Table of Contents
Requirements of a Protective Device:
- It should take no current during normal power frequency condition. The breakdown strength should be above the normal power frequency voltages and permissible over voltages (normally 5 to 10 %).
- Transient over voltages of value more than insulation flash over should be diverted to earth. It should breakdown as quickly as possible.
- It should not only protect the equipment for which it is used but should discharge the surge current without damaging itself. Lightning arrester should be in a position to absorb the energy without getting damaged.
- The voltage across arrester during discharge (residual voltage) should not be too high.
- Normal condition should be restored soon after the surge has been diverted.
- It should interrupt power frequency follow current after the surge is discharged to ground. The magnitude of this follow up current depends upon the power system.
The insulation breakdown is not only a function of voltage, but it depends upon the shape and size of the electrodes used. The steeper the voltage wave, the larger will be the magnitude of the voltage required for breakdown, this is because an expenditure of energy is required for rupturing of any dielectric whether gaseous, liquid or solid and energy involves time. The energy criterion for various insulations can be compared in terms of a Impulse Ratio which is defined as the ratio of breakdown due to an impulse of specified shape to the breakdown voltage at power frequency. The impulse ratio of an sphere gap is unity due to uniform field, whereas for needle gap it may be 1.5 to 2.3 depending upon the frequency and gap. The impulse ratio of a gap of given dimension is greater with solid than with air dielectric. The insulators should have a high impulse ratio for economic design, but a lightning arrester should have a low impulse ratio, so that a surge incident on the lightning arrester may be by-passed to the ground instead of passing it on to the apparatus.
Types of Arresters
The protective devices can be classified according to their placement as:
OUTDOOR: The arrester should be located as near as possible to the equipment protected. On OH lines, it is the first apparatus from line to the substation.
INDOOR: These are kept inside the building / panel etc.
Here the devices mainly used are:
Horn Gap: It consists of two horn shaped rods separated by a small distance. One end of this is connected to the line and the other to the earth with or without series resistance as shown in fig. The inductor placed between the equipment and horn gap serves two purposes: steepness of the incident wave on the equipment is reduced and it reflects the voltage surge back to the horn.
Whenever a voltage exceeds the breakdown voltage of the gap, an arc is setup between the gap which acts like a flexible conductor and rises upwards under the influence of electromagnetic forces. The major drawbacks of horn gap is that its time of operation is very large and on isolated neutrals the horn gap may constitute a vicious kind of arcing ground.
Rod Gap: This surge diverter is the simplest, cheapest and most rugged one. The figure and characteristics are shown in fig below:
Even though the rod gap is the cheapest form of protection, it does not satisfy one of the basic requirement of a lightning arrester: it does not interrupt the power frequency follow current. This means for every operation of rod gap results in L-G fault and breaker must operate to de-energise the circuit to clear the flashover. Owing to this it is used as a backup.
Lightning Arrester: These are of two types:
Expulsion Type: An improvement of rod gap is expulsion tube which consists of (i) a series gap external to tube which is good enough to withstand normal system voltage, thus avoiding leakage across the tube, (ii) a tube which has a fibre lining on the inner side, (iii) a spark gap inside the tube and (iv) an open vent at the lower end for gases to be expelled. This is shown in fig.
When a surge voltage is incident a arc is formed between the electrodes within the tube, vapourising the organic material and causing the arc to be extinguished by the pressure. This type of lightning arrester can interrupt power voltage after flashover.
Valve Type: It is an improved but expensive surge divertor also called non-linear surge divertor. Here a porcelain bushing consists of a number of series gaps, coil units and the valve elements of non-linear element, usually made of silicon carbide disc. An schematic is shown in figure (only one element is shown in figure). (Modern ZnO arresters are without gap elements.) The resistor offers non-linear resistance such that for normal power frequency voltage the resistance is very high and for surge voltages the resistance is low. The characteristics is usually expressed as I = K × Vn , n lies between 2 and 6 and K is a constant, a function of geometry and dimension of the resistor. A grading ring or high resistance is connected across the disc so that the system voltage is evenly distributed across the discs and also keeps the inner assembly dry due to some heat generated. When a surge traveling along transmission line reaches the terminals of the arrester, at a particular voltage, arrester sparks over. This is called impulse spark over voltage.
The insulation build up ofter the flashover should be more than RRRV.
Surge Absorbers: A surge absorber is a device which absorbs energy contained in a traveling wave, and corona is one such wave. A short length of cable between the equipment and overhead line absorbs energy in the traveling wave because of its high capacitance and low inductance. Another method of absorbing energy is the use of ferranti surge absorber which consists of an air core inductor connected in series with the line and surrounded by an earthed metallic sheet called a dissipator.
LIGHTNING ARRESTER SPECIFICATIONS:
- Rated Voltage: Maximum permissible RMS voltage between the line and earth.
- Power Frequency Spark Over Voltage: As per BS it should be 1.6 times the rated voltage of the arrester, to avoid frequent discharges through lightning arrester under insufficient over voltages.
- Residual (Discharge) Voltage: The voltage that appears between the line and earth terminal of the arrester during the passage of current.
- Nominal Discharge Current: Surge current which flows through the arrester after spark over.
SELECTION OF RATINGS:
Voltage Rating: As during earth faults, the voltage of line to earth increases to √3 times the normal voltage, so the lightning arrester should withstand this value. Also as per IE rule, the system highest voltage is generally 110%, so voltage of arrester shoud be:
Voltage of arrester = Highest L-L rms voltage x coefficient of earthing. = L-L rms voltage x 1.1 x coefficient of earthing.
Where the coefficient of earthing is defined as:
Coefficient of Earthing = It is the ratio of highest rms L-E voltage of the healthy line to the L-L rms Voltage during earth fault on one line. For effectively earthed system the coefficient of earthing is taken as 0.8 and for non-effectively earthed system it is taken as 1.0.
So for a 11 kV system having non-effective earthing, the rms rating of arrester should be : 11 × 1.1 × 1.0 = 12.2 kV.
Nominal Discharge Current: As per BS 2914 : 1957, four ratings of L.A. has been specified: 10, 5, 2.5 and 1.5 kA. 10 kA current rating is considered for major power stations and substations having normal system voltage more than 66 kV, 5 kA rating is used for large substations having voltage rating not exceeding 66 kV. The 2.5 kA and 1.5 kA are used for small and rural substations having voltage not exceeding 33 and 22 kV, respectively.
INDOOR SURGE PROTECTORS:
The standard impulses for protection of lightning and other surges, in reference to whom specifications for various items are given are: 10/350 μs, 8/80 μs, and 8/20 μs, the first letter denoted the rise time and second the fall time. Mainly 10/350 and 8/20 pulses are used to give ratings. The current associated with the first may vary from 50 to 200 kA and for second from 5 kA to 25 kA. A sketch of these is given below:
1 is 10/350 pulse and 2 is 8/20 pulse.
The voltage waveform is 1.2/50 μs, 10/700 μs, and 10/1000 μs.
CLASSIFICATION OF ARRESTERS: Arresters are classified in the following manner. The arresters should confirm to international standard on lightning protection (LEMP – lightning electriomagnetic pulses) as per IEC 61312 – 1 and DIN VDE 0185 Part 103.
|Requirement Class as per VDE 0675 – 6.||Requirement of arrester as per||Location of Use|
|A||For use in low voltage overhead line||Impulse withstand voltage class IV as per DIN VDE 0110||As per DIN VDE 0675, Pt 1. (iSN = 5 kA [8/20])||
|B||For lightning protection equipotential bonding||Impulse withstand voltage class IV||As per DIN VDE 0675, Pt 1. (iIMP = 0.5 to 50 kA [10/350])||
|C||For overvoltage protection in stationary installation||Impulse withstand voltage class III||As per DIN VDE 0675, Pt 1. (iSN = 5 kA [8/20])|
|D||For overvoltage protection in non-stationary/stationary installation||Impulse withstand voltage class II||As per DIN VDE 0675, Pt 1. (iSN = 1.5 kA [8/20])||
IMPULSE WITHSTAND VOLTAGE CLASS AS PER DIN VDE 0110 Pt I / IEC 60664 -1
|CLASS||WORKING VOLTAGE||SURGE WITHSTAND VOLTAGE OF INSULATION||TYPICAL PROTECTION LEVEL|
|I||230 V||1500 V||1000 V|
|II||230 V||2500 V||< 1500 V|
|III||230 / 400 V||4000 V||< 2500 V|
|IV||400 V||6000 V||< 4000 V|
RISK CLASS ACCORDIND TO IEC 61024 – 1
|Current Parameters||Protection Class|
|Peak Current (kA)||200||150||100|
|Charge of impulse current Qs (C)||100||75||50|
|Specific Energy (MJ/Ω)||10||5.6||2.5|
|Negative initial strike||100 %||100 %||100 %|
|Positive strike||90 %||85 %||80 %|
|Sum of all lightning discharges||99 %||98.5 %||98 %|
EQUIPOTENTIAL BONDING AS PER IEC 61312 – 1
The 100% of the lightning energy breaks as follows:
50% to ground
50% as below (approximately):
10% to water pipe (metal)
10% to gas pipe (metal)
10% to oil pipe (metal)
10% to sewage pipe
10% to incoming feeder
Max. 5% or 5 kA shared across all data lines.
LIGHTNING PROTECTION ZONE CONCEPT AS PER IEC 61312 - 1.
In the case of direct strike into a lightning protection system, the direct lightning current is discharged to the earthing system via the existing down conductors. The electromagnetic fields generated here are another threat to the elctronic equipment within a building. To minimize these potential dangers, lightning protection zones are defined in any number.
These zones are established by a complete building or room screening. The screens can be formed from the existing armours or metal fronts. The lines passing a lightning protection zone must be equipped with appropriate protection device at the zone interface. These devices are then connected to the local equipotential bonding.
The lightning protection zones according to IEC are defined as follows from the outside to inside:
- Zone 0A: Direct lightning may occur here. The electromagnetic field is effective without limit.
- Zone 0B: This is the area that is protected from direct strikes. The electromagnetic field is effective without limit.
- Zone 1: In this zone only partial lightning current can flow through e.g. arresters (class B) or equipotential bonding lines. The elctromagnetc field is damped by screen 1 (building screen).
- Zone 2: The lightning partial currents occuring here are reduced by Zone 1. The electromagnetic field is further reduced by screen 2 (room screen). Class C arrester is recommended between Zone 1 and Zone 2.
- Zone 3 etc: Further reduction of the lightning partial currents and the electromagnetic fields. Class D and other data protective devices are required.
All metallic pipes/parts must be bonded together. Computers, telecom and control cables must be covered by the equipotential bonding by surge protection device at their zone interface.
A typical diagram of equipotentialisation is given below:
Tolerance capacity for surges of various equipments are:
|Rectifier: 2 kV (Normally rectifiers comes with 320 V MOV)|
|Telecom Cable: 5 to 8 kV|
|Signal Cable: 20 kV|
|Power Cable: 30 kV|
TYPES OF ARRESTERS AND TYPICAL APPLICATIONS: The arresters are classified into following based on the technology:
SPARK GAPS: Spark gaps are arresters in which two or more electrodes in series are opposed to each other. The electrodes consists of incombustible material. If a spark gap fire, the operating voltage collapses to anode-cathode drop voltage. The flashover voltage depends on the shape and distance between the elctrodes.
The spark gaps have a very high discharge capacity and are used as lightning arresters but here follow currents may occur.
VARISTORS: These are voltage dependant resistors with highly non-linear VI charecteristics. This property arises from a large number of micro-varistors (sintered zinc oxide grains with additives of mettalic oxide) connected in parallel and series. Transition takes place between these micro varistors under the influence of overvoltages.
MOV’s as they are popularily called have excellent clamping property and fast response time. They can be used for both DC and AC. They have very low leakage current and large current withstand capability, though not as much as a spark gap. Also they cannot withstand surges for long time.
GAS DISCHARGE TUBES: These acts as a voltage dependant switch and as sson as the voltage applied to the arrester exceeds the spark over voltage, an arc is formed in the hermetically sealed discharge region within nano-seconds and is extinguished under pressure. After discharge, the internal resistance returns to very high value.
Though the GD tubes are fast, bur they are not as fast as the MOV’s.
DIODES: Transzorb diodes (also suppressor diodes) are diodes that limit both positive and negative voltages. They are extremely fast and switch in picosecond region and are especially well suited for use in data line protection device.
SELECTION OF LIGHTNING ARRESTERS:
CLASS B ARRESTERS: They are rated for 10/350 μs waveform and are mainly of spark gap type, however upto 25 kA, some manufactures are having MOV type also. Only spark gap should be used between neutral and earth as MOV are clamping devices. They are rated upto 125 kA.
These are further classified as blowout and encapsulated type. In the blowout type, the spark gap blows hot ionized gases.Also the enclosure size should be bigger so that blow out area should not hit the enclosure and the enclosure should be fire proof.
Encapsulated types are sealed type, so they can be mounted without separate enclosure in distribution boards and no special enclosure is required.
The voltage protection level Up (also called spark over voltage) is available from 900 V to 4 kV.The advantage of selecting a lesser Upis that lesser distance between class B and class C is required as given in the table below:
Follow up current eliminating capability between 15 and 25 kA without the backup fuse should be there. Maximum continuous withstand voltage mostly should be between 275 to 320 Volts.
The discharge current rating of N-PE arrester should be more than L-N arrester e.g. if L-N arresters are having discharge currents of 50 kA, N-PE should have 100 kA, as sum of total current passes through this conductor.
Table: Relation between Up and Coupling Inductance
Voltage Protection Level of Class B Arrester Inductance Value / Cable Length Device between L & N ≤ 1300 V Not Required Device between N & PE ≤ 1300 V Device between L & N ≤ 2500 V 5 μH / 5 meters. Device between N & PE ≤ 2500 V Device between L & N ≤ 4000 V 15 μH / 15 meters. Device between N & PE ≤ 4000 V Device between L & N ≤ 900 V 15 μH / 15 meters. Device between N & PE ≤ 4000 V Device between L & N ≤ 4000 V 15 μH / 15 meters. Device between N & PE ≤ 1500 V
Also the inductance value/cable length depends on whether PE conductor is integrated in the cable or not and requirement is as below:
Min. cable length
≥ 5 meters if PE conductor is not integrated into the cable. ≥ 15 meters if PE conductoe is integrated into the cable.
CLASS C ARRESTERS: They are rated for 8/20 μs waveform and for discharge current upto 25 kA. Based on MOV and spark gap technology. Only spark gap should be used between line and earth.
The discharge rating of N-PE arrester should be more than L-N arrester e.g if L-G arresters are having 15 kA as discharge current, N-PE should have 50 kA.
CLASS D ARRESTERS: They are rated for 8/20 μs waveform and for discharge currents of 2.5 kA. Based on varistor technology with gas arresters (Y – circuits).
INSTALLATION: Typical installation for 3 phase systems are shown below and following legends have been used:
Arrester Backup fuses are required only when the fuse provided in the installation (line fuse) exceeds the value given in the technical data of the arrester.
FOR TN NETWORKS:
Here operational earthing RB is directly earthed. In the TN-C Network, the arrester between N-PE on B side is ommited and is shown below:
FOR TN-C-S NETWORKS:
Here operational earthing RB is directly earthed. The neutral conductor and protective conductor are combined in one conductor, the PEN conductor, for entry to the building. In the building PEN is split into N and PE. This is used predominantly in heavily populated areas and new installations.
FOR TT NETWORKS: Here operational earthing RB is directly earthed. The components of electrical installation are not directly connected to the operational earth via a cable or a conductor. This leads to higher earth resistance. The connection between installation earthing point and operational earthing is made via the ground. This is used predominantly in rural areas.
Now if the method used for TN system is used for TT system and if current (e.g. leakage current) flows as a result of arrester failing, a contact voltage arises on the earthing system as installation resistance may be high. This problem is overcome by connecting spark gaps between the neutral line and the earth system and connecting the phase arresters to the neutral line, as shown in fig below:
COORDINATION BETWEEN STAGES: A basic model of two arresters is illustrated below:
Suppose min operating voltage of B stage is 2 kV and residual voltage across C arrester is 1.3 kV. Because of low residual voltage, the C arrester will respond first. As the discharge capability of C stage is limited, for high partial lightning currents, the C arrester will be destroyed. By including a decoupling impedance, we have:
UB = UC + UL = UC + L di/dt.
So considering one meter length of line correspond to 1 μH, we need five meters of cable length for proper coordination. Otherwise inductor may be used.
Installation Check List:
- Installations should be based on IEC 60364-5-534.
For class B arresters following precautions will be taken:
- Only surge protection devices based on a spark gap will be used. It is not permissible to connect varistors in parallel.
- It must be possible to load seal the protective insulated housing of surge protection devices in accordance with the Load sealing requirements.
- The surge protection devices must be checked every four years to ensure their perfect working.
- Lightning arresters should be installed in accordance with the lightning protection zone concept close to the entrance of the building. Their use before the meter is essential.
- Avoid running unprotected conductors in parallel with protected conductors.
- The earth of the arrester must always be bonded with the earth of the consumer installation. If the PE busbars of a distribution boards are used, the PE busbar must be connected to the installation earthing point by a connection capable of carrying lightning current. (16 mm² on the PAS).
- The lines to the arrester elements should not be more than 0.5 m long, so that no excessive voltage take place in serious cases.
If it is not possible to apply the recommended line length of 0.5 m, the surge protection line should not be connected with a spur line, but in a V-shape. Care should be taken to run the outgoing and return lines as far apart as possible.
A safe clearance of 150 mm between all live non-insulated parts must be observed for installation of class B arresters. This clearance is required for arcing space for class B arresters.
|CLASS||Class B L-N lightning arrester||Class B N-PE lightning conductor||Class C L-N surge arrester||Class C N-PE spark discharger|
|Rated Voltage UC (Max. permissible operating voltage)||255 V / 50 Hz||255 V / 50 Hz||255 V / 50 Hz||255 V / 50 Hz|
|Nominal Voltage||230 V / 50 Hz||230 V / 50 Hz||230 V / 50 Hz||230 V / 50 Hz|
|Follow current dis charge at UC||25 kA without backup fuse||100 A RMS||100 A|
|Discharge capacity IIMP|
|(i) 10/350||50 kA||125 kA|
|(ii) 8/20||15 kA||50 kA|
|Protection level UP||≤ 4 kV||≤ 2.5 kV||≤ 1.5 kV||≤ 1.2 KV|
|Response time tA||≤ 100 ns||≤ 100 ns||≤ 25 ns||≤ 100 ns|
|Max short corcuit protection, if not already provided in the system||250 A gl||125 A gl|
|Short circuit capability at max back-up protection level||50 kA / 50 Hz|
|Temperature range||-40 °C to 80 °C||-40 °C to 80 °C||-40 °C to 80 °C||-40 °C to 80 °C|
|Degree of protection||IP 20||IP 20||IP 20||IP 20|
|Conductor cross section||10 to 50 mm²||10 to 50 mm²||1.5 to 35 mm²||1.5 to 35 mm²|
SPECIFICATIONS FOR LIGHTNING COORDINATOR:
|Rated Voltage UN||< 500 V, 50 Hz.|
|Rated Current IN||63 A|
|Inductance LN||5 μH ± 10%|
|DC Resistance RCU||1 mΩ|
|Temperature Rise||45 K at 63 A|
|Maximum required series fuse||63 A gl|
|Operating Temperature||-40 °C to 80 °C|
|Degree of protection||IP 20|
|Connection cross section||10 to 35 mm²|