Selection & Application Of Surge Arrester
Electrical equipment like transformer, generator, CTs, PTs and motor requires to be protected from over voltages. Surge arrester is used to protect these equipments from lightning and switching over voltages. Modern day’s surge arrester uses Metal oxide varistor as an active element and provides exceptional overvoltage protection to the equipment connected to the power system...
Electrical equipments connected in power system are exposed to many stresses, and one of major concern is protecting them from transient over voltages. Transients over voltages are caused by lightning discharges and switching operations.
A surge arrester is a protective device connected in parallel with system equipment to be protected. Overvoltages at the protected equipment are limited by the arrester that conducts energy associated with surge to ground and protect the equipment. The highly non-linear characteristics of an arrester allow the arrester to limit the voltage across its terminal nearly a constant value over a wide range of arrester current. The voltage across the equipment to be protected is almost same as voltage across arrester (Unless there is large separation distance between surge Arrester and equipment to be protected & large lead lengths).
During conduction of surge current, arrester exhibits very low impedance & forms a voltage divider to applied surge voltage in conjunction with line surge impedance. During the time arrester is in conduction, a large percentage of surge voltage appears across line surge impedance and not across equipment to be protected.
By properly applying the arrester, the equipment insulation will not be exposed to damaging voltages, thus eliminating the opportunity for insulation failure. It is important to correctly select arrester parameters so that it can do the desired protection function without causing any nuisance in system.
Over Voltages In Power Networks
A. Temporary overvoltage
These occurs due to earth faults, load rejections, resonance and ferroresonance or combinations of above. For insulation coordination purpose the representative temporary over voltage is considered to have the shape of the standard short duration (1 min) power frequency voltage. Generally, their amplitude does not significantly exceed 2.5p.u and duration varies from few cycles to several hours depending on system configuration.
(1 p.u = √2 Us / √3)
Where Us = maximum system voltage
B. Switching overvoltage
They generally arise from
• Line energization & re-energization
• Faults & fault clearing
• Load rejections
• Switching of capacitive and inductive currents
• Distant lightning strokes to the conductor of overhead line
The representative voltage shape is standard switching impulse (250 / 2500 µs). Generally the amplitude can go up to 3 p.u.
Steeper impulses with very high du/dt in the range from 0.1 to 10 µs & magnitude up to 4.0 p.u are possible in switching operations in inductive power circuits.
C. Lightning over voltages
They are caused by direct strokes to the phase conductor or back flashover or are induced by lightning strokes to earth close to the line.
Induced Lightning surge generally cause over voltage below 400 kV on the over head line and are, therefore of most importance for medium voltage networks.
The representative shape of lightning wave form is 1.2/50 µs. In a medium voltage network the amplitude of lightning voltage can go up to 10 p.u
Selection Of Surge Arrester
The user, application engineers should be aware of important surge arrester parameters and how to select it with reference to system parameters.
A) Continuous operating voltage, Uc –
is the maximum permissible value of a sinusoidal power frequency voltage, which may be continuously applied between the arrester terminals.
Uc is selected with reference to the highest actual system voltage Us or if this voltage is not known or it changes in the course of time, the highest voltage for the equipment Um should be taken as reference.
Normally, arresters are connected phase to ground so Uc should be equal to or greater than Um/√3. Additionally, a factor of 1.05 can be taken for harmonic distortion.
B) Rated voltage, Ur –
It has no particular practical significance from the user point of view. It is defined as maximum permissible 10 second power frequency rms over voltage that can be applied between the arrester, as verified in the TOV and the operating duty test.
Ur is selected with reference to temporary power frequency over voltages expected on system.
In three phase system temporary over voltages can occur due to earth faults. Single line to ground earth fault is considered to be severe condition where other two phases get over voltage & its magnitude depends on earthing of networks.
The magnitude of the expected temporary over voltage is often defined using the earth fault factor, Kd.
The typical range of factor Kd for various network configurations is listed in table-1.
It is also important to note that the grounding of the neutral at the source transformer is the configuration referred to in determining the voltage rise of the system.
C) Selection of continuous operating voltage Uc, rated voltage, Ur & TOV of surge arrester
a) Ungrounded or isolated neutral systems–Kd 1.73 (up to 2.3)
Generally, in these networks the phase to ground voltage of healthy phase would not exceed Um. There is no earth fault clearing so this voltage can remain there till the time fault gets cleared manually.
Continuous operating voltage
Uc ≥ Um (considering Kd = 1.73)
It must be noted; however the Kd factor can reach higher values under certain circumstances as a result of resonance phenomena. In such cases the Uc value should be increased accordingly.
Rated voltage – Once the Uc value selected as above there is no special attention needed to select Rated Voltage, Ur. Generally there is margin of 20% between Uc& Ur. So Ur can be selected as
Ur = 1.25* Uc
e.g 11kV isolated neutral system
Uc ≥ 12 kV
Ur = 15 kV
b) High Impedance grounded system (with earth fault clearing) - Kd 1.73
Here the magnitude of temporary over voltage is the same as in isolated neutral system but as there is earth fault clearing, lower values of Uc& Ur can be selected giving better protection margins.
Ur = Um * Kd / √3
Uc = 0.8 * Ur
e.g 11kV impedance grounded system
Ur = 12 * 1.73 / √3 = 12 kV
Uc = 12*0.8 = 9.6 kV
c) Solidly grounded three wire system - Kd 1.4
Provided a sufficient number of transformers have low-impedance earthed neutrals, the Kd factor will not exceed the value 1.4 for thisnetwork; clearance in such networks is very rapid so here also lower values of Uc & Ur can be selected giving better protection margin.
Ur = Um * Kd / √3
Uc = 0.8 * Ur
e.g. 11kV solidly grounded system
Ur = 12*1.4/√3 = 9.6 kV
Nearest rating can be selected as 9kV or 10kV
Ur = 10kV
Uc = 8 kV
Temporary Overvoltage Curve, TOV
Generally the arrester manufacturer defines the curve temporary over voltage vs time. It means arrester can withstand specified over voltages for specified duration. The over voltage is defined per unit of rated voltage Ur and also with & without prior energy.
e.g. TOV withstand with prior energy
Once the Ur & Uc values selected as per network configuration user should check the values defined in TOV curve of arrester datasheet with the expected over voltages & its duration at arrester location. User should keep in mind that at all time the arrester guaranteed TOV values should be greater than the expected TOV at arrester location, if not, arrester Ur & Uc to be increased accordingly.
In fact Uc, Ur & TOV of arrester are all linked parameters to be derived fromsystem highest voltage, system temporary over voltages and fault clearing time.
D) Nominal Discharge Current, In
The nominal discharge current is the peak value of lighting current impulse with wave shape as 8/20µs, which is used to classify an arrester, It is also a basis for calculating lightning impulse protection level, LIPL of a surge arrester.
Standard ‘In’ values are 2.5 kA, 5kA, 10kA & 20kA but the value of the nominal discharge current alone does not give enough information about the performance of the arrester. Additional information about the application either Distribution or Station class and duty low, medium or high along with repetitive charge transfer ‘Qrs’ rating is required to be specified.
E) Impulse and Thermal Energy Ratings -
Repetitive charge transfer rating, Qrs – is the maximum specified charge transfer capability (in coulombs C) of an arrester, in the form of a single event or group of surges that may be transferred through an arrester without causing mechanical failure or unacceptable electrical degradation to the MO resistors.
The repetitive charge transfer testing shows the capability of arrester to withstand repetitive discharges of lightning or switching surges.
This is basically impulse energy handling capability mentioned in Coulombs.
Thermal charge transfer rating. Qth - maximum specified charge that may be transferred through an arrester or arrester section within 3 minutes in a thermal recovery test without causing a thermal runaway. This is defined only for distribution class arresters.
Thermal energy rating, Wth - maximum specified energy, given in kJ/kV of Ur, that may be injected into an arrester or arrester section within 3 minutes in a thermal recovery test without causing a thermal runaway. This is defined only for station class arresters.
Both Qth and Wth relates to thermal energy handling of the arrester.
As per IEC 60099-4 ed. 3.0 a new concept of arrester classification and energy withstand testing is introduced: theline discharge classification was replaced by a classification based on repetitive chargetransfer rating (Qrs), as well as on thermal energy rating (Wth) and thermal charge transfer rating (Qth), respectively for statin & distribution class arrester. Requirements depend on the intended arrester application, being either a distribution class arrester (of In = 2,5 kA; 5 kA or 10 kA) or a station classarrester (of In = 10 kA or 20 kA). The new concept clearly differentiates between impulse and thermal energy handling capability, which is reflected in the requirements as well as in the related test procedures.
Now arrester nominal discharge and energy ratings can be simply selected based on application of surge arrester as per the table below.
The letters "H", "M" and "L" in the designation stand for "high", "medium" and "low" duty, respectively.
This new classification system now replaces the old classification system Class 1, 2, 3, 4 & 5.
Comparison of old classification and approx. equivalent of new classification is mentioned in Annexure L, Table L.3 of new IEC 60099-4 ed.3.0
As per this table now Old LDC class 1 will be equivalent to DH of distribution class and old LDC Class 2, 3 & 4 equivalent of SL, SM & SH of station class respectively as per new IEC.
Selection of correct Thermal Energy Rating ‘Wth’ for station Class arresters –
Metal-oxide surge arresters must be able to absorb the energy due to transient overvoltagesin the system.
With this new energy rating system, the required energy rating of an arrester can be determined by first calculating the level of energy the system will discharge into the arrester and then selecting the arrester with a Thermal Energy Rating Wth that is above the system discharge energy. The prospective energy that a system will require of an arrester can be determined using transient analysis software, but if that is not available a simplified formula as given in IEC 60099-5
L = Line length
C = Speed of light
Zs = Surge impedance of line
Ups = is the arrester residual voltage at the lower of the two switching impulse currents
Urp = is the representative maximum switching voltage
If the calculated system energy as per above is 7 kJ/kV of Ur then the desired ‘Wth’ rating should be minimum 7 kJ / kV of Ur.
F) Protection Level& protective Margins -
Lightning impulse protection level, LIPL or Upl
the maximum residual voltage of the arrester for the nominal discharge current
Switching impulse protection level, SIPL or Ups
the maximum residual voltage of the arrester for the switching impulse discharge current specified for its class
Steep current impulse protection level, STIPL
the maximum residual voltage of the arrester for a steep current impulse of magnitude equal to the magnitude of the nominal discharge current.
The residual voltages as mentioned above of the selected surge arresters should be well below the equipment withstand level.
E.g., for a 33kV system the BIL of transformer is 170kV and LIPL of surge arrester is 90kV then 80 kV is the protective margin. The protective margin should be high enough to take care of lead lengths, separation distance and aging effects of equipment to be protected.
G) Selection of Arrester Housing –
The arrester housing protects the internal active elements from environment and also provides the necessary creepage distance. It can be porcelain or polymeric type.
The housing should be tested for lightning impulse voltage, power frequency withstand voltage and switching impulse voltage (For > 245 kV).
If altitude is more than 1000 metre, then correction should be applied as below
Ka = e ^ m * (H/8150)
Ka = Altitude correction factor
H = Altitude above sea level in meters
m = 1.0 for lightning and power frequency withstand voltages
for switching voltage the value of m depends on the magnitude of switching voltage – and can be referred from curve given in IEC 71-2
Creepage distance of housing – can be selected as per pollution level at arrester location
Recommended creepage distance as below:
H) Short circuit Current
Rated short circuit current, Is of a surge arrester is defined as the highest tested power-frequency current that may develop in a failed arrester as a short-circuit current without causing violent shattering of the housing or any open flames for more than two minutes under the specified test conditions. Table 7 of IEC 60099-4 specifies the required current for short circuit test based on nominal discharge current of arrester.
Users should first find out the system short circuit current at arrester location and then select arrester with equal or higher short circuit rating.
I) Mechanical Considerations
In service surge arrester get subjected to various mechanical loading like
a) Terminal connectors – along with line conductors impose a load on the terminals as well as bending moment at the arrester base.
b) Wind Load – Heavy wind increases the horizontal loading on the arrester.
c) Seismic Load – the application of arrester in earth quake prone zone
d) Use of arrester as support
f) Tensile loading
g) Torsional loading if any
Users should study all these site conditions & correctly specifies the SLL, SSL, terminal torque values. Manufacturer type tested values should be more than the service conditions.
SLL = specified long term load
SSL = specified short term load
Both porcelain and polymeric arrester undergoes bending moment test and seismic test as applicable for the required voltage class of arrester as per IEC 60099-4
E.g., Bending Moment Test in polymer housed arrester (Us > 52kV consist of below sequence
Total no of samples - 3
1) 1000 cycles of bending moment (SLL loading) – All 3 samples
2) Bending moment test – 2 samples from step 1
3) Mechanical thermal preconditioning – Balance 1 sample from step 1
Terminal torque preconditioning for 30 seconds
4) Water immersion test – All 3 sample from step 1
4) Water immersion test
5) Test conclusion
a) No physical damage
b) Less than 20% change in power loss
c) PD less than 10 pC
d) Residual voltage changes less than 5%
e) Deviation in 2 impulse of residual voltage less than 2%
f) Reference voltage changes less than 2%
A) Surge Arrester for transformer neutral
One of the most widely used special applications of arresters is for the protection of transformer neutrals.
Each unearthed neutral brought out through a bushing should be protected against lightning and switching overvoltages by an arrester. The neutral insulationmay be overstressed in case of incoming multiphase lightning overvoltages, or in case ofswitching overvoltages due to asymmetrical faults in the power systems.
The Uc of surge arrester for transformer neutral should be selected as given hereafter.
Uc ≥ Um/√3 and the energy capability should be same as line to earth surge arrester.
High Impedance grounded system (with earth fault clearing)
Uc ≥ Um/(√3 *T), where T = 1.25 considering fault clearing within 10 seconds and margin of 20% between Uc& Ur.
Low Impedance grounded system
Uc ≥ 0.4 * Um / T, where T = 1.25 considering fault clearing within 10 seconds and margin of 20% between Uc& Ur.
B) Protection of Rotating machines -
If a generator under load condition disconnected from network, the generator voltage will rise sharply till the time regulator acts and readjust it. If surge arrester is connected at generator side then special care should be taken during selection as this temporary over voltage in tune of 1.5 time normal voltage will appear across arrester for up to 10 seconds. The arrester Ur & Uc should be selected accordingly for proper functioning of surge arrester.
High voltage motors connected through VCB can experience high voltage surge during switching operations especially over voltage on account of multiple re-ignition.
Surge due to current chopping can have low magnitude but very steep.Surges due to current chopping can have very high magnitude but with lower steepness that can be handled by surge arrester.
The Ur & Uc of the arrester can be selected as mentioned in ‘C’.
C) Surge Arrester for capacitor switching
Arresters are installed at capacitor banks due to a variety of reasons.
• To prevent capacitor failures at a breaker restrike
• To limit the risk of repeated breaker restrikes
• To prolong the service life of the capacitors by limiting high overvoltage
• For overall limitation of transients related to capacitor bank switching, which can be transferred further in the system and cause disturbances in sensitive equipment
• To serve as protection against lightning for capacitors banks connected to lines
The possible arrester discharge energy is the most important parameter to be considered.
This energy depends on capacitor bank design, grounded – ungrounded, arrester installation phase-ground or phase neutral, breaker performance.
Arrester energy W can be roughly estimated as below
C is the single-phase capacitance of the bank Uo is the phase-to-earth operating voltage; (peak voltage)
Ur is the rated voltage of the arrester (r.m.s. value).
The factor “3” comes from the assumption of a breaker restrike with full voltage of opposite polarity on the capacitor due to a previous break.
Furthermore, the operating voltage at the capacitor bank may be 5 to 10% higher than at other locations due to series reactors which may be used either to limit current when switching-in with parallel banks or to form a filter with the capacitors. This enhanced voltage must be considered when selecting the continuous operating voltage of the arrester.
D) Protection of the cable sheath
Surge arresters for cable sheath protection are sometimes called Sheath Voltage Limiters (SVL). Due to thermal reasons (power losses in the cable sheath) cable sheaths of power cables in high voltage systems are earthed on one side only (majority of the cases depending on circuit length). The open cable sheath has to be protected against overvoltage.
The continuous operating voltage Uc of surge arrester to be selected based upon induced voltage in the sheath during short circuit conditions. Normally the short circuit current is defined for 3 seconds so TOV capability of selected surge arrester for 10 seconds should be greater than calculated sheath voltage during short circuit.
E) Surge arresters between phases
Considerable overvoltage between the phase terminals of transformers or reactors may occur when a reactor or a reactive loaded transformer is switched off. The withstand voltage of the reactor or the transformer between phases may be exceeded without operation of the phase-to-earth arresters. If such switching operations are expected, surge arresters should be applied between phases in addition to those applied phase-to-earth. The phase-to-phase arresters should have a continuous operating voltage equal to or higher than 1.05 times the highest system voltage
F) Transmission line arrester, TLA – non gapped
TLA are in most cases directly suspended from the line conductor close to an insulator. The ground connection is connected to the tower steel structure. It’s important to understand the objective of TLA installation. There shall be one or more targets possible as listed below, hence the selection should be appropriate to address the needs.
· Reduce the total number of trips for the line to a target level.
· Reduce or eliminate double circuit tripping.
· Reduce number of shielding failures.
Employment of disconnector in series with TLA is essential. The electrical characteristics of the disconnector are in general different from those of disconnectors for distribution arresters, because the operating duties, where disconnection shall not occur, are harder. It must be ensured that after disconnection no part (swinging in the wind) will be able to produce a flashover to ground.
Selection of Ur –
The Ur shall be selected so that the lightning and switching surge protective levels are coordinated below the LIWV, and SIWV of the line insulation respectively. Ur of TLA is normally higher than substation arrester Ur as this will reduce the risk of TLA unnecessary getting stressed by system power frequency high voltage, here there is no benefit of extra protection margin as the purpose is to avoid flashover of line insulators by shielding failure or back flashover.
Selection of energy –
On shielded lines TLA typically have a nominal discharge current of 5 kA or 10 kA according to IEC with energy equivalent to classes 1 to 3, depending on their application.
Fault clearing & disconnector for TLA –non gapped
Disconnectors are used to facilitate fast reclosing as TLA are connected directly across the line insulators, which are self-restoring. Disconnectors are usually not permitted to disconnect high voltage substation arresters automatically in the event of an arrester failure since theinsulation of the substation equipment is generally not self-restoring and should not be switched in without protection.
These disconnectors in series with the TLA also serve as indicators making it simple to find overloaded TLA with visual inspection.The TLA disconnector must be capable of withstanding both higher impulse currents as well aslonger duration impulses compared to disconnectors for distribution arresters; in fact the disconnectors must pass all the type tests that the TLA is capable of and must know when disconnector shall operate and when it shall not operate. Below are the requirement to be complied for satisfactory TLA fail safe operation.
· The disconnector design shall be such that it shall always continue its opening operation once it is triggered to operate even if the system voltage trips.
· As the disconnector typically reacts on heating from power frequency currents. It cannot distinguish between TOV currents that the arrester withstands or real short-circuit currents thus it’s important to always select a high enough rated voltage so that the NGLA do not see TOV stresses that can interfere with its disconnector operation.
· Disconnector shall operate before the line trips and also shall operate before fast reclosing of the line.
The disconnector device is often mechanically weaker than the rest of the installation. Hence, the conductor connecting the TLA to ground or the phase conductor must be sufficiently long to ensure that the movements of the arresters and/or the transmission line will not risk that the disconnector device may break off by mechanical fatigue.
This article briefed about selection of surge arrester for various application conditions based on assumptions of some system parameters, however actual service conditions at arrester location may be worse or good than assumed.
For critical application user should be well aware of system conditions like TOV, line discharge energy etc which can help to correctly dimension the surge arrester.
Only a correctly dimensioned surge arrester can survive during normal conditions and protect system in the event of abnormal switching & lighting conditions and repeat these functions over its life time.
Abhijit Dhamale, P Kirushnaraj
Raychem RPG Pvt Ltd, Mumbai, India