**1. Introduction**

Wind diesel power systems (WDPS) are isolated microgrids which combine wind turbine generators (WTGs) with diesel generators (DGs) to supply electrical power to remote consumers. All WDPSs have two modes of operation [1]: Diesel-Only (DO) mode, where the DGs supply all the power (active and reactive) to the isolated consumers, and Wind-Diesel (WD) mode, where both the WTGs and the DGs supply active power. In both DO and WD modes, the system frequency regulation is performed by speed governors included in each diesel engine (DE) and system voltage regulation is performed by automatic voltage regulators (AVR) included in each synchronous machine (SM). High penetration WDPSs can also work in Wind-Only (WO) mode, where the WTGs are the only active power suppliers and the DGs do not run.

WDPSs are low inertia isolated power systems, where the balance between power generation and consumption is difficult to achieve due to the uncontrolled WTGs power production and consumer loading. As a result, the WDPS frequency and voltage can have significant deviations. The WDPS stability and power quality have been dealt with in literature mostly by the dynamic simulation of different WDPS architectures. Ref. [2] shows how the variations in load and WTG power affects the

power quality of a no-storage WDPS. In the no-storage WDPS of ref. [3], a static reactive compensator and a synchronous machine voltage regulator are coordinated to control the system voltage. In the WDPS of [4], distributed resistive loads are controlled to support the speed governor of the diesel engine in the regulation of frequency. When a short-term Energy Storage System (ESS) is added to a WDPS, several benefits such as voltage and frequency support and increasing stability [5] are obtained. Previous benefits are greater in WDPS than in large power systems that have much bigger inertia. The WDPS frequency in [6] is stabilized by an ultra-capacitor based ESS. In [7] a battery-based ESS (BESS) in a WDPS supplies the active power needed to prevent temporarily a DG overloaded situation, so that load shedding is avoided and the WDPS reliability is increased. In [8] the simulations in WD mode of a WDPS with a flywheel based ESS (FESS) show that the WDPS power quality is improved by the addition of the FESS.

WDPS are isolated microgrids and the following microgrid studies are related to WDPS: [9] shows a BESS providing frequency support to a microgrid with high penetration of renewable energy sources, and the BESS in [9] is also used as an uninterrupted power supply for critical loads, a working mode that can be used in the ESS employed in a WDPS; in [10], a BESS supports voltage regulation by counteracting the voltage variations resulting from power fluctuations of renewable power sources; ref. [11] shows how a BESS included in a DGs-based isolated ship power plant smooths the active power variation, so that this study can be applied to the DO mode of a WDPS; ref. [12] shows, with simulation results, the coordination between a supercapacitor and a battery ESSs to balance the active power in an isolated microgrid with only a WTG as a generator. The simulations in [12] can be applied to the WO mode of a WDPS. The "El Hierro" island Diesel-Hydro-Wind power system includes hydropower pumped-storage and has been simulated with the DGs shut-o ff (Wind-Hydro mode) in the following publications: ref [13] shows the system frequency regulation by using the variable and fixed speed pumps integrated into the hydropower pumped-storage; ref [14] shows, among other frequency control schemes, how the frequency regulation is improved by adding a FESS to the system.

The first part of this article presents the modelling of the WDPS shown in Figure 1, which can work in DO and WD modes. In addition to a DG, WTG and consumer load, the WDPS includes a dump load (DL) and a flywheel ESS (FESS). The second part and main goal of this article aims to present solutions for the WD mode of the WDPS of Figure 1 in the case where the WTG produced power *PT* exceeds the load consumption *PL*, a situation that makes the WDPS unstable. The WTG power excess situation (*PT* > *PL*) is simulated in WD mode in three cases, namely DL and FESS-o ff (DL and FESS are turned off), only-DL (DL actuates but FESS is turned o ff) and only-FESS (FESS actuates but DL is turned o ff). In the DL and FESS-o ff simulation case, it is shown that to prevent the isolated power system collapse that the WTG power excess provokes, the solution is to trip the WTG circuit breaker (IT in Figure 1). In both the only-DL and only-FESS simulation cases, it is shown how the DL/FESS are commanded to consume controlled power, avoiding the WTG circuit breaker trip necessary in the DL and FESS-o ff case so that the WDPS absolute stability is increased. A previous paper [15] deals with simulations in WO mode of a system that comprises a WTG, a grid forming SM, load and a FESS. Ref [15] does not include a diesel engine, therefore this article's isolated power system architecture is di fferent. In a previous paper [8], the simulated WDPS includes a FESS, but no DL is considered. Furthermore, the WTG power levels in [8] are below the load consumption ( *PT* < *PL*) in all the simulations, so [8] does not deal with the WTG power excess situation. In addition, [8] focuses on the control of the FESS power converter. Previous papers [5] and [7] use a battery ESS instead of a flywheel ESS, so the used power converter and ESS variables shown in the simulations are di fferent. Additionally, [5] and [7] do not use the DL. The WD mode simulations in [7] do not consider the WTG power excess situation dealt in this paper. Ref. [5] deals with a high penetration WDPS and its main simulations are in WO mode.

**Figure 1.** Layout of the Wind Diesel Power System (WDPS) with Dump Load (DL) and Flywheel Energy Storage.

After this introduction, this article contains the following sections: Section 2 presents the modelling of the WDPS components: DG, WTG, DL and FESS. The WTG power excess situation is analysed and simulated in Section 3 for the DL and FESS-off case and in Section 4 for the only-DL and only-FESS cases. Section 4 also compares its simulation results with the ones in Section 3 and makes a comparison between the only-DL and only-FESS cases. The last section contains conclusions, summarizing the benefits in terms of greater stability and reliability of using the DL/FESS.
