Calculator
An Inverse Defined Minimum Time (IDMT) Calculator is an online (or) Excel-based tool that calculates the operation time of protective relays using the inverse time characteristics of overcurrent protection systems.
These tools assist in determining the trip time of relays if a fault current exceeds a predefined threshold.
According to the IDMT curve, the trip time decreases as the fault current increases.
These calculators are frequently utilized in electrical protection coordination, allowing engineers to quickly establish relay parameters for best protection in distribution & power systems.
Inverse time overcurrent protection relays are utilized to provide faster tripping times for the higher fault currents, thereby improving system protection.
The magnitude of fault current & their settings determine relays’ operating time.
The specified minimum time calculation implements different standard curves based on the current multiplier & time multiplier setting (TMS).
Key Variables
Setting Current (Iset): The reference current for relay operation, commonly set as a percentage of the transformer rating or CT secondary rating.
Time Multiplier Setting (TMS): The setting on the relay that influences the time delay for tripping. Lower TMS values result in speedier operations.
CT Primary: The primary current of the current transformer (CT), typically in amperes.
CT Secondary: The secondary current of the current transformer (CT), usually 1A (or) 5A.
Formula for Time Calculation
The tripping time for an inverse-time overcurrent relay is calculated using the following formula:
T=TMS x [K/ (I/Iset)P – 1]
Where
T – Tripping time in seconds
I – Actual fault current
Iset – Relay’s Setting Current
(TMS) – Time Multiplier Setting
For the type of curve,
K – Constant (varies for every inverse characteristic)
P – Exponent – Depending on the type of inverse curve (Normal, Very, Extreme, or Long-term)
Curve Types
IEC Normal Inverse: K = 0.14, 𝑃 = 0.02,
IEC Very Inverse: K = 13.5, 𝑃 = 1.0,
IEC Extreme Inverse: K = 80.0, 𝑃 = 2.0,
IEC Long-Term Inverse: K = 120.0, 𝑃 = 1.0.
How to Calculate the Tripping Time?
Step 1: Calculate the fault current (I) by multiplying it by the setting current (Iset).
Multiples of Iset = I/ Iset
Step 2: Determine the appropriate inverse curve for the protection requirement (e.g., IEC Normal, IEC Very Inverse, etc.).
Step 3: Enter K, P, & TMS values into the formula for the desired curve type.
Step 4: Determine the tripping time based on the fault current multiple.
Example of Time Calculation
Assuming you are employing the IEC Normal Inverse Curve, the overall formula becomes:
T=TMS x [0.14/ (I/Iset) 0.02 – 1]
This formula calculates the tripping time for a fault current (I) in seconds based on the time multiplier (TMS) and current multiple.
The general technique for each curve type is the same, with their specific constants (K) and exponents (P).
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