1. Overview The Laiwu Steel 1500mm hot-rolled strip steel project is powered by 10kV. The main loads include the roughing mill, finishing mill, auxiliary drives, and power loads. The roughing mill has two AC-AC variable frequency speed-regulating motors, each with a power of 4500 kW; finishing mills F1 to F3 have three AC-AC variable frequency speed-regulating motors, each with a power of 4000 kW; finishing mills F4 to F6 have three AC-AC variable frequency speed-regulating motors, each with a power of 3500 kW; DC auxiliary drive loads total 1500 kW; flying shear DC drive loads total 435 kW; DC drive loads for drums, loopers, etc., total 680 kW; and other loads are AC-DC variable frequency drive loads, totaling approximately 7300 kW. The load characteristics are that the rolling mill has a large load capacity, which is the root cause of power grid fluctuations, especially the primary rolling mill, which has the largest impact load. The loads of the R1 primary rolling mill and the F1 to F6 finishing rolling mills are both AC-AC frequency converter loads, which have side-frequency harmonics. The rectifier load is configured with 12-pulse rectification, and its characteristic harmonics are 12k±1, with the 5th and 7th harmonics being relatively small, and the 11th and 13th harmonics being relatively large. The reactive power and harmonic quantities generated vary greatly depending on the rolling specifications, varieties, and processes. It is necessary to design overall dynamic reactive power compensation and harmonic filtering for the above loads. The reactive power compensation and harmonic filtering design needs to solve the reactive power and harmonics generated by the rolling mill and its auxiliary drives in order to improve the operating power factor, significantly reduce reactive power loss, effectively eliminate or suppress harmonic content and voltage distortion, and meet the requirements of national standards. 2. Reactive Power Compensation Design Application According to the power supply system diagram of the Laiwu Steel 1500mm hot-rolled strip project, the power supply is a 10kV two-section busbar. Section I mainly includes the main drive components of the R1 primary rolling mill and the F1~F6 finishing mills. The main challenges are reactive power impact, voltage fluctuations, and harmonic filtering. Section II mainly includes auxiliary drives and other auxiliary loads. The main challenge is reactive power compensation. Because Section I contains the R1 primary rolling mill and the F1~F6 finishing mills, the load impact is significant; Section II, being an auxiliary load, is relatively stable. Therefore, this plan proposes to install an FC+TCR type SVC (Static Dynamic Reactive Power Compensation and Harmonic Filtering Device) system on Section I and an FC filter device on Section II. The advantages of this method are: 1) Installing an SVC system on the I-section busbar grid allows for continuous adjustment of reactive power compensation, with a fast power-state response. This addresses the severe impact of the main mill on the grid, stabilizes the grid voltage, filters harmonics, and fully leverages the advantages of SVC, maintaining the power factor of the I-section grid above the set value of 0.95. 2) The main load on the II-section busbar is the auxiliary drive, resulting in minimal load impact. Installing a static filter can save investment while meeting grid requirements. The power factor compensation index for the II-section busbar is designed to be >0.92. 3) The combined effect of the two sections is dynamic reactive power compensation and harmonic filtering. 2.1 System Composition The system includes the main power supply circuit, FC filters (H3, H5, H7, H11 filter circuits), and TCR phase-controlled reactors (TCR phase-controlled valves, main reactors). The main power supply circuit includes: 2 high-voltage vacuum switchgear cabinets, 1 high-voltage PT cabinet, 1 high-voltage PT cabinet, and 4 high-voltage disconnect switchgear cabinets. The FC filter (H3, H5, H7, H11 filter circuits) includes: 4 sets of filter capacitors (with external fuses and internal discharge resistors), 4 sets of filter reactors (hollow core), 4 sets of high-voltage resistors, and several protection devices. The TCR phase-controlled reactor (TCR phase-controlled valve, main reactor) includes: 1 set of TCR phase-controlled valve (using a water-cooled system), 1 set of main reactor (hollow core, three-phase horizontal placement), 1 set of water-cooled system, and several protection devices. The control system includes: a TCR phase-controlled valve control system (using Siemens' SIMADYN-D CNC system), a triggering system (high and low voltage opto-isolation, pulse triggering, and BOD protection), and a relay protection system (including various protections for the FC and TCR systems). 2.2 Compensation Principle of FC+TCR Type SVC for Section I Busbar The FC+TCR device consists of 3 parts: an FC filter, a TCR phase-controlled reactor, and a control and protection system. The system block diagram is shown in Figure 1. FC filters are used for reactive power compensation and harmonic filtering. These filters should perform two tasks: providing the capacitive reactive power required by the system and filtering out harmonics generated by the load and the TCR system itself. TCR phase-controlled reactors are used to balance the inductive reactive power generated by load fluctuations in the system. These phase-controlled reactors should perform two tasks: providing the inductive reactive power required by the system and stabilizing voltage fluctuations caused by load surges. The control and protection system includes two aspects: protection and control of the entire system; and phase-adjustment control of the phase-controlled reactor. [ALIGN=CENTER] Figure 1 FC+TCR Compensation Principle Diagram [/ALIGN] The basic structure of a TCR is a controllable valve body composed of thyristors connected in series with a reactor, using a delta connection. When this circuit is connected in parallel to the power grid, it is equivalent to the structure of an AC voltage regulating circuit for an inductive load. The absorbed harmonic current and reactive power decrease as the control angle α increases, achieving the purpose of regulating reactive power. The load change is balanced by the change in reactive power generated by the TCR, so that the sum of the two is always kept constant. This constant inductive reactive power is canceled by the capacitive reactive power of the FC, and finally the power factor of the grid is kept at the set value, such as 0.95. (1) FC filter The FC filter is composed of a series filter reactor on a parallel capacitor. According to the different harmonic content of the load, several different LC compensation filter circuits are formed. The harmonics to be filtered by the FC filter mainly come from two aspects: one is the harmonics generated by the load, mainly from the rectifier equipment, and its characteristic harmonics are the 5th, 7th, 11th, 13th, etc.; the other is from the TCR phase-controlled reactor, mainly the 3rd harmonic. (2) TCR phase-controlled reactor The TCR phase-controlled reactor is composed of a thyristor phase-controlled valve and a reactor connected in series. The three-phase connection adopts a delta connection. This connection form has a smaller harmonic content in the current than other connection forms. In addition, in practical engineering, when the capacity is large, the reactor is divided into two parts of equal capacity and connected in series across the two ends of the thyristor phase-controlled valve. This provides additional protection for the thyristor when the reactor fails. 2.3 Compensation Principle of FC Filter Device on Section II Bus The FC of Section II bus consists of two parts: an FC filter and a control system. The FC filter is used for reactive power compensation and harmonic filtering. This filter should perform the following two tasks: provide the capacitive reactive power required by the system and filter out the harmonics generated by the load. The control system provides relay protection for the entire system. The harmonics to be filtered out by the FC filter mainly come from the rectifier equipment of the load, and their characteristic harmonics are the 5th, 7th, 11th, and 13th harmonics. However, since there are fewer harmonic sources in the load of Section II bus, designing the filter as a 5th and 7th high-pass two-loop filter can meet the requirements of the power grid, ensuring effective harmonic filtering and preventing resonance with the background harmonics of the power grid. 2.4. Capacity Calculation of Reactive Power Compensation and Harmonic Filtering System (1) Theoretical Basis for Harmonic Calculation 1) Rated Capacity of Motor Where K[SUB]Uu[/SUB] —— Transformer voltage calculation coefficient, take 1; K[SUB]1[/SUB] —— Transformer current calculation coefficient, take 0.789 for 12 pulses; K[SUB]t[/SUB] —— Motor overload multiple, take 2.5 for rough rolling and 1 for fine rolling; P[SUB]dn[/SUB] —— Rated power of motor, kW; η[SUB]d[/SUB] —— Efficiency, take 0.92. 2) Harmonic Estimation Characteristic Harmonic Current Content of 12-Pulse Converter Non-Characteristic Harmonic Content TCR (Triangle Connection) (2) Selection of Compensation Capacity The capacity calculation of this reactive power compensation and harmonic filtering device is based on PSCAD simulation software, which is widely used worldwide. Authoritative power system agencies in China, such as Tsinghua University and China Electric Power Research Institute, are using it. The advantages of using simulation technology are: the power technical indicators of the compensation and filtering device can be predicted before it is put into operation, and after adjustment, a more ideal reactive power compensation and harmonic filtering device can be designed, avoiding blindness and ensuring the design quality of the device. 1) The average power factor of the FC+TCR type SVC system after the I-section busbar is put into operation is calculated as 0.95, and the technical parameters of the device are selected as follows: device name: AriSVC36/10-2-TCR (28.8/36); compensation point: 10kV busbar; rated voltage: 10kV; device capacity: FC 36Mvar, TCR 28.8Mvar. 2) The average power factor of the FC filter device after the II-section busbar is put into operation is calculated as 0.92, and the technical parameters of the device are selected as follows: device name: AriFC——3/10-2 -FC; compensation point: 10kV busbar; rated voltage: 10kV; device capacity: FC 3Mvar. (3) Capacity calculation and simulation curve of the SVC device of section I busbar. According to the harmonic calculation theory, the capacity of the equipment was calculated, and the specific values ​​are as follows. Rough rolling 4500 kW×2=9000 KW, overload multiple is taken as 2.5. Finishing rolling 4009kWx3+3500kWx3=22509 kW, overload multiple is taken as 1. The total apparent capacity S[SUB]N[/SUB] is 66844 KVA. The harmonic content of the fundamental frequency current is very small due to the three-phase symmetry of the system, and is taken as 60A. The TCR is connected in a diagonal, with a total capacity of 28.8 Mvar. The total harmonic current reactive power device capacity is 36 Mvar. Among them, the 3rd is 4.2Mvar; the 5th is 13.8Mvar; the 7th is 9Mvar; the 11th is 9Mvar. A high-pass filter is used, and the quality factor is taken as 5. [ALIGN=CENTER] Table 1 Calculation of SVC device parameters for section I bus of Laiwu Steel [/ALIGN] The simulation of the SVC device for section I bus was carried out. The simulation curve of the SVC device for section I bus is shown in Figure 2, which shows the effect of harmonic filtering before and after the SVC device for section I bus is put into operation. The green line is the test curve before filtering and the red line is the test curve after using the ramp filter. It can be seen that the filtering effect is very obvious. 1) Total harmonic voltage distortion rate before filtering (exceeds the standard) After filtering (meets the standard) 2) Residual harmonic current injected into the system 3) 10 kV bus voltage fluctuation before filtering (exceeds the standard) After filtering (meets the standard) 4) Power factor is 0.95 (4) Comparison of the results of the filtering device for section II bus For AC-DC-AC frequency conversion loads, due to their high power factor, they are not included in the capacity calculation. DC auxiliary drive, total load 1509 kW. Flying shear DC drive, total load 435 kW. Drum, looper and other DC drive loads total 680 kW. Other loads are AC-DC frequency conversion driven, with a total load of approximately 7300 kW. The calculation results are shown in Table 2. [ALIGN=CENTER] ' Figure 2 Simulation curve of SVC device for section I bus [/ALIGN] [ALIGN=CENTER] Table 2 Calculation of parameters of SVC device for section II bus of Laiwu Steel before and after using the filter device: [/ALIGN] 1) Total harmonic voltage distortion rate before filtering (exceeds the standard) After filtering (meets the standard) 2) Residual harmonic current injected into the system 3) Power factor is 0.95 (5) Protection and control system of section I bus The protection and control system of the device includes two aspects: the first is the relay protection control of the entire system; the second is the phase adjustment control of the phase-controlled reactor. The relay protection control and the conversion of the trigger signal are completed by the protection control cabinet, and the phase adjustment control of the phase-controlled reactor and the protection of the TCR system are completed by Siemens' SLMADYN-D CNC system. The relay protection system uses a PLC as the central controller to monitor the FC filter and electrical parameters, and simultaneously shapes, amplifies, and converts the phase-controlled trigger signal, ultimately outputting it as an optical signal to the thyristor switch cabinet. The FC+TCR type SVC device features overvoltage protection, undervoltage protection, overcurrent protection, capacitor overvoltage and overcurrent protection, unbalanced voltage protection, unbalanced current protection, and grounding protection. When a fault occurs, it operates as follows: For minor faults, it first reports a minor fault; after the minor fault persists for a certain period, it reports a major fault; when a major fault occurs, it automatically shuts down, activates an alarm buzzer, and displays the fault status on the display panel. Even after the major fault signal is cleared, the display panel continues to alarm until manually reset. 3. Conclusion The system employs dynamic reactive power compensation and harmonic filtering technology. After commissioning, it operates well, with high control accuracy, convenient operation, stable and reliable operation, low interference, and few problems, causing minimal damage to the power grid. The SVC system installed on the 10kV busbar of Section I of the power grid can continuously adjust the reactive power compensation, with a fast dynamic response speed. This solves the problem of severe impacts on the power grid from the R1 primary rolling mill, stabilizes the grid voltage, filters harmonics, and fully utilizes the advantages of SVC, keeping the power factor of the Section I 10kV power grid above the set value of 0.95. During the rolling mill's production process, the load impact on the Section II 10kV busbar is smaller compared to that on the Section I 10kV busbar. The power factor compensation index for the Section II busbar is designed to be 0.92. After commissioning, there were no shutdowns due to grid disturbances; the grid operated normally, reducing potential losses from downtime and creating significant economic benefits. (Article excerpted from "Energy Saving Innovation 2006—Proceedings of the First National Electrical Energy Saving Competition")