SURGE ARRESTERS SPECIFICATION FOR 132 KV & 33 KV LINE
What Is Lightning Arrester Or Surge Arrester?
In a simple and shortly we can say surge arrester or lightning arrester is a device used in electrical power system to protect others equipment from switching surges or high energy lightning surges.
If you are hearing the terms “lightning arrester” or “surge arrester” very first time and thinking, it’s like something “police arrest terrorist”! Yes, you not quite wrong; lightning or surge like a destructive terrorist for electrical power system, surge arrester works like a police man, catch it and send to ground before destruct the system equipment.
Don’t confuse lightning or surge arrester with lightning rod, surge suppressor or TVSS.
Lightning Rod is a grounded potential device that divert the lightning surge;
Suppressor is a lightning diverter device that uses in low voltage (below 1000V) system;
TVSS is Transient Voltage Surge Suppressor used for divert the transient surge in low voltage (below 100v) system.
So as summary, we can say surge or lightning arrester does not absorbed the lightning or stop the lightning, it does divert the lightning to the ground and limit the lightning voltage produced by the lightning.
Why Lightning Arrester Is Required For Power System?
Before know the causes of use surge or lightning arrester in electrical power transmission system, let we know the causes of surges.
There are various reason for electrical surge in power system, two major surge in electrical system occurs due to lightning from external onto the system and switching within the system.
Lightning Surge is with ultra-rapid transient phenomenon and highly destructive energy;
Switching Surge is with repetitive phenomenon leading to premature aging and malfunction which may even result in permanent damage.
Electrical charge cloud contains huge electrical charge energy, when these charge rapidly discharge through lightning, it release gigantic electrical energy as a very high voltages, and if this happen near to electrical transmission line then this massive energy travels through the electrical wire and destroy the electrical equipment.
On the other way switching surges may happen due to operating switching within the system. The main effect of surge is electrical over-voltage which exposes the electrical installations to a serious risk of malfunction, destruction of equipment or unavailability of production etc.
Hope, already it made sense that if the electrical power transmission line operate without lightning arrester and lightning strikes on the system the equipment will damage and power will go shut-down.
So, Electrical transmission system with lightning or surge arrester means, if lightning or any destructive surge strikes, surge will be diverted and sensitive equipment will be saved by arrester.
How Does Lightning Arrester Work?
The core part of the modern Lightning arrester is the MOV, MOV is shorten of Metal Oxide Varistor, and in simply it is a variable resistor. The MOV is a semiconductor disc that sensitive to voltage. At normal operating voltage MOV acts like an insulator, but in higher voltage it works as a conductor, this property makes the arrester divert the surge voltage to ground.
The heart portion of lightning arrester MOV that formed by numbers of solid zinc oxide disc arranged one by one inside a cylinder, compressed by end compression spring and attached to end cap, connector boss for end termination line and earth are located in top and bottom respectively (refer to the figure for details).
The number of MOV discs on lightning arrester is depends on the voltage rating and the arrangement is housed in polymer or porcelain cylindrical housing.
Working principle of MOV will be clear if we consider to magnify the zinc oxide disc, we will find in each disc contains a numbers of billion MOV grains that acts like switch itself formed between grains and their junctions, these switch is as open circuit at operating voltage and close circuit at higher voltage. So, lightning arrester is a collective of billions micro switches that operate (ON/OFF) in micro seconds to divert the surged high voltage to protect the power system equipment.
Working principle of various lightning arresters are almost same that provide high resistive path for normal operating voltage and high conductive path for instantaneous high voltage; but construction may vary to ensure better performance and higher sensitivity.
Types Of Lightning Arrester
Very first generation of lightning arrester just electrode gap in rod or pipe were used to limit or divert the surge voltages, but in latter engineering and technological knowledge are added and resistive elements are using now.
According constructional view types of lightning arrester may be as:
Road Gap Arrester;
Horn Gap Arrester;
Multi Gap Arrester;
Expulsion Type Arrester;
Valve Type Arrester;
Silicon Carbide Arrester;
Metal Oxide Arrestor
Road Gap Arrester
Is very simple construction, just two copper rods are bended in right angle and placed the rod ends in a straight line with arrangement in an insulator with a small gap. Upper rod connected with line and lower rod connected with earth. During surge voltage rod ends limit or divert the voltage by arcing between rods gap.
Horn Gap Arrester
Is also construction with two horn shaped metal rods with a small gap between them, the horns are constructed such a way that the gap between them gradually increased, one end connected with line through a choke coil and a resistor where other end directly grounded. During surge horn arcing and divert the voltage to ground as well as choke not allow to transients to enter the lines.
Multi Gap Arrester
Is as constructed a series of metallic cylinders that separated multiple air gaps. First cylinder connected with line and last one with ground, these series resistor limit the arcs.
Expulsion Type Arrester
Is constructed in such a way that consists a rod gap within the fiber tube. The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth. During arc vaporizes some of the fiber of tube walls resulting in the production of neutral gas that leaves the tube violently and carries away ionized air around the arc.
Valve Type Arrester
Valve type arrester construction may consist- series spark gaps, or non-linear resistor discs in series that tightly assembled in porcelain cylinder.
In spark gap assembly system a multiple number of identical spark gaps are consists in series. Each gap contains of two electrodes with a fixed gap spacing.
In non-linear resistor disc system thyrite or metrosil compound discs are connected in series. The non-linear resistors have the property of high resistance to current flow at normal system voltage but a low resistance to flow the high surge currents.
Silicon Carbide Arrester
Silicon carbide (SiC) is used for the nonlinear resistive elements, but not suitable to use so longer due to some unusual electrical characteristics- like very high resistance to low voltage and very low resistance to high-voltage. Major disadvantage of this arrester is operating over a period of time, the arrester gap will break down at small over voltages or even at normal operating voltages.
Metal Oxide Arrestor
Is used the MOV and it is gapless constructed arrester that usually used today. MOV arrester has duty cycle and maximum continuous operating voltage rating.
Duty cycle: the designated maximum permissible operating voltage between arrester’s terminals at which it is designed to perform its duty cycle.
MCOV: maximum voltage the device can withstand before conduction (clamping) begins. When applying metal oxide arrester, the minimum value of this voltage is usually the maximum system line-to-ground voltage.
Selection Criteria Of 132 KV & 33 Surge Arrester
Before selecting surge arrester following criteria may asked
2. Model Number
4. Continuous operating voltage kV rms
5. Rated voltage kV rms
6. Standard nominal discharge current kV
7. Reference current at ambient temperature mA
8. Reference voltage for above kV rms
9. Steep current impulse residual voltage kV pk
10. Lightning impulse residual voltage at
5 kA kV pk
10 kA kV pk
20 kA kV pk
11. Duty Class
12. Discharge Class
13. Pressure relief class
14. Nominal Diameter of resistor blocks mm
15. Number of resistor blocks connected electrically in parallel
16. Number of separately housed units pre phase
17. Overall height of arrester (without supporting structure) m
18. Overall height of arrester including grading ring if applicable
Phase to earth (from centre line) mm
Phase to phase (centre line to centre line) mm
20. Overall height of arrester (without supporting structure) kg
21. Maximum cantilever strength Nm
22. Maximum force due to wind (at maximum specified gust speed) Nm
23. Minimum creepage distance over insulator mm
Case Study (Surge Arrester Requirements for 132kV and 33kV Underground Cable Line Projects)
Surge arresters should be of the type employing nonlinear metal oxide (like ZnO) resistors without spark gaps. The Servicer should demonstrate by calculations that the surge arresters will adequately protect the switchgear arrangement proposed.
Arresters should be designed and tested in accordance with the requirements of IEC 99 4. Any departure should be the subject of agreement between the Engineer and the Servicer. Routine tests should be carried out in accordance with the requirements of specification.
Surge arresters should be housed in porcelain insulators designed to withstand extremes of the environment described. The insulation should have a minimum creepage distance of 25 mm/kV rated system phase to phase voltage. Porcelain should comply with IEC 233. The method of sealing against the ingress of moisture should be of a type well proven in service and the manufacturing procedures should include an effective leak test which can be demonstrated to the inspecting authorized engineer if required.
The internal components of arresters should be arranged to minimize radial voltage stresses, internal corona and to ensure minimal capacitive coupling with any conducting layer of pollutant on the outside of the porcelain housing except where approved, organic materials are not permitted.
Good electrical contact should be maintained between resistor blocks taking account of any thermal expansion and contraction of the block or mechanical shock during transport and erection, by installing a well proven clamping system.
Metal oxide arresters installed outdoors should be able to dissipate, when new, twice the energy generated in the resistor blocks when energized at their maximum continuous operating voltage immediately having been subjected to the discharge duties specified in IEC 99 4 and assuming that the porcelain housing and the surrounding air is at least 5˚C higher than the maximum ambient air temperature specified.
Good quality control of the manufacturing process of the resistors should be ensured by rigorous testing procedures. The procedures should ensure that the characteristics of the blocks are, and will remain, within the specified limits when new and throughout the anticipated life of the arresters. Samples may be selected at random by the authorized Engineer for special tests to be agreed with the manufacturer.
All surge arresters should be fitted with a pressure relief diaphragm which should prevent explosive shattering of the porcelain housing in the event of an arrester failure and the arrester should have been tested according to the high and low current tests specified in IEC 99 1.
Arresters should be supplied complete for installation in an outdoor switchyard, including supporting structures, insulating bases and surge counters, one per phase, and, if applicable, grading rings. The material used for terminals should be compatible with that of the conductors to which they are to be connected.
Each arrester should be identified by a rating plate in accordance with the requirements of IEC 99 4. In addition an identification mark should be permanently inscribed on each separately housed unit of a multi-unit arrester so that units can be replaced in the correct position in the event of them being dismantled.
Surge counters should have an internal assembly which is matched to the line discharge capability of the arrester and should include a leakage current meter with a bi linear scale for ease of reading. Auxiliary contacts are to be provided to signal remote indication of counter operation.