China Power System Transformation |
Power system transformation and flexibility |
case in point. If left unchecked, the growth in EVs could have a number of undesirable effects on the power system, such as increases in peak demand, overloading of distribution grids and increased difficulty in integrating VRE. For example, a recent study of grid integration of renewable energy in Thailand (IEA, 2018f) found that unmanaged EV charging could reduce the capacity credit of solar energy, while smart charging would help to improve it (Figure 11). This example also points to the important relationship between EVs and system integration. The key concept in this context is power system flexibility, which is the focus of the next section.
Figure 11. Impact of EVs on capacity credit of solar PV in Thailand, 2036
Note: Scenario assumes 10.87% annual average solar PV penetration, peak demand of 48 GW and a total number of EVs of 1.2 million.
EV charging needs to be managed in order to maximise benefits to the power system.
Flexibility as the core concept of power system transformation
The flexibility of a power system is a measure of its ability to reliably, and cost-effectively, manage the variability and uncertainty of supply and demand across all relevant timescales. The increased relevance of system flexibility is largely due to the rapid deployment of VRE (IEA, 2018a). In order to understand the requirements for power system flexibility, it is useful to consider a number of basic aspects of system integration of VRE. First to consider are the inherent properties of VRE generators, which increase the need for flexibility, followed by their effect on the power system through various deployment phases. Then it is important to understand the different time horizons of flexibility requirements, as well as the deployment framework. Last to be addressed are the associated effects on redefining the roles of system resources.
Properties of VRE generators
VRE generators have five technical properties that differentiate them from more traditional forms of power generation, i.e. large-scale thermal power plants. First, their maximum output fluctuates according to the availability of the underlying resource (e.g. wind and sunlight). Second, the ability to accurately predict fluctuations depends on the lead time, with generally more accurate forecasts possible a few hours ahead than a few days ahead. Third, they connect
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China Power System Transformation |
Power system transformation and flexibility |
to the grid via power converter technology.22 This can be relevant in ensuring the stability of power systems, such as following the unexpected shutdown of a generator. Fourth, they are more modular and are deployed in a much more distributed fashion. Finally, unlike fossil fuels, wind and sunlight cannot be transported, and locations with the best resources are frequently at a distance from load centres. Despite these general similarities, wind and solar PV also show a number of differences (Table 3).
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Table 3. |
Overview of differences between wind power and solar PV |
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Wind power |
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Solar PV |
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Variability at plant |
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Often random at subseasonal |
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Planetary motion (days, seasons) with |
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timescales; local conditions may |
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level |
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statistical overlay (clouds, fog, snow etc.). |
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yield pattern. |
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Variability when aggregated Uncertainty when aggregated
Ramps
Modularity
Technology
Capacity factor
Usually with a strong geographical smoothing benefit.
Shape and timing of generation unknown.
Depends on resource; typically few extreme events.
Community and above. Non-synchronous grid connection and mechanical power generation. Approximately 20% to 50%.
Once “bell shape” is reached, limited benefit.
Unknown scaling factor of a known shape.
Frequent, largely deterministic and repetitive, steep.
Household and above. Non-synchronous grid connection and electronic power generation. Approximately 10% to 25%.
Source: IEA (2017e), Getting Wind and Sun onto the Grid: A Manual for Policy Makers.
Phases of system integration
The properties of VRE interact with the broader power system, which gives rise to a number of relevant integration effects. These effects do not appear abruptly, but rather increase over time along with the increase in VRE penetration. The IEA has developed a phase categorisation to capture changing impacts on the power system and resulting integration issues:
Phase 1: The first set of VRE plants are deployed, but they are basically insignificant at the system level; effects are very localised, for example at the grid connection point of plants.
Phase 2: As more VRE plants are added, changes between load and net load23 become noticeable. Upgrades to operating practices and making better use of existing system resources are usually sufficient to achieve system integration.
Phase 3: Greater swings in the supply–demand balance prompt the need for a systematic increase in power system flexibility that goes beyond what can be fairly easily supplied by existing assets and operational practice.
Phase 4: VRE output is sufficient to provide a large majority of electricity demand during certain periods (high VRE generation during times of low demand); this requires changes in both operational and regulatory approaches. From the operational perspective, it is related to the way the power system responds immediately following system disturbances. This phase thus concerns power system stability. From the regulatory perspective, it may involve rule changes so that VRE has to provide frequency response services such as primary and secondary frequency regulation.
22With the exception of older wind turbine technologies, which connect directly to the grid.
23Net load is the difference between forecast load and generation from VRE.
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Power system transformation and flexibility |
Phase 5: Without additional measures, adding more VRE plants means that their output frequently exceeds power demand and structural surpluses of negative net load appear, leading to an increased risk of curtailment of VRE output. Shifting demand to periods of high VRE output and creating new demand via electrification can mitigate this issue. Another possibility is to enhance interchange with neighbouring systems. In this phase it is possible that, in some periods, demand is entirely covered by VRE without any thermal plants on the high-voltage grid.
Phase 6: The main obstacle to achieving even higher shares of VRE now becomes meeting demand during periods of low wind and sun availability over extended periods (e.g. weeks), as well as supplying uses that cannot be easily electrified. This phase thus can be characterised by the potential need for seasonal storage and use of synthetic fuels such as hydrogen.
Most countries around the world are currently in Phases 1 and 2. However, as VRE deployment accelerates over the coming five years, this will shift with more and more countries moving into Phases 3 and 4. It is important to note that a country-wide characterisation – especially for large and diverse countries – can provide only a rough indication. For example, while the overall impact of VRE in China is still moderate on a national scale, there are provinces that already experience issues associated with more advanced phases (Figure 12).
Figure 12. Overview of VRE system integration phases for different countries and selected provinces, 2017
% VRE generation
60%
50%
40%
30%
20%
10%
0%
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Korea South Africa Thailand Mexico India Japan China United States Australia Kyushu Sweden Texas |
Belgium China-Xinjiang California Italy Greece China-IM China-Ningxia United Kingdom Uruguay China-Gansu Germany China-Qinghai Spain Portugal Ireland South Australia Denmark |
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Phase 1 |
- No relevant impact on system integration |
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Phase 2 |
- Drawing on existing system flexibility |
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Phase 3 |
- Investing in flexibility |
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Phase 4 |
- Requiring advanced technologies to ensure reliability |
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Countries around the world are at different levels of system integration. Regions within one country can be at a higher or lower phase than the national average.
Using this framework for system integration, it is then possible to consider different challenges that need to be addressed for system integration to be successful (Table 4).
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China Power System Transformation Power system transformation and flexibility
Table 4. |
Summary of impacts associated with Phases 1 to 4 of system integration |
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Attributes (incremental with progress through the phases) |
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Phase 1 |
Phase 2 |
Phase 3 |
Phase 4 |
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Characterisation |
VRE capacity is |
VRE capacity |
Flexibility |
Stability becomes |
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from a system |
not relevant at the |
becomes |
becomes relevant |
relevant; VRE |
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perspective |
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all-system level |
noticeable to the |
with greater |
capacity covers |
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system operator |
swings in the |
nearly 100% of |
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supply–demand |
demand at certain |
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balance |
times |
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Impacts on the |
No noticeable |
No significant rise |
Greater variability |
No power plants |
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existing |
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difference |
in uncertainty and |
of net load; major |
are running |
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generator fleet |
between load and |
variability of net |
differences in |
around the clock; |
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net load |
load, but there are |
operating |
all plants adjust |
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small changes to |
patterns; |
output to |
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operating patterns |
reduction in power |
accommodate |
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of existing |
plants running |
VRE |
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generators to |
continuously |
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accommodate |
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VRE |
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Impacts on the |
Local grid |
Very likely to |
Significant |
Requirement for |
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grid |
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conditions near |
affect local grid |
changes in power |
grid-wide |
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points of |
conditions; |
flow patterns |
reinforcement and |
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connection, if any |
transmission |
across the grid, |
improved ability of |
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congestion is |
driven by weather |
the grid to recover |
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possible, driven by |
condition at |
from disturbances |
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shifting power |
different |
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flows across the |
locations; |
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grid |
increased two-way |
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flows between |
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highand low- |
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voltage parts of |
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the grid |
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Challenges |
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Local conditions in |
Match between |
Availability of |
Ability of system |
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depend mainly |
the grid |
demand and VRE |
flexible resources |
to withstand |
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on |
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output |
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disturbances |
Source: IEA (2017e), Getting Wind and Sun onto the Grid: A Manual for Policy Makers.
The impact of VRE on the power system increases gradually from one phase to the next.
Different timescales of system flexibility
Keeping the lights on requires the continuous balancing of supply and demand across all timescales, from moments to years; it is thus useful to consider flexibility across these timescales. To help understand different flexibility needs, as well as the different mechanisms for meeting them, this report groups flexibility requirements on the basis of timescales, ranging from short term (subseconds to hours) to medium term (hours to days) and long term (days to years) (Table 5).
As regards the short term, these flexibility needs are driven by technical power system characteristics relating to voltage and frequency management, which are essential to system stability. Longer-term flexibility needs come from weather system and seasonal drivers and are related to the availability of appropriate capacity and resources.
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