The development of Renewable Energy Sources (RES) is rapidly growing worldwide and they will play an increasingly important role in electricity power systems in the coming years. The majority of this is expected to be from Variable Non-Synchronous Renewable (VNSR) generation sources. According to the 2020 targets, significant volumes of VNSR generation are estimated across the world. The level of average annual electricity consumption from VNSR should exceed the share of 25% in many countries, such as in Spain (26%), Germany (27%), Denmark (32%) and Ireland (37%). In Italy this level should reach around 14%, with instantaneous value well over 50%. The integration of a high amount of this kind of generation involves a significantly revision in the procedures, policies, rules and tools implemented by the power system operators.
Integration of significant amounts of RES represents a challenge for TSOs. RES behavior is different from conventional power plants due to variability, uncertainty, dispersion and reduced participation in voltage control, frequency regulation, system inertia and grid support during faults. These characteristics could represent a threat for the system security, if specific operational practices are not pursued.
According to International and Italian experience, the key issues related to operation and dispatching with RES are: Forecasting, Operational Scheduling and Dispatch (review and/or new paradigms and strategies), Voltage and Transient Stability, Inertia and Fault Levels, reinforcement and design of the Network Infrastructure, System Operation Policies and Rules (e.g. Grid Code Requirements), Interactions and information exchange between Transmission and Distribution Operators, Storage and impact on traditional generation.
From CESI experience the solution and actions that has to be taken, in order to deal with the impact of the integration of high RES in the network, are:
In most countries, Grid Codes have been reviewed in order to harmonize the rules regarding the design, connection and control of VNSR power plants. Generally speaking, system operators appreciate if these new VNSR generators behave like conventional installations, providing inertia, frequency control, voltage/reactive power control, fault level control and fault ride-through ability. High penetrations of VNSR generation can be reached if the overall controllability of the power system improves and therefore the capabilities of all generation improve.
The management of intermittency can be achieved through a combination of improved forecasting, increased control capability of conventional and renewable generation, and new operational strategies to manage uncertainty. Forecasting is anyway a key requirement for a successful VNSR integration. The accuracy of generation forecasts, particularly at higher VNSR penetration levels, materially impact the efficient and secure operation of the power system.
There may be also benefit in the large deployment of Storage devices to manage variability and uncertainty but it should at least be based on an objective economic analysis.
New dispatch paradigms and/or new operational strategies. For example, photovoltaic generation is usually connected to the distribution system. Firstly, this offsets a part of the grid load observed by TSOs that are in charge of the balancing activities: TSOs are required to change their traditional balancing methods which take only into account schedules of production for conventional power plants, national forecasts of consumption and exchanges (imports and/or exports) with other countries. Then, the connection of VNSR generation to distribution networks alters the control capability of the portfolio itself since the performances required to generation plants connected to distribution networks are generally less extensive than those for generation connected to transmission systems, particularly for reactive and active power control.
Furthermore VNSR generation is grid connected with different technologies than conventional synchronous generation units. These include full converter electronic photovoltaic cells, wind turbines and non-synchronous doubly fed induction generators. The level of penetration of these technologies can actually alter the characteristics of the power system affecting synchronizing torque, fault levels, voltage stability and frequency control.
High density of VNSR generation may impact maintenance activities of the grid: some periodic controls and operations which are usually performed on the main elements (transformers, breakers, etc.) generally require a local decrease in the load rate. This is usually done operating configuration changes in the grid and/or with special agreements with the producers. Such kind of agreement becomes hardly possible to conclude in the perspective of high distributed number of power plants and therefore producers and require to change the methods of scheduling maintenance operations.
Other issues are also expected from the evolution of market activity. In the future, VNSR will be required to provide ancillary services and in the same time, conventional power plants will have to improve their operating flexibility and thus more frequently operate at inefficient partial load or shut-down and restart. This will cause increased losses, additional fuel consumption and emissions, reduced operating hours, reduced lifetime of the plants with the final consequence of a relevant increase of specific costs.
Last but not least, the VNSR sector rapidly changes and moves, especially under the pressure of incentive scheme implementation and reducing costs due to large scale deployment and technological improvements; fast reactivity of system operators is thus required and a continuous assessment of the adequacy of the technical, economic and regulatory frames with respect to the rapid evolution and change of their power system should be put in place.
According to CESI experience, it is essential that any TSO keeps monitored and assess the adequacy of their technical rules, with particular regards to functional requirements: protection, regulation and control systems. Typically the integration of small amounts of VNSR generation is not considered as a major issue, because power systems are usually able to incorporate it without significant problems. On the other hand, significant amounts of VNSR generation can instead impact on the operation of a power system. The extent of this impact is related to the existing nature and characteristics of the power system, the level of penetration to be achieved and to any additional investments being made into the control capability of the system.