Overvoltage (electrical engineering)
Overvoltage is an electrical voltage in electrical systems that exceeds the tolerance range of their nominal voltage .
Overvoltages lead to a malfunction or failure if they destroy components or parts of the system. Overvoltages can occur symmetrically , that is, between the two supply lines, or asymmetrically - both supply lines carry overvoltage to earth.
causes
Causes for long-term overvoltages in the range of seconds to hours can be:
- poor regulation by the energy supplier
- sudden drop in load in the energy supply network, generally with a voltage source , for example through
- Consumer behavior (e.g. symbolic power-saving campaigns, end of a football game)
- Power outages in the network neighborhood, e.g. B. with shutdowns triggered by lightning strikes
- asymmetrical load or short circuit of an outer conductor in three-phase alternators , power sets , non-
- Decrease in consumption during the night
Causes of transient overvoltages can be:
- Electrostatic discharges (pulse rise times typically <1 ns)
- Shutdown of inductive loads in particular , switching sparks on switching contacts, brush fire of large electrical machines ( burst , pulse trains with rise times of around 5 ns)
- Lightning strike in the neighborhood, switching operations in the power grid ( surge , pulse rise times a few µs, pulse duration several 10 µs)
Also NEMP and solar activity on Earth caused magnetic storms cause power surges.
Transient overvoltages from neighboring sources of interference can also couple capacitively (due to influence) or inductively into supply or signal lines and can also be caused by strong radio waves .
consequences
- Consequences of transient (short-term) overvoltages
- Breakdowns and damage to insulating materials
- Destruction of semiconductor components due to microscopic thermal overloads
- temporary malfunction or failure of electronic circuits ( latching )
- Consequences of prolonged surges
- thermal overload z. B. of transformers due to increased no-load current (core saturation)
- premature failure z. B. Incandescent lamps
remedy
Long-term overvoltages in the supply network can only be prevented by the consumer with a network regulator . Such network regulators are, however, common and useful only for individual, particularly sensitive and expensive electrical consumers .
Transient overvoltages can be kept away from endangered components with surge arresters (see also surge protection ):
- in the field of energy suppliers with spark gaps and varistor surge arresters
- at the house with lightning protection , common earth rails for power supply, media and communication as well as with varistor surge arresters
- in the field of telecommunications with suppressor diodes and gas discharge tubes
- within electronic devices with suppressor diodes, varistors or protection diodes as well as with interference protection capacitors
On the mains voltage side, insulation, air and creepage distances are still oversized in order to avoid electrical breakdown in the event of high voltage pulses .
Electronic components as well as electrical end devices for mains voltage operation are tested as part of the electromagnetic compatibility test with regard to their immunity to transient overvoltages of 500 to 4000 volts (sometimes up to 15,000 volts) and specified accordingly or, in the case of end devices, certified according to standards for industrial, medical or home use . This includes:
- ESD test (pulse rise time approx. 1 ns) according to the human body model or the machine model
- Burst test (pulse trains with 5 ns rise time and 50 ns duration)
- Surge test (impulses with 1.2 / 50 µs or 8/20 µs rise / fall time)
Overvoltage categories
The overvoltage category according to DIN EN 60664-1 VDE 0110-1 describes the overvoltage resistance of electrical equipment (electrical installation, measuring device, electrical device, power supply, etc.). There are four categories, denoted by Roman numerals from I (lowest) to IV (highest). Since the power grid itself spreads overvoltage events, the closer the equipment is to the distribution network, the higher the requirements. In detail, the classes mean:
- Category I: Devices beyond a mains transformer (mostly low voltage consumers); Rated impulse voltage 1500 V
- Category II: Devices with IEC plugs , typical household devices , hand-held electrical devices ; Rated surge voltage 2500 V
- Category III: devices connected directly to the network (permanently installed); Rated impulse voltage 4000 V
- Category IV: Equipment directly at the point of supply in the building; Rated impulse voltage 6000 V
The surge voltage resistance is ensured by appropriate air and creepage distances, the dielectric strength of interference suppression capacitors or the group and type of insulation material .