirements and considering the actual operation, and at the same time considering the need to reduce the overall cost as much as possible, the current value of the active filter equipment is adjusted to 600A, which can meet the requirements. 4 Conclusion According to the statistical data in Table 3, the curve is plotted as follows (Figure 3). This paper analyzes the harmonic content and magnitude of the inverter load in the low-voltage system of the air-cooled island of the power plant of large-capacity thermal power generating units using power system professional simulation software. At the same time, it also refers to the simulation analysis of the inverter manufacturer. The results of the simulation calculation also refer to the harmonic measurement and data analysis of similar projects. It has certain reference value for the harmonic analysis and harmonic control design and implementation of the air-cooled system of the power plant in similar domestic thermal power projects. References [1] Jiang Qirong, Zhao Dongyuan, Chen Jianye. Active power filter. Beijing: Science Press, 205, 10 [1] Xiaolong Liu. Hannonic Estimator RePort-one Line, Rockwell Automation. 2 (X) 5, 12 [1] Editorial Board of Power Design Manual for Iron and Steel Enterprises. Power Design Manual for Iron and Steel Enterprises (Volume 1 and Volume 2). Metallurgical Industry Press, 19%. About the author Liu Chundong (1968-), male, Master of Engineering, senior manager, mainly engaged in electrical technology management of power generation projects. Wei Xian'an (1966-), male, senior engineer, has long been engaged in the design and consulting of electrical systems for thermal power plants.