Electrical Plant Dimension Ing

ELECTRICAL PLANT DIMENSIONING

CIRCUIT BREAKER SELECTION

ABB SACE

Protection of transformers Generalities 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 CB and the setting depend (In); •the max estimated short circuit current in the installation point which defines the minimal breaking power of the protection circuit breaker (Isc).

ABB SACE

Protection of transformers MT-BT switchboard with one single transformer

Sn U20

In Isc

ABB SACE

Protection of transformers 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]

ABB SACE

Protection of transformers 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]

ABB SACE

Protection of transformers

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

ABB SACE

Protection of transformers Choice of the circuit breaker 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.

ABB SACE

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

E1B12

E2B16

E2B20

E3B32

E4S40

E6H50

SACE Emax

E1B08

E3B25

(1) For values of the percent short circuit voltage U’cc% different from the Ucc% values as per table, the rated threephase 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 following k times:

U’20 k

ABB SACE

[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 lines The choice of the circuit breakers for switching and protection of the lines means the perfect knowledge of: •rated operating line current lB •max admissable cable current lZ •presumed short circuit current in the point of installation of the circuit breaker Icc

ABB SACE

Protection of lines The correct circuit breaker must be apt 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 •the let through energy (l2t) that flows through the circuit breaker must be lesser or equal to the maximal one allowed by the cable (K²S²)

ABB SACE

Protection of lines As far as the verification required by the IEC 364 standard, 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 SACE CBs since those are IEC 60947-2 standard compliant and the value is less than 1,3 ln.

ABB SACE

Primary and secondary distribution Selective protection

A

B

C

ABB SACE

Primary and secondary distribution The example emphasizes the need of co-ordination of the intervention between the two A and B circuit breakers so that, in case of fault in C, only the B circuit breaker trips, thus leaving complete continuity to the rest of the plant supplied by the circuit breaker A.

ABB SACE

Primary and secondary distribution Selectivity might be total or partial: •total selectivity: only the circuit breaker B trips for every current value lesser or equal to the max short circuit current foreseen in C; •partial selectivity: the circuit breaker B opens only according to fault current lower than a certain value; values that are equal or greater than this will give the intervention of the two A and B circuit breakers

ABB SACE

Primary and secondary distribution Amperometric selectivity is obtained by setting on different values the instantaneous tripping currents of the circuit breakers’ chain (greater values for upstream circuit breakers)

ABB SACE

Primary and secondary distribution

B

ImB

A

ImA

ImA is the selectivity limit !

ABB SACE

Primary and secondary distribution Chronometric selectivity is obtained by introducing intentionally always greater delays in the intervention tripping timings of the upstream circuit breakers in the chain.

ABB SACE

Primary and secondary distribution

B

A

t

Total selectivity

ABB SACE

Primary and secondary distribution Switchboard A

2500 kVA (fault current 57,5 kA)

E4S40 with PR112 400V E2N20 with PR111

Switchboard B

E2N20 MS (disconnector) 400V

T5H 630 with PR222

ABB SACE

Primary and secondary distribution

ABB SACE

ABB SACE

ABB SACE

Primary and secondary distribution Back-up protection

A

B

C

ABB SACE

Primary and secondary distribution In the figure, the circuit breaker B, downwards in respect with A, might have a short circuit breaking capacity lesser than the presumed short circuit current in case of fault in C if the circuit breaker A satisfies at all the two following conditions: •it own correct short circuit power (greater or equal to the presumed short circuit current in its installation point and obviously greater than the short circuit current in C) •in case of fault in C with short circuit values greater than the short circuit breaking capacity of circuit breaker B, the circuit breaker A must limit the let through energy by tapering it to a correct value than can be stood by the circuit breaker B and by the protected lines

ABB SACE

Primary and secondary distribution The back-up protection is to be used in electric plants where operation continuity is not a main need: that is the tripping of an upstream circuit breaker will include in the black-out also those parts of the plant that are not interested in the faulty current. This co-ordination solution is used by those who need to contain the plant costs by reducing the general performance in case of fault.

ABB SACE

Primary and secondary distribution

Co-ordination table for back-up protection

ABB SACE

Upstream circuit breaker

Short circuit breaking capacity

E2L - E3L

130 [kA] (@ 380/415 V)

Downstream circuit breaker

Breaker capacity on back-upped loads

T4N

65 [kA]

T4S - T5N - E1B - E2B

85 [kA]

T4H - T5S - T5H - S7H - E2N

100 [kA]

Back up protection application example

T4L 250

100 kA !!!

T1N 160

T1N 160

T1N 160

* T1N 160 is 36 kA only mccb

ABB SACE

ABB SACE

Solution for selectivity

S6L 800 100 kA

T4L 250

ABB SACE

T4L 250

T4L 250

Selectivity Table

ABB SACE

ABB SACE