Overvoltage definition

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).

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

Generally

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.

Galvanic coupling

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.)

Capacitive coupling

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.

Inductive coupling

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.

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.  

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.