Type of Discrimination in Electrical System Protection


Discrimination is a critical aspect of electrical system protection. It refers to the ability of the protective devices to distinguish between a fault that is localized and one that is extensive. In other words, discrimination allows the protection system to isolate and de-energize only the part of the system that is affected by the fault, while leaving the rest of the system energized. This is essential because it minimizes the disruption to the overall system, reduces the risk of damage to the unaffected components, and ensures the safety of personnel working nearby. 


Discrimination can be achieved through careful selection and coordination of protective devices, such as fuses, circuit breakers, and relays, based on their operating characteristics and time-current curves. 


This is a fundamental principle of electrical system protection that enhances system reliability, continuity, and safety. The protections in a system work together to keep the system safe. They are designed to quickly identify any problems in the system and only turn off the part that is not working properly, while leaving the rest of the system running normally. This way, the system can continue to function without interruption.


There are several means that can be implemented to ensure proper discrimination in electrical system protection: 

  • current discrimination.
  • time discrimination.
  • discrimination by data exchange, referred to as logic discrimination.
  • discrimination by the use of directional protection devices.
  • discrimination by the use of differential protection devices.



Related Article: Fundamentals of Protection Practice: Power System Protection and Relaying Techniques


Current Discrimination

Current discrimination anchored in a principle that the further away a fault is from the power source, the weaker the fault current will be. To protect the system from faults, current-based protection devices are installed at the beginning of each section, set at a level lower than the minimum short-circuit current caused by a fault in that section and higher than the maximum current caused by a fault downstream.


Figure 1. Current Discrimination | Source: Merlin Gerin


To achieve current discrimination, protective devices are rated to trip at different current levels. Devices closer to the fault have a lower current rating than devices further away. For example, the protection device closest to the fault may be set to trip at a lower current level than the next protective device downstream. This way, when a fault occurs, the protective device closest to the fault will operate first, quickly isolating the faulted section. The protective devices further upstream will not trip, allowing the rest of the system to continue functioning.


This way, each device only operates for faults located immediately downstream and is not triggered by faults beyond that point. While it can be challenging to define the settings for two cascading protection devices without a significant decrease in current, this system is useful for sections separated by a transformer since it is straightforward, cost-effective, and quick to operate.


Time Discrimination

Time discrimination consists of setting different time delays for the current-based protection devices distributed throughout the system. The closer the relay is to the source, the longer the time delay. To achieve time discrimination, protective devices are selected and set to operate with a time delay. The time delay is set in such a way that the protective device closest to the fault will operate first, quickly isolating the faulted section. Protective devices further upstream will have a longer time delay, allowing them to operate only if the fault persists beyond the time delay of the device closest to the fault.



Figure 2. Time Discrimination | Source: Merlin Gerin


The settings of protective devices for time discrimination are based on the time-current characteristics of each protective device. For example, a protective device with a lower current rating will typically have a shorter time delay than a device with a higher current rating. This way, the protective device closest to the fault will trip faster than the devices further upstream.


Related Article: What is the Advantage of IDMT in Protective Relaying?


In some cases, it may be challenging to define the settings for two cascading protection devices when there is no notable difference in time delay between two adjacent areas. However, for most sections of a power system, time discrimination can be easily applied, making it a popular and effective protection method.



Logic Selectivity

Logic selectivity is a protection technique used in electrical power systems to ensure that only the faulty part of the system is disconnected during a fault. It involves the use of logic-based relays and circuit breakers to respond selectively to faults at different points in the system based on their location and other system parameters.


To achieve logic selectivity, protective devices are equipped with digital relays that are programmed with specific logic settings. These settings determine the behavior of the relay based on inputs from the power system. The inputs can include system voltage, current, frequency, and other parameters that help the relay determine the location and severity of a fault.


Figure 3. Logic Selectivity | Source: Merlin Gerin
 


When a fault occurs, the digital relays receive input from the system and execute their programmed logic. This logic determines which protective device should operate and how quickly it should operate. For example, the protective device closest to the fault may be set to operate first, followed by devices further upstream with longer time delays. The relays can also communicate with other relays upstream and downstream to ensure coordination and selectivity.


The settings of protective devices for logic selectivity are based on the physical and electrical characteristics of the power system, including cable lengths, transformer sizes, and other system parameters. The relay settings are carefully coordinated to ensure that only the faulty part of the system is disconnected during a fault, minimizing disruption to the rest of the system.


In a radial system, the protections located upstream from the fault point are activated; those downstream are not. The fault point and the circuit breaker to be controlled can therefore be located without any ambiguity.


Each protection activated by a fault sends:

  • a blocking input to the upstream stage (order to increase the upstream relay time delay),
  • a tripping order to the related circuit breaker unless it has already received a blocking input from the downstream stage. Time-delayed tripping is provided for as back-up.


Directional Discrimination

Directional discrimination is achieved using directional relays. These relays are designed to respond only to current that flows in a specific direction. They are typically installed at strategic locations in the power system, such as at the end of a transmission line or at the point where a distribution feeder branches off from the main circuit.

When a fault occurs, the directional relay measures the direction of the fault current and compares it to the direction of the normal current flow. If the fault current flows in the opposite direction to the normal current flow, the directional relay will operate, triggering the protective device closest to the fault to isolate it.

Figure 4. Directional Discrimination | Source: Merlin Gerin


The settings of protective devices for directional discrimination are based on the physical layout of the power system, the type of fault, and the direction of the fault current. Protective devices can be set to respond to different fault currents flowing in different directions, ensuring that only the faulty part of the system is disconnected during a fault.


Overall, protection techniques such as current discrimination, time discrimination, logic selectivity, and directional discrimination are essential for maintaining the stability, reliability, and safety of electrical power systems. These protection techniques allow for the selective isolation of the faulty part of the system during a fault, minimizing the impact on the rest of the system and reducing the risk of damage to equipment and personnel. 

The choice of protection technique depends on the characteristics of the power system, the type of fault, and other factors specific to the application. Implementing an effective protection scheme requires careful coordination and selection of protective devices, along with appropriate settings and programming of relays. 


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  • Title: Protection Guide | page 15-19 (discrimination)
  • Source: Merlin Gerin 

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