Download Islanded

PHILOSOPH OF ISLANDING
• Inter-Connected operation of Modern Power Systems has
increased vulnerability of widespread propagation of any
disturbance.
• It is therefore essential to frame a reliable defense plan
• Faulty portion of the system should be quickly isolated or
“Islanded” to save the rest of the system
• Designing of successful islanding schemes requires
consideration of an enormous number of contingencies.
• Load-Generation balance by UFR load shedding is to be
done prior to islanding
Philosophy of Relay Actuated shedding
• Manual Response time often too large; low frequency actuated
relays are thus essential
• Shedding to be done in stages and quantum of shedding may be
decided assuming loss of generation from the largest unit in
operation
• Severity of Disturbance can sometimes be assessed from the rate
of frequency decline and relays capable of sensing df/dt prove
highly useful in such cases
• Sufficient quantum of load should be shed at a frequency much
higher than the islanding frequency
• Proper calibration of UFRs to be ensured to the extent possible
• Loads identified for the specific islands should not be touched
during normal UFR shedding
Philosophy of Islanding
• Islanding of a sub-system and in particular, a power station, should be
resorted as the last alternative to save the system.
• Load to be included in an island should preferably be radial and neither
fluctuating nor unbalanced in nature
• At least one machine in each island to be kept in free governor operation and
automatic combustion control.
• For selective islanding, sensing of reverse power and building of logic
alongwith frequency, may be a correct criteria
• Relays on priority loads like coal and traction should have a setting of 1 to 1.5
HZ lower than the islanding frequency
• All efforts to be made to maintain reactive power balance in the island since
demand would be fluctuating in most cases, leading to unstable operation of
the generators
• Generator capability limits are considerably modified when operated at a
voltage and a frequency significantly different from the nominal
• To ensure adequate reactive absorption capability, special attention needs to
be paid on G.T. tap position and setting of AVR.
POSSIBLE SYSTEM PROLEMS
UNIT
FAILURE
TRANSFORMER/
LINE FAILURE
REDUCED
NETWORK
REDUNDANCY
LOSS OF
GENERATION
BUS
ISOLATED
LINE OVERLOAD
OR
UNSATISFACTORY
BUS VOLTAGE
INSUFFICIENT
GENERATION
LOSS OF
LOAD
SYSTEM
COLLAPS
E
ISLANDING