Time Current Characteristic Curves for Selective Coordination

Time Current Characteristic Curves for Selective Coordination

Objective of Power System Protection:

Types of Sub

Principles of Power System Protection:

  • Sensitivity: Protection equipment should be sensitive in accurately detecting all kinds of faults.
  • Speed: Speed in tripping (cutting of supply from healthy region)
  • Economics: Less expensive. Should not cost greater than 25% of the overall cost.
  • Simplicity: should not make the overall system to look complex
  • Selectivity: identifying the correct faulty part so that the least part gets affected. For example, a university has its main breaker and each department has its own breakers, suppose if a fault occurs in a department, It should not trip the main breaker of the university, instead the main breaker of that department should trips.

Time Current Curves (TCCs)

Time Current Curves (TCCs)
  • In a TCC, current is mentioned on the x-axis while time on the y-axis.
  • A TCC is plotted on a logarithmic scale so that all values of current and time are easily incorporated. For example: in a system, a minimum fault of 100 A should be cleared within 10 s and for a system with a maximum fault of 5000 A it must be cleared within 50 ms. Logarithmic scale in TCC ensures that both extreme values of current and time are present.
  • Relay curves are sharper and thinner than fuse and breakers because relays are only used to sense a fault and then issue a trip signal to the breakers. They are typically used in MV and HV systems.

TCC of a Circuit Breaker:

Solid State Trip
  • Long Time Ampere Rating: It is the continuous current rating at which breaker shows no tripping. For example, a breaker is rated at 1000 A and maximum current that will flow through the breaker is 800 A. Therefore, long-time ampere setting will be adjusted to 800 A.
  • Long Time Delay: This setting refers to delay due to inrush current of transformer and starting current of motor. This delay is given in the form of a slope.
  • Short Time Pickup: It is 1.5 to 10 times the long time ampere rating. The setting at which breaker tends to trip after some delay.
  • Short Time Delay: Delay gave due to check if downstream devices cleared the fault so no trip issues or else after reaching delay breaker trips. It has two settings available
  • Instantaneous Pickup: Used when tripping is required without any delay. Its setting can vary from 2 to 40 times of long-time ampere rating.
  • Minimum Clearing Time: It is the time at which breaker senses a fault.
  • Maximum Clearing Time: It is the time at which breaker issues a trip signal.
Thermal Magnetic Trip
  1. Delay Trip
  2. Instantaneous Trip

What is Selective Coordination?

Fault below CB5
Figure 1: Fault below CB5

How Selective Coordination is done?

How Selective Coordination is done?

Achieving Selective Coordination using ETAP:

  1. Select the part of system of which TCC is to be obtained. Then from below shown module bar (Fig 01), we will select Star Protective Protection Then as shown in Fig 2 we will select Create Star View.
Achieving Selective Coordination using ETAP
Coordinating CB1
Figure 2: Coordinating CB1
Coordinating CB1
Coordinating CB2
Figure 3: Coordinating CB2
Coordinating CB2
90% OFF — Power System Protection Fundamentals

Rules of Selectivity:

Creating selectivity by proper selection of pickup settings
Fig. 2 — Creating selectivity by proper selection of pickup settings.
Creating selectivity by proper selection of delay settings
Fig. 3 — Creating selectivity by proper selection of delay settings.
Identifying complete selectivity
Fig. 4 — Identifying complete selectivity
non-selective system
Fig. 5 — An example of a non-selective system

TCC of a Fuse:

TCC of a Fuse
Figure 4: TCC of a Fuse

Cable Damage Curves:

A typical cable damage curve
Figure 5: A typical cable damage curve
Protecting a cable
Figure 6: Protecting a cable

TCC of a Transformer:

Coordinating with transformer inrush and damage curves
Figure 7: Coordinating with transformer inrush and damage curves

Nuisance Tripping due to Inrush Current:

Transformer Damage Curve:

Thermal capability curve of a transformer
Figure 8: Thermal capability curve of a transformer

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AllumiaX Engineering

AllumiaX Engineering

Leaders in Industrial & Commercial Power Systems Engineering