Autoreclosing in Transmission and Distribution Systems

Introduction

Two main functions of Utility Electric System Operations department are Control and Protection.

Control function deals with all operational aspects involved in efficient and optimal dispatching of available real power (with reactive power support) from the Generating units (and reactive sources) to geographically distributed consumers through available system of Transmission and distribution (T&D) network (s).

Some of these functions are performed automatically and some manually by system operators. Various monitoring and alarming systems help perform these control functions.

Protection function deals with automatic detection of faults or abnormal/unsafe system operation conditions and automatically disconnects involved circuits or equipment. Thus protective system helps in minimizing the damage to circuit components and equipment. Circuit components and equipment are thus available for reuse in most cases.

Automatic Reconnection

Once a circuit component or equipment is automatically disconnected by protective functions, its reconnection is mostly a control function. Reconnection of a circuit component or equipment (after its trip due to protective device action) is performed either manually or automatically.

It is often essential to make sure that reconnecting it will not thermally degrade the life of major equipment or damage it beyond acceptable level or results in major financial burden.

Protection action for equipment like Generators, Transformers, Reactors, Capacitor banks, Filter banks, Power cables etc. are always results in ‘Locked out’ (meaning are blocked from closing both manually or automatically). This is because it is always a permanent fault needing some kind of maintenance/repair.

Every utility will have well decided operation policy in regard to reconnection of equipment or circuit component after a protection trip. Before reconnecting a major equipment considerable testing and maintenance procedures are followed (requires some equipment down time).

Overhead (OH) Transmission and distribution circuits are the only circuits which are often reconnected automatically without any testing or maintenance performed after its trip due to protection action. This is because most often the protection action is due to a fault which is transient in nature and the fault vanishes once it is de-energized.

Depending on the magnitude of fault current and strength of electric potential (voltage level), some ionization occurs in the surrounding dielectric media (air) which neutralizes with time and regains required dielectric strength making it available for successful reconnection. Thus Automatic reconnection called Auto-reclosing, can be performed for OH Transmission and Distribution circuits.

From Auto-reclosing perspective, it becomes very important therefore to understand types of faults which can occur in OH T&D circuits and amount of risk involved in auto-reclosing and its advantages.

I guessed your answer… if you are a Protection Engineer, you will surely say it is a Control function and… he he, if you are a Control system Engineer (operation person) you will surely say it is a Protection function!!!!

Because it is performing a closing operation not a trip action, a Protection Engineer says it is a control function. Control Engineer says otherwise because it is performed by a protective relay or device following a definitive and sequential action decided (major part of sequence and conditions for reclosing action) by Protection Engineers.

Resource: Autoreclosing in Transmission and Distribution Systems by mr. S.R. Javed Ahmed