Overcurrent and Inverse-Time Relay Selection Calculator – IEEE, IEC

Overcurrent and inverse-time relays are critical components in power system protection, ensuring safety and reliability. Selecting the correct relay settings requires precise calculations based on standards like IEEE and IEC.

This article explores the technical aspects of overcurrent and inverse-time relay selection calculators, detailing formulas, tables, and real-world applications. It provides a comprehensive guide for engineers and professionals in power system protection.

Artificial Intelligence (AI) Calculator for “Overcurrent and Inverse-Time Relay Selection Calculator – IEEE, IEC”

• Calculate relay operating time for a 150 A fault current with IEC standard settings.
• Determine pickup current and time dial setting for IEEE inverse-time relay.
• Compute coordination time interval between two overcurrent relays in a distribution network.
• Evaluate relay characteristic curve parameters for a 200 A load using IEC standard.

Comprehensive Tables for Overcurrent and Inverse-Time Relay Selection – IEEE and IEC Standards

Fundamental Formulas for Overcurrent and Inverse-Time Relay Selection

Relay coordination and timing depend on several key formulas derived from IEEE and IEC standards. Understanding these formulas is essential for accurate relay setting and system protection.

• Pickup Current (Ip): The minimum current at which the relay starts to operate. Usually set as a multiple of the relay’s rated current (In).

Ip = k × In

• Where k is the pickup multiplier, typically between 1.0 and 20.
• In is the relay rated current.

• Operating Current Ratio (I/Ip): Ratio of fault current to pickup current, critical for time calculation.

Operating Current Ratio = I / Ip

• Inverse-Time Relay Operating Time (t): Time delay before relay operation, inversely proportional to the magnitude of current.

t = TDS × (A / ((I/Ip)^B – 1)) + C

• TDS: Time Dial Setting, adjustable from 0.05 to 1.0 (or higher depending on relay).
• A, B, C: Constants defined by the relay characteristic curve (Standard Inverse, Very Inverse, Extremely Inverse).
• I: Fault current magnitude.
• Ip: Pickup current.

• Coordination Time Interval (CTI): Minimum time difference between two relays to ensure selectivity.

CTI = t_downstream – t_upstream ≥ 0.3 s (typical)

• Ensures downstream relay operates before upstream relay to isolate fault locally.

Explanation of Variables and Typical Values

Real-World Application Examples of Overcurrent and Inverse-Time Relay Selection

Example 1: Feeder Protection Using IEC Standard Inverse-Time Relay

A distribution feeder is protected by an IEC standard inverse-time overcurrent relay. The relay rated current (In) is 10 A, and the pickup current (Ip) is set at 1.2 × In = 12 A. The time dial setting (TDS) is 0.5. A fault current of 60 A occurs on the feeder. Calculate the relay operating time.

• Given:
• In = 10 A
• Ip = 12 A
• TDS = 0.5
• I = 60 A
• Characteristic constants for IEC standard inverse: A = 0.14, B = 0.02

Step 1: Calculate operating current ratio

I/Ip = 60 / 12 = 5

Step 2: Calculate operating time using formula

t = TDS × (A / ((I/Ip)^B – 1))

t = 0.5 × (0.14 / (5^0.02 – 1))

Calculate 5^0.02:

5^0.02 ≈ e^(0.02 × ln(5)) ≈ e^(0.02 × 1.609) ≈ e^(0.03218) ≈ 1.0327

Then:

t = 0.5 × (0.14 / (1.0327 – 1)) = 0.5 × (0.14 / 0.0327) = 0.5 × 4.28 = 2.14 seconds

The relay will operate approximately after 2.14 seconds, providing adequate time coordination with upstream devices.

Example 2: Coordination Between Two Overcurrent Relays Using IEEE Very Inverse Curve

Two overcurrent relays protect a motor feeder and its upstream feeder. The downstream relay has In = 20 A, Ip = 1.5 × In = 30 A, and TDS = 0.3. The upstream relay has In = 50 A, Ip = 1.2 × In = 60 A, and TDS = 0.5. A fault current of 90 A occurs. Calculate the operating times and verify coordination with a minimum CTI of 0.3 seconds.

• Given:
• Downstream relay: In = 20 A, Ip = 30 A, TDS = 0.3
• Upstream relay: In = 50 A, Ip = 60 A, TDS = 0.5
• Fault current I = 90 A
• Characteristic constants for IEEE Very Inverse: A = 13.5, B = 1.0

Step 1: Calculate operating current ratios

Downstream: I/Ip = 90 / 30 = 3

Upstream: I/Ip = 90 / 60 = 1.5

Step 2: Calculate operating times

t_downstream = TDS × (A / ((I/Ip)^B – 1))

t_downstream = 0.3 × (13.5 / (3^1 – 1)) = 0.3 × (13.5 / (3 – 1)) = 0.3 × (13.5 / 2) = 0.3 × 6.75 = 2.025 seconds

t_upstream = 0.5 × (13.5 / (1.5 – 1)) = 0.5 × (13.5 / 0.5) = 0.5 × 27 = 13.5 seconds

Step 3: Calculate Coordination Time Interval (CTI)

CTI = t_upstream – t_downstream = 13.5 – 2.025 = 11.475 seconds

The CTI is well above the typical minimum of 0.3 seconds, confirming proper coordination between relays.

Additional Technical Considerations for Relay Selection and Coordination

• Relay Setting Margins: Pickup current should be set above maximum load current to avoid nuisance tripping.
• Time Dial Setting (TDS): Adjusted to coordinate with upstream and downstream devices, balancing speed and selectivity.
• Fault Current Calculations: Accurate fault current estimation is critical; use system studies and short-circuit analysis tools.
• Relay Characteristic Curves: Selection depends on application; motors require very inverse curves, while feeders often use standard inverse.
• Standards Compliance: Follow IEEE C37.112 and IEC 60255-151 for relay characteristics and testing procedures.
• Coordination Studies: Use software tools to simulate relay operation and ensure selectivity and reliability.

Understanding these factors ensures optimal protection system performance, minimizing downtime and equipment damage.

References and Further Reading

• IEEE C37.112-2014 – Inverse-Time Overcurrent Relays
• IEC 60255-151 – Measuring Relays and Protection Equipment
• Electrical Engineering Portal – Overcurrent Relay Characteristics
• PSERC – Relay Coordination and Protection