IEC Standards & Power Factor Correction

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Circuit Breakers Standards Guidelines IEC 60947-2

Agenda



IEC 60947-2

Circuit Breaker Standard, for industrial application –

Definitions for MCCBs and ACBs

–

Choice criteria based on rated and limit values

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Standard for LV apparatus 

IEC 60947 Standard for industrial application

–

International Standard

IEC 60947

–

European Standard

EN 60947

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

IEC 60947-1

Part 1: General rules



IEC 60947-2

Part 2: Circuit breakers



IEC 60947-3

Part 3: Switch disconnectors



IEC 60947-4-1

Part 4: Contactors



IEC 60947-5-1

Part 5: Control circuit devices



IEC 60947-6-1

Part 6: Multifunction devices



IEC 60947-7-1

Part 7: Auxiliary materials

IEC Standard definitions 

Circuit Breaker - IEC 60947-2

A mechanical switching device capable of breaking, carrying and making currents under normal circuit conditions and also making, carrying, for a specified time, and breaking currents under specified abnormal circuit conditions such as those of short-circuit.

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

BREAKING

Breaking Capacity



WITHSTAND

Short time withstand



MAKING

Making Capacity

IEC Standard definitions 

Switch Disconnector - IEC 60947-3

A mechanical switching device capable of breaking, making and carrying currents under normal circuit conditions but only making and carrying, for a specified time, currents under specified abnormal circuit conditions such as those of short-circuit.

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

BREAKING

Breaking Capacity



WITHSTAND

Short time withstand



MAKING

Making Capacity

IEC Standard definitions

Moulded case circuit breaker (MCCB): a circuit breaker having a supporting housing of moulding insulating material, forming an integral part of the circuit breaker (Tmax-XT).

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IEC Standard definitions Air circuit breaker (ACB): a circuit breaker having a supporting housing of moulding insulating material and a metallic frame, forming an integral part of the circuit breaker (Emax & Emax 2).

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Current limiting circuit breaker 

Current limiting circuit breaker (IEC 60947-2 def. 2.3)

A circuit breaker with a break-time short enough to prevent the short-circuit current from reaching its peak value.

A current-limiting circuit breaker is able to reduce the stress, both thermal and dynamic, because it has been designed to start the opening operation before the shortcircuit current has reached its first peak, and to quickly extinguish the arc between the contacts.

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Current limiting circuit breaker

A

R

A

I

R

A = Direction of arc due to the magnetic field R= Repulsion of moving contacts due to the short circuit current

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Energy limitation Current

Time

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Peak limitation curves Value of the limited peak of the short circuit current according to the value of the symmetrical short circuit current Irms.

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I2t curves

Value of the let-through energy according to the value of the symmetrical short circuit current I rms.

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Energy limitation Protection against short-circuit (IEC 60364) To protect a cable against short-circuit, the specific let-through energy of the protective device must be lower or equal to the withstanding energy of the cable: Specific let through energy curve LLL

1E3MA²s

100MA²s

where – I2 t is the specific let-through energy of the protective device which can be read on the curves supplied by the manufacturer; – S is the cable cross section [mm 2]; in the case of conductors in parallel it is the cross section of the single conductor; – k is a factor that depends on the cable insulating and conducting material.

10MA²s

1MA²s

0.1MA²s

1E-2MA²s

0.1kA

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1kA

10kA

100kA

Choice criteria



Rated values (Iu, Ue) 

Limit values (Icu, Ics, Icw, Icm)



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Insulation values (U i, Uimp)

Rated value I u 

Rated uninterrupted current Iu the rated uninterrupted current of an equipment is a value of current, stated by the manufacturer, that the equipment can carry in uninterrupted duty (at 40 °C)

IEC 60947-1 def. 4.3.2.4

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Rated value I u The rated uninterrupted current I u is different from the rated current In, which is the rated current of the thermomagnetic or electronic trip unit and is lower or equal to I u. 

A new concept for setting the current In: the rating plug

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Rated value I u 

Rated uninterrupted current I u Some factors may reduce the I u of a circuit breaker like temperature, altitude or frequency. XT1 160

XT4 250

Rated value U e 

Rated operational voltage U e the rated operational voltage of an equipment is a value of voltage which, combined with a rated operational current, determines the application of the equipment and to which the relevant tests and the utilization categories are referred.

IEC 60947-1 def. 4.3.1.1

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Rated value U e 

Rated operational voltage U e Breaking capacity is always referred to the operational voltage; the breaking capacity decreases when the voltage increases.

Rated value U e 

Some factors may reduce the Ue of a circuit breaker

Choice criteria



Rated values (Iu, Ue)



Limit values (I cu, Ics, Icw, Icm)



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Limit value Icu Icu = RATED ULTIMATE SHORT CIRCUIT BREAKING CAPACITY

IEC 60947-2 def. 4.3.5.2.1

Breaking capacity according to a specified test sequence. Do not include after the short circuit test, the capability of the circuit breaker to carry its rated current continuously. - test sequence: O - 3 min - CO - dielectric withstand at 2 x Ue - verification of overload release at 2.5 x I1

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Limit value Ics Ics = RATED SERVICE SHORT CIRCUIT BREAKING CAPACITY

IEC 60947-2 def. 4.3.5.2.2

Breaking capacity according to a specified test sequence. Include after the short circuit test, the capability of the circuit breaker to carry its rated current continuously - test sequence: O - 3 min - CO - 3 min – CO - dielectric withstand at 2 x Ue - verification of temperature rise at I u - verification of overload release at 1.45 x I1 - verification of the electrical life

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Limit values I cu and Ics 

Relation between I cs and Icu

This relation is always true!!!

Ics ≤ Icu The service breaking capacity I cs can be expressed as a value of breaking current, in kA; a percentage of I cu, rounded up to the lowest whole number, in accordance with the table (for example Ics = 25% Icu). Standard ratios between

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and I I cs cu

When is Icu required? 

Where continuity of service is not a fundamental requirement.



For protection of single terminal load.



For motor protection.





Where maintenance work is easily carried out without much disruption. Generally for circuit breaker installed on terminals part of plant.

When is Ics required? 

Where continuity of service is a fundamental requirement.



For installation in power center.



Where is more difficult to make maintenance.



When is difficult to manage spare breakers.



Generally for installation in main distribution board immediately downstream transformer or generator.

Limit values I cu and Ics 

Icu and Ics: selection criteria

Main circuit breakers or circuit breakers for which a long out-of-service period can not be accepted (for example naval installation) Ics CB selection based on Icu

circuit breakers tor termlnal circuits or circuit breakers for economic application

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Icu or Ics ?



Application of Icu / Ics circuit breakers

When Isc = 100 % of Icu is not necessary ?



When the real short circuit current in the point of installation is lower than the maximum Ics breaking capacity. Breaker A: Icu =100 kA with Ics = 100 % of Icu Breaker B: Icu = 100 kA with Ics = 75 % of Icu Please also consider that short circuit current at the end of the line is still lower

A

70 kA B

50 kA !!! U

LOAD

When Isc = 100 % of Icu is not necessary ? 

Motor Protection according to IEC 60947- 4-1

Duty cycle: O - 3mins - CO at “Iq” current (maximum short circuit current) O - 3mins - CO at “r” current (critical short circuit current depending from the contactor size)

Where: O: Tripping of the circuit breaker under short circuit condition. CO: Closing by the contactor under short circuit condition and tripping of the circuit breaker.

Icu or Ics ? Conclusion 





Consider that not always Ics = 100% of Icu for all the employ voltage range, i.e. (from 220 V a.c. to 690 V a.c.duty, and 250 V d.c.). Selection of circuit breaker with breaking capacity Icu or Ics must be done according to the real technical installation requirement. Independently from the duty cycle selected the safety of the plant is strictly dependent from the maximum circuit breaking capacity (in most of cases Icu).

Limit value Icw IEC 60947-2 def. 4.3.5.4

Icw = RATED SHORT-TIME WITHSTAND CURRENT Example of use of category B circuit breakers in electrical plant Trafo 630kVA Ucc%=4%

ACB E1B12 400V

22.7kA MCCB XT4

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MCCB XT3

The upstream circuit breaker can withstand the fault current up to 1 sec, thus guaranteeing an excellent selectivity with downstream apparatus

Limit value Icw CATEGORY B CIRCUIT BREAKER

IEC 60947-2 Table 4

Circuit breakers specifically intended for selectivity in short circuit conditions in relation to other protection devices in load-side series, that is with an intentional delay (adjustable) applicable in short circuit conditions. These circuit breakers have a specified rated short-time withstand current I cw.

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Limit value Icw CATEGORY A CIRCUIT BREAKER

IEC 60947-2 Table 4

Circuit-breakers “not specifically” intended for selectivity under short circuit conditions with respect to other protection devices in series on the load side, that is without intentional short-time delay provided for selectivity under short-circuit conditions. These circuit-breakers have not a specified rated short-time withstand current value I cw.

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Limit value Icw Icw = RATED SHORT-TIME WITHSTAND CURRENT

IEC 60947-2 Table 3

It is the value of short-time withstand current assigned to the circuit-breaker by the manufacturer under specified test conditions. This value is referred to a specified time (usually 1s or 3s). It must be stated when the circuit-breaker is classified in category B and its value must be greater than: 

The highest value between 12 I u and 5 kA

for CBs with Iu  2500A



30 kA

for CBs with Iu > 2500A

Circuit breakers without I cw value are classified in category A

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Selectivity Categories

Limit value Icm Icm = RATED SHORT-CIRCUIT MAKING CAPACITY

IEC 60947-2 def. 4.3.5.1

Making capacity for which the prescribed conditions according to a specified test sequence include the capability of the circuit breaker to make the peak current corresponding to that rated capacity at the appropriate applied voltage. It is always necessary to verify that: Icm  Ipeak

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Limit value Icm For a.c. the rated short-circuit making capacity of a circuit-breaker shall be not less than its rated ultimate short-circuit breaking capacity, multiplied by the factor n of the table.

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IEC 60947-2 Table 2 Icm ≥ n x Icu

Current limiting circuit breaker Example Peak 105kA 100kA

T6L800 In800 54kA

16,8kA

XT2L 160 In160

10kA

10kA

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50kA

100kA

Irms

Limit value Icm

If the cos  of the plant is higher than the standard prescribed value, it is not necessary to take into account the rated shortcircuit making capacity of the circuit-breakers (Icm).

If the cos  of the plant is lower than the standard prescribed value, usually near to the transformer and/or generator, it is necessary to verify Icm  Ipeak.

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Limit value Icm Sometimes it can happen Short circuit current of the plant is Icc = 75kA ; The used circuit breaker has an Icu = 75 kA; According to the table 2, cosk=0.2 and n=2,2 so Icm = n x Icu = 165 kA.

If the cos k of the plant is equal to 0.16 (lower than the standard prescribed value) the evaluated Ip = 175 kA.

Since Ip > Icm the CB selected is not correct. I will use a CB with a greater value of Icu in order to have an Icm value suitable to the peak current of the plant.

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Limit value Icm

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Choice criteria



Rated values (Iu, Ue)



Limit values (Icu, Ics, Icw, Icm)



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Limit value Ui IEC 60947-1 def. 4.3.1.2

Ui = RATED INSULATION VOLTAGE

The rated insulation voltage of an equipment is the value of voltage to which dielectric tests and creepage distances are referred.

It shall be always verified that:

Ue < Ui

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Limit value Uimp Uimp = RATED IMPULSE WITHSTAND VOLTAGE

IEC 60947-1 def. 4.3.1.3

The peak value of an impulse voltage of prescribed form and polarity (1,2/50ms) which the equipment is capable of withstanding without failure under specified conditions of test and to which the values of the clearances are referred. It shall be always verified that: Uimp > transient overvoltage in the plant

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Temperature-rise for terminals and accessible parts IEC 60947- 2 Table 7

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Overload protection

IEC 60947- 2 Table 6

t

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i

Short circuit protection

IEC 60947- 2 8.3.3.1.2

t

S I

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i

Type Tests IEC 60947- 2 8.3

The tests to verify the characteristics of circuit breakers are: • type tests carried out on samples:

Type Tests

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Routine Tests IEC 60947- 2 8.4

• routine tests carried out on all circuit breakers and including the following tests:

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Annex F - J Tests of EMC for circuit breakers with electronic overcurrent protection •Electrostatic discharges •Radiated radio-frequency electromagnetic fields

Immunity

•Electrical fast transients/bursts •Surges •Conducted disturbances induced by radio-frequency fields •Harmonics •Voltage fluctuations

Emission

•Conducted disturbances •Radiated disturbances •Dry heat test Damp heat test

Climatic tests

•Temperature variation cycles at a specified rate of change

CE Marking

According to european directives:



© ABB Group



Low Voltage Directive 73/23 EEC



Electromagnetic Compatibility 89/336 EEC

Annex H

Test sequence for circuit-breakers for IT systems This test is intended to cover the case of a second fault to earth in presence of a first fault on the opposite side of a circuit breaker when installed in IT systems.

In this test at each pole the applied voltage shall be the phase-to-phase voltage corresponding to the maximum rated operational voltage of the circuit breaker at which it is suitable for applications on IT systems.

Circuit Breakers Standards Guidelines IEC 60898

IEC Standard definitions Miniature Circuit Breakers MCB International Standard References IEC 60898

Applicable to circuit-breakers for protection of wiring installation in buildings and similar applications, and designed for use by uninstructed persons, and for not being maintained. Part 1: Circuit-breakers for a.c. operation Part 2: Circuit-breakers for a.c. and d.c. operation

(additional requirements)

Choice criteria



Rated values (In, Ue)



Limit values (Icn, Ics)

Rated value I n Rated uninterrupted current (I n): the rated uninterrupted current of an equipment is a value of current, stated by the manufacturer, which the equipment can carry in uninterrupted duty, at a specified reference ambient air temperature (30 °C). 



The rated current doesn’t exceed the 125A.

IEC 60898-1 def. 5.2.2

Rated value U e

Rated operational voltage (U e): • The rated operational voltage of a circuit-breaker is the value of voltage, assigned by the manufacturer, to which its performances (particularly the short-circuit performance) are referred. • The rated operational voltage doesn’t exceed the 440Vac 220Vdc. IEC 60898-1 def. 5.2.1.1

Choice criteria



Rated values (In, Ue)

 

Limit values (Icn cn, Ics cs)

Limit value Icn Icn = RATED SHORT CIRCUIT CAPACITY

IEC 60898-1 def. 5.2.4

The rated short-circuit capacity is the value of the ultimate short-circuit breaking capacity for which the prescribed conditions, according to a specified test sequence, do not include the capability of the circuit-breaker to carry 0.85 times its non-tripping current for the conventional time. test test sequ sequen ence ce:: O - 3 min min - CO - leakage leakage current current at at 1.1 1.1 U e (< 2 mA) - dielectri dielectric c strength strength test test at 900 900 V - verificat verification ion of overloa overload d release release at 2.8 x I n The rated short circuit capacity doesn’t exceed the 25kA in ac and 10kA in dc

Limit value Ics Ics = RATED SERVICE SHORT CIRCUIT CAPACITY CAPACITY

IEC 60898-1 def. 3.5.5.2

The service short-circuit capacity of a circuit-breaker is the value of the breaking capacity for which the prescribed conditions according to a specified test sequence include the capability of the circuit-breaker to carry 0.85 times its nontripping current for the conventional time.

Limit value Ics Service Short Circuit capacity (I cs): - test seq. :

O - 3 min - O - 3 min – CO O - 3 min - CO - 3 min – CO - leakage current at 1.1 Ue (< 2 mA) - dielectric strength test - verification of no tripping at 0,85 x In

(for one or two poles cb) (for three or four poles cb)

A circuit-breaker with a rated short-circuit capacity (Icn) has a corresponding service shortcircuit capacity (I cs) as from this table: The circuit breaker with

Icn < 6000A 6000A < Icn < 10000A Icn > 10000A

Ics is equal to 1xIcn Ics is equal to 0,75xIcn Minimum value of Ics is 6000A. Ics is equal to 0,5xIcn Minimum value of Ics is 7500A.

Ics Test

Tripping Curves Overload characteristics The main difference between the overload protection curve of the CBs responding to IEC 60947 or IEC 60898 are referred to the conventional non tripping current. The prescibed conditions are given in this table:

Tripping Curves Magnetic characteristics The CBs according to IEC 60947 usually have the instantaneous threshold at 5 or 10 times the rated current with a tolerance of + 20%.

The CBs according to IEC 60898-1 (ac applications) have different instantaneous threshold referred to the type B , C , D as indicated in the table below:

Tripping Curves

Tripping Curves

In some cases, the conditions IB < In < IZ and I2 < 1.45 IZ do not guarantee complete protection, e.g. when overcurrents are present for long periods which are smaller than I2. They also do not necessarily lead to an economical solution. It is therefore assumed that the circuit is designed so that minor overloads of a long duration will not occur regularly.

IEC 60364-4-43

Tripping Curves

Comparison IEC 60947-2 vs IEC 60898 IEC 60947-2

IEC 60898-1

People

Instructed

Uninstructed

Maintenance

Possible

Not possible

< 1000 Vac

< 440 Vac

< 1500 Vdc

< 220 Vdc

40° C

30° C

Rated Voltage (Ue) Ambient Temperature

No limits Rated Current

(Iu < 6300 A) Short circuit breaking current

No limits for Icu

In = 125 A Icn = 25 kA (ac) Icn = 10 kA (dc)

Selection of protective Devices



Generalities about the main electrical parameters 

Don’t forget   

Ue  Un Icu or Ics  Ik Icm  Ip

Ue, Icu, Ics, Icm?




Protection of feeders 

against overload

I ≤ In or I1 ≤ Iz

 b



against short-circuit 

In Iz S

I2t ≤ k2S2

Ib

Selection of protective Devices The correct circuit breaker must be selected to satisfy the following conditions: •It

must own short circuit breaking power (lcu or eventually lcs) greater or equal to the short circuit current lcc •It

must use a protection release so that its overload setting current ln (l1) satisfies the relation lB < ln < lZ let through energy (l 2t) that flows through the circuit breaker must be lesser or equal to the maximal one allowed by the cable (K²S²) •The



As far as the verification required by IEC 60364, according to which the overload protection must have an intervention current lf that assures the operation for a value lesser than 1,45 lz (lf < 1,45 lz), we must state that it is always verified for ABB Circuit breakers, since according to IEC 60947-2 the required value is less than 1,3 ln.





Protection of generators I ≤ I1  I3 or I2 ≤ 2.5 -4 x Ingen  ngen

G




Protection of transformers I ≤ I1  Upstream CB  nT



I3 or I2  Iinrush

Selection of protective Devices 20kV 

Steps determining the short-circuit currents  choosing the CB  setting of the MV overcurrent protection …  setting of the LV overcurrent protection … 

400V

Selection of protective Devices 20kV

400V

Selection of protective Devices 20kV

400V

Protection of Transformers

As to be able to protect LV/MV transformers LV side, we must mainly take into account: Rated current of the protected transformer, LV side, from which the rated current of the circuit breaker and the setting depend on (In); •

The maximum estimated short circuit current in the installation point which defines the minimal breaking power of the protection circuit breaker (Isc). •

Protection of Transformers Switchboards with one transformer

Sn

U20

In Isc

The rated current of the transformers LV side is defined by the following expression Sn x 103 In = 3 x U20 where Sn = rated power of the transformer [kVA] U20 = rated secondary voltage (no load) of the transformer [V] ln = rated current of the transformer, LV side [A]


The full voltage three-phase short circuit current immediately after the LV side of the transformer can be expressed by the following relation once we suppose infinite power at the primary: Isc =

In x 100 Ucc %

where Ucc %= short circuit voltage of the transformer [%] ln

= rated current, LV side, [A]

lsc = three-phase rated short circuit current, LV side, [A]


The short circuit current is normally lesser than the preceding deduced value if the circuit breaker is installed at a certain distance by means of a cable or bar connection, according to the connection impedance.


The following table shows some possible choices within the SACE Emax ACB range according to the characteristics of the CB to protect. Attention Those indications are valid at the conditions that we declare in the table; different conditions will lead us to repeat calculations and modify the choices.

Protection of Transformers Sn

[kVA]

500

630

800

1000

1250

1600

2000

2500

3150

Ucc (1)

%

4

4

5

5

5

6,25

6,25

6,25

6,25

In (2)

[A]

722

909

1154

1443

1804

2309

2887

3608

4547

Isc (2)

[kA]

18

22.7

23.1

28.9

36.1

37

46.2

57.7

72.7

E1B12

E2B16

E2B20

E3B25

E3B32

E4S40

E6H50

SACE Emax

E1B08

E1B12

(1) For values of the percent short circuit voltage U’cc % different from the Ucc% values as per table, the rated three-phase short circuit current I’cn becomes: I’sc =

Isc

Ucc % U’cc %

(2) The calculated values refer to a U20 voltage of 400 V. for different U’20 values, do multiply In and Isc the f ollowing k times:

U’20

k

[V]

220

380

400

415

440

480

500

660

690

1.82

1.05

1

0.96

0.91

0.83

0.8

0.606

0.580

Protection of Transformers Switchboards with more than 1 transformer in Parallel

3

2

1 Isc2 + Isc3

I2

I1

C i r c u i t b r e a k e r A

I3

Isc1

I4

I5

Isc1 + Isc2 + Isc3

C i r c u i t b r e a k e r B


As far as the calculation of the rated current of the transformer is concerned, the rules beforehand indicated are completely valid. The minimum breaking capacity of each circuit breaker LV side must be greater than the highest of the following values: (the example refers to machine 1 of the figure and it is valid for the three machines in parallel): •lsc

1 (short circuit current of transformer 1) in case of fault immediately downstream circuit breaker 1; •lsc2

+ lsc3 (short circuit currents of transformer 2 and 3) in case of fault immediately upstream circuit breaker 1;


Circuit breakers l4 and l5 on the load side must have a short circuit capacity greater than lsc1 + lsc2 + lsc3; naturally every transformer contribution in the short circuit current calculation is to be lessened by the connection line transformer - circuit breaker (to be defined case by case).

Low voltage selectivity with ABB circuit breakers Selectivity definitions and Standards

Agenda Low voltage selectivity with ABB circuit breakers



Definitions and Standards



Selectivity techniques



Back-up protection

Introduction What is selectivity? Selectivity (or discrimination)

A is the supply side circuit breaker (or upstream)

is a type of coordination of two or more protective devices in series.

Selectivity is done between one circuit breaker on the supply side and one circuit breaker, or more than one, on the load side.

B and C are the load side circuit breakers (or downstream)

Introduction Protection system philosophy  

Reduce the stress and prevent damage Minimize the area and the duration of power loss

DAMAGE REDUCTION



Better selectivity

FAULT

CONTINUITY OF SERVICE



Fast fault elimination

Main purposes of coordination Selectivity purpose Selective coordination among devices is fundamental for economical and technical reasons It is studied in order to: 









rapidly identify the area involved in the problem; bound the effects of a fault by excluding just the affected zone of the network; preserve the continuity of service and good power quality to the sound parts of the network; provide a quick and precise identification of the fault to the personnel in charge of maintenance or to management system, in order to restore the service as rapidly as possible;

achieve a valid compromise between reliability, simplicity and cost effectiveness.

Standards definition Selectivity The definition of selectivity

IEC 60947-1 Standard: “Low voltage equipment Part 1: General rules for low voltage equipment”

IEC 60947-1 def. 2.5.23

“Trip selectivity (for overcurrent) is a coordination between the operating characteristics of two or more overcurrent protection devices, so that, when an overcurrent within established limits occurs, the device destined to operate within those limits trips whereas the others do not trip”

Overcurrent selectivity Example In occurrence of a fault (an overload or a short circuit) if selectivity is provided only the downstream circuit breaker opens.

Overcurren Overcur rentt sel select ectivi ivity ty Example In occurrence of a fault (an overload or a short circuit) if selectivity is not provided both the upstream and the downstream circuit breakers could could open open All the system is out of service!

Standards definition Partial and total selectivity IEC 60947-2 def. def. 2.17.2 2.17.2 - 2.17.3 2.17.3

A and B connected in series: part partia iall sele select ctiv ivit ity y and and tota totall sele select ctiv ivit ity y.

Standards definition Partial selectivity “Partial selectivity is an overcurrent selectivity where, in the presence of two protection devices against overcurrent in series, the load side protection device carries out the protection up to a given iven leve levell of overc vercu urre rrent, with witho out makin king the the othe otherr devi device ce trip trip..”

Is is the ultimate selectivity value!

B opens only according to fault current c urrent lower than a certain current value; values equal or greater than I s will give the t he trip of both A and B. Is = ImA

Standards definition Total selectivity “Total selectivity is an overcurrent selectivity where, in the presence of two protection devices against overcurrent in series, the load side protection device carries out the protection without making the other device trip.”

B

A

Only B trips for every current value lower or equal to the maximum short-circuit current. Is = Ik

Standards definition Partial and total selectivity Upstream circuit breaker A T4N 250 PR221DS In = 250 (Icu = 36kA) Downstream circuit breaker B

S 294 C100 (Icu = 15kA)

Selectivity analysis Time-current curves 

Overload zone Thermal protection L protection

Time-current selectivity



Short-circuit zone Magnetic protection S, D, I and EF protections

Current, time, energy, zone, directional, zone directional selectivity

Selectivity analysis Real currents Real currents circulating through the circuit breakers

A

I>

B

I>

A

B

I A = IB tA

I>

A

I>

I>

B

I>

I A = IB + Iloads

I>

I>

I>

I>

I>

I A = (IB + Iloads) / 2

tA tA

tB tB

tB

IA=IB

IB IA

IA

IB










Back-up protection

Introduction Selectivity techniques 

Current selectivity



Time selectivity



Energy selectivity



Zone (logical) selectivity

Current selectivity Base concept 

Current selectivity: closer to the t he power supply the fault point is, higher the fault f ault current is 3kA 

In order to guarantee selectivity, the protections must be set to different values of current thresholds 1kA

tA



tA

B

A

The ultimate selectivity value is equal to the instantaneous trip threshold of the upstream protection device

Ultimate selectivity value tB



ImB

ImA

Other methods are needed to have a total selectivity

Current selectivity Example Circuit breaker A will be set to a value which does not trip for faults which occur on the load side of B. (I3Amin >1kA)

Circuit breaker B will be set to trip for faults which occur on its load side (I3Bmax < 1kA) 104s

Is

103s

Is = I3Amin

A

102s 10s

Here the selectivity is a total selectivity, selectivity, because it is guaranteed up to the maximum value of the short-circuit current, 1kA.

1s B 10-1s 10-2s

0.1kA

1kA

3kA

10kA

Current selectivity Plus and minus Plus Easy to be realized Economic Instantaneous

CURRENT SELECTIVITY Minus Sele Select ctiv ivit ity y is often ften only part partia iall Curr rren entt thre thresh sho olds lds rise ise very very quickl ickly y

Time selectivity Base concept 

Time selectivity is based on a trip delay of the upstream circuit breaker, so to let to the downstream protection the time suitable to trip



B

A



Setting strategy: progressively increase the trip delays getting closer to the power supply source On the supply side the S function is required

Time selectivity Example A will be set with the current threshold I2 adjusted so as not to create trip overlapping and with a trip time t2 adjusted so that B always clears the fault before A

B will be set with an instantaneous trip against short-circuit

I k 104s

Is

The ultimate selectivity value is:

103s 102s 10s

B

1s

I2 t2

10-1s 10-2s 0.1kA

1kA

10kA

100kA

Is = Icw A

(if function I = OFF)

Is = I3minA

(if function I = ON)

Time selectivity Example Which is the problem of time selectivity?

In the case of fault occurring at the busbars, circuit breaker A takes a delayed trip time t 2

I k

The network must withstand high values of let-through energy!

104s 103s 102s 10s

If there are many hierarchical levels, the progressive delays could be significant!

B

1s

t2

10-1s 10-2s 0.1kA

1kA

10kA

100kA

Time selectivity Plus and minus Plus Economic solution Easy to be realized

TIME SELECTIVITY Minus Quick rise of setting levels High values of let-through energy

Energy selectivity Base concept Energy selectivity is based on the currentlimiting characteristics of some circuit breakers



104s 103s A

102s 10s 1s B 10-1s 10-2s

0.1kA



1kA

The ultimate current selectivity values is given by the manufacturer (Coordination tables)

10kA

Current-limiting circuit breaker has an extremely fast trip time, short enough to prevent the current from reaching its peak

Energy selectivity Example

104s 103s

Is = 20kA

A

Circuit breaker A conditions:

102s B 10s

I3=OFF

1s

S as for time selectivity

10-1s 10-2s

0.1kA

1kA

10kA

Energy selectivity Plus and minus PLUS High selectivity values Reduced tripping times Low stress and network disturbance

ENERGY SELECTIVITY MINUS Increasing of circuit breakers size

Zone selectivity Base concept 

Zone selectivity is an evolution of the time selectivity, obtained by means of a electrical interlock between devices locking signal





The circuit breaker which detects a fault communicates this to the one on the supply side, sending a locking signal

Only the downstream circuit breaker opens, with no need to increase the intentional time delay

Fault

Zone selectivity Example

1 e n o Z

A

B Does Not Open

2 e n o Z

3 e n o Z

A Does Not Open

B

C

C Opens

Zone selectivity Specifications Zone selectivity needs: 

a shielded twisted pair cable



an external source of 24V



dedicated trip units 

PR223EF for Tmax T4, T5 and T6



PR332/P for Tmax T7 and T8



PR122/P and PR123/P for Emax



PR332/P and PR333/P for X1

1 e n o Z

2 e n o Z

3 e n o Z



Is up to 100kA for Tmax



Is up to Icw for Emax



It is possible to obtain zone selectivity between Tmax and Emax

Zone selectivity Plus and minus

PLUS Trip times reduced Low thermal and dynamic stress High number of hierarchical levels Can be made between same size circuit breakers

ZONE SELECTIVITY MINUS Cost and complexity of the installation Additional wiring and components










Back-up protection

Back-up protection What is back-up protection? Back-up protection (or cascading) is a type of coordination of two protective devices in series which is done in electrical installations where continuous operation is not an essential requirement.

Back-up protection excludes the use of selectivity!!!

Back-up protection Standards definition The definition of back-up is given by the IEC 60947-1 Standard: “Low voltage equipment Part 1: General rules for low voltage equipment”

IEC 60947-1 def. 2.5.24

“Back -up is a coordination of two overcurrent protective devices in series, where the protective device on the supply side, with or without the assistance of the other protective device, trips first in order to prevents any excessive stress on downstream devices”.

Back-up protection Base concept







Back-up is used by those who need to contain the plant costs The use of a current-limiting circuit breaker on the supply side permits the installation of lower performance circuit breakers on the load side

Both the continuity of service and the selectivity are sacrificed

Back-up protection Application example

T4L 250

Ik = 100 kA Icu (T4L+T1N) = 100kA

T4L 250

T4L 250

T4L 250

Icu = 120kA

T1N 160

T1N 160

T1N 160

Icu = 36kA

Back-up protection tables

Back-up protection Application example

T4L 250 A

B

C

Ik = 100kA

Icu (T4L+T1N) = 100kA

D

Ik = 100kA T1N 160

T1N 160

T1N 160

General power supply is always lost

Back-up protection Plus and minus

Plus

Economic solution Quick tripping times

BACK-UP PROTECTION Minus

No selectivity Low power quality

Example of Selectivity

~ T5H 630A

70kA

T3N 160A

36kA

Incoming = T5H 630A (70kA rating) Outgoing = T3N 160A (36kA rating) Results: The co-ordination resulted in a conditional shortcircuit of 65kA for the T3 mccb!

Iz

The discrimination is up to 20kA.

65kA

Example of Selectivity Discrimination

Example of Selectivity Back-Up

Example of Selectivity Meaning of Selectivity Value

T5H 70kA

T5H

70kA

T3N 36kA T3N

Fault level at Y is 20kA Y is 20kA

36kA

Example of Selectivity Meaning of Selectivity Value T5H

T3N

70kA

T5H 20kA T3N

36kA


5kA fault

T5H ON

T3N Trip

T5H

70kA

T3N

36kA

5kA


5kA fault

T5H ON

T3N Trip

10kA fault

ON

Trip

T5H

70kA

T3N

36kA

10kA


5kA fault

T5H ON

T3N Trip

10kA fault

ON

Trip

20kA fault

Trip

Trip

T5H

70kA

T3N

36kA 20kA


5kA fault

T5H ON

T3N Trip

10kA fault

ON

Trip

20kA fault

Trip

Trip

36kA fault

Trip

Trip

T5H

70kA

T3N

36kA 36kA


5kA fault

T5H ON

T3N Trip

10kA fault

ON

Trip

20kA fault

Trip

Trip

36kA fault

Trip

Trip

65kA fault

Trip

Trip

T5H

70kA

T3N

36kA

65kA

MV/LV Transformer Substations Selection of Protective & Control Devices 

Motor co-ordination ABB offers co-ordination tables



MV/LV Transformer Substations Selection of Protective & Control Devices 

Co-ordination between CBs and switch-disconnectors 400V

T2S160

T1D160

Power Factor Correction

Power Factor Correction Generalities on Power Factor Correction In alternating current circuits, current is absorbed by a load which can be represented by two components: 



The Active component 

In phase with the supply voltage



Directly related to the output

The Reactive component 





© ABB Group

Quadrature to the voltage Used to generate the flow necessary for the conversion of powers through the electric or magnetic field

In most installations the presence of inductive type loads, the current lags the active component (IR).

Power Factor Correction Generalities Generalities on Power Factor Correction In order to generate and transmit active power (P) a certain reactive power (Q) is essential for the conversion of the electrical energy but is not available to the load. The power generated and transmitted make up the apparent power (S). Power factor (cos ) is defined as the ratio between the active component (IR) and the total value of current (I).  is the phase angle between the voltage and the current.

© ABB Group

Power Factor Correction Generalities Generalities on Power Factor Correction

© ABB Group

Power Factor Correction Typical Power Factors of some electrical equipment Generalities

© ABB Group

Power Factor Correction Advantages Generalities of Power Factor Correction

© ABB Group

Power Factor Correction Advantages Generalities of Power Factor Correction 

Better utilization of electrical machines 



Better utilization of cables 

© ABB Group

Generators & transformers are sized according to the apparent power (S). With the same active power (P), the smaller the reactive power (Q) delivered, the apparent power will be smaller.

The reduction in current allows the use of smaller cables in the installation.

Power Factor Correction Generalities 

Reduction in losses 



Reduction in voltage drop 

© ABB Group

By improving the power factor, power losses is reduced in all parts of the installation.

The higher the power factor the Voltage drop will be lower at the same level of Active power.

Power Factor Correction Generalities 

Economical savings 

© ABB Group

Power supply utilities apply penalties for energy used with poor factor. An improved power factor will reduce such penalties from the utilities.

Power Factor Correction Advantages Generalities of Power Factor Correction 

Improve capacity of transformers and cables 

By improving the power factor, you reduce the kVA load on the transformer and the current carried by the cables

Apparent Power (VA) e.g 2MVA Transformer At 100% capacity

Real Power (W) eg. 500kW Load





© ABB Group

Reactive Power (VAR) e.g Motors (inductive) 100kW at 0.7pf = 102kVAR Reactive Power (VAR) eg. 50kVAR Capacitors

Thus additional transformer capacity is available if upgrade or expansion is required in the future Or new cables might not be needed if new loads are connected to an existing switchboard

Power Factor Correction Different Methods 

Distributed power factor correction 

© ABB Group

It is achieved by connecting a capacitor bank properly sized according to the load and is connected directly to the terminals of the load.

Power Factor Correction Different Methods 

Group power factor correction 

© ABB Group

It is achieved by connecting a capacitor bank properly sized according to a group of loads and is connected to the upstream of the loads to be corrected.

Power Factor Correction Types of Power Factor correction Different Methods 

Centralized power factor correction 

© ABB Group

It is achieved by installing an automatic power factor correction bank capacitor bank directly to the main distribution boards.

Power Factor Correction Types of Power Factor correction Different Methods 

Combined power factor correction 

This solution is derived from a compromise between a distributed & centralized power factor correction. 

© ABB Group

Distributed power factor correction is used mainly for higher loads and a smaller centralized power factor correction is used for the small loads.

Power Factor Correction Switching Protection Capacitor and Switching 

Electrical switching phenomena 



© ABB Group

The switching of a capacitor bank causes an electric transient due to the phenomena of electric charging of the bank.

The overcurrents at the moment of switching depends greatly on both the inductance of the upstream network as well as from the number of connected capacitor banks.

Power Factor Correction Switching Protection Capacitor and Switching 

© ABB Group

Choice of protective device

Power Factor Correction Capacitor Switching

Resistance

Motor

In

In

In AC-1

Capacitor

AC-3

AC-6b

Power Factor Correction Capacitor Switching Single step capacitor

30 times In

In

Power Factor Correction Capacitor Switching Multi steps capacitor bank

> 100 times In

In

Power Factor Correction Contactor Sizing Contactor sizing: Thermal current + peak current Thermal current Up to 30% for harmonics and voltage fluctuations on main Up to 15% for tolerances on capacitor power Contactor have to support I th

I = 1.3 1. 3 x 1. 1.15 15 x I = 1.5 I t h n c n c

Power Factor Correction Example Example kVARh kVARh is billed if it is higher than the contracted contract ed level.

kVA kVA kVar

kW

Apparent power (kVA) (kVA) is significantly higher higher than the Active power (kW) The excess current causes losses (kWh) which is billed.

1MVA

The design of the installation has to be over-dimensioned. The installation requires 850kW at power factor of 0.75.

400V

The transformer will have to be overloaded to 850k / 0.75 = 1.133MVA. Current taken by the system is P I = = 1636A 3 * U * Cos  Losses in the cables P = I2R Cos  = 0.75 850kW Load © ABB Group

I = 1636A

Cos  = 0.75 The Transformer, Transformer, Circuit breaker & Cable has to be increased.

Power Factor Correction Example Example kVA kVA

kVARh kVARh is reduced to lower than the t he contracted level or eliminated.

kVar

kW

Apparent power (kVA) (kVA) is significantly higher than the Active power (kW) The charges based on the contracted kVA demand demand is close to the active power.

1MVA

The installation requires 850kW at a power factor of 0.9. 400V

The transformer will not be overloaded to 850k / 0.90 = 945 kVA. Current taken by the system is P I = = 1364A 3 * U * Cos  Losses in the cables P = I2R

Cos  = 0.90 850kW Load © ABB Group

I = 1364A

Cos  = 0.90 There is not need to increase the t he Transformer, Transformer, Circuit breaker & Cable.

Power Factor Correction Technical Application Paper

© ABB Group

IEC Standards & Power Factor Correction

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