The safe operation of electrical equipment in a factory enterprise depends mainly on the level of insulation of electrical equipment and the voltage acting on the insulation. The purpose of overvoltage protection is to prevent the insulation of electrical equipment from being damaged by overvoltage. Under the action of overvoltage, if no measures are taken, the insulation of electrical equipment will be broken, resulting in equipment damage and power outages, and other accidents.

External Overvoltage

Overvoltage caused by lightning discharge in the atmosphere is called external overvoltage or atmospheric overvoltage, also known as lightning overvoltage, divided into direct lightning overvoltage induction lightning overvoltage, and lightning intrusion wave. Lightning overvoltage is caused by atmospheric thunderclouds on the ground (including lines, equipment) discharge, lightning discharge can cause a short circuit in the substation bus, thus causing a major accident. Atmospheric overvoltage makes the substation equipment, especially the transformer's internal insulation damage, its to burn transformers, burn overhead lines, resulting in power supply, damage to the electricity system, so it must take protective measures against overvoltage.

1. Direct lightning overvoltage

Lightning directly to the transmission line or electrical equipment discharge, causing a strong lightning flow through the line or equipment into the earth, thus producing a very destructive thermal and mechanical effects, known as direct lightning overvoltage. If the thundercloud is directly on the wire discharge, thundercloud in a large number of charges will be gathered on the wire, it will produce direct lightning overvoltage, direct lightning overvoltage amplitude of up to millions of volts. Overhead lines and open-air substations suffer more opportunities for direct lightning.

2. Induced lightning overvoltage

When the thundercloud is not directly hit on the transmission line wire, but to the line near the ground or the lightning line on the main discharge, in the discharge process due to the rapid changes in the spatial electromagnetic field and not directly affected by lightning on the electrical equipment induced overvoltage called induction overvoltage.

3. Lightning intrusion wave

Also known as high-potential intrusion wave, it refers to the direct lightning and induction lightning due to overhead lines or overhead metal pipelines and the generation of high-voltage impact lightning charge

May intrude along the line or pipeline into the room. According to statistics, in the power system, due to lightning intrusion waves and lightning accidents, accounting for about half of the total number of lightning damage.

Internal overvoltage

Due to the internal operation or fault of the power supply and use system, the energy transformation or transfer caused by overvoltage, known as internal overvoltage. Internal overvoltage

voltage can be divided into operating overvoltage, arc grounding overvoltage, and resonance overvoltage. Internal overvoltage generally occurs in the power system parameters change in the transition process, or after the change in the power grid parameters in the frequency resonance or other resonance of the steady-state. Such as a substation resonant overvoltage, its casing flashover will make its internal overvoltage multiplier to 4 times the working frequency voltage.

1. Operating overvoltage

Capacitive and inductive components in the power system are energy storage components. When the operation or fault in the system so that its working state changes, will produce electromagnetic energy oscillation of the transition process. In this process, the magnetic energy stored in the inductive components will be converted to electric field energy stored in the capacitive components at a certain instant, producing several times the power supply voltage of the transition process voltage, the circuit breaker cut off the no-load transformer will produce a very high overvoltage, the stronger the circuit breaker interrupting ability, the more over-voltage accidents cut off the no-load transformer.

2. Arc grounding overvoltage

Also known as intermittent arc grounding overvoltage, when the neutral point is not directly grounded system occurs single-phase intermittent arc ground fault, due to the unstable intermittent arc many times constantly extinguished and reignited, in the fault phase and non-fault phase of the inductance, capacitance circuit will cause high-frequency oscillation overvoltage, non-fault phase overvoltage amplitude is generally up to 3.15 ~ 3.5 times the phase voltage, this overvoltage is due to the system to the ground This overvoltage is due to the accumulation and redistribution of charge on the system to the ground capacitor many times, is intermittent, instantaneous and high amplitude overvoltage, the power system equipment is extremely dangerous.

3. Resonant overvoltage

Resonant overvoltage refers to some inductive and capacitive components in the power system that can form various oscillation loops when the system is operated or when a fault occurs, and under the action of certain energy, it will produce series resonance phenomenon, resulting in serious overvoltage of some components of the system.

1) Linear resonant overvoltage

Resonant circuit without the core of the inductive components (such as transmission line inductance, transformer leakage) or excitation characteristics close to the linear inductive components with iron core (such as arc extinguishing coil) and the capacitive components of the system is composed.

2) Ferromagnetic resonant overvoltage

The resonant circuit consists of inductive components with iron core (such as no-load transformers, voltage transformers) and capacitive components of the system. Due to the saturation phenomenon of the core inductor element, the inductance parameter of the circuit is nonlinear, and this circuit containing nonlinear inductive components will produce ferromagnetic resonance when certain resonance conditions are met.

3) Parameter resonance overvoltage

By the inductance parameters for periodic changes in the inductive components and system capacitive components (such as a no-load line) composed of the circuit, when the parameters with, through the inductance of the shield periodicity changes, constantly to the resonant system energy delivery, resulting in parameter resonance overvoltage.