Frequency control of hybrid wind-hydro power plants with long conduits in isolated power systems

Abstract

The increase in the penetration of renewable energy is a reality in the vast majority of countries, both in Europe and in the rest of the world. This increase is not only occurring in continental power systems, but also on small islands that have high wind and/or solar potential. However, a high renewable energy penetration may compromise the stability of power systems, especially in the case of islands, which constitute isolated power systems. This is due to the inherent characteristics of renewable resources (uncertainty, intermittency and independence between availability and demand). These drawbacks are amplified by the power system inertia reduction caused by the fact that renewable generators are normally decoupled from the grid through a power electronics interface that do not add inertia to the system. It is well known that the long-term variability of renewable energy is well managed by pumped storage hydropower plants (PSHP). Thus, PSHP play an important role in the renewable resources integration, especially in isolated systems, because they can mitigate the energy fluctuations caused by the variations in the renewable resources availability. There are small isolated power systems equipped with diesel units, wind farms and pumped storage power plants. The critical scene takes place when power demand is only supplied by renewable energy: PSHP in turbine mode and variable speed wind turbines (VSWTs) not aimed at providing frequency regulation. Therefore, the PSHP is aimed at contributing to frequency regulation without any other technologies. In this case, the hydropower plant must adapt its operating point to make generated power match demanded power. One of the objectives of this Thesis is to analyse the frequency control in isolated systems when it is only provided by hydro generators. Furthermore, power plants operating in this mode may be equipped with long conduits due to special topographic conditions. As a case study, two islands belonging to the Canary archipelago have been selected: El Hierro and La Palma. The El Hierro power system is composed of several diesel generators and a hybrid wind-hydro power plant commissioned with the aim of reducing the consumption of fossil fuels. This hybrid power plant is composed of both a pumped storage hydroelectric plant with a 2577 meter length penstock and four 2.83 MW generation units coupled to Pelton turbines, as well as five 2.3 MW wind turbines. The La Palma power system is composed of several thermal groups, photovoltaic units (4.6 MW) and wind turbines (7 MW). However, the island has several renewable energy expansion plans with the aim of reducing fossil fuels dependence. In fact, the construction of new wind farms is planned. Since renewable energy penetration increases needed energy storage systems, the construction of a PSHP is also scheduled. Therefore, this Thesis has assumed these expansion plans including a PSHP designed to cover the island power demand without using fossil fuels. The 50 MW hydropower plant could have a 1950 meter length tail-race tunnel to take advantage of a natural lagoon as an upper reservoir without a surge tank due to environmental constraints. Frequency control becomes more complex when hydroelectric power plants are equipped with long conduits. In this case, the conduits’ dynamics may significantly influence the power plant response making the governor tuning difficult, which is a problem that has not been deeply studied in the scientific literature. Both isolated power systems (El Hierro and La Palma) have been mathematically modelled including the main components of which they are composed. Due to the conduits’ length, water compressibility and the conduits’ elasticity are taken into account since the associated pressure waves influence the system dynamic response. To analyse the frequency control in these isolated systems, classical control techniques have been used. These techniques allow the establishment of a relationship between the dynamic response of the hydropower plant and the design parameters of the control system. In order to apply classical control techniques, the non-linear mathematical model proposed for each of the configurations is linearized around an equilibrium operating point. Thus, the dynamic matrix of the linearized system has been used to develop a modal analysis. Modal analysis allows the investigation of the consequences of different control strategies. Through a strategic placement of the poles that represent the dynamic response of the hydroelectric power plant, the values for the controller's gains that improve the damping ratio of the oscillations, as well as the decoupling of the oscillation modes, are findable. This implies better behaviour of the power system in general, and of the generators in particular. However, sometimes pumped-hydro storage systems need complementary technologies able to absorb or inject energy in the short-term to contain the frequency within the limits established by the transmission system operator. On a high wind potential island, energy fluctuations can also be mitigated if (VSWTs) are aimed at contributing to frequency regulation, although this contribution is not yet implemented in all power systems. For the El Hierro power system, the VSWT capacity to provide frequency regulation is considered. Therefore, wind–hydro joint frequency regulation has also been analysed for that power system in this Thesis. The results obtained based on the linear model are applied to the non-linear model since the load-power regulation sometimes requires variations of the initial equilibrium conditions that exceed the small perturbation assumption. In fact, exhaustive searches of the hydro and wind governors’ gains have been carried out in the non-linear model for the El Hierro power system showing results similar to those obtained from the linear model. Based on the proposed obtained governors’ gains, the hydropower plant and the VSWT dynamic response are analysed simulating, in the non-linear model, realistic events related to renewable energy fluctuations and power demand variations. Using different widely used system frequency and PSHP operating quality indicators, the system dynamic response, assuming the proposed recommendations, is compared with the one that would be produced using classic criteria settings, both for the hydropower plant governor and for the VSWT control loops. These classic criteria were proposed assuming hypotheses that do not include the phenomena described above such as conduits elasticity, fluid compressibility or the isolated systems requirements. In all the studied cases, a clear improvement of the system dynamic response, maintaining frequency within the limits established by the transmission system operator, and in the hydropower plant dynamics, is observed, thanks to the recommendations proposed in this Thesis.