Directional protection and directional zone selectivity

Source: ABB 

The definition of selectivity is given by the ANSI C37.17 Standard, “American National Standard for Trip Devices for AC and General-purpose DC Low voltage Power Circuit Breakers”. Zone protective interlocking provides a selective trip system which obtains shorter tripping times for upstream circuit breakers for faults located between two or more circuit breakers, while providing coordination of upstream and downstream circuit breakers for through faults. ~ABB 

Related Article: Protection Relays in Power System

Zone protective interlocking may operate on the short-time-delay trip function and/ or the ground fault trip function. It requires communication between the direct-acting trip devices comprising the zone protective interlocking system. Selection of the protection system of the electrical installation is fundamental both to guarantee correct economical and functional service of the whole installation and to reduce to a minimum the problems caused by abnormal service conditions or actual faults. 

Particularly, a good protection system must be able to:

  1. Sense what has happened and where, discriminating between abnormal but tolerable situations and fault situations within its zone of competence, avoiding unwanted trips that cause unjustified stoppage of an undamaged part of the installation. 
  2. Act as rapidly as possible to limit the damage (destruction, accelerated ageing, etc.) safeguarding power supply continuity and stability.

Directional Protection

Directional protection is a type of protective relay system that operates by detecting the direction of current flow in an electrical power system. This allows it to distinguish between faults that occur within a protected zone (such as a single feeder or transmission line) and those that occur outside of the zone. By doing so, it can selectively trip only the breakers that are necessary to isolate the fault, without tripping other breakers that are not affected by the fault. This helps to minimize the extent of the power system that is affected by a fault and reduces the duration of the power outage.

Directional protection can be achieved using various types of protective relays, including electromechanical relays, static relays, and microprocessor-based relays. The relay typically has a set of directional elements that are designed to operate when the current flow is in a particular direction, and a set of non-directional elements that are designed to operate regardless of the current direction.

Directional Zone Selectivity

Directional zone selectivity is a concept that involves dividing an electrical power system into multiple zones of protection, with each zone having its own directional protection relay. The zones are arranged such that the relays nearest to the source of power have the highest fault current settings and the smallest operating time delay, while those farther away have lower settings and longer time delays. This creates a cascading effect where the nearest relay will trip first, followed by the next closest relay, and so on. By coordinating the settings of the directional relays and the time delays, it is possible to achieve a high level of selectivity, such that only the circuit breaker closest to the fault will trip, while the others remain unaffected.

Directional zone selectivity is an important concept in power system protection, as it allows faults to be quickly isolated without affecting other parts of the system. This helps to maintain the reliability of the power system and minimize the impact of faults on customers.

The Role of Numerical Relays

Nowadays, numerical relays are the most convenient to use for this purpose. Numerical relays operate circuit breakers in a directional protection scheme by sending a trip signal to the appropriate circuit breaker when a fault is detected within its zone of protection. The directional protection is achieved by analyzing the current waveform and determining the direction of current flow, which enables the relay to determine the location of the fault.

Numerical relays are advanced microprocessor-based relays that offer several advantages for directional protection in electrical power systems. Some of the key roles of numerical relays in directional protection include:
  1. Accurate and reliable detection of fault direction - Numerical relays use complex mathematical algorithms to analyze the current waveform and accurately detect the direction of current flow. This allows them to provide precise directional protection and avoid unnecessary tripping of healthy circuit breakers. 
  2. High-speed operation - Numerical relays can operate in a matter of milliseconds, which is essential for fast and reliable fault detection and isolation. They can also operate in high-speed applications such as generator protection and line differential protection. 
  3. Flexibility and programming capabilities - Numerical relays can be programmed with complex logic and customized settings to meet specific protection requirements. They can also be remotely monitored and controlled, which allows for easy maintenance and troubleshooting. 
  4. Advanced communication capabilities - Numerical relays can communicate with other relays and control systems using various protocols such as IEC 61850 and DNP3. This allows for coordinated protection and improved system reliability. 
  5. Self-monitoring and fault recording - Numerical relays can monitor their own performance and provide diagnostics and fault records for analysis. This helps to improve system reliability and reduce downtime.
Once the relay has identified the location of the fault, it sends a trip signal to the circuit breaker closest to the fault. The trip signal is transmitted to the circuit breaker over a communication network, such as an Ethernet or serial communication link. The circuit breaker then operates and isolates the fault, thereby protecting the system.

The communication link between the numerical relay and the circuit breaker allows for advanced features such as zone coordination and adaptive protection. Zone coordination involves coordinating the trip settings and time delays between multiple circuit breakers to ensure that only the breaker closest to the fault trips. Adaptive protection involves adjusting the relay settings in response to changes in the power system conditions, such as changes in load or fault location.

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  • Title: Directional protection and directional zone selectivity
  • Source: ABB

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