Transient overvoltage causes
Transient overvoltages are sudden, short-duration voltage increases, typically less than 10 µs, which can reach peaks of 6000 V and even exceed 1000 V in industrial circuits. They originate from both external and internal phenomena to the electrical system and represent a threat to equipment insulation and operation.
Causes of Transient Overvoltages
Section titled “Causes of Transient Overvoltages”Atmospheric Discharges (Lightning)
Section titled “Atmospheric Discharges (Lightning)”A direct lightning strike on a distribution line can generate overvoltages exceeding 50 000 V. Distribution transformers attenuate these pulses with a factor ranging from 1 to 6, so the overvoltage that finally reaches internal circuits can still be several thousand volts. Lightning also induces overvoltages without direct contact, due to the electromagnetic field that accompanies the discharge.
Load Switching (Interrupter and Contactor Operations)
Section titled “Load Switching (Interrupter and Contactor Operations)”The connection or disconnection of inductive loads — motors, electromagnets, power supplies — causes abrupt current variations. When opening a loaded circuit, the “current chopping” phenomenon transiently raises the voltage. Switching a motor can generate peaks up to 2500 V. On unloaded transmission lines, when energizing or de-energizing the line, the voltage can momentarily double relative to the nominal value.
Insulation Faults and Ground Arcing
Section titled “Insulation Faults and Ground Arcing”When an active conductor accidentally contacts ground, the sudden grounding causes a rapid voltage redistribution that results in a transient. Insulation faults in cables, transformers, or rotating machines also induce short-duration transient overvoltages.
Resonance
Section titled “Resonance”If the voltage waveform is distorted by high-order harmonics (fifth harmonic or higher), a resonance condition can occur in which the inductive and capacitive reactance of the system cancel each other. At that instant, the impedance is minimal and the voltage can increase significantly, generating a transient of a frequency different from the fundamental.
Typical Magnitudes of Transient Overvoltages
Section titled “Typical Magnitudes of Transient Overvoltages”| Origin | Maximum Voltage | Typical Duration |
|---|---|---|
| Direct lightning strike on distribution line | >50 000 V / >50 kV | <10 µs |
| Lightning-induced overvoltage in internal circuits | 6000 V / 6 kV | <10 µs |
| Switching of electric motors | 2500 V / 2,5 kV | <10 µs |
| Switching of unloaded transmission lines | 2 × V_nominal (e.g., 460 V for 230 V) | <10 µs |
Consequences of Transient Overvoltages
Section titled “Consequences of Transient Overvoltages”Transient overvoltages seek the path of least resistance to ground and generate heat in circuit components, which accelerates insulation degradation and can cause premature failures. In sensitive electronic equipment, a single high-energy transient is enough to damage semiconductors or destroy printed circuit board traces. Additionally, repetitive peaks gradually reduce equipment service life and increase the risk of fire due to localized overheating.
Protection Against Transient Overvoltages
Section titled “Protection Against Transient Overvoltages”The most effective protection against external transients (lightning) is achieved with a combination of surge protective devices (SPDs) — also called lightning arresters or surge limiters —, shield wires on transmission lines, and grounding screens in substations. For internal transients, transient voltage surge suppressors (TVSS) are used in distribution panels and protection devices in sensitive equipment, such as varistors, transient voltage suppression diodes (TVS), and line filters. Coordination of these devices allows diverting the transient energy to ground and restoring insulation once the voltage returns to its normal value.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is a transient overvoltage?
Section titled “What is a transient overvoltage?”It is a sudden voltage increase lasting less than 10 µs, with peaks that can exceed several thousand volts, caused by lightning, load switching, or system faults.
What are the most common causes of transient overvoltages?
Section titled “What are the most common causes of transient overvoltages?”Atmospheric discharges (lightning) and the connection/disconnection operations of inductive loads (motors, transformers) account for most transients in electrical systems.
How does a transient overvoltage differ from a temporary overvoltage?
Section titled “How does a transient overvoltage differ from a temporary overvoltage?”The transient lasts microseconds and is characterized by a high-frequency pulse; the temporary (or power-frequency) overvoltage persists for several cycles, even seconds, and is usually due to system imbalances or sustained faults.
What effects do transient overvoltages have on electronic equipment?
Section titled “What effects do transient overvoltages have on electronic equipment?”They cause premature aging of components, random failures, unwanted restarts, and in severe cases, immediate destruction of semiconductors and integrated circuits.
How is an electrical system protected against transient overvoltages?
Section titled “How is an electrical system protected against transient overvoltages?”By installing surge protective devices at the service entrance, transient voltage surge suppressors in secondary panels, and local protection devices on sensitive equipment, in addition to maintaining proper grounding and equipotential bonding.
Which device is most effective for protecting against lightning?
Section titled “Which device is most effective for protecting against lightning?”The surge protective device (lightning arrester) coordinated with a low-impedance grounding system is the primary defense; it diverts lightning current to ground and limits the residual voltage to a safe level for connected equipment.
References
Section titled “References”- electrical4u.com: https://www.electrical4u.com/overvoltage-protection/