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—APPLIC ATION NOTE 2 .0
TransformersOvervoltage protection
The APPLICATION NOTES (AN) are intended to be used in conjunction with the
APPLICATION GUIDELINESOvervoltage protectionMetal-oxide surge arresters in medium-voltage systems.
Each APPLICATION NOTE gives in a concentrated form additional and more detailed information for the selection and application of MO surge arresters in general or for a specific equipment.
First published December 2018
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1 Introduction
The most efficient protection against overvolt-ages is the installation of gapless MO surge arresters on both sides (primary and secondary side) of the transformer. The MO surge arresters must be installed as close as possible to the bush-ings. In medium-voltage distribution systems mainly lightning incidents are critical.
Figure 1 shows a medium-voltage transformer in a very simplified way. The transformer is in star connection with open star point on the medium- voltage side (3phase, three-wire system) and the low-voltage side (3phase, five-wire system).
Overvoltage protection has to be considered for:• The bushings and insulation on the medium-
voltage side• The neutral of the transformer (star point)• The bushings and lines on the low-voltage side.
Concentrating on the medium-voltage side we have the situation shown in Figure 2 with MO surge arresters between phase and earth (phase arresters) and between neutral and earth (neutral arrester). The possible coupling of transient over-voltages from the medium-voltage side to the low-voltage side is shown in Figure 3 and de-scribed below.
—Overvoltage protection of transformers
All transformers in high-voltage and medium-voltage systems must be protected against transient overvoltages resulting from lightning and switching events.
—Figure 1: Principle outline of a medium- voltage transformer in star connection
—Figure 2: Medium- voltage transformer in star connection
MV LV
U
L1
L2
L3
L1
L2
L3
i i
NP
Mp, neutral
U
Trafo
Mp
L1
L2
L3
neutral arrester phase arrester
Trafo
4 A PPLI C ATI O N N OTE TR A NSFO R M ER S
2 Coupling of transient overvoltages through a transformer
Up to 40% of fast front overvoltages (e.g. light-ning overvoltages) can be transmitted capacitive from the primary to the secondary side. That is why it is necessary to protect the secondary side, even though the line on this side may not be di-rectly lightning endangered.
The resistive coupling of the overvoltage in a sub-station is also to be taken into account. Depend-ing on the execution of the earthing at the medium-voltage and the low-voltage side, over-voltages can be transmitted from one side to the other over the earthing system. In Figure 3 the possible voltage transmissions are depicted in a strongly simplified manner.
Capacitive couplingThe height of the possible transmitted impulse voltage can be roughly estimated with a simple observation. In a system having a maximum system voltage of Us = 24 kV and an insulated neutral, the MO surge arrester with, for example, a continuous operating voltage Uc = 24 kV is di-rectly connected at the medium voltage bushing of the transformer. This arrester may have a typical lightning impulse protection level of Upl = 80 kV (POLIM-K). Therefore, the insulation of the transformer with LIWV = 125 kV is very well protected on the medium-voltage side. If 40% of the voltage is coupled through the transformer, an overvoltage of theoretical 32 kV occurs on the low-voltage side. The insulation of the trans-former is not likely to be endangered, but the bushings on the low-voltage side and the con-nected lines can be destroyed or a flash over may occur.
Resistive couplingLet us consider the possible resistive transmis-sion of the overvoltage. The lightning current of In = 10 kA peak value flows according to the Fig-ure 3 through the arrester and over the earthing resistance RE to earth. If we take a typical earth-ing resistance of RE = 10 Ω, a temporary potential rise of the transformer housing of 100 kV occurs.
This potential difference is also to be found on the low-voltage side between the conductor and the earthing system.
This very simplified examinations do not provide an absolute statement about the level of the over-voltages that are transmitted, but explain the problems very well.
Therefore, overvoltages on both sides of the transformer are to be considered in any case.
3 Selection of the MO surge arresters
The MO surge arresters have to be selected as described in the Application Guidelines and the Application Notes, see Application Note 1.1.
The examples given below guide through the principle of the selection process step by step. Other system configurations are possible and have to be considered from case to case.
Depending on the expected stresses, electrical and environmental, and the importance of the equipment to be protected it is necessary to de-cide which characteristics of the MO surge arrest-ers are most important to provide best protec-tion. In this way the type of arrester (arrester class etc.) can be chosen from the beginning. In the example below high thunderstorm activity is mentioned, and consequently an MO surge ar-rester with sufficient energy handling capability should be chosen, in this case the type POLIM-K. The following practical example guides through the selection process for the arresters.
3.1 Selection example: Transformer protection in an outdoor substation Supplied information• System voltage Us = 24 kV• Star point high ohmic insulated with automatic
earth fault clearing after a maximum of 60 s• Installation at 3 600 m above sea level• High thunderstorm activity, seasonally
dependent• Line discharge class 2
0.4 res
MV LV
�
res
E
Ui
R i �U
U U
C
i
—Figure 3: Coupling of a lightning overvoltage through a medium-volt-age transformer
5
AssumptionsFirst: Line discharge class 2 is the old arrester classification acc. IEC 60099-4, Ed. 2.2! Application Note 1.1 gives us the equivalent of line discharge class 2: arrester class SL acc. IEC 60099-4, Ed.3.0 is the correct choice.
If no further information is provided we have to assume • Um = 24 kV• LIWV = 125 kV
(see Application Note 1.1) or IEC 60071-1• Duration of the earth fault t = 60 s• Nominal discharge current In = 10 kA• Short circuit current of the system Is = 20 kA• Degree of pollution: light
3.2 Phase arresters
Following the steps given in selection flow chart Application Note 1.1 A1 it follows:
Step a) Continuous operating voltage Uc The choice of the continuous operating voltage according to Application Note 1.2 is UsUc ≥ ---- T
The type POLIM-K is chosen with the assumption of arrester class SL (station low) on the basis of increased thunderstorm activity. For t = 60 s, this results in a factor of T = 1.275 out of the TOV curve for POLIM-K. The continuous voltage is thus calculated as:
24 kVUc ≥ ---- -- --- = 18.8 kV 1.275 Adding a safety margin for Uc of 10% this results in Uc = 20.7 kV
Therefore, chosen is (according data sheet): POLIM-K with Uc = 21 kV
Step b) Rated voltage UrAccording data sheet the rated voltage is Ur = 26.3 kV
Step c) Nominal discharge current InThe nominal discharge current for MO surge arresters class DH, SL, SM is In = 10 kA, for class SH the nominal discharge current is In = 20 kA. The type POLIM-K is of class SL with In = 10 kA. See also data sheet.
Step d) Charge and thermal rating Qrs and WthBased on the given information “high thunder-storm activity” the type POLIM-K was chosen instead of the type POLIM-D.
POLIM-K is an MO surge arrester with arrester class SL (station low) with • Repetitive charge transfer rating Qrs = 1.0 C and• Rated thermal energy Wth = 5.6 kJ/kVUc ,
see data sheet.
Step e) Check lightning impulse protection level Upl and withstand voltage LIWV
Control of the protection level.Required is:
Upl ≤ LIWV / Ks
With LIWV = 125 kV and Ks = 1.15, the maximum allowed voltage at the electrical equipment results in 108.7 kV.
The POLIM-K 21 has an Upl of 70.0 kV and meets the demands with a good additional safety margin.
With the steps a) to e) the active part of the MO surge arrester is selected. Now follows the selec-tion of the arrester housing and confirmation of mechanical data.
Step f) Creepage distance According to the assumption, there is low pollu-tion (pollution class b-light acc. IEC/TS 60815-1) to be considered. Therefore the minimum recom-mended specific creepage distance between phase and earth is 27.8 mm/kV.
—Note: In previous definitions the creepage distance was related to the system voltage phase-to-phase and defined as “specific creepage distance” SCD. In this case the SCD would be 27.8/√3 = 16 mm/kV.
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6 A PPLI C ATI O N N OTE TR A NSFO R M ER S
Pollution effects on insulators or housings are long term effects. Short TOVs as for instance for 60 s need not to be considered. Therefore, the needed creepage distance in this example can be based on the phase-to-earth voltage Us/√3 = 24 kV/√3 = 13.8 kV.
This results in a minimum requirement of 384 mm creepage distance. With silicone housing and light pollution (pollution class b, see Table 4 in the APPLICATION GUIDELINES), the creepage distance can be reduced by 30%. This ultimately results in a creepage distance of 269 mm. The POLIM-K 21-50 has a creepage distance of 745 mm according to the datasheet and offers large reserves here as well.
Step g) Flashover distanceThe minimum necessary withstand values of the empty arrester housing are calculated according to IEC 60099-4, Ed. 3.0 as:
Lightning voltage impulse 1.2/50 μs: 1.3 × Upl = 1.3 × 70.0 kV = 91.0 kV
a.c. voltage test 1 min., wet: 1.06 × Ups (switching current impulse 500 A => Ups = 53.8 kV) = Utest,pv = 57.1 kV,pv
This results in a withstand value of 57.1 kV / √2 = 40.3 kV, rms, 1 min., wet
The proved withstand values according to the datasheet are:
Lightning discharge voltage 1.2/50 μs: 180 kV
a.c. voltage test: 80 kV, rms, 1 min. wet.
Therefore, the housing of POLIM-K 21-50 has higher withstand values than are required accord-ing to IEC.
Taking into consideration the installation height of 3600 m, it must be checked whether an in-creasing of the flash over distance of the arrester housing is necessary. According the guide lines for altitude correction, the flash over distance should be increased with 10% per 1000 m above an installation height of 1800 m, which means that a corresponding higher withstand voltage must be proven. Thus, for installation in an alti-tude of 3600 m a correction factor of 18% has to be considered.
For the minimum required withstand voltage, this results in: Lightning discharge voltage 1.2/50 μs: 91.0 kV. An increase of 18% results in 107.4 kV.
a.c. voltage tests 1 min., wet: 40.3 kV rms. An increase of 18% results in 47.6 kV rms
Both calculated values according to the altitude correction lie below the proved withstand values. Therefore, it is not necessary to extend the hous-ing.
Step h) Consider short circuit rating IsThe POLIM-K 21-50 is proved with a short circuit current of 50 kA and easily meets the demands for a short circuit current of 20 kA, as it was assumed.
Step i) Consider mechanical loadsSpecial requirements for mechanical loads are not given. Therefore, no further considerations necessary.
—It follows: the POLIM-K 21-50 is the right arrester from all points of view for this application.
3.3 Neutral arresterOne of the most widely used special application of MO surge arresters is for the protection of transformer neutrals. Each not directly earthed neutral brought out through a bushing should be protected against lightning and switching over-voltages by an arrester. Without protection, the neutral insulation may be overstressed by over-voltages due to lightning events or to asymmetri-cal faults or switching operations in the power system.
The charge transfer rating or the energy handling capability of neutral arresters should be at least the same as required for the phase-to-earth arresters.
The selection of the neutral arrester has, in prin-ciple, to be done in the same way as for the phase arresters. But, as said above, the neutral arrester should have the same specific ratings as the phase arresters. Therefore, we have to go for a POLIM-K.
Step a) Continuous operating voltage Uc The power frequency voltage at the neutral cannot be higher than Us/√3. It follows for contin-uous operating voltage of the neutral arrester
USUc ≥ ---------- T x √3
The continuous voltage for the phase arrester was calculated to Uc = 20.7 kV. This includes already a safety margin of 10%, so we can writeUc,neutral = Uc,phase /√3 = 20.7 kV / √3 = 11.95 kV. From the data sheet follows POLIM-K 12-20.
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Step b) Rated voltage UrAccording data sheet the rated voltage is Ur = 15.0 kV
Step c) Nominal discharge current InIn = 10 kA for type POLIM-K
Step d) Charge and thermal rating Qrs and WthRepetitive charge transfer rating Qrs = 1.0 C, andRated thermal energy Wth = 5.6 kJ/kVUc
Step e) Check lightning impulse protection level Upl and withstand voltage LIWV
If no further information is given one can assume that the lightning and switching impulse with-stand voltages of the neutral insulation are the same as for the phases.
Control of the protection level.
Required is:Upl ≤ LIWV / Ks
With LIWV = 125 kV and Ks = 1.15, the maximum allowed voltage at the electrical equipment results in 108.7 kV.
The POLIM-K 12 has an Upl of 40.0 kV and the demands with a good additional safety margin.
Step f) Creepage distance According to the assumption, there is low pollu-tion (pollution class b-light acc. IEC /TS 60815-1) to be considered. Therefore, the minimum recom-mended specific creepage distance between neutral and earth is theoretical 27.8 mm/kV. In our example the voltage between neutral and earth is virtually zero, except for a short UTOV = 13.8 kV for 60 s As pollution effects are long term effects we don’t need to worry about the creepage distance for the neutral arrester.
Step g) Flashover distanceThe minimum necessary withstand values of the empty arrester housing are calculated according to IEC 60099-4, Ed. 3.0 as:
Lightning voltage impulse 1.2/50 μs: 1.3 × Upl = 1.3 × 40.0 kV = 52.0 kV
a.c. voltage test 1 min., wet: 1.06 × Ups (switching current impulse 500 A => Ups = 30.8 kV) = Utest,pv = 32.7 kV,pv.
This results in a withstand value of 32.7 kV / √2 = 23.1 kV, rms, 1 min., wet.
The proved withstand values according to the datasheet are:Lightning discharge voltage 1.2/50 μs: 110 kV.
a.c. voltage test: 50 kV, rms, 1 min. wet.
Therefore, the housing of POLIM-K 12-20 has higher withstand values than are required accord-ing to IEC.
Taking into consideration the installation height of 3600 m, it must be checked whether an in-creasing of the flash over distance of the arrester housing is necessary. According the guide lines for altitude correction, the flash over distance should be increased with 10% per 1,000 m above an installation height of 1800 m, which means that a corresponding higher withstand voltage must be proved. Thus, at 3600 m it must be cor-rected by 18%.
For the minimum required withstand voltage, this results in: Lightning discharge voltage 1.2/50 μs: 52.0 kV. An increase of 18% results in 61.36 kV.
a.c. voltage tests 1 min., wet: 23.1 kV rms. An increase of 18% results in 27.26 kV rms
Both calculated values according to the altitude correction lie below the proved withstand values. Therefore, it is not necessary to extend the hous-ing.
Step h) Consider short circuit rating IsThe POLIM-K 12-20 is proved with a short circuit current of 50 kA and easily meets the demands for a short circuit current of 20 kA, as it was assumed.
Step i) Consider mechanical loadsSpecial requirements for mechanical loads are not given. Therefore, no further considerations necessary.
—It follows: the POLIM-K 12-20 is the right arrester from all points of view for this application.
8 A PPLI C ATI O N N OTE TR A NSFO R M ER S
4 Low-voltage arresters
High-voltage and medium-voltage arresters are designed and tested according the IEC 60099 series. The scope of the 60099 series is limited to MO surge arresters for AC power circuits with Us above 1 kV. Therefore, the IEC 60099 series are not applicable for low-voltage arresters.
For low-voltage systems IEC 61643-11 “Low-volt-age surge protective devices - Part 11: surge pro-tective devices connected to low-voltage power systems – Requirements and test methods”, and other standards of the 61643 series are applica-ble. Figure 4 illustrates the situation.
The practice for overvoltage protection of the low-voltage side of the transformer (protection of the bushings and connected lines) is the same adopted for medium-voltage overhead lines.
Overvoltage protection in buildings and struc-tures (including lightning protection structur) starts at the meter and is to be done according to the IEC 61643 series, and not subject to the ABB application guidelines. However, some special applications as for instance protection of trans-former bushings on the low-voltage side, motors, cable sheath, low-voltage overhead lines etc. are considered in separate application notes.
Typical low-voltage power systems are three-phase four-wire systems with system voltages 230/400 V and 400/690 V.
5 Application considerations
Information about the assembling and installa-tion, maintenance, transport, storage and dis-posal of MO surge arresters is to be found in the operating instructions (manual) for each surge arrester.
There are some points to be especially observed so that a MO surge arrester can fulfill correctly its function.
Overhead line
C
T
1
C
T
2
C
T
3
1: Poor. The connection leads are too long and the transformer and the MO surge arrester do not have the same earthing point.
2: Good. Common earth of MO surge arrester and transformer. The connection leads are much shorter.
3: Very good. The MO surge arrester is earthed directly at the transformer tank. The loop is very short. In this way the inductance is kept to a minimum.
—Figure 4: Principle of over-voltage protection in a medium-voltage and low-voltage power system.
—Figure 5: Examples of good and poor con-nection principles for MO surge arresters in distribution systems.
Overhead line
MVSA: Medium-voltage MO surge arresterLVSA: Low-voltage MO surge arresterSPD: Surge protective deviceLPS: Lightning protection structureLPZ0 and LPZ1: Lightning protection zones
MVSA LVSA
IEC 60099-series IEC 61643-series
LPZ1LPZ0
SPDs
LPS
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5.1 Connections
At distribution levels the MO surge arresters of-ten can be located very close to the equipment to be protected, e.g. transformers. The connections must be as short and straight as possible, on the medium-voltage side as well as on the earth side. This is because inductive voltages appear at each conductor due to the self-inductivity of the leads during the flowing of the impulse current.
The specified residual voltages, which are given in the data sheets, are always the voltages between the arrester terminals only. The additional induc-tive voltage drop Ui is to be calculated asUi = L × di/dt.
L is the inductance of the loop and di/dt the rate of rise of the impulse current. Figure 5 gives hints for good and poor connection principles. If possi-ble connection principle 3 should be used. In no case connections as in 1 should be made.
5.2 Earthing considerations
Low earth resistance is essential, and it should be as low as possible in order to limit the earth potential rise at the earth terminal, and hence mitigate safety hazards and flash over on the low-voltage side of the transformer. A value for earth resistance of 10 Ω or less is considered to be sufficient. See also Figure 3.
Additional information about connecting MO surge arresters and induced voltages is provided in Technical Note 2.3.
6 Summary
The provided information in the example was limited to some basics. Sometimes specifications refer to old standards, which are not applicable anymore. Therefore, assumptions have to be made. In any case, it must be clear with the offer on which information and assumptions the offer is based.
For our example the technical data of the phase arresters are summarized below. For completion, the data for the neutral arrester and some infor-mation for the overvoltage protection on the low-voltage side is given as well.
Based on the information provided and the assumptions made, the technical data for the chosen MO surge arrester are, see Table 1:
Information and assumptions Technical data Comments
Phase arresters
Highest system voltage Us = 24 kV, k = √3Fault clearing time 60 s
Uc = 21 kVUr = 26.3 kV
Nominal discharge current In = 10 kA
LD 2 => old arrester class new class: SL (station low) selected POLIM-K 21-50
Repetitive charge transfer rating Qrs = 1.0 C according data sheet
Rated thermal energy Wth = 5.6 kJ/kVUc according data sheet
Lightning protection level Upl = 70 kV according data sheet
Um = 24 kV LIWV = 125 kV assumed acc. AN 1.1 A2
All other technical data according datasheet.
Neutral arrester
Uc = 12 kVUr = 15 kV
In = 10 kA
class SL selected POLIM-K 12-20
Qrs = 1.0 C
Wth = 5.6 kJ/kVUc
Upl = 40 kV
Um = 24 kV LIWV = 125 kV assumed acc. AN 1.1 A2
All other technical data according datasheet
Low-voltage arresters
If MO surge arresters for the protection of the low-voltage bushings and connected installations are required, the types MVR or POLIM-R..N can be a solution. For a 230/400 V low-voltage system MO surge arresters with Uc = 440 V are recommended.
—Table 1: Technical data of selected MO surge arresters
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© Copyright 2018 ABB. All rights reservedSpecifications subject to change without notice
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abb.com/arrestersonline
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