كيفية اختيار ال C.B

 عوامل اختيار القواطع الكهربية

(CBs)

 تحدبد القيم المقننه التالية: 

1- The steady short circuit current

2- Initial symmetrical RMS current

3- The maximum possible dc. Component of short circuit current

4- The making capacity of the circuit breaker in kA

5- The RMS value of the symmetrical breaking current.

6- Asymmetrical breaking current

7- The interrupting capacity in MVA.

وسوف نعطى مثال لكيفية حسابات هذة القيم بفرض وجود مولد له البيانات التالية

Three phase alternator

Power rated = 15 MVA

Voltage terminal = 6.6 KV

Subtransient reactance = 0.1 pu

Transient = 0.15 pu

Synchronous = 0.6 pu

وتم توصيل هذا المولد بقاطع دائرة من النوع الزيتى الصغير 5 Cycle – oil circuit breaker

وتكون خطوات الحساب كالتالى.


In power system, the circuit breakers provided should have ample capacity to make as well as break the circuit at the service voltage.

The breaking capacity ratings are expressed in terms of:

1- The symmetrical RMS current which can be broken all poles.

2- The max. Symmetrical RMS that can be interrupted on any one pole.

The former may be referred to in terms of MVA which is the product

1.73 * Line KV * symmetrical breaking current in KA.

The short circuit current against which the circuit breaker can remain closed during the first cycle of short circuit is called the ‘making current ’which may be taken as about

1.8X1.414= 2.546 Times the symmetrical RMS breaking current

Or 1.8 Times the peak value symmetrical breaking current

For 5-cycle circuit breaker the asymmetrical breaking current may be taken as about 1.4 times symmetrical breaking current.

In this problem, rated current of the alternator is

1 pu = 15000 / 1.73 * 6.6 = 1312 Amp.

1 pu = 6.6 KV

Subtransient reactance = 0.1 pu

Transient = 0.15 pu

Synchronous = 0.6 pu

1 – Steady short circuit = 1.0 / 0.6 = 1.66 pu

= 1.66 * 1312 = 2.18 KA

2 - Initial symmetrical RMS current = 1pu / Subtransient reactance

= 1.0 / 0.1 = 10 pu

= 10 * 1312 =13.12 KA

3 – The maximum possible dc. Component of short circuit current

= peak value symmetrical breaking current

= 1.414 * 13.12 = 18.55 KA

4 – The making capacity of the breaker =1.8 * 18.55 =33.4 KA

5 – RMS value of the symmetrical breaking current = E / transient reactance

= 1.0 pu / 0.15 = 6.66

= 6.66 * 1.312 = 8.75 KA

6 - Asymmetrical breaking current = 1.4 * 8.75 = 12.25 KA

7 - The interrupting capacity of the circuit breaker = 1.73 * 6.6 *12.25 = 140 MVA

و هذا باختصر شديد مع مراعاة انه يجب الرجوع الى المواصفات الهندسية مثل BS و IEC و ما شابه لاأختيار القيم الثابته و المعاملات حيث انها تختلف حسب مستوى الجهد.

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