5. What is System protection?
System protection is the art and science of detecting
problems with power system components and isolating
these components.
6. Need for the protection
The power system must maintain acceptable operation
24 hours a day
Voltage and frequency must stay within certain limits
Protect the public
Improve system stability
Minimize damage to equipment
Protect against overloads
13. Components to be protected
Generators
Transformers
Lines
Buses
14. GENERATOR
PROTECTION Failure of Prime-mover
Failure of Field
Over-current
Over-speed
Over-voltage
Unbalanced Loading
Stator Winding Faults
15. UNBALANED LOADING
Unbalanced loading arises from fault to earth or faults
between phases on the circuits external to the
alternator.
Unbalanced current may burn the mechanical fixing of
the rotor core or damage the field winding.
16.
17. OPERATION
Under normal operating condition, algebraic sum of
three currents flowing through the relay is zero and
relay does not operate.
When unbalancing occur, resultant current flows
through the relay and relay trips the circuit breaker to
disconnect the alternator from the system.
18. STATOR WINDING
FAULTS
Fault between phase and
ground
Fault between phases
Inter-turn faults involving turns
of same phase winding
19. Differential Protection
It provides protection against phase to phase and
phase to ground
Current at two ends of the protected sections are
compared
Under normal operating conditions, these currents are
equal, hence no current flows through the relay.
When a fault occurs in the protected zone, currents at
two ends of the CT becomes unequal. Differential
current flowing the the relay isolates the protected
section from the system.
20.
21. DIFFICULTY AND REMEDY
In most of the cases, alternator is located at a
considerable distance from the switchgear.
the relays are located close to the circuit breaker,
balancing resistances are inserted in shorter length of
pilot wire to balance the impedance on both side of the
relay.
22.
23. Balanced earth fault protection
Balanced earth fault protection is used for those
alternators in which neutral ends of the three phase
are connected internally to a single terminal.
It provides no protection against phase to phase fault.
26. What can go wrong?
● Winding-to-winding faults
● Winding-to-ground faults
● Bushing faults
Protection Methods:
Fuse
Differential
Buchholz relay
27. Fuses
Fuses commonly protect small distribution
transformers typically up to ratings of 1MVA at
distribution voltages. In many cases no circuit breaker
is provided, making fuse protection the only available
means of automatic isolation.
The fuse must have a rating well above the maximum
transformer load current to withstand the short duration
overloads that may occur. Also, the fuses must
withstand the magnetising inrush currents drawn when
power transformers are energized.
28.
29. Differential protection
Current transformers on the primary and secondary
sides are connected to form a circulating current
system.
30. BUCHHOLZRELAY
It is a gas actuated relay. When a fault develops
slowly, it produces heat, thereby decomposing solid or
liquid insulating material in the transformer.
The decomposition of the insulating material produces
inflammable gases. The Buchholz relay gives an alarm
when a specified amount of gas is formed.
The analysis of the gas collected in the relay chamber
indicates the type of the incipient fault.
31.
32. WORKING
• When an incipient fault such as a winding-to-core fault or an inter-turn fault
occurs on the transformer winding, there is severe heating of the oil.
• This causes gases to be liberated from the oil around 350°C. There is a
build-up of oil pressure causing oil to rush into the conservator.
• A vane is placed in the path of surge of oil between the transformer and the
conservator. A set of contacts, operated by this vane, is used as trip
contacts of the Buchholz relay. This output of Buchholz relay may be used to
trip the transformer.
