With the widespread use of 6kV to 35kV medium and low-voltage switchgear in substation distribution systems, the number of accidents caused by arc fault damage to power supply and distribution equipment is increasing. When a medium and low-voltage switchgear experiences a short-circuit fault internally, an arc is inevitably produced, and the arc is the energy source that leads to equipment damage. Currently, the number of unattended substations in the power grid is rapidly increasing, but they have the weakness of inadequate protection. Due to the failure to promptly clear arc fault damage in switchgear, it can escalate into a medium and low-voltage bus fault, expanding the scope of the accident. The high temperature, high pressure, and high energy of arc faults often result in the destruction of an entire section of bus equipment, causing severe power outages in the substation's supply area, leading to significant economic losses. Therefore, it is highly necessary to install rapid and reliable arc fault protection devices in 6kV to 35kV medium and low-voltage switchgear, to quickly detect and isolate arc fault damage within the switchgear, prevent the expansion of accidents, and enhance the reliability of power supply.
This article analyzes the causes and characteristics of arc fault failures in the Xinjiang Oilfield power grid, which resulted in power outages due to arc-over short circuits. It also introduces technical measures to reduce power outages caused by arc fault failures through the use of arc protection devices in actual engineering applications.
Causes, Characteristics, and Hazards of Arc Flash Short Circuit Fault
Arc Flash Faults are relatively common in electrical systems, with causes including insulation damage, foreign object entry, human operation errors, equipment aging, and system issues. An arc is a phenomenon that occurs during the discharge process, and it happens when the voltage between two points exceeds their working frequency insulation strength limit. As the voltage between two points rises, positive and negative ions in the air near the electrodes are accelerated by the electric field. During their movement, these ions collide with other air molecules, creating new ions, a process known as "ionization." Simultaneously with ionization, the temperature rises sharply, producing an arc, which is a type of discharge called arc discharge. Arc discharges generally do not require high voltages and are characterized by low voltage and high current discharges. An arc will continue to occur as long as the energy provided by the voltage at the two ends of the discharge point is sufficient to compensate for heat losses and maintain appropriate temperature conditions. If the arc is elongated and cooled, it will extinguish due to the lack of necessary conditions. Arc discharges can be classified into different types based on the parts of the arc where they occur, such as between charged conductors, between charged conductors and ground, and leakage on insulating surfaces. The severity of the harm caused by an arc fault depends on the arc current and the time to disconnection. The energy produced by the arc increases exponentially with time (as shown in Figure 1). According to IEC 62271-200 and IEEE C37.20.7 standards, switchgear can withstand internal discharge and arc for only 100ms. Currently, switchgears on the market are primarily manufactured according to IEC standards. When the total disconnection time exceeds 100ms, it will cause varying degrees of damage to the equipment. In practice, when an arc flash fault occurs within medium and low-voltage switchgear, due to the arc resistance, the short-circuit current often does not reach the overcurrent and quick-break setting values, resulting in slow action to cut off the fault. The continuous burning of the arc releases a tremendous amount of energy. If not cleared in time, it can develop into a medium-voltage busbar fault, burn out the switchgear, and cause a power station blackout.
The process of forming an electric arc inside a medium and low voltage switchgear cabinet can be divided into four stages. 1) Compression stage: the arc occupies the entire air space, and the air inside the cabinet is heated; 2) Expansion stage: the pressure inside the cabinet increases, reaching a high value in this stage, and begins to decrease due to the release of hot air; 3) Emission stage: due to the continuous release of arc energy, almost all the air inside the cabinet is forced out by pressure; 4) Heating stage: after expelling the air, the temperature inside the cabinet almost reaches the temperature of the arc until it extinguishes. At this point, all metals and insulators merge together after being eroded by the generated gases, smoke, and particles of corrosive substances.
An arc fault can reach local temperatures of 2,000°C to 3,000°C, easily igniting nearby flammable materials. This not only poses a risk of destroying switchgear, wires, and cables, but also could lead to severe electrical short circuits and fires. In occupied operating areas, the arc light may cause personnel to suffer from intense light, burns, radiation, and toxic gases, posing personal injury risks.
Analysis of Arc Fault Short Circuit in Oil Field Power Grid
In recent years, the Xinjiang Oilfield Power Grid has experienced multiple arc fault short-circuit accidents in unattended transformer substations, leading to fires that destroyed electrical equipment and caused power outages. Taking the most recent arc fault short-circuit fire incident in the Xinjiang Oilfield Power Grid as an example, a 35kV voltage transformer in the 35kV Lu Liang substation supplying power to the Xinjiang Oilfield experienced a phase C grounding. This caused the phase voltage of the non-grounded phases to rise, piercing the vulnerable insulation point of the transformer's phase B, resulting in a BC phase short-circuit. The protection system of the upper-level 110kV substation activated, causing the 35kV line to be de-energized, and the BC phase short-circuit was eliminated. Subsequently, the line reclosed, and power was restored to the 35kV substation. However, a single-phase grounding fault persisted for about 20 minutes, leading to another BC phase short-circuit. The protection system of the upper-level 110kV substation activated, tripping the circuit breaker to disconnect the fault current, and initiating the reclosing. The bus protection system then activated, disconnecting the fault current again. As this 35kV substation was unattended, when the operations personnel arrived on the scene, the 35kV distribution room within the substation had already caught fire. Most of the switchgear on the 35kV bus segment was destroyed, causing all oilfield production facilities within the substation's power supply area to shut down due to the outage, resulting in significant economic losses to the oilfield production.
The reason lies in the fact that the 6kV to 35kV neutral point of the Xinjiang Oilfield power grid system operates ungrounded. According to the "Electrical Accident Handling Regulations," when a single-phase metallic grounding occurs, the system can operate for 1 to 2 hours; when a single-phase arc grounding occurs, it can operate for 15 minutes. The aforementioned substation continued to operate for about 20 minutes after a single-phase grounding short circuit occurred. The arc grounding fault was not promptly cut off, leading to the burning of switchgear equipment due to phase-to-phase arc grounding, which triggered the protection action of the upper-level substation. This demonstrates that in actual operations, once arc grounding occurs, overvoltage and large grounding currents can cause rapid damage to electrical equipment. This incident's short-circuit point occurred on the voltage transformer connected to the 35kV line, and the fault quickly spread to the busbar. However, in the current protection configuration, there is no busbar protection in the medium and low-voltage switchgear, and the presence of a short-circuit fault on the busbar can only rely on the protection action of the upper protection (such as the transformer's backup overcurrent protection), which takes 1.4 to 2.0 seconds, and is unable to meet the requirements for rapid busbar fault removal, resulting in the expansion of the accident and the damage to switchgear equipment. From a macro perspective of the power grid, with the increasing use of power cables in the power grid over the past few years, there is an equivalent capacitance between the power cables and the ground, and the single-phase grounding capacitance current has reached a significant level. The single-phase grounding current of the substation involved in the accident is calculated to be approximately 22A, which is right in the range of 10A to 30A where single-phase arc grounding current is extremely prone to occur, thereby triggering arc grounding faults.
In recent years, the Xinjiang Oilfield Power Grid has experienced power outages caused by arc fault accidents at 35kV substation. Analysis reveals that these outages were triggered by single-phase arc grounding, which led to overvoltage. The arc grounding at the grounding point causes intermittent arc combustion, resulting in overvoltage multiples reaching 3 to 3.5 times the normal phase voltage amplitude. This overvoltage significantly exceeds the voltage withstand capability of the transformer insulation and the air insulation gap to the ground, leading to breakdowns in the weakest insulation points. This causes phase-to-phase short circuits, forming arc faults between phases, eventually resulting in fires and the destruction of distribution switchgear.
In summary, substation arc fault short circuits typically originate from single-phase grounding due to the non-grounded neutral operation, which permits the system to continue operating shortly after a single-phase grounding occurs. During this period, an unstable intermittent arc grounding can easily form at the grounding point. This leads to overvoltage and breakdown of the insulation at vulnerable spots within the system. Simultaneously, the intense arc can cause two-phase or phase-to-phase short circuits, resulting in inter-phase arc fault short circuits. This can severely damage electrical equipment and threaten power supply.
Arc Light Protection Applications
3.1 Arc Protection Principle
Arc Flash Protection primarily controls the total fault clearing time within the switchgear's withstandable arc flash duration by detecting arc flashes and fault currents, thereby minimizing the hazards of arc flash short circuits. The Arc Flash Protection System measures parameters of light and electricity that possess completely different physical properties. The system utilizes the principle of criteria for the correlation between light and electricity, and when all criteria meet the action requirements, it issues a tripping command to operate the circuit breaker (as shown in Figure 2). The current response speed of the Arc Flash Protection System, from arc flash detection to relay tripping output, only requires 5ms to 7ms.
3.2 Application of Arc Flash Protection in Oilfield Power Grid
Arc protection for medium and low-voltage switchgear began to be utilized internally for fault protection in the early 1990s abroad. Currently, arc protection is widely employed in Europe and the United States. China started using imported busbar arc protection after 2000, and products with domestic independent intellectual property rights have also entered the market after 2010.
In the actual construction and operation of the Xinjiang Oilfield power grid, transformer stations with high reliability requirements and significant losses in the event of power outages, such as the ~35kV transformer stations in the Xinjiang Oilfield Corporation and the 110kV Transformer Station 1 of Karamay Petrochemical Company, have all installed arc light protection devices in their 6kV to 35kV switchgears. The project utilizes arc light protection devices (as shown in Figure 3), installing one device per section of the 10kV high-voltage switchgear within the transformer station, which enables rapid protection of busbars and cable terminations under arc fault conditions. This can prevent the severe injuries caused by arc light during arc faults to equipment and personnel. Arc light point sensors are separately configured in the busbar rooms and cable termination rooms of each circuit. In the event of an arc fault in the busbar room, the incoming line circuit breaker and bus tie breaker for that section are quickly opened; when an arc fault occurs in the cable termination room, the outgoing circuit breaker is quickly opened without affecting the busbar or other circuits, minimizing the scope of the fault and reducing power outages.
The arc fault device consists of a control main unit, I/O unit, arc fault sensor, and communication connection cables. The main function of the main control unit is to receive arc fault signals from the direct arc extension unit, detecting both three-phase currents and zero-sequence currents and voltages. By processing the received data, it programs and issues tripping commands based on dual criteria of current overflows and arc faults. It also continuously self-inspects all units, providing circuit breaker failure protection and arc fault location functionality. The arc extension unit primarily serves to connect multiple arc fault sensors (up to 12 sensors), addressing the issue of limited connection interfaces for the main control unit's arc fault sensors. After receiving the arc fault signals, it transmits them to the main control unit and completes the tripping output for cable head arc protection. The arc sensors have a 270° detection range, directly connected to the arc extension unit for real-time arc monitoring. Communication cables transmit power, monitoring signals, and tripping signals between the main control unit, extension units, and arc sensors. Currently, the setting value of the arc protection device should be less than the I-section current setting of the main transformer's backup protection, and the protection action time should be less than 5ms.
The installation and operation of the equipment have been stable for nearly half a year, with no malfunctions or failures observed.
Arc Flash Protection Product Selection Guide: Ankoer ARB5-M
ARB5 - Arc Light Master Unit
*(1) *Indicates an optional attachment, which incurs an additional charge of 1,500 RMB.
(2) The total number of main control boards and collection boards cannot exceed 4.
(3) The length from the arc probe to the collection board must not exceed 20 meters.
(4) Please specify any special requirements.
Ankorree ARB5-M Arc Flash Protection Product: Features and Technical Specifications
AnkoRay ARB5-M Arc Protection Product Field Installation
The arc protection master control unit and probe installation diagram are as follows.
7 Conclusions
As the power grid continues to evolve, the role of medium and low voltage switching equipment as a "hub" between the grid and consumers has become increasingly significant. Arcing faults not only damage medium and low voltage switching equipment but also severely impact the operation of the power system. Arc protection, as a technology that can quickly interrupt arc faults within medium and low voltage switchgear, boasts its simplicity in principle and reliability in operation. Therefore, with the deeper understanding of arc protection, its operation and tuning will become more mature and refined, and arc protection will be more extensively promoted and applied in engineering practice. Eliminating the impact of arcing faults on medium and low voltage switching equipment and ensuring the reliability of electricity supply to consumers has significant practical importance.
