An inverse current relay is a type of protective relay whose operating time is inversely proportional to the magnitude of the fault current flowing through it; the higher the current, the faster the relay trips.
Understanding Inverse Time Relays
Inverse-time relays are crucial components in power system protection. Their primary function is to detect overcurrent conditions (faults) and initiate a tripping action to isolate the faulted section of the system. The key differentiating factor is their inverse time characteristic.
How They Work
Unlike instantaneous relays which trip almost immediately when a pre-set current threshold is exceeded, inverse time relays introduce a deliberate time delay. This delay is inversely related to the current magnitude. This means:
- High Current: A large fault current results in a short operating time.
- Low Current: A smaller overcurrent results in a longer operating time.
Why Use Inverse Time Relays?
The inverse time characteristic provides several key advantages:
- Coordination: Inverse time relays are often used in a coordinated protection scheme. Relays closer to the fault trip faster, isolating only the affected section. Upstream relays (further away from the fault) have longer time delays, providing backup protection if the downstream relays fail to operate.
- Discrimination: This allows for better discrimination between temporary overloads (e.g., motor starting currents) and actual faults. The relay will not trip for short-duration overloads, preventing unnecessary outages.
- Selectivity: Enables selective tripping, minimizing the impact of faults by isolating only the faulted zone.
Relay Settings
Inverse time relays typically have two adjustable settings:
- Pick-up Current (Ipickup): This is the minimum current level at which the relay starts to operate. Below this current, the relay will not trip.
- Time Multiplier Setting (TMS) or Time Dial Setting (TDS): This setting adjusts the overall operating time of the relay. It essentially scales the inverse time curve, making the relay trip faster or slower for a given current level. A lower TMS results in faster tripping times.
Mathematical Representation
The operating time (T) of an inverse time relay can be approximated by the following formula:
T = TMS * (K / ( (I / Ipickup)n - 1))
Where:
- T = Operating time
- TMS = Time Multiplier Setting
- K = Constant (depends on the relay's curve characteristic)
- I = Fault current
- Ipickup = Pick-up current
- n = Constant (determines the inverse time characteristic; e.g., n = 0.02 for standard inverse, n = 1 for very inverse)
Types of Inverse Time Relays
Different inverse time characteristics are available, each with a different response curve. Common types include:
- Standard Inverse: A moderate inverse characteristic.
- Very Inverse: A more pronounced inverse characteristic, tripping much faster for high currents.
- Extremely Inverse: An even more aggressive inverse characteristic.
- Long-Time Inverse: Longer tripping times, suitable for specific applications.
- Definite Minimum Time (DMT): Combines an inverse time characteristic with a minimum operating time limit.
Applications
Inverse current relays are widely used in various power system applications, including:
- Distribution feeders: Protecting distribution lines and equipment from overcurrent faults.
- Motor protection: Protecting motors from overload and short-circuit conditions.
- Transformer protection: Providing overcurrent protection for transformers.
- Generator protection: Protecting generators from faults within the generator or in the connected system.
Example
Imagine a distribution feeder protected by an inverse time relay. If a short circuit occurs near the end of the feeder, a high current will flow through the relay. Due to its inverse characteristic, the relay will quickly trip, isolating the faulted section. However, if a temporary overload occurs (e.g., during motor starting), the current will be lower, and the relay will allow the overload to subside without tripping, avoiding unnecessary service interruption.
