Abstract
Electrical generators are designed to perform best under permanent rotation velocity and fixed loads conditions. However, such ideal conditions are not practically feasible during the operation of real wind turbines. Generally, the voltage output of electrical generators can be regulated without redesigning the electrical or/and mechanical parts constituting such a system, by simply changing the connection of the generator to the grid from Star to Delta or by using combined windings. The present work attempts to investigate the behavior of grid-connected wind turbines with Star-Delta, Delta, and Star connection switches in a variety of simulation scenarios, by taking into consideration the influence of both internal and external factors such as the inertia factor and the wind speed.
1. Introduction
The wide use of conventional fuels has caused an increase of the emissions of Greenhouse gases, which are considered to be the main cause of climate change and global warming [1]. Renewable energy resources constitute a significant approach for power generation, without challenging further environmental concerns. The renewable energy resources which have the most enhanced exploitation, thanks to relevant advances in modern scientific research, are solar, hydropower, geothermal, biomass, and wind energy [2, 3]. All of these energy sources, apart from solar energy, demand electromechanical energy converters in order to convert the available energy sources via prime movers to rotation and finally to electricity via generators [4].
Wind energy is considered to be, so far, the most mature renewable energy source [5]. However, the exploitation of wind energy is particularly challenging with numerous research works carried out in this field [6–12]. For instance, many problems in electricity generation from wind are due to the fact that wind power may be highly variable at several timescales, that is, hourly, daily, or seasonally. This is why modern research in wind turbine technology focuses on the development of variable speed turbines. Systems of this type enable variable speed working conditions depending on the present wind speed in order to maximize the energy captured from wind.
In order to cope with this, instead of applying control on the mechanical parts of a wind turbine, it is possible to induce control on the electrical side, allowing for voltage and frequency output control. This alternative is obviously the most efficient for wind turbines, as wind speed (and hence the engine speed) constantly varies with time. Variable speed turbines enable the control of active and reactive powers generated and the grid voltage. Moreover, they have relatively higher energy capture than the turbines of fixed speed. The disadvantage of variable speed wind turbines is that they require the decoupling of the electrical grid frequency and the mechanical rotor frequency and ask for the presence of a power converter that increases the overall system cost and control complexity [13].
On the other hand, the typical design of electric generators is not optimized to accommodate wind turbine aerodynamics and often suffers due to the requirement for constant rotational velocity. Moreover, typical small-sized generators are designed to reduce cogging torque for startup thus making the operating characteristics suffer as a result [14].
Therefore, it becomes evident that voltage control is required in order to achieve the optimal performance of wind generators. Star-Delta connection switch may provide a generator output adjustment method which allows the regulation of the generator exit voltage. Actually, Star and Delta forms are two of the most commonly used in electrical networks arrangements for the connection of three wires [15]. In Star connection the three wires are connected to a common point to form a Y-like pattern. On the other hand, in Delta connection three wires are connected in a way that they form a triangular closed loop. When a 3-phase motor's three windings are connected in a Star configuration, both the voltage applied to each winding and the current per winding are considerably reduced when compared to Delta configuration. So by using Star connection the startup procedure to supply power to a motor is smoother and allows for thinner 3-phase line wires. However, leaving a 3-phase motor running in Star connection is disadvantageous because both the power and the torque produced by the motor are 1/3 of the corresponding quantities it can produce when running in Delta.
The aim of this research paper is to investigate the overall behavior of a grid-connected wind park, in which wind turbines make use of Star-Delta connection switches, compared to standard Delta or Star connections in a number of simulation scenarios.
The rest of the paper is ordered as follows: Section 2 focuses on the use of Star-Delta coupling in wind turbine motors. In Section 3 the configuration of the simulation model built is described. The results of the simulation tests executed are both presented and discussed in Section 4. Finally, Section 5 concludes the paper.
2. Star-Delta Connection in Wind Turbines
It is true that in a normal induction motor, where the stator is connected through a converter to the network and load is relatively low, it is feasible to decrease the magnetic losses by reducing the size of the magnetic flux. However, this is not practical in a wind turbine having Doubly Fed Induction Generator (FDIG) because stator is directly connected to the network which in turn has constant voltage and thus unchanging magnetic flux [16].
Nevertheless, it is still achievable to reduce the magnetic losses in a wind turbine by coupling the stator windings with Delta connection at high wind speeds and a Star connection at low wind speeds. For this reason a so called Star-Delta switch may be incorporated in the overall system structure in order to shift from Delta to Star coupling of the stator and thereby reduce the stator winding voltage when the wind speed and consequently power are low. With reduced stator winding voltage, less voltage is required on the rotor, and the voltage capacity of the power converter and the rotor windings can be utilized to increase the slip approximately with a factor
Generally, Star-Delta conversion is in the center of recent research works which focus on the utilization of novel control schemes which enable the integration of wind turbines with the main power grid, by integrating the generated electricity to the main power grid [18]. This task by default is obstructed due to the asynchronous character of the wind turbines.
However, despite the profound benefits provided by the utilization of Star-Delta coupling in the performance of a wind turbine the shift from Star to Delta and vice versa is correlated with transient phenomena the form and size of which depend on the configuration of the wind turbines and the existing wind conditions. Specifically, wind turbine operation during transient situations is strongly influenced by a wide set of mechanical factors, such as the inertia constant [19].
Actually, the inertia constant has a significant and direct effect on the wind farms transients and thus this value is of great importance [20]. Even though the rotor, gearbox, and other components contribute cumulatively to the total system inertia, their contribution is insignificant when compared to that of the blade weight. Additionally, the transient response of a wind turbine is dependent on the magnitude of the present wind speed.
For these reasons this research paper studies the utilization of Star-Delta winding as a feasible configuration for the generator output adjustment for wind turbines connected to the grid in correlation with the amount of system inertia and the wind speed.
3. Model Configuration
In order to investigate the behavior of grid-connected wind turbines utilizing a Star-Delta connection switch as the wind speed changes, a simulation model was developed by using Mathworks Matlab software environment [20, 21]. Additionally, In order to make the model represent a real-world application, its parameters were chosen so as to correspond to a real grid in rural Greece and the calculation blocks were reconfigured so as to include the function of Star-Delta switching [22].
Specifically, a parametric study was conducted, analyzing the behavior of grid-connected wind turbines using three types of connections: a Star-Delta switch, permanent Star connection, and permanent Delta connection. Moreover, three different wind parks were studied, each utilizing wind turbines of different sizes for each simulation round. All wind turbines are asynchronous and have the same operation frequency.
On the other hand, they differ in other technical features such as the power output, the diameter of the rotor, the cutin and cutout wind speeds, the output voltage, and the inertia constant. Specifically, the inertia constant was considered due to its importance for the wind turbine performance which was declared in Section 2. For each wind turbine modeled the inertia constant has been calculated by taking into account the rotor diameter and the power magnitude [19]. The specifications of the three wind turbine types used are summarized in Table 1.
Technical features of the wind turbines modeled.
The simulation model used represents a complete grid-connected small wind farm formation, consisting of 6 wind turbines. The farm is connected to an 11 kV distribution system transferring power through a 2 km long 11 kW feeder to a 150 kV grid. All wind turbines examined in this study use squirrel-cage induction generators.
The stator winding is connected to the grid by using either a Star-Delta switch, a permanent Star connection, or a permanent Delta connection while the rotor is driven by a variable-pitch wind turbine. A STATCOM of appropriate characteristics for each wind farm regulates the output [13]. Figure 1 illustrates the simulation model used.
Wind farm grid-connected simulation model.
Each one of the wind turbines incorporated is supposed to have a protection system monitoring voltage, current, and machine speed. Additionally, reactive power absorbed by the generators is partially compensated by power factor correction (PFC) capacitors connected at each wind turbine low voltage bus. The PFC capacitor banks were chosen so as to correct the power factor (PF), that is, be as close to unity as possible. The rest of the reactive power which is required to maintain the 11 kV voltage at bus B20 close to 1 pu is provided through the utilization of a MVAr controller with a droop setting equal to 3%.
While Star-Delta is the type of configuration which is in the center of the research performed in this paper, relational operators are also used to determine whether the wind turbines are connected in Star or in Delta configuration according to the existing wind speed. More specifically, when the wind speed is between 3 m/s and 7 m/s the wind turbines are connected in Star, while when the wind speed is between 8 m/s and 26 m/s the wind turbines switch to Delta connection. The switching speed of the Star-Delta switches is 50 m/s for the 100 kW/275 kW turbines and 150 m/s for the 1.5 MW turbines.
Each formation has been simulated for two wind speed scenarios, one with abrupt wind changes and one with smooth wind changes, which were extracted from real climatic data recorded at a wind park in Attica, Greece. The duration of each one of these wind speed scenarios is 1 minute.
4. Results and Discussion
In the following subsections of Section 4 the outcomes of the simulation tests carried out are both described and commented on. Specifically, the performance evaluation of the modeled scheme is performed in correlation with either the influence of the wind speed or the system inertia.
For this reason three different operation scenarios were simulated correspondingly investigating abrupt wind changes, smooth wind changes, and the inertia deviation.
4.1. Performance Evaluation in Abrupt Wind Speed Changes Scenario
The abrupt wind speed changes scenario refers to swiftly changing Aeolian dynamics, with great and sudden changes on the wind speed within very short periods of time, as illustrated in Figure 2.
Abrupt wind speed changes pattern.
The application of this pattern of wind changes in a wind park having Type 1 wind turbines which use either (a) Star-Delta, or (b) Star or (c) Delta connection results to a total power output form is depicted in Figure 3.
Total wind park power output in abrupt wind speed changes scenario, with 100 kW turbines using (a) a Star-Delta connection switch, (b) a permanent Star connection, and (c) a permanent Delta connection.
As shown in Figure 3, the use of the Star-Delta connection switch generates significant transient phenomena but the system appears to be still capable of stabilizing relatively quickly after each shift. However, while the utilization of this switch imposes transient phenomena as shown in Figure 4, it also allows the wind farm to generate power while the wind speed ranges between 7 m/s and 20 m/s.
Star-Delta switching induced transient phenomenon detail referring to main line RMS current in a 100 kW turbines wind park in abrupt wind speed changes scenario.
The system with the wind turbines permanently connected in Star is not capable of generating power at high wind speeds, with the safety system decoupling the generators due to the very high internal currents, in order to insure the reliability of the system.
On the other hand, if the wind turbines were to be permanently connected in Delta, a wind speed of 7 m/s is not sufficient for the system to produce any significant level of power.
Even though the switching from Star to Delta connection (and vice versa) takes only 50 ms, once the switch takes place the generator slip induces a very strong transient phenomenon, where the reactive energy absorbed by the out-of-phase engines leads to the generation of very high currents. The bulk of the transient effect lasts for about 380 ms, a relatively short period of time, yet measures to limit the inrush current already appear necessary.
On the other hand, if the wind turbines were to be permanently connected in Delta, a wind speed of 7 m/s is not sufficient for the system to produce any significant level of power.
Even though the switching from Star to Delta connection (and vice versa) takes only 50 ms, once the switch takes place the generator slip induces a very strong transient phenomenon, where the reactive energy absorbed by the out-of-phase engines leads to the generation of very high currents. The bulk of the transient effect lasts for about 380 ms, a relatively short period of time, yet measures to limit the inrush current already appear necessary.
The application of great and abrupt changes on the wind speed within very short periods of time in a wind park having Type 2 wind turbines which use either (a) Star-Delta, (b) Star, or (c) Delta connection results in a total power output form depicted in Figure 5.
Total wind park power output in abrupt wind speed changes scenario, with 275 kW turbines using (a) a Star-Delta connection switch, (b) a permanent Star connection, and (c) a permanent Delta connection.
As Figure 5 suggests, the results are not as positive as when the system was enlarged and designed for 275 kW wind turbines. Although there is real power generation at low wind speeds when using a Star-Delta switch if the system is allowed to stabilize, the large inertia of the wind turbines does not allow for the quick stabilization of the system between switches; therefore, the use of a Star-Delta switch hardly improves the performance of the system, while the disconnection and reconnection of the generators introduce severe transient phenomena.
Although the Star-Delta switches used on the 275 kW wind turbines still have a switching speed of 50 ms, due to the size of the system and inertia of the generators, the transient phenomenon is significantly more severe than before, both in terms of magnitude and length. The strongest transient phenomenon lasts for about 3 seconds, as shown in Figure 6, with the inrush current reaching nearly 200 A per turbine and exceeding 2 kA momentarily.
Star-Delta switching induced transient phenomenon detail referring to main line RMS current in a 275 kW turbines wind park in abrupt wind speed changes scenario.
The corresponding simulation results referring to the Star-Delta switching induced transient phenomenon when 1500 kW turbines are used in the wind park are illustrated in Figure 7.
Star-Delta switching induced transient phenomenon detail referring to main line RMS current in a 1500 kW turbines wind park in abrupt wind speed changes scenario.
The examination of this figure indicates that the use of a typical Star-Delta switch is not a feasible option with large and very large wind turbines. The size and inertia of the turbine in conjunction with the slower mechanical switch create catastrophic transient phenomena, during which the current peaks at over 3.2 kA and lasts for nearly 9 seconds. Such a phenomenon would surely cause electrical and/or mechanical damage to the wind turbines or to the distribution equipment, removing the possibility of using Star-Delta switches in such installations.
4.2. Performance Evaluation in Smooth Wind Speed Changes Scenario
The smooth wind speed changes scenario depicts relatively steady Aeolian dynamics, with the wind speed changing significantly but smoothly over time, as illustrated in Figure 8.
Smooth wind speed changes scenario pattern.
The use of this pattern of wind changes in a wind park with Type 1 turbines using either (a) Star-Delta, (b) Star, or (c) Delta connection results in a power output form depicted in Figure 9 while the induced transient response of current is shown in Figure 10.
Total wind park power output in smooth wind speed changes scenario, with 100 kW turbines using (a) a Star-Delta connection switch, (b) a permanent Star connection, and (c) a permanent Delta connection.
Star-Delta switching induced transient phenomenon detail referring to main line RMS current in a 100 kW turbines wind park in smooth wind speed changes scenario.
The observation of these two figures shows that, similar to the first wind speed scenario, the use of a Star-Delta connection switch allows the wind turbines to generate power at lower wind speeds and still reach their maximum power output potential at higher wind speeds. If the wind turbines were permanently coupled in Delta connection, a wind speed of 8 m/s is the minimum required for the wind park to produce notable levels of power, whereas a permanent Star connection disconnects the generators if the wind speed is higher than 14 m/s in order to protect the engine from the high internal currents. The smooth change of the wind has a very positive impact on the transient phenomenon induced by the Star-Delta switching as the generator slip is affected to a lesser extend during the switch. As a result, the time that the transient effect lasts is reduced down to about 140 ms and the bulk of the effect lasts for about 50 ms.
The simulation results referring to the Star-Delta switching induced transient phenomenon when 275 kW and 1500 kW turbines are used in the wind park are correspondingly illustrated in Figures 11 and 12.
Star-Delta switching induced transient phenomenon detail referring to main line RMS current in a 275 kW turbines wind park in smooth wind speed changes scenario.
Star-Delta switching induced transient phenomenon detail referring to main line RMS current in a 1500 kW turbines wind park in smooth wind speed changes scenario.
The observation of the simulation results which is illustrated in Figures 11 and 12 makes it profound that the utilization of a Star-Delta switch induces extremely strong and persistent transient phenomena even in the cases that the changes of wind speed are very smooth. Therefore, it becomes obvious that the utilization of Star-Delta switch with even average-sized wind turbines is an unrealistic option.
4.3. Performance Evaluation with respect to the Inertia
In this subsection the influence of the inertia on the transient phenomena induced by the Star-Delta switching is investigated.
Specifically, the RMS value of main line current is recorded when the inertia is considered to be reduced by 10% and 20% compared to its initial value for the case that wind turbines of Type 1 are used. The corresponding results are illustrated in Figures 13 and 14.
Star-Delta switching induced transient phenomenon detail referring to main line RMS current in a 100 kW turbines wind park in abrupt wind speed changes scenario with inertia reduced by 10%.
Star-Delta switching induced transient phenomenon detail referring to main line RMS current in a 100 kW turbines wind park in abrupt wind speed changes scenario with inertia reduced by 20%.
By observing these two figures it can be deduced that inertia of the wind turbine assembly has a notable influence on the length but not the magnitude of the transient phenomena caused by the utilization of Star-Delta switching.
More precisely, as it can be seen in Figure 13, the reduction of the inertia constant of the 100 kW wind turbine by 10% can decrease the length of the transient phenomenon by about 47%, from 380 ms down to about 200 ms. Similarly, reduction of the inertia constant by 20% can reduce the transient phenomenon down to 140 ms, as seen in Figure 14.
Nevertheless, although the reduction of the inertia constant has a significant effect on the transient phenomena of small wind turbines, this benefit diminishes as the size of the wind turbine increases.
As it can be seen in Figure 15, the reduction of the inertia constant of a 275 kW wind turbine by 25% will reduce the length of the transient phenomenon by about 650 ms but the overall length of the phenomenon remains above 2.2 seconds, a value which still is greatly intolerable for any power system.
Star-Delta switching induced transient phenomenon detail referring to main line RMS current in a 275 kW turbines wind park in abrupt wind speed changes scenario with inertia reduced by 25%.
5. Conclusions
The outcomes of the simulations have showed that Star-Delta switches are feasible configurations for small wind turbines with the use of relatively simple transient filters. The utilization of a Star-Delta switch improves the operational range of the wind turbines, allowing them to generate power at lower wind speeds.
However, the transient phenomena caused by the Star-Delta switching will most certainly be catastrophic for larger wind turbines. Thus, the use of Star-Delta switches may only be used with low power, lightweight turbines, and switching needs to take place at relatively low wind speeds.
Therefore, although simple Star-Delta switches cannot be used with wind turbines of medium size and above, they appear to be an attractive option for increasing the power output of small and very small wind turbines. This is especially important for the small residential level wind turbines, allowing small private installments to produce more power and maintain an output at lower wind speeds thus aiding the growth-distributed energy generation.
The reduction of the inertia constant has no practical effect on average and large wind turbines, as the length of the transient phenomenon remains greatly above any acceptable level. However, the effect of the lower inertia is significant to the 100 kW turbines. The reduction of the wind turbine's blades weight by using more advanced materials and processes could in time make the use of Star-Delta switches a more attractive option.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.