Executive SummaryIntroducing a rapid arc protection system and its working principle, system structure, and discussing the field engineering application of a 10kV busbar arc protection system installed at a 110kV substation.
Keywords:Medium and low voltage; busbar fault; busbar protection; arc light protection
0 Preface
Currently, in the power system, only the low voltage buses of substation transformer units with voltage levels of 220kV and above are required to be equipped with bus protection devices. For substation transformer units with voltage levels of 110kV and below, the buses with voltage levels of 35kV and below are not equipped with bus protection. Due to various reasons, the failure rate of medium and low voltage buses is quite high. However, for a long time, once a fault occurs on the bus, only a backup protection with a longer delay can be used to remove the fault on the medium and low voltage bus, making it impossible to quickly isolate the bus fault. As a result, there are often cases where the extended duration of medium and low voltage bus faults leads to the expansion of faults, causing burnouts of busbars and other equipment inside switchgear, and even explosions, which endanger nearby equipment and personnel. Statistical data shows that electrical forces generated by main transformer external circuit breakers are prone to deform the main transformer windings and cause damage, resulting in significant economic losses and severely threatening the stable operation of the power system. In order to protect the stable operation of transformers and bus equipment, and to reduce the degree of equipment damage during bus faults, the power system urgently needs an economical and reliable solution for medium and low voltage bus protection. Among the numerous options, an arc light protection system is an ideal solution.
Arc Light Protection Working Principle
When a busbar equipment failure occurs, it will result in high currents and produce an arc light up to tens of thousands of lux. Arc light protection is achieved by detecting the arc light and high currents during the fault. Arc light protection detects the arc using an arc light probe or sensor, and simultaneously detects the current through an overcurrent element. When an arc signal is detected and the overcurrent element operates, the protection is activated and a trip command is issued to disconnect the power input line and the bus tie breaker of the protected busbar, isolating the fault point. At the same time, a protection action signal is sent and the fault point is located. The principle is shown in Figure 1.
Composition of the Arc Light Protection System
The arc protection system is composed of units, expansion units, arc sensors, and arc probes interconnected via fiber optic cables or network cables, as illustrated in Figure 2. One arc protection system safeguards a section of busbar, and each system consists of a unit, several expansion units, and several arc probes. The units of the arc protection systems for two busbars communicate with each other through fiber optic cables, exchanging light signals, overcurrent signals, and trip signals. Units are connected to expansion units via network cables, where the units supply working power to the expansion units and receive signals from them. Expansion units are connected to arc probes installed in various circuit breaker intervals through fiber optic cables. Multiple expansion units on the same busbar are linked via network cables to enable communication and obtain working power.
2.1 Unit
The unit is the core component of the arc light protection system, primarily responsible for protection and communication functions. It is mainly composed of current monitoring elements and light signal detection elements. The current monitoring element can be configured to use three-phase overcurrent criteria, two-phase overcurrent criteria, or zero-sequence overcurrent criteria to implement overcurrent protection. The overcurrent element acts instantaneously when the current exceeds the set overcurrent value. The light signal detection element detects the arc action signal transmitted by the expansion unit, and upon receiving the signal, it is determined that an arc has occurred.
2.2 Expansion Unit
The expansion unit is primarily used to increase the number of interfaces for the arc probe. Its internal photo-detection elements amplify the light captured by the arc probe and compare it with a pre-selected reference level. If the photoelectric level exceeds the preset reference level, it is deemed an arc occurrence, and an arc action signal is then sent to the unit. The expansion unit connects to the arc probe via fiber optics, ensuring that the arc signal's intensity is attenuated during transmission.
3 Application Examples
A 110kV substation has been equipped with an arc protection system on each of its two 10kV busbars. The two 10kV busbars are connected via a bus tie breaker. The structure of the 10kV electrical main circuit and the two installed arc protection systems are shown in Figure 3.
3.1 Data Collection for Protection Criteria
To achieve the functionality of the arc light protection system, the overcurrent element collects the 10kV side current of the main transformer, and the current winding used is the same as the one used for the backup protection of the 10kV side of the main transformer. The arc light signal is provided to the unit by the expansion unit, which collects the arc light signals transmitted from the arc light probes installed in each circuit breaker interval of the air-insulated armored switchgear through optical fibers.
3.2 Arc Flash Protection System Setting
1) Adjusted the protection action conditions, which can be set to require only light signals without the need for overcurrent signals, or to require both light and overcurrent signals simultaneously. The arc light protection system's action would cause a section of busbar to lose power, thus to prevent false protection actions, the conditions are set to require both light and overcurrent signals.
2) Select the optical reference level, choose the optical compensation method, and set to automatic background light compensation as recommended by the manufacturer.
3) The set value for overcurrent protection is adjusted, with the option to select either phase current or zero-sequence current. Considering adaptability to various faults, phase current is chosen, and the overcurrent set value is adjusted based on the primary transformer's backup protection overcurrent I-phase set value.
4) Tune the fiber channel, setting the working mode (input or output) for each fiber interface and the type of signal to be received or transmitted (optical signal, overcurrent signal, tripping signal). As shown in Figure 3, to ensure that one main transformer is taken out of service, while another main transformer supplies power to both bus sections through the bus tie, if one unit detects an overcurrent, the protection should also act correctly, and the signal type should be set to overcurrent signal.
3.3 Arc Flash Protection System Operation Logic
When arc signal is detected and the overcurrent conditions are met, the protective action trips the corresponding bus incoming circuit breaker and the bus tie breaker for that bus through two high-speed breaker outputs.
2) The arc protection should lock the 10kV standby power auto-reclosing device to prevent a fault on the 10kV bus during independent operation of the two sections of busbars. After the arc protection trips the main transformer's 10kV side circuit breaker, the 10kV backup auto-reclosing device will connect the 10kV bus to the faulty 10kV bus. (The arc protection system must provide at least two high-speed tripping outputs for tripping operating circuit breakers, as well as two sets of conventional relay contacts for signaling and locking the backup auto-reclosing.)
3.4 Arc Light Protection System Calibration
Based on the design requirements of this protective measure, the following debugging items were carried out during the installation and commissioning process:
1) Illuminating each circuit breaker cubicle's arc probe with a strong light source.
Inspect the arc sensor operation, record the signal light numbers corresponding to each interval's arc probe, and label the signal lights with circuit breaker numbers for easy fault location in the future.
2) Verify if the action logic meets the requirements: one is to only require light signals without current signals, and the other is to require both light and current signals simultaneously, with protection under both action conditions.
3) Inspected the action values, return values, and return coefficients of the current sensing elements to ensure they meet the regulatory requirements.
4) Verify the correctness of the uploaded signals.
5) The breaker was tested with transmission, and the protection action logic was correct, the breaker's action behavior was proper, with a correct action rate of 100%, and the 10kV backup power supply was correctly locked out, with the protection action time not exceeding 3ms. Consequently, it can be concluded that the arc light protection has tremendous superiority in action time compared to other medium and low voltage busbar protections, as shown in Table 1. Equipment damage is listed in Table 2.
Ankore ARB5-M Arc Protection Product Selection Guide
ARB5 - Arc Control Unit
*(1) *Indicates an optional attachment, which will incur an additional charge of 1,500 RMB.
(2) The total number of masterboards and acquisition 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.
Ankorri ARB5-M Arc Flash Protection Product: Features and Technical Specifications
Ankorri ARB5-M Arc Protection Product Field Installation
Arc Light Protection Main Control Unit and Probe Installation Diagrams are as follows.
7 Closing Remarks
In summary, controlling the bus short-circuit fault time to 100ms or even shorter can significantly reduce the damage to bus equipment and the main transformer, as shown in Table 2. Additionally, reducing the extent of equipment damage can also decrease the downtime for switchgear maintenance. A relatively ideal protection solution that achieves this goal is an arc light protection system, which is simple in principle, operates quickly and reliably, and has no special requirements for existing equipment, specifically designed for the fault characteristics of medium and low-pressure switchgear.
Reference:
Yu Guang, Wang Jiyu. Application of Arc Light Protection System in 10kV Busbar[J].
Jin Wenlong, Chen Jianhua, Li Guangfan. Statistical Analysis of Short Circuit Failure Accidents in Power Transformers of 110kV and Above [J]. Electric Power Grid Technology, 1999, 23(6), 70-74.
[3] Zhang Bo, Application of Arc Suppression System in Medium and Low Voltage Busbar Protection [J], Electrical Engineering Technology, 2008(5), 12-13.
[4] An Initial Exploration of Measures to Shorten the Fault Clearing Time for 10kV Busbar Short Circuit in Substation [J] Relay, 12007, 35(16), 61-63.
[5] Ankorri Enterprise Microgrid Design and Application Handbook, 2022.5 Edition.
