< Back | 3 October 2019

Grid connection of new generators: simulation

Grid connection of new generators: modelling and simulation

By 31 December 2019, the Spanish electricity system will welcome an additional 9,000 MW of generation capacity, all of which will be renewable. These MW come from the last three renewable capacity auctions, held in 2016 and 2017, but they will not be the only ones. More renewable generation will be needed to meet the ambitious European commitments for 2030.

New generators will have to satisfy, among others, the requirements defined in the operating procedures (PP.OO.), which are the Spanish grid codes, a total of 15 rules that apply to transmission grids and distribution grids that affect transmission. The O.O.P. have been formulated in anticipation of a massive irruption of renewable and non-controllable generation sources, with specific requirements such as the one relating to overcoming voltage dips (O.P. 12.3 [1]). Requirements such as this are intended to ensure the smooth operation of the system by avoiding large-scale disconnection of renewable generation sources following a fault in the electricity system. In this article we will review how to use simulation models to obtain authorisation from the transmission system operator and distribution system operators for the commissioning of new renewable generators.

The operators of modern electricity systems systematically use complex simulation models that allow them to carry out safety studies with great accuracy: contingency analysis, stability studies, protection adjustment, among others. Moreover, they can do so by considering a multitude of scenarios, foreseeing any possible circumstance in the future operation of their networks; this allows them to identify weaknesses in the electricity system and mitigate them, anticipate problems and solve them, know their limits and establish safety margins, reconstruct past incidents and prevent them from recurring. All of this means that operators are becoming increasingly demanding of those who want to connect to their networks and request models that are compatible with their simulation systems, as well as in other cases simulations associated with the operating modes they consider most relevant. And this has only just begun.

In the last 20 years, the contact of developers with simulation models has been gradually increasing as the level of detail of the O.P. requirements has increased. Currently, models appear throughout the whole process of processing a new wind farm. Thus, in P.O. 12.1 for grid access [2], the best available model estimate of the future generators to be installed is required. P.O. 9 on information exchanged by the system operator [3] extends this obligation during the connection process to all those plants connected to the transmission grid and to those of more than 10 MW (1 MW in island systems) that are connected to the distribution grid, which are practically all new plants. Recently, the need to submit the suitability validation report with the model was included. This point is more clearly explained in the descriptive guide of the commissioning and service procedure [4], which states that in order to obtain the authorization for commissioning and service (APES), the following must be provided:

  • Certification of compliance with P.O. 12.3
  • Harmonic characterization, in case of connection to the transmission grid (P.O. 9).
  • Dynamic models and model suitability report (P.O. 12.1 and P.O. 9).

Let’s look at each of these points in detail:

First of all, certifying compliance with P.O. 12.3 regarding the voltage gap [1] requires that a generator has been tested in the field, that this test has been used to validate a simulation model of a generator and that this simulation model has been extrapolated to the farm level to confirm compliance with the voltage gap. To this end, once the simulation model of the generator supplied by the manufacturer has been validated, it is scaled up to the total power of the wind farm and other elements such as the reticulation of the wind farm’s power lines, power transformers, equivalent impedance at the grid connection point and wind farm regulation loops are added. This model is simulated to evaluate the wind farm’s compliance with the requirements set by P.O. 12.3. The final certification of compliance will be carried out by an accredited company in accordance with the Verification and Validation and Certification Procedure (PVVC) [5]. It is worth mentioning that the proposed new P.O. 12.2 [6] regarding the implementation of the European grid code (Regulation 2016/631 [7]) contemplates new voltage gap curves, so the certification will be made based on a future technical standard for monitoring the technical requirements of that regulation on which work is currently being done in the compliance monitoring groups led by Red Eléctrica de España (REE) [8].

Secondly, developers must characterize the harmonics emitted by their installations. This requirement applies exclusively to installations connected to the transmission grid. The characterization is performed by means of harmonic load flow simulations in tools such as Power Factory. This involves extrapolating the harmonic emissions at the terminals of a wind turbine or solar inverter, supplied by the manufacturer, to those produced by an entire wind farm at its grid connection point. For this purpose, the propagation of harmonics through the lines and transformers of the wind farm is simulated.

Third and finally, the developers have to deliver to the system operator dynamic models of the generator and its controls, at local and farm level, of all wind farms and solar plants that are connected to the transmission grid and those over 10 MW (1 MW in island systems) that are connected to the distribution grid. These models may be sent directly by the manufacturer to the system operator for confidentiality reasons. Also, according to the current regulation, P.O. 9, the simulation models must be finally compatible with PSS/E and their delivery must be accompanied by descriptive documentation, as well as a validation report of the suitability of these models against real wind farm measurements [8]. The system operator provides a list of supported standard models, which consist of generic models with parameters that can be adjusted using real measurements to reproduce the response of each wind farm. However, it may happen that the generator does not fit the structure of any of these predefined models, in which case it is necessary to create so-called PSS/E user models, which are coded in Fortran. These user models must meet the requirements specified in [10].

As an example, the following figure shows the validation of the simulation model of a 2 MW doubly fed wind turbine (DFIG) against real measurements. Fig. 1 compares the measured response (blue) of the wind turbine to a voltage dip to zero of 500 milliseconds duration, and its comparison with the simulated responses (green and red) with different control settings.

Fig. 1 Validation of the IEC 61400-27 [11] simulation model of a 2 MW doubly fed wind turbine (DFIG) against real measurements. Measured responses (blue) and simulated responses for two pitch control configurations (green and red) are compared for a voltage dip to zero of 500 ms duration.

With the growing weight of renewables in the energy mix, grid access for these generation sources is becoming increasingly demanding. At Norvento we help developers and engineering companies to generate the models and perform the simulations that allow them to comply with access regulations and adequately justify such compliance, and transmission and distribution grid operators to validate through simulation the proper integration of new assets.

Acknowledgments: The authors would like to thank Sergio Martínez Villanueva, an engineer in REE’s system reliability department, for his interesting contributions that have helped to enrich this article.

Sources:

Luis Díez Maroto & Inmaculada Saboya Bautista

Luis Díez Maroto

Luis holds a PhD in Electrical Energy from the Universidad Pontificia de Comillas, and develops microgrid projects and network studies at Norvento. Contact with Luis

Inmaculada Saboya Bautista

Inmaculada has a PhD in Electrical Energy from the Universidad Pontificia de Comillas, and works on microgrid projects and network studies at Norvento. Contact with Inmaculada

< Carbon Capture Megaplants Renewable Microgrids: A Case Study >