LINE PROTECTION

Power System Protection and switchgear

Minia Univ., Faculty of Engineering Electrical Eng. Department

4 ^th Power & Machines Section

Prof. Dr. Abou-Hashema Mostafa El-Sayed

LINE PROTECTION

LINE PROTECTION

Most faults experienced in a power system occur on the lines connecting

generating sources with usage points. Just as these circuits vary widely

in their characteristics, configurations, lengths, and relative importance,

so do their protection schemes.

LINE PROTECTION

Classification of Electrical Power Line

Alternating current lines are commonly classified by function, which is related to voltage level. While there are no utility-wide standards, typical classifications are as follows:

1- Distribution (2.4 – 34.5 kV): Circuits transmitting power to the final retail outlets.

2- Subtransmission (13.8 – 138 kV): Circuits transmitting power to distribution substations and to bulk retail outlets.

3- Transmission (69 – 765 kV): Circuits transmitting power between major substations or interconnecting systems, and to wholesale outlets. Transmission lines are further divided into:

I- High voltage (HV): 115 – 230 kV

II- Extra high voltage (EHV): 345 – 765 kV

III- Ultra high voltage (UHV): greater than 765 kV.

LINE PROTECTION

Direct-current systems can be classified as follows:

1- Low voltage (24 – 250 V): Auxiliary power in power plants and substations; control circuits and, occasionally, utilization power in some industrial plants.

2- Medium voltage (300 – 600 V): Transportation industry.

3- High voltage (greater than 600 V): Long distance bulk transmission,

submarine, and major system interconnections.

LINE PROTECTION

Techniques for Line Protection

There are seven protective techniques commonly used for isolating faults on power lines:

a- Instantaneous overcurrent b- Time-overcurrent

c- Directional Instantaneous and/or time-overcurrent d- Step time-overcurrent

e- Inverse time-distance f- Zone distance

g- Differential current h- Pilot relaying.

LINE PROTECTION

Selecting a Protection System

Several fundamental factors influence the final choice of the protection applied to a power line:

1- Type of circuit: Cable, overhead, single line, parallel lines, multiterminals, etc.

2- Line function and importance: Effect on service continuity, realistic and practical time requirements to isolate the fault from the rest of the system.

3- Coordination and matching requirements: Compatibility with equipment on the associated lines and systems.

To there three considerations must be added economic factors and the relay

engineer’s preferences based on his or her technical knowledge and experience.