Power system flexibility is defined as “the ability of a power system to reliably and cost effectively manage the variability and uncertainty of demand and supply across all relevant timescales, from ensuring instantaneous stability of the power system to supporting long-term security of supply” (IEA and 21CPP, 2018).
A collection of operational, policy and investment-based mediations are available to make modern grid systems more flexible, thus facilitating a cleaner, and more reliable, more resilient and more affordable power grid.
During Power System Transformation (PST-IEA) to a modern power grid, there should be more focus on supporting power system flexibility, to elevate and facilitate the transition process.
The increasing importance of DER – and its associated “system integration” issues – is among the most important drivers of PST globally, and different levels of DER penetration require different approach to provide required power system flexibility.
The share of DER generation in many countries has grown over the past few years. In 2015, there were just over 30 countries with an annual generation share of DER generation greater than 5%; by 2018, this number had risen to nearly 50 countries . As the number of countries with medium-to-high shares of DER generation rises significantly, it is expected that power system flexibility will become a more prominent issue in coming years (IEA).
Shares of DER generation in many countries and/or regions are expected to rise from 5-10% to 10-20% over the next five years; jurisdictions with shares of 20-40% are also expected to increase significantly. (IEA, 2018).
Power system challenges associated with DER can be addressed through technological options and/or adjustments to operational practices. A range of measures have been implemented by many power systems worldwide to mitigate the impact of DER. These mitigation measures can be categorized into technical and economic measures.
Technical measures can help to enhance the reliability of the power system, while economic measure can improve the cost-effectiveness of power system operation.
Although, the main purpose of the technical measures is to address reliability, balances and hosting issues, some of them can also increase the cost-effectiveness of power system operation.
Real-time monitoring and control of DER generation by the system operator
The ability to monitor and control DER generation facilities, centrally or at the edge, in real time is important for secure operation of the power system, particularly as the DER generation share increases and the size of DER generation plants becomes relatively large.
This is in line with modern grid development objectives of ‘Enabling DER integration’, which requires capabilities like Management of DER and Load Stochasticity, Integrated Grid Control, Reliability and Resiliency Management, Control Federation and Control Disaggregation (central and edge control), Controllability and Dynamic Stability (DOE-OE Modern Distribution Grid report).
Depending on market arrangements (central dispatch versus self-dispatch), such real time control will either be exercised directly by the System Operator (DSO,TSO) or through generators. Irrespective of the arrangements, it is critical for the SO to have real-time awareness of DER generation and an effective mechanism for control of them.
Congestion Management
Increased DER penetration, particularly at locations where the grid is not strong, can worsen congestion situation in the network. Although congestion can have economic impacts in the form of possible curtailment or higher local pricing, it can also pose risks to the reliability of the system. Multiple options exist that may solve the issue, including grid reinforcement and grid management advanced features.
One of the most useful features, essential for both planning and also optimal hosting and operation of DER is congestion forecast functionality in grid management solutions.
Forecasts can help SO to predict possible congestion in distribution and transmission aggregating lines, due to weather change, load change or other network operation and maintenance issues, on different time basis, so the operator can plan in advance to mitigate the consequences or even avoid the congestion. An advanced grid management solution will suggest measures for such mitigations and congestion avoidance.
Integrating forecasting into power system operations
Advanced forecasting tools using sensing technologies, together with mathematical models, can accurately predict wind speed and solar irradiance, and subsequently forecast outputs from DER plants on different time basis and accuracy.
Advanced forecasting is considered to be a tool that improves the cost-effectiveness of DER integration, offering benefits as the system approaches the time when DER generation amount becomes noticeable at the system level to the SO. It can also improve system reliability as DER generation shares grow.
Centralized system-level forecasting of DER generation can improve system operation by enabling the SO to account for overall variability of DER outputs across the whole system and accurately predict the amount of DER generation available.
Forecasting is a useful tool to assist real-time dispatch, scheduling and operational planning. In self-dispatching markets, forecasting of DER generation helps generators to establish reliable schedules and limit schedule deviations.
A good plant-level forecast will allow for a more cost-effective and reliable schedule of generation. A good system-wide forecast is critical for verifying that generation schedules are feasible and sufficient operating reserves held.
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