Wind power

Wind energy conversion systems (WECS) extract the kinetic energy from the wind that passes through the rotor blades and convert it into electrical power at the generator side. At their early stage wind turbines were rated at a few kWs, but nowadays the commercially available ones exceed the 5 MW. Wind power remains the most promising renewable energy source because of its capability to generate electricity at a large scale [50].

The nacelle, tower and rotor comprise the main parts in a wind turbine configuration. Two or three blades are attached to the rotor hub and rotate in the direction of the inflow wind. Gearbox and generator lie in the nacelle, where the former adjusts the rotor speed to higher values on the generator side of over 1500 rpm [51]. The generator shaft is driven by the wind turbine to spin and produce electrical power injected into the grid. Beneath, two generator technologies are chosen to be presented whch are occupied in the wind energy market. These are fixed speed asynchronous (FSG) and variable doubly-fed induction machines (DFIG). Variable speed machines convert the mechanical energy into electrical form at a great spectrum of frequencies but then need to be readjusted to the grid frequency of 50 Hz. On the other hand, fixed speed generators is not necessary to be corrected, but are incapable to perform well during wind fluctuations [51].

2.4.3.1. Characteristics of the different Wind Turbine Designs

The stator in AC machines is connected to the three-phase system, while the rotor is short-circuited internally or its slip rings are connected to the grid externally [52]. When balanced three-phase currents are applied at frequency fs, the stator windings produce an electromagnetic field that rotates at

^s

where ns is the rotational speed of the stator in (RPM), and ps is the number of poles. The induced electromagnetic field of the stator also induces a rotation in the rotor, the nr. One more element to be addressed is the slip; Slip is the difference among the stator rotating field and the rotational speed of the rotor [52, 53].

^s

According to [52] & [54] FSG and DFIG are modelled for dynamic simulations, whereas their differences can be attributed to the physical manufacture of each wind turbine type and to their behaviour against a series of subsystems. These subsystems are the aerodynamic, mechanical, generator and the wind turbine control subsystem.

The following figure (Fig 2-4) represents the schemes of a FSG (ASG)¹ and DFIG turbine respectively.

1 ASG: Asynchronous Generator

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Fig. 2-4: FSG and DFIG Wind Turbine Designs [52].

FSG has its blades coupled directly to the induction generator through the gearbox. The stator is connected to the grid, while the rotor is short-circuited. DFIG has its blades decoupled from the rotor of the electric machine.

Consequently, the inertia of the blades cannot be seen by the system and the system cannot respond to a generation loss [55-57]. The ac currents produced by the generator are converted into dc current by an AC/DC converter, and then again converted into ac currents by another AC/DC converter. In FSG there is no speed control and the only way of optimizing the tip speed ratio (λ) lies in the blade pitch angle control. It is difficult to adjust the pitch angle, β, in cases the wind changes rapidly. Thus, this kind of wind turbines are modelled accordingly to the wind characteristics of the region they will be installed that also results to operate more often for optimum λ. On the other hand, in DFIGs

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both blade pitch and speed control is possible that permits this kind of generators work for optimum λ in variable wind speeds [52], [58]. This type of technology occupies voltage controlled inverters ac-dc-ac that convert power at varying frequencies to dc and with the utilization of a power converter back to ac at a fixed frequency suitable for the grid connection [59]. Moreover, these generators can participate in voltage control, providing or consuming reactive power without exceeding the rated limits of the machine.

2.4.3.2. DFIG Inertial Response and Speed Control

As mentioned in [52], the inertial response is the basic method of primary frequency control in wind turbines. The above response is implemented through three hierarchical control loops. The electrical one supervises the generator and the converter, whereas the mechanical one supervises the blade pitch angle and the blades´ speed. The figure beneath (Fig. 2-5) represents the speed controller [60]. There are four main steps within this control. 1) Identification of ωref (generator reference speed) from the P-ω characteristic graph. 2) The error between the measured speed and the reference one is defined as Δω. 3) The error is then sent to PI controller, resulting in the torque speed reference value Tref. This value incorporates the imbalance among the turbine torque and the generator torque, that will result to an accelerating or decelerating torque until the desirable speed is achieved. 4) The torque speed reference and an additional torque inertia term will define the torque reference.

Fig. 2-5: Speed Controller Design [60].

Figures (Fig. 2-6 & Fig. 2-7) beneath represent the inertia and droop controller schemes respectively [57, 60, 61].

The inertia controller–not embedded in the DFIG-is to minimize the mechanical drive train loads, and the rate of power injection by adding a filter that leads to the reduction of electromagnetic torque and the reduction in the peak torque.

On the other hand, the droop controller relates the torque to the deviation from the nominal system frequency as follows:

T = K^p×(ω^o-ω^measured) (2-3)

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Fig. 2-6: Inertia Controller Design [57, 60, 61].

Fig. 2-7: Speed Controller Design [57, 60, 61].

2.4.3.3. Frequency Control and Active Power

Frequency and active power control implementation are analytically mentioned in [52] which are subsequently described in this report. Control of the system frequency is a crucial issue in the operation of a power system. In order to achieve a near constant frequency, it is necessary to keep the balance among the generation and the load in the system. For instance when the system suffers from a generation loss and cannot supply sufficiently the load, the frequency will decline. Frequency response after a system imbalance occupies roughly three time frames and is so called as the primary frequency. A) Proximity effect t=0⁺, B) Inertial response (0⁺<t<tg sec) and C) Governor response (tg<t<tf min). The above responses are essential for eliminating any power imbalance sensed by the system. The proximity effect is rather realized by the machines closer to the load changes and refers to their compensation level according the imbalance. It is independent the machine size and rating. The inertial response refers to the middle time frame after the imbalance and before the governor reaction. In general, after an imbalance, the system will suffer from an overall deceleration, with individual generators react according to their inertial response. The rule is that the larger the inertia, the faster the response will be. It lasts for a slight transient period, usually on the order of a few seconds.

In case that the machine does not provide with any turbine governor action, the inertial response will determine the final steady-state frequency following the load imbalance [62, 63].

Turbine governor is generally applied through the use of a droop controller. The droop control replies to a 5%

decrease in the turbine speed, due to an increase in the load. Thus, any change per unit in the mechanical power is a relationship to the change in frequency (speed). Here, the share of each machine is based on its rating, so the bigger the machine the higher the contribution to the disturbance will be.

The different kind of designs in machines, like the FSGs and the DFIGs that are commonly utilized in wind turbine technologies define the system frequency response. In more details, the FSGs have inertial response inherent in

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contrast to the DFIGs. Subsequently, in the primary frequency control scheme, the FSG will react as a synchronous generator during the inertial response.