34 Reliability of Future Power Grids
The following points highlight some of the current limitations of the PoweRisk tool, and hence suggest ideas for further research. ¾¾ A power-flow simulation assumes that the system is in steady state. In fact, transients (due to, e.g., large generator or line trips) could cause instability even if the post state is considered steady. A stability analysis must be done in order to test the system security for a given transient. It is worth noting that no commercial tool presently exists for probabilistic security studies of power systems [9]. Including functionality to capture the more severe transients and performing stability analysis on these is a topic for further work. ¾¾ A power-flow simulation is a snapshot of the system state and does not consider the history of events leading up to the present situation. Hence, limitations on generator ramping and re-dispatch are not taken into account. ¾¾ Load shedding is conducted by dispatching loads continuously downwards. In reality, loads would be shed in discrete steps. Therefore the results tend to be optimistic with respect to load shedding. Block dispatching of loads would, however, lead to a mixed integer problem that is substantially more complex and time-consuming to solve than the linear Optimal Power flow (OPF). ¾¾ The generator maintenance schedule lacks intelligence. A more sophisticated model for maintenance should be developed that postpones maintenance on main components if the system is severely stressed. The following points list some main shortcomings of the study and some recommendations for studies of real systems: ¾¾ Correlation between wind and load is not captured, as the IEEE-RTS is a hypothetical power system. In studies of real systems it is essential that time-correlated wind and load data are used for the same area. Wind-load correlation could significantly impact wind power’s contribution to system adequacy. ¾¾ The study assumed equal wind conditions on all wind parks in the system. Studies of real systems should use wind generation data for individual parks, or, at least, parks that are in relatively close proximity of each other. This would capture the actual wind power infeed at each bus and the
resulting power flows in the grid more accurately. This is especially important if the transmission grid is constrained and if the system under study spans a large geographical area. Further development of the PoweRisk tool is foreseen to include: ¾¾ Implementation of all relevant functionalities for solving AC power flow (steady-state) problems with the possibility of including point-to-point HVDC transmission lines. ¾¾ Development of a conceptual methodology for solving power flow in multi-terminal HVDC (MTDC) grids and consideration of the possibility for simulating combined AC/MTDC power systems. ¾¾ Assessment of the possibility of combining the AC-based tool with dynamic (time-domain) simulations. ¾¾ It is important to note that the PoweRisk tool will not be developed as commercial software. The intention is that it is to be used and maintained internally in DNV GL for consultancy services.
STOCHASTIC POWERFLOW In 2013 DNV GL started a research project, Stochflow, which aimed at exploring the use of stochastics in power system analysis and developing tools for future services. Using stochastics is seen as one way forward in addressing the increasing amount of uncertainty in planning and operating conditions. The primary objective of the work for 2013 was defining the way ahead for DNV GL’s research in this field. As a part of this a proof-of-principle tool and case study were developed for power-flow calculations. It should be noted that the method used could be equally applicable to other types of analyses, for example, a short circuit calculation. The objective of the proof-of-principle case was to find the full cumulative distribution functions of all currents and voltages in a generic grid, given stochastic inputs for wind generation, solar generation, and demand. The brute force Monte Carlo Simulation (MCS) approach is traditionally used to perform “standard” power-flow calculations for each combination of inputs. The large number of calculations (100,000 – 1,000,000) that are required for the desired accuracy requires