Protection Coordination in Industrial Facilities Calculator – IEEE

Protection coordination ensures selective and reliable operation of protective devices in industrial electrical systems. It calculates settings to prevent unnecessary outages and equipment damage.

This article explores the IEEE-based protection coordination calculator, detailing formulas, tables, and real-world industrial applications. Learn to optimize device settings effectively.

Artificial Intelligence (AI) Calculator for “Protection Coordination in Industrial Facilities Calculator – IEEE”

• Calculate coordination time interval between upstream and downstream relays for a 5 MW motor feeder.
• Determine the minimum fault current for setting an overcurrent relay in a 480 V industrial panel.
• Compute the time dial setting for a relay protecting a 600 A feeder with known fault current.
• Evaluate the coordination margin between two protective devices in a 13.8 kV distribution system.

Common Values for Protection Coordination in Industrial Facilities

Key Formulas for Protection Coordination in Industrial Facilities

Protection coordination relies on precise calculations of relay operating times and settings. Below are the essential formulas used in IEEE standards for industrial protection coordination.

1. Relay Operating Time Calculation (Inverse Time Overcurrent Relay)

The operating time (T) of an inverse time overcurrent relay is calculated as:

• T: Operating time of the relay (seconds)
• TDS: Time Dial Setting (unitless, typically 0.05 to 1.0)
• K: Constant depending on relay characteristic (e.g., 0.14 for IEEE standard inverse)
• PSM: Plug Setting Multiplier = Fault Current / Pickup Current (unitless)
• α: Exponent depending on relay type (e.g., 0.02 for IEEE standard inverse)

Typical values for K and α for common IEEE relay characteristics:

2. Plug Setting Multiplier (PSM)

The Plug Setting Multiplier is a critical parameter for relay coordination:

• I_fault: Fault current at relay location (Amperes)
• I_pickup: Relay pickup current setting (Amperes)

3. Coordination Time Interval (CTI)

CTI is the intentional time margin between the operation of downstream and upstream protective devices to ensure selectivity:

• T_upstream: Operating time of upstream relay (seconds)
• T_downstream: Operating time of downstream relay (seconds)
• 0.3 seconds: Minimum recommended coordination margin per IEEE standards

4. Minimum Fault Current for Selectivity

To ensure selectivity, the minimum fault current must be greater than the pickup current multiplied by a margin factor:

• I_min_fault: Minimum fault current to be cleared selectively (Amperes)
• I_pickup: Relay pickup current (Amperes)
• 1.2: Safety margin factor recommended by IEEE

Real-World Application Examples of Protection Coordination in Industrial Facilities

Example 1: Coordination Between Downstream Motor Feeder Relay and Upstream Feeder Relay

An industrial facility has a 480 V motor feeder protected by an inverse time overcurrent relay. The motor feeder relay has a pickup current of 100 A and a time dial setting of 0.2. The upstream feeder relay protects a 600 A feeder with a pickup current of 300 A and a time dial setting of 0.4. The fault current at the motor feeder relay location is 1200 A, and at the upstream relay location is 1500 A. Calculate the operating times of both relays and verify the coordination time interval.

Step 1: Calculate Plug Setting Multipliers (PSM)

• Downstream relay PSM = 1200 A / 100 A = 12
• Upstream relay PSM = 1500 A / 300 A = 5

Step 2: Calculate Operating Times Using IEEE Standard Inverse Characteristic (K=0.14, α=0.02)

Calculate 12^0.02:

12^0.02 ≈ e^(0.02 × ln(12)) ≈ e^(0.02 × 2.4849) ≈ e^(0.0497) ≈ 1.0509

Then:

T_downstream = 0.2 × 0.14 / (1.0509 – 1) = 0.028 / 0.0509 ≈ 0.55 seconds

Calculate 5^0.02:

5^0.02 ≈ e^(0.02 × ln(5)) ≈ e^(0.02 × 1.6094) ≈ e^(0.0322) ≈ 1.0327

Then:

T_upstream = 0.4 × 0.14 / (1.0327 – 1) = 0.056 / 0.0327 ≈ 1.71 seconds

Step 3: Calculate Coordination Time Interval (CTI)

CTI = T_upstream – T_downstream = 1.71 – 0.55 = 1.16 seconds

Since 1.16 seconds > 0.3 seconds, coordination is acceptable.

Example 2: Setting Overcurrent Relay for a 13.8 kV Industrial Distribution Feeder

A 13.8 kV feeder in an industrial plant is protected by an overcurrent relay with a rated current of 800 A. The maximum fault current at the relay location is 12,000 A. The relay pickup current is set at 1.2 times the rated current. Determine the plug setting multiplier, and calculate the relay operating time using a time dial setting of 0.3 and IEEE Very Inverse characteristic (K=13.5, α=1.0).

Step 1: Calculate Pickup Current

I_pickup = 1.2 × 800 A = 960 A

Step 2: Calculate Plug Setting Multiplier (PSM)

PSM = I_fault / I_pickup = 12,000 A / 960 A = 12.5

Step 3: Calculate Operating Time

The relay will operate in approximately 0.35 seconds under maximum fault conditions, ensuring fast clearance.

Additional Technical Considerations for Protection Coordination

• Relay Characteristic Selection: Choosing between standard inverse, very inverse, or extremely inverse characteristics depends on the fault current magnitude and system selectivity requirements.
• Coordination Margin: IEEE recommends a minimum coordination time interval of 0.3 seconds to account for relay operating time tolerances and breaker opening times.
• Impact of CTI on System Stability: Excessive CTI can delay fault clearance, risking equipment damage; insufficient CTI can cause miscoordination and unnecessary outages.
• Use of Directional Relays: In complex industrial systems with multiple sources, directional overcurrent relays improve selectivity by considering fault current direction.
• Integration with Motor Protection: Motor starting currents and thermal limits must be considered when setting relay pickup currents to avoid nuisance tripping.
• Coordination with Fuses and Circuit Breakers: Coordination studies must include all protective devices to ensure proper cascading operation.

References and Further Reading

• IEEE Std C37.112-1996 – IEEE Inverse-Time Characteristic Relays
• IEEE Guide for Protective Relay Applications to Industrial and Commercial Power Systems
• National Fire Protection Association (NFPA) Standards for Electrical Safety
• Eaton Overcurrent Relay Technical Resources