application note 1.2 continuous operating voltage uc …€¦ · application note 1.2 continuous...

—APPLIC ATION NOTE 1 . 2

Continuous operating voltage UcOvervoltage 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

3OV ER VO LTAG E PR OTEC TI O N

1 Basic considerations

Because the overvoltage levels on distribution systems are not well monitored and, in many cases, not well known they are assumed as worst case during a system earth fault. The worst case voltage rise depends on the system neutral earth-ing configuration and this will impact directly on the selection of the Uc of the MO surge arrester.

The earth fault factor k is the ratio of the highest power-frequency phase to earth voltage ULE , f on a healthy phase during an earth fault to the power frequency phase to earth voltage ULE in absence of the fault at the same location in the system, see Figure 1 and Figure 2. The earth fault factor only refers to a particular point of a three-phase system, and to a particular system condition. This means that different earth fault factors can exist in the same system, depending on the point of failure. The magnitude of the earth fault factor depends on the way the neutrals of a system are earthed.

ULE,fk = ---------- ULE

The earth fault factor is calculated using the com-plex impedances Z1 , Z2 and Z0 of the positive, negative and zero sequence system (symmetric components), taking into account the fault resis-tance R.

A system is considered effectively earthed if the earth fault factor k does not have a value higher than 1.4 anywhere in the system (also described as solid or directly earthed). If the earth fault fac-tor is higher than 1.4 at any point in the system, then this is considered as being ineffectively earthed. In this case the star point is insulated (also described as open) or compensated.

2 Failure mode and TOV considerations

In medium-voltage systems attention must be paid to potential temporary overvoltages UTOV. They occur during earth faults, and they depend on the treatment of the star point of the trans-formers and the system management. Thus, gen-erally the demand for the continuous operating voltage Uc is as follows:

UTOVUc ≥ ---------- T

T is the factor given in the TOV curves, supplied by the manufacturer.

For each of the below cases an earth fault factor k is used to determine the temporary overvoltage rise (TOV) on the system on the unfaulted phases. The voltage rise is the line to earth voltage times the earth fault factor k. The Uc is then selected with the help of TOV data.

The following figures, Figure 1 to Figure 9 indi-cate the earth fault conditions and the appro-priate Uc for different grounding configurations.

—Continuous operating voltage Uc

For the correct choice of MO surge arresters, the system preconditions must be known. The earthing of the transformer neutral and the failure conditions in the system determine the continuous operating voltage Uc

4 A PPLI C ATI O N N OTE CO NTI N U O US O PER ATI N G VO LTAG E U C

3 Three wire non-earthed system

As a rule, these are systems of small extension, auxiliary power systems for power stations or station services.

The basic circuit of a medium voltage transformer with star connection and open star point is shown in Figure 1. Specified are the voltages and currents in case of a symmetrical load, i.e. in an undisturbed service case. All voltages ULE are equally high. The voltage of the star point UMp-E relative to earth is zero. The voltage triangle is provided on the right for better understanding. The situation in case of a single pole earth fault is shown in Figure 2. One line touches the earth, in this example line L3. The voltage UL3-E moves towards zero, the voltages UL1-E and UL2-E move to the value of the system voltage Us, as it is to be seen in the voltage triangle. The voltage of the star point of transformer is UMp-E = Us / √3. The earth fault current ICe is defined through the capacities Ck of the lines L1 and L2 towards the surrounding earth. A capacitive earth fault cur-rent ICe of about 5 A to 30 A flows in this case. The earth fault factor is k ≈ √3 = 1.732. Because a fail-ure current of 30 A will not affect the power trans-former the duration of the failure may last up to several hours and can be considered as continu-ous.

In case of intermittent earth faults, the earth fault factor can reach values up to k ≈ 1.9. A typical case of an intermittent earth fault is if during a storm branches of a tree are touching an over-head line several times one after another. In such cases Uc should be increased by about 10%.

For this reason, the continuous operating voltage for MO surge arresters in such systems has to be chosen to Uc ≥ Us for the arresters between phase and earth. The voltage at the transformer neutral can reach a maximum of Us / √3. This results in Uc ≥ Us /√3 for the MO surge arrester between transformer neutral and earth.

—Summary System ineffectively earthed, earth fault factor k ≈ √3 = 1.732

Fault duration: continuous operation

Uc ≥ Us

for arresters phase to earth

Uc ≥ Us / √3 for arresters neutral to earth

—Figure 1: Three phase system with insulated (open) star point

—Figure 2: Single pole earth fault in system with open star point

Linienstärken für Grafiken

Linie oben: 1 pt, schwarz

Linie innen: 0.5 pt, grey03

Linie unten: 1 pt, grey03

Balkengrafiken:72 x 47 mm Aussenmass

Pfeil 7, 45 %

LL = s

L1

L2

L2

L3

L1

L3

LE

LL

LEMp-E = 0

Trafo

Mp

U U

i

UU U

U






Pfeil 7, 45 %

i

Mp-E = LE

Ce ≤ 30A

Trafo

Mp

Ce

LL= s

L1 L2L1

L2

L3

L3

= 0

E

= s = √3 × LE

k

Ce

L1-E L2-E

Mp-E

UU I U

I

U U U

R

C

I

U

U

U

U U

5

4 System with earth fault compensation

These are mostly overhead line systems with system voltages between 10 kV and 110 kV. Figure 3 shows the same system as in Figure 2, but with a high-ohmic earthed star point Mp of the transformer. The inductance L is called Pe-tersen coil. If in this case a single pole earth fault occurs the fault current can flow back into the system through the inductance L. The earth fault residual current Ires arises from the currents Ic and IL, which have opposite directions. Because the inductive current IL “compensates” a part of the capacitive current Ic this arrangement is called a system with earth fault compensation.

The remaining failure current is in maximum in the range of 60 A, which will not affect the trans-former and the system management dramatically. For this reason a single pole earth fault can last up to 30 min or longer, which can be considered continuous service. The same equations for Uc are used for the MO surge arresters between phase and earth and between transformer neutral Mp and earth as in case of the non-earthed system.

The advantage of this arrangement is that in case of an earth failure in the system the current through the Petersen coil indicates immediately the failure, and with appropriate relays, the fail-ure can be located in the system and the involved part disconnected.


Fault duration: continuous operation

Uc ≥ Us


Uc ≥ Us / √3 for arresters neutral to earth

OV ER VO LTAG E PR OTEC TI O N

—Figure 3: Single pole earth fault in a system with earth fault compensation






Pfeil 7, 45 %

iTrafo

Mp

resMp-E

LL= s

L1 L2L1

L2

L3

L3

= 0

E

= s

k

C

L1-E L2-E

Mp-E

I U

I

U U

R

C

I

U

U

U

U U

res = IC – IL ≤ 60A

Trafo

Mp

E

k

IUIIL

L

ERE


5 System with earth fault compensation and automatic earth fault clearing

These are mostly overhead line systems with system voltages between 10 kV and 110 kV. Figure 4 shows the same case as Figure 3. Sys-tems with earth fault compensation have mostly also an automatic earth fault clearing. This means that after a given time the failure is cleared auto-matically. The automatic earth fault clearing enables a reduction of Uc by the temporary over-voltage factor T. Naturally, it is decisive to know the level of the possible temporary overvoltage as well as the maximum time for the clearing of the earth fault. Making use of the TOV curve this results in Uc ≥ Us / T for the MO surge arrester between phase and earth, and Uc ≥ Us / (T × √3) for the MO surge arrester between transformer neutral and earth.


Fault duration: TOV (time duration t) has to be provided

USUc ≥ --------- T


USUc ≥ ---------- T x √3

for arresters neutral to earth

—Figure 4: Single pole earth fault in a system with earth fault compensation






Pfeil 7, 45 %

iTrafo

Mp

resMp-E

LL= s

L1 L2L1

L2

L3

L3

= 0

E

= s

k

C

L1-E L2-E

Mp-E

I U

I

U U

R

C

I

U

U

U

U U

res = IC – IL ≤ 60A

Trafo

Mp

E

k

IUIIL

L

ERE

OV ER VO LTAG E PR OTEC TI O N 7

6 Systems with direct star point earthing

This kind of star point earthing is principally used for all the systems with system voltages of Us = 220 kV and above, but it can also be found in medium voltage systems. In these types of systems, there are so many transformers with direct neural earthing that during an earth fault, the phase voltage in the complete system never exceeds 1.4 p.u. There-fore, the earth fault factor is k (0.75 … ≤ 0.8) × √3, i.e. k ≤ 1.4. These systems are considered to have “direct” or “solid” star point earthing. In case of a failure a short circuit current flows that can be in medium-voltage systems as high as Is = 20 kA, and consequently the failure has to be cleared in less than 0.5 s. However, under worst case condi-tions, and considering some safety margins, it can be assumed that in medium-voltage systems the clearing time of the earth fault is t = 3 s at the most. In Figure 18 of the Application Guidelines the described TOV curve for an MO surge arrester with class SL (e.g. an arrester type MWK) lists T = 1.343 as a result, so that it may be written

k × Us 1.4 × Us 1.05 × UsUc ≥ ---------- = ------ -- --- ≈ ----- ------ T × √3 1.343 × √3 √3 for arresters between phase and earth.

This simple equation can be generally used as a rule of thumb for systems with direct earthed neutral.

The voltage of the neutral of the unearthed transformers in the system reaches a maximum of UTOV = 0.4 × Us. This results in:

0.4 × UsUc ≥ ---- -- --- = 0.3 × Us 1.343 for arresters between transformer neutral and earth.

—Summary System effectively earthed, k ≤ 1.4

Fault duration: ≤ 3 s 1.05 x UsUc ≥ ---- - √3

for arresters between phase and earth.

Uc ≥ 0.3 x Us

for arresters between the unearthed transformer neutral and earth.

Both equations are rules of thumb, see explana-tion above.

—Figure 5: Single pole earth fault in systems with direct earthing of transformer neutral






Pfeil 7, 45 %

i

Mp-EU

Trafo

Mp

LL= s

L1 L2

L1

L2

L3

L3

= 0

L1-E L2-E

Mp-EU

I

U

U

U

U U

S ≤ 20 kA

Trafo

Mp

IIK

ERE

IIS= LE√3 U U =


7 Systems with low impedance earthing

A system with low ohmic star point earthing is provided if the star point of one or more trans-formers are earthed through current limiting im-pedances. These are typical cable systems in towns with system voltages between 10 kV and 110 kV. The reason for low ohmic earthing of the transformer neutral is the limitation of the (short circuit) current in case of an earth fault. The sys-tem protection is set up so that even a single line-to-earth fault at any place in the system causes an earth fault clearing within t < 0.5 s. In unfavor-able situations, the duration of the failure can last up to 3 s in maximum in medium-voltage sys-tems. The fault current is in the range of 500 A to 2,000 A.

One has to distinguish between inductive earthing (neutral reactor) and resistive earthing (earthing resistor), see Figure 6 and Figure 7.

7.1 System with low ohmic (inductive) earthing of the transformer neutral

The earth fault factor is in this systems k ≤ 1.4 The result is therefore UTOV ≤ 1.4 × Us / √3 It can be assumed as worst case condition a clearing time of 3s. The TOV curve of an MO surge arrester type MWK (class SL) gives a TOV factor of T = 1.343

As a result for the continuous operating voltage can be written.

k × Us 1.4 × Us 1.05 × UsUc ≥ ---------- = ------ -- --- ≈ ----- ------ T × √3 1.343 × √3 √3 for arresters between phase and earth.

0.4 × UsUc ≥ ------ ----- = 0.3 × Us 1.343 for arresters between transformer neutral and earth.


Fault duration: ≤ 3 s 1.05 × UsUc ≥ ---- - √3 for arresters between phase and earth.

Uc ≥ 0.3 × Us

for arresters between the transformer neutral and earth.


—Figure 6: Single pole earth fault in systems with inductive earthed transformer neutrals






Pfeil 7, 45 %

iTrafo

Mp

LL= s

L1 L2

L1

L2

L3

L3

= 0 x LE

L1-E L2-E

Mp-EU

I

√3 U

U

U

U

U U

U =

S = 500... 2000A

Trafo

Mp

IIS

ERE

IIS

L

Mp-EU

OV ER VO LTAG E PR OTEC TI O N 9

7.2 System with low ohmic (resistive) earthing of the transformer neutral In this systems the earth fault factor is

k = (0.8…1.0) × √3

For arresters in the vicinity of low ohmic resistive earthed transformers, an earth fault factor of k ≤ 1.4 is applicable, and the same equations for Uc as for direct and low ohmic inductive earthed transformers can be chosen.

k × Us 1.4 × Us 1.05 × UsUc ≥ ---------- = ------ -- --- ≈ ----- ------ T × √3 1.343 × √3 √3


0.4 × UsUc ≥ ------ ----- = 0.3 × Us 1.343 for arresters between transformer neutral and earth.

In some cases, a system with low ohmic neutral transformer earthing the earth fault factor may not be in the whole system k ≤ 1.4. Care is re-quired if the MO surge arresters are located just a few kilometers from the transformer. This can be the case if, for instance, a cable is connected to an overhead line, and the cable bushing is pro-tected with a MO surge arrester.

In case of very dry soil or rocks (such as in desert regions or mountains) the earthing resistance is very high and it is possible that at the point of MO surge arrester installation the phase to earth volt-age comes very close to the system voltage Us. In this case the continuous operating voltage should be chosen to Uc ≥ Us / T

It may also be possible that the fault current in case of an earth fault is so small that no auto-matic clearing occurs. In such cases, it is better to choose the Uc for the arrester similar to the sys-tem voltage, which means Uc ≥ Us


Fault duration: ≤ 3 s 1.05 × UsUc ≥ ---- - √3 for arresters between phase and earth.

Uc ≥ 0.3 × Us

for arresters between the transformer neutral and earth.


—Figure 7: Single pole earth fault in systems with resistive earthed transformer neutrals






Pfeil 7, 45 %

iTrafo

Mp

LL= s

L1 L2

L1

L2

L3

L3

= 0 = LE

L1-E L2-E

Mp-EU

I

U = √3 U

U

U

U

U U

S = 500... 2000A

Trafo

Mp

IS

ERE

LIISMp-EU


8 Four-wire multi-earthed-wye systems

In some countries a four-wire system is used in special cases. In this systems a fourth wire is con-nected to the earthed neutral point of the trans-former and additionally to earth at several points along the line, see Figure 8. In such systems an earth fault factor of k = 1.25 can be assumed. It can be assumed that the clearing time in case of a failure is less than t = 0.5 s.

The continuous operating voltage should be chosen to

Uc ≥ 1.25 × Us / √3 = 0.72 × Us

for the MO surge arresters between phase and earth.

—Summary System effectively earthed, k = 1.25

Fault duration: ≤ 0.5 s

Uc ≥ 0.72 × Us


9 Distribution system with delta connection

Transformers in delta connection have naturally no neutral or star point, see Figure 9. In case of an earth fault in such systems of one of the phases the MO surge arresters connected to the sound phases will be stressed with the system voltage Us. An earth fault factor of k = √3 = 1.732 has to be considered. The fault clearing time is t ≤ 0.5 s.

The continuous operating voltage should be chosen to

Uc ≥ 1.05 × Us

In case additional phase-to-phase protection is required the same equation applies.

—Summary Earth fault factor k = 1.732

Uc ≥ 1.05 × Us

for the arresters between phase and earth and phase to phase.

—Figure 8: Basic circuit of a four-wire, multi-earthed-wye system






Pfeil 7, 45 %

LL = s

L1

L2

L2

L3

L1

L3

U LE

ULL

LE

UMp-E = 0

Trafo

Mp

Mp/N

U

U

U U

—Figure 9: Basic circuit of a distribution system with delta connection






Pfeil 7, 45 %

LL = s

L1

L2

L2L3

L1

L3

U U

11OV ER VO LTAG E PR OTEC TI O N

— A PPLI C ATI O N NOTE A N N E X 1 . 2 A 1

Selection table Uc

System configurationEarthing of neutral

Earth fault factor k

Ucphase arrester

Ucneutral arrester

Three wire non-earthed (insulated, open) ineffectively √3 = 1.732 Uc ≥ Us

UsUc ≥ ----- √3

Earth fault compensation (Petersen coil) ineffectively √3 = 1.732 Uc ≥ Us

UsUc ≥ ----- √3

Earth fault compensation (Petersen coil) with automatic earth fault clearing ineffectively √3 = 1.732

UsUc ≥ ----- T

UsUc ≥ -------- T × √3

Low impedance earthing (direct) effectively 1.4

1.05 × UsUc ≥ ------------ √3

Uc ≥ 0.3 × Usfor the unearthed transformers in the system

Low impedance earthing (inductive) effectively 1.4

1.05 × UsUc ≥ ------------ √3 Uc ≥ 0.3 × Us

Low impedance earthing (resistive) effectively 1.4

1.05 × UsUc ≥ ------------ √3 Uc ≥ 0.3 × Us

Four-wire multi-earthed-wye effectively 1.25

1.25 × UsUc ≥ ------------ √3 n.a.

Delta connection n.a. √3 =1.732

Uc ≥ Us n.a.

As a rule, 10% as safety margin should be added to the calculated value for Uc and then the next higher value from the data sheet chosen.






Pfeil 7, 45 %

L2L1

L3Mp

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3Mp/N

L2L1

L3






Pfeil 7, 45 %

L2L1

L3Mp

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3Mp/N

L2L1

L3






Pfeil 7, 45 %

L2L1

L3Mp

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3Mp/N

L2L1

L3






Pfeil 7, 45 %

L2L1

L3Mp

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3Mp/N

L2L1

L3






Pfeil 7, 45 %

L2L1

L3Mp

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3Mp/N

L2L1

L3






Pfeil 7, 45 %

L2L1

L3Mp

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3Mp/N

L2L1

L3






Pfeil 7, 45 %

L2L1

L3Mp

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3Mp/N

L2L1

L3






Pfeil 7, 45 %

L2L1

L3Mp

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3

L2L1

L3Mp/N

L2L1

L3

1HC

013

88

61

EN

AA

© Copyright 2018 ABB. All rights reservedSpecifications subject to change without notice

—ABB Switzerland Ltd.PGHVSurge ArrestersJurastrasse 45CH-5430 Wettingen/Switzerland Tel. + 41 58585 2911Fax + 41 58585 5570Email: [email protected]

abb.com/arrestersonline

Additional informationWe reserve the right to make technical changes or modify the content of this document without prior notice. With regard to purchase orders, the agreed particulars shall prevail. ABB AG does not accept any responsibility whatsoever for potential errors or possible lack of information in this document.

We reserve all rights in this document and in the subject matter and illustrations contained therein. Any reproduction, disclosure to third parties or utilization of its contents – in whole or in parts – is forbidden without prior written consent of ABB AG.

application note 1.2 continuous operating voltage uc …€¦ · application note 1.2 continuous...

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