Transformer Overcurrent and Earth fault Relay setting Calculation Excel

Transformer IDMT Over Current & Earth Fault Relay Setting

The Inverse Definite Minimum Time (IDMT) Overcurrent and Earth Fault relays are protection devices used in power systems to safeguard transformers, lines, and other electrical equipment from overcurrent and earth faults. These relays operate with a time delay, which varies inversely with the fault current magnitude, ensuring selective and coordinated protection.

1. IDMT Overcurrent Relay

The overcurrent relay is designed to protect the transformer from faults caused by excessive current, which may occur due to short circuits or overloading. The relay operates when the current flowing through the transformer exceeds a set threshold, known as the pickup current.

Key Settings for Overcurrent Relay:

• Pickup Current (I> or I>>): This is the minimum current level at which the relay starts operating. The pickup setting is typically set between 110% - 150% of the rated transformer current. The value depends on the transformer’s rated current and the expected fault levels.

• Time Multiplier Setting (TMS): The TMS adjusts the operating time of the relay once the current exceeds the pickup value. A lower TMS results in a faster relay operation for the same overcurrent value. The TMS is set based on the system’s protection coordination requirements.

• Characteristic Curve: The relay's operating time is based on a predefined curve, typically following one of the following standards:

  • Standard inverse
  • Very inverse
  • Extremely inverse The inverse time characteristic ensures that the relay operates faster for higher fault currents but delays tripping for smaller overcurrent conditions, which might be tolerable for short periods.

2. IDMT Earth Fault Relay

The earth fault relay protects the transformer from earth or ground faults, which occur when there is an unintended connection between a phase conductor and the earth. These faults can result in severe damage due to large fault currents, which often have lower magnitude compared to phase faults but still pose serious risks.

Key Settings for Earth Fault Relay:

• Pickup Current (I₀ or I>>>): The pickup current for the earth fault relay is typically set lower than that of the overcurrent relay. It usually ranges from 10% to 40% of the rated current depending on the system grounding and fault current levels.

• Time Multiplier Setting (TMS): Similar to the overcurrent relay, the TMS for the earth fault relay controls the operating time. A proper setting ensures that the relay operates fast enough to clear faults while coordinating with other protective devices.

• Characteristic Curve: The same type of inverse time curves is used as for the overcurrent relay, but with a different range to accommodate the nature of earth faults.

3. Relay Coordination

For optimal performance, relay coordination is crucial. Coordination ensures that only the closest protective device to the fault operates, while others remain in service. The overcurrent and earth fault relay settings must be configured in such a way that the relay nearer to the fault clears it faster than those further away.

To achieve proper coordination:

• Set the TMS and pickup values considering the coordination time intervals (CTI) with upstream and downstream devices.
• Choose appropriate characteristic curves (standard inverse, very inverse, etc.) based on system design.

4. Setting Procedure:

1. Determine Rated Current of Transformer: Calculate the rated current of the transformer using the formula:

   $$I_{\text{rated}} = \frac{S_{\text{rated}}}{\sqrt{3} \times V_{\text{rated}}}$$

1. where $S_{\text{rated}}$ is the transformer rating (in kVA or MVA) and $V_{\text{rated}}$ is the rated voltage.

2. Select the Pickup Current: Set the relay pickup current to around 1.1 to 1.5 times the rated current.

3. Set the TMS: Set the TMS value to ensure selective coordination. Use time-current curves to verify proper relay operation for various fault levels.

4. Check System Coordination: Ensure the settings allow the relay to operate within the desired time frame and coordinate with upstream and downstream relays.

5. Earth Fault Settings: Adjust the earth fault relay pickup and TMS to detect low-level faults, ensuring adequate protection against earth faults while avoiding nuisance tripping.

5. Advantages of IDMT Relays:

• Selective protection: The inverse time characteristic provides selective protection, ensuring faster clearing of severe faults and slower tripping for minor, temporary overcurrent conditions.
• Coordination: Easily adjustable settings allow for coordination between multiple relays, ensuring that only the nearest relay to the fault trips.
• System stability: Time delay prevents nuisance tripping for transient overcurrent conditions, ensuring stable operation of the power system.

In conclusion, properly setting up an IDMT overcurrent and earth fault relay is essential for the effective protection of transformers and electrical systems. These settings should be done carefully, considering system load, fault levels, and coordination with other protection devices to ensure both equipment protection and system reliability.

Write about the 11kv/415v 1600kva Transformer IDMT Over Current _ Earth Fault Relay Setting 

The 11kV/415V 1600kVA transformer is a common step-down transformer used to reduce high voltage (11kV) to low voltage (415V) for distribution in industrial, commercial, or residential areas. It is crucial to protect this transformer from faults such as overcurrent (OC) and earth faults (EF) using protection relays. The Inverse Definite Minimum Time (IDMT) Overcurrent and Earth Fault Relay is typically used to provide this protection.

1. IDMT Overcurrent Relay Settings

The overcurrent relay protects the transformer against phase faults and overloading. The key parameters for setting an IDMT overcurrent relay are:

• Current Setting (Pick-Up Current): This is the threshold at which the relay starts to operate. For a 1600kVA transformer, the full load current on the 415V side can be calculated as:

  $$I_{full load} = \frac{1600}{\sqrt{3} \times 415} \approx 2228 \, A$$

On the 11kV side, the current will be:

$$I_{11kV} = \frac{1600}{\sqrt{3} \times 11} \approx 84 \, A$$

• The overcurrent relay is typically set at 120% to 150% of the full load current to avoid tripping under normal load conditions.

• Time Multiplier Setting (TMS): The TMS determines how quickly the relay operates in response to overcurrent. The TMS is chosen based on coordination with other relays to ensure selective tripping. A typical setting might range from 0.1 to 1.0, depending on the desired time delay.

• Curve Selection: The IDMT relay has several characteristic curves (e.g., standard inverse, very inverse, extremely inverse) that determine how the time delay decreases as the fault current increases. The selection depends on the system coordination study and fault clearing time requirements.

2. Earth Fault Relay Settings

The earth fault relay detects ground faults, where one phase comes in contact with the ground, which can cause severe damage to the transformer if not cleared quickly. The earth fault relay settings are usually lower than those of the overcurrent relay.

• Current Setting (Pick-Up Current): The pick-up setting for the earth fault relay is usually a percentage of the full load current, typically between 10% and 20%. This is because earth faults often involve much lower current than phase faults.

• Time Delay: The time delay is usually shorter for earth faults than for overcurrent conditions, as earth faults can cause significant damage if left unchecked. The TMS for earth faults is set to coordinate with other protection devices to allow selective tripping.

3. Example of Settings for 11kV/415V 1600kVA Transformer

• Overcurrent Relay (OC)

  • Current setting: 120% of full load current (e.g., 100A on the 11kV side)
  • Time multiplier setting: 0.2 to 0.5, depending on coordination study
  • Curve type: Standard Inverse

• Earth Fault Relay (EF)

  • Current setting: 10%-20% of full load current (e.g., 20A on the 11kV side)
  • Time multiplier setting: 0.05 to 0.2
  • Curve type: Standard Inverse

4. Coordination Considerations

Proper coordination between the overcurrent and earth fault relays is crucial to ensure the system operates selectively. This means that the nearest protective device to the fault should operate first to isolate the fault, leaving the rest of the system unaffected.

5. Conclusion

The IDMT overcurrent and earth fault relay settings for an 11kV/415V 1600kVA transformer must be carefully calculated based on the transformer's rated current, fault current levels, and system coordination. Proper settings ensure the transformer is adequately protected while minimizing nuisance trips and ensuring selective tripping during fault conditions.

Transformer Differential Protection Relay Setting

Transformer differential protection is a key protective scheme used to safeguard transformers from internal faults, such as winding short circuits, phase-to-phase faults, or phase-to-ground faults. The differential protection relay works by comparing the current entering and leaving the transformer. If the difference (or differential) in current exceeds a predetermined threshold, the relay operates to isolate the transformer from the system.

Key Components of Transformer Differential Protection:

1. CTs (Current Transformers): Installed on both sides (primary and secondary) of the transformer to measure the current.
2. Differential Relay: Receives input from the CTs and calculates the difference between primary and secondary currents.
3. Restraint and Operate Current: The relay usually works on a bias or restraint principle to prevent operation during external faults or inrush conditions.

Relay Setting Parameters

1. CT Ratio Matching:

   • Since the transformer steps up or steps down the voltage, the current on the primary and secondary sides will differ proportionally. Therefore, the CT ratios must be selected to match the transformer ratio so that both the primary and secondary currents appear equal when there is no fault.
   • Example: For a 10 MVA transformer with a 33 kV/11 kV voltage rating, the primary and secondary CT ratios may be 300:1 and 900:1, respectively.

2. Bias or Restraint Settings:

   • To prevent unnecessary tripping during inrush currents or external faults, a bias or restraint characteristic is used. It ensures that the relay operates only when the differential current is high compared to normal load conditions.
   • Bias Current (Ib): This is typically the average of the current seen by the CTs on the primary and secondary sides of the transformer.
   • The relay will have multiple bias slopes to handle different levels of unbalance or system conditions.

3. Percentage Differential Setting:

   • This setting determines how sensitive the relay is to differential current. It’s usually expressed as a percentage of the rated current.
   • A typical setting might be 15-30% of the rated current. A lower setting makes the relay more sensitive, while a higher setting prevents tripping during normal operating conditions like load unbalances.

4. Operate Current Threshold:

   • The operate current is the differential current that causes the relay to trip. This is the difference between the primary and secondary current beyond which the relay will act.
   • It is often set to detect internal faults while being secure against normal operation currents, external faults, or transformer inrush currents.

5. Inrush Current Detection:

   • Transformer energization can cause high inrush currents that are not due to faults. To prevent unnecessary tripping during inrush conditions, the relay uses harmonic restraint, typically the second harmonic (which is prominent during inrush). If the second harmonic content is high, the relay will restrain from tripping.
   • This is often set at around 15-20% of the second harmonic threshold.

6. Through-Fault Current Withstand:

   • The relay must be set to allow through-fault currents (currents that pass through the transformer without causing internal faults). A proper setting ensures that the relay doesn’t trip during external faults.

7. Zero Sequence Current Elimination:

   • To avoid maloperation due to ground faults external to the transformer, the relay may have settings to eliminate the effect of zero-sequence currents, especially if a delta-wye transformer is used.

Example of Relay Setting

For a 20 MVA, 66/11 kV transformer:

• CT ratios: 300:1 (primary) and 1500:1 (secondary).
• Percentage differential setting: 25%.
• Operate current setting: 0.25 times rated current.
• Bias setting: Slope 1: 15%, Slope 2: 30% (to accommodate higher through-faults).
• Inrush restraint: 2nd harmonic threshold at 15%.

Testing and Commissioning

• Stability Test: This tests the relay's stability under normal conditions and external faults. The relay should not trip for these conditions.
• Sensitivity Test: This tests the relay's ability to detect internal faults, ensuring it trips when the differential current exceeds the set threshold.

In summary, transformer differential protection relay settings involve careful selection of CT ratios, differential settings, bias characteristics, harmonic restraint, and other factors to ensure accurate fault detection while avoiding nuisance trips during normal operation.

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Transformer Overcurrent and Earth fault Relay setting Calculation Excel