Mastering Distance Protection and Calculations Part 1: Advice and Serious Warnings

Demystifying Distance Protection

In 2004, I embarked on an intensive nine-month training program in protection and instrumentation. This experience laid the foundation for my deep understanding of substation protection systems, combining both theoretical knowledge and practical application. The first protection scheme we delved into was overcurrent protection — a concept that is straightforward yet crucial.

Overcurrent protection is a familiar safeguard in electrical systems, responding to faults where short-circuit currents surge beyond normal levels. The principle is simple: when an overcurrent is detected by the relay, it triggers the circuit breaker to isolate the faulted section, thus protecting the system from damage.

We encounter this in various forms, from the miniature circuit breakers (MCBs) at low-voltage levels to overcurrent protection schemes in high-voltage (HV) systems. The calculations for overcurrent protection seemed intuitive, and its effectiveness was evident.

However, as the training progressed, we were introduced to distance protection—a more sophisticated form of protection designed for high-voltage (HV) and extra-high-voltage (EHV) transmission lines and cables. The complexity of distance protection immediately raised questions:

I dedicate this work to my esteemed teachers, Mr. Zulfat Shah, the Principal of Tarbela Engineering Academy, and Mr. Shumail Khan, who not only taught us the fundamentals but also guided us through the practical implementation and testing of these settings in the laboratory.

Table of Contents:

1. A Word About Overcurrent Protection
2. Relay Coordination Study (Time/Current):
   1. Example №1 – 13kV Feeder
   2. Example №2 – A Real-Life Case Study of Relay Coordination in 11kV Network
3. Concept of Source Impedance
4. Source Impedance and the Performance of Overcurrent Protection
   1. Challenges in Relay Coordination
5. Distance Protection
6. Line Impedance and Line Angle
7. BONUS (PDF) 🔗 Download Implementation Guidance For Generator Voltage Protective Relay Settings

1. Overcurrent Protection

Overcurrent protection is one of the most fundamental and widely used forms of electrical protection. It serves as the primary defense against short-circuit currents, which can cause significant damage to electrical equipment and pose serious safety risks. Short circuits occur when there is an unintended connection between phases or when a phase is grounded, causing the load impedance to be bypassed.

This results in a sudden and often dramatic increase in current, which must be swiftly detected and interrupted to prevent equipment damage and ensure system stability.

Overcurrent protection is categorized into two main types: instantaneous overcurrent protection and time-delay overcurrent protection. Instantaneous overcurrent protection operates without intentional time delay, tripping the circuit breaker immediately when the current exceeds a pre-set threshold. This type of protection is effective for dealing with severe short circuits that require immediate isolation to prevent catastrophic damage.

Time-delay overcurrent protection, on the other hand, introduces a deliberate delay before tripping, allowing it to distinguish between temporary overcurrents (such as those caused by inrush currents during motor startup) and sustained faults. Time-delay overcurrent protection is further divided into inverse time and definite time protection. Inverse time protection means that the tripping time decreases as the fault current increases, providing a balance between sensitivity and selectivity.

Definite time protection operates with a fixed time delay, regardless of the magnitude of the fault current, making it useful in certain coordination schemes.

Despite these challenges, overcurrent protection remains a cornerstone of electrical safety, offering a straightforward and dependable means of protecting against short-circuit currents.

Highly Recommended – Overcurrent Protection Course

Overcurrent Protection Course: Principles, Relays, Schematics and Settings

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2. Relay Coordination Study

Before diving deeper into the technical aspects of protection systems, it’s essential to understand the concept of relay coordination and its critical role in power system reliability. Relay coordination refers to the strategic arrangement and setting of protective relays within a power system to ensure that only the device closest to a fault operates, thereby isolating the faulty section with minimal disruption to the rest of the network.

Power systems are divided into various protection zones, each of which is safeguarded by specific protective devices such as relays and circuit breakers. These zones can include transmission lines, transformers, buses, and other critical components.

The primary goal of relay coordination is to ensure that when a fault occurs within a specific zone, the protection system isolates only that particular zone, leaving the rest of the power system unaffected. This selective tripping is crucial for maintaining system stability and minimizing the impact of faults on electricity supply.

Such events can have significant economic and operational consequences, making proper relay coordination a necessity in power system design and operation.

Figure 1 – Relay coordiantion example

There are two primary methods of achieving relay coordination: time coordination and current coordination.

Time coordination involves setting relays with time delays that increase as you move upstream from the fault location. The idea is that the relay closest to the fault will trip first due to its shortest time delay, while upstream relays will have longer delays to ensure they only trip if the downstream relay fails to clear the fault.

Current coordination relies on setting relays to operate at different current levels. Relays closer to the load side are set to trip • at lower current thresholds, while those further upstream are set at higher thresholds. This method is primarily used in overcurrent protection schemes, where it ensures that relays respond correctly to varying levels of fault current.

In summary, relay coordination is an essential aspect of protection system design, ensuring that faults are isolated efficiently and effectively, minimizing the impact on the overall power system. Properly coordinated relays protect the integrity of the system, prevent unnecessary outages, and maintain the continuity of service to the maximum extent possible.

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2.1 Example №1

13 kV feeder

Figure 2 depicts a standard 13 kV feeder consisting of multiple wires at a distribution substation. The power is provided by a 115 kilovolt line through a 15/20/25 MVA power transformer, which is protected by a fuse on the high side. Only one of the four feeders is depicted and represents a typical example, whereas the loading and protection of the remaining feeders are comparable but distinct. The fault magnitudes are expressed in amperes at a voltage of 13.09 kilovolts for solid faults occurring at the indicated sites.

Starting from the high-side fuse, the protection is established and coordinated as follows. The maximum load capacity for the 25 MVA tap is:

25,000√3 × 115 = 125.5 A at 115 kV

The 125E fuse was selected for the transformer bank primary. Its operating time close to 250 A is 600 seconds, which should override cold-load and magnetizing inrush transients.