Systems New Flashcards
Good for GS iteration
http://ocw.ump.edu.my/pluginfile.php/1335/mod_resource/content/1/CHP_4.pdf
Why generate AC power instead of DC?
-Voltage levels of AC easily be transformed in AC systems therefore more flexible, have wider ranges of distance as less losses etc.
- AC generators are much simpler than DC generators
- are easier and cheaper to generate and have a higher efficiency
-AC motors are much simpler and cheaper
What is the physical meaning of real power P and reactive power Q?
P - Average value of the instantaneous power absorbed by the resistive part of the load
Q - Maximum value of the instantaneous power absorbed by the reactive part of the load
What are the advantages of three phase over single phase?
Reduced capital cost
Reduced operating costs
Better voltage regulation
Total instantaneous power delivered by a generator under balanced conditions is constant
What are the three operating objectives of voltage and reactive power
control?
- Voltage limits
- bus voltage magnitudes should be within acceptable limits since electric power equipment + customers equipment can only be safely operated at a voltage very close to its rating
- outside of this limit may not perform correctly or damage
- system stably enhancement
- system transient stability ad voltage stability can usually be enhanced by proper voltage control and relative power (VAR) management
- Minimise reactive power flows
- reactive power flow should be minimised such that active and reactive power losses should be reduced. Also by product of minimised restive power flows can actually reduce voltage drop across transmission lines and transformers
What are the advantages of per-unit?
For apparatus of the same type the pu voltage drops and losses are in the same order regardless of size
The use of root(3) is reduces in 3-phase calculations
By choosing appropriate voltage bases networks containing several transformers are facilitated
Per-unit is easier for digital computation
Why calculation of per unit values within changes of bases is required?
- Power systems consist of consumers such as generators, transformers, transmission lines and loads.
- the parameters of these components such as voltages, current and impedances are sometimes given in per unit with respect to their own bases rather than a common base for a whole system
- in this situation, we need to choose a common base for the whole system, then all per unit value based on their own bases should be converted into the new per unit values with respect to the new base of system
What are the steps to calculate the impedance and load powers of bus 2 and bus 4 (in a 4 bus system)?
- calculate Zbase
- calculate Znewpu (Z for bus2 = Z for bus4)
- calculate Slpu for both buses
- calculate Zl
How do you calculate the pu voltages and currents for each bus in a 4 bus system?
- set V4 to a flat start
- calculate I4pu (using (Sl4/V4)*
- I23 = I34 = I4
- calculate V3 = V4 + Z4*I34 (same for V2)
- calculate I2pu = (Sl2/V2)*
- I12pu = I2pu + I23
- V1 = V2 + I12pu*Z1
