substation protection.

• SUBSTATION PROTECTION

 Atul sontakke.

OUTLINE • Introduction • Nature and Causes of faults • Components of Protection • Fuse and its types • Relay and its types • Circuit breaker • Earthing • Isolator

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Introduction • A Substation covers various equipment such as Circuit Breakers, Isolators, Earth Switches, Lightning Arresters, Grounding System and etc. Protection scheme is designed to limit the effects of disturbances in the power system which is allowed to persist, may damage plant and interrupt the supply of electric energy. The greatest threat to the security of a power supply station is short circuit which imposes sudden and violent change in he power system. Rapid isolation of the fault by nearest switchgear will minimize the damage. It covers various types of protection used in switchyard such as relays, circuit breakers and etc.

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NATURE AND CAUSES OF FAULTS • Insulation failure. • Conducting path failure. • Overvoltage's due to lightening or switching surges. • Puncturing or breaking of insulators. • Failure of conducting path due to broken conductors. • Faults due to poor quality of system components or because of faulty system design. System design.

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COMPONENTS OF PROTECTION • FUSES • MCB/MCCB • RELAYS • CIRCUIT BREAKERS • EARTHING • ISOLATOR

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FUSES • Fuse- Fuse is a piece of wire of a material with a very low melting point. When a high current flows through the circuit due to overloading or a short circuit wires gets heated and melts. As a result, the circuit is broken and current stops flowing.

• Types of fuses. •

Low voltage fuse. KIT-KAT fuse. HRC (High rupturing fuse). Semiconductor fuse.

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High voltage fuse. Drop out fuse.

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MCB (Miniature Circuit Breaker)

Type

Tripping Current

perating Time

Type B Type C Type D

3 To 5 time full load current 5 To 10 times full load current 10 To 20 times full load current

0.04 To13 Sec 0.04 To 5 Sec 0.04 To 3 Sec

Application of MCB • B Curve MCB-This type MCB is used for resistive load. e.g. Heater, Oven.

• C curve MCB- This type MCB is used for inductive load. e.g. Fan, Washing M/C, Fridge, motor, AC, Transformer,

• D Curve MCB- This type MCB is used where High inrush current is available. e.g. Induction motor, Transformer.

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MCCB (Moulded Case Circuit Breaker) • MCCB is used for protecting of low-voltage electrical wires and connected equipment, against overloads and short-circuits. • Rated current up to 1000 A. • Trip current is adjustable. • The advantage of MCCBs for very large ratings is their compact size. An ACB is physically larger, but more cost-effective for higher ratings.

RCCB(Residual Current Circuit Breaker) • Phase (line) and Neutral both wires connected through RCCB. • RCCB trips the circuit when there is earth fault current. • The amount of current flows through the phase (line) should return through neutral. • Any mismatch between two currents flowing through phase and neutral detect by -RCCB and trip the circuit within 30Miliseconed

ELCB (Earth Leakage Circuit Breaker) • An ELCB had a current sensing coil in series with the ground cable, so the supply would be tripped if current occurred in the ground lead. This was often combined with a voltage sensing coil on the frame also, which used a separated ground some distance away. • The original use was for larger motors and generators/transformers, where the insulation can fail, and the voltage on the frame of the machine can rise dangerously above ground due to the ground resistance and fault current.

Relays • Relays are electrical switches.

Types of Relay.       

Over current relay. Under voltage relay. Earth fault relay. Phase sequence relay. Time delay relay. Buchholz Relay. Frequency Relay.

Buchholz Relay.

High voltage circuit breakers • open or close a circuit under all conditions. Viz. no load full load and fault condition. • it can be operate manually and automatically under fault conditions. • Under normal operating condition, the contact remains close and the circuit breakers carries the full load-current continuously.

Classification of circuit breaker. Oil circuit breaker- Which employ some insulating oil for arc extinction. Air-blast circuit breaker- in which high pressure air  –blast is used arc extinction. SF6-Sulfer hexafluoride gas is used for arc extinction. Vacuum CB- Vacuum is used for arc extinction. 33kV.

Oil circuit breaker. Advantages of Oil Circuit Breaker (OCB)  As an Arc extinguishing medium oil has the following advantages: 1. Oil produces hydrogen during arching. The hydrogen helps extinguish the arc. 2. Oil provides insulation between the contacts after the arc has been extinguished Disadvantages of Oil Circuit Breaker (OCB) 1. Oil is inflammable and may cause fire hazards. When a defective circuit breaker fails under pressure, it may cause an explosion 2. The hydrogen generated during arcing, when combined with air, may form an explosive mixture 3. During arcing, oil decomposes and becomes polluted by carbon particles, which reduces its dielectric strength. Hence, it requires periodic maintenance and replacement.

Vacuum Circuit Breakers Advantages • Very long lifetime of the contacts (This provides longer breaker life.) • Less maintenance required • Less moving parts in mechanism • Less force needed to separate the contacts (since the distance between them is shorter.) • Environment friendly. Since interruption takes place in vacuum medium, VCBs do not require gas or liquid addition. This reduces the possibility of leakage of gas (or any material) that can be harmful for environment.

Air blast Circuit breaker.

SF6 circuit breaker. Advantages Simple construction, less cost. SF6 gas is non flammable, non toxic Maintenance free  Ability to interrupt low and high fault Excellent Arc extinction.

EARTHING • Types of Earthing Plate Earthing

Pipe Earthing

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Measurement of Earth Resistance

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ISOLATOR Circuit--Isolator Isolators are available in ratings from 11 kV to 132kv. in 1200 A, 1600 A, and 2000 A versions. Style like Side--break, vertical- break, centre break styles. can be furnished, to suit almost any substation layout.

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ELECTRICAL SLD

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Thank you