DIFFERENTIAL PROTECTION

DIFFERENTIAL PROTECTION


Differential protection is a very reliable method of protecting generators, transformers, buses,
and transmission lines from the effects of internal faults.



In a differential protection scheme in the above figure, currents on both sides of the equipment
are compared. The figure shows the connection only for one phase, but a similar connection is
usually used in each phase of the protected equipment. Under normal conditions, or for a fault
outside of the protected zone, current I1 is equal to current I2 . Therefore the currents in the
current transformers secondaries are also equal, i.e. i1 = i2 and no current flows through the
current relay.
If a fault develops inside of the protected zone, currents I1 and I2 are no longer equal, therefore
i1 and i2 are not equal and there is a current flowing
through the current relay.



Differential Protection of a Station Bus
The principle of the differential protection of a station bus is the same as for generators.
The sum of all currents entering and leaving the bus must be equal to zero under normal
conditions or if the fault is outside of the protected zone. If there is a fault on the bus, there will
be a net flow of current to the bus and the differential relay will operate.





Percentage Differential Relays
The disadvantage of the current differential protection is that current transformers must be
identical, otherwise there will be current flowing through the current relays for faults outside of
the protected zone or even under normal conditions. Sensitivity to the differential current due to
the current transformer errors is reduced by percentage differential relays.
[size=16]In percentage differential relays, the current from each current transformer flows through a
restraint coil. The purpose of the restraint coil is to prevent undesired relay operation due to
current transformer errors. The operating coil current | i1 - i2 | required for tripping is a
percentage of the average current through the restraint coils. It is given by
where k is the proportion of the operating coil current to the restraint coil current. For example
if k = 0.1, the operating coil current must be more than 10% of the average restraint coil current
in order for the relay to operate.

الزعيــم كتب:

شكرا على هذا الموضوع الممتاز
ولكن توجد ملاحظة بسيطة
بعض الشركات المصنعة للمولدات تقوم بقفل نقطة الstar point
....وهذا اكبر خطأ ويجب عليك كمهندس مسئول رفض استلام المولد
لاحظ ان الست اطراف الناتجة عن لف المولد
ماهى الا ثلاث بدايات وثلاث نهايات
الخرج 3 فاز كل فاز بداية ونهاية ... المجموع = 6 اطراف
ومن ثم عندما تضع محولات التيار على الثلاث بدايات
وتضع محولات التيار على الثلاث نهايات تكون عملت وقاية على المولد
وهذه الحماية ياجماعة مهمة جدا جدا جدا جدا
بل اهم حماية على المولد
اما بالنسبة للمذكور اعلاه بخصوص ان هذه الحماية حساسة جدا لاى تغيير فى التيار
جميع اجهزة الوقاية المستخدمة فى السوق حاليا تتيح لك تغيير القيم لما تراه انت مناسبا
وانت اللى هتشيل ياريس

شكراً يازعيم ويا ريت تفيدنا من خبراتك