Integration - Tutorial Flashcards
If a generator connection is constrained within a network, typically what options are available to the developer and network company to allow it to connect? What are the downsides of each?
a) Rebuilding relevant network sections with higher rating or capacity components. Tends to be expensive and in case of transmission system may take many years to design, get planning permission, and construct.
b) Installing smaller than intended capacity. Will still be some grid connection costs and these plus loss of economies of scale may affect viability of remaining generation project.
c) Use of control systems to modulate output at key times, e.g. curtailment, voltage control, etc. Historically distribution companies were uncomfortable with active control but transmission systems used things like intertripping frequently as well as dispatching generation and/or demand to ensure constraints managed. Distribution companies now much more interested in ‘smart grid’ solutions.
What are the main technical constraints on generator connections? Which are most applicable at distribution level and which at transmission?
All of the constraints ultimately apply at transmission and distribution levels but in most cases they are more common in one than another indicated by T = more common at transmission; D = common at distribution. Thermal limits on line/cable – T, Voltage limits – D, Fault level limits, Stability – T. Plus harmonics – D, unbalance – D, voltage step change – D, etc.
What drives the choice of connection for offshore wind farms? Give examples.
Mostly distance from shore (or at least length of cable), capacity of farms. Cables expensive so developers generally want a single cable to shore. Small schemes (few MW) near to shore will get away with single AC cable at 11 kV. Medium sized schemes (tens of MW) further offshore will need to step up to 33 kV. Larger farms (low 100s of MW) will need 132 kV connections. Very large schemes may need multiple connections and those of extreme size and distance will likely need HVDC connections able to ship 1 GW.
Briefly explain the terms: intermittent, variable and firm.
Intermittent – occurring at irregular intervals, not continuous or steady.
Variable – liable to change, does not have a fixed pattern.
Firm – firm capacity is the amount of energy that can be guaranteed to be
available at a given time.
Describe briefly what capacity credit means and what it might be used for.
Capacity credit measures the contribution of a given generator to meeting demand. It is a measure of the extent of the match between renewable generation and demand. It is quite a narrow measure as it focuses on risk of demand exceeding demand – which biases it towards peak demand periods.
What is the process of determining the capacity credit of renewable generation?
Basically create probability density function of available conventional generation, work out likely frequency of demand exceeding demand, net off renewable generation from demand to determine lower risk value, and determine how much conventional generation would be equivalent to reducing risk by that much.
If 1 GW of wind is deemed to be able to replace 150 MW of conventional generation, what is its capacity credit?
Capacity credit is amount of wind capacity that could replace conventional generation, normally expressed in percent. Here 1 GW is able to replace 150 MW conventional so capacity credit 0s 150/1000 = 15%.
A wave farm of capacity factor 35% is one of the first connected. What would you expect its capacity credit to be?
For most renewables, the initial connections have a capacity credit that is similar to their capacity factor. In this case this might be around 35%.
How would capacity credit change with cumulative installed
capacity?
It would tend to go down. If the spread of capacity around the country stays the
same but the overall capacity goes up then the nice smooth curves in the notes are
the result.
What are the three major issues in integrating renewable generation capacity?
Are the renewables in the right places? What happens when you connect them Can renewables meet demand?
What is net demand?
This is demand with variable renewable generation subtracted from it. It is what
the conventional generators must meet
How will growing renewable capacities affect existing thermal
power plant?
Renewable generation tends to:
- Have modest effect on peaking plant.
- Reduce production from mid-merit plant.
- Have minimal effect on base load unless renewable capacity is large and demand is low.
Storage is considered as part of the solution to renewable variability. Explain why this would be the case.
Allows some of the mismatch between renewable supply and demand to be buffered by filling storage when renewable supply > demand and emptying storage when demand > renewable supply. It is theoretically possible to handle variability with just storage but at large scale the amount would be extreme and expensive. Some smaller systems, e.g. islands like Eigg do this or rather nearly do this using so called hybrid systems, often using diesel as a backup. Interconnection, flexible conventional plant and demand management will probably be cheaper than storage alone.
What are the potential applications of electrical energy storage in power grids?
EVs?
Outline the key features of one of the following storage
technologies: pumped hydro or CAES.
Pumped storage (PEHS): Pumped storage is currently the most established utility scale method for energy storage with approximately 99% of the world's grid energy storage being pumped storage. A pumped storage scheme uses the differential in height between two reservoirs to store energy. Pumped storage schemes can either operate using reversible pump-turbines or hydro turbines and separate pumps depending on the conditions at the site. During periods when electricity demand is lower, electricity is purchased from the grid and used to pump water from the lower reservoir to the higher one, during periods of high demand this water is released and allowed to return through the pumps now acting as turbines in order to generate electricity.
Developing areas for PHES include:
- Underground pump storage.
- Use of the sea as a lower reservoir.
- Offshore energy islands.
Compressed Air Energy Storage (CAES):
CAES involves compressing and storing air in order to store energy, either using geological underground voids or purpose made vessels. When the stored energy is needed, the released air is heated via combustion using natural gas and is expanded in order to drive a gas turbine to generate electricity.
The compression of air for storage generates heat. Current installed CAES plants are termed diabatic storage and dissipate the generated heat as waste to the atmosphere. Installed CAES installations have a similar thermal efficiency to the most efficient combined cycle gas plants and a significantly higher efficiency than open cycle gas plants.
Research is being performed into the use of adiabatic CAES. The head generated during the compression of the gas is stored and used for heating the compressed air prior to the expansion turbine. This heating could either drastically reduce or even eliminate the requirement for fuel.
