28-12-2012, 06:49 PM
Power System Operation, Black Outs & Start Up
1Power System.ppt (Size: 381 KB / Downloads: 61)
Power System Control
Objective of Control:
Voltage and Frequency Control
Net Inter System Exchange Control, in real time
Loose Power Pools
Tight band of frequency
Primary Control (Governor)
Secondary Control (AGC / ALFC)
Tertiary Control
Why constant frequency?
Most types of ac motors run at speeds directly related to frequency
Electric clocks and recorder charts are driven by synchronous motors and their time accuracy will be the integrated frequency error.
Power system performance is highly influenced by frequency deviation
Constant frequency operation results in less risk of collapse.
Characteristics of system elements are dependant on frequency, e.g. Capacitors draw higher current to deliver higher VAR.
Load-Frequency relation
Various types of driven loads are in direct relation to frequency (speed in rps)
Example of effect of frequency on load:
A centrifugal pump with no process side control (like discharge valve, recirculation etc) e.g.. Agricultural pump has these relation
Q = k1 x N; H = k2 x N2;
Power = k1 x k2 x N3
Purely resistive loads e.g. incandescent lamp, resistance heater etc are directly immune to frequency, but are influenced indirectly.
Load-Frequency Mechanism
Momentary surplus / deficit of Generation over Consumption (Load + Losses) will result in increase / decrease of the system speed.
The rate of change of speed (frequency) depends on the quantum of surplus / deficit and inertia of the system.
All rotating machines will absorb / release Kinetic energy, during the period of speed change.
Windage / friction losses will increase / decrease in all machines including Turbines.
All tens of thousands of motors will speed up to meet higher load torque, and draw more power from the system (For instance an agricultural pump Power changes as the 3rd power of speed).
The resultant increase / decrease in the consumption soon equals the surplus generation, at a new higher / lower frequency.
Establishing Start Up Power
Close the Start Up source CB with permission of LDC / Remote S/S
Isolators may have to be closed manually, if DG power is not extended to Switch Yard ACDB
Good idea to retrofit a DG source. Prevent worries about battery charge, CBV Operation etc
Close Transformer feeders one by one.
Start up two dissimilar units if possible
GTs cannot be back charged with weak sources, where GCB is provided