International audienceIn this paper we investigate the sine qua non condition of existence of equilibria for electrical systems with external (AC or DC) sources furnishing constant power to the loads, which is a scenario encountered in modern applications. Two general cases are considered, when the system is i) linear time-invariant or ii) nonlinear, with dynamic behavior described by a port-Hamiltonian model with constant dissipation and switching interconnection matrix. The latter class includes the practically important case of power converters. For both cases necessary and sufficient conditions for existence of equilibria are given, which give an upper bound on the power dissipated in steady-state that should exceed the extracted constant power. The existence of the equilibrium is ensured if and only if the inequality is satisfied

In this work, a decentralized PI passivity-based controller (PI-PBC) is applied to Modular Multilevel Converters (MMCs) to ensure global stability of a multi-terminal HVDC system. For the derivation of the controller, an appropriate model with constant steady-state solutions is obtained via a multifrequency orthogonal coordinates transformation. The control design is next completed using passivity arguments and performance guarantees are established by a small-signal analysis. The obtained results are validated by means of detailed timedomain simulations both on a single-terminal and a four-terminal benchmark.

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Abstract:The high pollution coming from the use of gas turbines in oil and gas (O&G) platforms is calling for more sustainable solutions. One of those is to use wind turbines (WT) to supply power to the water injection systems (WIS) of offshore O&G installations and explore the potential of smart energy management in providing more efficient and green solutions to the O&G sector. The effect of WT integration into the local power system and the coordination with the local gas turbines, need to be carefully analysed since the natural intermittency of the wind resource can jeopardize the stability and efficiency of the offshore grid. In general, the existence of flexible loads interfaced by variable speed drives (VSD), such as water injection systems, can help overcome some of the challenges related to wind intermittency and local rotor angle stability: suitable control of such non-essential loads and load segregation can be implemented to reduce the effect of wind power fluctuations, balance power generation and consumption and contribute to maintaining the optimal efficiency of the gas turbine adjusting its loading conditions. The basic assumption is that the WIS can follow the generation available from the wind turbines. The assumption is reasonable since WISs do not require a fixed injection rate. This work will investigate the system dynamics in the event of short-term wind-induced power fluctuations, analysing the evolution of electrical variables, such as active power and generator rotor speed/frequency, under different operating conditions, to specifically evaluate the possible arise of low-frequency oscillations. The impact of an adequate WIS load control to counteract wind variations and increase the system damping is also further explored. Moreover, in order to prevent the event of critical rotor oscillations in the gas turbine and other directly-connected rotating machines due to the WT dynamics, an algorithm for system damping estimation is proposed and its performance studied in the selected test cases.

The design of microgrids at the level of distribution systems requires a stable behavior for multiple operation states. The tools used to study the stability of such systems require the estimation of the grid impedance. By the use of the grid impedance estimation around an operation point, it is possible to define a space variable-parameter to obtain a qualitative or quantitative measure from the operation to the unstable boundary. This study presents a comparison of the Kalman filter and the recursive least squares method for the estimation of the grid impedance. The grid impedance is estimated by the technique based on two neighbor operation points. The results were validated by a hardware in the loop and an experimental setup. Finally, the estimated values of the grid impedance of a microgrid are used with a large signal stability study of a dc constant power load.

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Abstract:In DC and hybrid microgrids (MG), the DC-bus regulation using Energy Storage Devices (ESD) is important for the stable operation of both the generators and loads. There are multiple commercial voltage levels for both ESD and DC-bus; therefore, the ESD voltage may be higher, equal or lower than the DC-bus voltage depending on the application. Moreover, most of the ESD converter controllers are linear-based, hence they ensure stability in a limited operation range. This paper proposes a system to regulate the DC-bus voltage of an MG accounting for any voltage relation between the ESD and the DC-bus voltage. The proposed system is formed by an ESD connected to a DC-bus through a bidirectional Buck-Boost converter, which is regulated by a Sliding-Mode Controller (SMC) to ensure the system stability in the entire operation range. The SMC drives the Buck-Boost charger-discharger to regulate the DC-bus voltage, at the desired reference value, by charging or discharging the ESD. This paper also provides detailed procedures to design the parameters of both the SMC and the charger-dischager. Finally, simulation and experimental results validate the proposed solution and illustrate its performance.

In this work, a decentralized PI passivity-based controller (PI-PBC) is applied to the Modular Multilevel Converters (MMCs) to ensure global asymptotic stability of a multiterminal MT-HVDC system. Since continuous MMC state-space models naturally have time-periodic steady-state solutions, a first step towards the derivation of the controller is the formulation of an equivalent model characterized by constant steady-state solutions, obtained via a multi-frequency orthogonal coordinates transformation. For the design of the controller, the overall system is represented in an appropriate port-Hamiltonian formulation, which allows the derivation of the stabilizing control law using passivity-based arguments. The results are validated on a threeterminal simulation benchmark.