Power System Protection Part - 1 | Powerplantguruji

  What is power system protection

• Power-system protection is a branch of electrical power engineering.
• It deals with the protection of electrical power system from fault.
• The objective of a protection scheme is to keep  the power system stable by isolating only the componets that are under fault.
• The device that are  used to protect the power system from the faults are called protection device

Basic Idea Of Protection Relay - Why , What and How ?

Power System Protection 

Why ?

A good electric power system should ensure the availability of electrical power with out any interruption to every load connected to it.

What ?

• Generally power is transmitted through high voltage transmission line and lines are exposed.
• Therefore, there may be chances of their breakdown due to strom, falling of external objects, and damage to the insulators, etc. These can result not only mechanical damage but also in an electrical fault.
• Protective relays and relaying systems detects abnormal conditions like faults in electrical circuits and automatically operate the switchgear to isolate faulty equipment from the system as quickly as possible.

How ?

• This limits the damage at the fault location and prevents the effects of the fault spreading into the system.
• The switch gear must be capable of intrrupting both normal current as well as fault current.
• The protective relay on other hand must be to recognize an abnormal condition in the power system and take suitable steps so that there will be least possible disturbance to normal operation.

Nature Of Faults - Defination

• The nature of fault simply implies any abnormal condition which causes a reduction in the basic insulation strength between phase conductors, between phase conductor and earth or any earth screen sorrounding the conductors.
• Power systems mainly consist of generator, switch gear transformer and distribustion system.
• The probability of failuer is more on the power system due to their greater length and exposure to atmosphere.

Nature Of Faults - Different Conditions

Breakdown due to normal voltage

Breakdown at normal voltage may occurs on account of :

• The deterioration of insulation.
• Damage due to unpredictable causes such as perching of birds, accidental short circuiting by snakes, tree branches, etc.

Breakdown due to Abnormal Voltage

Breakdown may occur because of abnormal voltages caused by:

• Switching surges
• Surges caused by lightning

Causes of Faults

Here are a few causes for the occurrnce of faults.

Though the present practice is to provided a high insulation level of the order of 3 to 5 times the normal voltage, but still faults occur:

• The pollution on an insulator string caused by deposited soot or cement dust in industrial area.
• Salt deposited wind borne sea spray in coastal area. These will initially lower the insulation resistances and cause a small leakage current to be diverted, thus hastening the deterioration.

Secondly , even if the insulation is enclosed, such as sheathed and armoured, the deterioration of the insulation occurs because of:

• Ageing effect
• Void formation in the insulation compound of underground cable due to unequal expansion and contractions caused by the rise and fall of temperature.

Thirdly, insulation may be subjected to transient over voltages because of switching operation.

• The voltage which rises at a rapid rate may achieve a peak value which approaches three times phase to neutral voltages.
• Lightning produces very high voltage in the power system in the order of million volts. These surges travel with the velocity of light in the power circuit. The limiting factors are the surges impedance and the line resistance.

Consequences of Faults

• A great reduction of the line voltages.
• Damage caused to the element of the system the electrical arc.
• Damage to the other parts due to overheating.
• Disturbance to the stability of the electrical system and this may even lead to a complete shutdown of the power system.
• Reduction in the voltage may fail pressure coil of the relay.
• Considerable reduction in the voltage on healthy feeder connected to the system having fault.

Abnormal Conditions - Types Of Faults

Power System Protection Part -1

Voltage Related Fault

• Over Voltage Protection :

It is very dangerous for any electrical equipment. The results of this fault are insulation deterioation, insulation failure and sometimes fire. So the protection from over voltage is required. The set value is decided based on the system and equivalent standard.

• Under Voltage Protection :

It is also dangerous for some specific electrical equipment. The result of this abnormality leads to damage of the equipment specially the motor. So the protection from under voltage is required. The set value is decided based on the system and equivalent standard.

Frequency Related Fault 

• Over Frequency :

It is the effect of unstable electrical power system and one of the reason is sudden cutting of huge electrical load.

• Under Frequency :

Similarly, It is also the effect of unstable electrical power system and one of the reason is sudden insertion of huge electrical load.