IDMT Relay Working, types, and applications
An IDMT (Inversely Definite Minimum Time) relay is a type of protective relay that is commonly used in electrical systems to protect circuits from faults such as overloads or short circuits. Its operation is based on the inverse time principle, which means that the response time of the relay is inversely proportional to the magnitude of the fault current. In simple terms, the higher the fault current, the faster the relay will trip. IDMT relays are an important component in protecting electrical equipment and ensuring the stability and safety of power systems.
Working Principle of IDMT Relay
IDMT relays work on the principle that the tripping time of the relay decreases as the fault current increases. This is in contrast to traditional time-based protection, where a fixed tripping time is set for any given fault. IDMT relays use an inverse time characteristic, meaning that for small fault currents, the relay will take longer to operate, while for higher fault currents, it will trip very quickly.
The characteristic curve of a relay is defined by two basic elements:
Defined Minimum Time (DMT): This is the minimum time that the relay takes to operate under any fault condition, even if the fault current is very low. This ensures that the relay will not take longer than necessary to trip for higher fault currents.
Inverse Time Characteristic: The time to trip of the relay is inversely related to the magnitude of the fault current. As the fault current increases, the operating time of the relay decreases, allowing the faulty part of the system to be disconnected more quickly.
Types of IDMT Relay
IDMT relays come in different types, which differ based on their characteristic curves. The most common curves are:
Standard inverse (SI):
In the realm of electrical protection systems, relays play a vital role in protecting electrical circuits and equipment from faults, ensuring system reliability, and preventing damage. Among the various types of protective relays, the inverse definite minimum time (IDMT) relay is one of the most widely used relays. Within the IDMT category, the standard inverse type is one of the most common forms. Understanding its operation, characteristics, and applications can help engineers design more effective and reliable protection schemes.
1-Introduction to IDMT Relay
An inverse definite minimum time (IDMT) relay is a time-overcurrent relay designed to operate based on the magnitude of the fault current. The primary function of an IDMT relay is to disconnect the faulty part of the electrical system by opening the circuit breaker after a predetermined time delay. The characteristic of an IDMT relay is that the time delay decreases as the fault current increases. This ensures that the relay operates quickly in the event of high fault current and slowly for lower fault currents, offering both flexibility and protection.
The standard inverse type IDMT relay is one of the most widely used types in the IDMT category. It provides a time-current characteristic that is specifically designed to operate with a time delay that is inversely proportional to the fault current.
2-Time-current characteristics of standard inverse IDMT relays
The characteristic of a standard inverse type relay is the time-current curve that shows how the operating time varies with the fault current. The curve is designed in such a way that, following the inverse relationship, the operating time decreases as the fault current increases.
The time-current characteristic curve generally follows this pattern:
At low fault currents, the time is longer, and the relay will take longer to trip. This avoids tripping for minor fault currents that may not cause potential damage.
At high fault currents, the time becomes very short, and the relay trips quickly to avoid excessive damage or equipment failure.
The curve is usually divided into several zones according to different fault levels, each zone providing the appropriate operating time for different fault conditions.
Key zones in the time-current curve:
Minimum time zone: This represents the region where the fault current is significantly higher than the pick-up current. The relay operates almost immediately, reducing the effects of high fault currents.
Inverse time zone: This is the region where the fault current is close to the pick-up value. The operating time is inversely proportional to the fault current and increases smoothly as the fault current approaches the pick-up current.
Defined minimum time zone: At the lower end of the characteristic curve, the relay maintains a minimum operating time that is independent of the fault current. This is particularly useful for ensuring reliable operation for small faults or disturbances.
3-Types of IDMT curves and their applications
IDMT relays, including the standard inverse type, can have different time-current characteristic curves depending on the application requirements. These curves are generally classified as follows:
Standard Inverse Curve (SI): As mentioned above, the standard inverse curve follows a typical inverse characteristic where the operating time decreases as the fault current increases. It is widely used for general protection of feeders and circuits.
Very Inverse Curve (VI): In this curve, the relay trips more quickly for high fault currents than in the standard inverse curve, making it suitable for fast protection of equipment in critical areas.
Extremely Inverse Curve (EI): This curve is even more sensitive to high fault currents, which is ideal for applications that require immediate protection in the event of high faults, such as transformers and large motors.
4-Standard Inverse IDMT Relay Settings
To ensure proper operation of a standard inverse IDMT relay, several settings need to be configured based on the system requirements and protection scheme:
Pickup Current (I_r): The current level at which the relay is set to start operating. It is usually selected based on the normal operating current of the system or the protection of the equipment.
Time Multiplier Setting (TMS): This is a factor that adjusts the time delay. It allows the user to fine-tune the time characteristics to suit the protection requirements of the system.
5-Advantages of Standard Inverse IDMT Relay
Versatility: Standard Inverse IDMT relay can be adjusted to different fault conditions, making it a flexible solution for protecting various electrical systems.
Coordination: Inverse time delay ensures that the relay can effectively coordinate with other protective devices in the system, providing selective tripping and reducing unnecessary power outages.
Reliability: The time-current characteristic ensures that the relay responds quickly to high fault currents, reducing the possibility of significant damage to electrical equipment.
Cost-effective: Due to its wide application and simple design, the standard Inverse IDMT relay is cost-effective and easy to maintain.
6-Applications of Standard Inverse IDMT Relays
The standard inverse IDMT relay is commonly used in various protection applications, including:
Feeder Protection: The relay is often used for feeder protection in power distribution networks. This ensures that faults are quickly isolated, minimizing unnecessary interruptions.
Transformer Protection: For transformers, where fault currents can vary greatly, the standard inverse IDMT relay provides the time delay necessary for integrated protection.
Motor Protection: Motors can experience inrush currents during start-up, which the IDMT relay helps to manage by ensuring that only a permanent fault causes the relay to trip.
Very inverse (VI):
In electrical protection systems, the importance of selecting the right relay type cannot be overstated. One of the most common types of relays used in electrical protection is the inverse definite minimum time (IDMT) relay. It is primarily used in overcurrent protection, where its primary purpose is to protect electrical circuits from damage caused by overcurrent conditions. Among the various types of IDMT relays, the very inverse type IDMT relay holds a prominent position because of its sensitivity to high short-circuit currents and its ability to provide instant disconnection in such cases.
Very Inverse IDMT Relay Features
The Very Inverse IDMT relay is designed with a step inverse characteristic, meaning that for large fault currents, the relay will operate almost immediately. It has the following key features:
Time-Current Current: The time-current characteristic curve of the Very Inverse IDMT relay is steeper than that of the standard inverse type, meaning that the relay will trip more quickly for large fault currents.
Sensitivity to High Fault Current: The Very Inverse relay operates very quickly when the fault current is well above the set limit. This helps to provide protection against large fault currents, such as those occurring in short-circuit conditions.
Time Delay for Small Faults: For small fault currents (which are close to the pick-up value), the relay has a longer operating time, allowing the system to withstand minor transient overcurrent conditions without unnecessary tripping.
Fixed minimum time: Even at very high fault levels, the relay will not trip immediately. Instead, it ensures a minimum time before tripping, which prevents unnecessary operation during non-critical faults or sudden but short increases in current.
Working principle of a very inverse IDMT relay
The very inverse type IDMT relay operates on the basis of the inverse time characteristic. It incorporates a time current that is designed to provide protection that is fast for severe faults and sufficiently delayed for minor overload conditions.
When a fault occurs, and the current increases above the set pick-up value, the relay starts monitoring the time for which the overcurrent is sustained.
For large fault currents, the time delay is significantly reduced. The greater the magnitude of the fault current, the faster the relay will trip, ensuring rapid disconnection of the faulty circuit. This is very important in scenarios where large fault currents can cause extensive damage if not stopped quickly.
For small overloads or faults, the relay operates with a longer time delay. This allows the system to handle minor fluctuations or overloads without unnecessarily disrupting service.
The constant K used in the time-current formula determines how steep the curve is. In the case of a very inverse relay, this constant is chosen to ensure that the relay operates quickly for large faults but with an appropriate time delay for small ones.
Applications of Very Inverse Type IDMT Relays
Very Inverse Type IDMT Relays are used in various electrical protection applications, especially in cases where the protection needs to be sensitive to large fault currents and react quickly. Some common applications include:
Feeder Protection: In a distribution system, feeders carry power to different parts of the grid. If a short circuit occurs, a very inverse IDMT relay provides rapid protection to isolate the faulty section, preventing widespread damage.
Motor Protection: Motors, especially large industrial ones, can be prone to overloads or faults. A very inverse IDMT relay can be used to protect motors from excessive current while allowing some tolerance during normal operations.
Transformer Protection: Transformers are expensive and important devices. Very inverse IDMT relays are often used to protect transformers from short circuits and overloads, ensuring that they are disconnected from the circuit before any significant damage occurs.
Protection in complex systems: In power systems where high fault currents are common, such as in substations or industrial plants, a very inverse IDMT relay ensures that the protection system responds effectively and quickly.
Advantages of Very Inverse Type IDMT Relays
The very inverse type IDMT relay offers several different advantages:
Fast response to severe faults: The very inverse characteristic ensures that the relay responds quickly to large fault currents, providing immediate isolation of the faulty section.
Selective protection: The time-current characteristics of the relay can be adjusted to the specific system requirements, providing selectivity in protection and ensuring that only the faulty section of the system is disconnected.
Prevents damage: By quickly disconnecting the circuit during short circuit conditions, the fully inverse IDMT relay helps prevent damage to equipment and reduces the risk of fire or explosion.
Improved system stability: The relay provides fast fault clearance, which can improve overall system stability by preventing faults from affecting other parts of the system for a long time.
Reliability: The relay is designed to be used in critical protection schemes, ensuring reliable performance in the event of a fault.
Limitations of the fully inverse IDMT relay
Despite being very effective in many scenarios, the fully inverse type IDMT relay has some limitations:
Not suitable for low fault current applications: The fully inverse type relay may not be ideal for situations where detection of faults with low fault currents is required, as the fast delay in such conditions can result in poor sensitivity.
Challenges identified: The performance of a relay is highly dependent on its settings, which require careful calibration. Incorrect settings can lead to either unnecessary trips or failure to trip during faults.
Coordination complexity: In a network with multiple protection devices, coordinating the operation of a highly inverted relay with other protection devices can be difficult and requires careful planning.
Extreme inverse (EI):
The Extremely Inverse (EI) type IDMT relay is a special type of overcurrent relay where the time delay for tripping decreases rapidly as the fault current increases. Unlike the normal inverse type, which has a moderate inverse characteristic, the extremely inverse IDMT relay responds even more rapidly to large fault currents, providing a high level of protection for systems where immediate isolation of faults is essential.
The relay uses an extremely fast inverse time characteristic curve. The important aspect of this type of relay is that, under high fault conditions, the operating time of the relay is minimized, ensuring that the protection system responds almost instantly to severe faults. Conversely, for smaller faults, the relay provides a longer time delay, which helps to prevent unnecessary tripping for temporary or minor overload conditions.
Applications of extreme inversion type IDMT relay
The extreme inversion IDMT relay is mainly used in applications where fast protection against large fault currents is required. Some common applications include:
1-Transformers
Transformers are important components in the electrical power system, and their protection requires fast response to high fault currents to avoid damage. The extreme inversion IDMT relay can provide quick isolation of faults, preventing damage to transformer windings and insulation from short circuits.
2-Busbars
Busbars are essential for the distribution of electrical power in substations and power plants. In the event of a fault, the extreme inversion relay ensures rapid disconnection of the faulty parts, preventing damage to other components of the system.
3-Motors and Large Industrial Equipment
In industrial settings, motors and large machinery can experience faults, and the highly inverted IDMT relay provides immediate protection against severe overcurrent conditions, minimizing downtime and preventing motor burnout.
4-Feeder Protection
For feeders in electrical distribution networks, highly inverted relays are ideal for protecting feeder cables from faults. The fast response ensures that fault conditions are cleared before significant damage occurs to the system.
5-Short Circuit Protection
In systems with high short circuit currents, the highly inverted IDMT relay provides a fast and reliable means of protection, ensuring that short circuits are isolated before they can cause widespread damage.
Advantages of the extreme inversion type IDMT relay
The extreme inversion IDMT relay offers several advantages, especially in high-speed fault isolation applications. These advantages include:
1-Instant fault isolation
Due to its extremely fast inversion time characteristic, the EI relay is ideal for quickly isolating faults, which helps protect sensitive equipment and prevent further damage.
2-Selective coordination
The EI relay provides selective coordination with other relays in the network. This means that it can isolate a faulty section of the network without affecting the entire system, ensuring that the rest of the system continues to operate normally.
3-Improved protection for high fault conditions
In the event of high fault currents, the extreme inversion relay operates more quickly than other types of relays, ensuring that the equipment is protected from catastrophic failure.
4-Reduced risk of damage
Since the relay reacts quickly to high fault conditions, there is less risk of permanent damage to electrical equipment such as transformers, motors and busbars.
5-Prevents cables from overheating.
In the event of severe faults, where delayed tripping can result in cables overheating, the EI relay reduces the chances of damage by ensuring a fast response to overcurrent events.
Disadvantages of Extremely Inverse Type IDMT Relays
Despite its many advantages, Extremely Inverse IDMT relays have some disadvantages, including:
1-Increased sensitivity to minor faults
The EI relay can trip on minor faults or transients that do not pose a significant risk to the equipment. This can lead to nuisance tripping, although this can be mitigated with appropriate settings.
2-Complex setting configuration
The extremely steep characteristic of the relay requires precise configuration of settings such as time multiplier and current settings to ensure proper operation. This can be difficult for engineers who are not familiar with the relay’s features.
3-High cost
Due to its sophisticated design and functionality, the Extremely Inverse IDMT relay can be more expensive than other types of protection relays.
Long-time inverse (LTI):
Long Time Inverse (LTI) type IDMT relay is a subclass of IDMT relay designed to respond to overcurrent conditions that develop over a long period of time. Compared to other types of inverse time characteristics (such as standard inverse or very inverse), the long time inverse characteristic has a slower response to low-level faults but provides extended protection to avoid tripping during transient conditions or minor overloads.
The key feature of a long-time inverse relay is its operational time curve. The curve starts slowly and becomes steeper as the fault current increases, providing a delay to avoid unnecessary tripping during overloads. This delay ensures that only sustained faults, which pose a significant threat to the system, are detected and cleared by the relay.
How does a long time inverse IDMT relay work?
The operation of a long-time inversion relay can be described in a few key steps:
Fault detection: The relay continuously monitors the current flowing through the protected circuit. If the current exceeds a set limit, the relay starts the fault condition timing. The current setting can be adjusted to determine when the relay timing will start.
Inverse time delay: Upon detection of an overcurrent condition, the relay’s internal mechanism begins to operate based on an inverse time delay. The operating time of the relay is inversely proportional to the magnitude of the fault current. Therefore, the higher the fault current, the faster the relay will trip. However, for lower fault currents, the relay provides a much longer time delay.
Time-current characteristic: The long-term inversion type is characterized by a relatively slow response to moderate overcurrent’s. This means that when the fault current is just above the threshold, the relay will take longer to trip (low-level fault). However, if the fault current increases, following the inverse time characteristic, the trip time decreases rapidly.
The curve usually looks like a long S-shape, where the initial part of the curve is flat, indicating a long time delay for small faults. As the fault current increases, the curve steepens, indicating a fast trip time for large faults.
Relay Trip: Once the time delay reaches the predetermined trip time, the relay will issue a trip signal to the circuit breaker, which will stop the current flow in the faulty section of the system. This helps to protect the system from damage caused by prolonged overcurrent conditions.
Time-Current Characteristics of LTI IDMT Relays
The time-current characteristic of a long-time inverse relay follows a minimum-time (DMT) principle, with the inverse behavior being more pronounced at higher fault currents. Here is a more detailed look at how these time-current curves are designed:
Minimum operating time: The LTI relay will only trip when the current exceeds a certain threshold, often called the pickup current. For faults at or near this pickup value, the operating time is relatively long to avoid unnecessary trips. The minimum operating time is a constant value that ensures that the relay provides protection only after a sustained overcurrent condition is detected.
Long-time protection: LTI relays are particularly effective for applications where faults do not occur instantaneously and develop over time, such as in long-distance transmission lines or large transformers. These faults may include high fault resistance or may be intermittent, requiring the relay to delay its operation to ensure that tripping is only initiated if the fault persists for an acceptable time.
Applications of Long Time Inverse IDMT Relays
The Long Time Inverse (LTI) type IDMT relay is primarily used in situations where a delay is necessary before a fault can be cleared, preventing unnecessary interruptions. Some common applications include:
Transmission lines: Long distance transmission lines, due to their physical length, can experience faults that take time to develop. LTI relays help provide protection by ensuring that a fault does not cause a trip unless it has been present long enough that it poses a serious threat.
Large transformers: Transformers with large inrush currents or internal fault characteristics benefit from the long-time delay offered by LTI relays. This ensures that only sustained overcurrent’s, which indicate a true fault, cause the relay to operate.
Industrial and Commercial Systems: In large industrial plants or commercial buildings, circuits often experience transient overloads due to fluctuating loads. LTI relays help prevent unnecessary trips due to short-term overloads while protecting the system from long-term faults.
Motor Protection: Motors can sometimes experience overloads due to sudden mechanical stress. LTI relays allow time for the overload condition to clear itself, avoiding unnecessary trips that could damage the motor or cause operational disruption.
Features of IDMT Relay
The main features of IDMT relays include:
Inverse time current characteristics:
The main feature of IDMT relays is its inverse time characteristic. When the fault current increases, the trip time decreases. This helps to avoid unnecessary delay in disconnecting the faulty part of the system.
Minimum trip time:
Even for small fault currents, the IDMT relay will not take more than a specified minimum time to trip. This ensures that even in the event of a fault with a low current intensity, protection is provided within a reasonable time.
Time current curve settings:
IDMT relays generally allow operators to adjust the time current according to the requirements of the system. These adjustments are very important for fine-tuning the protection of the system, especially when dealing with a range of fault conditions.
Selectivity:
One of the advantages of IDMT relays is their ability to offer selectivity. By adjusting the settings of multiple IDMT relays in a system, operators can ensure that only the relays closest to the fault trips, without affecting the rest of the system.
Wide range of application:
IDMT relays are versatile and can be applied in a variety of systems, from low-voltage distribution systems to high-voltage transmission lines, making them important in modern electrical grids.
Applications of IDMT relay
IDMT relays are used in many applications in various sectors:
Overload protection:
IDMT relays are often used to protect circuits from overload conditions. By adjusting the time settings, they can ensure that equipment is not damaged by prolonged overloads.
Motor protection:
Motors, especially large industrial motors, are often protected using IDMT relays. These relays can disconnect the motor from the supply in the event of an overload, short circuit, or other fault.
Feeder Protection:
IDMT relays are commonly used in feeders to protect the power distribution network. They are designed to operate based on fault magnitude, allowing for effective isolation of the faulted sections while keeping the rest of the system operational.
Transformer Protection:
Transformers are another important part of power system that are protected using IDMT relays. The relays help prevent damage to transformers in the event of faults, overloads, or short circuits, ensuring grid stability.
Feeder and Line Protection:
In long feeder lines, it is essential to use IDMT relays for fault detection and isolation. The relay will respond more quickly to faults occurring closer to the source and will protect the power system from damage.
Advantages of IDMT Relay
IDMT relays offer several advantages:
Fast response to high fault currents: Due to their inverse time characteristics, they can respond to high fault currents at high speed, minimizing damage to electrical equipment.
Flexibility: Adjustable settings allow fine-tuning of the relay to the specific safety requirements of the system.
Selectivity: They offer excellent coordination with other protective devices, ensuring that only the faulty part is isolated, and the rest of the system remains operational.
Low risk of damage: The minimum time ensures that even for low fault currents, the system will be protected within a predetermined period, preventing damage to equipment.
Challenges and Considerations
Although IDMT relays are widely used, there are some challenges and considerations when applying them:
Coordination: Proper coordination of time settings across multiple relays is essential to ensure selectivity and avoid unnecessary tripping of upstream or downstream relays.
Maintenance and Testing: Regular testing and maintenance of relays is essential to ensure their correct operation. This includes checking the time settings and resetting the relay if necessary.
Response Time: Although relays are designed to provide fast response, for systems with significant loads, additional protection methods may be required to further reduce the response time.
Frequently Asked Questions
1-What does IDMT stand for in IDMT relay?
Answer:
The abbreviation IDMT refers to Inverse Definite Minimum Time. This pertains to the relay characteristic in which the operating time diminishes with the rise of fault current, yet there is a specific minimum time.
2-What is the principle of operation of an IDMT relay?
Answer:
It works on the basis that the fault current’s magnitude and the speed of tripping are directly related, up to a specified limit. The time varies inversely with the magnitude of the current.
3-Where is the IDMT relay used?
Answer:
IDMT relays are widely utilized for overcurrent protection in distribution networks, transformers, and feeders.
4-What are the types of IDMT relays?
Answer:
Common types include:
1-Standard Inverse
2-Very Inverse
3-Extremely Inverse
4-Long-time Inverse
5-How does the IDMT characteristic improve system protection?
Answer:
It enables selective coordination and grading with upstream/downstream relays, guaranteeing minimal disruption and improved fault isolation.
6-What is the “pickup current” in IDMT relay?
Answer:
This is the lowest current at which the relay begins to function. The relay will not trip if the value is below this threshold.
7-What is meant by ‘definite minimum time’?
Answer:
It’s the shortest time the relay will take to trip, even at extremely high current levels.
8-Can IDMT relays be used for both phase and earth faults?
Answer:
Yes, separate IDMT elements can be utilized for phase and earth fault protections.
9-What is meant by ‘Time Multiplier Setting (TMS)’?
Answer:
TMS modifies the relay’s operating time. Lower TMS allows for quicker operation.
10-What is the use of Current Setting (CS) in IDMT relays?
Answer:
The level of the pickup current is determined by CS. It is usually established as a percentage of the CT secondary current (for example, 50% or 100%).
