Part 3: Electricity market and Power system integration Flashcards
In a power system there is normally a synchronously spinning inertia available. Where does it origin from?
From stored kinetic energy in rotating mass of the synchronous machines and turbines connected to the grid.
In which way is this inertia important for the frequency control?
The inertia is important in order to keep the balance between load and production. When the load is to large the spinning becomes faster and when there is a deficiency of production the spinning is reduced.
In which way may power system inertia be a larger challenge in power systems with high share of wind power.
Traditionally wind power does not contribute to the inertia and frequency control. In the future one might use wind to control. An example could be to use pitch control to reduce production when needed. Another version is is to keep a margin which can be used when needed. Conceptually there is kinetic energy in the turbines so it might be possible to design a system where this can be used.
What is the difference between a strong and a weak grid?
A strong grid will not change voltage when a load or generator is connected but a weak grid will.
What impacts the strength of a grid? Why is it “strong” or “weak” in a certain connection point.
The short-circuit capacity determines the strength of the grid. Generally high voltage, high production and low impedance provide a strong grid.
What is the capacity factor?
It is the ratio between the actual energy produced during a year and the energy produced in one year if it was running a nominal power.
CF=(energy produced within a year) (kWh/year)/(36524P_rated) (kwh/year)
What is the capacity credit?
The capacity credit instead gives a measurement of the ‘stability’ of the plant and estimate how much conventional power generation can be substituted by wind power. How much power can be guaranteed.
Assume a point in a grid (point A) with a short circuit capacity of 800 MVA, purely inductive. To this point a 20 MW wind farm is connected in point B via a 20 km transmission line with R=X= 0.4Ω/km. We now study the connection point (point B) the wind farm. Is the grid in this point weaker or stronger or has the same strength as in point A? Motivate the answer.
-Grid at point B is weaker than that at point A.
-Due to the impedance between point A and point B.
A strong grid can be modeled as an ideal voltage source behind zero impedance.
When the wind is connected there will be more variance in voltage in B compared to point A => B is a weaker point than A.
Assume that one in the 20 MW wind farm can control the reactive power production. Does this have any impact on the strength of the grid, and if so, in which way? Motivate the answer.
No, the definition of a strong grid is that the voltage is not affected by the loads. Controlling the wind farm will not affect the strength of the grid.
Assume a 50 MW wind farm which is connected to a 20 kV grid via a transmission line with R= 0Ω and X= 6Ω. When the active power production increases from 20 to 40 MW, what happens with the active power losses on the transmission line? Do they increase, decrease of remain the same. Motivate the answer.
P_loss=R*(P_kj^2+Q_kj^2)/U_k
WT-k——-j-Grid
Since the resistance in the transmission line is zero the loss will remain the same as before. P_loss=0. The resistance of the line is independent of the production.
What happens with the voltage in Sunwind Village if the power factors for the consumers and/or in the wind power station increases? Motivate the answer.
Voltage is strongly dependent on the reactive power. Therefore it would be in the best interest to produce only as much reactive power as the load will consume. Transmitting reactive power to the grid will also increase the losses.
When the power factor of the wind increases the production of reactive power is reduced. This will lead to the voltage being reduced. When the power factor of load increases, it will consume less reactive power hence the voltage will increase.
Assume that the short circuit capacity changes and also that solar power production changes. Does this have an impact on the voltage in Sunwind Village? Motivate your answer.
If the short circuit capacity changes, this will change the short circuit impedance which in turn will change the over all impedance of the system. It will therefore change the voltage at Sunwind.
When the solar production is changed the power flow (P_kj) will change and therefore the voltage will change-
What are the steps to be taken when checking the voltage in the village k?
Village k——————Grid j
-Calculate all reactive and active powers.
-Voltage in k without production and load is the voltage at the grid.
-Calculate Z_grid Rgrid=U_grid^2/S_sccos(phi) Xgrid=U_grid^2/S_scsin(phi)
-calculate Z_tot=Z_grid+Z_line
-Use formula to calculate U_k
B=0, P [MW], U_j [kV]
