Power System Protection Part -2 | Power Plant Guruji

  Essential Qualities Of Protection

Speed

Protective relaying should disconnected a faulty element as quickly as possible.

Principal Reasons:

• Improves power system stability
• Decreases the amount of damage incurred
• Decreases the outage time for power consumer
• Decreases the possibility of severe fault in the up - stream

Selectivity

Selectivity is the ability of the protective system to determine the point at which the fault occurs by selecting the proper type of protective relay. This gives trip command to the nearest circuit breakers for cleaning the fault and no damage the system.

Sensitivity 

Sensitivity of a protective scheme refers to the smallest value of actuating quantity at which the protection start operating in relation with the minimum value of fault current in the protection zone.

Reliability

Reliability means trustworthiness. The relay should not fail to operate in the event of fault in the protective zone. It should not operate unnecessarily without sensing any fault.

Simplicity

Contruction of the relay should be simple, good in quality, correct in design for installation and less maintenance or supervision prone.

Economy

As with all good engineering designs , economics play a major role. It is required to achieve all general requirements together at minimum compromising cost.
Too much protection as is bad as too little. So the relay engineer most strike a sensible compromise with due regard to pratical situation considered.

Basic Principles Of Protection

Essentially, the system or the equipment should be able to discriminate the appropriate disconnecting device.

There are two methods of discriminating fault:

• As per the location of the fault
• Types of fault

The different ways of discriminating the types of faults are : 

• Discrimination by time
• Discrimination by current magnitude
• Discrimination by time and direction
• Discrimination by distance measurement
• Time as addition to current magnitude or distance discrimination
• Current balance discrimination
• Powerdirection comparision discrimination
• Phase Comparision discrimination

Commonly Used Components Of Protection

Power System Protection Part - 2

Some of the commonly used components of the protective schemes  are described here in brife.

They are :

• Current Transformer ( C T )
• Voltage Transformer
• Circuit Breaker (C B )
• Tripping And Auxiliary Supplies
• Relays

Current Transformer - Importance

Current Transformer : 

• Reduce power system current to lower value for measurement.
• Insulate secondary circuit from the primary.
• Permit the use of standard current ratings for secondary equipment.

Remember : The relay performance depends on the C.T which drive it.

Types Of C Ts as per Construction

There are two types of CTs as per construction - Bar Type and Wound Type. 

Identifying Polarity Of CTs - Flick test

Polarity plays a vital role for some application like differential protection, directional over current protection etc.

Fick test is used to identify polarity in power system.

In this test : 

• Battery (6 V - 9 V ) + lead connected primary terminal should be marked P1.
• If the needle of the ammeter moves towards clockwise direction then the meter +ve lead is connected to S1.

C Ts Polarity - Directional Protection

• C T polarity plays a vital role here to sense the direction of current with reference of voltage transformer.
• The directional protection will not work if C T connection is changed or is wrongly connected.
• Directional Protection : It means the relay will become operational in one direction of current. But it will not sense any fault in opposite direction of current.

C Ts Polarity - Differential Protection

• C T Polarity plays a vital role in differential protection as well.
• Differential protection senses internal fault only.

Knee Point Voltage

The voltage at which a 10% increase in applied voltage increase the magnetizing current by 50% is referred to as knee point voltage.

Groups of C Ts as per operation 

Based on their operation, C Ts are devided into two groups - Measurement C Ts and Protection C Ts

Measurement C Ts :

• Limits are well-defined
• Require good accuracy up to approx 120% rated current
• Require low saturation level to protect instrument thus, use nickel iron alloy core with low exciting current and knee point at low flux density
• Measuring C T is more accurate than Protection C T

Protection C Ts :

• Operate on a wide range of current
• Accuracy not as important as above
• Require accuracy up to many times rated current. Thus use gain orientated silicon steel with high saturation flux density
• The saturation point denoted by Vk is more than Measuring C T