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Rapporto RSE 17001170

Analysis of new grid services aimed at frequency stability



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M. Rapizza (RSE SpA), S. Canevese (RSE SpA), A. Iaria (RSE SpA)

MERCATI ELETTRICI 2016 - Evolution of interconnected electrical systems and integration of the markets

As to grid frequency control, the introduction of fast primary and inertial control (synthetic inertia) actions is taken into consideration. The sensitivity of frequency transient behaviour with respect to the controller parameters is studied, in particular in terms of frequency nadir, time to nadir and initial ROCOF (Rate Of Change Of Frequency) after a sudden loss of generation.

Due to the increasing penetration into the power system of renewable source-based generation units, energy storage systems and new transport technologies, such as those involved in high voltage direct current connections (HVDC), two interesting issues arise: on the one hand, such devices can be considered as innovative tools to support more effective control of the power system; on the other hand, designing new control actions, i.e. new ancillary services, which could be carried out also by the mentioned devices, could be beneficial for system stability, security and reliability. This report focuses on grid frequency control, by taking into consideration (1) the so-called fast primary control (or fast frequency response that is a primary frequency control responding faster than the one usually deployed by conventional power plants)and (2) inertial control (or derivative control or synthetic inertia). These new control actions can be implemented by means of static components (batteries, converters in HVDC links and, under suitable conditions, generators interfaced with the grid by means of inverters, such as wind turbines and photovoltaic panels). In order to study the impact of such services on the behaviour of the system, a basic control theory approach is adopted. More specifically, for different controllers configurations, the three main topics of the analysis are: i) testing whether there exist some penetration limits of the considered services beyond which the system could lose asymptotic stability and become unstable; ii) evaluating the sensitivity of frequency dynamics with respect to the regulation parameters; iii) determining the mutual influence of the choice of the controllers parameters and the choice of the secondary controller parameters.

The main results obtained can be summarised as follows. As for asymptotic stability, there are restrictions only in case the system had a predominant share of traditional generation with negative transient response to a sudden power demand; this, anyway, is not the case of the European power system. Moreover, in order to evaluate the sensitivity of frequency transients in case of load disturbances, the values of some performance indicators are computed while changing the controls parameters. This analysis shows that the maximum frequency error is influenced mainly by the fast frequency response, and the time of maximum frequency error (i.e. the frequency peak time) and the frequency gradient (Rate Of Change Of Frequency - ROCOF) are mainly influenced by the synthetic inertia. Furthermore, in some situations the presence of a large amount of fast primary control causes an oscillatory frequency behaviour; the compensation of this effect, by varying the amount of synthetic inertia, suggests that it would be useful to operate an optimized choice of the parameters of the controls. Finally, as regards the interaction of the new frequency regulations with the secondary one, the analysis highlights some conditions that must be guaranteed in order to ensure system stability. These rules depend on the time constants that characterize the transfer function of the equivalent power plant adopted in the overall system model and the parameters of the considered traditional and new frequency regulations.