What is overvoltage?

The core of overvoltage is the instantaneous or short-term abnormal increase in voltage, and its key measurement criteria are multiples and waveforms.

 Multiple: The ratio of the overvoltage amplitude to the peak value of the system's highest operating phase voltage. For example, a 2.0 multiple overvoltage means that the voltage has reached twice the normal operating peak value.

 Waveform: It determines the energy and destructive power of overvoltage. For instance, lightning impulse waves (with steep slopes in the microsecond range) test the insulation's ability to withstand impulse voltages; while temporary overvoltages (lasting from milliseconds to seconds) test the insulation's ability to withstand prolonged high voltages and thermal stability.

Lightning is the most concentrated source of overvoltage in nature, with currents reaching tens to hundreds of kiloamperes.

1. Direct lightning overvoltage

◦ Occurrence: Lightning directly strikes the towers, lightning conductors or conductors of transmission lines. The huge lightning current enters the ground through impedance, generating extremely high voltage at the strike point.

Characteristics: Extremely high amplitude, up to several thousand kilovolts; extremely steep, with a wavefront time of 1-4 microseconds, posing the greatest threat to insulation. It is the focus of lightning protection for power lines.

2. Induced lightning overvoltage

◦ Occurrence: Lightning does not directly strike the line but discharges to the ground near the line.

Mechanism:

.Electrostatic induction: During the leader stage of a thundercloud, a large amount of bound charges of opposite polarity to the thundercloud will be induced on the conductors of the line. When the main discharge occurs, these bound charges are suddenly released, forming overvoltage waves that propagate along the conductors.

.Electromagnetic induction: The powerful lightning current generates a rapidly changing magnetic field around the discharge channel. This magnetic field passes through the conductor loop, inducing an electromotive force.

◦ Characteristics: The amplitude is usually lower than that of direct lightning strikes (generally not exceeding 300-400 kV), but it poses a significant threat to distribution lines of 35 kV and below and weak electrical equipment (such as communication and monitoring systems) because their insulation levels are relatively low.

This is caused by internal energy conversion or parameter changes within the system and is proportional to the system's rated voltage.

1. Switching overvoltage

◦ Generation: Due to the operation of circuit breakers or system faults, the circuit state changes abruptly, causing electromagnetic energy oscillations.

◦ Main types:

▪ Overvoltage from switching off no-load lines: When a circuit breaker cuts off a capacitive current (such as an unloaded long line), "reignition" occurs, causing electromagnetic oscillation, and the voltage can reach 3 to 4 times the normal level. This has been significantly reduced with the adoption of "no-reignition" circuit breakers in modern times.

▪ Overvoltage from switching on no-load lines: When a line with residual charge is closed, it is equivalent to charging a capacitor, which may generate high-amplitude overvoltage. This is one of the controlling factors in insulation design for ultra-high and extra-high voltage systems.

▪ Overvoltage from switching off no-load transformers: When a small inductive current (magnetizing current) is cut off, the magnetic field energy is converted into electric field energy, generating overvoltage on the equivalent capacitance of the equipment. Surge arresters are commonly used for protection.

▪ Arc grounding overvoltage: In a system with an ungrounded neutral point, when a single-phase ground fault occurs, the arc at the fault point repeatedly extinguishes and reignites, leading to energy exchange between the system's capacitance and inductance, resulting in overvoltage throughout the system, with amplitudes reaching up to 3.5 times. This can be suppressed by changing to a neutral point grounded through an arc suppression coil or a small resistor.

2. Temporary overvoltage

◦ Occurrence: Overvoltage with a frequency of power frequency or close to power frequency, and a relatively long duration (0.1 seconds to several seconds).

◦ Main types:

▪ Power frequency voltage rise: such as the long line capacitance effect (the voltage at the end of the line is higher than that at the beginning), the voltage rise of the healthy phase caused by asymmetrical short circuit, and the voltage rise caused by load shedding, etc. This is the "base voltage" of operational overvoltage, which determines the continuous operating voltage of the surge arrester.

▪ Ferroresonant overvoltage: when the system contains nonlinear inductance (such as the core of a voltage transformer) and capacitance (line-to-ground capacitance, series capacitance, etc.), it may be excited to generate resonances with very high amplitudes and frequencies that are fractions of the power frequency (such as 1/2, 1/3, etc.) when disturbed (such as after the removal of a single-phase ground fault). It lasts for a long time and is highly hazardous.

1. Direct damage to insulation: It causes breakdown of solid, liquid or gaseous insulation in electrical equipment, resulting in short circuits.

2. Acceleration of insulation aging: Continuous overvoltage that does not reach the breakdown value will accelerate the aging of insulation materials and shorten the service life of equipment.

3. Malfunction or failure to act of protective devices: It may interfere with the normal operation of relay protection devices and automation equipment.

4. "Soft damage" to electronic equipment: Especially lightning surges, which may lead to performance degradation of integrated circuits, data errors or loss and other imperceptible damages.

For different types of overvoltage, protection is a systematic project:

1. Protection against direct lightning strikes:

◦ Lightning arresters: Lightning rods and lightning wires (overhead ground wires) attract lightning to themselves.

A good grounding device: It can quickly and with low impedance discharge the lightning current into the earth, reducing the potential.

2. Protection against lightning surge and operating overvoltage:

◦ Valve-type/Gapless Metal Oxide Surge Arresters: Core protective devices. They present high resistance under normal conditions but quickly change to low resistance when overvoltage occurs, discharging the overvoltage energy to the ground and clamping the voltage on the protected equipment below a safe level (protection level). They are the last and most critical line of defense against both external and internal overvoltages.

Surge protectors: They are used for multi-level fine protection in low-voltage distribution systems and electronic information systems.

3. Suppression of internal overvoltage:

◦ Parallel resistance of circuit breaker: Insert a resistor in series during the closing/opening process to dampen oscillations.

Installing shunt reactors: Compensating for the capacitive effect of long lines, suppressing power frequency voltage rise and operation overvoltage.

Neutral point grounded through arc suppression coil / small resistance: Suppress arc grounding overvoltage.

The use of high-performance zinc oxide arresters is the most economical and effective way to limit various types of internal overvoltages.

Summary: Overvoltage poses a significant threat to the safe operation of power systems. Modern power systems have established a multi-level and deep defense system from power generation, transmission, transformation to distribution and consumption through a comprehensive strategy of "diversion, discharge, clamping and damping".

VS1-12 vacuum circuit breaker is an indoor switch equipment with a rated voltage of 12kV and AC 50/60Hz. It adopts an integrated frame operating mechanism and is suitable for various industrial and mining enterprises, as well as power grid equipment. It can be used as a handcart unit for use with KYN28A-12 switchgear, or as a fixed unit with relevant mechanical interlocking, making it suitable for XGN2 and other fixed cabinets.

  Shaanxi West Power Tongzhong Electrical Co., Ltd.

Contact: Ms.Grace Liu (Director of Sales Department)

Tel: +86 917 3661109 Fax: +86 917 6739416

Mobile: +86 18091765882(WhatsApp/Wechat/facebook )

Website:https://www.xdtzelectrical.com

Add: Nanpo Village, Chencang Avenue Jintai District Baoji City, Shaanxi Province, China.