Protection Criteria for Medium Voltage Networks

A Comprehensive Technical Guide from ABB

Designing a reliable and safe medium voltage (MV) distribution network requires a deep understanding of fault currents, network configurations, and the protection systems that safeguard both equipment and personnel. ABB’s technical guide, "Protection Criteria for Medium Voltage Networks," is an essential resource for engineers, providing a detailed overview of the principles, devices, and schemes used to protect MV systems. This article summarizes the key content from this comprehensive document.

Why Protection Criteria Matter

The guide emphasizes that the primary objectives of a protection system are to:

• Limit damage to people and plant equipment.
• Guarantee maximum service continuity for the parts of the network not affected by a fault.
• Activate necessary automatisms (like automatic changeover).

However, it also warns that an excessive number of protections can be harmful. Even if they operate correctly during a fault, they can also operate untimely when there is no fault, causing more widespread disturbances that are sometimes more damaging than the faults themselves.

Understanding Network Schemes

The protection system must adapt to the type of network. The guide analyses the main network schemes, highlighting their advantages and disadvantages:

Single Radial Network

• Description: The simplest and least costly tree-like structure where loads are distributed along a single path.
• Advantages: Simplicity and economy.
• Disadvantages: Low reliability. Maintenance or a fault will put the entire load side of the network out of service.

Double Radial Network

• Description: Two equal, alternative paths are created by duplicating a basic radial network.
• Advantages: High service continuity. Maintenance can be performed without plant stoppages, and faults cause limited outages.
• Disadvantage: High cost.

Ring Network

• Description: A network with at least one more path (n+1) than the minimum needed, providing two power supplies for each substation. It can be run open or closed.
• Run Open: The network operates as a radial type. A fault puts the load side out of service, but service can be restored relatively quickly by counter-supplying from the sound side.
• Run Closed: Theoretically, provides no outages for faults within the ring. It requires more complex protection systems, like directional overcurrent relays and differential protections, to isolate only the faulty trunk.

Meshed Network

• Description: The typical scheme of transmission networks with many connections among nodes, allowing for alternative power supply routes.
• Application: This scheme does not have any particular applications in industrial plants.

Selection of Switching Devices

The guide explains the role of key switching devices and their impact on protection strategies:

• Circuit-breakers: Able to close and interrupt short-circuit currents. Compulsory when using differential protections for rapid fault elimination.
• Switch-disconnectors: Can open rated current (or not). When used with fuses, they provide protection against short-circuits, but cannot offer safe and rapid protection against ground faults in the secondary of a transformer.
• Contactors with fuses: Can provide protection against overloads and ground faults via protection relays that control contactor opening. However, the limited rated currents of fuses can be a limitation for this type of switching device.

Current and Voltage Transformers

Accurate measurement is fundamental for protection. The guide details the selection and application of instrument transformers:

Current Transformers (CTs)

• Instrument CTs: Used for measurement. They must saturate at currents just above the rated primary current to protect instruments. It is crucial that their real secondary load is at least 25% of the rated performance to guarantee the precision class.
• Protection CTs: Used for relays. They must not saturate until protection tripping is guaranteed, defined by an accuracy limit factor (FL). The guide provides examples showing how oversizing CTs can lead to destruction of relays or instruments during a fault.
• Ring CTs: These measure the vectorial sum of currents (homopolar current) by enclosing all three phases. They are recommended for earth fault protection due to their precision and stability.

Voltage Transformers (VTs)

• Precision and Load: Like instrument CTs, the precision class of VTs is only guaranteed if the secondary load is at least 25% of the rated performance.
• Ferroresonance: A critical phenomenon in cable networks with isolated neutral. The guide recommends measures to prevent it, such as using VTs with high permeability iron or damping resistances.

Non-Inductive Sensors

• Technology: Modern digital relays have extremely low consumption, making air CTs (Rogowski coils) and voltage dividers viable alternatives.
• Advantages: They eliminate saturation, have linear response, and do not suffer from ferroresonance. However, their precision class does not yet match that of inductive types.

Short-Circuit Currents

Calculating short-circuit currents is essential for:

• Sizing apparatus and plant elements.
• Defining protection relay settings.
• Protecting people and plants.

The guide references standards EN 60909-0 for three-phase AC systems and highlights that more than 80% of faults start as a single-phase fault to ground. Therefore, the ability to identify and selectively clear ground faults is crucial to prevent them from evolving into more damaging two- or three-phase faults.

The Status of the Neutral

The method of grounding the neutral dictates how ground faults are detected and cleared.

• Isolated Neutral: No intentional homopolar current circulation. Fault current is from capacitive contributions. Identification is only possible via homopolar voltage measurement, which is not selective (it indicates a fault exists but not its location).
• Solidly Grounded: Ground fault current is high, similar to short-circuit current. Simple and selective identification using homopolar current (or even phase) protection is possible.
• Grounded via Resistance: Provides a definite fault current value. Allows selective protection. A lower fault current reduces machine damage, but a higher current makes identification easier.
• Grounded via Impedance (Petersen Coil): Compensates capacitive currents, reducing fault current to small values (e.g., 40-50 A). Allows selective protection. The current is tuned, and the angle between voltage and current is theoretically zero.

Protection Relay Codes and Settings

The guide uses the numerical codes from the IEEE C37-2 Standard (e.g., 50 for instantaneous overcurrent, 51 for inverse-time overcurrent, 87 for differential protection, 27 for undervoltage).

Philosophy of Protection Settings

• Selectivity: The goal is to isolate only the faulty section. This can be achieved through time selectivity (grading times), current selectivity (grading pickup currents), differential protection, or logical selectivity.
• Coordination: A study is required to ensure proper coordination between all protections. The protection trip current must always be lower than the minimum short-circuit current at the point of connection.

Protections for Specific Equipment

The guide dedicates a significant section to the specific protections required for different network components:

• Synchronous Machines (Generators): Includes differential protection (87G), thermal overload (49), overcurrent (51), loss of field (40), reverse power (32), negative sequence (46), and rotor/stator ground fault (64R/64S) protections.
• Transformers: Covers thermal overload (49), overcurrent (51/50), differential (87T), restricted ground differential (87N), and Buchholz relay (63) for oil-filled transformers.
• Motors: Includes thermal image (49), negative sequence (46), short-circuit (50), ground fault (51G), prolonged starting (48), locked rotor (51LR), and undervoltage (27) protections.
• Capacitors: Protections include overcurrent (51), short-circuit (50), ground fault (51G), overvoltage (59), and negative sequence (46) protections.
• Ring Networks and Automatic Changeover: Discusses protections for ring networks and schemes for automatic changeover using undervoltage (27), synchronism check (25), and other relays to ensure supply continuity.

Conclusion

ABB’s "Protection Criteria for Medium Voltage Networks" is a foundational guide that systematically covers the principles and practices of MV protection. It emphasizes that a well-designed protection system is not just about installing relays; it requires a deep understanding of the network’s structure, its neutral grounding, and the specific characteristics of the equipment being protected. This ensures that faults are cleared quickly and selectively, minimizing damage and maximizing the reliability of the power supply.

Download the Full Guide

This overview is based on the ABB technical guide. The full document provides detailed diagrams, in-depth discussions of protection functions, and practical examples for setting and coordinating protection relays.

The guide is available to download for free directly from ABB:

Download the Protection Criteria for Medium Voltage Networks Guide

Disclaimer: This article provides a summary of a third-party technical publication. While efforts have been made to accurately represent the content, please refer to the official ABB document for complete technical details. Final design and protection decisions remain the responsibility of qualified professionals.