Fundamentals of Protection Practice: Power System Protection and Relaying Techniques
The purpose of an electrical power system is to generate and supply electrical energy to consumers. The system should be designed to deliver this energy both reliably and economically. Frequent or prolonged power outages result in severe disruption to the normal routine of modern society, which is demanding ever-increasing reliability and security of supply. As the requirements of reliability and economy are largely opposed, power system design is inevitably a compromise.
Power System Protection
Power system protection is essential for the safe and reliable operation of power systems. Protection equipment is used to detect faults in the power system and isolate those faults to minimize damage to equipment and prevent downtime.
The following provides brief insight on why power system protection is so important:
- Prevents equipment damage and failure - Protection equipment is designed to detect faults in the power system and isolate those faults before they can cause damage to equipment. Without proper protection, faults could cause equipment to fail, leading to costly repairs or even the need for complete replacement of equipment.
- Minimizes downtime - Power system protection is critical in minimizing downtime. Faults in the power system can cause equipment to fail or damage the electrical infrastructure, leading to a loss of power. Protection equipment is designed to isolate faults to specific areas of the power system, minimizing the impact of a fault and ensuring that the power system remains operational.
- Ensures safe operation - Power system protection is essential for ensuring safe operation. Faults in the power system can cause damage to equipment, create a hazardous environment, and even pose a risk to the safety of personnel. Protection equipment is designed to detect faults and isolate them to minimize the risk of equipment damage or injury to personnel.
- Increases system reliability - Power system protection is essential for increasing system reliability. By detecting and isolating faults, protection equipment can prevent equipment failure and minimize downtime. A more reliable power system can lead to increased productivity and profitability for businesses that rely on electricity.
- Compliance with regulatory standards - Power system protection is necessary to comply with regulatory standards. Regulatory agencies require that power systems have proper protection equipment in place to ensure the safe and reliable operation of the system.
Related Article: Basic Guide for Power System Protection
Protection Relays
Relays are essential components of power systems, used to detect faults and protect the system from damage. Relays can be classified according to the technology used, with each type offering its own unique advantages and disadvantages. Relays frequently measure complex functions of the system quantities, which may only be readily expressible by mathematical or graphical means.
Here is an overview of the four main types of relays based on technology:
Electromechanical Relays
Electromechanical relays are the oldest type of relay technology and have been used for over a century. They work by using a mechanical switch to open or close a circuit in response to changes in voltage or current. Electromechanical relays are reliable and have been proven over many years of use. However, they are relatively slow compared to other types of relays, and the mechanical components can wear out over time.
 |
| Figure. Electromechanical Protection Relay |
Static Relays
Static relays use solid-state components, such as diodes, transistors, and thyristors, to perform the same function as an electromechanical relay. They offer faster response times than electromechanical relays and are more reliable since they have no moving parts. However, they can be more expensive than electromechanical relays.
 |
| Figure. Static Protection Relay |
Digital Relays
Digital relays use microprocessors and digital signal processing to detect and respond to faults in the power system. They offer greater flexibility than static relays since the settings can be easily changed, and they can provide more information about the fault that occurred. However, they can be more complex to program and troubleshoot than other types of relays.
 |
| Figure. Digital Relays |
Numerical Relays
Numerical relays are the latest type of relay technology, using microprocessors and digital signal processing to detect and respond to faults in the power system. They are similar to digital relays, but they offer even greater flexibility and accuracy. Numerical relays can provide detailed information about the fault that occurred and offer advanced protection functions. However, they can be expensive and complex to program and troubleshoot.
In many cases, it is not feasible to protect against all hazards with a relay that responds to a single power system quantity. An arrangement using several quantities may be required. In this case, either several relays, each responding to a single quantity, or, more commonly, a single relay containing several elements, each responding independently to a different quantity may be used.
Zones of Protection
To limit the extent of the power system that is disconnected
when a fault occurs, protection is arranged in zones. Zones of protection are essential in power systems to ensure that faults in one area of the system do not affect other areas. By defining specific zones, protection systems can isolate faults to specific areas, minimizing the impact of a fault on the entire power system.
Related Article: Zones of Protection in Power Systems
Without proper zones of protection, faults can spread to other areas of the system, causing damage to equipment and leading to costly downtime. Proper zones of protection can also help to ensure selectivity, ensuring that only the equipment affected by the fault is disconnected while other areas of the system remain operational. This can prevent unnecessary outages and minimize the impact of a fault on the entire power system.
 |
| Figure 1. Sample of Protection Zones |
Important Characteristics of Protective Relaying
Protective relays are essential components of power systems, used to detect and respond to faults and other abnormal conditions. There are five characteristics that would serves as benchmark of good practice in protective relaying.
Selectivity
Protective relays must be selective in responding to faults, ensuring that only the equipment affected by the fault is disconnected, while other areas of the system remain operational. Selectivity is achieved by coordinating the protection settings of different protection equipment in the system to ensure that the protection equipment closest to the fault responds first. When a fault occurs, the protection scheme is required to trip only those circuit breakers whose operation is required to isolate the fault. This property of selective tripping is also called 'discrimination' and is achieved by two general methods.
Speed
Protective relays must respond quickly to a fault to minimize damage to equipment and ensure that the power system remains operational. Speed is achieved by selecting protection equipment with fast response times and setting the protection parameters correctly. As the loading on a power system increases, the phase shift between voltages at different busbars on the system also increases, and therefore so does the probability that synchronism will be lost when the system is disturbed by a fault. The shorter the time a fault is allowed to remain in the system, the greater can be the loading of the system.
 |
| Figure 2. Typical power/time relationship for various fault types |
Figure 2 shows typical relations between system loading and fault clearance times for various types of faults. It will be noted that phase faults have a more marked effect on the stability of the system than a simple earth fault and therefore require faster clearance.
Stability
The term ‘stability’ is usually associated with unit protection schemes and refers to the ability of the protection system to remain unaffected by conditions external to the protected zone, for example through-load current and faults external to the protected zone. Protective relays must be stable and not trip unnecessarily, causing unnecessary downtime or damage to equipment. Stability is achieved by setting the protection equipment to operate within specific parameters and ensuring that it remains within those parameters during a fault condition.
Sensitivity
Protective relays must be sensitive enough to detect faults but not so sensitive that they trip unnecessarily, leading to downtime or damage to equipment. Sensitivity is achieved by setting the protection equipment to operate within specific parameters and ensuring that it remains within those parameters during normal operation and fault conditions.
Reliability
Protective relays must be reliable and operate correctly during a fault condition. Any failure in protection equipment could lead to a loss of power or damage to equipment, leading to costly repairs or downtime. Reliability is achieved through proper design, installation, and maintenance of the protection equipment.
Reliability is dependent on the following factors:
- incorrect design/settings
- incorrect installation/testing
- deterioration in service
The design of a protection scheme is of paramount importance. This is to ensure that the system will operate under all required conditions, and refrain from operating when so required. This includes being restrained from operating for faults external to the zone being protected, where necessary. Due consideration must be given to the nature, frequency and duration of faults likely to be experienced, all relevant parameters of the power system and the type of protection equipment used. Of course, the design of the protection equipment used in the scheme is just as important. No amount of effort at this stage can make up for the use of badly designed protection equipment.
Testing should cover all aspects of the protection scheme, reproducing operational and environmental conditions as closely as possible. Type testing of protection equipment to recognised standards is carried out during design and production and this fulfils many of these requirements, but it will still be necessary to test the complete protection scheme (relays, current transformers and other ancillary items). The tests must realistically simulate fault conditions.
Subsequent to installation, deterioration of equipment will take place and may eventually interfere with correct functioning. For example, contacts may become rough or burnt due to frequent operation, or tarnished due to atmospheric contamination, coils and other circuits may become open circuited, electronic components and auxiliary devices may fail, and mechanical parts may seize up. All these factors can contribute to the deterioration in service.
Coordination of Protection Devices
The coordination of protective devices is a critical aspect of power system protection. Protective devices must be coordinated to ensure that they work together effectively in response to faults in the power system. Coordination involves setting the protection parameters to ensure that the closest protective device responds first, minimizing the impact of a fault on the entire power system.
In a coordinated protection system, protective devices are arranged in a specific sequence, with the closest protective device responding first and any backup devices responding if the first device fails to operate. Coordination involves selecting the appropriate protective device, setting the operating parameters for each device, and adjusting the time-current characteristic curves to ensure that the devices operate in the proper sequence.
 |
| Figure 3. Coordination |
The primary objective of protective device coordination is to minimize the impact of a fault on the power system while maintaining the highest level of system reliability. Proper coordination of protective devices can prevent unnecessary outages and minimize the impact of a fault on the entire power system.
Several factors must be considered when coordinating protective devices, including the type of fault that may occur, the characteristics of the protective devices, and the system configuration. Coordinating protective devices involves complex calculations and modeling of the power system to ensure that each protective device operates within its designated zone and sequence.
View or Download PDF
- Title: Fundamentals of Protection Practice
- Source: General Electric
.webp)
No comments: