Shunt capacitors for a.c. power systems having a rated voltage above 1000 V - Part 3: Protection of shunt capacitors and shunt capacitor banks

IEC/TS 60871-3:2015(E) which is a technical specification, gives guidance on the protection of shunt capacitors and shunt capacitor banks. it applies to capacitors according to IEC 60871-1. This second edition cancels and replaces the first edition published in 2005. This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition:
- Clearer writing of formulas on energy limitation for expulsion fuses;
- Updated normative references and bibliography;
- A new clause for synchronized switching has been added. Keywords: shunt capacitors, shunt capacitor banks

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Status
Published
Publication Date
24-Jun-2015
Current Stage
PPUB - Publication issued
Start Date
25-Jun-2015
Completion Date
25-Jun-2015
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IEC TS 60871-3:2015 - Shunt capacitors for a.c. power systems having a rated voltage above 1000 V - Part 3: Protection of shunt capacitors and shunt capacitor banks
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IEC TS 60871-3
Edition 2.0 2015-06
TECHNICAL
SPECIFICATION
Shunt capacitors for AC power systems having a rated voltage above 1 000 V –
Part 3: Protection of shunt capacitors and shunt capacitor banks
IEC TS 60871-3:2015-06(en)
---------------------- Page: 1 ----------------------
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IEC TS 60871-3
Edition 2.0 2015-06
TECHNICAL
SPECIFICATION
Shunt capacitors for AC power systems having a rated voltage above 1 000 V –
Part 3: Protection of shunt capacitors and shunt capacitor banks
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.240.99; 31.060.70 ISBN 978-2-8322-2755-8

Warning! Make sure that you obtained this publication from an authorized distributor.

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TS 60871-3:2015 © IEC 2015
CONTENTS

FOREWORD ........................................................................................................................5

1 Scope ...........................................................................................................................7

2 Normative references ....................................................................................................7

3 Terms and definitions ....................................................................................................7

4 Internal fuses ................................................................................................................7

4.1 General ................................................................................................................7

4.2 Fuse characteristics ..............................................................................................8

4.2.1 Rated current .................................................................................................8

4.2.2 Rated discharge capability .............................................................................8

4.2.3 Disconnecting capability ................................................................................8

4.2.4 Voltage withstand capability after operation ....................................................8

4.3 Influence of capacitor element configuration on capacitor life.................................8

4.3.1 Capacitor with all elements connected in parallel ............................................8

4.3.2 Capacitor with elements connected in series and parallel ...............................8

5 External fuses ...............................................................................................................8

5.1 General ................................................................................................................8

5.2 Fuse characteristics ..............................................................................................9

5.2.1 Rated current .................................................................................................9

5.2.2 Rated voltage ................................................................................................9

5.2.3 Time-current characteristics ...........................................................................9

5.2.4 Discharge capability ..................................................................................... 10

5.3 Fuse types .......................................................................................................... 10

5.3.1 General ....................................................................................................... 10

5.3.2 Expulsion fuses ........................................................................................... 10

5.3.3 Current-limiting fuses ................................................................................... 11

5.3.4 Combination current-limiting/expulsion fuses ............................................... 11

5.4 Influence of capacitor bank configuration on fuse selection .................................. 11

5.4.1 Single series section grounded star and delta banks .................................... 11

5.4.2 Single series section ungrounded star banks ................................................ 11

5.4.3 Multiple series section banks ....................................................................... 11

5.5 Coordination with case rupture curves ................................................................. 11

6 Unbalance detection .................................................................................................... 12

6.1 Operation ............................................................................................................ 12

6.2 Types of unbalance protection ............................................................................. 12

6.2.1 Neutral current (Figure 3) ............................................................................. 12

6.2.2 Neutral voltage (Figure 4) ............................................................................ 12

6.2.3 Current unbalance between neutrals (Figure 5) ............................................ 13

6.2.4 Phase voltage unbalance (Figure 6) ............................................................. 13

6.2.5 Voltage difference (Figure 7) ........................................................................ 13

6.2.6 Current unbalance in bridge connection (Figure 8) ....................................... 13

6.3 Current and voltage transformers ........................................................................ 13

6.3.1 Current transformers .................................................................................... 13

6.3.2 Voltage transformers .................................................................................... 14

6.4 Relays and protection settings ............................................................................ 14

6.5 Sensitivity ........................................................................................................... 14

6.6 Initial unbalance .................................................................................................. 15

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IEC TS 60871-3:2015 © IEC 2015 – 3 –

7 Overload current ......................................................................................................... 15

7.1 Operation ............................................................................................................ 15

7.2 Protective arrangement ....................................................................................... 15

7.3 Current transformers ........................................................................................... 15

7.4 Relays ................................................................................................................ 15

7.5 Protective settings .............................................................................................. 16

8 Over and undervoltage ................................................................................................ 16

8.1 Operation ............................................................................................................ 16

8.2 Overvoltage protection ........................................................................................ 16

8.3 Undervoltage protection ...................................................................................... 16

8.4 Reclosing ............................................................................................................ 16

9 Other protection .......................................................................................................... 17

9.1 Surge arresters ................................................................................................... 17

9.1.1 General ....................................................................................................... 17

9.1.2 Operation .................................................................................................... 17

9.1.3 Lightning transients...................................................................................... 17

9.1.4 Switching transients ..................................................................................... 17

9.1.5 Temporary overvoltages ............................................................................... 17

9.1.6 Rated voltage .............................................................................................. 17

9.1.7 Energy absorption ........................................................................................ 18

9.2 Damping devices ................................................................................................ 18

9.2.1 Capacitor switching ...................................................................................... 18

9.2.2 Inrush currents ............................................................................................ 18

9.2.3 Voltage transients ........................................................................................ 19

9.2.4 Ratings ........................................................................................................ 19

9.3 Synchronized switching ....................................................................................... 19

9.3.1 Operation .................................................................................................... 19

9.3.2 Breaker contacts delay................................................................................. 19

10 Safety ......................................................................................................................... 19

10.1 Discharging devices ............................................................................................ 19

10.1.1 General ....................................................................................................... 19

10.1.2 Internal resistors .......................................................................................... 20

10.1.3 External discharge devices .......................................................................... 20

10.1.4 Discharging after disconnection ................................................................... 20

10.2 Dead metallic parts ............................................................................................. 20

Bibliography ....................................................................................................................... 25

Figure 1 – Fuse types ......................................................................................................... 10

Figure 2 – Typical case rupture curves for approximately 30 000 cm³ case volume ............. 21

Figure 3 – Star connection with the neutral grounded through a current transformer ........... 21

Figure 4 – Star connection with voltage transformer between neutral and ground ................ 21

Figure 5 – Star connection with ungrounded neutral and voltage transformers

connected in an open delta ................................................................................................ 22

Figure 6 – Double-star connection with ungrounded neutral ............................................... 22

Figure 7 – Star connection with grounded neutral and voltage transformers connected

in differential measurement ................................................................................................. 22

Figure 8 – Bridge connection .............................................................................................. 22

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– 4 – IEC TS 60871-3:2015 © IEC 2015

Figure 9 – Line overcurrent relays for capacitor bank, grounded .......................................... 22

Figure 10 – Line overcurrent relays for capacitor bank, ungrounded .................................... 23

Table 1 – Melting currents for type-K (fast) fuse links, in amperes ....................................... 23

Table 2 – Melting currents for type-T (slow) fuse links, in amperes ...................................... 24

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IEC TS 60871-3:2015 © IEC 2015 – 5 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SHUNT CAPACITORS FOR AC POWER SYSTEMS HAVING
A RATED VOLTAGE ABOVE 1 000 V –
Part 3: Protection of shunt capacitors and
shunt capacitor banks
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

all national electrotechnical committees (IEC National Committees). The object of IEC is to promote

international co-operation on all questions concerning standardization in the electrical and electronic fields. To

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2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an

international consensus of opinion on the relevant subjects since each technical committee has representation

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3) IEC Publications have the form of recommendations for international use and are accepted by IEC National

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6) All users should ensure that they have the latest edition of this publication.

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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is

indispensable for the correct application of this publication.

9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of

patent rights. IEC shall not be held responsible for identifying any or all such patent rights.

The main task of IEC technical committees is to prepare International Standards. In

exceptional circumstances, a technical committee may propose the publication of a technical

specification when

• the required support cannot be obtained for the publication of an International Standard,

despite repeated efforts, or

• the subject is still under technical development or where, for any other reason, there is

the future but no immediate possibility of an agreement on an International Standard.

International Standard IEC 60871-3, which is a technical specification, has been prepared by

IEC technical committee 33: Power capacitors and their applications.

This second edition cancels and replaces the first edition published in 1996. This edition

constitutes a technical revision.
---------------------- Page: 7 ----------------------
– 6 – IEC TS 60871-3:2015 © IEC 2015

This edition includes the following significant technical changes with respect to the previous

edition:
a) Clearer writing of formulas on energy limitation for expulsion fuses;
b) Updated normative references and bibliography;
c) A new clause for synchronized switching has been added.
The text of this technical specification is based on the following documents:
Enquiry draft Report on voting
33/545/DTS 33/563/RVC

Full information on the voting for the approval of this technical specification can be found in

the report on voting indicated in the above table.

This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

A list of all parts in the IEC 60871, published under the general title Shunt capacitors for a.c.

power systems having a rated voltage above 1 000 V, can be found on the IEC website.

The committee has decided that the contents of this publication will remain unchanged until

the stability date indicated on the IEC website under "http://webstore.iec.ch" in the data

related to the specific publication. At this date, the publication will be
• transformed into an International standard,
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.
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IEC TS 60871-3:2015 © IEC 2015 – 7 –
SHUNT CAPACITORS FOR AC POWER SYSTEMS HAVING
A RATED VOLTAGE ABOVE 1 000 V –
Part 3: Protection of shunt capacitors and
shunt capacitor banks
1 Scope

This part of IEC 60871, which is a technical specification, gives guidance on the protection of

shunt capacitors and shunt capacitor banks. it applies to capacitors according to IEC 60871-

2 Normative references

The following documents, in whole or in part, are normatively referenced in this document

and are indispensable for its application. For dated references, only the edition cited applies.

For undated references, the latest edition of the referenced document (including any

amendments) applies.
IEC 60549, High-voltage fuses for the external protection of shunt capacitors

IEC 60871-1, Shunt capacitors for a.c. power systems having a rated voltage above 1 000 V

–Part 1: General

IEC 60871-4, Shunt capacitors for AC power systems having a rated voltage above 1 000 V –

Part 4: Internal fuses
3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC 60549, IEC 60871-

1 and IEC 60871-4 apply.
4 Internal fuses
4.1 General

Internal fuses for shunt capacitors are selective current-limiting fuses arranged inside a

capacitor. As defined in IEC 60871-4, they are designed to isolate faulted capacitor elements

or capacitor unit, to allow operation of the remaining parts of that capacitor unit and the bank

in which the capacitor unit is connected.

The operation of an internal fuse is initiated by the breakdown of a capacitor element. The

affected element is instantaneously disconnected by the operation of the element fuse

without interruption in the operation of the capacitor.

The number of externally parallel connected capacitors and the available short-circuit current

of the supply system should not affect the current-limiting of internal fuses.

It should be noted that internal fuses do not provide protection against a short circuit between

internal connections or a short circuit between active parts and casing, both of which may

lead to case rupture.
---------------------- Page: 9 ----------------------
– 8 – IEC TS 60871-3:2015 © IEC 2015
4.2 Fuse characteristics
4.2.1 Rated current
There is no definition or test method existing for element fuses.

Element fuses are, in general, designed for much higher currents than the maximum

permissible element current. They are meant to disconnect only faulty elements. The faulty

elements and their fuses are not intended to be replaced.
4.2.2 Rated discharge capability

IEC 60871-4 and IEC 60871-1 specify that the capacitor be subject to five undamped

. For special applications, where inrush currents
discharges from a d.c. charge level of 2,5 U
and/or peak voltages are limited, lower discharge requirements are applicable.
4.2.3 Disconnecting capability

Requirements and test procedures are given in IEC 60871-4. These tests verify that the fuse

has a current-limiting action.
4.2.4 Voltage withstand capability after operation
Requirements and test procedures are given in IEC 60871-4.
4.3 Influence of capacitor element configuration on capacitor life
4.3.1 Capacitor with all elements connected in parallel

After the breakdown of an element, the respective fuse will melt in less than a millisecond

owing to the discharge current from the parallel connected elements and capacitors and the

power frequency current from the supply. The capacitor may, however, continue operating

with a correspondingly reduced output.

If the capacitor is operated at a fixed bus voltage, no variation in operating voltage on the

remaining healthy elements will occur.
4.3.2 Capacitor with elements connected in series and parallel

After the breakdown of an element, all parallel connected elements discharge their stored

energy or part of it into the faulty element. The power frequency current is limited by the

remaining healthy elements connected in series.

After the disconnection of the faulty element, the capacitor continues operating with a

correspondingly reduced output. The remaining healthy elements of the group are then

stressed with a voltage approximately m × n /[m (n – 1) + 1] times the initial voltage, where n

is the number of parallel connected elements per group and m the number of series-

connected sections per unit. In certain cases the voltage may be higher, for example due to

neutral shift with an ungrounded star configuration.
5 External fuses
5.1 General

External fuses for shunt capacitors are defined in IEC 60549 as intended to clear faults inside

a capacitor unit and to permit continued operation of the remaining parts of the bank in which

the unit is connected. They will also clear an external capacitor bushing flashover.

---------------------- Page: 10 ----------------------
IEC TS 60871-3:2015 © IEC 2015 – 9 –

The operation of an external fuse is generally determined by the power frequency fault

current and by the discharge energy from capacitors connected in parallel with the faulty

capacitor.

The initial breakdown is usually of an individual element within a capacitor. This invariably

becomes a short circuit which removes all elements in parallel with it and eliminates one

series section from the capacitor. Should the cause of the initial failure continue, failure of

successive series sections (which see an increasing voltage with each series section

removed) will occur. This causes an increase in the current through the capacitor to the point

where the fuse operates removing the failed capacitor from the circuit.

It should be noted, particularly in the case of paper or paper/film dielectric capacitors, that

the capacitor case may occasionally rupture in the event of failure. This occurs when the

initial element failure has high resistance between the shorted electrodes due to the

presence of paper and sustained arcing generates gas which swells the case to the point

where it may rupture before the protecting fuse can disconnect the capacitor.

Capacitors with all-film dielectric have a lower incidence of case rupture because the film

melts and generally allows a low resistance short between the electrodes. However, case

rupture may still occur due to arcing when there is a broken internal connection and when

there is excessive stored energy available in parallel capacitors and/or high power frequency

fault current.
5.2 Fuse characteristics
5.2.1 Rated current

The rated current of the selected fuse should be consistent with the criteria used for the

selection of a switch or circuit-breaker for the same bank. From the various national

standards the minimum accepted rating is 1,35 times the rated capacitor current.

In a steady-state basis, there is no need for the fuse capability to exceed that for the switch

or circuit-breaker. However, transient conditions such as currents associated with system or

bank switching should be considered. It is common to use a fuse with a current rating of 1,65

times the rated capacitor current.

IEC 60549 specifies that the fuse rated current be at least 1,43 times the capacitor rating.

This falls between the two values mentioned above of 1,35 and 1,65. For some banks, the

fuse rating may be higher than 1,65 times the capacitor rated current to avoid spurious fuse

operation due to switching transients and for mechanical reasons.

NOTE The continuous rating of the fuse is not necessarily its nameplate rating. For example, an expulsion fuse

link with a rating much smaller than the rating of the fuse holder can carry 150 % of its nameplate rating on a

continuous basis. It is extremely important that the actual current rating of the fuse link be known. Typically, fuse

holders are available in two current ratings, one for up to 50 A and the other for up to 100 A, whereas fuse links

used in these holders are rated from 5 A to 100 A. These holders also vary in voltage rating, e.g. up to 9 kV, 9 kV

to 16 kV and 16 kV to 25 kV.
5.2.2 Rated voltage

The rated voltage of the fuse should be not less than 1,1 times the rated voltage of the

capacitor with which it is associated in order to meet the requirements of IEC 60549.

5.2.3 Time-current characteristics

Time-current characteristics are available from most fuse manufacturers to assist in

coordination.
This information is sometimes available in table form.
---------------------- Page: 11 ----------------------
– 10 – IEC TS 60871-3:2015 © IEC 2015
5.2.4 Discharge capability

The external fuse should be capable of withstanding inrush transients and currents due to

external short circuits. IEC 60549 specifies tests to verify the I t to which the fuse may be

subjected for 5 and 100 discharges.
5.3 Fuse types
5.3.1 General
The different types of fuse are indicated in Figure 1.
External fuses
Expulsion Current-limiting
Com
...

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