Introduction to Electrical Power System Protection: Functions of Relays, Circuit Breakers, and Fuses

 Introduction

The electric power system is a critical infrastructure supporting modern society. This system encompasses the generation, transmission, distribution, and consumption of electrical energy. During operation, the electric power system must operate reliably and safely. To ensure this reliability and safety, a robust protection system is required.

Power system protection aims to detect faults such as short circuits, overloads, or insulation failures, and then take action to isolate the affected section to prevent damage to the entire system. The main components of this protection system include protective relays, circuit breakers, and fuses.

This document will discuss these three components in depth, from their operating principles and types to their application in electric power systems.

1. Basic Concepts of Power System Protection

1.1 Protection Objectives

The objectives of a protection system include:

• Protecting electrical equipment and installations from damage due to disturbances.
• Ensuring human safety against electrical hazards.
• Minimizing the area affected by disturbances by isolating damaged parts.
• Ensuring continuity of electrical energy service.

1.2 Types of Disturbances in Power Systems

Some common types of disturbances:

• Three-phase faults: the most serious and potentially damaging faults to equipment.
• Two-phase to ground faults.
• Single-phase to ground faults: the most common faults in distribution systems.
• Overloads.
• Short circuits

1.3 General Principles of Protection Systems

A protection system consists of:

• Sensors: usually current transformers (CTs) and voltage transformers (VTs).
• Protection relays: devices that detect faults and send trip signals.
• Circuit breakers: devices that open an electrical circuit when they receive a signal from a relay.
• Fuse: automatic thermal disconnecting device (fusible link).

2. Protection Relays

2.1 Definition of Relay

A protection relay is a device that detects abnormal conditions in a power system and sends a command to the circuit breaker to open the circuit if necessary. Relays operate based on parameters such as current, voltage, frequency, and time.

2.2 Types of Relays

a. Based on Operating Energy:

• Electromechanical relays: use electromagnetic principles. Still widely used, although they are being replaced by digital technology.
• Static relays: use electronic components.
• Microprocessor relays: computer-based, capable of data processing and communication.

b. Based on Function:

• Overcurrent relays: operate when the current exceeds a specified limit.
• Overvoltage and undervoltage relays.
• Distance relays: measure the impedance between the fault point and the relay location.
• Differential relays: detect the difference in current entering and leaving a protected zone.
• Frequency relays: operate when the system frequency changes significantly.

2.3 Relay Operating Principle

A relay receives a signal from a CT or VT. When the monitored parameter value (e.g., current) exceeds the set limit, the relay sends a signal to the circuit breaker to open. Relays also have a time delay setting to prevent malfunctions due to temporary disturbances.

2.4 Relay Characteristics

Inverse time: The greater the disturbance, the faster the relay operates.

Definite time: The operating time remains constant, regardless of the magnitude of the disturbance.

Instantaneous: Without delay, operates immediately upon detection of a disturbance.

3. Circuit Breakers

3.1 Definition of Circuit Breaker

A circuit breaker is an automatic switch that opens and closes an electrical circuit manually or automatically to protect the system from damage caused by faults. Circuit breakers are capable of interrupting large currents in a short time without causing damage to themselves.

3.2 Types of Circuit Breakers

a. Based on Arc Extinguishing Media:

• Air Circuit Breaker (ACB): uses air as the extinguishing medium.
• Oil Circuit Breaker (OCB): uses insulating oil.
• SF6 Circuit Breaker: uses sulfur hexafluoride (SF6) gas as the insulating and extinguishing medium.
• Vacuum Circuit Breaker: uses a vacuum to extinguish the arc.

b. Based on Operating Voltage:

• Low-voltage (LV) circuit breaker.
• Medium-voltage (MV) circuit breaker.
• High-voltage (HV) circuit breaker.

3.3 Circuit Breaker Operation Principle

When a fault occurs, the relay will send a trip signal to the circuit breaker. The circuit breaker then opens its contacts and cuts off the current. The arc that occurs during the break is extinguished by the extinguishing medium (air, oil, gas, or vacuum).

3.4 Important Specifications

• Breaking capacity: The maximum capacity to interrupt current.
• Making capacity: The ability to close a circuit under a fault.
• Closing time and tripping time.
• Number of operations: The number of operating cycles before requiring maintenance or replacement.

4. Fuses

4.1 Definition of Fuse

A fuse is a single-use safety device that protects electrical equipment from overcurrent. A fuse consists of a thin metal wire that melts and breaks the circuit when the current exceeds a safe limit.

4.2 Types of Fuses

• Cartridge fuse: used in low-voltage systems.
• Drop-out fuse: used in medium-voltage distribution systems.
• High-rupturing capacity (HRC) fuse: used for high-current protection.
• Resettable fuse (PTC): a fuse that can return to its original state after cooling.

4.3 Advantages and Disadvantages of Fuses

Advantages:

• Inexpensive.
• Reacts quickly to overcurrent.
• Simple and requires no maintenance.

Disadvantages:

• Can only be used once.
• Cannot be reset like a circuit breaker.
• Less accurate in selective protection than relays and circuit breakers.

5. Protection System Coordination

5.1 Selectivity

Selectivity is the ability of a protection system to isolate only the faulted section without affecting other parts of the system. This is important to prevent a local fault from causing a system-wide outage.

5.2 Relay and Circuit Breaker Coordination

In complex protection systems, coordination between various relays and circuit breakers is required to ensure the system operates in the desired sequence. For example, relays at distribution substations must operate later than relays at the end of the distribution network to prevent a fault in the network from tripping the substation first.

5.3 Current and Time Settings

Relay and circuit breaker settings are determined based on a protection coordination study, taking into account:

• Relay characteristic curves.
• Load current and short-circuit current.
• Selective time delay between zones.

6. Protection Applications in Power Systems

6.1 Generator Protection

Involves differential relays, overvoltage relays, low-frequency relays, and ground fault relays.

6.2 Transformer Protection

Uses differential relays, overcurrent relays, and Buchholz relays to detect internal faults.

6.3 Transmission Line Protection

Distance relays, overcurrent relays, and pilot protection are used to maintain the reliability of high-voltage lines.

6.4 Distribution Network Protection

Uses fuses, reclosers, and overcurrent relays to detect faults in the consumer network.

7. Latest Technology in Protection

7.1 Digital Relays and IEDs (Intelligent Electronic Devices)

Modern microprocessor-based relays are capable of providing protection, control, measurement, and communication in a single device. They are used in SCADA systems and smart grids.

7.2 Protection Communication

Using protocols such as IEC 61850 for real-time data exchange between protection devices.