ABB - Application Guide Low Voltage
20
OVERVOLTAGE PROTECTION Dimensioning, testing and application of metal oxide surge arresters in low-voltage power distribution systems
Transcript of ABB - Application Guide Low Voltage
APPL
ICAT
ION
GUID
ELIN
ESO
VE
RV
OL
TA
GE
PR
OT
EC
TI
ON Dimensioning,
testing and applicationof metal oxide surge arresters
in low-voltage powerdistribution systems
Up until 1998 no international standards existed for surge arresters in low-voltage power systems. This situation presented two difficulties: firstly it leadto specifications which were adapted from other standards, for example, IEC-99-1 and IEC-99-4, which are applied for high voltage surge arresters, with and without spark gaps; secondly, declaredrating, parameters and tests performed by different manufacturers were not clear, and therefore not really comparable.
In the past, different committees of IEC worked (and are still working) on standards and guidelines, as in IEC SC 28A:Insulation co-ordination of low-voltage installations; SC 37A: Surge protective devices (SPDs) in low-voltage power dis-tribution systems; TC 64: Electrical installations of buildings; SC 77B: Electromagnetic compatibility – high frequencyphenomena; and TC 81: Lightning protection. This did not make a clear and easy situation. Joint Working Group (JWG)31 of TC 64 has taken the task to co-ordinate the work of the different technical committees and sub-committees underthe title: Surge overvoltages and surge protection.
In 1998 the standard IEC 61643-1 (First edition 1998-02), was released with the title: Surge protective devicesconnected to low-voltage power distribution systems- Part 1: Performance requirements and testing methods
Mr. Bernhard Richter, Product Manager of the surge arrester division of ABB High Voltage Technologies Ltd, gladly tookon the task to describe in a short and clear form the technical bases and application of surge protective devices for low-voltage power systems, concentrating on Metal-Oxide surge arresters (MO-arresters) without gaps for outdoor and spe-cial applications.
Mr. Richter is an active member in different working groups of IEC SC 37A and TC 81. His activity field includes mainlythe development, testing and application of surge arresters for use in all voltage systems of power supply.
We hope, that you as a reader, will find this booklet useful. We welcome amendments, suggestions and qualified hints,which may help us to cover all the demands of our customers.
ABB High Voltage Technologies LtdWettingen, April 2001
First published: May 2001All rights reserved.No parts of this booklet may be reproduced or translated in any manner without the express written consent of ABB High Voltage Technologies Ltd.
© ABB High Voltage Technologies LtdDivision Surge ArrestersWettingen / Switzerland
Foreword
1
2
Contents
1 Introduction
2 Overvoltages in low-voltage supply networks
2.1 Overvoltages due to direct flashes
2.2 Induced overvoltages
2.3 Overvoltages due to coupling
2.4 Transferred overvoltages through transformers
2.5 Probability of overvoltages
3 Low-voltage networks
3.1 System voltages in low-voltage networks
3.2 Insulation categories
3.3 Low-voltage earthing systems
3.4 Temporary overvoltages (TOV) in low-voltage systems
4 Surge protective devices (SPDs)
4.1 Principle function of surge arresters
4.2 Definitions
4.3 Classifications
4.4 Service conditions
5 Low-voltage MO-surge arresters from ABB
5.1 MO-resistors
5.2 MO-surge arresters
5.3 Technical data of the arresters
6 Tests
6.1 Type tests
6.2 Special tests
6.3 Routine tests
6.4 Acceptance tests
7 Selection of MO-surge arresters
7.1 Selection of Uc
7.2 Selection of Up
7.3 Selection of the energy capability
8 Coordination of surge arresters
9 MO-surge arresters for d. c. systems
10 Installation of surge arresters
Bibliography
3
1 Introduction
Overvoltages in electrical supply networks result from effects of lightningstrokes and switching actions, and cannot be avoided. They endanger theelectrical equipment and due to economical reasons, the insulation can-not be designed for all possible cases. Therefore, a more economical andsafer on-line network calls for extensive protection of the electrical equip-ment against unacceptable overvoltages. This applies to high voltage aswell as to medium and low voltage networks.
Overvoltage protection can be basically achieved in two ways:
– Avoiding lightning overvoltages at the point of origin, for instancethrough shielding earth wires.
– Limit overvoltages near the electric equipment, for instance throughsurge arresters in the vicinity of the electrical equipment.
In low voltage systems the earth wire protection is generally not veryeffective. A lightning would hit not only one wire (the earth wire), but all,including the phase wires, and induced and transferred overvoltagescould not be avoided.
The most effective protection against overvoltages in low voltage net-works is therefore the use of surge arresters in the vicinity of the equip-ment.
For general information, and especially with regard to medium voltagenetworks, we refer to our APPLICATION GUIDELINES: Dimensioning,testing and application of metal oxide surge arresters in medium voltagenetworks [1]. Overvoltage protection in railway facilities, a. c. and d. c., isdescribed in: Dimensioning, testing and application of metal oxide surgearresters in railway facilities [2].
Lightning overvoltages are the greatest threat to the low voltage net-works. Overvoltage protection must be arranged in such a way that theovervoltage is limited to non-damaging values.
2 Overvoltages in low-voltage supply networks
Lightning surge overvoltages in electrical systems may be classifiedaccording their origin as follows [3]:
– overvoltages due to direct flashes to overhead lines– induced overvoltages on overhead lines due to flashes at some
distance– overvoltages caused by resistive, inductive and capacitive coupling
from systems carrying lightning currents.
In [4] is discussed in detail the case of transferred overvoltages througha distribution transformer from the medium voltage to the low voltageside.
2.1 Overvoltages due to direct flashes
The overvoltage is determined by the effective impedance of the line andthe lightning current. For a flash to an overhead line conductor, the impe-dance is in the first moments determined by the characteristic impedan-ce (surge impedance) Z0 of the line.
The impedance Z0 is normally in the range of 400 to 500Ω for one con-ductor. As shown in Figure 1 the lightning current is diverted in two, eachpart travelling along the line. The generated voltage is calculated
U = Z0 x i /2
Assuming Z0 = 450Ω and a typicall current of i=20kA (80% probability,see Table 1), the prospective voltage will reach U = 4500 kV. On lowvoltage lines, therefore, flashovers will occur between all the line con-ductors, and usually also a flashover to earth at the closest pole of theline. After flashover the effective impedance is reduced, depending onthe earth resistance involved. Even with a low impedance of 10Ω, andthe current being at 10 kA, the voltage will still be U = 100 kV, travellingalong the line. Therefore further flashovers can occur along the line.
2.2 Induced overvoltages
Due to the electromagnetic field changes caused by a lightning flash,overvoltages are induced in overhead lines of all kinds. As a rough appro-ximation, the prospective overvoltage between the line conductors andearth can be estimated according to Rusck [5]
Umax = Z0 x Imax x H / D
Imax is the peak value of the lightning currentZ0 is the effective impedance (assumed to be 30 Ω)H is the height of the lineD is the distance of the flash location from the line
Considering a height of 5 m for low voltage overhead lines, a lightningcurrent of 20 kA, and a distance of 100 m, the induced voltage is calcu-lated
Umax = 30 kV.
overhead lineearth
u
Z0
ui/2i/2 i
i : lightning currentU : generated overvoltageZ : surge impedance of the line0
Negative downward
Percentage
Current peak value
98%
> 4 kA
95%
> 6 kA
80%
> 20 kA
50 %
> 34 kA
20 %
> 55 kA
5%
> 90 kA
Figure 1
Lightning overvoltage caused by a direct lightning flash to an overhead line.
Table 1
Probability of lightning peak values.
4
With a distance of 1000 m between the line and the flash location, theinduced voltage has a value of Umax = 3 kV.
The above calculated values of induced voltages in low voltage overheadlines show that this kind of surge is of primary concern for low voltagedistribution systems.Lightning induced overvoltages occur mainly between the conductorsand earth. The voltage difference between the conductors is initiallysmall, especially when twisted conductors are used. However, due to dif-ferent loads on phase conductors (depending on low voltage system),interactions of surge protective devices, flashovers, etc., considerableline-to-line stresses can also occur.An example illustrating induced overvoltages line-to-line in low voltagesystems is shown in Figure 2. Twisted conductors, including neutral, areassumed. The neutral is earthed on both ends of the line. The voltages ata certain point of the line show a high frequency dumped oscillation(ringing wave). The period of the oscillation corresponds to twice the tra-vel time of a span, a span being the distance between two poles.Furthermore, it is found that the highest voltage occurs in the middle ofthe span. In the given example the voltage reaches up to 23 kV in themiddle of the span, and up to 5 kV directly at the pole, where consumersmay be connected.
2.3 Overvoltages due to coupling
A lightning flash to earth can result in an earth potential of high value atthe point of the strike, as well as in the vicinity. This phenomenon willcause overvoltages in electrical systems, using this point of earth as refe-rence for their earthing system. Figure 3 shows the principle of this phe-nomenon. The potential rise of the earthing system is determined by thelightning current and the effective earthing impedance. In the firstmoment the earth electrode potential is determined by the local impe-dance, for instance 10 Ω. This means that a high voltage is generatedbetween the earthing system and electrical installations inside the buil-ding, or other installations close to the earthing system. With a high pro-bability this overvoltages will cause either flashovers, insulation breakdownor operation of surge protective devices. Following such events, currentimpulses can flow into the various systems, mainly determined by theirimpedance to earth. In this way overvoltages are produced in the powersupply system as well as in other services (telecommunication, data andsignalling systems, etc.). Furthermore, overvoltages are transferred toother buildings, structures and installations.
Due to the high electromagnetic fields caused by the lightning current,inductive and capacitive coupling to electrical systems close to a light-ning path can also cause overvoltages of concern, causing failures ormalfunctions.
2.4 Transferred overvoltages through transformers
Overvoltages generated in the medium voltage (MV) system are transfer-red to the low voltage (LV) system in two ways,
– by capacitive and magnetic coupling through the MV / LV transformer– by earth coupling (see Figure 4).
The magnitude of the transferred overvoltage depends on many para-meters and some important differences can exist between differentcountries, due to differences in the transformer design and the LV earth-ing systems (T T, T N, IT).
Figure 2
Induced overvoltage line-to-line. Calculated values, assuming twisted conductors.
Figure 3
Example of resistive coupled overvoltages in electrical systems. In the electrical installati-
ons in the building, as well as in close installations (and all conductuing parts) in the earth
high overvoltages can be generated.
Figure 4
Overvoltages in the Low voltage system
0
5
-5
1 2 3 4
250 m
150 m
32 kA
0 1 2
6 7 t (µs ) 8
-10
10
15
20
25 U(kV )U2
U1
U0
5
L1
100%Lightning protection system (LPS)
earthing impedance LV cable
U
L2
i
U
0.5 i⋅
0.5 i
i
L3N
PE
∆ ∆
Medium voltage line (MV)
A1 by direct lightning to the MV lineA2 by indirect lightning to the MV line (induced voltage)B1 by direct lightning to the LV lineB2 by indirect lightning in the LV line (induced voltage)C by capacitive coupling through the transformer
Low voltage line (LV)
Transformer
L1C
A1
A2 B2
B1
L1
L2 L2
L3 L3
5
a) without representation of the users installation(no load assumed)
b) user installations represented by lumped capacitances
10U (kV)
5
0
0
-5
10
5
0
0
5
5
10
10
15
15
20
20
t (µs)
t (µs)
U (kV) b)
a)
Figure 5a / 5b
Typical wave shape of overvoltage transferred to the LV line (calculated).
The high frequency components of the overvoltage are transferred capa-citively from the MV to the LV side of the transformer [4]. Figure 5ashows a typical wave shape of the overvoltage transferred to the LV line.Being the transferred overvoltage characterized by high frequency oscil-lations, the natural capacitance of the load can reduce very effectivelythe peak overvoltages, as shown in Figure 5b. The calculated voltages inthe given example reach peak values of 10 kV (without load, Figure 5a),and 3 kV (with load, Figure 5b).In case of direct lightning to the MV line, the surge arrester operation oran insulator flashover diverts the surge current through the earthingsystem, and can produce a resistive earth coupling between the MV andLV system. An overvoltage is transferred to the LV system as shown inthe typical case of Figure 6a. Depending on the earthing impedance, thisearth coupling overvoltage can be much higher than the capacitive cou-pling through the transformer. Separating the earthing electrodes, as inFigure 6b, avoids this problem. Practically it is not possible to have real-ly separated earth systems, due to the short distance and the conduc-tivity of the earth.
2.5 Probability of overvoltages
The frequency of lightning flashes to an overhead line, or in the vicinityof the line, depends on the local flash density, line type (especially theheight) and possible shielding effects of the surroundings [3], [4], [5]. Forlines in an open area the number of flashes can be calculated as follows
N = A x Ng x 10-6
A = 6 x H x L
A = effective area for direct lightning to the line (in m2)H = height of the line ( in m)L = length of line (in m)Ng = local flash density per km2 and year
For a line of 5 m height and assuming Ng = 1, N is found to be 0,03 perkm of line and year, that means three direct flashes per 100 km of linelength and year. This gives a rough estimate of number of direct flashesto low voltage overhead lines.
The number of induced and transferred overvoltages is certainly muchhigher than the overvoltages due to direct flashes in the line. Especiallythe local lightning density and the different possibilities of generatingovervoltages, including switching, has great influence on the occuringnumber of dangerous overvoltages.
In Figure 7 a typical low voltage system with overhead line is given.Calculated figures are presented for induced overvoltages which may beexpected in this network, [3]. The ground flash density was assumed tobe 2,2 flashes per km2 per year, all loads were modelled by frequencyindependent resistors. Table 2 shows the calculated results. The lastcolumn (> 20 kV) shows high levels of overvoltages, but these occur onlyin case of direct lightning to the low voltage line. The probability of occur-rence of such surges in this example is once in 22 years. But the over-voltages in the range of 1,5 kV to 6 kV can occur several times a year ina low voltage network, depending on the type of installation.
b) Separate earthing for MV and LV side of the transformer.
a) MV and LV side of the transformer have same earthing point. Thisgenerates, in case of arrester operation, an overvoltage U on the LVsystem ( U = R i + L ⋅di/dt)(no load assumed)
g
g ⋅
U = U + R⋅i + L di/dt0
U = U0
Equipment
Equipment
Soil
i
i
Soil
MV Arrester
MV Arrester
(R, L)Transformer
Earthing
TransformerEarthing
ArresterEarthing
InstallationEarthing
InstallationEarthing
∆
Figure 6a / 6b
Overvoltage on the low voltage side due to earth coupling.
6
3 Low-voltage networks
Around the world very different low voltage networks exist. They differ inthe system voltage, the number of wires, the handling of the neutral andthe protective measures. The nominal voltages of the supply systems arebasically given in publication IEC 60038 (1983-01) and amendments:IEC standard voltages.
In IEC 60664 [6] is given a good overview of the nominal voltages pre-sently used in the world, depending on the type of network, see Table 3.
3.1 System voltages in low-voltage networks
As seen in Table 3, there is world wide a variety of existing voltages. Thestandard voltages in Europe, for instance, are given in [7]. The systemvoltages, according to the harmonization document, are 230 / 400 V,where 230 V is the line to neutral voltage, and 400 V is the line to linevoltage. Other existing common voltages in Europe are 240 / 415 V and220 / 380 V.Considering an allowed tolerance of 10 %, the highest voltages to beexpected for the 400 V system are
U0max = 253 V (line to neutral voltage) and
UNmax = 440 V (line to line voltage).
Figure 7
Typical low voltage network with overhead line.
Arrangement used for calculating the values in Table 2.
Table 2
Line-to-earth prospective overvoltage levels in the LV installation, occurrences per year.
Note 1: The numbers shown in the table were obtained for an overhead twisted cable dis-
tribution system. For a distribution system with overhead open conductors in air,
the voltage levels can be expected to be twice as high for the same probabilities.
Note 2: In this example, when performing a variation of the model to represent a TN
system, it was found that the value of the earthing impedance had no significant
influence because the LV neutral is directly connected to earth.
MV/LV station
Line connection
Installation earthing50 ohms
Neutral earthing30 ohms
Neutral earthing30 ohms
Conductive parts earthing30 ohms
Consumer’searthing
20 kV line
MV arresters240 m 240 m 30 m
25 m
230/400 V linetwisted cable (3 phases + neutral)
Unloaded TT system
Loaded TT system
Loaded TN system
> 1,5 kV
6
4
1
> 2,5 kV
3
17
0,6
> 4 kV
18
1
0,35
> 6 kV
1
0,5
0,25
> 20 kV
0,045
0,045
0,045
3.2 Insulation categories
The concept of overvoltage categories is used for equipment energizeddirectly from the low voltage mains. For the different categories the insu-lation levels are specified. According to [6] the definitions of the catego-ries are as follows:
– Equipment of overvoltage category IV is for use at the origin of theinstallation (e. g. overhead lines, cables, bus bars, meters, primaryovercurrent protection equipment).
– Equipment of overvoltage category III is equipment in fixed installationsand for cases where he reliability of the equipment is subject to spe-cial requirements (e. g. mainly fixed indoor installation).
– Equipment of overvoltage category II is energy-consuming equipmentto be supplied from the fixed installation (e. g. appliances, portabletools and other household and similar loads).
– Equipment of overvoltage category I is equipment for connection to cir-cuits in which measures are taken to limit transient overvoltages to anappropriate low level (e. g. protected electronic circuits).
Table 3
Nominal voltages presently used world wide.
50
100
150
300
600
1000
Voltageline-to-neutralderived from
nominalvoltages
a.c or d.c.up to andincluding
Three-phasefour-wiresystems
with earthed neutral
* Practice in the United States of America and in Canada.
Three-phasethree-wiresystems
unearthed
Single-phasetwo-wiresystems
a.c. or d.c.
Single-phasethree-wiresystems
a.c. or d.c.
66/115
120/208*127/220
220/380, 230/400240/415, 260/440
447/830
347/600, 380/660400/690, 417/720
480/830
66
125, 120127
220, 230, 240260, 277, 347380, 400, 415
440, 480
500, 577, 600
600690, 720
830, 1000
12,5 2425 3042 48
60
110, 120
220
480
1000
30–60
110–220120–240
220–440
480–960
V V
E
V V V
Table 4 gives the four insulation categories. The rated impulse voltagegives the insulation withstand capability for the different categories,depending on the line to neutral voltage of the systems derived from thenominal voltages a. c. or d. c., based on IEC 60038.
3.3 Low voltage earthing systems
There are a number of methods used to provide an earth connection orsystem. The different arrangements and standard definitions are givenbelow. Each is defined by a coding which contains the following letters:
T : terre, direct connection to earthN : neutralC : combinedS : separateThe different principle earthing arrangements are shown in Figure 8. Forsimplification single line diagrams are used.
TN - S system (Figure 8a)The incoming supply has a point of connection between the supply neu-tral and earth only at the supply transformer. The lines have separateneutral and earth protective conductors.
TN - C system (Figure 8b) The neutral and earth wire are combined within the premises, and areearthed at the supply transformer or close to it.
7
50
100
150
300
600
1 000
Voltageline-to-neutralderived from
nominalvoltages
a.c or d.c.up to andincluding
Rated impulse voltagefor equipment
Insulation category
330
500
800
1 500
2 500
4 000
500
800
1 500
2 500
4 000
6 000
800
1 500
2 500
4 000
6 000
8 000
1 500
2 500
4 000
6 000
8 000
12 000
V
V
I II III IV
Table 4
Insulation categories for low voltage systems.
Figure 8a
ElectricalEquipment
TN-SSystem
CustomerConnection
Point
N
LV line
Electrical Power Source
E
L
TN - C-S system (Figure 8c)The supply neutral is earthed at the source and points in the network.Supply lines have a combined neutral and earth wire. Supply within thecustomer premises would have separate neutral and earth wire, connec-ted only at the service position.A protective neutral bonding (PNB) arrangement may be used to providean earth terminal connected to the supply neutral. With this arrangement,the neutral will be connected to earth at the source point only, at or nearto the customers supply point. The arrangement is generally restricted toa single customer with it`s own transformer. See Figure 8d.
TT system (Figure 8e)The transformer is connected directly to earth, the customers installati-on is earthed via a separate electrode. This will be independent of anysupply point electrode.
ElectricalEquipment
TN-CSystem
CustomerConnection
Point
N & E
LV line
Combined PEN Conductor
LElectrical Power Source
Figure 8b
Figure 8c
ElectricalEquipment
TN-C-SSystem
CustomerConnection
Point
N
LV line
Electrical Power Source
Combined PEN Conductor
E
L
Figure 8d
ElectricalEquipment
TN-C-SSystem(PNB)
CustomerConnection
Point
N
LV line
Electrical Power Source
Combined PEN Conductor
E
L
8
IT system (Figure 8f)This arrangement has no direct system connection between live partsand earth, but the exposed conductive parts of the customers installati-on and its equipment is earthed.
3.4 Temporary overvoltages (TOV) in low-voltage systems
In case of a failure on the medium voltage side of the MV / LV transfor-mer, due to an internal fault of the transformer or a sparkover of a gap orinsulator, a current flows through the earthing impedance of the trans-former. Depending on the connection between this earth impedance andthe low voltage network a temporary overvoltage with power frequencycan stress the low voltage network for a given period of time, equal to theclearing time of the fault in the medium voltage network. This can be bet-ween some 10 µs up to some hours. For a detailed discussion of tem-porary overvoltage conditions see IEC 60364 [8].
Depending on the earthing system of the low voltage network differentTOV can occur. Table 5 gives an overview about the considered systemsand the possible TOV between the different lines. Two values are given,the minimum TOV value for 5 sec, and the TOV values for 0,2 sec.Corresponding test procedures are described in the amendment of IEC61643-1 [9].
The test procedure depends on the intended application of an SPD in alow-voltage power installation system according to the installationinstructions given by the manufacturer.
4 Surge protective devices (SPDs)
SPDs are devices for surge protection against direct and indirect effectsof lightning or other transient overvoltages. They contain at least one
Figure 8e
Figure 8f
ElectricalEquipment
AlternativeLocationfor EarthTerminal
T TSystem
CustomerConnection
Point
N
LV line
Electrical Power Source
E
L
ElectricalEquipment
AlternativeLocationfor EarthTerminal
I TSystem
CustomerConnection
Point
N
LV line
Electrical Power Source
E
L
nonlinear component that is intended to limit surge voltages and divertsurge currents. The discussed SPDs are typically for use in low-voltagepower systems, providing protection from the low-voltage bushing of theMV / LV transformer up to the plugs in buildings. In the course of this gui-delines we will talk mainly about metal oxide surge arresters (MO-arre-sters) without gaps for outdoor and indoor application.
4.1 Principle function of surge arresters
There are two different designs for surge arresters: a voltage limitingtype, and a voltage switching type. The voltage limiting type is a nonline-ar resistor, generally a metal oxide resistor, without any spark gap inseries. This types are sometimes called MOV, which is an abbreviation ofmetal oxide varistor. The voltage switching type is a spark gap, or a sparkgap with a nonlinear resistor (MO or SiC) in series or parallel.
Figure 9 shows the principle difference in the function of the two types.
t
v
v
v
v
t
time scale 10 µs/div time scale 25 µs/div
Gapped arrester MO-arrester
Figure 9
Difference in function of gapped arresters (left), and MO-surge arresters without gaps
(right). Both types were tested with switching voltage impulses of the wave shape
250/ 2500 µs .
The voltage scale is the same in both cases. It is to be seen that in case of the MO-arre-
ster the residual voltage is only half of the one given by the gapped arrester (same Uc for
both types of arresters).
Table 5
TOV values in low voltage systems.
TN-systemsConnected L- (PE)N or L-NConnected N-PEConnected L-L
Connected L-PEConnected L-NConnected N-PEConnected L-L
Connected L-PEConnected L-NConnected N-PEConnected L-L
Connected L-PEConnected L-(PE) NConnected N-PEConnected L-L
TT-systems
IT-systems
TN, TT and IT-systems
SPDs connected to: Minimum U for 5s:T TOV values for 0,2s:
1,45 U⋅ 0
––
√3 U⋅ 0
1,45 U⋅ 0
––
–1,45 U⋅ 0
––
√3 U⋅ 0
1,45 U⋅ 0
––
–––
1200V + U0
–1200V
–
1200V + U0
1200V + U0
1200V
1200V
–
–
–
–
Nominal a. c. voltage of the system U0
U0 is the nominal line to neutral voltage of the a. c. system (rmsvalue).
Continuous operating current Ic
The current flowing through the arrester when energized at the maximumcontinuous operating voltage Uc.
Follow current If
Current supplied by the electrical power system and flowing through thearrester after a discharge current impulse.Note: the follow current is significantly different depending on the designof the arrester. For MO-surge arresters without gaps the follow current isgenerally in the range of some 10 mA in maximum.
Reference current of an arrester Iref
The reference current is the peak value of the resistive component of apower frequency current used to determine the reference voltage of thearrester. The reference current should be high enough to have a cleardominating resistive component, so that capacitive influences can beneglected. The reference current is specified by the manufacturer, andgenerally in the range of 1 mA to 10 mA, depending on the cross sectionof the MO-resistor used in the arrester.
Reference voltage of an arrester Uref
The reference voltage of an arrester is the peak value of the power fre-quency voltage divided by √2 which has to be applied to the arrester toobtain the reference current Iref.The reference voltage at a given reference current is used to determinea point on the u-i-characteristic of an arrester in the low current range.
Voltage protection level Up
A parameter that characterizes the performance of the arrester in limit-ing the voltage across its terminals, which is selected from a list of pre-ferred values. This is generally the guaranteed value given by the manu-facturer.
Residual voltage Ures
The peak value of voltage that appears between the terminals of the arre-ster due to the passage of discharge current.
Protection ratio Up / Uc
The protection ratio gives the relation between the voltage protectionlevel Up at In and the maximum continuous operating voltage Uc. Up isgiven as a peak value and Uc is given as a rms value. The lower the ratioUp / Uc , the better the protection given by the arrester.
Temporary overvoltage The maximum a. c. (rms) or d. c. overvoltage that exceeds the maxi-mum continuous operating voltage of the network for a specified timeduration.Note: It has to be made a clear distinction between the temporary over-voltage UTOV occuring in the network at a given location, and the tempor-ary overvoltage UT an arrester can withstand. The power frequency volta-ge versus time characteristics of an arrester (TOV-characteristic), provi-ded on request by the manufacturer, is in low-voltage systems normallyused only in case of special applications of the arrester.
Combination waveThe combination wave is delivered by a generator that applies a1,2 / 50µs voltage impulse across an open circuit and an 8/20µs cur-rent impulse into a short circuit. The voltage, current amplitude andwaveforms that are delivered to the arrester depend on the impedanceof the arrester to which the surge is applied.
Surge arresters which contain only spark gaps, or spark gaps with non-linear resistors in series, have the disadvantage that the voltage collap-ses suddenly when the sparkover-voltage of the device is reached. Thisvery high du/dt may cause EMC problems in data-lines which are closeto the power lines, or lead to failures in inductive loads. Furthermore, thespark-over voltage depends on the steepness of the overvoltage.Because the spark gap fires only at very high voltage levels, it can hap-pen that overvoltages bypass the surge arrester, and downstreamconnected instruments or installations are over-stressed.Surge arresters containing only MO-resistors have no sparkover-voltage.The turn on time is in the range of 15 ns, and the voltage is limited accor-ding to the extremely nonlinear voltage-current characteristic of the MO-material. A bypassing of these arresters is not possible.
The advantages of MO-surge arresters are mainly the constant low pro-tection level independent on the steepness and polarity of the incomingsurge, the very good ageing behaviour, and the high energy capability.Possibilities of coordination of parallel MO-surge arresters are describedin chapter 8.
4.2 Definitions
In the new standard family of IEC 61643 the special requirements forsurge arresters for application in low-voltage power systems are consi-dered. In the following, the most important definitions are given with refe-rence to [9], concentrating on MO-surge arresters without gaps.For the purpose of this guidelines some definitions with reference to [11]are added.
The surge arresters addressed in this guidelines are to be connected to50 / 60 Hz a. c. and d. c. power circuits, and equipment rated up to1000 V a. c. (rms) or 1500 V d. c.
Surge Protective Device (SPD)A device that is intended to limit transient overvoltages and divert surgecurrents. It contains at least one nonlinear component.Note: as mentioned above, in the course of this guidelines this is thesame as a surge arrester, or short arrester.
Nominal discharge current In
The crest value of the current through the arrester having a current waveshape of 8 /20µs. This is used for the classification of the arrester forclass II test and also for preconditioning of the arrester for class I and II tests.
Impulse current Iimp
It is defined by a current peak value Ipeak and the charge Q, tested accor-ding to the test sequence of the operating duty test. This is used for theclassification of the arrester for class I test. A typical waveshape that canachieve the parameters is that of a unipolar impulse current with awaveshape of 10 / 350µs. An other waveshape or impulse combinationis acceptable, as long as they obtain the peak value Ipeak within 50µs andthe charge Q within 10 ms.
Maximum discharge current Imax for class II testCrest value of a current through the arrester having a 8/20µs waveshape and magnitude according to the test sequence of the class II ope-rating duty test. Imax is greater than In and declared by the manufacturer.It is used in the operating duty test to prove the correct function and ther-mal stability of the arrester.
Maximum continuous operating voltage Uc
The maximum a. c. (rms) or d. c. voltage which may be continuouslyapplied to the arresters terminals. This is equal to the rated voltage.
9
Thermal runawayAn operational condition when the sustained power dissipation of anarrester exceeds the thermal dissipation capability of the design, leadingto an increase in the temperature of the internal elements culminating infailure.
Thermal stability An arrester is thermally stable if after an energy input causing a tempe-rature rise the temperature of the arrester decreases with time underapplied continuous operating voltage.
DegradationThe change of original performance parameters as a result of exposureof the arrester to surges, service or unfavourable environment.
DisconnectorA device for disconnecting an arrester from the system in the event ofarrester failure. It is to prevent a persistent fault on the system and to givevisible indication of the arrester failure.
Type testsTests which are made upon the completion of the development of a newarrester design. They are used to establish representative performance andto demonstrate compliance with the relevant standard. Once made, thesetests need not to be repeated unless the design is changed so as to modifyits performance. In such a case, only the relevant tests need to be repeated.
Routine testsTests made on each arrester or parts of it to ensure that the productmeets the design specifications.
Acceptance testsTests which are made when it has been agreed between the manufactu-rer and the purchaser that the arrester or representative samples of anorder are to be tested.
4.3 Classification
In [9] the surge protective devices (or short arresters) are classifiedaccording– the number of ports (one or two)
A one port device has two terminals, a two port device has four termi-nals. The two port device may contain internal decoupling elements.
– the design topology (switching type, limiting type, or combination type)– the test method (class I, class II, or class III test method)– the location (outdoor or indoor)– the accessibility (accessible or out-of-reach)– the mounting method (fixed or portable)– the disconnector (with or without)– the backup overcurrent protection (specified or not specified)– the temperature range (normal or extended)As long as the arresters are installed at different locations in a system orinstallation the stresses to be expected are very different. Therefore, thearresters are classified with respect to the expected stresses, and con-sequently the test methods, in three classes. See Table 6.
The class I test is intended to simulate partial conducted lightning cur-rent impulses. Arresters subjected to class I test methods are generallyrecommended for locations at points of high exposure, e.g. line entran-ces to buildings protected by lightning protection systems (LPS).These devices are called lightning current arresters.In addition to nominal discharge current In, information is required for theimpulse current Iimp.
10
Arresters tested according to class II test methods are generally subjec-ted to impulses of shorter duration than class I arresters. The typicalapplication is the overvoltage protection of low-voltage overhead linesand cables, as well as the protection of indoor installations. The expec-ted stresses are originated by direct or indirect lightning to overheadlines or cable junctions.Required information is the nominal discharge current In and the maxi-mum discharge current Imax.
Arresters tested according, to class III test methods are subjected toimpulses of lesser energy content than class I and class II arresters. Theyare recommended for locations with less exposure, mainly indoor.The information required is the open-circuit-voltage Uoc of the combina-tion wave generator.
4.4 Service conditions
The normal service conditions are – the applied continuous voltage between the terminals of the arrester
should not exceed the maximum continuous operating voltage Uc
– frequency between 48 Hz and 62 Hz a. c., or d. c. voltage– altitude up to 2000 m– operating and storing temperatures
normal range: - 5 °C to + 40 °Cextended range: - 40 °C to + 70 °C
– relative humidity up to 90 % for indoor temperature conditions
Exposure of the arrester to abnormal service conditions may require spe-cial considerations in the design or application of the arrester, and shouldbe called to the attention of the manufacturer. Abnormal conditions maybe extreme ambient temperatures (minus or plus), mechanical stresses,shock and vibration, etc.
For outdoor arresters exposed to solar radiation, air pollution, bad weat-her conditions, additional requirements may be necessary.
5 Low-voltage MO-surge arresters from ABB
A MO-surge arrester is made of two parts: the active part, which consistsof a MO-resistor, and an insulating housing including the terminals.
5.1 MO-resistors
The voltage-current (u-i) characteristic of a metal oxide resistor is extre-mely nonlinear. That is the reason why arrester designs without sparkgaps are possible [1], [10]. Figure 10 shows a typically u-i-characteristicof a MO-surge arrester with In = 10 kA. The voltage is normalized to theresidual voltage at In.
lightning currentarresters
lightning protectionin connection withlightning protection
structures
I (10 / 350 µs)imp
1 kA … 20 kA
surge arrester
overvoltage protectionenergy supply
I (8 / 20 µs)max
> 0,05 kA … 50 kA
surge arrester
overvoltage protection“down stream”
Uoc
(2 Ω)
Class I Class II Class III
Table 6.
Classification of low-voltage surge arresters. The given values are typical ratings.
All used materials are UV-resistant and perform well under extreme wea-ther conditions. Safety and ecological aspects are specially taken intoconsideration with all arresters.Figures 11 to 14 show a selection of different types of MO-surge arre-sters from ABB.
The diameter of the MO-resis-tors decides the carrying capacity of thecurrent, the height of the voltage, and the volume of the energy capa-city. Table 7 shows the main data of the MO-resistors. For low-voltageapplication the same high-quality MO-material is used as for distributi-on and high voltage application.MO-resistors are compressed and sintered in the form of round blocksof different metal oxides in powder form.The diameters of the MO-resistors from ABB for low-voltage applicationare between 30mm and 75mm, covering even the highest energy require-ments. The height of the blocks is between 1 mm and 10 mm, covering avoltage range from 120 V a. c. to 1500 V d. c. For special applicationsMO-resistors with a rectangular shape can be produced.
5.2 MO-surge arresters
As long as very different applications and ratings for low-voltage surgearresters exist, different designs are needed.ABB offers a great variety of different arrester types for all kind of appli-cations.The main design principle is always the same: A MO-resistor, as theactive part, and the terminals are moulded completely in an insulatinghousing.Depending on the application and rating of the arresters the physicalshape and housing material may be different. The general, surge arre-sters for outdoor application (e. g. overhead lines, MV / LV transformers)have a housing of polyamide; arresters for outdoor and indoor applicati-ons (e. g. railway applications) have a housing of silicon, and arresters ofolder design have housings of PUR. All arresters are moulded to be com-pletely sealed and waterproof.
11
U
[p.u.]
1.0
0
0.5
10-4
10-3
10-2
10-1
100
101
102
103
104
4/10µs1/5µs
30/60µs2000µs
I [A]
8/20µs
Figure 10
Normalized voltage-current-characteristic of a MO-surge arrester with In = 10 kA.
Table 7
Main data of ABB MO-resistors used in ABB MO-surge arresters for low-voltage applicati-
on. The values are given as tested in the operating duty test to prove the thermal stability
of the respective surge arrester. Other values are possible in other arrester designs.
Diameter of blocksin mm
Nominal currentI 8 /20 µs in kAn
I 8 /20 µs acc.class II test in kAmax
I (10 /350 µs) acc.class I test in kApeak
Energy capabilityin kJ /k VUc
30
5
25
–
2,5
41
10
40
–
4,0
47
10
32
–
4,5
75
10 /20
50
10
12,0
Figure 11
MO-surge arrester type LOVOS.
This type was developed for outdoor application and can be used under all weather condi-
tions. It is available with In = 5 kA or 10 kA, with or without disconnector.
Uc = 280 V, 440 V and 660 V.
Figure 12
MO-surge arrester POLIM-R.
Very high energy capability. Can be used for a. c. and d. c. networks. This type is, besides
other applications, used in d. c. railway networks. Uc range from 140 V d.c. to 1000 V d. c.,
and 110 V a. c. to 780 V a. c.. Tested according test class I and test class II.
Figure 13
MO-surge arrester MVR.
Used in low-voltage systems and railway equipment. For a. c. and d. c. application.
Available for In = 5 kA and 10 kA, with Uc = 440 V, 660 V and 800 V.
5.3 Technical data of the arresters
Table 8 presents main electrical data of the arresters. The ratings aregiven according to [9], see also the definitions in chapter 4.2. All descri-bed MO-surge arresters are of the voltage limiting type. The energycapability, as given in the table, is the value as tested in the operatingduty tests to prove the thermal stability of the arrester with the maximumcontinuous operating voltage applied. It is not the limiting value thatwould destroy the arrester.
6 Tests
All tests for ABB low voltage arresters follow internationally agreed uponrecommendations. For low voltage arresters in power systems the inter-national standard IEC 61643-1 [9] is valid. For some special cases, forinstance surge arresters for railway systems with d. c. voltage, otherstandards are applicable [2].
12
The standard IEC 61643-1 does not mention a high current impulse witha waveshape of 4/10µs and a rectangular current with a time durationof some ms. The high current impulse 4/10µs, as known from IEC60099-4 [11], was intended to represent a severe direct lightning to theline very close to the arrester location. Direct lightning, and the relevantparameters, are covered more realistically by the impulse current Iimp,which is used for testing lightning current arresters (class I test).Rectangular currents are generated by discharges of a loaded transmis-sion line of typically some hundred km of length. Such a current wave-shape, coming from a line discharge, is not relevant for low-voltage networks.
6.1 Type tests
Type tests are performed after completion of the design to prove the per-formance and specified characteristics of the product. The type tests aredescribed in detail in the relevant standards. In the frame of this guideli-ne, the main electrical tests for MO-surge arresters without gaps for out-door application are described briefly. In general each test series is per-formed on three new test samples. The tests are performed in free air atroom temperature (20 °C ± 15 °C)
Test procedure to measure the residual voltage with 8/20µs current impulsesThe voltage-current characteristic of the MO-surge arrester is measuredwith 8 / 20µs current impulses in the range 0,1 to 2 times In. The resultis given in form of a table or curve to show the protection performancedepending on the current magnitude.
Operating duty testThe operating duty test has two parts: the preconditioning and the evi-dence of the thermal stability of the MO-surge arrester. It is a test inwhich service conditions are simulated by the application of a stipulatednumber of specified impulses to the MO-surge arrester while it is ener-gized at the maximum continuous operating voltage Uc.For the preconditioning test, 15 times In in three groups of five impulseseach, are applied to the test samples which are energized at Uc. Eachimpulse shall be synchronized to the power frequency. Starting from 0°the synchronisation angle shall be increased in steps of 30° intervals.The interval between the impulses is 1 min; the interval between thegroups is 25 to 30 min. For practical reasons it is not required that thetest sample is energized between the groups.In the operating duty test itself, e.g. to prove the thermal stability, the testsample is energized at Uc, and current impulses up to Ipeak (test class I) orImax (test class II) are superimposed. The power frequency voltage isapplied for 30 min after each impulse to prove the thermal stability. Thesuperimposed current impulses should be of positive polarity and initia-ted in the corresponding positive peak value of the power frequency vol-tage. The value of the current impulse is increased from 0,1 to 1,0 Ipeak orImax. The intermediate values are 0,25; 0,5 and 0,75 Ipeak or Imax.The arresters have past the test if thermal stability was achieved and theresidual voltage at In measured before and after the test sequence hasnot changed by more than ± 10 %.
Disconnector testsArresters with an integrated or external disconnector are tested togetherin the operating duty test. During the complete sequence of preconditio-ning procedure and operating duty test the disconnector remains non-functioning.
Thermal stability test (of disconnector)This test shows the disconnecting characteristic and the safety perfor-mance of overstressed surge arresters with disconnectors. The arresterwith the disconnector is heated electrically with constant current untill
Figure 14
MO-surge arrester MVR...ZS.
For low-voltage systems. Only indoor application. Suitable for fixing on DIN racks.
In = 5 kA, Uc = 140 V, 250 V and 440 V.
Table 8
Electrical main data of the ABB surge arresters for low-voltage systems. The arresters of
type POLIM-R have been tested according both class I and class II tests. The arresters of
type MVR and POLIM-R can be used in d. c. systems as well, see chapter 9.
Arrester Type I I EnergyCapability
8/20 µs 8/20 µs kJ/kVkA
Test class II I I Energy8/20 µs capability
special applications kA kA kJ/kV
n U /Up c
U /Up c
U /Up c
max
Uc
n max
Uc
Test class IIfor a. c. systems kA
for a. c. systems and 8/20 µs
Test class I I I (10/350 µs)for a. c. systems and 8/20 µs I Charge Qspecial applications kA kA As
n imp
peak
LOVOS - 5 4,1LOVOS - 10 10 4,1 40 4,0POLIM-R...1N 10 3,1 50 12,0POLIM-R...2N 20 3,1 100 24,0
MVR...-5 5 3,5 15 3,0MVR...-10 10 3,64 4,5MVR...ZS 5 3,5 15 3,0
POLIM-R...1N 10 3,1 10 5POLIM-R...2N 20 3,1
5 25 2,5
32
20 10
thermal equilibrium is reached or the disconnector operates. If the dis-connector functioned, there should be clear evidence of effective andpermanent disconnection by the device. The surface temperature of thedevice during the entire test should be below 120 °C, and there shouldbe no evidence of burning or ejected parts. The pass criteria depend onthe classification of the arrester, e. g. whether it is indoor, outdoor, acces-sable or not accessable.
6.2 Special tests
Additionally to the type tests given by the applicable standard, it may benecessary to conduct tests covering special requirements, (i. e. long termbehaviour of the MO-material or the behaviour of the housing materialunder severe weather conditions).
Accelerated ageing testThis test has to show that the power losses of the arrester in the networkunder applied continuous operating voltage does not increase with time.An increase of the power losses would lead with time to a thermal runa-way, and consequently to a failure of the arrester.In the accelerated ageing test the complete arrester is to be tested underincreased stress, e. g. under increased ambient temperature of +115 °C. During the whole test period of 1000 h the power losses aremeasured. It is vital that the power losses do not increase with time, butremaining constant at the lowest reached level. Because the materialaround the MO-resistor may influence its long term performance, it isimportant that the complete surge arrester is tested and not only the MO-resistor. The test has to be performed with power frequency voltage forsurge arresters with a.c. systems, and with d.c. voltage for surge arrestersfor application in d.c. systems. Ageing tests carried out with a.c. voltageare not transferable to the application in d.c. networks. The acceleratedageing test is performed with reference to the test procedure given in [11].
All ABB MO-resistors or MO-surge arresters, which are to be installed ind. c. networks, fulfill the most strict demands towards the long-term sta-bility under d. c. voltage stress.
UV radiation testIn regions with strong solar radiation it is important to determine thebehaviour of polymeric materials under UV radiation stress. The energyof the radiation can crack the surface of the insulator made of a synthe-tic material, and as a result the insulator may erode and finally fail. ABBsurge arrester housing materials (silicon, polyamide and PUR) have suc-cessfully withstood UV radiation tests with time duration of 1000 h.
Water immersion testThis test is performed to show the tightness of design against water per-meation. It is performed with reference to [12]. The test samples are keptin a vessel with deionized boiling water with 1 kg / m3 NaCl for 42 hours.
6.3 Routine tests
Routine tests are carried out on every arrester or parts of it (e. g. on theMO-resistors) in order to ascertain that the product meets the require-ments of the design specification. The test method and the pass criteriaare declared by the manufacturer.
All above mentioned MO-surge arresters for low-voltage applicationmade by ABB are tested to 100 % in the routine test. On each arresterthe reference voltage Uref is measured at the declared reference currentIref. Additional the arresters are checked to be free of internal partialdischarges or contact noise.
13
To ensure the long term stability of the MO-resistors, from each produ-ced batch two MO-resistors are taken and tested in a time-reducedaccelerated ageing test.
6.4 Acceptance tests
Acceptance tests are made upon agreement between manufacturer andcustomer. If acceptance tests are agreed upon they are then to be per-formed on the nearest lower number to the cube root of the number ofarresters to be supplied.
If not otherwise specified, the following acceptance tests are performed:– verification of identification by inspection– verification of marking by inspection– verification of electrical parameters, for instance repetition of routine
tests.
7 Selection of MO-surge arresters
For selecting a MO-surge arrester three main electrical parameters haveto be evaluated:
– continuous operating voltage Uc
– voltage protection level Up
– energy capability
Additionally we need to be informed which modes should be protected.Table 9 shows the possible modes of protection, depending on the earth-ing practise in the low-voltage network.Depending on the application and the environment it has to be decidedwhether a disconnector is needed, which mechanical requirements needto be fulfilled (vibration and shock resistant, other mechanical stresses),and which ambient conditions have to be considered (increased tempe-rature, solar radiation, rain, saltfog, etc.).
7.1 Selection of Uc
The maximum continuous operating voltage Uc of an arrester has to beselected with respect to the power frequency voltages which can occurein the low-voltage system. Maximum system voltage has to be conside-red and possible temporary overvoltages in the network.
Uc shall be equal or higher than the maximum power frequency voltageUcs occuring in the system.
Uc ≥ Ucs
SPD connectedbetween: TT TN-C TN-S IT
Line and neutral X
Neutral and PE
X X*
Line and PE X X X
Line and PEN X
X X X*
Line to line X X X X
Power system type
* When the neutral is distributed
Table 9
Possible protection modes in low-voltage systems.
The temporary overvoltage withstand capability UT of the arrester has tobe higher than the temporary overvoltage UTOV coming from the system.
UT > UTOV
If a transformer failure occures in a solidly earthed MV system, a tem-porary overvoltage can result in a UTOV of up to 1200 V in the LV system.It may be impossible to find surge arresters providing acceptable protec-tion. In such cases surge arresters have to be used which have safe over-load conditions.
Considering an upper tolerance in the system voltages of 10 %, seechapter 3.1 and [7] the continuous operating voltage of the arrestershould be chosen to
Uc ≥ 1,1 × UN for arresters to be connected line-to-line
and
Uc ≥ 1,1 × UN /√3 for arresters line-to-neutral or line-to-earth.
As standard values (preferred values) for the system voltages 220/380 V,230/400 V, 240/415 V (Table 3) the following values for Uc are proposedfor outdoor application on overhead lines:
Uc = 280 V for the protection phase to neutral and neutral to earth (TT and TN systems)
Uc = 440 V for the protection phase to neutral and neutral to earth (IT system)
Uc = 440 V for protection phase to phase (TT, TN, IT systems)
MO-surge arresters with the above given Uc values will cover almost allpossible temporary overvoltages in the low-voltage network with suffi-cient safety margin, providing in the same time a good protection ratioUp / Uc.
7.2 Selection of Up
The purpose of surge arresters is to protect an installation or a specificpiece of electrical equipment against overvoltages.Overvoltages can destroy the insulation of the installation or connectedelectrical equipment like transformers, cables, motors, etc., and they canlead to malfunction or destruction of connected electronic equipment.
The protection level Up of the arrester has to be below the voltage with-stand capability of the equipment to be protected.The required impulse withstand voltages for the four insulation catego-ries are given in Table 4. For category IV (fixed outdoor installation, forinstance from the LV-bushing of a MV / LV transformer via an overheadline to a building) 6000 V impulse withstand is required. Comparing thisvalue with the voltage protection level Up = 1800 V of a MO-surge arre-ster with In = 10 kA, for instance type LOVOS with Uc = 440 V (Table 8),shows the excellent protection of the insulation provided by the arrester.
However, it has to be considered, that the distance between the arresterand the equipment to be protected (e.g. a transformer or a meter in thebuilding) has a great influence on the overvoltage occuring at the equip-ment to be protected. This is known as the protective distance of thearrester [1]. As a rule of thumb it can be said, that the arrester should beinstalled as close as possible to the equipment to be protected.
14
In cases when the insulation withstand capability is lower than the valuesgiven in Table 4, or the overvoltage should be limited to a particular valueto protect sensitive equipment in a special application, then the voltageprotection level Up has to be calculated case by case.
As a general rule the voltage protection level Up of the arrester and themaximum allowed impulse voltage at the point of protection should havea safety margin of at least 20 %.
An important parameter to characterize a surge arrester is the ratio bet-ween the voltage protection level Up and the maximum continuous ope-rating voltage Uc. This ratio Up / Uc depends on the technology used and,in case of MO-surge arresters, on the diameter of the MO-resistors andthe nominal current In.For MO-surge arresters available today on the market typical values ofUp / Uc are in the range from 3 to 5.The lower the ratio Up / Uc of an arrester, the higher the provided pro-tection level against overvoltages. Good engineering design is requi-red when arresters are connected in parallel with coordinated arre-sters.
7.3 Selection of the energy capability
The energy capability of arresters is in principle defined by the nominaldischarge current In and the impulse current Iimp for class I arresters or Imax
for class II arresters. According Table 6 the arrester has to be chosen withrespect to the place of installation and the expected stresses or surges.For class II arresters typical values for the nominal current are In = 5 kAor 10 kA. As long as there is no fixed relation given between the nominalcurrent In and the maximum discharge current Imax, both values have to bespecified.
From lightning statistics [5] it is known that about 95 % of the lightningcurrents have peak values of up to 14 kA, and 5 % up to 80 kA.Considering that in distribution and low-voltage systems a direct light-ning will struck not only one phase, but all three (due to the shortdistance between the phases), and that the current from the lightning willtravel in both directions of the line, the lightning current can be dividedby 6 (as a first approximation). With this we result for the 95 % value at2,3 kA, and for the 5 % value at 13 kA as a peak value for one phase(e. g. one arrester).
Comparing this values with the technical data of the arresters ( Table 8),an arrester with In = 5 kA (covering 95 % of the events), and Imax = 25 kA(covering the very rare 5 % values) is fully complying with the occuringstresses.
Therefore, as a standard type for outdoor application on overhead linesan ABB arrester of type LOVOS-5 is proposed with
In = 5 kA and Imax = 25 kA.
If higher stresses from lightnings are expected, or for regions with veryhigh isokeraunic level, an ABB arrester of type LOVOS-10 is proposedwith
In = 10 kA and Imax = 40 kA.
Wherever an arrester is used to protect equipment which can store ener-gy, as for instance capacitor-banks, cables, inductances in filters, etc. themaximum energy stored in these elements should be used to determinethe right arrester.
15
8 Coordination of surge arresters
The energy capability of MO-arresters can be increased by connectingMO-resistors in parallel [13]. Using identical u-i- characteristics of theMO-resistors, an even current sharing (and energy sharing) can be rea-ched. This is possible due to an exact classification of the MO-resistorsduring the routine test of the MO-resistors.It is possible to connect two or more MO-resistors in parallel internally ina MO-surge arrester, or connect two or more MO-surge arresters in par-allel thereby increasing the energy capability of the devise. In the lattercase the MO-surge arresters have to be installed close to one another toavoid decoupling effects.
An other possibility of coordinating MO-surge arresters is shown inFigure 15. Three MO-surge arresters with slightly different u-i-characte-ristics are coordinated in such a way that the arrester A1 has the highestenergy capability and the lowest voltage protection level Up, arrester A2has a lower energy capability than A1 but a higher Up, and arrester A3has again a lower energy capability and a higher Up than A2.
In Figure 16 the u-i-characteristics of the arresters A1 to A3 are given.Under the same overvoltage stress from incoming surge arrester A1 willconduct most of the current to earth at the entrance of the installation,where as the arresters A2 and A3 will recieve a much lower stress,keeping the occuring overvoltage in the whole installation on a low level.
Figure 17 illustrates the protection principle. The given values in Figure17 are measured results from a realized installation in a civil defenceconstruction in Switzerland [14]. Starting with an injected impulse cur-rent of approximately 23 kA (28 / 50µs) at the entrance of the installati-on close to arrester A1, in the first distribution box a current 0,4 kA wasmeasured, in the second distribution box a current of 0,08 kA and in thejunction box almost no current. The residual voltage Ures was in the wholeinstallation kept below 1,6 kV.This example shows the effectiveness of a protection concept withseveral steps in an energy supply, realized with MO-surge arresters withcoordinated u-i-characteristics.
Buildings and structures equipped with lightning protection systems(LPS), as for instance franklin rods, need special protection measures. Itis generally assumed that a direct lightning hits the LPS, and that part ofthe lightning current is transferred into the structure or the building. Insuch cases the concept of Lightning Protection Zones (LPZ) has to beconsidered. The LPZ concept is described for instance in [15] andvarious other publications.The concept of LPZ requires that surge arresters are installed, wheneveran electrical line crosses the boundary between two zones. These surgearresters have to be well coordinated to effectively reduce the lightningthreat down to the surge withstand capability of the equipment to be pro-tected. For this LPZ concepts, which are realized inside buildings, diffe-rent types of arresters are used, as spark gaps, gas discharge tubes,varistors (MO-resistors), diodes, and combinations of these. The princi-ple is the same as mentioned above, the energy content of the surge hasto be reduced step-by-step with cascaded surge protective devices. Thewhole structure is subdivided into a series of LPZs, thus successivelyreducing the interference level from the primary lightning threat down tothe basic immunity of the electronic equipment.
Installation protection
Terminal box
Incomingcurrentsurge
Energy-consumer
M
Distribution box Junction box
Consumerprotection
Public energyisupply
A1 A2 A3
102 2
1,3
1,2
1,1
1,0
0,9
0,8
0,7
p.u.
Ures
U ( I )p n
5 103 2 5 104 32 5 105 A 2
I ( A3 )n I ( A2 )n
I ( A1 - 1 )n
I
I ( A1 - 2 )n
A1
-2-1
A2A3
Terminal box Firstdistribution box
Seconddistribution box
Junction box
A1 - 2 A2
L = 15 m
Ures1 Ures2 Ures3 Ures4
U5i1
i1 22 kA i2 0,4 kA i3 0,08 kA i4 0 kA
Ures1 1,45 kV Ures2 1,5 kV Ures3 1,1 kV Ures4 1,4 kV U5 1,6 kV
i2 i3 i4
L = 24 m L = 18 m L = 3 m
A2 A3
Figure 15
Components and borders in an EMP-protection system with coordinated MO-surge arre-
sters. A1, A2, A3, see Figure 16.
Figure 16
Coordination of u-i-characteristics of different dimensioned MO-surge arresters. The used
arresters in this coordination concept are (Table 8):
A1-2: POLIM-R...2N with In = 20 kA; Up / Uc = 3,1; E` = 24 kJ / kVUc
A1-1: POLIM-R...1N with In = 10 kA; Up / Uc = 3,1; E` = 12 kJ / kVUc
A2: MVR...-5 with In = 5 kA; Up / Uc = 3,5; E` = 3,0 kJ / kVUc
A3: MR...ZS with In = 1 kA; Up / Uc = 4,25; E` = 0,6 kJ / kVUc
The arresters of type POLIM-R and MVR are standard arresters producted by ABB. The type
MR...ZS was specially developed according to existing special requirements for the men-
tioned coordination concept.
Figure 17
Current distribution in a NEMP-protected civil defence construction. Measured values of i
and U. Arresters A1-2 to A3 see Figures 15 and 16. The injected current (incoming surge)
had a peak value of approximately 23 kA.
16
9 MO-surge arresters for d. c. systems
MO-surge arresters without spark gaps are especially suitable for appli-cation in low-voltage d. c. systems, because they do not conduct any fol-low current like spark gaps. Due to the extreme nonlinear u-i-characte-ristic of the MO-resistors the current after limiting the overvoltage isimmediately again in the range of less than 1 mA. It is not necessary toextinguish any d. c. current arc.
Tests for surge arresters for d. c. application are under discussion in work-ing group 5 of IEC SC 37A. For the time being the type tests for a. c.application apply. In special cases the purchaser should contact themanufacturer for clarification.
As pointed out in chapter 6.2 it is very important to ensure that the MO-resistors used in surge arresters for d. c. application are tested in theaccelerated ageing test with d. c. voltage. MO-resistors for a. c. applica-tion are not generally long term stable under d. c. voltage stress.
Most of the d. c. current networks are railway networks. Arresters for usein railway networks are described in [2]. Other examples of d. c. applica-tions are in power electronics, chemical industry and data transmission.
D.C. voltage can be subjected to superimposed voltage peaks (i. e. fromcommutation of converter stations) and may have strong voltage fluctua-tions, presenting a difficult determination of continuous operating volta-ge Uc. The selection of MO-surge arresters for d. c. railway application isgiven in [2]. For other d. c. applications the user should contact themanufacturer for selecting the right surge arrester.
Table 10 gives the main electrical data of ABB MO-surge arresters foruse in d. c. systems.
10 Installation of surge arresters
National requirements and regulations apply to the installation of surgearresters. Surge arresters for outdoor application are in most cases outof reach. The IP degree of the surge arresters depends on the accesso-ries used.
ABB offers a variety of accessories for different methods of installation,including fully insulated connections. For details please refer to ABB.
As a general rule the surge arresters should be installed avoiding con-stant mechanical stresses on the terminals. One terminal should beconnected with a flexible lead. It is not important whether this is the vol-tage side or the earth side, though normally, the earth connection is fle-xible.
As long as MO-surge arresters have a symmetrical characteristic it is,from the electrical point of view, not important which terminal is connec-ted to the voltage and which to the earth.
For optimal protection, the arrester should be installed as close as pos-sible to the equipment to be protected, with connections as short andstraight as possible.
Arrester Type I U / U ,d. c. . I EnergyCapability
8/20 µs 8/20 µs kJ/kV d.c.kA
n p c
U / U ,d. c.p c
max
UcTest class IIfor d. c. Systems kA
Test class I I I (10/350 µs)for d. c. systems 8/20 µs I Charge Q
kA kA As
n imp
peak
POLIM-H...ND 20 2,7 50 6,0
POLIM-R...2ND 20 2,4 100 12,0MVR...5 5 2,8 15 2,4MVR...10 10 2,9 32 3,6
POLIM-R...1ND 10 2,4 10 5POLIM-R...2ND 20 2,4
POLIM-R...1ND 10 2,4 50 6,0
20 10
Table 10
Electrical main data of the ABB MO-surge arresters for the application in d.c. networks. The
type POLIM-H...ND is mechanically a very strong arrester, especially used in railway systems.
The maximum continuous voltage Uc for the types POLIM-R...ND ranges from 140 V to
1000 V, offering a large variety of applications.
[1] Application Guidelines Overvoltage ProtectionDimensioning, testing and application of metal oxide surge arresters in medium voltage networks, 3rd revi-sed edition July 1999. ABB High Voltage Technologies Ltd. Wettingen / Switzerland
[2] Application Guidelines Overvoltage ProtectionDimensioning, testing and application of metal oxide surge arresters in railway facilities, 1st edition June2000. ABB High Voltage Technologies Ltd. Wettingen / Switzerland
[3] J. Huse; Compact Course: Lightning Surge Protection in Low Voltage Electric PowerDistribution Systems, Including Consumers Installations and Equipment.V International Symposium on Lightning Protection,Sao Paulo – Brazil, May 17th – 21st 1999
[4] C. Mirra, A. Porrini, A. Ardito, C.A. Nucci; Lightning Overvoltages in Low Voltage Networks. 14th CIREDConference, Birmingham, U.K., June 1997
[5] Joint CIRED/CIGRE Working Group 05: Protection of MV and LV networks against lightning. Part I: BasicInformation. 14th CIRED Conference, Birmingham, U.K., June 1997
[6] International Standard IEC 60664-1, Edition 1.1 (2000-04); Insulation coordination for equipment withinlow-voltage systems – Part 1: Principles, requirements and tests
[7] CENELEC publication HD 472 S1 (1988); Nominal voltages for low voltage public electricity supply systems
[8] International Standard IEC 60364-4-442 (1993-03); Electrical installations of buildings – Part 4:Protection for safety – Section 442: Protection of low-voltage installations against faults between high-vol-tage systems and earth
[9] International Standard IEC 61643-1, First edition, 1998-02; Surge protective devices connected to low-voltage power distribution systems- Part 1: Performance requirements and testing methods
[10] F. Greuter, R. Perkins, M. Rossinelli, F. Schmückle; The metal-oxide resistor – at the heart of modern surgearresters. ABB Review 1/89
[11] International standard IEC 60099-4: Surge arresters – Part 4 : Metal-oxide surge arresters without gapsfor a. c. systems
[12] Amendment 2 to IEC 60099-4; IEC TC 37/231/CDV
[13] B. Richter, W. Schmidt, K. Tanner; Protection against high energy surges with MO-surge arresters: a newconcept for low voltage systems. 10th International Zurich Symposium on electromagnetic compatibility1993. Paper 70K5, pages 383 to 388.
[14] K. Tanner, P. Bertholet, B. Richter, W. Schmidt; NEMP-Protection in the Energy-Supply of Civil DefenceConstructions. Federal Office of Civil Defence, Material Division. CH-Bern, August 1992.
[15] P. Hasse, P. Zahlmann, J. Wiesinger, W. Zischank; Principle for an advanced coordination of surge protec-tive devices in low voltage systems. 22nd ICLP 1994, Budapest, paper R5-04.
17
Bibliography
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ABB High Voltage Technologies LtdDivision Surge ArrestersJurastrasse 45CH-5430 Wettingen 1SwitzerlandTel.: ++41 56 / 205 29 11Fax: ++41 56 / 205 55 70
Creation ZHPrinted in Switzerland 2001-06
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