introductoin to p.s.p

The objectives of power system protection are to:

Limit the extent and duration of service interruption
thought the protection zone

Minimize damage to the system components

Fault sequence of events

1. Fault occurs somewhere on the system, changing the system
currents and voltages parameters.

2. Current transformers (CTs) and potential transformers (PTs)
sensors detect the change in currents/voltages

3. Relays use sensor input to determine whether a fault has
occurred

4.If fault occurs relays open circuit breakers to isolated the fault .

Protection systems must be designed with :
 primary protection and
 Backup protection

In designing power system protection there are two
main types of systems that need to be considered:

1. Radial: The protection systems usually use inversetime overcurrent relays

2. Ring : power can flow in both direction ,in this case
directional overcurrent relay is used.

Protection against faults and abnormalities

a) Faults : The principal electrical system faults are:
short circuits and overloads.

Short circuits: failure of insulation , mechanical
damage to electrical distribution equipment, failure of
equipment as a result of overloading or other. be solid or has relatively low
impedance

Typical Short-Circuit Type Distribution
Single-Phase-Ground: 70 - 80 %
Phase-Phase-Ground: 17 - 10 %

Phase-Phase: 10 - 8 %

Three-Phase: 3 - 2 %

Causes of Short-Circuit Faults:
• Insulation breakdown, Birds and animals ,diggers for
underground cables, poles collapsing, conductors

breaking, vehicle impact ,wind effect ,incorrect

operation by personnel..and so

Effects of Short-Circuit Type Faults

1) Large current can be involved. This cause
equipment and generators would be damaged. Only be allowed to flow for a very short time as 10ms
up to say 3 seconds .

2) Arcs, sparking and the heating effect can start fires

3) large mechanical forces which have potential to
break or damage equipment.

4) Electric current leakage flow that could create a
hazard to people

Subtopic

b) Abnormalities

Under and over frequency

Power swings.

Overload and over temperature

Overvoltage or under voltage.

Under excitation of synchronous machines

Over fluxing of power transformers

Asynchronous operation of synchronous machines

Mechanical defects i.e. leaking oil, tap changer
mechanism faults etc

Planning system protection minimize the effects of faults and abnormalities
occurring on the system

a) Quickly isolate faulted zone

b) Minimize the magnitude of short-circuit current and,
minimize potential damage to the system.

c) Provide alternate circuits, automatic transfers, or automatic
reclosing devices, in order to minimize the duration
outages.

Performance Requirements of Protection System

• Discriminate between normal and abnormal
conditions.

• No unprotected zones "blind spots

• Selectivity – by coordinate with other protection
systems.

• Fast enough to prevent damage and hazards - but not
too fast.

• High degree of reliability and availability

• Secure against incorrect operation

• Should be easy to operate.

What Info is Required to Apply Protection

One-line diagram ,Impedances and connections of power
system , system frequency, voltage level and phase
sequence, existing schemes ,maximum allowed clearance

times ,system fault studies, maximum load and system

swing limits ,CTs and VTs locations, connections and ratios,

future expansion and any special considerations for

application

Basic Protection Scheme Components