Calcium carbide furnace renovation

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The calcium carbide production unit of Ningxia Western PVC Co., Ltd. (referred to as Western PVC Company) is equipped with 4×20,000 kV.A semi-hermetical calcium carbide furnaces, with a designed capacity of 160,000 tons per year. The transformers for these calcium carbide furnaces are of the HCDSPI-6700/35 single-phase type, produced by Shanghai Chemical Machinery Factory No. 3. The electrodes use combined holders, and the hydraulic system is supplied by Yuci Hydraulic Co., Ltd. for control purposes; an automated feeding system is implemented using a PLC automation control system developed by Shanghai Jamie Automation Equipment Engineering Co., Ltd. Power supply status: The electric arc furnace plant substation of Western PVC Company is equipped with 2 power transformers of 110 kV/35 kV capacity (each with a capacity of 63,000 kV.A, giving a total capacity of 126,000 kV.A). The 110 kV power supply system has one incoming line; it is of the single-bus type and installed outdoors, with the power being supplied by the thermal power plant of Western PVC Company via power cables. The 35 kV power supply system is of the single bus sectionalized type, indoor, and is equipped with a bus tie switch. The 35 kV power supply system has a total of 8 outgoing circuits: 2 for power transformers, 4 for calcium carbide furnace transformers, and 4 for 35 kV filtering devices. According to the requirements of the production process, the secondary side of the 4 calcium carbide furnace transformers can be connected in a Y configuration or a Δ configuration using a Y/△ conversion cabinet. 2 Application of overvoltage absorption devices: In calcium carbide production, the power supply system generates harmonic overvoltages and operational overvoltages. Therefore, the 35 kV power supply system for the calcium carbide furnaces at Western PVC Company is equipped with 4 sets of TAL35-9600/200AKW type 35 kV AC filtering devices, distributed evenly across the I and II busbars, in order to eliminate harmonics and compensate for the power factor. However, both the capacitors and inductors in these AC filtering devices are energy storage elements; when the power supply system is switched on or off or when its operating conditions change during production, a transition process occurs. During the transition process, as the power supply continues to supply energy and the magnetic energy stored in the capacitor is converted at a certain moment and stored in the system’s capacitors in the form of electric field energy, harmonic overvoltages or operating overvoltages that can be several times the voltage of the power supply are generated. These are transient overvoltages that disappear after 1–100 ms. This type of transient overvoltage poses a serious threat to electrical equipment; it often damages such equipment, leads to production instability, and requires power outages for maintenance, thereby increasing the energy consumption in calcium carbide production facilities. To address this issue, Western PVC Company installed three-phase combined overvoltage protectors on the AC filtering units: TBP-C-42F/630W (on the capacitor bus) and TBP-B-42F/630W (at the input and output terminals of the reactors in the AC filtering units). Data on overvoltages recorded by the multi-functional harmonic suppression device for the 35 kV Phase I and Phase II power supply systems: 35 times before Phase I was equipped ； Before the installation of Section II, 24 times ； After Section I is installed, 1 time ； After the installation of Section II, 0. The statistical data of the counter for the three-phase combined overvoltage protector in the AC filtering device are shown in Table 1. Table 1: Counter statistics of the AC filtering three-phase combined overvoltage protector. Filtering devices: 1# filtering device, 2# filtering device, 3# filtering device, 4# filtering device. Phases to which they are connected: A, B, C; A, B, C; A, B, C; A, B, C. Number of discharges: ?2, 158, 97, 65, 43, 27, 9, 8, 17, 186, 104, 23. As can be seen from the data in Table 1, the three-phase combined overvoltage protector provides excellent protection against overvoltages in the power supply system; overvoltage problems in the 35 kV power supply system are essentially eliminated. The overvoltages on the capacitors and inductors of the AC filtering devices are also removed by this protector, thus preventing any harm to the 35 kV power supply system. No overvoltage damage to electrical equipment has occurred since the installation of the AC filtering device. 3 Application of current-limiting reactor coils: In calcium carbide production, shutdowns are necessary for various reasons, and power outages and restorations occur frequently. Given the requirements and characteristics of calcium carbide production, these shutdowns and power restorations generally take place under full load. The inrush current generated when the calcium carbide furnace and capacitor banks are switched on poses a serious threat to the main transformer; if no measures are taken, this can lead to severe deformation of the coils on the 35 kV side of the transformer due to prolonged exposure to high instantaneous forces, and may even result in damage to the transformer, causing losses for the enterprise. Solution: Connect a current-limiting reactor in series on the valve side (secondary side) bus of the main transformer, using the principle that the current-limiting reactor limits short-circuit currents to protect the main transformer. Western PVC Company uses the CKSGK4186/38.5 type current-limiting hollow reactor produced by Liaoning Huaye Group to limit the impact of short-circuit currents on transformers. After analyzing the deformation test data of the transformer’s secondary coil after the installation of the reactor, it was concluded that no deformation occurred in the secondary coil, indicating that the current-limiting reactor played a significant role in protecting the transformer. It is worth noting that when installing current-limiting reactors, short-circuit current, voltage drop, and resonance points must be carefully calculated in order to reduce the adverse effects and resonance phenomena resulting from their installation. 4 Application of the automatic tracking compensation device for arc suppression in dry-type grounded arc-suppression transformers: In the electric arc furnace substation of the PVC plant in the western region, the distance between the switches of the electric arc furnaces and the Y/Δ conversion cabinets of their transformers is greater than 500 meters. All power supply lines for the 4 electric arc furnaces use 150 mm2 single-core cables, with an average of 2,300 meters of cable required per electric arc furnace. Measurements showed that the capacitive current of the 35 kV power supply system to ground reached 28 A, which is far above the specified value of 10 A. In the 35 kV power supply system of Western PVC Company, which is a system with an ungrounded neutral point, the hazards caused by capacitive current to the equipment are mainly manifested in the following 3 aspects. (1) During intermittent arc grounding, high-amplitude overvoltages are generated as a result of the charge accumulation on the relative ground capacitance, typically 2.0–3.5 times the phase voltage. (2) During the transient process of single-phase grounding, a large grounding current is generated, which can damage the inter-phase insulation at the grounding point and cause inter-phase short circuits. (3) Single-phase ground arcing can cause a surge in the excitation current of the voltage transformer, which is an important cause of damage to the voltage transformer. In a grid with an ungrounded neutral point, one method of controlling excessive single-phase grounding capacitive current is to install an arc-suppression coil at the neutral point. The function of the arc-suppression coil is to provide an inductive current I when a ground fault occurs in the power grid, thereby compensating for the capacitive current IC resulting from the ground fault. This reduces the total ground current and also speeds up the reduction of the voltage across the phase that is grounded, thus achieving automatic extinguishment of the fault. Western PVC Company uses an automatic tracking compensation device for arc suppression developed by Shanghai Siyuan Electric Co., Ltd. The dry-type grounding transformer is of the DKSC-1100/35 type, while the dry-type arc suppression coil is of the XHDCZ-1100/35 type. After installing an automatic tracking compensation device for arc suppression in a dry-type grounded arc suppression transformer, the capacitance current compensation value reached 30 A, while the capacitance current to ground was less than 2.5 A. This effectively addressed the problem posed by capacitance current to the 35 kV power supply system, thereby stabilizing operations. 5 Conclusion In 2006, Western PVC Company’s average comprehensive power consumption per ton of calcium carbide produced was 3,579.25 kW·h, with an operating rate of 79.4% ； From January to August 2007, the average comprehensive power consumption for producing 1 ton of calcium carbide was 3,553.89 kW·h, with an operating rate of 87.6%. Using 1 ton of calcium carbide reduces power consumption by 25.36 kW·h, and increases the operation rate by 8.2%. Practice has shown that after implementing three measures in the power supply system of its calcium carbide plants, Western PVC companies are able to serve production better.