Combined hydro-wind frequency control scheme: Modal analysis and isolated power system case example

Grid operational challenges are significant to increase securely the wind penetration level. New embedded control functions are therefore required in order to make participate wind generators in power system management. In this paper the implementation of inertial response and primary frequency control in a wind turbine controller are investigated. Main factors affecting the performances of the frequency regulation are identified and characterized. The influence of control parameters and the turbine operating point on the inertial response are analyzed through obtained performances in an islanded power system. The combined control scheme using both controllers is also developed and the potential of the obtained grid service at partial load is discussed.

The increasing penetration of variable-speed wind turbines in the electricity grid will result in a reduction of the number of connected conventional power plants. This will require changes in the way the grid frequency is controlled. In this letter, a method is proposed to let variable-speed wind turbines emulate inertia and support primary frequency control. The required power is obtained from the kinetic energy stored in the rotating mass of the turbine blades.

Coordinated reactive power management in power networks with wind turbines and FACTS devices

Variable-speed wind turbines (VSWTs) typically use a maximum power-point tracking (MPPT) method to optimize wind-energy acquisition. MPPT can be implemented by regulating the rotor speed or by adjusting the active power. The former, termed speed-control mode (SCM), employs a speed controller to regulate the rotor, while the latter, termed power-control mode (PCM), uses an active power controller to optimize the power. They are fundamentally equivalent; however, since they use a different controller at the outer control loop of the machine-side converter (MSC) controller, the time dependence of the control system differs depending on whether SCM or PCM is used. We have compared and analyzed the power quality and the power coefficient when these two different control modes were used in fluctuating wind speeds through computer simulations. The contrast between the two methods was larger when the wind-speed fluctuations were greater. Furthermore, we found that SCM was preferable to PCM in terms of the power coefficient, but PCM was superior in terms of power quality and system stability.

This paper presents a combined control scheme, comprising of the well-known PID controller augmented with a radial basis function neural network (RBFNN) for the control of servo mechanical systems. A second-order linear dominant model is considered with an unmodeled part of dynamics that is possibly nonlinear and time-varying. The PID part of the controller is designed to stabilize the dominant model. The RBFNN is used to compensate for the deviation of the system characteristics from the dominant linear model to achieve performance enhancement. The advantage of this combined control scheme is that it can cope with strong nonlinearities in the system while still using the PID control structure which is well-known to many control engineers.

As increasing the penetration level of wind power, a power system operator requires the frequency support to a wind turbine generator (WTG). To support the frequency control, the conventional frequency control schemes adopt a fixed gain or the variable gain in the frequency deviation loops, but these schemes conduct during the operating region of the rotor speed of a WTG. Thus, the additional device needs to support the frequency control when the rotor speed of a WTG is out of the operating range. This paper proposes the coordinated frequency control scheme of a WTG and battery to enhance the frequency nadir when a large disturbance occurs in a power system. The WTG and battery are in charge of different roles. The proposed frequency control scheme of a WTG uses a droop loop with a variable gain depending on the rotor speed and frequency deviation. In addition, the battery discharges the stored energy depending on the state of charge and the rotor speed of a WTG. The performance of the proposed scheme is investigated in IEEE 14-bus system. As the results, the proposed scheme can improve the frequency nadir for a synchronous generator trip by releasing more stored energy.

With the development of isolated power system (IPS), stability analysis of IPS attracts wide attention. It is necessary to consider the load types in the small signal stability analysis of IPS because IPS is lack of the support from external power grid. In this paper, two different load types are considered: resistive and inductive load and constant power load. Based on the state-space model of IPS, the impact of droop parameters, connected line and load capacity is discussed. Then, it is essential to increase the droop parameters so that the power is well distributed among different inverters. But this will decrease the stability margin of IPS. To ensure IPS is still stable with good power distribution, a power system stabilizer (PSS) is introduced. Through the analysis of the small signal stability based on the state-space model, it is shown that the PSS can improve the stability of IPS. The advantage of proposed method is that there is no need to change the original control parameters, and the parameters of PSS can be calculated step by step. Finally, the PSCAD\EMTDC simulation verifies the effectiveness of the proposed method.

The increasing penetration of renewable energies in the electrical systems, particularly in small and isolated systems, like the case studied in this paper, Terceira Island in Azores, Portugal, creates challenges in the dispatch related to the variability and the difficulty to forecast the renewable resources. One way to deal with such issues is to use Water Pumping Storage Systems (WPSS) to regulate the system electricity production by storing energy surplus in low load periods and returning it back in high load periods, reducing at the same time the need to curtail wind. This paper describes and compares a deterministic and a metaheuristic methodology, using Particle Swarm Optimization (PSO), used to determine the best configuration of the WPSS in terms of number and unit power of pumps and turbines, and upper and lower reservoir capacity that lead to the best economic value, determined by the Net Present Value (NPV) of the investment.

This paper deals with a new Autonomous Wind Energy Conversion System (AWECS) employing parallel operated Permanent Magnet Synchronous Generators (PMSGs) driven by variable speed wind turbines and feeding three-phase four-wire local loads. The proposed system utilizes three Pulse Width Modulated (PWM) Insulated Gate Bipolar Transistors (IGBTs) based voltage source converters (VSCs) with a battery energy storage system (BESS) at their dc link. The system utilizes two separate machine side converters one for each PMSG for maxi-mum power tracking (MPT) through their rotor speed control. However a common dc bus, a battery bank, and a common load side converter are used for voltage and frequency control (VFC) at the load terminals. The proposed electro-mechanical system using PMSGs, MPT controllers and a voltage and frequency con-troller (VFC) are modeled and simulated in MATLAB using Si-mulink and Sim Power System (SPS) set toolboxes. The perfor-mance of the proposed AWEC system is presented to dem-onstrate its capability of MPT, control of load voltage magnitude and frequency, harmonic elimination, and load balancing.

An adaptive neuro-fuzzy inertia controller for variable-speed wind turbines

This paper presents a voltage-source-converter (VSC)-FfVDC-supplied industrial system with onsite generation where the VSC-HVDC and generator are taking part in the voltage and frequency control. The reason for using a frequency controller in the converter is to increase the ridethrough capability of the VSC-HVDC-supplied industrial system by exploiting the inertia energy of rotating masses in case of voltage disturbances. The investigation is performed using the PSCAD/EMTDC program and the dynamic performance of the system is evaluated during grid faults and load changes. Three comparisons are done to analyze the system response when the inverter uses different frequency-control strategies, the impact on the system when the turbine operates in different modes, and the VSC-HVDC's ability to mitigate voltage dips from the grid. Simulation results show that the use of a VSC-HVDC for feeding industrial plants can mitigate voltage dips in the grid. Different frequency-control strategies have different responses on the system. The best frequency response is achieved when the turbine is also equipped with a frequency controller.

In this paper, the hysteresis modeling and compensation are carried out and verified for a piezo-driven XY parallel micropositioning stage aiming at a sub-micron precise motion tracking control. Specifically, inverse modified Prandtl-Ishlinskii (MPI) model-based feedforward in combination with a proportional-integral-derivative (PID) feedback control algorithm is implemented for the real-time control. The MPI model is identified by optimizing the weight parameters through particle swarm optimization (PSO) to match the model output to experimental data. For the purpose of comparisons, a feedforward controller using the inverse MPI model solely is realized as well. The performance of feedforward plus feedback over stand-alone feedforward control is examined by experimental studies conducted on the prototype micropositioning stage. Results show that the combined control scheme can reduce the nonsymmetric hysteresis to a negligible level and produce a sub-micron accuracy motion tracking, which provides a sound base of practical control of the micropositioning system for micro/nano scale manipulation.

In this paper, the adaptive finite-time attitude tracking control problem for rigid spacecraft with upper bounds unknown external disturbances is studied by using a novel combining control scheme of adaptive control technique and adding a power integrator (AAPI) technique. An adaptive finite-time control law with boundary layer is first established under the proposed combined control scheme, which can attenuate the influences of disturbances and contain the advantages of AAPI technique. Then, another adaptive finite-time control law without boundary layer is further given, which can simply the control design. It is proved that both the two control laws can ensure the tracking errors converge to the desired regions in finite-time. Moreover, since the established two control laws are both designed with continuous control architecture, the chattering problem is eliminated, which are more adopt to practical engineering applications. An example is included to show the effectiveness of the proposed methods.

New structure of sliding mode control for variable speed wind turbine

Frequency and voltage of embedded variable speed wind turbine (VSWT) driving a permanent magnet synchronous generator (PMSG) is strongly affected by wind speed fluctuations. In practice, power imbalance between supply and demand is also common, especially when VSWT-PMSG is connected to a weak micro grid (MG). If load demand fluctuations become high, isolated MG may be unable to stabilize the frequency and voltage so that battery storage needs to be installed into the MG to adjust energy supply and demand. To allow flexible control of active and reactive power flow from/to battery storage, grid-supporting inverters are used. For a system that contains highly nonlinear components, the use of conventional linear proportional-integral-derivative (PID) controllers may cause system performance deterioration. Additionally, these controllers show slow, oscillating responses, and complex equations are required to obtain optimum responses in other controllers. To cope with these limitations, this paper proposes PID-type fuzzy controller (PIDfc) design to control grid-supporting inverter of battery. To ensure safe battery operating limits, we also propose a new controller scheme called intelligent battery protection (IBP). This IBP is integrated into PIDfc. Several simulation tests are performed to verify the scheme’s effectiveness. The results show that the proposed PIDfc controller exhibits improved performance and acceptable responses, and can be used instead of conventional controllers.