The essentials of directional protection and selectivity in modern networks

Detecting and disconnecting

Protection equipment has the basic role of detecting an electrical fault and disconnecting that part of the network in which the fault occurs limiting the size of the disconnected section as far as possible.

In modern medium-voltage (MV) distribution lines and in almost all high voltage transmission lines, a fault can be in two different directions from a relay and it is highly desirable for a relay to respond differently for faults in the forward or reverse direction.

In fact, in almost all situations the relay should respond only when the fault is on one side, while for failures on the other side it remains inactive.

Directional protection enables better discrimination of the faulty part of the network than with overcurrent protection.

It is necessary to use it in the following conditions:

1. in a system with several sources
2. in closed loop or parallel-cables systems
3. in isolated neutral systems for the feedback of capacitive current
4. and to detect an abnormal direction of flow of active or reactive power (generators)

Directional protection is used for all network components in which the direction of flow of power could change, for example for the short circuit between phases or for an earthing fault (single phase fault):

• Phase directional protection is installed to protect two connections operated in parallel, a loop or a network component connected to two power sources
• Earth fault directional protection is sensitive to the direction of flow of the current to earth. It is necessary to install this type of protection equipment whenever the phase to earth fault current is divided between several earthing systems.
• Active and reactive directional power protection equipment is used to detect abnormal power flow other than the one due to a short circuit.
  
  For example, in the event of the failure of the prime mover, a generator will continue to run as a synchronous motor, drawing power form the system.

The IEEE device number used to signify a directional element is 67-directional overcurrent, generally based on the phase relationship of V (voltage) and I (current), with no distance to fault capability.

In the following pages of this seminar paper, all above mentioned usages of directional protection are described in more details and while describing the usages, the operating principle of relay will be explained as well.