What is Feeder Coupling in electrical systems?

In electrical power distribution systems, coupling two feeders is a common practice to enhance reliability, load sharing, and redundancy. By connecting two separate feeders, we create a parallel path for power flow. In this article, we delve into the details of coupling feeders, including its purpose, methods, criteria, and precautions.

Feeder coupling refers to the process of interconnecting two or more feeders (power distribution lines) to form a composite system. The goal is to improve system performance, maintain continuity of supply, and optimize load distribution.

How Is Feeder Coupling Done?

Feeder coupling can be achieved through various methods:

  1. Bus Tie Breaker: A bus tie breaker connects two adjacent busbars (bus sections) in a substation. When one feeder experiences a fault or maintenance, the bus tie breaker transfers the load to the other feeder. This method ensures uninterrupted power supply.
  2. Sectionalizing Switches: These switches allow selective isolation of faulty sections. By opening or closing sectionalizing switches strategically, we can couple or decouple feeders as needed.
  3. Ring Main Units (RMUs): RMUs are compact switchgear units that combine circuit breakers, load disconnectors, and earthing switches. They facilitate feeder coupling by allowing manual or automatic switching between different feeders.

Criteria for Feeder Coupling

When deciding whether to couple feeders, consider the following criteria:

  1. Voltage Levels: Coupling is feasible when the voltage levels of the feeders match. Mismatched voltages can lead to instability and equipment damage.
  2. Phase Matching: Couple feeders with matching phases, both feeds should have a same source or synchronized.
  3. Load Balancing: Coupling should balance the load distribution between feeders. Uneven load sharing may strain one feeder excessively.
  4. Fault Currents: Coupled feeders should have compatible fault current levels. Mismatched fault currents can affect protection coordination.
  5. Cable Sizes: Ensure that the cable sizes of the coupled feeders can handle the combined load.

When to Couple Feeders

Feeder coupling is beneficial in the following scenarios:

  • Redundancy: Coupling provides an alternative path in case of feeder failure.
  • Load Growth: As demand increases, coupling accommodates additional load.
  • Maintenance: During maintenance of one feeder, the other continues to supply power.

What Shouldn’t Couple?

While feeder coupling offers advantages, certain situations warrant caution:

  1. Dissimilar Systems: Avoid coupling feeders with significantly different characteristics (e.g., radial and loop systems).
  2. Non-Synchronized Feeders: Non-synchronized feeders have different frequencies or are not in phase with each other. They shouldn’t be coupled, because:
      • Frequency Mismatch: Non-synchronized feeders cannot share load effectively due to varying frequencies.
      • Voltage Instability: Voltage fluctuations occur when non-synchronized feeders are coupled.
  3. Overloading: Coupling should not overload cables or equipment beyond their ratings.
  4. Protection Coordination: Improper coupling can disrupt protection schemes. Ensure proper coordination of protective devices.

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