Frequency services Flashcards
Describe the power balance equation in the power system
T_mec-T_el = J*dw/dt
T_mec = mechanical torque from the turbine
T_el = electric torque from the generator
J = inerita from rotating electrical machines
w = rotation speed of the generator = frequency of the voltage
What happens to the frequency if the production suddenly decreases?
The frequency will drop
Which power production units helps keeping the frequency constant?
- Hydro power is good for frequency regulation since it can dispatch large amounts quickly to match supply and demand
- Nuclear power turbines is good since it adds extra inertia to the system
- Wind turbines can also contribute by pitching angle, reducing power coefficient and thereby also the active power output
Describe the different frequency restoration services
- FFR (Fast Frequency Response)
- First reserve to manage initial deviation due to small inertia
*FCR-U (Frequency Containment Reserve-Up)
- Frequency Containment Reserve when the frequency is too low
- FCR-D (Frequency Containment Reserve-Down)
- Frequency Containment Reserve when the frequency is too high
*FCR-N (Frequency Containment Reserve Normal)
- Frequency Containment Reserve for
*aFRR (automatic Frequency Restoration Reserve) - Automatic Frequency Restoration Reserve to fix frequency once the drift has stopped
*mFRR (manual Frequency Restoration Reserve)
- manually activated frequency restoration reserve with the goal to restore the frequency when the drift has stopped
Depending on which type of reserve that is needed, different technologies will be used. The reason for this is that depending on if it is FFR or FCR, different magnitudes of power is needed
Why is frequency control needed?
- To keep the frequency in a power grid - balance between consumption and production
- Frequency control by wind turbines is to control the power output from the wind turbine in a not optimal energy way and get payed for the energy losses or more
How do you spill wind power?
You pitch the blades of the turbine since that will reduce the power coefficient and thus also the active power output
What is the frequency set point value?
50 Hz, but is okay to vary around 49,9-50,1 Hz
Describe the equation for wind turbines to control the grid voltage
V1 = V2+(RP+XQ)/V2
V1 = turbine voltage
V2 = grid voltage
R = Equivalent circuit resistance
X = Equivalent circuit reactance
P = Active power
Q = Reactive power
How do you calculate the transformer reactance?
X = uk*V^2/St
uk = reactive leakage of the transformer
V = voltage
St = Apparent power rating of the transformer
How do you calculate the grid parameters?
Xg = Vhv/(sqrt(3)Isc)*(Vlv/vhv)^2
lv = low voltage
hv = high voltage
Isc = short circuit current
What is adequacy?
- Generation and grid capacity adequacy
What is reliability?
Balance & stability
Explain the concept grid capacity
Grid capacity is the capacity that we are able to transport on our transmission lines. The production takes place in northern sweden but consumption is mostly in the southern parts
Explain the concept generation capacity
Generation capacity is about making sure that we have sufficient production to cover consumption (daily, hourly, weekly, monthly and yearly)
Divided into 2 parts
1) Energy capacity adequacy:
- generation in TWh = demand in TWh
- Annual, seasonal, weekly, daily
2) Power capacity adequacy
- hourly generation in GWh/h = hourly demand in GWh/h
- To meet peak load in cold winter
- Nordpool market: day-ahead energy market + intraday market
No one is really responsible for generation capacity, i.e. it is driven by market. If price of electricity increases they will start to build production units
Which services are included in the umbrella term “reliability”?
- Balancing and stability
- Frequency reserves
1) Balancing services (MW & MWh)
- FRR
- RR
2) Power system stability services
- Frequency control & stability
- Voltage control & stability
- Rotor angle stability
If there is a fault in the grid, is the system still able to be connected to the grid once the fault has been fixed?
3) Other ancillary services
- Black-start capability (if the whole grid is out, they are able to start on their own)
- Island operation capability (some producers that are a bit away from the grid are afraid thath if there is a fault in the grid they might become islanded. Thus this one is about if they would be able to survive that or not)
- Other stability issues
- Procedures and mechanisms
- How to design grid code & market to provide these services
- Robustness
- N-1 contingency
- Should be able to lose any component (1 at the time) & still be able to provide
Which two factors are the RoCoF determined by?
- Disturbance size
- System inertia
Why is RoCoF needed?
- Higher inertia is good for not having these quick fluctuations
What is frequency nadir and why is it important?
Frequency nadir is the frequency at which the first initial drop starts stabilizing, i.e. the lowest frequency after which the frequency starts restoring.
- It depends on disturbance size
- System inertia
- Frequency containment process
- Load frequency dependence
Too low frequency can caise mechanical vibrations in a long shaft, which is one reason to why frequency nadir is needed
How does renewables impact inertia?
Renewables decreases the inertia (it’s however also dependent on generator size).
During summer nights we have the lowest inertia which can harm the nuclear reactors
What is voltage stability?
- ABility of a power system to maintain steady voltages close to nominal value at all bbuses in the system after being subjected to a disturbance
- Linked to the voltage drop that occurs when P&Q flow through inductive reactances of the transmission network
Why is DC more commonly used for transporting electricity than AC?
DC has much lower losses than AC, and the longer the lines becomes, the more losses there will beW
Which are the 5 most common principles for voltage control?
- Generator excitation control
- Tap-changing transformer
- Shunt capacitors & reactors
- FACTS devices: Flexible ac transmission system
- Inverter from wind turbines and solar PV, & energy storage systems
