Application of air circuit breaker delayed tripping technology in light hydrocarbon plants

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Preface: The Second Gas Treatment Plant of Zhongyuan Oil & Gas High-Tech Co., Ltd. is equipped with three sets of advanced processing units for natural gas condensate light hydrocarbons, capable of handling approximately 50,000 tons per year of such hydrocarbons. The production units and auxiliary equipment operate continuously, powered by electricity. Due to its location at the edge of the northern Henan power grid, voltage fluctuations are significant and lightning occurrences are frequent. Lightning often causes the complete or partial shutdown of power equipment such as pumps; the restart of these shut down devices must be carried out manually. With a large number of operating power devices, the time required for restart operations is long, and the system’s operational parameters deviate from normal values for extended periods, which severely affects the continuous operation of the light hydrocarbon production facility. Adjusting the system operation parameters generally takes 8 to 12 hours; each adjustment period consumes approximately 40 to 60 m3 of light hydrocarbon feedstock and 4000 to 6000 m3 of natural gas, resulting in significant economic losses. To address the issue of sudden shutdowns of power equipment such as pumps caused by lightning, a scientific and systematic analysis was conducted on the causes of lightning. The technology of delayed operation for air switches was applied, and technical modifications were made to the two air switch cabinets in the power supply system of the production facility. This enabled the air switches in the high-voltage distribution system to operate with a delay in response to lightning strikes, thereby preventing sudden shutdowns of power equipment like pumps. As a result, the impact of lightning on the continuous operation of light hydrocarbon processing units was significantly reduced. 2 Causes of lightning and its impact on the production of light hydrocarbon plants 2.1 Causes of lightning Since high-voltage overhead power distribution lines are equipped with automatic reclosing systems, once a fault occurs and causes the line to trip, the system will automatically attempt to reclose it. If the fault is resolved at this point, power supply to the high-voltage distribution system returns to normal (this process is characterized by a brief drop in system voltage followed by its return to normal, with a duration of no more than 1.25 seconds). If the line trip fault persists, the switch will trip again after reclosing to cut off the high-voltage distribution line. A successful reclosing operation results in what, for the downstream users, appears to be a lightning strike in the power supply system. According to statistics, around 80% of the faults in the high-voltage overhead lines operated by the Zhongyuan Oilfield Power Supply Company in 2003 were transient faults; in other words, the reclosing system had an success rate of about 80%. 2.2 Impact of lightning on the production of the light hydrocarbons plant: The power required for the operation of this plant comes from lines 611 and 618 at Liutun Substation. The 6kV high-voltage electricity is transformed by transformers to provide 400V of power voltage, thereby ensuring the continuous operation of the light hydrocarbons plant. As shown in Figure 1, during a lightning strike on the high-voltage power distribution system, although the duration is very short, generally not exceeding 1.25 seconds, the system voltage drops. As a result, the loss-of-voltage trip device in the air switch (DW15—1600 type universal circuit breaker) loses power instantly, causing the air switch to trip and cut off the power supply. After the high-voltage system voltage is restored promptly following a lightning strike, the low-voltage power supply system lacks automatic reclosing functionality due to the presence of circuit breakers; therefore, manual operation is required to close the switches and restore power supply, with a time requirement of around 1 minute. A power outage in this system inevitably leads to the shutdown of power equipment such as pumps. Given the large number of such devices in operation, restarting them after a lightning strike also requires manual intervention, taking approximately 5 minutes. If the power equipment remains shut down for an extended period, it will cause the system parameters to deviate significantly from their normal values for a long time, leading to disruptions in the production of light hydrocarbons. It is therefore necessary to adjust the operating parameters of the light hydrocarbon plant until they return to normal, so that the plant can resume normal production. 3 Analysis of Air Circuit Breaker Tripping Issues and Solutions 3.1 Analysis of Air Circuit Breaker Tripping Issues The voltage-loss trip device in the automatic air circuit breakers of power supply systems is an electromagnet; when power is lost, the armature is released under the action of a spring, which in turn activates the tripping mechanism, causing the air circuit breaker to trip. During lightning strikes in high-voltage distribution systems, if the voltage loss trip device can delay its activation by a few seconds, the power supply system can maintain normal operation once the voltage in the high-voltage system returns to normal, thereby significantly reducing the impact of lightning on the production of light hydrocarbon facilities. To prevent the armature of the high-voltage system’s lightning-induced voltage drop trip device from releasing, three technical solutions were proposed after analysis: ① Securing the armature of the electromagnetic voltage drop trip device to prevent it from releasing; this can prevent the circuit breaker from tripping during lightning strikes. However, in the event of a permanent power loss in the system, the circuit breaker will also not function, losing its purpose, so this approach is not viable ； ②The method of using a UPS system to power the voltage loss trip device has been tried on multiple occasions; however, due to the complex wiring of the equipment, its poor reliability, and the inability to achieve delayed operation in a stable manner, this approach is not viable ； ③The power supply for the coil of the voltage-loss trip device is changed to a DC power source, with an energy-storing capacitor connected in parallel to this coil. When the system voltage drops too low, the capacitor automatically supplies electrical energy to the coil of the voltage-loss trip device, allowing it to remain in its engaged state for a certain period of time. Once the energy-storing capacitor is discharged, the voltage-loss trip device loses power, and the air switch then carries out the delayed tripping operation automatically. Its modification method is similar to that of AC contactors, featuring AC starting and silent DC operation; it has a simple wiring scheme and high reliability as proven by tests, which is why it is adopted. 3.2 Basis for technical modification: In order to measure the discharge delay time of energy storage capacitors with different capacitances, we connected a spare circuit breaker of the same model (see Figure 2 for the circuit schematic), and determined in batches the time it takes for the voltage-loss trip device to remain engaged when using capacitors with different capacitances (see Table 1). According to the test data, using a capacitor of 2200μF ensures that the voltage-loss release stays engaged for 5 seconds, which allows the automatic air switch to operate with a proper delay during lightning strikes; therefore, a 2200μF energy-storing capacitor is chosen. The measured DC resistance of the coil of the voltage loss release device is 540Ω, with an AC voltage of 380V and a current of 0.045A. Because the attracting force remains constant when the alternating current coil is subjected to the same direct current and alternating current ； When the alternating current supply is replaced with a direct current supply, the inductor has no inductive reactance, and its total impedance is equal to its DC resistance. Therefore, the required DC voltage = current × resistance = 0.045 × 540 = 24.3 V. After analysis, the electrical components to be used are listed in Table 2. 3.3 Solution: We first choose to install a 50W control transformer to convert 400V alternating current into 24V alternating current ； It then passes through a bridge rectifier stack, which converts the voltage of the overload release coil from 24V AC to 24V DC ； Finally, a 2200 uF energy storage capacitor is connected in parallel in the circuit formed by the bridge rectifier stack and the coil of the voltage-loss trip device, to supply power to the coil of the voltage-loss trip device during lightning strikes (see Figure 3 for the circuit schematic). When a lightning strike causes an instantaneous loss of voltage in the power supply system, the energy storage capacitor automatically releases electrical energy into the coil of the voltage-loss trip device, keeping it in its engaged state. Since the air switch takes 5 seconds to trip, if the power supply system can return to normal within these 5 seconds, the air switch will not trip. This effectively ensures a delay in the tripping action of the air switch; once the voltage in the high-voltage system returns to normal, the power supply system can continue to supply power normally. At the same time, it prevents sudden shutdowns of power equipment such as pumps due to lightning, thereby significantly reducing the impact of lightning on the continuous operation of light hydrocarbon processing units. 4 Review: The light hydrocarbon deep processing unit at the second gas treatment plant has achieved a delay in the operation of air switches following lightning strikes, thanks to improvements in the delay-starting technology for these switches. This has enhanced the stability of the power supply system, ensuring continuous and stable operation of the light hydrocarbon processing unit after lightning events. The impact of lightning on this unit has been minimized, resulting in significant economic benefits. The delayed tripping technology of air switches is applied in the production of light hydrocarbon plants, yielding significant efficiency improvements. It is highly suitable for petrochemical enterprises located in areas where lightning occurs frequently. It can also provide useful insights for domestic companies that operate similar continuous production facilities, offering broad application prospects. Author profile: Zhang Yonghua (born 1979), male, bachelor’s degree, engineer, mainly engaged in research in the field of natural gas chemistry ； Postal Code: 457061 Contact Address: Second Gas Treatment Plant, Zhongyuan Oilfield Natural Gas Treatment Plant, Liutun, Puyang County, Henan Province e-mail: PYZYH@126.COM