pharos university ee-385
DESCRIPTION
Pharos University EE-385. Electrical Power & Machines “Electrical Engineering Dept” Prepared By: Dr. Sahar Abd El Moneim Moussa. PROTECTION. IMPORTANCE OF PROTECTION Any electric circuit is subjected all the time to up normal conditions such as short-circuit. - PowerPoint PPT PresentationTRANSCRIPT
Dr. Sahar Abd El Moneim Moussa 1
Pharos UniversityEE-385
Electrical Power & Machines“Electrical Engineering Dept”
Prepared By:Dr. Sahar Abd El Moneim Moussa
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PROTECTION
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IMPORTANCE OF PROTECTION
• Any electric circuit is subjected all the time to up normal conditions such as short-circuit.
• Hence, any circuit needs protection against these up normal conditions, otherwise, it would fail
• The function of any protection system is to sense the fault and to clear it through disconnect the faulted section.
• Many devices and systems are found such fuses and circuit-breakers
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ARCING PHENOMENON
- When protection devices disconnect the circuit at any point, an arc is generated between the separation points due to the discharged energy stored in the network.
- This arc ionizes the separating medium between the
two points of separation due to its extreme high temperature and this means that the S.C. current still found.
- Hence, the task of the disconnecting device is to disconnect the faulted section and extinguish the generated arc
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- The engineer must select the suitable disconnecting device which can perform this task properly and safely
- The selection depends on the value of the S.C. current and the system voltage at the separation point
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PROTECTIVE DEVICES
(a)FUSES
A fuse is a current interrupting device which opens the circuit by a fusing element (it is the part which is designed to melt when the fuse operates) when the current in the circuit exceeds a certain value
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Important Definitions:
• Rated Current: It is the maximum r.m.s. current at the rated frequency which can pass through the fuse without increasing its temperature.
• Rated Voltage: It is the maximum r.m.s. voltage for the fuse to put on. The operating voltage must be equal or less than the rated voltage.
• Intterrupting rating: It is the maximum r.m.s. current that the fuse can interrupt safely under rated voltage .
• I2t Value: It is a measure of the heat capacity of the fuse.
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Major Types of Fuses:
1- Rewirable semi-enclosed Fuse2- Quartz sand-filled Cartridge HRCF3- HRC fuse type4- Expulsion fuse type5- Current Limiting Fuse
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Low voltage Fuses
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High voltage Fuses
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PROTECTIVE DEVICES
(b) Circuit Breakers
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Any LVCB has the following principal parts:
• Trip Unit: This unit is responsible for sensing the fault and giving signal to the latching . This unit may be adjustable or unadjustable in time or current or both. It has a time-current C/C’s which determine the operation of the CB. This unit may be mechanical or electronic.
• Latching Mechanism: This is a mechanical part which receives order from the trip unit and separates the two poles of the CB which results in opening the metallic parts of circuit.
• Arc Extinguish Chamber: This is a chamber provided with means to put out the generated arc.
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Low voltage Circuit-Breakers
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Major Types of LVCB:
1- Miniature-type Circuit Breaker (MCB)
2- Molded Case-type Circuit Breaker (MCCB)
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1- Miniature-type Circuit Breaker (MCB)
• Type: 4-poles.• Overload protection Ir : is set fixed at rated current
value In.• Short-circuit protection Im : is set fixed at standard
setting of 5InIm10In
• Application: domestic use and illumination systems in commercial or industrial use.
• Standard current ratings In : 15, 20, 30, 40, 65, 80 and 100 A.
• Standard short-circuit protection Im : 4-kA (low-setting), 10-kA (standard setting) and 15-kA (high setting).
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2- Molded Case-type Circuit Breaker (MCCB)
• Characteristics: heavy-duty industrial circuit breakers of large current ratings up to 1200A and breaking short circuit capacity up to 75kA.
• Type of protective relay: thermal-magnetic or electronic.
• Overload & short-circuit protection mode of operation: fixed or adjustable.
• Auxiliary: can be equipped with shunt tripping coil to receive tripping signals from other relaying devices.