During thunderstorms, in power supply lines equipped with power arresters, line protection equipment often trips. Especially in power supply and distribution systems located in open areas, tripping occurs frequently. In severe cases, equipment is damaged by lightning breakdown, which brings a lot of inconvenience to daily work. Due to various reasons, the circuit breaker connected in series at the front end of the arrester often operates, causing the arrester to lose its protective effect. Therefore, some people complain that the arrester has become a decoration and does not work at all.
This article will explain the role of lightning arresters in power supply lines and the working nature of circuit breakers and leakage circuit breakers. Combined with the actual trip situations encountered by the author at work, it will analyze the reasons for the tripping of various protective equipment in lines equipped with lightning arresters.
1. Working principles of various protectors
Surge protectors are commonly known as lightning arresters. Lightning arresters in low-voltage distribution lines are mainly composed of semiconductor components and air gaps. They are essentially a limit switch. When there is no lightning wave, both ends of it are in an open circuit state, which has no impact on the power supply and signals. When the lightning wave invades and exceeds a certain value, it quickly becomes a path state, clamping the voltage within a safe range, and discharging most of the lightning current to the ground. When the lightning current passes, the arrester returns to the high-impedance state to ensure the safe and normal operation of the back-end equipment.
2. Working principle of circuit breaker
Miniature circuit breaker consists of operating mechanism, contacts, protection devices (various releases), arc extinguishing system, etc. Its main contact is closed manually or electrically. After the main contact is closed, the free tripping mechanism locks the main contact in the closing position. The coil of the overcurrent release and the thermal element of the thermal release are connected in series with the main circuit, and the coil of the undervoltage release is connected in parallel with the power supply.
When a short circuit or serious overload occurs in the circuit, the armature of the overcurrent release is closed, causing the free tripping mechanism to operate and the main contact to disconnect the main circuit. When the circuit is under-voltage, the armature of the under-voltage release is released, which also causes the free tripping mechanism to operate.
3. Working principle of leakage circuit breaker
The leakage circuit breaker consists of three parts: zero sequence current transformer TAN, amplifier A and voltage-down circuit breaker. When the equipment is working normally, the three-phase current of the circuit is symmetrical and the vector sum of the three-phase current is zero. Therefore, there is no magnetic flux in the core of the zero-sequence current transformer and it does not operate.
When a leakage or single-phase ground fault occurs, since the vector sum of the three-phase currents in the circuit is not zero, there is zero-sequence magnetic flux in the core of the zero-sequence current transformer, and there is current on the secondary side. After being amplified by the amplifier, this current flows into the switch trip coil, causing the low-voltage circuit breaker to trip, cutting off the faulty circuit, and avoiding electric shock accidents.
4. By analyzing the working principles of the three protectors in the line, we can summarize three reasons why various protection equipment (including the protection equipment at the front end of the arrester) in the line equipped with a power arrester trip during thunderstorm weather.
When a miniature circuit breaker is connected in series with the front end of the power arrester
In order to prevent the equipment from being damaged by ground short-circuit fault current when the power supply arrester fails and ensure personal safety, in lightning protection engineering applications, small circuit breakers are often connected in series at the front end of the power supply SPD as the front-end protection device of the SPD.
The failure modes of power arresters can be divided into two categories: open circuit failure mode and short circuit failure mode.
a. Open circuit failure mode: due to the nonlinear components of the SPD itself or caused by the internal or external protection equipment connected in series with the SPD and the power supply disconnection. At this time, the continuity of the power supply is guaranteed in the case of SPD failure. b. Short circuit failure mode: caused by the SPD itself or caused by an additional device, then the power supply will be interrupted due to the subsequent protection of the system. At this time, the power supply system is protected, but the system no longer supplies power.
PD is the overcurrent protection device of the surge protector; the arrester is the surge protector; E/I is the electrical device or equipment protected by the surge protector.
Therefore, whether the priority is to ensure the continuity of power supply or the continuity of overvoltage protection depends on the installation position of the front-end protection device that disconnects the power arrester when the power arrester fails.
In the open-circuit failure mode, when the overcurrent passing through the arrester lasts too long, that is, when the power arrester cannot discharge all the lightning current to the ground within microseconds, the protection device connected in series at the front end of the power arrester will judge it to be an overcurrent or short-circuit fault and take action. At this time, although the continuity of power supply is guaranteed, when overvoltage occurs again, neither the electrical devices nor the equipment will be protected, and the continued overcurrent will cause the circuit breakers in the power supply line, especially the circuit breakers installed at the main distribution office, to operate in an overvoltage state, causing the system power supply to be interrupted.
Short-circuit failure mode: In this failure mode, the protection device connected in series in front of the power arrester will cause the circuit breaker in the power supply line to directly operate when it is judged to be an overcurrent or short-circuit fault.
In the above two failure modes, if the parameters selected for the protection equipment at the front end of the power arrester are inconsistent with the relevant parameters of the arrester, the power supply line circuit breaker will also operate. In particular, the protection equipment at the front end of the arrester is more likely to operate, thus reducing the protection effectiveness of the arrester and the continuity of power supply.
Of course, we do not want the power supply to be interrupted. Therefore, the open circuit failure mode wiring method is often used in lightning protection projects.
In addition to the failure mode of the arrester, the installation process and installation location of the arrester will also affect the continuity of line power supply during thunderstorms.
