IDMT Curve Calculator Excel Download



An IDMT Curve Calculator is a tool used in the field of electrical engineering to determine the operating time of protection relays based on an Inverse Definite Minimum Time (IDMT) characteristic. IDMT relays are typically used for overcurrent protection in power systems, where they are designed to operate more quickly when the magnitude of the fault current is higher, but still have a definite minimum time to avoid unnecessary tripping due to transient conditions.

Key Aspects of IDMT Curve Calculators:

  1. IDMT Curve Characteristics: The IDMT relay operates on a predefined curve that relates the fault current (usually in multiples of the rated current) to the operating time. The higher the fault current, the shorter the operation time, but the curve ensures a minimum delay to prevent nuisance trips. There are different types of IDMT curves, including:

    • Standard Inverse
    • Very Inverse
    • Extremely Inverse
    • Long-Time Inverse

  2. Time Multiplier Setting (TMS): The Time Multiplier Setting is an adjustable parameter on the IDMT relay that shifts the relay's operating curve. By changing the TMS, the overall operating time of the relay can be adjusted to coordinate with other relays in the system. Lower TMS values cause faster operation, while higher values delay the response.

  3. Plug Setting (PS): The Plug Setting determines the current level at which the relay begins to operate. It is typically a multiple of the relay’s rated current. The fault current is expressed as a multiple of the plug setting current.

  4. Operating Time Calculation: The IDMT Curve Calculator uses inputs like the fault current, relay settings (TMS and PS), and the type of IDMT curve to compute the operating time of the relay. The formula used for the calculation varies depending on the type of curve but follows a general format like:

    Operating Time=TMS(Fault Current/Plug Setting)Curve Constant−1\text{Operating Time} = \frac{TMS}{(Fault \, Current / Plug \, Setting)^{Curve \, Constant} - 1}

    The constants and formulas differ for each type of curve (standard inverse, very inverse, etc.).

  5. Relay Coordination: An IDMT Curve Calculator is crucial for relay coordination in power systems. It ensures that the relay closest to the fault operates first, while upstream relays only operate if the fault is not cleared, maintaining system stability.

Practical Applications:

  • Power Distribution Systems: IDMT relays protect transformers, transmission lines, and generators by clearing overcurrent faults effectively.
  • Relay Testing and Commissioning: Engineers use IDMT Curve Calculators to test the settings of relays during commissioning and maintenance of electrical systems.
  • Coordination Studies: In complex power systems, relays must be coordinated to minimize outage impacts. The calculator helps in fine-tuning relay settings to avoid overlap in relay operations.

Overall, the IDMT Curve Calculator simplifies the process of determining the appropriate response time for relays, ensuring both system protection and reliable operation.

The Inverse Definite Minimum Time (IDMT) curve is a characteristic used in the operation of protective relays, particularly in power system protection schemes. It defines how the relay responds to overcurrent conditions, determining the time delay before it trips a circuit breaker in the event of a fault. The IDMT curve is widely used in protection systems to balance the need for quick fault clearance with system stability.

Key Features of the IDMT Curve:

  1. Inverse Nature: The time delay is inversely proportional to the magnitude of the fault current. As the fault current increases, the relay operates faster, meaning the higher the current, the shorter the operating time.

  2. Definite Minimum Time: Despite being inverse, the curve has a minimum limit on the operating time, ensuring that the relay does not trip instantly for minor overcurrents. This definite time prevents unnecessary operations due to transient conditions.

  3. Standardized Curves: There are several standard IDMT curves, such as:

    • Normal Inverse: The relay operates with moderate speed as the fault current increases.
    • Very Inverse: The relay operates faster for higher overcurrents but slower for lower currents.
    • Extremely Inverse: This curve provides the quickest response for very high fault currents and the slowest for low currents.

Applications:

  • Overcurrent Protection: IDMT relays are commonly used to protect electrical equipment such as transformers, feeders, and motors from overcurrent conditions.
  • Coordination: The IDMT curve allows for better coordination between upstream and downstream protection devices. Devices closer to the fault operate faster, while those further away operate slower, ensuring only the faulty section of the system is isolated.

By combining time delay and current sensitivity, the IDMT curve helps ensure reliable and efficient fault protection in power systems.

An Inverse Definite Minimum Time (IDMT) relay is a type of overcurrent relay commonly used in power systems for protection against faults such as short circuits or overloads. Its unique characteristic lies in its time-current relationship: the relay operating time decreases as the fault current increases, which makes it ideal for selective and effective protection of electrical systems.

Working Principle

The IDMT relay operates on the principle that the higher the current, the faster the relay will trip. It has an inverse time characteristic, meaning that at lower current levels, the relay takes a longer time to operate, but as the current increases beyond the set threshold, the operation time decreases rapidly. This is achieved through the use of a time-current curve, which is inverse in nature.

The main components involved in the working of an IDMT relay are:

  1. Current Sensing Element: The relay measures the current flowing through the system. If the current exceeds a preset value (pickup current), the relay begins its timing function.

  2. Time Delay Mechanism: The time delay is inversely proportional to the magnitude of the overcurrent. As the current increases, the time to trip decreases, preventing damage by isolating the fault quicker.

  3. Definite Minimum Time: The relay includes a minimum operating time to avoid tripping during transient conditions like short-lived current spikes that don't indicate actual faults. Even at extremely high fault currents, the relay will trip after a fixed minimum time.

Operating Characteristics

The operating time is defined by specific curves, such as:

  • Normal inverse: For general overcurrent protection. The time to trip decreases significantly with increasing fault current.
  • Very inverse: Used in situations where greater sensitivity is needed for higher currents, such as near generators.
  • Extremely inverse: Suitable for protecting equipment like motors, where damage from higher current is more critical.

Settings of IDMT Relay

  1. Pickup Current: This is the minimum current that causes the relay to start its operation. It is set as a multiple of the rated current.

  2. Time Multiplier Setting (TMS): Adjusts the time taken by the relay to trip once the pickup current is exceeded. A higher TMS results in a longer time delay for the same fault current, allowing coordination with other protection devices downstream.

  3. Current Setting (Plug Setting): This sets the threshold level for overcurrent detection, usually set as a percentage of the rated current.

Advantages of IDMT Relay

  • Selectivity: Different relays in a network can be set to operate with different time delays and current settings to ensure only the relay closest to the fault operates.
  • Coordination: IDMT relays work well with other protective devices in the system, providing backup protection.
  • Sensitivity: The relay is highly sensitive to overcurrent conditions and offers a dynamic response based on fault severity.

Applications

  • Distribution Networks: Protecting feeders and transformers in distribution networks.
  • Generators and Motors: Protecting generators and motors from overloading or short circuits.
  • Transmission Lines: Acting as backup protection for high-voltage transmission lines.

In summary, IDMT relays provide efficient protection in power systems by dynamically adjusting their operating time based on the magnitude of the fault current, ensuring fast and selective fault clearance.

The Inverse Definite Minimum Time (IDMT) relay is widely used in electrical protection systems, including the protection of transformers, to detect overcurrent and short-circuits. For an 11kV/415V, 1600kVA transformer, it plays a critical role in safeguarding the transformer from damage due to abnormal electrical conditions.

Key Parameters in IDMT Relay Calculation

To calculate IDMT relay settings for an 11kV/415V, 1600kVA transformer, the following parameters are typically considered:

  1. Transformer Rating:

    • Voltage: 11kV/415V
    • Power: 1600kVA


    • CT Ratio:

      • Current Transformers (CTs) are used to step down the high primary currents to manageable levels for relay input. For example, if the CT ratio is 100/1, 84A on the primary will correspond to 0.84A on the relay side.

    • Overcurrent Setting (Plug Setting):

      • The overcurrent relay is typically set at a percentage of the transformer full load current. This is known as the plug setting. For example, the relay may be set to operate at 120% of the transformer full load current.

    • Time Multiplier Setting (TMS):

      • The TMS adjusts the time delay of the relay. Lower values of TMS result in faster operation for a given fault current, while higher values increase the delay.
      • The time-current characteristic of an IDMT relay follows a predefined curve (such as standard inverse, very inverse, or extremely inverse). This curve defines the relationship between fault current and relay operation time.

      • Step-by-Step Calculation Example

      Let’s assume the following for an 11kV/415V, 1600kVA transformer:

      • CT Ratio = 100/1
      • Plug Setting = 1.2 (120% of full load current)
      • TMS = 0.2
      • Standard Inverse Characteristic

      1. Determine Fault Current:
        Assume a fault on the secondary side of the transformer. If the fault current is 6 times the full load current (6 × 2224 = 13,344A on the secondary side), the current on the relay side would be:

        Ifault_relay=13,344 A100=133.44 AI_{fault\_relay} = \frac{13,344 \, \text{A}}{100} = 133.44 \, \text{A}

      2. Calculate Pickup Current:
        The full load current on the relay side is:

        Ifull_load_relay=84 A100=0.84 AI_{full\_load\_relay} = \frac{84 \, \text{A}}{100} = 0.84 \, \text{A}

        The relay is set to operate at 120% of the full load current, so the pickup current is:

        Ipickup=1.2×0.84=1.008 AI_{pickup} = 1.2 \times 0.84 = 1.008 \, \text{A}

      3. Fault Multiple:
        The fault current multiple is:

        M=Ifault_relayIpickup=133.441.008=132.38M = \frac{I_{fault\_relay}}{I_{pickup}} = \frac{133.44}{1.008} = 132.38

      4. Relay Operating Time:
        For a standard inverse relay, the operating time is given by:

        t=TMS×0.14(M0.02−1)t = TMS \times \frac{0.14}{(M^{0.02} - 1)}

        Substituting the values:

        t=0.2×0.14(132.380.02−1)≈0.2×0.14(1.208−1)=0.2×0.140.208≈0.2×0.673=0.1346 secondst = 0.2 \times \frac{0.14}{(132.38^{0.02} - 1)} \approx 0.2 \times \frac{0.14}{(1.208 - 1)} = 0.2 \times \frac{0.14}{0.208} \approx 0.2 \times 0.673 = 0.1346 \, \text{seconds}

      Conclusion

      For a fault current 6 times the full load current, the IDMT relay will operate in approximately 0.1346 seconds, depending on the actual relay settings and fault conditions. These calculations ensure that the transformer is protected in a timely manner while preventing unnecessary tripping under normal operating conditions.

      The relay settings need to be coordinated with other protection devices in the system to avoid maloperation and ensure selectivity.

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      IDMT Curve Calculator Excel Download

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