Renewables Flashcards
What are the challenges for a 100% renewable power system?
- Increased consumption
- electrification of transports and industry
- Power from wind and solar
- Intermittent production, harder to match demand and supply
- Power quality
- Power system stability
- Peak load
- Decentralised power system
- Grid
- More connections to other countries are needed
- Matching production with consumption geographically
- Reactive power
- Use existing grid more efficiently
What impact does renewables have on power quality?
More renewable power production in the power system has led to more power electronics. If these are not handled right, it can lead to poor power quality
- Harmonics
- Transients
Describe how inertia affects the power quality and how different power sources can contribute to inertia
Disturbances in power production causes grid frequency to decrease. If the frequency drops too low, more power production will disconnect. Therefore, there are mechanisms for preventing the frequency drop. One important is inertia
If a system has inertia, J, it will help decrease the frequency change
- Nuclear, hydro and CHP all have turbines with synchronous machines, directly connected to the grid. This means that they will contribute with inertia directly if there is a fault in the grid.
- Wind and solar have induction machines and are connected to the grid through a converter. This means that they will not do anything when a fault occurs unless the converter is programmed to do that
- With the right control, wind turbines can easily produce both inertia and extra power. The extra power comes from the rotation mass of the rotor.
- Solar power doesn’t have any moving parts but can still produce inertia with the converter
- The inertia from wind and solar is disconnected from the grid and are sometimes referred to as synthetic inertia. A better name is controlled inertia response
What is peak load?
Peak load - the hour of the year with the highest consumption. The risk of having a lot of renewables is that the peak load demand might not be met
The peak load is met by using gas turbines that are used only a few hours a year to cover the demand during the peak load hours.
It is expected to increase in the future due to higher electricity demand
How can the problems with peak load be mitigated?
- The need for peak load capacity is dependent on how much demand side management and storage there is in the power system. WIth more flexible loads and more storage, the demand for peak load capacity goes down.
- An easy way to solve the peak load capacity is to import more electricity, but here the question will be if situations with many countries with import demand at the same time will occur. Would it be possible to be self-sufficient with power?
- In reality it will probably be both, depending on import price and availability to import
The more reewables we have in the system, the less flexible the power system will become unless we add storage and DSM etc.
What is a decentralised power system?
A power system where we are going from few large power stations to many small(er)
- Wind farms in Sweden are decentralised
What are the best option to fully utilize renewables gobally?
To build more connections between countries.
- When there is a lot of wind in one area, they will need to export the electricity to other countries.
- The wind then moves to another area, which will then export their surplus
- IT would be expensive if all countries strive to be self-sufficient from electricity
- Better to build more cables between countreis and export and import depending on where the wind blows and the sun shines
Matching production with consumption geographically
- To minimize the number of cables that is needed: match production with consumption geographically
- It is expensive and takes time to build more grid
- Both consumption from household and heavy industry should be taken into account
Use existing grid more efficiently
- Connect more wind and solar to existing grid by more and better monitoring of the grid,
- Maximise the hosting capacity of the distribution system
- Dynamic line rating
- Dynamic transformer rating
Maximise the hosting capacity of the distribution system:
- Worst case scenario
- The local grids are passive with no active monitoring
- Therefore, grid pwners calculate worst case scenarios when wind turbines should be connected
- Worst cases are overloading and overvoltage which occur when the load is minimal and the production is maximal
- Active management strategies:
- Since both load and production varies a lot over time, the probability for the worsta case scenario to occur is very low
- This means that unnecessary restrictions on connection capacity are used.
- To increase and maximise the hosting capacity active management strategies can be used:
- Reactive power compensation
- Coordinated on-load tap changer voltage control
- Wind energy curtailment
What are the problems with long power transmission?
If a lot wind power is installed in the north of Sweden together with the hydro power already there and no nuclear power in the south: long distance power transmission is needed.
This would affect the voltage in the grid and in order to keep the voltage reactive power compensation will be needed.
Reactive power compensation needs to be handled regionally, since it can’t be transferred for long distances, since it would lead to congestion of the lines
Explain the principle of coordinated on-load tap changer voltage control
- For some grids, the number of tap changes increases when the amount of installed wind power increases: grids with low X/R
A low ratio between X and R means that the voltage will vary a lot when a lot of wind power is installed
X and R is the impedance (Z) for the power cable connecting the wind farm and the transformer
Explain the principle of wind energy curtailment
Curtail the amount of power that the wind farm can produce to ensure that no overvoltage or overloading occur
Explain the principle of dynamic line rating
Dynamic line rating is a way of measuring the temperature of the OH-lines to see when power should be curtailed to keep within the temperature limits
Factors that increase temperature are:
* The sun is shining on the cable
* High air temperature
* High power is transferred through the cable
This will cause the cable to expand and might dip too low
Factors the decrease temperature are:
* It’s blowing at the cable
* Less power is transferred through the cable
This optimizes the hosting capacity of the lines
Explain the working principle of dynamic transformer rating
Dynamic transformer rating (DTR) looks at load and temperature variations to increase the rating of the transformer while maintaining in safe operation
Why use DTR?
* The transformer is an expensive component, big part of CAPEX
* Transformers operates too safe today
* More efficient use of component
* Potential for wind farm expansion
* Cheaper grid connection for wind farms
Transformer maximum capacity is highly dependent on wather and load conditions. Use weather prediction tools to better design the transformer
The result will be a smaller transformer with DTR:
- Lower cost
- Increased system and component reliability
- Decreased price of wind electricity (LCOE)
