How To Select A Qualified Surge Backup Protector?

The primary function of a surge backup protector (SPD) in a power line is to interrupt the short-circuit follow current flowing into the SPD, preventing it from catching fire. When a lightning surge current arrives, the SPD does not trip, allowing the lightning current to be discharged smoothly to the ground, thus protecting the equipment from damage due to the surge.

However, after multiple surges or a single large current surge, the varistor chip inside the surge protective device gradually deteriorates. Its starting voltage decreases, and its ability to interrupt follow current and return to a high blocking state weakens. This can lead to an inability to promptly interrupt the power frequency follow current after a surge, or frequent activation by system transient overvoltages, causing the SPD to remain in an on-state for extended periods. This allows power frequency current to flow through the SPD for a prolonged period, potentially leading to SPD fire.

Although most SPDs have built-in backup fuses for backup protection in fault conditions, the reaction time of these fuses to small short-circuit currents is still insufficient to guarantee SPD safety in fault situations.

Therefore, a mechanism is needed as backup protection after SPD deterioration. This mechanism needs to meet the following conditions:

1. It has a fast response capability to small currents, and can quickly disconnect the SPD from the line when a small power frequency current flows through it;

2. It can distinguish between lightning current and power frequency current, and selectively allow lightning current or impulse current to be discharged to ground through the SPD;

3. It has a reliable high breaking capacity, and can withstand and break the expected short-circuit current of the transmission line;

4. It is safe and secure, and can withstand the impact stress when breaking high short-circuit currents without bursting;

5. It will not bring additional high residual voltage to the SPD when discharging impulse overvoltage or overcurrent energy.

However, what factors in the line can lead to the danger of the SPD? Generally speaking, transient overvoltage or transient voltage will occur when the following situations occur in the line:

(1) Neutral point grounding of the distribution transformer is disconnected

When the neutral point grounding wire of the distribution transformer is disconnected, regardless of whether the three-phase load is symmetrical, the change in the system load state will cause the neutral point potential of the low-voltage distribution network to shift, resulting in an increase in the voltage of one phase.

(2) Reversed neutral and C-phase live wires

In this case, the loads of phases A and B are subjected to 380V, causing equipment burnout. This situation is most likely to occur during major overhauls of the distribution network, service line upgrades, and replacement of three-phase four-wire energy meters.

(3) Neutral wire breakage

Similar to a break in the neutral point grounding of a distribution transformer, causing neutral point potential drift.

(4) Short circuit or open circuit to ground in one phase live wire, leading to three-phase load imbalance. The load of the short-circuited or open-circuited phase decreases, and the impedance increases.

(5) Instantaneous energy injection, such as lightning striking a phase wire or neutral wire.

(6) Operational overvoltage caused by the opening and closing of the switching mechanism in the line.

When the above situations occur, overvoltages of up to several thousand volts will be generated on the line. When the overvoltage exceeds the start-up voltage of the SPD, the overcurrent on the line will be directly discharged into the ground through the SPD. However, transient overvoltages and transient surge protectors (SPDs) have significantly different effects. Transient overvoltages are usually short-lived and do not damage the SPD. Transient overvoltages, on the other hand, are mostly caused by transmission line faults and generally last for a long time until the fault is manually cleared. Therefore, once a transient overvoltage exceeds the SPD's activation threshold, it will keep the SPD in a continuously conducting state until the SPD fails or even catches fire. Due to the characteristics of surge protector varistors—they can withstand instantaneous lightning currents but cannot withstand continuous power frequency currents—it is particularly important to install surge backup protectors (SCBs) upstream of the SPD. When the SPD encounters a power frequency current, it can quickly disconnect the SPD from the line, preventing the power frequency current from continuously damaging the surge protection device and preventing the SPD from catching fire.

Currently, there are two typical SCB design schemes on the market: bypass trip SCBs and main circuit trip SCBs. In practical applications, what are the differences between the two different SCB structures? Which one is better?

When a transient overvoltage exceeds the SPD's start-up voltage threshold, the line current rapidly flows through the bypass and is directly discharged to ground via the SPD. Since the transient overvoltage only stops after the line fault is resolved, the power frequency current will continuously flow through the SPD, preventing the SCB from tripping and disconnecting the circuit, ultimately leading to the SPD catching fire.

When power frequency current flows through the SCB's main circuit, it can quickly cut off the power frequency current within 0.1 seconds, preventing the SPD from catching fire; when lightning current flows through the main circuit, the SCB does not trip, allowing the lightning current to be smoothly discharged to ground, ensuring the continuous effectiveness of the equipment's lightning protection.

In summary, due to the different internal structures of the products, the protection effects are drastically different. SCBs with bypass tripping structures have design flaws. Customers must inquire about the internal structure from the manufacturer before making a selection; otherwise, it may lead to safety hazards in the project.