—
A P P L I C AT I O N N OT E 2 .0
Transformers
Overvoltage protection
The APPLICATION NOTES (AN) are intended to be used in
conjunction with the
APPLICATION GUIDELINES
Overvoltage protection
Metal-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
3
O V E R V O LTA G E P R OT E C T I O N
—
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.
1 Introduction
The most efficient protection against overvoltages 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 bushings. 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 mediumvoltage side (3phase, three-wire system) and the
low-voltage side (3phase, five-wire system).
—
Figure 1: Principle
outline of a mediumvoltage transformer
in star connection
Overvoltage protection has to be considered for:
• The bushings and insulation on the mediumvoltage 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 overvoltages from the medium-voltage side to the
low-voltage side is shown in Figure 3 and described below.
Trafo
L1
i
MV
LV
i
L1
L2
L2
L3
L3
U
N
P
U
Mp, neutral
—
Figure 2: Mediumvoltage transformer
in star connection
Trafo
L1
Mp
L2
L3
neutral arrester
phase arrester
4
A P P L I C AT I O N N O T E T R A N S F O R M E R S
2 Coupling of transient overvoltages
through a transformer
This potential difference is also to be found on
the low-voltage side between the conductor and
the earthing system.
Up to 40% of fast front overvoltages (e.g. lightning 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 directly lightning endangered.
The resistive coupling of the overvoltage in a substation is also to be taken into account. Depending on the execution of the earthing at the
medium-voltage and the low-voltage side, overvoltages 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 coupling
The 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 directly connected at the medium voltage bushing
of the transformer. This arrester may have a
­t ypical 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 transformer is not likely to be endangered, but the
bushings on the low-voltage side and the connected lines can be destroyed or a flash over may
occur.
Resistive coupling
Let us consider the possible resistive transmission of the overvoltage. The lightning current of
In = 10 kA peak value flows according to the Figure 3 through the arrester and over the earthing
resistance RE to earth. If we take a typical earthing resistance of RE = 10 Ω, a temporary potential
rise of the transformer housing of 100 kV occurs.
—
Figure 3: Coupling of a
lightning overvoltage
through a medium-voltage transformer
This very simplified examinations do not provide
an absolute statement about the level of the overvoltages 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 decide which characteristics of the MO surge arresters are most important to provide best protection. 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 arrester 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
LV
MV
C
0.4 Ures
i
Ures
i
RE
i
U
U
5
O V E R V O LTA G E P R OT E C T I O N
Assumptions
First: 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
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.
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
Us
Uc ≥ ---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 kV
Uc ≥ ---- -- --- = 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 Ur
According data sheet the rated voltage is
Ur = 26.3 kV
Step c) Nominal discharge current In
The 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 Wth
Based on the given information “high thunderstorm activity” the type POLIM-K was chosen
­instead of the type POLIM-D.
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 selection of the arrester housing and confirmation of
mechanical data.
Step f) Creepage distance
According to the assumption, there is low pollution (pollution class b-light acc. IEC/TS 60815-1)
to be considered. Therefore the minimum recommended 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.
6
A P P L I C AT I O N N O T E T R A N S F O R M E R 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 P
­ OLIM-K 21-50 has a creepage distance of
745 mm according to the datasheet and offers
large reserves here as well.
Step g) Flashover distance
The 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:
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 housing.
Step h) Consider short circuit rating Is
The 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 loads
Special 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 arrester
One 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 overvoltages by an arrester. Without protection, the
neutral insulation may be overstressed by overvoltages due to lightning events or to asymmetrical faults or switching operations in the power
system.
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 according to IEC.
Taking into consideration the installation height
of 3600 m, it must be checked whether an increasing 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 altitude 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.
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 continuous operating voltage of the neutral arrester
US
Uc ≥ ---------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 write
Uc,neutral = Uc,phase /√3 = 20.7 kV / √3 = 11.95 kV.
From the data sheet follows POLIM-K 12-20.
O V E R V O LTA G E P R OT E C T I O N
7
Step b) Rated voltage Ur
According data sheet the rated voltage is
Ur = 15.0 kV
The proved withstand values according to the
datasheet are:
Lightning discharge voltage 1.2/50 μs: 110 kV.
Step c) Nominal discharge current In
In = 10 kA for type POLIM-K
a.c. voltage test: 50 kV, rms, 1 min. wet.
Step d) Charge and thermal rating Qrs and Wth
Repetitive charge transfer rating Qrs = 1.0 C, and
Rated 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 withstand 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 pollution (pollution class b-light acc. IEC /TS 60815-1)
to be considered. Therefore, the minimum recommended 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 distance
The 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.
Therefore, the housing of POLIM-K 12-20 has
higher withstand values than are required according to IEC.
Taking into consideration the installation height
of 3600 m, it must be checked whether an increasing 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 corrected 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 housing.
Step h) Consider short circuit rating Is
The 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 loads
Special 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 P P L I C AT I O N N O T E T R A N S F O R M E R S
4 Low-voltage arresters
Overvoltage protection in buildings and structures (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 transformer bushings on the low-voltage side, motors,
cable sheath, low-voltage overhead lines etc. are
considered in separate application notes.
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-voltage surge protective devices - Part 11: surge protective devices connected to low-voltage power
systems – Requirements and test methods”, and
other standards of the 61643 series are applicable. Figure 4 illustrates the situation.
Typical low-voltage power systems are threephase four-wire systems with system voltages
230/400 V and 400/690 V.
5 Application considerations
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.
Information about the assembling and installation, maintenance, transport, storage and disposal 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.
—
Figure 4:
Principle of overvoltage protection
in a medium-voltage
and low-voltage
power system.
IEC 60099-series
IEC 61643-series
LPS
Overhead line
LPZ0
MVSA
LPZ1
LVSA
SPDs
MVSA: Medium-voltage MO surge arrester
LVSA: Low-voltage MO surge arrester
SPD: Surge protective device
LPS: Lightning protection structure
LPZ0 and LPZ1: Lightning protection zones
—
Figure 5: Examples of
good and poor connection principles for
MO surge arresters in
distribution systems.
Overhead line
T
T
C
C
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.
1
2
T
C
3
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.
O V E R V O LTA G E P R OT E C T I O N
9
5.1 Connections
At distribution levels the MO surge arresters often 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 inductive voltage drop Ui is to be calculated as
Ui = 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 possible connection principle 3 should be used. In no
case connections as in 1 should be made.
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 information 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:
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.
—
Table 1: Technical data of selected MO surge arresters
Information and assumptions
Technical data
Comments
Phase arresters
Highest system voltage Us = 24 kV, k = √3
Fault clearing time 60 s
Nominal discharge current
LD 2 => old arrester class
Repetitive charge transfer rating
Rated thermal energy
Lightning protection level
Um = 24 kV
Uc = 21 kV
Ur = 26.3 kV
In = 10 kA
new class: SL (station low)
selected POLIM-K 21-50
Qrs = 1.0 C
according data sheet
W th = 5.6 kJ/kV Uc
according data sheet
Upl = 70 kV
according data sheet
LIWV = 125 kV
assumed acc. AN 1.1 A2
All other technical data according datasheet.
Neutral arrester
Uc = 12 kV
Ur = 15 kV
In = 10 kA
class SL
selected POLIM-K 12-20
Qrs = 1.0 C
W th = 5.6 kJ/kV Uc
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.
—
ABB Switzerland Ltd.
PGHV
Surge Arresters
Jurastrasse 45
CH-5430 Wettingen/Switzerland
Tel. + 41 58585 2911
Fax + 41 58585 5570
Email: sales.sa@ch.abb.com
abb.com/arrestersonline
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© Copyright 2018 ABB. All rights reserved
Specifications subject to change without notice
1HC0138866 EN AA
Additional information
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