Abstract : This article introduces the application of an SVG (model FGSVG-C12.5/35) from Xinfengguang Electronic Technology Co., Ltd. in compensating for the load of an electric arc furnace at a steel plant in Guangdong. The field application of the high-voltage dynamic reactive power compensation generator demonstrates that it has a very good compensation effect on the electric arc furnace.
1 Introduction
In early December 2016, relevant national departments issued a "Notice on Resolutely Curbing Illegal New Production Capacity in the Steel and Coal Industries, Cracking Down on 'Inferior Steel', and Establishing Order in Production and Operation." Inferior steel refers to steel produced using scrap steel as raw material, melted in induction furnaces, and without effective control over its composition and quality. This type of steel is mainly used in construction and is considered to pose serious quality risks. The equipment primarily used for smelting this type of steel is medium-frequency furnaces.
A steel plant in Guangdong Province, primarily producing rebar coils, is a vertically integrated steel production enterprise encompassing steelmaking and rolling. In early 2017, to improve steelmaking quality, it underwent a power grid upgrade, dismantling the intermediate frequency furnace and installing an electric arc furnace. After the electric arc furnace was put into operation, it caused voltage fluctuations in the 35kV power grid, with flicker values exceeding standards, directly leading to the shutdown of a nearby hydroelectric power station.
While electric arc furnaces have a relatively simple system structure, they significantly impact grid voltage. During the melting phase, the charge is not yet melted, the arc is unstable, and arc interruption and reignition frequently occur, sometimes leading to short circuits between electrodes. In the initial melting stage, the current waveform is irregular, with high harmonic content, primarily low-order harmonics, sometimes exceeding 20%. Also in the initial melting stage, the three-phase unbalanced current contains a large negative sequence component; when one phase extinguishes the arc and the other two phases are short-circuited, the fundamental negative sequence current has the highest proportion. However, the most significant impact of electric arc furnaces is not harmonics, but rather voltage fluctuations and flicker. Voltage fluctuations and flicker severely affect the safe operation of the power grid and other electrical equipment.
2. Characteristics of on-site power consumption and compensation requirements of electric arc furnace
2.1 Primary Diagram of On-site Power Distribution System
Figure 1 shows the primary circuit diagram of the power distribution system at a steel plant in Guangdong.
Figure 1 Primary diagram of the on-site power distribution system
2.2 On-site load conditions
The on-site load conditions of this project are shown in Table 1.
Table 1 Load Conditions
Load Name Voltage Rating Power Number of Units Load Type Remarks
#1 Electric Arc Furnace 35kV 16000kVA 1 Inductive Load Impact
#2 Electric Arc Furnace 35kV 16000kVA 1 Inductive Load Impact
While converting the medium-frequency furnace to an electric arc furnace, the steel plant installed an FC compensation device, as shown in Table 2.
Table 2 FC Compensation Device
Load Name Voltage Level Installed Capacity Fundamental Capacity Tuning Times Filter Type
2nd FC 35kV 4100kvar 2500kvar 2 Single-tuned
3rd FC 35kV 5400kvar 4000kvar 3 single-tuned
4th FC 35kV 3100kvar 2200kvar 4 Single-tuned
5th order FC 35kV 5000kvar 3500kvar 5 single tuned
2.3 Power quality issues existing at the project site
After the electric arc furnace at the steel plant was put into operation, although an FC compensation device was installed, it was unable to handle the rapid reactive power surges from the furnace, and the compensation capacity was severely insufficient. The grid voltage flicker exceeded the standard, and the unstable grid voltage caused nearby hydroelectric power stations to malfunction, resulting in severe generator vibrations and drastic fluctuations in reactive and active currents. Furthermore, when the load on the electric arc furnace decreased, there was an overcompensation problem on the system side.
In order to reduce investment, steel mill owners only require that the electric arc furnace not affect the production of surrounding enterprises after it is put into operation, and that the power factor be increased to above 0.9.
2.4 On-site compensation plan for this project
Based on the on-site power quality and user needs, a reasonable compensation capacity was designed. Test data shows that the maximum reactive power impact of the electric arc furnace can reach more than twice the capacity of the electric arc furnace transformer, as shown in Figure 2.
Figure 2. Reactive power curve of electric arc furnace
Considering cost and on-site conditions, the compensation plan for this project is to add a 35kV direct-connected FGSVG to the existing compensation device (FC), with an FGSVG capacity of 12.5Mvar.
3. Introduction to the basic principles of SVG (Static Image Processing)
SVG is currently the most advanced reactive power compensation device both domestically and internationally. This compensation device, based on a voltage-source PWM converter, represents a qualitative leap in reactive power compensation methods. It no longer uses large-capacity capacitors and inductors, but instead achieves reactive power conversion through high-frequency switching of power electronic devices. The basic principle involves connecting a self-commutated bridge circuit in parallel with the power grid via a reactor. By appropriately adjusting the amplitude and phase of the AC output voltage of the bridge circuit, or by directly controlling its AC current, the circuit can absorb or generate the required reactive current, achieving dynamic reactive power compensation. SVG features a fast current response speed and strong voltage flicker suppression capability.
Figure 3 Schematic diagram of FGSVG structure
Figure 3 shows a schematic diagram of the structure of the new Fengguang 35kV direct-connected SVG. The main circuit adopts a chain-type series structure design, with each phase consisting of multiple identical power units. Each power unit consists of a bridge circuit composed of multiple high-power power electronic devices. The series connection of units and carrier phase-shifting technology make the output voltage waveform of the whole machine closer to a sine wave, avoiding many problems caused by large du/dt.
The FGSVG series products utilize imported power electronic modules as the main power devices, with multiple DSPs and FPGAs forming a powerful control system. The control algorithm employs instantaneous reactive power theory to achieve rapid and accurate detection of system/load reactive power. To achieve faster response speed and higher performance, direct current control technology and carrier phase-shifting technology are used to achieve rapid control of grid-connected reactive current and a better grid-connected current waveform, as shown in Figure 4. FGSVG can quickly and continuously provide capacitive or inductive reactive power, achieving appropriate voltage and reactive power control, ensuring stable, efficient, and high-quality operation of the power system.
Figure 4. Response time waveform from inductive rated current to capacitive rated current.
The FGSVG power unit adopts a redundant and modular design to meet the high reliability requirements of the system. The modular design offers high integration, good interchangeability of power units, simple on-site installation and maintenance, and a small footprint, typically only 50% that of a SVC of the same capacity. Multiple FGSVG units can be installed in parallel, easily expanding capacity; parallel operation utilizes high-speed fiber optic communication, providing fast communication speeds and meeting rapid compensation requirements. Furthermore, on-site operation is very simple, with a large touchscreen offering a rich display interface, including real-time status and analog quantity display, historical event recording, historical curve record query, unit status monitoring, system information query, and historical fault query functions. It also features power-on control system self-test, one-button start/stop, time-sharing control, oscilloscope (AD waveform recording), and fault instantaneous voltage/current waveform recording functions.
4. Compensation effect
4.1 FGSVG Debugging
On-site, the FGSVG is connected to the 35kV power grid via a high-voltage distribution cabinet. It uses PTs and CTs to detect the grid voltage and current on the 35kV system side and performs automatic tracking compensation. The FGSVG has automatic PT and CT identification capabilities. After the on-site distribution cabinet is closed, the FGSVG main controller automatically analyzes the PT and CT wiring, identifies the PT and CT phase sequence, and can then directly start automatic tracking compensation. The FGSVG has multiple automatic compensation modes, such as constant point reactive power mode, constant point power factor mode, constant point voltage mode, load compensation mode, and comprehensive compensation mode.
After the FGSVG is connected to the grid, to facilitate remote monitoring of the SVG's operating data, the user's original monitoring computer is connected to the SVG's external communication interface via a twisted-pair cable to achieve remote monitoring. The FGSVG has multiple communication interfaces, such as RS-485 and Ethernet interfaces, as well as various commonly used power system communication protocols, such as MODBUS_RTU, CDT91, and IEC104, enabling network communication with the backend. During production, on-site personnel only need to conduct periodic inspections and cleaning; operation and maintenance are very simple. A photo of the FGSVG in operation is shown in Figure 5.
Figure 5. FGSVG in operation on site
4.2 Compensation Effect
Before the SVG was put into operation, the maximum flicker value reached about 11.0 in a short time, as shown in Figure 6.
Figure 6 Short-time flicker values before SVG is put into operation
After FGSVG was put into operation, it significantly suppressed grid voltage fluctuations, with the maximum measured flicker value reduced to around 3.0, as shown in Figure 7. The range of grid voltage fluctuations was significantly reduced after FGSVG was put into operation. After the nearby hydroelectric power station was put into operation, the severe vibrations previously experienced by the generator units disappeared, the reactive current stabilized, and the active current fluctuated only slightly, no longer affecting production.
Figure 7 Short-time flicker values after SVG is put into operation
5. Conclusion
Electric arc furnaces are the equipment in the metallurgical industry that has the most serious impact on power grid quality. During operation, they generate a large amount of harmonics, negative sequence voltage, and flicker, with flicker being the most harmful. The degree of impact is highly dependent on the capacity of the electric arc furnace and the grid capacity. This steel plant, in an effort to reduce investment costs, selected a relatively small compensation capacity. However, after using the high-voltage dynamic reactive power compensation device (FGSVG) produced by Xinfengguang Electronic Technology Co., Ltd., the grid voltage flicker value significantly decreased. This not only eliminated the impact of the electric arc furnace on the hydroelectric power station but also improved the steel plant's monthly average power factor to over 0.96.
References:
1. Wang Zhaoan, Yang Jun, Liu Jinjun, et al. Harmonic Suppression and Reactive Power Compensation - 2nd ed. Machinery Industry Press, 2005.10.
2. Li Chun, Ma Xiaojun, Jiang Qirong, et al. Dynamic model test of improving system voltage regulation characteristics using STATCOM [J]. Proceedings of the CSEE, 1999, 19(9): 46-49.
3. User Manual for FGSVG Series High Voltage Dynamic Reactive Power Compensation Device (V2.1), Xinfengguang Electronic Technology Co., Ltd., April 2015.
About the author:
Pei Baofeng: Born in 1981, male, from Zaozhuang, Shandong Province, holds a bachelor's degree in Automation. He works at Xinfengguang Electronic Technology Co., Ltd., where he is engaged in new product research and development testing and on-site product testing.
Ren Qiguang: Born in 1984, male, from Laiwu, Shandong Province, holds a master's degree in power electronics and electric transmission. He works at Xinfengguang Electronic Technology Co., Ltd., where he is engaged in new product research and development testing and on-site product testing.
Su Liujun: Born in 1988, male, from Heze, Shandong Province, holds a master's degree in power electronics and electric drive. He works at Xinguang Electronics Technology Co., Ltd., where he is engaged in new product research and development testing and on-site product testing.
Address: Wenshang Economic Development Zone, Shandong Province
Postal code: 272500
Contact person for this article: Guo Peibin
Contact number: 0537-7237007
