Overvoltage is any voltage, whose peak value exceeds the appropriate peak value of the highest operating voltage in the LV power system. Overvoltage is usually an accidental phenomenon, which differs in time history and the place of its occurrence. Its parameters are defined by its cause (lightning stroke, switching in heavy-current network and so on) and also by electrical character of the circuit (wave resistance, ending impedance, discharge ability and so on). In the past few years the range of current and voltage courses for different uses has been standardized. These courses enable implementation of testing on equipment and constructive elements under the same conditions. In the following text the most important parameters of the most used standardized courses will be defined (according to EN 61 643-11, IEC 60-1 and CSN 34 5640).
|Peak value (amplitude) Umax,Imax||peak value is the maximal value of voltage or current
which is achieved by monitored impulse course
|Front of impulse||a part of voltage or current impulse before the peak
|Front time of current impulse T1||1,25multiple of the time interval between moments, when actual current value rise from 10% to 90% of the peak value|
|Front time of voltage impulse T1||1,67multiple of the time interval between moments, when actual voltage value rise from 30% to 90% of the peak value|
|Tail of impulse||a part of voltage or current impulse after the peak value|
|Time to half-value T2||the time interval between virtual beginning of impulse and the moment, when observed course reduces to 50% of its peak value|
Note: The virtual beginning is an intersection of time axis and bisector, which goes through points, where actual value of the front of impulse at first time reaches partly given lower value and partly given higher value …in detail see the following two figures.
Standardized testing current impulse
two basic types of testing current impulses are used during SPD tests:
- Testing impulse of lightning current Iimp(10/350) – it is used for simulation of lightning current (so-called test by lightning current)
- Testing current impulse Imax(8/20) – it is used for simulation of indirect effect of lightning and switching overvoltages. Arrester must discharge cca 17,5x higher charge during test by the testing impulse of lightning current Iimp(10/350), than during testing by the current impulse Imax(8/20) with the same amplitude. Also resulting in a different construction of the lightning current arresters tested by the lightning current impulse Iimp(10/350) and surge arresters tested by the current impulse Imax(8/20).
Course and parameters of lightning voltages and currents
In the chart shown below there are typical courses and parameters of lightning impulse voltages and currents, which occur in conductive parts of landscape, building constructions and metal lines in consequence of lightning stroke (taking in account influences caused by galvanic, inductive or capacitive coupling).The typical values of lightning impulse voltages and currents, which occur in conductive parts of landscape, building constructions and metal lines.
Testing current impulse in the waveform of 10/350μs is most often used for simulation of currents infiltrating into power lines and electric equipment in consequence of galvanic coupling. In case of inductive and capacitive coupling the voltage and current impulses are considerably shorter. The examination of interfering lightning effects in relation to inductive surges (currents) in consequence of inductive coupling is most often carried out by the testing current impulses in the waveform of 8/20μs. The examination of lightning effects in relation to interfering surges (currents) in consequence of capacitive coupling is similarly carried out by the testing voltage impulses in the waveform of 1,2/50μs.
Kinds of surge couplings
Disturbing energies (e.g. voltages, currents, fields) can infiltrate into the building by ways of different couplings whereas cabling and its layout represent an important part here.
During near and direct lightning strokes into the lightning conductors of buildings, the overvoltage shows in consequences of a galvanic coupling. The galvanic coupling is given by a different size of ground potentials along the building (earth electrodes, protective connection etc.)
There is always a capacitive coupling (parasitic capacity) between the source of interference and the receiver. The higher the front rate of rise of the disturbing voltage impulse (du/dt) is, the stronger its interference effect is.
There is always an inductive coupling (magnetic field) between the source of interference and the receiver. The higher the front rate of rise of the disturbing current impulse (di/dt) activating the magnetic field is, the higher the interference effect is.
Types of overvoltage
Direct Lightning Stroke
A lightning stroke is an electric discharge between an electrically charged cloud and earth surface (earth lightning), between two or more clouds and each other or between parts of one cloud (cloud lightning). Just a small percentage of strokes happens be tween the surface and the clouds. The lightning strokes originate in the „storm cells“, which stretch average out up to few kilometers. Every storm cell is active for up to 30 minutes and generates from two to three lightning strokes per minute. The storm cell often reaches the height of over 10 kilometers, whereas the bottom visible part of the clouds is usually at the height of one to two kilometers. In the centre of the storm cell there exists a strong rising air flow, which causes separation of positive and negative charges. The positive charge is usually binded on the frazils at the top of the storm cell, while negative charge is usually binded on water drops at the bottom of the cell. Nearby the earth the cell is charged with positive charge which is usually caused by discharge especially from forest. Beyond the storm cells originating from the summer heat there are storm cells originating from the frontal cloudiness as a result of big air masses movement. The storm frequency depends on the season. In summer months (July–August) there are on average 5 times more storms than in winter months (December–February). The environmental heating up supports the storm creation. In autumn warm water near the seacoast gives the necessary energy for the storm creation. According to IEC 1312-1:1995 and IEC 62305 it is possible to describe lightning charges by five basic parameters.
|Total impoulse lightning charge Qf||max.300 C|
|THe first stroke charge Qs||max.100 C|
|The first stroke peak current Iimp||max.200 kA|
|Specific energy the first stroke current W/R||max.10 MJ/Ω|
|Rate of rise of the current di/dt||max.200 kA/μs|
General distribution of lightning current when an object is thunderstruck
Protection system of LV power system composited of lightning current arresters and surge arresters SPD must be able to discharge lightning currents or their substantial parts without their damage. It is generally recommended to come out from the ohmic resistance of the building earthing, pipeline, power distribution system and so on for the purposes of establishing current distribution going through SPD in case of direct lightning stroke into a building protected by the outside lightning system. The following figure shows a typical example of lightning current distribution in an object hit by direct lightning stroke.
Where an individual evaluation is not possible, it can be assumed that:
- 50% of the total lightning current Iimp=200kA (10/350)…. IS1= 100kA (10/350) enters the earth termination system of the LPS (lightning protection system) of the structure considered
- 50% of Iimp=200kA (10/350)…. IS2= 100kA (10/350) is distributed among the services entering the structure (external conductive parts, el.power, communication lines, etc.) The value of the current flowing in each service Ii is given by IS/n, where n is the number of the above mentioned services (see the above figure). For evaluating the current Iv in individual conductors in unscreened cables, the cable current Iiis divided by m, the number of conductors, i.e. Iv = Ii /m.
For shielded cables, the current will flow along the shield. Requirement on dimensioning of protective system SPD in the most usual connection of the building and LV power system (TNC – system 230/400V/50Hz) results from this reasoning: For maximum lightning current size Iimp = 200kA (10/350) it is enough to dimension the protective cascade of each phase conductor entering the object on cca 4% Iimp, that is on cca 8kA (10/350) in most cases.
Distribution of protected area into the lightning protection zones
The standard IEC 13 12-1 and IEC 62 305 defines the lightning protection zones LPZ from the respect of the direct even indirect lightning effect. These zones are characteristic thanks to fundamental breaks of the electromagnetic conditions in their limited zones.
|LPZ OA||Zone where items are subject to direct lightning strokes, and therefore may have to carry up to the full lightning current; the unattenuated electromagnetic field occurs here.|
|LPZ OB||Zone where items are not subject to direct lightning strokes, but the unattenuated electromagnetic field occurs|
|LPZ OC||Zone where items are not subject to direct lightning strokes and where currents on all conductive parts within this zone are further reduced compared with zones 0B. In this zone the electromagnetic field may also be attenuated depending on the screening measures|
|LPZ 1||Zone where items are not subject to direct lightning strokes and where currents on all conductive parts within this zone are further reduced compared with zones 0B. In this zone the electromagnetic field may also be attenuated depending on the screening measures|
|LPZ 2||Zone where items are not subject to direct lightning strokes and where currents on all conductive parts within this zone are further reduced compared with zones 0B. In this zone the electromagnetic field may also be attenuated depending on the screening measures|
If a further reduction of conducted currents and/or electromagnetic field is required, subsequent zones shall be introduced. The requirement for those zones shall be selected according to the required environmental zones of the system to be protected. In general, the higher the number of the zones, the lower the electromagnetic environment parameters. At the boundary of the individual zones, bonding of all metal penetrations shall be provided and screening measures might by installed.
Note: Bonding at the boundary between LPZ 0A, LPZ 0B and LPZ 1 is defined in IEC 13 12-1 and IEC 62 305. The electromagnetic fields inside a structure are influenced by opening windows, by currents on metal conductors (e.g. bonding bars, cable shields and tubes), and by cable routing.
The following figure shows an example for dividing a structure into several zones.
There all electric power and signal lines enter the protected volume (LPZ 1) at one point, and are bonded to bonding bar 1 at the boundary of LPZ 0A, LPZ 0B and LPZ 1. In addition, the lines are bonded to the internal bonding bar 2 at the boundary of LPZ 1 and LPZ 2.
Furthermore, the outer shield 1 of the structure is bonded to bonding bar 1 and the inner shield 2 to bonding bar 2. Where cables pass from one LPZ to another, the bonding must be executed at each boundary. LPZ 2 is constructed in such a way that partial lightning currents are not transferred into this volume and cannot pass through it.
The above described segmentation of the protected object into protection zones gives possibilities of active protection of the LV power system thanks to insertion of the protective SPDs (usually at the zone boundary LPZ 0→1and LPZ 1→2) and other protective SPDs at the zone boundary LPZ 2→3. Standardly it is recommended to insert so-called 1st stage protection – surge arrester class I tested by lightning current Iimp(10/350) at the zone boundary LPZ 0→1. It is recommended to insert 2nd stage protection – surge arrester class II tested by testing impulse Imax(8/20) at the boundary zone LPZ 1→2. At the boundary of LPZ 2→3 and subsequently along the consequential circuit there is also recommended to shoulder after every cca 10m by so called 3rd stage protection class III also tested by testing impulse Imax(8/20) or UOC. For extra important protected equipment it is recommended to secure it by a quality continuous surge protection class III with high-frequency filter at the boundary of LPZ 2→3. If there are adjacent structures between which power and communication cables pass, the earthing system shall be interconnected, and it is beneficial to have many parallel paths to reduce current in the cables. A meshed earthing system fulfills this requirement. The lightning currents are further reduced, e.g. by enclosing all the cables in metal conduits or grid like reinforced concrete ducts, which must be integrated into the meshed earthing system.