1. Current Energy Situation
Energy and the environment are pressing issues for human survival and development. Conventional energy sources, primarily coal, oil, and natural gas, are not only finite resources but also cause severe air pollution. Therefore, wind energy, as a pollution-free and renewable green energy source, has attracted widespread attention. Currently, variable-speed constant-frequency wind power systems are particularly popular because they offer many advantages over traditional constant-speed constant-frequency systems. For example, at low speeds, they can adjust to changes in wind speed, maintaining the optimal tip speed ratio to maximize wind energy; at high wind speeds, they utilize changes in rotor speed to store or release some energy, resulting in more stable power output. Direct-drive wind turbines, where the rotor directly drives a low-speed synchronous generator, are widely welcomed. Direct-drive wind turbines offer advantages such as reduced investment, lower transmission chain losses and downtime, lower maintenance costs, and higher reliability. Furthermore, since permanent magnet synchronous generators do not require excitation windings and DC excitation power supplies, the faulty slip rings and brushes are eliminated, creating brushless motors. Therefore, this motor has a simple structure, reliable operation, and low price, making it very suitable for solving the electricity needs of farmers and herdsmen in remote areas of my country.
2. Characteristics of wind turbines
Wind turbines convert wind energy into mechanical energy through their blades, providing torque to drive mechanical loads. According to Betz's principle, the mechanical power captured by a wind turbine from the wind is:
In the formula, Cp is the wind energy utilization coefficient of the wind turbine, A is the swept area of the wind turbine, ρ is the air density, and ν is the wind speed.
In essence, Cp represents the efficiency of a wind turbine in converting wind energy into mechanical energy; it is a function of the tip speed ratio λ and the blade pitch angle α. Therefore, under constant wind speed, the input mechanical power Pmech obtained by the generator depends solely on the wind turbine's wind energy utilization coefficient Cp. Generally, for electrical regulation, α is a constant. Thus, Cp is only a function of λ. λ is a function of the ratio of the wind turbine blade tip linear velocity to ν.
In the formula, R represents the radius of the wind turbine, and ω represents the mechanical angular velocity of the wind turbine.
Substituting equation (2) into equation (1), and taking Cp as its maximum value Cpmax, we have:
In the formula, Pmax represents the optimal power.
There exists a tip speed ratio that maximizes Cp, i.e., Cpmax. A typical optimal tip speed ratio is λ=9, where Cp is maximized, i.e., Cpmax=0.43. Therefore, to achieve Pmax, ω must be adjusted promptly as wind speed changes to maintain the wind turbine operating at the optimal tip speed ratio.
3. Inverter Design
Figure 1 shows the topology of a wind power generation system. The system uses a permanent magnet synchronous generator directly coupled to the wind turbine. To address the rigid coupling between the synchronous generator's speed and the grid frequency, a frequency converter is used between the generator and the grid. The frequency converter consists of a rectifier, capacitors, a battery pack, and an inverter. As shown in Figure 1, the generator's output voltage fluctuates drastically with changes in wind speed. This characteristic affects the wind turbine's maximum power output. That is, at low to medium wind speeds, because the generator's output voltage is very low, active power cannot be guaranteed to flow to the grid or load. Instead, it can lead to the reverse flow of active power.
3.1 Implementation of the circuit for maximum wind energy utilization
The circuit topology of the wind power generation system shown in Figure 1 cannot fully utilize wind energy. Figure 2 shows a simple circuit of a wind power generation system that uses a Buck-Boost circuit to achieve maximum wind energy tracking, where the circuit within the dashed box is the Buck-Boost circuit.
During the ton period, VS is on and VD is reverse-biased off, at which time the inductor voltage UL = Uin. During the toff period, VS is off, the inductor's stored energy is released in the form of self-induced electromotive force, VD is on, and the average load voltage has the opposite polarity to the input voltage, and the inductor voltage UL = Uo. Considering that the integral average value of the inductor voltage over one cycle is zero, we can conclude that:
The duty cycle D is defined as:
From equations (4) and (5), we get:
Equation (6) shows that when D≤0.5, |Uo|≤|Uin|, and the output voltage Uo decreases; when D>0.5, |Uo|>|Uin|, and Uo increases, and the input and output voltages are reversed.
3.2 Maximum Power Search Algorithm
Figure 3 shows the output active power P and D characteristic curves of a wind turbine at different wind speeds. It consists of two parts, A and B. It can be seen that P is approximately proportional to D.
Figure 3. Output power characteristic curve of wind turbine
As shown in Figure 3, taking the output power characteristic curve at a certain wind speed as an example, in region A, dP/dD > 0. When dP > 0 and dD > 0, the operating point is approaching the maximum power level. At this time, to make the system operate at the maximum power level, D needs to be increased further. When dP < 0 and dD < 0, the operating point is moving away from the maximum power level. At this time, the direction of movement of the operating point needs to be changed, and D needs to be increased. In region B, dP/dD < 0. When dP > 0 and dD < 0, the operating point is approaching the maximum power level. At this time, to make the system operate at the maximum power level, D needs to be decreased further. When dP < 0 and dD > 0, the operating point is moving away from the maximum power level. At this time, the direction of movement of the operating point needs to be changed, and D needs to be decreased. Based on the above analysis, Figure 4 shows the software flow of the maximum wind energy utilization search algorithm.
Figure 4. Software flowchart of the maximum wind energy utilization search algorithm
4. Experimental Results
To facilitate the experiment, the following simplification measures were taken: ① A DC motor was used instead of a wind turbine; ② The inverter circuit and the power grid were replaced by a load RL. Figure 5 shows the simplified experimental circuit of the direct-drive wind power generation system. The parameters of the synchronous generator are: rated power Pe = 2.2kW, rated voltage Ue = 200V, and number of pole pairs p = 2. Figure 6a shows the output power waveform obtained by using the wind power generation system shown in Figure 1 under medium and low wind speeds. As can be seen from the figure, almost no active power flows to the load. This is because at medium and low wind speeds, the voltage output of the synchronous generator is not high enough, and therefore cannot guarantee that the current flows to the power grid or the load. Figure 6b shows the output active power waveform obtained by using the wind power generation system shown in Figure 2 under medium and low wind speeds and applying the maximum wind energy utilization control algorithm. As can be seen from Figure 6b, the output active power increases significantly. This is because, under medium and low wind speeds, although the output voltage of the synchronous generator is not high enough... However, by using the maximum wind energy utilization search algorithm to adjust the duty cycle of the Buck-Boost circuit accordingly, the current is forced to flow to the load or grid at near its maximum value, ensuring that the wind turbine operates at its maximum power factor.
5. Conclusion
The most significant problem with wind power systems employing direct-drive permanent magnet synchronous generators is the drastic voltage fluctuations of the synchronous generator at low to medium wind speeds, which prevents the wind turbine from operating at its optimal point. To address this issue, a simple Buck-Boost converter was used, and a maximum wind energy search algorithm was employed to maximize wind energy utilization. This system eliminates the need for wind speed and generator rotation speed monitoring, and its inverter structure is also simple, resulting in lower cost, easier implementation, and greater potential for widespread adoption in remote, power-deficient areas of my country.
