Protection Of Power System Using Relays And Fuses

Protection of power system

objectives • • • • • • •

Physical appearance Definition Working and construction How it protect power system? Applications Advantages and disadvantages Questioning about my topic

Physical appearance

What is fuse •

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A fuse is a short piece of metal inserted in a circuit, which melts when excessive current flows through it and thus break the circuit. The fuse element is generally made of materials having low melting point, high conductivity and least deterioration due to low oxidation e.g, silver, copper etc Under normal conditions the fuse element is at a temperature below its melting point. When a short-circuit or overload occurs, the current through fuse increase beyond its rating this melt the fuse

Protection of power system Power system Protection-Basic Components:       

Fuses: Self-destructing to save the equipment being protected. Relays earth screen Bus-bars Circuit Breakers: These are used to make circuits carrying enormous current, and also to break the circuit carrying the fault currents for a few cycle based on feedback from the relays.

DC batteries: These give uninterrupted power source to the relays and breakers that is independent of the main power source being protected.  Lightning Arresters  Surge Absorber  Over head ground wires

Working And Construction Function: A fuse is generally inserted into an electrical circuit for 1 of 2 reasons, either to protect the power source which includes the wire that connects the power supply to the electrical device, or to protect the electronic equipment. The electronic equipment manufacturers specify a fuse rated to open the electrical circuit before damage can be done to the device or open the circuit if the electronic device fails in some way (electronic devices may pull excessive current when they fail). If a fuse larger than the specified fuse is used, a small mistake when installing the equipment may cause catastrophic failure of the equipment.

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In the diagram below, you see that there is a fuse between the battery and the amplifier. In this configuration, the fuse can be used to protect the wire and the amplifier. If the fuse is the proper one for the amplifier, all you have to do is make sure that the wire segments 'A' and 'B' are rated to pass more current than the fuse and you'll be OK. Wire segment 'A' must be as short as possible because it is NOT protected by the fuse. •

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In the this diagram, things get a little more complicated. As you can see, wire 'A' is used to deliver power to the distribution block. Wire 'A' is of a large enough gauge to power both amplifiers. 4 gauge wire is commonly used as a main power wire. Fuse 'A' must be rated to protect wire 'A'. Again, fuse 'B' protects wire 'B' and fuse 'C' protects wire 'C'. Wire segments 'X' and 'Y' MUST be as short as possible because, unless they are of the same gauge as wire 'A' (or larger), they could be a fire hazard.

• In most cases, the wire size is reduced at the point of distribution. ANY time that the wire size is reduced, you must add a fuse in the line (at the point of distribution) to protect the smaller wire. Look at the following for more detailed info about changing wire sizes.

• Wire "A" is unprotected which is why you want the main (125 amp) fuse as close to the battery as possible. If this length of wire gets shorted, it WILL

Fuse Opening Time

ProtectionAdvantages of power system • • • • • •

It is the cheapest form of protection available. It requires no maintenance. Operation is completely automatic. It can break heavy short-circuit currents without noise or smoke. The inverse time-current characteristic of a fuse make it suitable for overcurrent protection. Time of operation shorter than circuit breakers

Disadvantages • Time is lost in rewiring or replacing a fuse after operation. • On heavy short-circuits discrimination between fuses in series can not be obtained. • The current-time characteristic of fuse can not always be co-related with that of protected apparatus.

Important terms • Fusing current • • • • •

It depends upon the following factors material of the fusing element Length, smaller the length greater the current diameter Previously history Size and location of terminal used

• Fusing factor fusing factor = min. fusing current current rating of fuse

Important terms • • • • • •

Prospective current. Cut-off current. Pre-arcing time. Arcing time Total operating time Breaking capacity

Types of fuses 1. Low voltages fuses 2. High voltages fuses

low voltages fuses • • •

Semi-enclosed rewireable fuse High rupturing capacity cartridge fuse H.r.c. fuse with trippping device

High voltages fuses • • •

Cartridge type Liquid type Metal clad fuses

H.R.C. cartridge fuse Advantages 2. 3. 4. 5. 6.

They are capable of clearing high as well as low fault currents They do not deteriorate with age They have high speed of operation They provide reliable discrimination They require no maintenance

Disadvantages • They have to be replaced after each operation • Heat produced by the arc may affect the associated switches

H.R.C. fuse with tripping device Advantages 2.

3. 4.

In case of a single phase fault on a three-phase system, the plunger operates the tripping mechanism of circuit breaker to open all three phases and prevents single phasing It also deals with very small fault currents This permits the use of a relatively inexpensive circuit breaker

Liquid type fuse • These are filled with carbon tetrachloride • Having widest range of application to h.v. systems • Used for circuit upto100A rated current on 132 kv • Breaking capacities of 6100a

Relays

Physical Appearance of Relays

What is Relay •

A relay is a device that detects the fault and initiates the operation of circuit breaker to isolate the defective element from the rest of the system

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A relay is a simple electromechanical switch made up of an electromagnet and a set of contacts. Relays are found hidden in all sorts of devices. In fact, some of the first computers ever built used relays to implement Boolean gates.

Construction and working • A relay is used to isolate one electrical circuit from another. It allows a low current control circuit to make or break an electrically isolated high current circuit path. The basic relay consists of a coil and a set of contacts. The most common relay coil is a length of magnet wire wrapped around a metal core. When voltage is applied to the coil, current passes through the wire and creates a magnetic field. This magnetic field pulls the contacts together and holds them there until the current flow in the coil has stopped. The diagram below shows the parts of a simple relay.

The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This

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lever moves the switch contacts

Single pole single throw double throw

Double pole single throw

single pole

Double pole 2 throw

Relay specificati ons • There are two specification s that you must consider when selecting a relay for use in an automobile, the coil

Protective relay And Construction •

A protective relay is a complex electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault was found. Design and theory of these protective devices is an important part of the education of an electrical engineer who specializes in power systems. Today these devices are nearly entirely replaced (in new designs) with microprocessor-based instruments (numerical relays) that emulate their electromechanical ancestors with great precision and convenience in application

Operating Principle •

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Most of the relays in service on electric power system today are of electro-mechanical type They work on the following two main operating principles Electromagnetic attraction Electromagnetic induction

Attracted armature type relay

Solenoid type relay

Balanced beam type relay

Induction type Relays Over . current relay (non-directional)

Over current relay (directional)

Directional power relay

Others types of functional relays 1. 2. 3.

Distance relays Differential relays Translay system

Current differential relay

Voltage balance differential relay

Types of protection 1. Primary protection 2. Back-up protection

Advantages of relays • • • •

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Relays can switch AC and DC, transistors can only switch DC. Relays can switch high voltages, transistors cannot. Relays are a better choice for switching large currents (> 5A). Relays can switch many contacts at once.

disadvantages of relays • •

Relays are bulkier than transistors for switching small currents. Relays cannot switch rapidly (except reed relays), transistors can switch many times per second. • Relays use more power due to the current flowing through their coil. • Relays require more current than many ICs can provide, so a low power transistor may be needed to switch the current for the relay's coil.

Busbar protection

Techniques Differential protection

Fault bus protection

References • www.bcae1.com (for protection project) • www.bcot1.com