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: Components :
Fuses: Self-destructing to save the Fuses: equipment being protected. Relays earth screen Bus-bars Circuit Breakers: 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: 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 electr onic 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 electr onic device fails in some way (electronic devices may pull excessive current when they fail). If a fuse larger than t han the specified fuse is used, a small mistake when installing the equipment may cause catastrophic failure of the equipment.
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 t he 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
Fuse Opening Time
ProtectionAdvantages of power system •
It is the cheapest form of protection available.
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It requires no maintenance.
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Operation is completely automatic.
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It can break heavy short-circuit currents without noise or smoke.
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The inverse time-current characteristic of a fuse make it suitable for overcurrent protection.
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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
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Length, smaller the length greater the current
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diameter
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Previously history
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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.
They are capable of clearing high as well as low fault currents
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They do not deteriorate with age
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They have high speed of operation
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They provide reliable discr criimin ina atio ion n
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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.
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
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It also deals with very small fault currents
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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 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. 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 curr current ent flow in the coil has stopped. The diagram below shows the
The animated picture shows a working relay with its coil and switch contacts. You 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,
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 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 •
Most of the relays in service on electric power system today are of electro-mechanical type
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They work on the following f ollowing two main operating principles
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Electromagnetic attraction
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Electromagnetic induction
Attracted armature type relay
Solenoid ty type re relay
Balanced ced be beam ty type re relay
Induction type Relays Ov.er current relay (non-directional)
Over current relay (directional)
Directional power relay
Others types of functional relays 1.
Distance relays
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Differential relays
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Translay system
Current differential relay
Voltage balance differential relay
Types of protection 1. Primary pr protection 2. Back-up pr protection
Advantages of relays • • • •
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Relays can switch AC and DC, DC, transistors can only switch DC. Relays can switch high voltages, 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 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 power due to the current flowing through their coil. Relays require more current than many ICs can provide, 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 www.bcae1.com (for protection project) • www.bcot1.com
