IntroductionProtectionengineers define

dependability as the tendency of the protection system to operate

correctly for in-zone faults.

In an electric power system, a fault or fault current is any abnormal electric current. For example, a short circuit is a fault in which current bypasses the normal load. An open-circuit fault occurs if a circuit is interrupted by some failure. In three-phase systems, a fault may involve one or more phases and ground, or may occur only between phases. In a "ground fault" or "earth fault", current flows into the earth. The prospective short circuit current of a predictable fault can be calculated for most situations. In power systems, protective devices can detect fault conditions and operate circuit breakers and other devices to limit the loss of service due to a failure.

Faults In Power System

Conductor clashing Conductor clashing is the phenomenon where conductors come in contact with one another during high wind speeds or gusts Short Circuit

Short circuit is a connection between two nodes that forces them to be at the same voltage

Types Of Faults

Arc Flash An arc flash is the light and heat produced from an electric arc supplied with sufficient electrical energy to cause substantial damage, harm, fire, or injury. Types of Arc Flashes There are four different arc flash type events to be assessed when designing safety programs: Open Air Arc Flashes Ejected Arc Flashes Equipment Focused Arc Flashes (Arc-in-a-box) Tracking Arc Flashes

Power-system protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the isolation of faulted parts from the rest of the electrical network. The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible still in operation

POWER SYSTEM PROTECTION

Generator sets In a power plant, the protective relays are intended to prevent damage to alternators or to the transformers in case of abnormal conditions of operation High-voltage transmission network

At a basic level, protection looks to disconnect equipment which experiences an overload or a short to earth. Overload and back-up for distance

Overload protection requires a current transformer which simply measures the current in a circuit.

Types of Protection

Earth fault Earth fault protection again requires current transformers and senses an imbalance in a three-phase circuit. Back-up The objective of protection is to remove only the affected portion of plant and nothing else.

MINIATURE CIRCUIT BREAKER

Components of Protection System

DIGITAL PROTECTIVE RELAY

CURRENT TRANSFORMER

Switchgear is the combination of electrical disconnects switches, fuses  or circuit breakers used to control, protect and isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.

SWITCHGEAR

Oil

Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil along the path of the arc. The vapor released by the arcing consists of  hydrogen gas. Mineral oil has better insulating property than air.

TYPES OF SWITCHGEAR

Vacuum Circuit breakers with interrupting vacuum have minimal arcing characteristics, so the arc quenches when it is stretched by a small amount. Near zero current the arc is not hot enough to maintain a plasma, and current ceases; the gap can then withstand the rise of voltage. Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 40,500 volts.

A Current Transformer (CT)is used for measurement of alternating electric current.

When current in a circuit is too high to apply directly to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments.

CURRENT TRANSFORMER

Voltage transformers (VT) are a parallel connected type of instrument transformer, used for metering and protection in high-voltage circuits or phasor

phase shift isolation. The primary may be connected phase to ground or phase to phase. The secondary is usually grounded on one terminal.

VOLTAGE TRANSFORMER

 A Protective Relay is a device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detection of abnormal operating conditions such as over-current, over-voltage, reverse power flow, over- and under- frequency.

PROTECTIVE RELAY

There are 6 types of relays: • Over voltage relays• Over current relays• I D M T  fault relay• Earth fault relay• Bucheloz’s relay• Differential relay

TYPES OF RELAYS

A circuit breaker is an automatically operated electrical switch 

designed to protect an  electrical circuit from damage caused by  overload or short circuit. Its basic function is to

detect a fault condition and interrupt current flow.

CIRCUIT BREAKERS

Sulfur Hexafluoride (SF6)

The sulfur hexaflouride gas (SF6) is an electronegative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of sulphurhexaflouride gas and an arc is struck betweenthem. The gas captures theconducting free electrons in thearc to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enoughinsulation strength to extinguishthe arc.

TYPES OF CIRCUIT BREAKERS

High Voltage Circuit Breaker Electrical power transmission networks are

protected and controlled by high-voltage breakers.

The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher

BUSBAR

In electrical power distribution, a Busbar is a metallic strip or bar that conducts electricity within a switchboard, distribution board, substation, battery bank, or other electrical apparatus. Its main purpose is to conduct a substantial current of electricity, and not to function as a structural member.

An aluminium smelter will have very large busbars used to carry tens of thousands of amperes to the electrochemical cells that produce aluminium from molten salts.