Overcurrent relay settings are critical for protecting electrical power systems from faults and overloads. Accurate calculation ensures system reliability and safety.
This article covers IEEE and IEC standards for overcurrent relay settings, including formulas, tables, and practical examples. Learn to optimize relay coordination effectively.
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- Calculate relay pickup current for a 100 A load with 1.2x safety margin.
- Determine time dial setting for a 5-second trip at 200 A fault current.
- Compute plug setting multiplier (PSM) for a relay with 150 A pickup and 300 A fault.
- Find operating time using IEC standard for a relay with TMS=0.1 and fault current 2x pickup.
Common Overcurrent Relay Settings – IEEE and IEC Standards
| Parameter | Typical Range | IEEE Standard Values | IEC Standard Values | Notes |
|---|---|---|---|---|
| Pickup Current (Ip) | 1.0 to 2.0 x Load Current | 1.2 x Load Current | 1.1 to 1.5 x Load Current | Set above max load to avoid nuisance tripping |
| Time Multiplier Setting (TMS) | 0.05 to 1.0 | 0.05 to 1.0 | 0.05 to 1.0 | Adjusts relay operating time |
| Plug Setting (PS) | 0.1 to 1.0 (per unit of max current) | 0.1 to 1.0 | 0.1 to 1.0 | Defines pickup current as fraction of max current |
| Operating Time (t) | 0.1 to 30 seconds | Depends on fault current and TMS | Depends on fault current and TMS | Calculated using standard inverse time curves |
| Fault Current (If) | 1.0 to 20 x Load Current | Varies by system | Varies by system | Determined from system short circuit studies |
Key Formulas for Overcurrent Relay Settings Calculation
Understanding the fundamental formulas is essential for accurate relay setting calculations. Below are the primary equations used in IEEE and IEC standards.
1. Pickup Current (Ip)
The pickup current is the minimum current at which the relay starts to operate.
- Ip = Pickup current (Amperes)
- K = Safety factor (typically 1.1 to 1.5)
- IL = Maximum load current (Amperes)
The safety factor ensures the relay does not trip during normal load variations.
2. Plug Setting Multiplier (PSM)
PSM is the ratio of fault current to pickup current, indicating how many times the fault current exceeds the pickup.
- PSM = Plug Setting Multiplier (unitless)
- If = Fault current (Amperes)
- Ip = Pickup current (Amperes)
3. Operating Time (t) – IEEE Inverse Time Relay
The IEEE standard inverse time relay operating time is calculated as:
- t = Operating time (seconds)
- TMS = Time Multiplier Setting (0.05 to 1.0)
- PSM = Plug Setting Multiplier (unitless)
This formula applies to the IEEE standard inverse characteristic curve.
4. Operating Time (t) – IEC Standard Inverse Time Relay
The IEC standard defines several inverse time characteristics. The general formula is:
- t = Operating time (seconds)
- TMS = Time Multiplier Setting (0.05 to 1.0)
- k and α = Constants depending on curve type
- Typical values for k and α:
- Standard Inverse (SI): k=0.14, α=0.02
- Very Inverse (VI): k=13.5, α=1.0
- Extremely Inverse (EI): k=80, α=2.0
IEC curves provide flexibility for different protection coordination requirements.
5. Time Dial Setting (TDS)
Time Dial Setting is often synonymous with Time Multiplier Setting (TMS) but can be adjusted based on coordination studies.
- TDS = Time Dial Setting (unitless)
- Used to fine-tune relay operating time to coordinate with upstream/downstream devices.
Detailed Real-World Examples of Overcurrent Relay Settings Calculation
Example 1: IEEE Standard Overcurrent Relay Setting for a Distribution Feeder
A distribution feeder has a maximum load current of 120 A. The relay must be set to avoid tripping during normal load but operate quickly during faults. The maximum fault current at the relay location is 600 A. Calculate the pickup current, plug setting multiplier, and operating time assuming a TMS of 0.2.
- Step 1: Calculate Pickup Current (Ip)
- Step 2: Calculate Plug Setting Multiplier (PSM)
- Step 3: Calculate Operating Time (t) using IEEE formula
- Step 1: Calculate Pickup Current (Ip)
- Step 2: Calculate Plug Setting Multiplier (PSM)
- Step 3: Calculate Operating Time (t) using IEC Very Inverse curve
- Coordination with Upstream and Downstream Devices: Relay settings must ensure selectivity to isolate only the faulted section.
- Load Variations: Pickup current should be set above maximum load current plus margin to prevent nuisance trips.
- Relay Characteristic Curves: Choice between standard inverse, very inverse, and extremely inverse curves depends on fault current magnitude and system requirements.
- Time Dial Setting (TDS) vs. Time Multiplier Setting (TMS): Some manufacturers use TDS terminology; both adjust relay operating time.
- Impact of CT Accuracy: Current transformer accuracy and saturation affect relay performance; settings must consider CT characteristics.
- Standards Compliance: IEEE C37.112 and IEC 60255-151 provide guidelines for relay characteristics and settings.
- Always verify system maximum load and fault current values from recent studies.
- Use manufacturer-specific relay curves and constants when available.
- Perform coordination studies to ensure selectivity and minimize outage impact.
- Consider CT ratio and accuracy class in setting calculations.
- Regularly update settings based on system changes or expansions.
- Use simulation software or AI calculators to validate manual calculations.
Using a safety factor K = 1.2:
Fault current If = 600 A
Calculate the denominator:
Therefore:
The relay will operate in approximately 0.9 seconds for a 600 A fault current.
Example 2: IEC Standard Overcurrent Relay Setting for a Substation Transformer Protection
A substation transformer has a rated load current of 200 A. The relay pickup current is set at 1.3 times the load current. The maximum fault current is 1000 A. Using the IEC Very Inverse (VI) curve with TMS = 0.1, calculate the operating time.
IEC VI constants: k = 13.5, α = 1.0
The relay will trip in approximately 0.47 seconds for the fault current of 1000 A.
Additional Technical Considerations for Overcurrent Relay Settings
Summary of IEEE and IEC Overcurrent Relay Characteristics
| Characteristic Curve | IEEE Parameters | IEC Parameters | Typical Application |
|---|---|---|---|
| Standard Inverse (SI) | t = TMS × 0.14 / (PSM^0.02 – 1) | t = TMS × 0.14 / (PSM^0.02 – 1) | General purpose protection |
| Very Inverse (VI) | N/A (IEC specific) | t = TMS × 13.5 / (PSM – 1) | Transformer and feeder protection |
| Extremely Inverse (EI) | N/A (IEC specific) | t = TMS × 80 / (PSM^2 – 1) | High fault current areas, motor protection |
Practical Tips for Using Overcurrent Relay Settings Calculators
For further reading, consult the IEEE C37.112-1996 standard and IEC 60255-151 for detailed relay characteristic definitions and testing procedures.
By mastering overcurrent relay settings calculation using IEEE and IEC standards, engineers can enhance power system protection, ensuring safety and operational continuity.