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Medium voltage products

Technical Application Papers No. 22Medium voltage generator circuit-breakers

11

Index

2 1 Introduction 4 2 Generator circuit-breakers 4 2.1 History of generator circuit-breakers 5 2.2 Vacuum generator circuit-breakers 7 2.2.1 Diffuse or contracted vacuum arc 7 2.2.2 Spiral shape of the contacts of ABB vacuum

interrupters 8 2.2.3 The VD4G family 8 2.2.4 Solutions in the switchgear and accessories 10 3 The standards governing generator

circuit-breakers 11 3.1 Main differences between Dual Logo Standard IEC/IEEE 62271-37-013 and Standard IEC 62271-100 13 3.2 Rated insulation levels 14 3.3 Degree of asymmetry for system-source short-

circuit currents 15 3.4 Degree of asymmetry for generator-source short- circuit currents 16 3.5 Rate of rise of transient recovery voltage (TRV) after

system-source fault 18 3.6 Rate of rise of transient recovery voltage (TRV) after

generator-source fault 19 3.7 Presence of fault currents due to closing in out-of-

phase conditions 20 3.8 Load current making and breaking 20 3.9 Summary of the comparison between IEC/IEEE 62271-37-013 and IEC 62271-100 in relation to TRV parameters 22 3.10 Short-circuit rated making current 22 3.11 Short-time withstand current 22 3.12 Rated operating sequence 23 3.13 Number of mechanical operations 23 3.14 Type tests required by the Standards 24 4 Choice of generator circuit-breaker 24 4.1 System-source short-circuit current 25 4.2 Generator-source short-circuit current 27 4.3 Making capacity 28 4.4 Choice of generator circuit-breaker class 29 4.5 Information to be given with enquiries, tenders and

orders 30 5 Example 30 5.1 Installation data 30 5.2 Simplified calculation of the symmetrical component

2

TREND

1.670

572

1.005

2

1. Introduction

Renewable

Nuclear

Fossil

Conventional and renewable concentrated power generating plants Transformer Transmission

Rail transport

Industry

Housing

Industry, services-providers and

storage

Individual homes

Decentralized and new-concept renewable power generating

stations

Low voltage230/400 V

Medium voltage 10-30 kV

High Voltage 60-150 kV

High Voltage 380 kV

FarmsSmall industries

One must also consider that over the past few years, governments have encouraged the liberalization of the energy market and that this has shifted the burden of investments onto the enterprises which directly (electricity companies) or indirectly (industry) generate electric power. Attention is focusing on whether or not a return on investment will be obtained but that's not all. Generation from renewable sources which, as described above, is on the rapid increase, is extremely variable and also poses the problem of continuity of supply. This means that as well as installing renewable generating plants, we must also create installations for storing the power and maintain or even add to the more flexible part of generation from conventional sources, such as hydroelectric or thermal power stations.In future, both large and small generators will be scattered throughout the country, where the former will supply backup power should the latter be unable to generate owing to the inevitable variability of the renewable sources.

Both the authorities and public opinion are paying increasing attention to climate changes. Since it has now been proved that greenhouse gas emissions are the cause behind the abnormal rise in global temperature, governments throughout the world are actively endeavouring to find possible solutions to the problem. One of these solutions is to increase the production of electric power from renewable sources. According to the most conservative estimates, the investments forecast in Europe during the 2010-2050 period will amount to more than 3,200 billion euros, of which a hefty 1,670 billion for power generation from renewable sources.

33

To get the most out of their investments, the main goal of enterprises that run (or would like to run) a generating plant will be to obtain the highest possible amount of power at the lowest cost. The reliability of an electric power generating plant depends on its typology, but most especially on the quality of the components installed in relation to the specific characteristics of the generator itself. This means that the circuit-breaker installed to protect the generator is of fundamental importance for safeguarding the investment, since it must:– protect the generator– protect the installation– simplify the operating procedures– generally improve the safety and reliability of the installation.

Since this component is so important, IEC recently issued a standard, the result of joint work with IEEE, which specifically deals with generator circuit-breakers.

This standard is IEC/IEEE High-voltage switchgear and controlgear – Part 37-013: Alternating-current generator circuit-breakers, Edition 1.0 2015-10.Requirements for generator circuit-breakers are much different from those of a generic circuit-breaker for distribution purposes, as defined in Standard IEC 62271-100. Indeed, the circuit-breaker installed between a generator and a transformer must meet decidedly more demanding requirements as to fault currents. In particular cases, fault currents can also present delayed current zero crossing. It can be very difficult and complicated to define the capability of the circuit-breaker to interrupt these currents. The certificates of the tests performed in laboratories may not contain sufficient information. This means that simulation research must be conducted by considering the effect of the circuit-breaker's arc voltage on the presumed short-circuit current.

44

2 Generator circuit-breakers

2.1 History of generator circuit-breakers The first circuit-breakers to be specifically designed as generator circuit-breakers date back to the early '70's. After this, these circuit-breakers were subjected to constant development, passing from interrupting solutions using compressed air to the '80's, when SF6 generator circuit-breakers were first introduced. In order to cover the increasingly higher generator power ratings, research was dedicated right from the start to solutions with segregated phases. There are now three-phase systems where each individual phase comes in a separate metal enclosure installed on a common structure with shared operating and monitoring systems. Remarkable limits have been reached with this solution, with rated current up to 57000 A, forced air cooling and up to 210 kA breaking capacities. These circuit-breakers are able to protect generators with up to 2000 MVA power ratings.

Meanwhile, vacuum technology in the distribution field became progressively established, being economical, compact and able to provide remarkable performance as to mechanical and electrical life. Later on, this method of interruption was also applied to generators, if only to the lower power ratings, where it provided extremely compact solutions in standard medium voltage switchgear.

5

1

3

4

7

8

9

10

6

2

5

5

1 Upper terminal 2 Vacuum interrupter 3 Housing/pole 4 Hub of moving contact 5 Lower terminal

6 Flexible connection 7 Spring fork of tie-rod 8 Tie-rod 9 Pole fastening 10 Connection to operating mechanism

Nowadays, the ABB range of generator circuit-breakers is able to cover the entire field of small, medium and high power generators with scalable solutions as to cost and size.

2.2 Vacuum generator circuit-breakersSimilarly to the vacuum circuit-breakers for distribution network, also generator vacuum circuit-breakers use encapsulated vacuum interrupters for the poles.

6

1

2

3

4

5

7

8

9

10

6

6


1 Terminal 2 Protection 3 Metal bellows 4 Interrupter case 5 Shield 6 Ceramic insulator 7 Shield 8 Contacts 9 Terminal 10 Interrupter case

This construction technique makes the poles of the circuit-breaker particularly sturdy and protects the interrupter from impact, dust and moisture. The vacuum interrupter houses the contacts and forms the breaking chamber.

Consequently, the vacuum circuit-breaker does not need a breaking and insulating medium since it does not contain ionizable material. The electric arc that generates when the contacts separate is merely formed by the fusion and vaporization of the contact material. Sustained by the external energy, the electric arc persists until the current annuls near to natural zero. In that instant, sharp reduction of the density of the conveyed charge and rapid condensation of the metallic vapour rapidly restore the dielectric properties and provide the capability to sustain the transient recovery voltage, thereby definitively extinguishing the arc. Since high dielectric strength can be reached in the vacuum even with minimum distances, circuit breaking is also guaranteed when separation is just a few millimeters (2–3 mm).

The special shape of the contacts and the material used, combined with the brief arcing time and low arc voltage guarantee long-lasting contacts with a minimum amount of wear. The vacuum also prevents the contacts from tarnishing and becoming contaminated.

77

2.2.1 Diffuse or contracted vacuum arcIn a vacuum interrupter, the electric arc begins the instant in which the contacts separate. The contact area diminishes at that moment and the current concentrates in points that become very hot, leading to localized fusion of the cathode surface. This leads to the formation of metallic vapours that support the arc itself. After this, the arc expands over the surface of the contact with evenly distributed thermal stress. These conditions are known as diffuse arc.The electric arc is always the diffuse type at the interrupter's rated current value. The contact is only eroded very slightly and the number of interruptions is very high.As the value of the current increases (beyond rated value, depending on the contact material), the electric arc tends to change from diffuse to contracted. The arc initially concentrates in a single zone at the anode, while there are several very close and moving concentration points at the cathode. Lastly, as the current increases to a further extent, the arc concentrates in a single point on both the anode and cathode. There is a temperature rise on a level with the affected contact areas, and the material is consequently subjected to thermal stress.To prevent the contacts from overheating and becoming eroded, the arc is made to rotate. By turning, the arc resembles a moving conductor through which current passes.

2.2.2 Spiral shape of the contacts of ABB vacuum interrupters

The special spiral shape of the contacts generates a radial magnetic field that acts on the arc column, pushing it onto the external circumferences of the contacts.In short, the electromagnetic force that self-generates, acts tangentially and causes the arc to spin rapidly around the axis of the contacts.This forces the arc to turn and affect a larger area than that of a fixed contracted arc. This behaviour limits the thermal stress to which the contacts are subjected, ensures that these latter are only eroded to a negligible degree and, above all, allows the breaking process to be controlled even with very high short-circuit current values.At the zero current instant, rapid reduction in current density and rapid condensation of the metallic vapours allow maximum dielectric strength to be re-established between the interrupter's contacts within a few thousandths of a second.

Contraction on the anode and on the cathode.

Diffuse arc.

Contraction of the anode

88


System-source breaking capacity[kA]

Generator-source breaking capacity[kA]

Out-of- phase breaking capacity [kA]

VD4G-50 50 50/37 25

VD4G-40 40 25/25 20

VD4G-25 25 16/16 12.5

The extremely compact circuit-breakers can be installed in standard medium voltage switchgear and create very interesting solutions as to space occupied and homogeneity, since they can be installed alongside the other medium voltage switchgear panels.

2.2.4 Solutions in the switchgear and accessoriesABB offers the VD4G circuit-breaker for switchgears, moduls and retrofitting solutions.UniGear ZS1 switchgears specifically offer the following range:– VD4G-50 circuit-breaker with 1000 mm panel width;– VD4G-40 circuit-breaker with 800 mm and 1000 mm panel

width for 3150 A rated current;– VD4G-25 circuit-breaker with 650 mm panel width.

2.2.3 The VD4G family

The VD4G family of generator circuit-breakers includes three apparatuses: VD4G-50, VD4G-40 and VD4G-25 for up to 15 kV voltage ratings, up to 4000 A rated current ratings and up to 50 kA for generator-source short-circuit breaking current. The circuit-breakers all conform to Standard IEC/IEEE 62271-37-013 “High-voltage switchgear and controlgear – Part 37-013: Alternating-current generator circuit-breakers”.The following table lists the breaking capacities of the family in the three conditions: system-source, generator-source and out-of-phase conditions. In the case of generator-source breaking capacity, the first value refers to maximum breaking capacity with 110% asymmetry and the second to 74% breaking capacity but 130% asymmetry (called class G1 in the Standard). The same value means that the circuit-breaker is able to interrupt at maximum breaking capacity with 130% asymmetry (called class G2 in the Standard).

9

G

Io

Io

3U

3I

3I

9

Use of UniGear ZS1 switchgear also allows a protected solution to be obtained against internal arc faults inside the switchgear itself, thus ensuring that work can be performed in the utmost safety. The switchgear is classified as IAC (Internal Arc Classified) AFRL according to Standard IEC 62271-200 “High-voltage switchgear and controlgear – Part 200: AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV”, where A indicates that the switchgear can only be accessed by authorized personnel and FRL that the protection covers the front, rear and sides. To meet specific requirements, the protection covers 16 to 50 kA fault currents for 0.1 to 1 s.UniGear ZS1 switchgear provides indisputable benefits as to continuity of service by achieving maximum class LSC-2B (Loss of Service Continuity), since the busbar, circuit-breaker and cable compartments are electrically segregated. This means that the circuit-breaker compartment can be accessed when the busbar and cable compartments are live, while the switchgear and, consequently, the installation continue to operate.

The relays in the Relion series include the most advanced protections for generators, which are obviously one of the most important components in an electrical installation. Generators are liable to many different types of faults; e.g., synchronous generators can also act as motors, thus faults may occur in the stator windings and rotor windings supplied by direct current. In short, generators are certainly the components of an electrical installation most likely to be affected by faults or malfunctions. In this case, the most suitable relay is REG630, since it is highly flexible, scalable and able to adapt to all types of generator. An example of application to a Diesel/Gas generator is given in the next diagram.

In addition, REG630 complies with standard IEC61850 for communication in substations and covers both horizontal and vertical communication, including GOOSE messaging. REG630 also supports communication protocols DNP3 (TCP/IP) and IEC 60870-5-103 (serial). These systems allow the apparatus to connect to various automation systems and SCADA.

REG630

Preconfiguration A

ANSI IEC

27 3U<

32R/320 P>

32U P<

40 X<

46G/46M I2>G/M

49T/G 3Ith>T/G

51BF/51NBF 3I>/Io>BF

51P-1/ 51P-2 3I>/3I>>

51V I(U)>

59 3U>

59G Uo>

60 FUSEF

67N-1/ 67N-2 Io>/Io>>

81U/81O/81R f</t>/∆f/∆t

87G/87M 3dl>G/M

1010

3 The standards governing generator circuit-breakers

International Standard IEC/IEEE 62271-37-013 was prepared by a joint working group comprising IEC and IEEE members and was consequently published with the dual logo IEC/IEEE. This standard follows IEEE Std C37.013-1997 (R2008) “IEEE Standard for AC High-Voltage Generator Circuit Breakers Rated on a Symmetrical Current Basis” and the successive amendment IEEE Std C37.013a-2007 “IEEE Standard for AC High Voltage Generator Circuit Breakers Rated on a Symmetrical Current Basis -Amendment 1: Supplement for Use with Generators Rated 10–100 MVA” which, for many years, was the only world-wide Standard on the subject. Since the dual logo Standard is the result of a compromise, there are certain, albeit minor differences.

Consequently, generator circuit-breakers must now be designed and tested in accordance with IEC/IEEE 62271-37-013, since the Standard governing high voltage alternating current circuit-breakers (IEC 62271-100 “High-Voltage Switchgear and Controlgear – Part 100: High-Voltage Alternating-Current Circuit-Breakers” specifically excludes generator circuit-breakers from its scope (see IEC 62271-100, chap. 1.1 Scope).

11

A B

G

11

3.1 Main differences between Dual Logo Standard IEC/IEEE 62271-37-013 and Standard IEC 62271-100

The first difference lies in the fact that generator circuit-breakers have two breaking capacities: system-source breaking capacity (A - in the event of faults between the generator and circuit-breaker) and generator-source breaking capacity (B - in the event of faults between the circuit-breaker and transformer).

Figure 1: Short-circuit current through the circuit-breaker owing to a fault in A, thus supplied by the system. Called System-source Fault by the standard

1212


Figure 2: Short-circuit current through the circuit-breaker owing to a fault in B, thus supplied by the generator. Called Generator-source Fault by the standard

The other differences, which depend on the type of application, are:– the degree of asymmetry for system-source short- circuits (A); – the degree of asymmetry for generator-source short- circuits (B);– the rate of rise of transient recovery voltage for system-

source faults;– the rate of rise of transient recovery voltage for generator-

source faults;– presence of fault currents due to closing in out-of-phase

conditions.

Standard IEC 62271-100 does not adequately cover these requirements, which become challenging in the case of generators owing to the strong presence of a direct current component, the degree of fault current asymmetry and the characteristics of the TRV.

13

7,2 8,25 12 15 15,5 17,5

7,2 8,25 12 15 15,5 17,5

120

100

80

60

40

20

0

50

40

30

20

10

0

IEC 62271-100 IEC 62271-100 NAM IEC/IEEE 62271-37-013

IEC 62271-100 IEC 62271-100 NAM IEC/IEEE 62271-37-013

13

Rated voltage Rated short-time withstand voltage at power frequency

Rated lightning impulse withstand voltage

Ur Ud Up

kV (r.m.s. value) kV (r.m.s. value) kV (peak value)

Ur ≤ 7.2 20 60

7,2 < Ur ≤ 12 28 75

12 < Ur ≤ 15 38 95

15 < Ur ≤ 17.5 50 110

3.2 Rated insulation levelsStandard IEC/IEEE 62271-37-013 (part 4.2.101, table 1) defines the following rated insulation levels for generator circuit-breakers, which are actually a compromise between the IEEE and IEC standard values:

within the 15 < Ur ≤ 17.5 range, higher withstand voltage values than those indicated by Standard IEC 62271-1 for 17.5 kV rated voltage.The following graphs illustrate the differences between the two standards for short-time withstand voltage in kV (first graph) and impulse withstand voltage in kVp (second graph) depending on the rated voltage in kV:

On the other hand, Standard IEC 62271-1 (part 4.2, table 1a and 1b), has two tables, one of which is additional for the values used in North America:

Rated voltage Rated short-time withstand voltage at power frequency


Ur Ud Up


7,2 20 60

12 28 75

17.5 38 95

24 50 125

Rated voltage(North America)

Ur

Rated short-time withstand voltage at power frequency

Ud


Up


8.25 36 95

15 36 95

15.5 50 110

27 60 125

As can be seen, the values correspond up to 15 kV rated voltage. After this, Standard IEC/IEEE 62271-37-013 adopts,

Short-time withstand voltage [kV]

Impulse withstand voltage [kVp]

14

100

90

80

70

60

50

40

30

20

10

0

Iaccs

Idccs

A'

A

N

O

E

C'

C IMC

B'E'

M

B

0 20 40 60 80 100

14


3.3 Degree of asymmetry for system-source short-circuit currents

Generally speaking, short-circuit current is characterized by two values: a) the root mean square value (r.m.s.) of the alternating

component, known as Isc;b) time constant τ of the direct component of the short-circuit

current shown by a certain degree of asymmetry the instant in which the contacts separate.

In the example illustrated in the figure below:

Time

Cur

rent

i

Time constant (r) = 133 ms

Time after fault starts ms

Deg

ree

of a

sym

met

ry (%

)

Figure 4: Degree of asymmetry depending on time after fault starts

Figure 3: Alternating component and degree of asymmetry upon separation

Idccs Iaccs

– is the root mean square value of the alternating component of current Isc the instant the contacts – Asycs is the degree of asymmetry the instant the contacts

separate, equal to Asycs = 100%

Standard IEC/IEEE 62271-37-013 (part 4.101.2) defines a 133 ms time constant for the direct component of the rated system-source short-circuit current. This corresponds to approx. 68% asymmetry the instant the contacts separate, e.g. at 50 ms.

Vice versa, Standard IEC 62271-100 defines a normal unidirectional time constant value of 45 ms (part. 4.101.2). All the short-circuit tests performed on the circuit-breaker are conducted with this time constant unless specific requests due to particular applications are involved. For test cycles T10, T30, T60 and T100s (at 10%, 30%, 60% and 100% of the breaking capacity, respectively), the percentage of the one-way component the instant the contacts separate must not exceed 10% of the alternating component (part 6.106). Iaccs

2

15

0 100 200 300 400 500

100

80

60

40

20

0

160

150

140

130

120

110

100

90

80

70

60

50

40

30

20

10

0

15

Peak value of A.C. component

D.C. component

Degree of asymmetry

A.C. component

Generator-source short-circuit current

Degree of asymmetry = 130%

Cur

rent

(kA

)

Deg

ree

of a

sym

met

ry (%

)

Time after fault initiation (ms)

3.4 Degree of asymmetry for generator-source short-circuit currents

Here again, the short-circuit current is characterized by root-mean-square value Isc of the alternating component and by time constant τ of the direct component. The degree of asymmetry the instant the contacts separate may be much higher than in the previous case. If the degree of asymmetry were to exceed 100%, “missing current zero” would occur, i.e. delay of the current to pass through zero, thus absence of the condition required for breaking (an indispensable condition). Standard IEC/IEEE 62271-37-013 deals with this issue (part 4.101.3 and in annex H). If the fault begins when the voltage in one phase passes through zero, the resulting fault current in that phase will have the maximum degree of asymmetry. Statistically, it has been noted that this degree of asymmetry can be very high and exceed 130%. The following figure, taken from the standard, shows that the degree of asymmetry increases quickly at the beginning owing to rapid damping of the alternating component, and reaches a value

The so-called class G1 circuit-breakers must be tested with 130% asymmetry degree at 74% breaking capacity of the alternating component and with 110% asymmetry at full breaking capacity.Class G2 circuit-breakers must be tested with 130% asymmetry at full breaking capacity of the alternating component.

Standard IEC 62271-100 does not envisage this situation since it is beyond its scope.

of about 148%. After this, the degree of asymmetry gradually decreases as the alternating component achieves a stationary condition. Since the cases encountered in real installations are extremely variable and also depend on the load status of the generator before the fault, adoption of the 130% asymmetry degree at the instant the contacts separate is established by the Standard as a requirement for the test.

Prospective generator-source short-circuit current (fault initiation at voltage zero) (IEC/IEEE 62271-37-013, figure H.1)

16

uc

u'

0 td t' t3

16


3.5 Rate of rise of transient recovery voltage (TRV) after system-source fault

Transient Recovery Voltage (TRV) is the voltage that appears between the open contacts of the circuit-breaker immediately after an interruption. The waveform of TRV is defined by the characteristics of the generator and circuit, mainly by those of the associated step-up transformer. Three-phase faults are generally more severe and generate a higher short-circuit overcurrent and the maximum rate of rise of the TRV. Another factor to bear in mind is that the first pole that interrupts the current is subjected to higher voltage at power frequency than that after interruption in all three poles. Consequently, the TRV will also be more severe in this case. Standard IEC/IEEE 62271-37-013 (part 4.105 and 8.103.7.4) establishes that the first pole to clear factor Kpp in the case of generators, is 1.5 for cases where the neutral is not effectively earthed. First pole to clear factor = 1.5 takes account of fault conditions limited by a transformer (e.g. for example, transformers with non-effectively earthed neutral in systems with effectively earthed neutral, or transformers with one side effectively earthed and the other connected to systems with non-effectively earthed neutral), or the typical case of an MV generator connected to an MV/HV step-up transformer.Lastly, again according to Standard IEC/IEEE 62271-37-013 (part 8.103.7.5), 1.5 is a realistic amplitude factor value, kaf so long as capacitances are not connected to the generator terminals.It is fairly safe to say that the waveform of transient recovery voltage is a single frequency damped oscillation. Two straight lines depict the lower and upper limits of the rising waveform of the TRV.

The upper line begins at the origin and is tangent to the TRV. This line terminates where it meets the horizontal line tangent to the highest point, uc, of the TRV. The time of the intersection point is called t3. The lower line is parallel to the first. It begins on the time axis in the point corresponding to delay time td and ends in the point of coordinates t’, u’.The two parameters used for representing TRV are therefore uc and t3. uc is the peak value of TRV and is calculated in the following way:

Time (µs)

Voltage (kV)

1717

As described above:

value uc/3 is preferably assumed for u’ and value td+t3/3 for t’.Ratio uc/t3 is called Rate-of-Rise-of-Recovery-Voltage or RRRV. For the power ratings of generators that affect VD4G circuit-breakers, the parameters of TRV for system-source faults are (IEC/IEEE 62271-37-013, table 3):

Transformer rating Recovery voltage (TRV)

Time t3 Peak value of TRV uo

RRRV

MVA µs kV kV/µs

10-50 0.58 Ur 1.84 Ur 3.2

51-100 0.53 Ur 1.84 Ur 3.5

101-200 0.46 Ur 1.84 Ur 4.0

201-400 0.41 Ur 1.84 Ur 4.5

401-600 0.37 Ur 1.84 Ur 5.0

601-1000 0.34 Ur 1.84 Ur 5.5

1001 or higher 0.31 Ur 1.84 Ur 6.0

On the other hand, Standard IEC 62271-100 (part 4.102.2) uses the same representation with two parameters, uc and t3, for voltage values of less than 100 kV.Circuit-breakers for interiors are normally designed for use in a cable system thus, according to the Standard, they belong to class S1.The following formula is used to define the TRV for this class of parameters:

Delay time td equals 1 µs.

where the first pole to clear factor Kpp is 1.5 while kaf is 1.4 for terminal faults in the case of cabled systems. The values of uc are slightly different and lower than those defined by IEC/IEEE 62271-37-013. Thus 62271-100 defines:

While td = 0.15 x t3, again for terminal faults.

TRV parameters for circuit-breaker terminal faults (from IEC 62271-100, table 1)

Rated voltage First pole to clear factor

Amplitude factor

Peak value of TRV

Time Delay Voltage Time RRRV

Ur kpp kaf uc t3 td u' t' uc/t3

kV p.u. p.u. kV µs µs kV µs kV/µs

3.6 1.5 1.4 6.2 41 6 2,1 20 0,15

4.76(b) 1.5 1.4 8.2 44 7 2,7 21 0,19

7.2 1.5 1.4 12.3 51 8 4,1 25 0,24

8.25(b) 1.5 1.4 14.1 52 8 4,7 25 0,27

12 1.5 1.4 20.6 61 9 6,9 29 0,34

15(b) 1.5 1.4 25.7 66 10 8,6 32 0,39

17.5 1.5 1.4 30 71 11 10 34 0,42

24 1.5 1.4 41.2 87 13 13,7 42 0,47

25.8(b) 1.5 1.4 44.2 91 14 14,7 44 0,49

36 1.5 1.4 61.7 109 16 20,6 53 0,57

Where the voltage values marked (b) are used in North America.

1818

3.6 Rate of rise of transient recovery voltage (TRV) after generator-source fault

In the case of short-circuits supplied by generators, Standard IEC/IEEE 62271-37-013 (part 4.105 and 8.103.7.3) defines the following parameters (table 4), using the same model as the previous case:


Generator power Prospective recovery voltage (TRV)


RRRV

MVA µs kV kV/µs

10 - 50 1.23 Ur 1.84 Ur 1.5

51 - 100 1.15 Ur 1.84 Ur 1.6

Delay time td equals 0.5 µs.

As can be seen, the rate of rise of recovery voltage is lower but still higher than the indications given by Standard IEC 62271-100 for interruption in cable systems. This latter standard does not envisage the case of generators since it is beyond its scope.

1919

3.7 Presence of fault currents due to closing in out-of-phase conditions

When it comes to TRV, fault current interruption due to closing in out-of-phase conditions is extremely challenging for the circuit-breaker. Moreover, if a generator is connected to the system in full phase opposition conditions (180° phase difference), the current will generally exceed the short-circuit current at the generator terminals. This condition can be very dangerous for the generator and must be absolutely avoided (by resorting to automatic synchronization for example).Generator circuit-breakers are not required to interrupt full phase opposition. In this case, the assigned breaking capacity is 50% of the system-source symmetrical breaking capacity, which corresponds to a 90° out-of-phase angle.When it comes to interruption in the presence of fault currents due to closing in out-of-phase conditions, Standard IEC/IEEE 62271-37-013 (part 8.103.9.2) uses the same model as the one for calculating TRV, but considers that recovery voltage at normal frequency is √2 times the maximum operating voltage of the generator. Thus:

The reference table taken from table 1 of Standard IEC 62271-100 is given below:

Rated voltage First pole to clear factor

Amplitude factor

Peak value of TRV

Time Delay Voltage Time RRRV

Ur kpp kaf uc t3 td u' t' uc/t3

kV p.u. p.u. kV µs µs kV µs kV/µs

3.6 2.5 1.25 9.2 82 12 3.1 40 0.11

4.76(b) 2.5 1.25 12.1 88 13 4 43 0.14

7.2 2.5 1.25 18.4 102 15 6.1 49 0.18

8.25(b) 2.5 1.25 21.1 104 16 7 50 0.2

12 2.5 1.25 30.6 122 18 10.2 59 0.25

15(b) 2.5 1.25 38.3 132 20 12.8 64 0.29

17.5 2.5 1.25 44.7 142 21 14.9 69 0.31

24 2.5 1.25 61.2 174 26 20.4 84 0.35

25.8(b) 2.5 1.25 65.8 182 27 21.9 88 0.36

36 2.5 1.25 91.9 218 33 30.6 105 0.42

Where the voltage values marked (b) are used in North America.

Generator power Recovery voltage (TRV)


RRRV

MVA µs kV kV/µs

10 - 50 0.87 Ur 2.6 Ur 3.0

51 - 100 0.79 Ur 2.6 Ur 3.3


On the other hand, Standard IEC 62271-100 (part 4.102.2) specifies that kaf equals 1.25, Kpp equals 2.5 and t3 equals twice the value of t3 for terminal faults.In the case of cable systems, the delay time for the out-of-phase condition is: td = 0.15 x t3.

The test for breaking in the presence of fault currents due to closing in out-of-phase conditions is not mandatory for Standard IEC/IEEE 62271-37-013. If required by the application, the parameters given in table 6 apply (model being equal):

2020

3.8 Load current making and breakingIn accordance with Standard IEC/IEEE 62271-37-013 (part 6.104), a generator circuit-breaker must be able to make and break load currents up to the continuous rated current of the generator. This operation may be required in emergency circumstances and is therefore occasional. In this situation, it is clear that both circuit-breaker terminals remain energized. The test can be performed in both three-phase and single-phase conditions with <20% degree of asymmetry upon contact separation. 3 breaking tests must be performed in three-phase conditions. The Standard establishes the following parameters with respect to TRV (table 5):

3.9 Summary of the comparison between IEC/IEEE 62271-37-013 and IEC 62271-100 in relation to TRV parameters

Outlining the issues discussed in the previous chapters concerning the parameters that define the trend of TRV for tests on circuit-breakers, the following graph illustrates the rate of rise of the transient recovery voltage tracked, by way of example, for 12 kV voltage and generators with 10 to 50 MVA power ratings. Note how the rate of rise is always sharper when it conforms to Standard IEC/IEEE 62271-37-013, as compared to IEC 62271-100.


Generator power Recovery voltage (TRV)


RRRV

MVA µs kV kV/µs

10 - 50 1,03 Ur 0,92 Ur 0,9

51 - 100 0,92 Ur 0,92 Ur 1,0


On the other hand, Standard IEC 62271-100 establishes that the electrical life expectancy of circuit-breakers intended for use without the rapid auto-reclosing function, such as in cable networks, is demonstrated by the performance of short-circuit test cycles without intermediate maintenance. Thus additional tests are not required.

21

35

30

25

20

15

10

5

0

uc [k

V]

t3 [µs]

0 5 10 15 20

td [µs]

0 20 40 60 80 100 120 140

21

Delay time td is the other important parameter to be considered since it is critical for the very first breaking instants.According to Standard IEC/IEEE 62271-37-013, time td is 1µs for system-source short-circuits, 0.5µs for supply by generators and 1µs for out-of-phase conditions.Standard IEC 62271-100 specifies that time td varies depending on the voltage. For example, in the case of 12 kV voltage it is 9µs for terminal faults and 18µs for out-of-phase conditions. Here again, Standard IEC/IEEE 62271-37-013 defines more stringent values for the circuit-breaker.

IEC/IEEE load current

IEC out-of-phase

IEC/IEEE out-of-phase

IEC/IEEE generator-source

IEC system-source

IEC/IEEE system-source

IEC/IEEE system-source IEC system-source IEC/IEEE generator-source

IEC/IEEE out-of-phase IEC out-of-phase IEC/IEEE load current

2222


3.10 Short-circuit rated making currentStandard IEC/IEEE 62271-37-013 (part 4.102 and 6.103.12), the rated short-circuit making current at rated frequency and with 133 ms time constant must be 2.74 times the root-mean-square value of the alternating component of its rated system-source short-circuit breaking current. If the circuit-breaker is assigned a breaking capacity for supply by generator and the breaking current is higher than the previous value, the breaking current must be assigned by the manufacturer.On the other hand, Standard IEC 62271-100 (part 4.103) requires application of the following values:– For 50 Hz rated frequency and 45 ms time constant,

2.5 times the root-mean-square value of the alternating component of its rated short-circuit breaking current.

– For 60 Hz rated frequency and 45 ms time constant, 2.6 times the root-mean-square value of the alternating component of its rated short-circuit breaking current.

Thus Standard IEC 62271-100 is less stringent.

3.11 Short-time withstand currentStandard IEC/IEEE 62271-37-013 refers to IEC 62271-1 (part 4.6 and 4.7) according to which a current equal to the short-circuit capacity must be brought to the closed position for 1s, but which also allows other preferential values such as 0.5, 2 and 3s.Standard IEC 62271-100 obviously refers to IEC 62271-1.

3.12 Rated operating sequenceStandard IEC/IEEE 62271-37-013 (part 4.106.1), the envisaged sequence of operations comprises two operating cycles CO with a 30-minute interval between them, thus:

CO-30 min-CO

Standard IEC 62271-100 (part 4.104) envisages two alternative sequences:a) O – t – CO – t' – CO with t= 3 min if rapid auto-reclosing is

not involved and t= 0.3 s with rapid auto-reclosing. t’ is 3 min in both

cases;b) CO – t'' – CO with t'' = 15 s for circuit-breakers not

intended for rapid auto-reclosing

2323

3.14 Type tests required by the Standards

Standard IEC/IEEE 62271-37-013 (chap.6, table 8) specifies the following type tests:

Type tests Parts of Std IEC/IEEE 62271-37-013

Dielectric tests 6.2

Measurements of main circuit resistance 6.4

Temperature-rise tests 6.5

Short-time withstand current and peak withstand current tests

6.6

Additional tests on auxiliary and control circuits 6.10

Mechanical operation tests at ambient temperature from 6.101.2.1 to 6.101.2.3

Sound pressure level tests 6.101.4

System-source short-circuit current making and breaking test

6.103

Load current switching tests 6.104

Generator-source short-circuit current making and breaking tests

6.105

Out-of-phase making and breaking tests 6.106

Standard IEC 62271-100 (chap.6, table 11) specifies the following type tests:

The generator-source short-circuit making and breaking test is not envisaged by IEC 62271-100 because it is beyond its scope and the out-of-phase making and breaking test is not mandatory but can be performed if required by the specific application.

Type tests Parts of Standard IEC 62271-100

Dielectric tests 6.2

Measurements of main circuit resistance 6.4

Temperature-rise tests 6.5

Short-time withstand current and peak withstand current tests

6.6

Additional tests on auxiliary and control circuits 6.10

Mechanical operation tests at ambient air temperature

from 6.101.2.1 to 6.101.2.3

Short-circuit current making and breaking tests from 6.102 to 6.106

Electrical endurance tests (only for Ur ≤ 52kV voltage values) (only for class E2)

6.112

3.13 Number of mechanical operationsStandard IEC/IEEE 62271-37-013 (part 4.108) defines two mechanical operation endurance capability classes: class M1 and class M2. The circuit-breaker must perform the number of operating cycles specified for each class, taking into account the maintenance schedule established by the manufacturer:

Standard generator circuit-breaker (normal mechanical endurance)Class M1

1000 operating cycles

Circuit-breaker for special service requirements (extended mechanical endurance)Class M2


Standard IEC 62271-100 (part 4.110) also includes the two mechanical endurance classes (M1 and M2), but with a more demanding number of operations. The circuit-breaker must perform the number of operating cycles specified for each class, taking into account the maintenance schedule established by the manufacturer:

Standard circuit-breaker (normal mechanical endurance)Class M1


Circuit-breaker for special service requirements (extended mechanical endurance)Class M2


24

0 20 40 60 80 100

100

90

80

70

60

50

40

30

20

10

0

24

Time constant = 133 ms

Time after fault initiation (ms)

Deg

ree

of a

sym

met

ry (%

)

4 Choice of generator circuit-breaker

In chapter 8 and Annex E, standard IEC/IEEE 62271-37-013 provides guidelines as how to calculate the short-circuit currents of an installation with a generator, for the purpose of choosing the protection circuit-breaker.The following parameters must be defined when choosing the circuit-breaker:a) System-source short-circuit current:

– Symmetrical breaking capacity– Asymmetrical breaking capacity– Short-time withstand current

b) Generator-source short-circuit current:– Symmetrical breaking capacity– Asymmetrical breaking capacity– Asymmetrical breaking capacity at maximum degree of

asymmetryc) For system or generator-source: making capacity

The first Xsys can be calculated as: Xsys ≅ c Umsys / ( 3 I”k sys) Where factor c for voltage up to 35 kV is 1.1 (IEC 60038, table III), Um sys is the system voltage and Ik” sys is the initial short-circuit current of the system.Reactance should thus be considered in relation to the voltage level on the generator side of the transformer by multiplying it by ratio (Ur /Um sys)2

For the reactance of the transformer, XT ≅ (ukr / 100%) (Ur2/SrT)

Where ukr is the short-circuit voltage at the rated current in %, Srt is the apparent power of the transformer.In the absence of other components (e.g. motors, cables, auxiliary system), the root-mean-square value of the system-source short-circuit current is obtained by:Ik” = c Ur / ( 3 (Xsys+XT))The percentage of direct component Idc cs of the short-circuit current must be calculated in order to define the asymmetrical breaking capacity. The degree of asymmetry upon separation of the circuit-breaker contacts is defined in the following way:Asycs = 100% • Idc cs/( 2 Isc)Where Isc is the root-mean-square value of the alternating component of the short-circuit current the instant the contacts of the circuit-breaker tcs separate.The standard time constant of the direct component defined by the Standard is τ=133 ms. Lastly, let us assume that the direct component is measured at the instant the circuit-breaker contacts separate, typically 50 ms (½ cycle=detection time of the protection system plus the minimum opening time of the circuit-breaker). With τ=133 ms, the degree of asymmetry can be easily obtained from the following curve proposed by the Standard:

4.1 System-source short-circuit current

As established by the Standard, the breaking capacity is related to the root-mean-square-value of the symmetrical component of the three-phase short-circuit. This is obtained beginning from equivalent reactance X given by the sum of the reactance due to the short-circuit current of the system and from the reactance of the transformer (if installed) considered in relation to the voltage level of the fault point in question.

25

-10

0

1

I"sc

I'scIsc

t

25

In the case of different time constants, the degree of asymmetry can be calculated in the following way. First the direct component is calculated:

Where Ik” is the initial symmetrical short-circuit current. Assuming that the root-mean-square value of this current is constant over time, one can affirm that the instant of contact separation Ik” = Isc

Lastly, τ can be calculated as:

where X is the equivalent reactance of the system with reference to the MV side of the transformer and R is the equivalent resistance with reference to the same point; ϖ equals 2πf, with system frequency f, and t is still the instant in which the circuit-breaker contacts separate.The asymmetrical breaking capacity is thus:

or also:

In short, to define the breaking capacity required in a generator circuit-breaker in the event of a system-source fault, any combination of alternating and direct components can be considered so long as:– the alternating component does not exceed the

symmetrical breaking capacity;– the asymmetrical short-circuit current does not

exceed the asymmetrical breaking capacity;– the degree of asymmetry is 100% or less.

4.2 Generator-source short-circuit currentIn accordance with the Standard, the short-circuit current for supply by generator that a circuit-breaker is called upon to break is the highest root-mean-square value reached by the symmetrical component in the case of a three-phase fault. However, it can be much lower that the system-source short-circuit current. The value is measured by means of the oscillation envelope of the current the instant in which the contacts separate, when the current source is completely via generator without transformation. The envelope will take account of the generator time constants since the alternating component dampens with the sub-transient and transient time constants of the generator, as shown by the diagram below.

The root-mean-square value of the alternating component of the generator-source short-circuit current can be calculated using (in no-load mode) the following equation:

where:UmG is the maximum phase voltage of the generator;SrG is the rated power of the generator;UrG is the rated voltage of the generator;x”d is the saturated value of the direct axis subtransient reactance in p.u.;x’d is the saturated value of the direct axis transient reactance in p.u.;xd is the saturated value of the direct axis synchronous reactance in p.u.;τ”d is the direct axis subtransient short-circuit time constant in s;τ’d is the direct transient short-circuit time constant in s.

time

Current

Beginning of contact separation

Separation of contacts

2626

If the fault begins when the voltage in one phase passes through zero, the resulting fault current in that phase will have the maximum degree of asymmetry. Once again, the alternating component dampens with the sub-transient and transient time constants while the direct component dampens with armature short-circuit time constant τa.


since, with fair approximation, x”d equals x”q quadrature-axis subtransient reactance, again in no-load mode, the formula becomes:

τa can be calculated in the following way:

τa = X”d/ϖ Ra (again in the case of X”d ≅ X”q)

where X”d is the subtransient direct axis reactance and Ra the armature resistance in d.c.

The asymmetrical short-circuit current for generator supply is normally calculated using appropriate calculation programs (e.g. EMTP, ElectroMagnetic Transient Program) especially in the case of over-excited or under-excited generators for which approximated formulas are not used. In short, to define the breaking capacity required in a generator circuit-breaker in the event of a fault supplied by generator, any combination of alternating and direct components can be considered so long as:– the alternating component does not exceed the symmetrical

breaking capacity for generator supply;– the asymmetrical short-circuit current does not exceed the

asymmetrical breaking capacity;– if the degree of asymmetry exceeds 100%, calculations are

made to demonstrate that the generator circuit-breaker is able to force the current to zero by means of its arc voltage within the maximum supportable arcing time.

If the alternating component of the fault current were to dampen faster than the direct component, the value of the direct component could be higher than the value of the alternating component for a certain period of time running from the instant the fault begins. This would mean that the degree of asymmetry of the current is more than 100%, which could lead to delayed current zeros for certain period of time, as shown in the diagram below:

27

A

C

B

IMC

27

The matter becomes even more complicated because the value of the alternating component and the degree of asymmetry may vary, depending on whether the generator was functioning in unloaded conditions, was over-energized (thus with a lagging or inductive power factor) or under-energized (leading, thus capacitive power factor) before the fault occurred.Analysis of a large number of generators has shown typical degree of asymmetry values for short-circuit current that can exceed 130%. The worst case is when the generator functions with a lagging power factor before the fault, since the short-circuit currents that occur have a lower alternating component but with a higher degree of asymmetry. This case must therefore be treated with care.When the current trend clearly shows that delayed current zeros could occur (with > 100% degrees of asymmetry), the capacity of the circuit-breaker to interrupt the current by forcing it to zero within the maximum supportable arcing time must be demonstrated by means of simulations that consider the effect of the generator circuit-breaker's arcing voltage on the presumed current. Depending on the current, the arc voltage must be transferred into a mathematical model so that circuit-breaker behaviour during current interruptions with lack of current zeroes can be simulated. This is done by using a non-linear resistor, thus variable over time with current Rarc (i,t) beginning the instant the circuit-breaker contacts separate.If the arcing time resulting from the presence of lack of current zeroes were to exceed the maximum arcing time and the maximum energy the circuit-breaker is able to withstand, one possible solution would be to delay the release signal of the circuit-breaker so as to return below that maximum value. This would clearly lengthen the time the installation would be exposed to short-circuit current. For that reason, this solution must be carefully assessed and agreed with the user.

4.3 Making capacityThe short-circuit current that the generator circuit-breaker must interrupt is whichever one is highest between the short-circuit current values for system-source and generator-source.The highest value is normally that for system-source.

Cur

rent

i

The ratio between the peak value of the short-circuit current Imc and the root-mean-square value of the alternating component of the system-source short-circuit-current Isc can be calculated with the following formula:

Where t is approximately worth ½ a cycle in ms.

On the other hand, if the short-circuit current for generator supply were higher than that of the system, the relative peak value would depend on dampening of the alternating component, i.e. on the time constants of the generator and, consequently, could vary depending on the application.

2828

4.4 Choice of generator circuit-breaker class

Standard IEC/IEEE 62271-37-013 provides a method for choosing the class (G1 or G2) of generator circuit-breaker.In practice, three-phase short-circuit faults occur when the generator starts after installation downtime for maintenance or when put into service for the first time. Taking account of the probability of faults occurring and for practical reasons, only the case of faults for generators operating in unloaded conditions will be considered here.

The following are defined:– Iscg_unl the root-mean-square value of the symmetrical

component of the presumed short-circuit current for faults supplied by unloaded generator conditions prior to the fault;

– Isc is the breaking capacity of the generator circuit-breaker in root-mean-square value of the symmetrical component

– the following hypotheses also apply: the calculations do not initially take account of the arcing voltage introduced by the circuit-breaker, that the fault begins with nil voltage on one phase (thus with the current corresponding to maximum asymmetry) and, lastly, that the values are calculated the instant the circuit-breaker contacts separate.

Three possibilities can be distinguished on the basis of the definition given for classes G1 and G2 (sect. 3.4):

– Case a): the degree of asymmetry calculated does not exceed 110%. In this case, a circuit-breaker in either class G1 or G2 can be chosen so long as Isc is not less than Iscg_unl

– Case b): the degree of asymmetry calculated is between 110% and 130%. This situation gives rise to three further possibilities.- b1) choice of a generator in class G1 with:

a breaking capacity Isc of not less than Iscg_unl; make sure that the asymmetric short-circuit current

calculated the instant the contacts separate does not exceed the breaking capacity for asymmetric current;

- b2) choice of a generator in class G1 with: a 0.74 • Isc value of not less than Iscg_unl;

- b3) choice of a generator in class G2 with: a breaking capacity Isc of not less than Iscg_unl;

– Case c): the degree of asymmetry calculated exceeds

130%. Again, there are three further possibilities.- c1) choice of a generator in class G1 with:


calculated the instant the contacts separate does not exceed the breaking capacity for asymmetric current;

- c2) choice of a generator in class G1 with: a 0.74 ∙ Isc value of not less than Iscg_unl; make sure that the asymmetric short-circuit current

calculated the instant the contacts separate does not exceed the value of

0,74 ∙ Iscg 1+2 ∙ 1.32 ;- c3) choice of a generator in class G2 with:


calculated the instant the contacts separate does not exceed the breaking capacity for asymmetric current.

In all cases, the capability of a generator circuit-breaker to interrupt a given short-circuit current which shows delayed current zeros can be considered as being demonstrated if the generator circuit-breaker is capable of forcing the current to zero within the maximum permissible arcing time or supporting the maximum energy permitted by the pole.


2929

4.5 Information to be given with enquiries, tenders and orders

The list of information required for requests for offers of generator circuit-breakers differs from that defined by Standard IEC 62271-1.

For example, Standard IEC/IEEE 62271-37-013 specifies that the following information is necessary:

Defined as necessary by Standard IEC/IEEE 62271-37-013

Why it is important….

Single-line diagram of the installation – Allows the contribution of motors or high loads to the short-circuit current to be identified;

– Allows the feasible installation configurations to be assessed and the choice of circuit-breaker to be optimized;

Rated, under and over voltage; – Allows the most critical conditions to be assessed;

Rated frequency –

Generator data: rated values, reactances, time constants, armature resistance, moment of inertia, operating capability curve, with indications of the limits in MW and MVAr;

– Allow the calculation of the fault current to be assessed;

– Allow the symmetrical component at the instant the contacts separate to be assessed;

– Allow the presence of delayed current zeros to be assessed;

– Allow the energy brought into play during the interruption to be assessed;

– Allow assessments to be made of the TRV for a fault supplied by generator and for the lack of synchronism condition;

Generator earthing system – Fundamental for identifying current evolution in the instants following current interruption on the first phase;

– Impact on maximum arcing time;

Power transformer data, if installed (rated values, reactances, resistance values and time constants)

– Fundamental for defining the short-circuit current on the switchgear busbars and for defining asymmetry;

Data of the voltage switch of the power transformer (if installed) and variations in the impedance when the actual switch operates

– If installed, important for defining its impact on the short-circuit current;

Maximum system-source short-circuit current on system side of transformer;

– Allows the contribution of the system to the short-circuit current to be assessed;

Time constants of the system; – Allow assessments to be made of the peak current and asymmetry by the system;

– Allow the worst case of the energy involved in the interrupter owing to a system fault to be considered;

Values of the surge capacitors, if installed – Allow the impact on the TRV to be assessed.

This information is absolutely essential if a generator circuit-breaker is to be chosen correctly, especially when plant situations and special generators require simulations to be performed in order to calculate the required breaking capacity.

3030

5 Example

5.1 Installation data

Power system:Scc=2000MVA X/R=10 Vn=150 kV

Transformer with 3 windings:V1 = 150 kV S1 = 150 MVA υcc_12 = 11.5% @ 55 MVAV2 = 11.5 kV S2 = 75 MVA υcc_13 = 11.1% @ 55 MVAV3 = 11.5 kV S3 = 75 MVA υcc_23 = 21% @ 55 MVA

Generators:Sn = 75.294 MVAVn = 11.5kVXd = 2.26 Xq = 2.06 Td’ = 0.71 Tq’ = 0.71Xd’ = 0.217 Xq’ = 0.26 Td’’ = 0.04 Tq’’ = 0.04Xd’’ = 0.155 Xq’’ = 0.19 Ra = 0.001309

The reactances and resistances are given in p.u. while the values of the time constants are given in seconds. According to standard IEC 60034-3, the permitted tolerances can be around ±15%, thus all reactances are decreased by that percentage as a precaution.

5.2 Simplified calculation of the symmetrical component

We will first analyze the symmetrical current at instant t=0 (i.e. the moment that short-circuit occurs) on the supply side and then on the load side of the generator circuit-breaker (GCB). After this, the capability of the circuit-breaker to eliminate a three-phase-earth short-circuit in the two above-mentioned points will be assessed.We will first consider a three-phase earthed fault between the GCB circuit-breaker and generator G1. Application of the MVA method allows the value of the short-circuit symmetrical current to be assessed in just a few steps. First, we must make sure that the vcc_12; vcc_13; vcc_23 values are given according to the same basis. After this, the values of the short-circuit impedances for each winding can be obtained from the following relations:

Now let us suppose that the transformer with three windings is like the one in the following equivalent diagram:

Two generators connected to the HV grid by means of a transformer with three windings are considered in the proposed installation. The starting condition will be that of an initially unloaded generator. A 1.05 voltage factor is considered for this installation.

Sn= 55 MVAvcc_1%= 0.8%Scc1= 6875 MVA

Sn= 55 MVAvcc_3%= 10.3%Scc3= 533.98 MVA

Sn= 55 MVAvcc_2%= 10.7%Scc2= 514.01 MVA

31

Scc1= 6875 MVA

Snet= 2000 MVA

G G

31

The MVA method can now be applied to the circuit in the diagram:

– The short-circuit current for a fault between the circuit-breaker and generator G1 will be calculated first:

11

3

k

22

Scc2= 514.01 MVA

Sg2= 571.49 MVA

Scc3= 533.98 MVA

Sg1= 571.49 MVA

32

GS GS

GCB

TR

G1 G2

3

1

2

Isff IG2

IGRID

32

Figure 5: Flow of short-circuit currents for a system-source fault

The symmetrical short-circuit current at instant t=0 can be obtained from this value.

After this, the behaviour of the short-circuit current will be assessed for two voltage phase angles, i.e. at 0 and 90 degrees.

.

. .

. .

.

5 Example

Ik" is the symmetrical short-circuit current value at time t=0. This value acts as a reference for successive simulation performed via computer using EMTP (Electromagnetic Transient Program) software. It also allows an initial estimation to be made of the size of the circuit-breaker required. The single-line diagram showing the flow of the short-circuit currents for system-source faults for this particular example is given below. Current Isff is the symmetrical short-circuit current to which value Ik" corresponds at time t=0.

3333

Figure 6: Short-circuit currents for a system-source fault

By assessing the current values (with the exception of the peak value) in the graph at instant t=45ms, one observes:

Note that the value of the symmetrical component is slightly different from the one observed at instant t=0. This is due to the contribution from the generator of the right-hand busbar which, in the absence of a constant symmetrical component, also changes the total symmetrical current value, although to a lesser extent.

The next data item to assess is the short-circuit current value in the case of a three-phase earthed fault supplied by the generator alone, considering the symmetrical component at instant t=0 and -15% tolerance on the reactance as explained previously:

Ip = 56,70 kA

Issf_sym = 21,38 kA

idc% = 53,75%

34

GS GS

GCB

TR

G1 G2

3

1

2

IGRID

Isff

34

Figure 7: Flow of short-circuit currents for a fault supplied by generator

5 Example

After this, the behaviour of the short-circuit current will be assessed for two voltage phase angles, i.e. at 0 and 90 degrees.

Here again, short-circuit current I"kg acts as the reference value for the following computer simulation. The single-line diagram showing the current flow for faults supplied by generator (Igff) is given below.

3535

Figure 8: Generator-source short-circuit current, 90° voltage angle

Considering a 45ms instant (with the exception of the peak value), the current values are as follows:

As explained in chapter 4 and on the basis of the asymmetry value, this result shows that case b1 applies. Functionality is therefore guaranteed by choosing a class G1 circuit-breaker, such as VD4G-50.

Both the graphs in figures 8 and 9 show the two types of asymmetry that must be considered when choosing the circuit-breaker, as clearly suggested by the new standard for generator circuit-breakers (Annex E). These graphs also show that the maximum peak value of the short-circuit current is 80kA. This means that according to the new standard IEC/IEEE 62271-37-013, the minimum symmetrical component that can be selected is 80kA/2.74=29.19kA. In this case, 31.5 kA is the next largest size of circuit-breaker. This assessment narrows the field and allows one of the lower limits to be obtained. In the case of system-source faults, thus owing to simultaneous contributions from the grid and generator, the ratio between peak value and the real symmetrical component the instant the contacts separate may exceed the value of 2.74 (standardized value in the product standard). Checks based on the peak value mentioned above therefore allow assessments to be made of the minimum size that can be selected.

Ip = 80 kA

Igff_sym = 22,52 kA

idc% = 118%

3636

Figure 9: Generator-source short-circuit current, 0° voltage angle

As mentioned before, the other limit to consider is the symmetrical component of the short-circuit current which was calculated at instant t=0 using the MVA method. Although approximate, in the absence of detailed data it may be useful to assess that component at instant t=0. The value of the symmetrical component varies over time in both points in which the short-circuit was calculated, i.e. on the supply and load sides of the circuit-breaker. This is due to the fact that a generator whose symmetrical component is not constant during the short-circuit phase is involved in both cases. Consequently, the value of the total symmetrical component assessed the instant the contacts separate is lower than the value assessed by the MVA method, i.e. at t=0. The exact calculation of this value depends on the generator's exact characteristic parameters being known.

If short-circuit current breaking is now considered, one notes that separation of the contacts typically occurs at 45 ms (time given by the protection detection time + time it takes the circuit-breaker to operate). Arcing time will commence at this instant and will continue until the current has been extinguished.

Supposing that the fault occurred at instant t=0, the different instants of the short-circuit current are described below with reference to figure 10.T1 = detection time of relayT2 = circuit-breaker operating timeT3 = Arcing time T1+T2 = Contact separation timeT1+T2+T3 = Short-circuit current extintion time

5 Example

37

T1 T2 T3

37

Figure 10: Time of current interruption

The instant the contacts separate, an electric arc with a non-linear characteristic and strictly resistive nature forms between them. The presence of this resistance in series substantially changes the unidirectional time constant of the current itself. Since the time constant of the unidirectional component is:

where X2: Negative sequence reactanceƒ : FrequencyRa : Stator resistance.

presence at instant T3 of an additional resistance due to the arc changes the unidirectional time constant by forcing the current towards zero, as illustrated in figure 11.

38

8

6

4

2

0

-2

-4

-6

Idc

0 20 40 60 80 100 120 140 160

38

Figure 11: Effect of arc resistance on the unidirectional component of the short-circuit current

This aspect is no small matter, since current passage through zero is a necessary condition if interruption is to take place, thus the presence of an additional resistance helps this condition to occur.From the instant the contacts separate to current zero, the arc dissipates a certain quantity of energy in the interrupter.

Rarc : Arc resistance

In the case of vacuum circuit-breakers, the arc voltage can initially be considered more or less constant, thus

so energy measurement can be easily attributable to a measurement of A.s.

This energy is proportional to the area under the more asymmetrical current, between instant T3 and current zero:

Thus:

.

.

5 Examplecu

rren

t pu

star

t of

con

tact

op

enin

g

t=time in ms

3939

Figure 12: Energy involved in an interrupter after separation of the contacts due to generator-source fault, 90° voltage angle

The capability of the interrupter to deal with this energy is of fundamental importance for assessing the applicability of the circuit-breaker, thus it is important for that energy not to exceed the limit of the interrupter itself.In the case of short-circuits supplied by generator but with 0° phase angle, the generator switches to two-phase operation owing to current interruption in phase 1. This change in configuration may result delayed current zeros and lengthen the arc extinction time. In such cases, the energy will consequently increase as shown in figure 13.

Thus both situations are important when assessing the operation of a circuit-breaker. That the VD4G-50 considered is able to interrupt the current without difficulty can be demonstrated. The interrupters of the past were able to withstand a lesser amount of energy than the current VD4G interrupters. Arc energy (blue area and green area), which lasts from the natural opening time of the contacts (45 ms) until current zero occurs (necessary condition for current interruption), could therefore have exceeded the interrupter's capacity to withstand it. In that case and since fault current comprises a symmetrical component and a decreasing unidirectional component, an opening delay was typically set by means of a relay so as to wait for the current and, thus, the arcing time and relative energy, to diminish.

4040

Figure 13: Energy involved in an interrupter after separation of the contacts due to generator-source fault, 0° voltage angle

These guidelines explain how other parameters are of fundamental importance when choosing a circuit-breaker, e.g. TRV assessment in the three specific cases:– System-source fault – Generator-source fault– Fault in out-of-phase conditions

As mentioned previously, TRV (Transient Recovery Voltage) occurs between the contacts of a circuit-breaker when it opens. By and large, the most critical TRV value occurs on the first pole that breaks. Under no circumstances may the TRV values be exceeded, otherwise re-ignition could occur with consequent failure to interrupt the current itself.

Owing to its nature, TRV depends on what happens on the supply and load sides of the circuit-breaker, i.e.: the characteristics of the connection cables between the generator and circuit-breaker and between this and the step-up transformer, the capacitance to earth of the generator, the leakage capacitances of the transformer, etc. In actual fact, the interruption technology may also influence the TRV trend, specially during the first microseconds after current interruption.

5 Example

4141

Figure 14. TRV for a system-source fault

After the following considerations, the TRV was initially assessed for a three-phase-earth fault supplied by the system, i.e. between the generator and relevant circuit-breaker. In this case, the trend was mainly determined by the characteristics of the connection cables, the parameters of the system and its capacitances to earth:

4242

Figure 15: TRV for generator-source fault

The leakage capacitances of the stator to earth are of fundamental importance for faults supplied by generator:

5 Example

4343

Faults in out-of-phase conditions are due to the circuit-breaker having closed in the absence of synchronism between the grid and the generator itself. This condition may occur if the system for paralleling the generator with the grid functions in a faulty way. The fault current that occurs in this case follows the characteristic trend in the figure, which mainly depends on the inertia of the rotor and relative turbine connected:

Figure 16: Fault current due to circuit-breaker closing in out-of-phase conditions at 90° phase difference.

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Figure 17: TRV for fault interruption in out-of-phase conditions

Although it may not seem so frequent, this type of fault must still be considered since its effects can be serious. Thus the capability of a generator circuit-breaker to deal with it is of fundamental importance.

The example analyzed describes a classic approach to choosing a generator circuit-breaker. Introduction of a universally acknowledged international standard for generator circuit-breakers like IEC/IEEE 62271-37-013, according to which VD4G have been type-approved, allows these circuit-breakers to be chosen knowing that the product complies with the best working standards.

5 Example

ABB S.p.A.ABB SACE DivisionMedium Voltage Products Via Friuli, 4I-24044 DalmineTel.: +39 035 6952 111Fax: +39 035 6952 874e-mail: [email protected]

www.abb.com

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The data and illustrations are not binding. We reserve the right to make changes without notice in the course of technical and product development.

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