Practically, in electric power systems (macrogrids or microgrids), the scheduling of generation resources is done on day-ahead or hour-ahead basis? Can you elaborate this issue? when the balancing is done? how non-dispatchable generating units such as wind and power take part in the scheduling?
Thank you
Regards,
Ahmad
In actual grids, the generation planning is done in multiple time frames, including day-ahead and hour-ahead, but also in shorter terms like 15-minutes ahead or similar.
Balancing load and generation must be done all the time, in all time frames, including real-time. If load (plus losses) do not perfectly match generation (in real-time) a frequency deviation will occur. Primary frequency control is provided by speed governors and shaft inertias on the generation side and the P/f sensitivity of the load, in the initial few seconds to up to 1 minute following a frequency event such as the trip of the largest generation unit in the grid. Secondary frequency control (AGC in North America) acts to correct frequency deviations and bring frequency back to nominal in a few minutes. Load shedding or generation shedding schemes are emergency actions to prevent collapse and further generation trip, not part of the control action.
Different countries have different energy market structures and their interaction with renewable generations varies. Wind energy generation presently receives jurisdiction support through price and quantity regulation in energy spot market besides voluntary and indirect support . In fixed price-oriented scheme, the wind energy producers are either paid an above-market price for the energy generation and the required quantity is determined by energy market. Example Germany, Denmark, Spain and France, Canada, Ireland and Greece. In legally obligated quotas or quantity based scheme, the share of the energy supply is guaranteed by government while the market determines the energy price. Examples, UK, Sweden, Poland, Italy, Belgium, Japan and several states of US. The capacity of installed wind energy systems is significantly different due to the diverse economics of the two schemes. There are some jurisdictions like Denmark and Spain, which operate both types of the schemes and is known as Green marketing. Green marketing or voluntary system programme is based on customer willingness to pay extra price for renewables. In all direct schemes, price for wind energy is directly or indirectly related with trading market schemes and respective pricing mechanism.
For scheduling and dispatching, I give specific example of Australia. Following are the excerpts from one of my paper under publication:
Australian TSO prepares forecasts of the available capacity of all semi-scheduled generators, to schedule sufficient generation in the dispatch process and Unconstrained Intermittent Generation Forecasts (UIGF) for reserve assessment purposes. The Australian Wind Energy Forecasting System (AWEFS), produces generation forecasts for all national electricity market (NEM) connected wind farms including semi-scheduled and non-scheduled ones. Maximum wind generation is included in central dispatch process as forecasted intermittent generation for timeframes ranging from five minutes’ advance to two years advance for load–generation balance. UIGF forecasts for individual unconstraint semi-scheduled wind generators dictates the available capacity which refers to the generation capability of a wind generator that is available for dispatch (without consideration of network limitations, price bids etc.). Semi-scheduled wind generators participating in NEM dispatch process must mandatory follow the NEMDE generated dispatch levels only when the semi-dispatch cap (SDC) is active otherwise wind farms are free to generate to any level. For an unconstraint wind farm, UIGF is based upon actual MW output received through SCADA system while in case of constraint wind farm, UIGF is based on meteorological forecast based upon number of available wind turbine, wind speed and MW set point. AWEFS performs three validation checks prior to every dispatch interval to determine if a wind farm’s output is being limited below its wind speed–based forecast (potential power): 1. Is the wind farm control system set point < the registered capacity of the wind farm? 2. Is the wind farm control system set point < Active power + 5% of registered capacity? 3. Is the wind farm control system set point < Potential power? If all three validation checks pass, AWEFS produces a wind speed–based UIGF. If any of these checks fail, AWEFS reverts to producing a UIGF based on the active power SCADA output. Figure represents the interactions between Wind farm SCADA, AWEFS and National Electricity Market Dispatch Engine (NEMDE) in NEM Dispatch market.
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