Protection and Backup Protection for High Voltage Cables

1. Introduction

High voltage (HV) cable protection is critical to ensure reliability and safety in power transmission systems. The primary protection scheme for HV cables is current differential protection (87L), while backup protection consists of overcurrent (51), directional overcurrent (67), earth fault (51N/67N), and breaker failure (50BF) protection. This document outlines the settings and coordination required for each protection scheme.

2. Example System Data and Assumptions

For the purpose of this analysis, we consider a 220 kV, 50 km XLPE underground cable with the following parameters:

Parameter	Value
Rated Voltage	220 kV
Cable Length	50 km
Rated Current (I_rated)	1000 A
Charging Current (I_charging)	15 A
Short-Circuit Current (I_sc)	15 kA
CT Ratio	2000:1
Fault Clearing Time (Primary Relay)	80 ms

3. Differential Protection (87L) Settings

Differential protection (87L) detects faults within the cable zone using current comparison at both ends.

• Provides selective and fast protection by comparing currents at both ends of the cable.

• Highly effective in detecting internal faults

  3.1 Principle of Operation

  • Differential protection operates by comparing the current entering and leaving the cable.

  • If the difference (I_diff) exceeds a preset threshold, it indicates an internal fault.

  • It uses Kirchhoff’s Current Law (KCL):

    Idiff​=Iin​−Iout​​

  • Ideally, in a healthy system:

    Iin​=Iout​⇒Idiff​=0

  • If a fault occurs inside the protected zone, the difference becomes significant, triggering a trip.

  3.2 Components & Implementation

  • Current Transformers (CTs) at both ends of the cable.

  • Communication Link (Fiber optic, pilot wire, or power line carrier).

  • Numerical Relay with phase-segregated or combined residual differential algorithms.

  3.3 Challenges in HV Cable Differential Protection

  1. CT Saturation:

     • Causes false tripping or maloperation.

     • Solution: Use CTs with high saturation withstand (e.g., Class X CTs per IEC 60044-1).

  2. Charging Current Compensation:

     • HV cables have significant charging current.

     • Solution: Numerical relays use a stabilizing resistor or compensation algorithm.

  3. Mismatch in CT Ratios or Vector Groups:

     • Requires proper CT selection and correction settings in relays.

  4. External Fault Stability:

     • High inrush currents during switching events may cause unwanted trips.

     • Solution: Use a percentage restraint scheme to prevent tripping on through-faults.

3A. Pickup Setting (I_diff_min)

• Set at 20-30% of the rated cable current:
• Formula
  Idiff_min​=(0.2−0.3)×Irated​
• Example: If the cable carries 1000 A, set the pickup to 200-300 A.
  Idiff_min​=0.2×1000A=200A

3B. Stabilization Factor (K_stab)

• Protects against CT saturation and through-faults.
• Recommended setting: 0.5 - 0.8.
• Selected value: 0.6.

3C. Slope (Percentage Bias)

• Prevents misoperation due to CT mismatch.
  • Slope1 = 35% (normal operation)
  • Slope2 = 70% (high through-faults)

3D. Charging Current Compensation

• Compensation factor:

  Icomp​=Icharging​×Compensation Factor

• Compensation Factor = 1.2 (depends on relay model).

• Compensation current:

  Icomp​=15A×1.2=18A

• This value should be added to the stabilization factor.

3E. Final 87L Relay Settings

Parameter	Setting
Differential Pickup (I_diff_min)	200 A
Stabilization Factor (K_stab)	0.6
Slope1	35%
Slope2	70%
Charging Current Compensation	18 A

4. Backup Overcurrent Protection (51) Settings

Time-delayed (51) for lower-magnitude faults.

4A. Pickup Current

• Overcurrent relay (51) should not trip for normal load but must clear faults.
• Set 1.5 × maximum load current:
  Ipickup​=1.5×Irated​
  Ipickup​=1.5×1000A=1500A
• Converted to secondary side (CT ratio = 2000:1):
  Ipickup(sec)​=2000/1500​=0.75A

4B. Time Delay (Inverse Curve)

• Standard Inverse Curve (IEC 60255):
• For fault current of 10 kA, we get 0.5 sec delay.

4Final 51 Relay Settings

Parameter	Setting
Pickup Current	1500 A (0.75 A secondary)
Time Delay	0.5 sec
Curve Type	Standard Inverse

5. Directional Overcurrent Protection (67) Settings

• Applied where power flow direction is crucial (e.g., parallel cables, ring networks).

• Ensures fault clearing in the correct direction.

5A. Pickup Current

• 1.2 × maximum load current:
• Secondary Current:

5B. Directional Angle

• 30° lag to detect forward faults.

5C. Time Delay

• 300 ms delay.

Final 67 Relay Settings

Parameter	Setting
Pickup Current	1200 A (0.6 A secondary)
Directional Angle	30° lag
Time Delay	300 ms

6. Earth Fault Protection (51N/67N) Settings

• Detects ground faults using zero-sequence current.

• Directional earth fault protection (67N) is used in systems with multiple grounding points.

6A. Residual Current Pickup

• Set at 0.2 × I_rated:

6B. Directional Settings

• Directional angle: ±90°.

6C. Time Delay

• 400 ms delay.

Final 51N/67N Relay Settings

Parameter	Setting
Pickup Current	200 A (0.1 A secondary)
Directional Angle	±90°
Time Delay	400 ms

7. Breaker Failure Protection (50BF) Settings

• Detects failure of circuit breakers to operate.

• Triggers upstream breakers to clear the fault.

7A. Current Supervision

• Set at 1.2 × I_rated:

7B. Time Delay

• 200 ms delay.

Final 50BF Relay Settings

Parameter	Setting
Pickup Current	1200 A (0.6 A secondary)
Time Delay	200 ms

8. Summary of Protection Settings

Protection	Pickup Current	Time Delay	Direction
87L (Diff Protection)	200 A	Instantaneous	-
51 (Overcurrent)	1500 A	0.5 sec	-
67 (Directional OC)	1200 A	300 ms	30° lag
51N (Earth Fault)	200 A	400 ms	±90°
50BF (Breaker Failure)	1200 A	200 ms	-

9. Conclusion

• Differential Protection (87L) provides fast and selective fault clearing.
• Backup protection (51, 67, 51N) ensures system reliability.
• Proper coordination avoids false trips and improves stability.

This article provides a structured guide for setting up HV cable protection schemes. If required, the settings can be refined by the Electrical Engineer for specific relay models such as SEL, ABB, Siemens, or Schneider.

10. Standards Governing HV Cable Protection

1. IEC 60287 – Electric Cables – Calculation of Current Rating

2. IEC 60840 – Power Cables with Rated Voltages Above 30 kV and Up to 150 kV

3. IEC 62067 – Power Cables with Rated Voltages Above 150 kV

4. IEC 60255 – Measuring Relays and Protection Equipment

5. IEC 60364 – Electrical Installations – Protection Requirements

6. IEEE 242 (Buff Book) – Protection and Coordination of Industrial and Commercial Power Systems

7. IEEE 400 – Guide for Field Testing and Evaluation of Cable Systems

8. IEEE C37.91 – Guide for Protective Relay Application to Power Transformers, Motors, and Cables