Circuit Breakers

ما الفرق بين القاطع الغازى و الزيتى و المفرغ من الهواء ؟

      

الفرق الرئيسى بين القواطع هو نوع المادة العازلة المستخدمة فى إطفاء الشرارة الكهربية اثناء فصل نقط التلامس الرئيسية للقاطع.

1- القاطع المفرغ من الهواء ( Vacuum Circuit Breaker):

هذا النوع من القواطع تكون غرفة أطفاء الشرارة مفرغة تماما من الهواء بدرجة عالية جدا جدا تصل الى

1000000000 / 1 Torr تحت الضغط الجوى و لذلك لا يمكن عمل صيانة داخلية للملامسات الرئيسية للقاطع و هذا يعتبر من عيوب هذا النوع من القواطع وعند اجراء الأختبارات على هذا النوع من القواطع و قياس المقاومة الداخلية للملامسات و وجد ان قيمتها غير سليمة يتم استبدال غرفة أطفاء الشرارة بالكامل مما يزيد من تكاليف الصيانة و هذا النوع يستخدم فى الجهود حتى 36 كيلو فولت.

2- القاطع الزيتي (Oil Circuit Breaker):

هذا النوع من اقدم انواع القواطع و مازال يستخدم حتى الآن و تكون غرفة أطفاء الشرارة مملؤة بزيت عازل يساعد على أطفاء الشرارة بين الملامسات الرئيسية و لكن يجب ملاحظة انه يجب عمل اختبارات دورية للزيت بعد عدة عمليات فصل للقصر و يتم تغيرة اذا لزم الأمر و يستخدم فى الجهود المنخفضة و المتوسطة و من عيوبة ان حجمة كبير جدا فى حالة استخدامة فى الجهد العالى.

3- القاطع الغازي (SF6 Circuit Breaker):

هذا النوع من القواطع اخذ فى الأنتشار فى الأونة الأخيرة لم له من مزايا كثيرة و متعددة و يستخدم فى جميع مستويات الجهود المختلفة حتى 1100 كيلوفولت.و فى هذا النوع يستخدم غاز سادس فلوريد الكبريت SF6 كوسط عازل داخل غرفة أطفاء الشرارة.

سداسي فلوريد الكبريت هو مركب غير عضوي صيغته SF₆ وهو عديم اللون، عديم الرائحة وغير سام، وهو غاز غير قابل للاشتعال (في الأحوال العادية). جزيء SF₆ له ثمانية سطوح ويتكون من ست ذرات فلور مرتبطة بذرة الكبريت الوسطية بشكل متناظر. وهو غاز ثقيل وينقل عموما كغاز مميع.

ومن خصائصه سداسي فلوريد الكبريت أنه غاز من صنع الإنسان (غير متواجد في الطبيعة) وهو معروف بخصائصه الفيزيائية المميزة

تفصيل أكثر.......

What is Circuit Breaker?

Is a switching device which can be operated manually as well as automatically for controlling and protection of electrical power system respectively.

Types of Circuit Breaker

According different criteria there are different types of circuit breaker. According to their arc quenching media the circuit breaker can be divided as:

  1.  Oil circuit breaker.

  2.  Air circuit breaker.

  3.  SF₆ circuit breaker.

  4.  Vacuum circuit breaker.

According to their services the circuit breaker can be divided as:

   1.  Outdoor circuit breaker.

   2.  Indoor breaker.

According to the operating mechanism of circuit breaker they can be divided as:

   1.  Spring operated circuit breaker.

   2.  Pneumatic circuit breaker.

   3.  Hydraulic circuit breaker.

According to the voltage level of installation types of circuit breaker are referred as:

   1.  High voltage circuit breaker.

   2.  Medium voltage circuit breaker.

   3.  Low voltage circuit breaker.

Oil circuit breaker

Working principle: 

(i) The hydrogen gas bubble generated around the arc cools the arc column and aids the desalinization of the medium between the contacts.

 (ii) The gas sets up turbulence in the oil and helps in eliminating the arcing products from the arc path.
 (iii) As the arc lengthens due to the separating contacts, the dielectric strength of the medium is increased.

Vacuum Circuit BreakerWorking principle:

The production of arc in a vacuum circuit breaker and its extinction can be explained as follows : 

• When the contacts of the breaker are opened in vacuum (10^-7 to 10^-5 torr), an arc is produced between the contacts by the ionisation of metal vapours of contacts. However, the arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength. 
• The salient feature of vacuum as an arc quenching medium is that as soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.

The working of Vacuum circuit breakers is briefly explained below:

• When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of arc is due to the ionisation of metal ions and depends very much upon the material of contacts. 
• The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields. 
• Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation (say 0.625 cm). 

Vacuum circuit breakers are employed for outdoor applications ranging from 22kV to 66kV. Even with limited rating say 60 to 100MVA, they are suitable for majority of applications in rural areas.

SF6 Circuit Breacker :

Working principle:

At this point we are aware that the medium in which arc extinction of the circuit breaker takes place greatly influences the  important characteristics and life of the circuit breaker. In the last article the working of a vacuum circuit breaker was illustrated. We already know that the use of vacuum circuit breaker is mainly restricted to  system voltage below 38 kV. The characteristics of vacuum as medium and cost of the vacuum CB does not makes it suitable for voltage exceeding 38 kV. In the past for higher transmission voltage Oil Circuit Breaker (OCB) and Air Blast Circuit Breaker (ABCB) were used. These days for higher transmission voltage levels  SF₆  Circuit Breakers are largely used. OCB and ABCB have almost become obsolete.  In fact in many installations SF₆  CB is used for lower voltages  like 11 kV, 6 kV etc.. 

Sulphur Hexafluoride symbolically written as SF₆  is a gas which satisfy the requirements of an ideal arc interrupting medium. So SF₆  is extensively used these days as an arc interrupting medium in circuit breakers ranging from 3 kv  upto 765 kv class. In addition to this SF₆ is used in many electrical equipments for insulation. Here first we discuss in brief, some of the essential properties of  SF₆ which is the reason of it's extensive use in circuit breakers

• SF₆ gas has high dielectric strength which is the most important quality of a material for use in electrical equipments and in particular for breaker it is one of the most desired properties. Moreover it has high Rate of Rise of dielectric strength after arc extinction. This characteristics is very much sought for a circuit breaker to avoid restriking.
• SF₆ is colour less, odour less and non toxic gas.
• SF₆  is an inert gas. So in normal operating condition the metallic parts in contact with the gas are not corroded. This ensures the life of the breaker and reduces the need for maintenance.
• SF₆ has high thermal conductivity which means the heat dissipation capacity is more. This implies greater current carrying capacity when surrounded by SF₆ .
• The gas is quite stable. However it disintegrates to other fluorides of Sulphur in the presence of arc. but after the extinction of the arc the SF₆  gas is reformed from the decomposition.
• SF₆ being non-flammable so there is no risk of fire hazard and explosion. 

The construction and working principles of SF₆ circuit breaker varies from manufacturer to manufacturer. In the past double pressure type of SF₆ breakers were used. Now these are obsolete. Another type of SF₆ breaker design is the self blast type, which is usually used for medium transmission voltage. The Puffer type SF₆ breakers of single pressure type are the most favoured types prevalent in power industry.  Here the working principle of Puffer type breaker is illustrated .