OPF for different time frames

Power system operations is a term which encompasses an entire range of activities performed by the different stakeholders. The activities of the operation fall within a time frame of several weeks, days or hours in advance, up to real time (figure 3.2, [54]).

Operations Operational planning Planning

Real-time operation

Milliseconds, seconds

Near real-time operation

Minutes, hours, days

Short-term Months, year (1) Mid-term Years (2-5) Long-term Years (5-15)

Market forces Regulation, legislation

Severe emergency conditions requiring

automatic and self- healing control and protective actions

Non-severe emergency conditions requiring

coordinated operator assisted control and optimised re-dispatch and balancing

Resource adequacy and grid development

gaps requiring

coordinated grid planning optimisation

Long-term goals for €green• resource adequacy and grid

development optimisation requiring

coordinated grid planning Uncertainties in variable production and demand; external forces, vulnerabilities (physical and cyber)

Steady state operation conditions requiring

preventive control and power flow optimisation

Legend: Operation, control and protection actions Coordinated control execution cycles Market effects transmitted in physical process dynamics

Figure 3.2: Smart Operation and Planning time frames

Months to days in advance, the operational planning of the power system focusses on maintenance, long-term generation scheduling and assessing the grid capacity between zones. Closer to actual operations (D-2), the guaranteed available system capacity between zones is determined and given to the market. Based on this input, market participants make offers to the market. The bids for the actual day come in the day before the actual operation (D-1), before gate closure. The different system operators perform the Day-Ahead Congestion Forecast (DACF) to determine whether the provided generation schedule can be maintained or whether there are adjustments needed. The DACF forecasts the system flows for each individual hour, while taking into account “N-1” constraints. The DACF also includes the expected generation from renewable energy sources. These adjustments can be done through market actions, or through TSO preventive actions such as the control of power flow controlling devices (PFCs). The DACF forms the basis for the security assessment done by the TSO. During the day itself, the TSO monitors the grid behaviour, which in normal operation always differs to a certain extent from the predicted state due to contingencies in the system, unforeseen generation shifts (possibly due to weather conditions), and changes in demand amongst others.

Furthermore, the system continuously changes its operating state because of the numerous variables in the system. If the predictions are close enough to the actual operation point, the TSO performs its planned operation. Generators (and other market participants) might also trade electricity intraday, resulting in possible changes from the foreseen schedules. Regular energy trade and balancing actions occur throughout the day. Larger shifts from the predicted operating point might occur as well. This can happen through large deviations in generation or load (due to an outage or unforeseen shifts in generation) or through outages in the grid. Such larger shifts can cause the system to move beyond the secure operating boundaries of the system. At such occasions, the system operator takes action through additional preventive actions or even corrective actions [55].

This discussion also reflects on the type of studies done with optimal power flow calculations. For long-term studies in an unbundled market situation, the generation of active power is in the hands of the generation companies and market players. The transmission system operator needs to develop his system in order to operate his network in a stable way. One of the aspects is having correct voltage ranges on each of the network nodes, taking into account different grid loading patterns. The nodal voltages are linked to the generation of reactive power. To manage this, the operator has proper means at hand: switching shunt reactors and capacitor banks. Another share of the reactive power generation comes from the generator units themselves. Optimal power flow calculations are used to estimate the minimum amount of reactive power from generator production units to guarantee correct bus voltages, taking into account sufficient margin on reactive power in case of grid emergencies. Securing sufficient transmission capacity, both internally and for international transit, taking security constraints into consideration, defines the second most important long-term studies from the operator point of view. Typically, these long-term optimisation calculations are run once each half to full year. The results are announced to the market players and serve as base cases for short-term studies.

The short-term calculations are typically performed one to two days before the studied date in a day-to-day cycle. They consider the actual state of the network with all current outages. The active power is nominated again by the market and the cross-border transfer capacities are maximised by the TSO. This type of optimisations and verification calculations is routinely done for each month, week and day.

The optimisation tools can also play a crucial rule for the real-time grid

operations: one example is the gradual curtailment of dispersed generation.

In order to provisionally accommodate more generation than can be allowed for safe operation in a security constrained situation, the owners of the generation units agree to curtail their output in case of a specific contingency. The grid operator sends maximum output setpoints which are the result of optimisation calculations initiated immediately after a grid incident has occurred. As such, more generation is allowed with the trade-off to be curtailed temporarily depending on the time to establish the safe state of the local network.

All calculation tools give at the desired time intervals a global picture on how the grid can be operated in an optimal way by means of setpoints for power and phase shifting transformers, output of generators and setpoints for converters. It is up to the human operator to perform the necessary switchings, tap position and setpoint changes in order to shift the grid state to its optimal point taking into account the inherent control restrictions. At all times, weather conditions and unforeseen outages are interacting with this process.