What is Interlocking in substations, and what is it for?
electrical-engineering-portal.com/what-is-interlocking-in-substations
December 16, 2024
By Muhammad Kashif Published on December 16th, 2024
Interlocking in substations
Interlocking in substations is a critical aspect of power system protection and control,
designed to ensure the safe and efficient operation of substation equipment. The primary
objective of interlocking is to prevent hazardous situations, such as energizing an earthed
circuit or operating isolators under load.
What is Interlocking in substations, and what is it for?
By enforcing logical and mechanical constraints, interlocking schemes minimize human
error and reduce the risk of system failures.
As substations evolve into smarter grids, modern systems increasingly employ software-
based interlocking, integrated with SCADA (Supervisory Control and Data Acquisition)
and RTU (Remote Terminal Unit) platforms. Additionally, the use of IEC 61850-based
interlocking with GOOSE (Generic Object-Oriented Substation Event) messaging has
enhanced system reliability, speed, and interoperability.
Interlocking schemes are categorized into electrical, mechanical, and software-based
logical interlocking. Electrical interlocking uses auxiliary contacts and relays,
mechanical interlocking employs physical locks, keys, and cam-operated devices, while
logical interlocking is implemented using PLCs (Programmable Logic Controllers) or
SCADA logic.
Each type has its specific role in ensuring operational safety and system reliability.
1/18
,This article provides a comprehensive overview of interlocking in substations,
covering its definition, purpose, and key equipment involved, such as isolators, circuit
breakers, and earth switches. It explains the various types of interlocking and their
practical applications in line bays and busbar systems.
The role of GOOSE messaging in IEC 61850-based interlocking is also discussed,
illustrating how modern substations achieve faster, more flexible, and more efficient
protection and control.
By the end of this article, readers will gain valuable insights into how interlocking
enhances safety, system reliability, and operational efficiency in substations.
Table of Contents:
1. What is Interlocking in Substations?
Interlocking is a critical safety mechanism implemented in substations to ensure the safe
operation of various control devices. To fully understand the concept of interlocking, it is
essential to first understand the functions, operational requirements, and characteristics
of key substation equipment. Interlocking ensures that substation equipment is operated
in a specific, predefined sequence.
Failure to follow this sequence can result in severe equipment damage, faults, or
hazardous conditions. Therefore, network engineers must be familiar with the sequence
of operation for various control devices.
Figure 1 – Substation Components Depicted in SLD and Elevation Plan for a Double Bus
Single Breaker Scheme Types of Interlocking
Figure 1 – Substation Components Depicted in SLD and
Elevation Plan for a Double Bus Single Breaker Scheme Types
of Interlocking
2/18
, Go back to Content Table ↑
1.1 Key Substation Equipment Involved in Interlocking
1.1.1 Isolator (Disconnector)
Definition: An isolator (or disconnector) is an off-load device, meaning it does not have
the capability to break load current or fault current. Its primary function is to provide a
visible physical break in the circuit for maintenance purposes.
Operation: Before operating an isolator, it must be ensured that no current is flowing
through the circuit.
Attempting to operate an isolator under load conditions will result in heavy arcing,
severe heating, and extremely high arc temperatures. This can lead to damage to the
isolator’s contacts and may even prevent the circuit from being successfully broken.
Interlocking Requirement: To avoid the risk of operating the isolator under load,
interlocking ensures that the isolator can only be operated after the circuit breaker has
opened and interrupted the current flow.
Figure 2 – High voltage disconnector
Figure 2 – High voltage disconnector
Go back to Content Table ↑
3/18
electrical-engineering-portal.com/what-is-interlocking-in-substations
December 16, 2024
By Muhammad Kashif Published on December 16th, 2024
Interlocking in substations
Interlocking in substations is a critical aspect of power system protection and control,
designed to ensure the safe and efficient operation of substation equipment. The primary
objective of interlocking is to prevent hazardous situations, such as energizing an earthed
circuit or operating isolators under load.
What is Interlocking in substations, and what is it for?
By enforcing logical and mechanical constraints, interlocking schemes minimize human
error and reduce the risk of system failures.
As substations evolve into smarter grids, modern systems increasingly employ software-
based interlocking, integrated with SCADA (Supervisory Control and Data Acquisition)
and RTU (Remote Terminal Unit) platforms. Additionally, the use of IEC 61850-based
interlocking with GOOSE (Generic Object-Oriented Substation Event) messaging has
enhanced system reliability, speed, and interoperability.
Interlocking schemes are categorized into electrical, mechanical, and software-based
logical interlocking. Electrical interlocking uses auxiliary contacts and relays,
mechanical interlocking employs physical locks, keys, and cam-operated devices, while
logical interlocking is implemented using PLCs (Programmable Logic Controllers) or
SCADA logic.
Each type has its specific role in ensuring operational safety and system reliability.
1/18
,This article provides a comprehensive overview of interlocking in substations,
covering its definition, purpose, and key equipment involved, such as isolators, circuit
breakers, and earth switches. It explains the various types of interlocking and their
practical applications in line bays and busbar systems.
The role of GOOSE messaging in IEC 61850-based interlocking is also discussed,
illustrating how modern substations achieve faster, more flexible, and more efficient
protection and control.
By the end of this article, readers will gain valuable insights into how interlocking
enhances safety, system reliability, and operational efficiency in substations.
Table of Contents:
1. What is Interlocking in Substations?
Interlocking is a critical safety mechanism implemented in substations to ensure the safe
operation of various control devices. To fully understand the concept of interlocking, it is
essential to first understand the functions, operational requirements, and characteristics
of key substation equipment. Interlocking ensures that substation equipment is operated
in a specific, predefined sequence.
Failure to follow this sequence can result in severe equipment damage, faults, or
hazardous conditions. Therefore, network engineers must be familiar with the sequence
of operation for various control devices.
Figure 1 – Substation Components Depicted in SLD and Elevation Plan for a Double Bus
Single Breaker Scheme Types of Interlocking
Figure 1 – Substation Components Depicted in SLD and
Elevation Plan for a Double Bus Single Breaker Scheme Types
of Interlocking
2/18
, Go back to Content Table ↑
1.1 Key Substation Equipment Involved in Interlocking
1.1.1 Isolator (Disconnector)
Definition: An isolator (or disconnector) is an off-load device, meaning it does not have
the capability to break load current or fault current. Its primary function is to provide a
visible physical break in the circuit for maintenance purposes.
Operation: Before operating an isolator, it must be ensured that no current is flowing
through the circuit.
Attempting to operate an isolator under load conditions will result in heavy arcing,
severe heating, and extremely high arc temperatures. This can lead to damage to the
isolator’s contacts and may even prevent the circuit from being successfully broken.
Interlocking Requirement: To avoid the risk of operating the isolator under load,
interlocking ensures that the isolator can only be operated after the circuit breaker has
opened and interrupted the current flow.
Figure 2 – High voltage disconnector
Figure 2 – High voltage disconnector
Go back to Content Table ↑
3/18
